Calibration Certification of Spectrophotometers

5/25/2018 Calibration Certification of Spectrophotometers

CALIBRATION & CERTIFICATION OF SPECTRO PHOTOMETERS

AuthorManish Kapadia, JISL Ahmedabad.(JISL belongs to Jaysynth Group, Mumbai, Pioneers in Textilecolorants, and early Indian manufacturers of spectrophotometers.)

1 IntroductionLike all instruments used for measuring physical quantities, spectrophotometersneed to be calibrated and characterized periodically. The procedure itself

must be certified, also periodically, to validate the use of the instrument. ISO

9000a series of quality management and quality assurance standardsrecommends the documentation of these procedures to maintain a history

trail of a process.

Since many people are unfamiliar with ISO 9000, the originalstandards in the series are outlined in a table:

1.1 TerminologyBefore proceeding any further, we need to agree on the terminology. Often we

use interchangeably terms like calibration, characterization, certification,qualification, and validation. However, these terms are all different.

Calibration. To correlate the readings of an instrument with a standard. Aftera calibration procedure the output of the instrument may be different (closer

to the standard).

Characterization. To relate the readings of an instrument to a standard. Thecharacterization procedure does not change the instruments output. Thecharacterization of a (calibration) standard, e.g., a reference tile, consists in

determining a physical property of the standard based on a physical observation.

If the characterization instrument has been calibrated with respect to a

natural constant or a primary standard, then the characterization is absolute,otherwise it is relative.

Certification. The creation of a document attesting that performance of aninstrument is within a pre-specified tolerance. The certification requires threeelements: an authority (person or apparatus skilled in the art), a characterization,

and the presence of a tolerance.

Qualification. The verification that an instrument can be used to fulfill a specifiedtask. The steps required to certify an instrument for a particular task.

Validation. Confirmation that an instrument or procedure performs as certi-fied. Validation occurs through peer review or through signature of a certifi-

cate after the assertion has been verified.


Traceability. The ability to relate an instrument to a natural constant or a primarystandard by transferring a calibration from one tile to another [9].

Precision. Random error in measurement.

1.2 Manufacturing issuesMost commonly CIE colorimetry is employed to determine whether the color

of a sample is a match to the aim-color of a product for an average observer

under specified viewing conditions. The tolerance of an instrument can thenbe just slightly lower than a just noticeable difference (jnd). In a different

application, some drifts in manufacturing processes can be detected more easily

with a colorimetric measurement than measuring a different parameter,

e.g., temperature* or acidity. In these cases jnds are irrelevant and precision ismuch more important than accuracy.

This report describes a verification procedure for spectrophotometers used in

process control based on trend analysis. This analysis is more valuable if theinstruments drift is tightly monitored, so it can be unequivocally established

that any detected drift is in the process under observation and not in the

instrument used for the measurement.

Verification implies a two step process:

a calibration to adjust the instruments scale, and the measurement of a sampleof known color to validate the calibration within a predetermined tolerance.

We recommend a much more frequent calibration certification than is usual

for spectrophotometers used in color matching applications. To make this feasible,

the procedure must be very simple and quick to perform; our procedure

fulfills both requirements.In support of ISO 9001 procedures we publish all data relevant to our instruments.

We make recommendations for a working methodology and recordkeeping

that significantly facilitate the introduction of ISO 9001 compliantprocedures.

1.3 Previous workTodays spectrophotometers can be easily calibrated making a single quickmeasurement of a white reference tile that is traceable to an absolute standard.

This procedure is sufficient for routine work, but periodically the instruments

calibration must be verified by measuring a number of different tiles.

Unfortunately this verification is often ignored.

In todays world of contract manufacturing, where parts are progressivelymanipulated by a number of suppliers and then assembled, quality control

can be a nightmare if it is not designed very meticulously. This is the main

goal of the ISO 9000 standards series mentioned earlier. The prescribed proceduresfor a spectrophotometer can consist for example in a verification with a

single color tile each three months and a larger set each year. This larger set

usually consists in a number of gray samples to determine linearity and variousvivid samples.


For process control we need a more frequent verification. If possible, we would

like to use the verification data to increase the precision of the measurement

data.Most research in this direction has been to improve the calibration of instruments.

The techniques (see e.g., [9]) consist in modeling the various errors,

such as white reference, black reference, wavelength errors. A set of referencesamples characterized with an absolute instrument by a standards body ismeasured with the instrument under consideration. The obtained data is used

to estimate the parameters for the error model. The analyst can then measure

a sample and apply the model to the measurement data to obtain a characterizationthat is traceable to a primary standard.

Such a procedure is quite difficult and requires a very high commitment. Fortunately

it is necessary only when calibration standards are manufactured,

because in calibration a scale is zeroed and accuracy is therefore very impor-tant.

An example is the standard for a corporate color that is used in all goodsmanufactured or distributed by a company. In most applications of spectrophotometry

accuracy is less important than precision, the ability to repeat a

certain measurement.

The focus of the work in this report is to design a validation procedure that isas simple as a calibration. Such a procedure does not need a high commitment.

5 Procedures5.1 OverviewEach of the sub-sections in this section is a self-contained step-by-step

description of a qualification procedure. This sub-section is an overview outlininghow the various procedures fit together. It shall also clarify why the procedures

have been defined and what benefits they provide. We generally followthe recommendations of Christopher Burgess [3] and of Gretag-Macbeth [10].It was not so many years ago, that the standard instrumentation for color

measurements consisted of a spectroradiometer, a set lamps, and a BaSO4

(barium sulfate) cake. The instruments sensor was cooled with a Peltier element, which caused condensation, requiring a silicagel cake in the instrument

that had to be baked periodically in an oven. A careful log had to be maintained

on the lamps on time so they could be renewed when a certain usage

time had past. Finally, the barium sulfate cake had to be inspected carefullyand sanded when necessary.

Setting up the instruments was also a challenge, because the geometry had to

be calibrated for each measurement session. The instruments were connectedto a minicomputer running simple software that could compute the colorimetricquantities and sometimes could produce simple diagrams or histograms.

Todays spectrophotometers are simpler to use than the remote controller for a

television set, are completely self-contained, ruggerized, and have a sophisticatedembedded software system that can compute a plethora of quantities,

evaluate them statistically, and plot them.

Unfortunately this can mislead users in thinking that measuring color is as


easy as flipping a light switch. The fundamental physics on which color measurements

are based have not changed; color has not become easier to measure,

and modern instruments merely provide more comfort and convenience.The main area of concern in practical colorimetry is the validation of color

measurements. As outlined by Burgess, this consists of the following points:

the fitness for purpose of the spectrophotometer for the task demonstrable compliance with the manufacturers performance criteria

demonstrable compliance with established standards and practices

documented evidence for continued operability and data integrity40 Spectrophotometer Calibration and CertificationFor HP Internal Use Only

These requirements for validation are met with the qualifications for design,

installation, operation, and performance. These qualification steps are summarizedin the following sections.

5.1.1 Design qualificationThe design qualification is aspecification for the instrument:

photometric requirements

environmental conditions

sample presentation

data acquisition and presentation needs

operability factors; health & safety issues

cost/benefit analysis

The photometric requirements include such parameters as precision, accuracy,

uniformity with respect to color, aperture, reading frequency, and geometry.

Color metrology is still a field of active study in the basic sciences; the

colorimetric calculations are still being improved considerably and standardizationbodies such as the ASTM, CIE, and ISO/IEC regularly update the recommended

procedures. The instrument should be current with the latestprogress in the technology, such as color matching function interpolation andcolor difference evaluation.

The environmental conditions cover parameters such as operational limits for

temperature and humidity, ruggedness, orientation in space, electrical supplystability, and the generation of heat and noise by the instrument.

The sample presentation refers to the instruments ability to reliably measure

the sample. This is not limited to the physical size of the sample, but also covers

considerations such as the ability to measure liquids, wet samples, cockledsamples, and to select the appropriate backing. If the sample is ink on a substrate,

the substrate can be opaque, translucent, or transparent; it can be

glossy or opaque.The data acquisition needs usually are a serial RS-232 or USB connection anda full specification of the protocol. In the past years there has been increased

demand for support of the http data transfer protocol. Depending on the users

skills and time, there are varying demands on a client implementation of theprotocol, which can range from a simple driver to a class library or a specialized

application.

The operability factors include documentation and training. Color is an illusion


and colorimetry is the art to predict an illusion from a physical measurement.

Each analyst using the instrument must be thoroughly trained in theFor HP Internal Use Only

basics of colorimetry and in the operation of the equipment. Refresher courses

must be given periodically, and a supervisor must be responsible for the analyststraining.

5.1.2 Installation qualificationThe second qualification is the compliance with the above specification, i.e.,

does the instrument work the way the manufacturer claims it should? This

reports provides methods and guidance for the following qualification steps:

written test protocol with acceptance criteria

certification from the installation engineer

full written records

adequate documentation and manuals

health and safety information from manufacturer

Regardless of their exterior ruggedness and environmental specifications,

internally spectroradiometers are very delicate instruments that should neverbe opened or disassembled.The installation engineer has also to assure that the analysts using the

instrument fully understand the operation of the instrument and the procedures.

Especially in the collection of historical data for process control, meticulousadherence to procedures and accurate records are essential for the ability

to perform time series analyses.

When several sites use the same process, then the procedures must be the

same, so it is possible to compare the data.

5.1.3 Operational qualificationThe operability items ensure that the instrument works for the specific applications:

training for staff

5.1.3 Operational qualificationThe operability items ensure that the instrument works for the specific applications:

training for staff

testing protocols for standards

testing protocols for subsystems

calibration procedure and defined test methods and procedures

challenging the system with known standards

record what is done and why it is done

ensure data transfer integrity to external software

In essence, the operational qualification requires that the procedures in thisreport be followed and that the described tools be used. If the application

changes, then this report must be revised as necessary, historical data must be

safeguarded as necessary, and the analysts must be retrained.The major immediate change is to port the database aspect from the simple

demonstration application described in this report to a database that is fully

integrated with the existing data storage and management system.


Another major change will be to upgrade the procedures to full ISO 9001 compliance.

Such a compliance does not only facilitate the business transactions

with suppliers and customers. It also entails a critical review of all proceduresand eventually leads to improved product quality, higher manufacturing

yields, and improved profits.

Accumulating data by itself does not serve any useful purpose. The linchpin ofthe process is the design of data analysis algorithms that allow the datas useto provide a feedback mechanism in the manufacturing line to keep the process

within specified tolerances. The quality of the collected data directly

determines the statistical confidence intervals for the correlation between colorimetricdata and manufacturing drifts.

Once sufficient historical data is available and data analysis algorithms are

designed, the tolerances described in this report will have to be reconsidered.

This reconsideration will produce a different set of qualifications.

5.1.4 Performance qualificationThe performance qualification regulates the ongoing compliance, ensuring

that the instrument continues to work in the intended manner. The items inthis qualification include:

regular service program

performance monitoring with warning and action limits

individual log books and databases for spectrophotometers

regular peer review

simple change procedure

The essence of this report is to define a good set of procedures for the performance

qualification, and the following sections will describe each procedure in

detail. These procedures ensure that each instrument performs within certainlimits and assures the validity of the data generated.

For HP Internal Use OnlyProcedures 43While the database designed for this demonstration deals only with measurements

per se, there should also be a database table to log all other events that

impact the data, such as

lamp changes

problems and service calls

instrument substitutions

firmware upgrades

optical upgrades

procedure changes

manufacturing line changes impacting the colorimetry manufactured good changes (e.g., ink formulation)

environmental changes

personnel changes

peer reviews

With the last item we imply that each site is visited on a regular basis and theprocedures and data be reviewed by a peer or a consultant. This ensures that

the data of the various sites is fully exchangeable and the sites do not drift


away from each other.

Changes in ink formulation are important because they can lead to metameric

matches. Metameric matches require accuracy, i.e., absolute certification,while the procedures described in this report are based on relative certification,

i.e., only precision is certified. This aspect may have to be reconsidered.

The log should not only exist digitally in a database, but should also exist inprinted form, because it is more convenient and robust to use during peerreviews. For this reason, when we designed our demonstration database we

took care to design a printable form layout of each record, allowing physical

collection in a log book.

5.2 White calibrationThe white calibration must be performed when one of the following events has

occurred:

begin a new measurement session

end of a pause

10 consecutive measurements have been performed

the ambient temperature has changed by more than 5C

Frequent white calibration is important in the case of process control, because

a drift in the instrument might be interpreted as a drift in the process. Whenan automatic stage such as the SpectroScan is used, the controller software

must be programmed to perform the periodic calibration automatically. With

the SpectroScan, the reference tiles should be lodged permanently in one of

the apposite receptacles in the stage and the entire set-up should be shieldedfrom direct sun light.

When the instrument is used manually, the white calibration must be performed

in the following steps:1. set the white reference parameter to absolute

2. set the filter wheel to the desired position (SPM-100) or apply the required

filter (Spectrolino)

3. check the white reference for cleanliness and place it on an even surface;for the Spectrolino, always insert the reference tile in the special calibration

base (see Figure 2 on page 18)

4. adjust the spectrophotometer to the white reference and initiate the white

calibrationUsing the calibration plate (9) and ensuring the instrument fits perfectly in

the positioning pins (6) is important. Ceramic tiles are never uniform and the

purpose of the base is to ensure you always measure the same spot of the tile.For this reason it is necessary to remove the positioning foot (see Fig. 3) from

the Spectrolino and measure directly on the calibration plate holding the clip

(7) with the white reference. For details, see Construction on page 18.If you have several instruments, make sure that the serial number on the

instruments identification plate is the same as thaton the white reference (5).

5.3 Quarterly certificationInstrument error is not the only source of incorrect measurements. More often


the error is in the measurement procedure. This can have many causes, such

as documentation error, personnel changes, etc. The best insurance against

this type of problems is to participate in the CTS Collaborative Reference Program

for Color and Appearance.

5.3.1 Background

Collaborative Testing Services Inc. (CTS) is an organization that conductsinterlaboratory tests. In such a test, the performance of a laboratory is not

compared to an absolute reference. Instead, multivariate comparative statisticalmethods developed in conjunction with the National Institute of Standards

and Technology (NIST) are used to gauge performance.

With these techniques a consensus value among a large number of participating

laboratories using a variety of instruments and methods is determined.

The values of each laboratory are then compared with this consensus value.

Such an analysis is also known as a real world analysis.The concept of formalized, large-scale interlaboratory testing was developed

in the late 1960s by NIST (then known as the National Bureau of Standards,NBS). In the mid 1970s, full responsibility for the laboratory proficiency testing

program was transferred to CTS. CTS maintains compliance withISOGuide 431996: Development and Operation of Laboratory Proficiency Testing.

It is important to understand that proficiency testing does not gauge the performance

of an instrument. Rather, as the name implies, it assesses the profi-ciency of a laboratory.

5.3.2 Color and color differenceAnalysts measure a set of three opaque color paint chips made by Munsell

Color, Macbeth Division. The colors are selected from throughout the full colorspectrum. The chips consist of a metameric and nonmetameric match with

small color differences.

The measurements are performed according to ASTM E 1349 immediatelyafter opening the sealed moisture-free barrier bag containing the specimens.

Each specimen is backed with the other specimens when making measurements.*

Each specimen is measured twice, rotating the specimen 90 for thesecond measurement. This latter step is very important for the Gretag instruments,

because they are based on 45/0 geometry, which is susceptible to

polarization.

When you receive the report with the results, use your lab code to find the

data you have contributed. You will find one of two data flags: 0 or X. If thedata flag is 0, your results are consistent with those of the other participants

and you may proceed. If the data flag is X, stop work immediately and consultan expert to review your data and your testing procedure. This review will

consist of four steps, as delineated in the decision tree in Fig. 16.

In the first step repeat the performance test and verify that you can approximately

reproduce the results in the report. With this you check for grosserrors, such as using the paint chips from an old test or measuring the chips


from the back side. We strongly recommend a peer review if this happened.

In the second step perform the certification procedure with your Green reference

tile. If this test fails, you probably have a defective instrument and

should repeat the CTS test with the backup instrument. If you would havepassed the test with the backup instrument, follow the procedure to replace a

defective instrument (not described in this document).

If the certification with your reference tile passes, proceed as follows with stepthree. Request the panic reference tile (see Section 2.2.1 on page 15) and use

the certification database to perform an absolute certification. If this certification

passes, you made a procedural error in performing the CTS test. Youshould obtain training in ASTM procedure E 134990 and the CTS test in particular.

All procedures in your laboratory should be reviewed by an expert.

Step four. If the absolute certification fails but the relative certification failed,

your laboratory is set up incorrectly and an expert should review all operations.Returning to the CTS report, after reviewing the data flag, examine the column

labelled E*. If the result is followed by an * your E is greater than 2but less than 3 standard deviations from the grand mean. You should review

your procedure and perform the relative certification procedure to establish ifyou can continue working.

If the result is followed by an X, your E is greater than 3 standard deviations

from the grand mean. In this case you should follow the decision tree inFig. 16.

5.3.3 Spectrophotometric analysisIn this test analysts report the reflectance data at 16 selected wavelengths inthe 400700 nanometer range (i.e., at 20 nm intervals) on one of the opaque

color chips. The same backing rule as in the previous test is followed. Also in

this test it is important to repeat the measurement after rotating the specimenby 90 because of polarization in the instrument.

The results are evaluated as in the color & color difference test by examining

the data flag and the procedure explained in the previous section can be followed.

The reflectance values at single wavelengths can also be flagged. Sinceour intended use of the spectrophotometer is just colorimetry, a few errors in

the reflectance values are not a cause for alarm.

5.3.4 Useful aidsOne of the advantages of the CTS program is that it takes only a few minutes

each quarter to complete. There are a few tools to make this work even easier.Copying the results from the screen into the CTS form is tedious and can

introduce errors. If you have the KeyWizard, download a convenient Acrobat

form from http://chroma.hpl.hp.com/Acrobat/Notebook/Colorimetry/cts.pdf (ifyou are viewing this document on-line, the link is active and you can click on

it). JavaScript is used to calculate the data required in the CTS form from the

data provided by the SMP-100 or Spectrolino.

If you have an older version of the Acrobat Reader, you may have to downloadthe free Acrobat Forms Fill-in Plug-in 3.5 Update from the Adobe Web site.

This two-page form will automatically fill out the CTS form, which can be

printed and sent to CTS.


If you have Acrobat Exchange, you can save the blank form after you have

edited the laboratory number and analyst name. You can also save the completed

form for future retrieval or to submit it to a document repository. In anycase, keep a hard copy in the log book.

If you want to further examine the data, there is an Excel spreadsheet to tabulate

the data, plot the spectra, and compute the resultant E of your laboratory.This spreadsheet is available from http://chroma.hpl.hp.com/color/CTS1098D65.xls.

Finally, the historical CTS data for Spectrolino serial number 10255 in Palo

Alto can be found at http://chroma.hpl.hp.com/color/cts.html.48 Spectrophotometer Calibration and CertificationFor HP Internal Use Only

5.4 Yearly calibrationOnce a year the instrument should be sent to the manufacturer for the following

operations:

install firmware and hardware updates (this is to ensure that all instruments

are comparable and interchangeable)

measure the full set of color ceramic tiles, each mounted in the calibration

base

compute the error compared to an absolute instrument traceable according

to ISO 9001

compute the error compared to the average of 10 instruments of the same

construction

measure 10 times the white tile to assess the short term repeatability

The manufacturer will return the instrument with a calibration certificate

that must be stored in the document repository. An example of the yearly calibration

certificate can be found on the intranet at http://chroma.hpl.hp.com/

color/absCal98.html. This is the same data as in Section 2.4, Spectrophotometer,on page 20.

5.5 Initial characterization of the Green tileWe have discussed earlier (Section 2.1.2 on page 13) the distinction betweenabsolute and relative certification. For the relative certification, when you

receive a new spectrophotometer you must first determine the aim values of

the green tile for this instrument. We refer to this preparation as the initialcharacterization.

Since the color reference tile has a life time of 5 years, we recommend to collect

the data for a number of illuminants and filters, because it is possible that

during such a long period of time the viewing conditions chosen as a Corporatestandard can change. We suggest to characterize at least for two illuminants,

D50 and D65, and two filter settings, no filter (U for unfiltered) and D65 filter.

The observer angle is 2 and the color space is CIELAB.Although it is sufficient to record theL*, a*, and b* coordinate values, for convenience

we require to record also the correlates for chroma and hue. We suggest

to also keep a record of the reflectance spectrum of the reference tile

measured with the various filters. The benefit of recording the entire spectrumis with the CTS test (see Section 5.3.3 on page 47). When there are several

flagged values, it is a good idea to compare the values at those wavelength


for the color reference with the initial values. Finally, the spectral data can be

useful for diagnostic purposes when an instrument fails.For HP Internal Use Only

The steps are similar to those discussed earlier for the white calibration. It the

demonstration database, select Set up relative certification for an on-lineguide.

1. set the white reference parameter to absolute

2. set the first illuminant3. set the observer angle

4. carry out the white calibration as described in Section 5.2 on page 43

5. create a new measurement set in the certification database6. measure the color reference tile 10 times at a 10 second interval

7. record the color coordinates in the colorreference table of the database

8. repeat for each filter and illuminant

9. back up all database filesIn step 6. we recommend to use the Color Measurement Set Acquisition view

in the measureset table of the database. If you cannot make 10 measurements,

we encourage to make at least 3 measurements at 10 second intervals and toenter the average values in the color reference table. The average values aredisplayed in the evaluation view of the measurement set table.

There is no command to transfer by program the color coordinates from the

measurement set table to the color reference table. This is intentional, so thecolor reference table is a database by itself that can be easily redistributed

within the Company any time it is necessary. For example, this table could be

maintained in a central location and made available on the intranet through a

servelet. Depending on the platform, the color coordinates can be transferredfrom one table to another using copy & paste or drag & drop.

In the color reference database, the part number for the clip with the green

tile that fits in the Spectrolino calibration plate is 36.65.25. It is important torecord the serial number because even if the ceramic tiles are from the same

batch, the uniformity is not perfect.

Always record the current ambient temperature. If the tile is stored in a closed

container, the container should be opened half an hour prior to use. The temperaturetolerance depends on the effect of thermochromism (see Section 6.4

on page 55) for the particular tile and the planned tolerance for the certification.

For example, the Green tile in the BCRA CCS II standard exhibits a change of

0.68 E*ab units when the temperature changes by 10C, the measurementgeometry is spinc 0/t, and the illuminant is D65 with the CIE 10 observer [8].

A tolerance of 2C in this case translates to a measurement uncertainty of0.68 * 0.4 = 0.27 CIELAB units. This is a large error in the case of process con-50 Spectrophotometer Calibration and CertificationFor HP Internal Use Only

trol, but it seems to be realistic in the real world. If the temperature excursioncan be reduced, then a smaller interval should be specified for the temperature

tolerance. The raw data in Table 16 on page 57 should be used to compute

the colorimetric differences for the particular instrument geometry and viewingconditions prescribed.


Conceptionally, we treat a reference tile as a different color reference for each

viewing condition. In the database we store a separate record for each combination

of illuminant and observer angle. Since the serial number must be aunique identifier, we have followed the convention of creating a serial number

by combining the serial number on the clip label (see Figure 2 on page 18,

item 5) with the illuminant and the observer angle.For example, the colorimetric data for reference tile 221 measured for illuminantA and the 2 CIE observer is stored in the record with the serial number

field 221-A-2. This convention is only a pro memoria and does not have to be

followed. (See Table 2 on page 16 for the colorimetric data.)In Appendix VI on page 77 we reproduce the colorimetric values from the initial

characterization of five Green tiles with instrument 10255. These values

are just examples and cannot be used with other instruments.

5.6 Accuracy checkThe accuracy check is a simple verification that must be repeated at the beginning

at every working shift for instruments that are used in process control.

For instruments used in less critical applications, such as color reproductionor ICC profile creation, the verification must be repeated at least every three

months.

The first preparatory step in this procedure is to start the certification database

and browse the references until the relevant reference record is found.Check the current room temperature and verify that it lies within the limits

indicated in the record, taking into account the tolerance of your thermometer.*

Complete all preparatory steps:1. check room temperature

2. set the white reference parameter to absolute white

3. set the illuminant

4. set the observer angle

* The tiles should be stored at ambient temperature and if they are in a closed container the

container should be opened 30 minutes before the measurement.

Return to the certification window and select the viewPerform certification. Inthis view follow the instructions on the screen:

1. click on theNewbutton to create a new record. The system generates a

new sequential certificate number and fills in the fields with the current

date and time2. enter the filter on the spectrophotometer. When you proceed to the next

field, on the right of the filter designation you will see the warning

WRONG. Ignore this warning for the moment; at the end of the certificationit will indicate if the unit has detected the same filter as you indicated.

This is a consistency check

3. enter the serial number of the color reference tile. The pop-up menu showsthe entries in your color reference database

4. carry out the white calibration as described in Section 5.2 on page 43.

Enter the current room temperature. The system will instantly verify thatit lies within the limits; the instrument check will fail if the temperature is


outside the limits

5. check the color reference for cleanliness and place it in the receptacle

6. enter the instruments serial number and a description of the instrumentand the reference

7. click the Gobutton to jump to the measurement set window and make sure

it is set to the data acquisition view. On the bottom on the screen are theinstructions for the KeyWizard; you have already performed the white calibrationand a new record has already been created, so these two steps must

be omitted

proceed to step 5 in the instructions on the screen and perform the prescribednumber of measurements. This number is usually the same as the

number of repetitions you normally perform in the instruments operation

at the end go to the evaluation view and verify that the standard deviation

is small. If necessary, you can add a comment for future reference8. return to the certification window, where you will notice that the system

has verified the settings on the instrument against the values in the color

reference databaseclick in the next two fields to refresh them

the first field shows the CIE94 color difference and the second field shows

the pass/fail message. Currently the tolerance for absolute measurements

is 0.8 and for relative measurements it is 0.39. Click on thePrintbutton to print a certificate and file it in the log book

If the test fails, check for the following conditions:

is the room temperature correct?

are the white reference and the color reference both clean?52 Spectrophotometer Calibration and CertificationFor HP Internal Use Only

do the reference tiles have the correct serial numbers?

if you are using the SpectroScan, are the two clips in the correct bays?

if you are using the Spectrolino in hand-held mode, did you remove the

positioning foot?

is the filter clean?

After checking all conditions, repeat the accuracy check. If it fails again, follow

the rules for a failed quarterly certification summarized in Figure 16 onpage 46.

6 Thermal issuesIn a manufacturing situation it is not always possible to control the room temperature.

For example, in a factory that has been built in four copies, the following

temperature ranges have been measured:At first glance, these ranges seem to quite benign, especially when compared

to the typical operating temperature range for spectrophotometers, which is

from10C to 40C. However, there are four important issues, especially when

ceramic tiles are measured: noise in the sensor, dimensional stability of themonochromator, heating the sample, and thermochromism in the sample.

6.1 Sensor noise


CCDs have a temperature-dependent leakage current that flows even when no

light impinges upon the element. This signal is called dark current. Modern

instruments no longer require cooling with a Peltier element and instead dissipateheat produced by the light source with passive systems.

Because customers want small instruments and short measurement intervals,

instrument designers tend to dimension the cooling elements small and themeasurement intervals short. In a well-balanced system, the noise will be justunder the precision stated in the instrument specifications.

The instrument specifications are tailored to the market in which the instrument

is sold. For example, an instrument sold in the graphic arts market willnot require a precision larger than 0.5 units. Since this is a relatively large

market, such instruments are inexpensive, and since the target users are not

technicians, the user interfaces tend to be very user-friendly.

If an isolated measurement is taken with such an instrument, the precision istypically much better than stated in the specifications. This leads us use

graphic arts instruments in process control applications. The important point

is not to perform measurements and the maximum rate allowed by the instru-Location Low temperature High temperatureDublin 19C 23C

Puerto Rico 21C 22C

Corvallis 22C 23C

Singapore 24C 26C

TABLE 13. Room temperature in various factories54 Spectrophotometer Calibration and CertificationFor HP Internal Use Only

ment, but to delay measurements until the heat generated by the lamp has

completely dissipated. In the case of the Spectrolino, this interval is 10 seconds.

This is why in the validation procedures we prescribe to wait for 10 secondsbetween measurements and why we log the time of each measurement.

6.2 Optics stabilityHeat also causes the optics in the instrument to expand. A common deviceused as a monochromator is a holographic diffraction grating. When the grating

expands, the spectrum interval detected by the sensor moves and the

instruments calibration is no longer valid. The instrument designer can minimizethis effect by choosing a material that has a very good dimensional stability,

but there is a cost trade-off.

The user has two choices to avoid this problem. The first choice is to recalibrate

the instrument frequently, say every 10 measurements. The secondchoice is to wait between measurements until the instrument has completely

cooled, which in the case of the Spectrolino is 10 seconds.

To err on the safe side, in our procedures we have recommended to do both,wait for 10 seconds and recalibrate every 10 measurements. It is now clear tothe reader that the calibration can be much less frequent. We have not performed

experiments to determine the exact required calibration frequency, but

it might be an order of magnitude larger.In the specific case of the Spectrolino, in the models designated as Rev. A or

newer the diffraction grating is on a new glass type that is dimensionally very

stable under temperature changes.* With this improvement it will be worthwhile


to relax the calibration requirements.

6.3 Sample heatingA typical light source for spectrophotometers is a gas filled tungsten lamp.Such a lamp has a strong radiation in the infrared part of the spectrum, which

can heat up the sample. In the Spectrolino, for example, the filament heats to

3000K on a surface of 1.5 mm 0.5 mm.If the measurement time is kept very short to minimize this effect, then only asmall number of photons will be available at the sensor and the dark current

will cause a very poor signal to noise ratio. The classical solution to this problem

is to keep the light on until the sensor saturates, and then to determine

the spectral power from the integration time of the radiant flux. With darksamples this time can be quite long and considerably heat the sample.* In the Spectrolino S/N 10255 used to obtain the data for this report we have upgraded the

grating to the new type.For HP Internal Use OnlyThermal issues 55We do not know how the Spectrolino and the SPM-100 work, so we have

assessed the problem with an experiment. We set the Spectrolino with serialnumber 10255 to the following conditions:

illuminant D65

2 observer

no filter

no data transfer

We measure the temperature of a green reference tile (serial number GBB-1),

then we perform 30 measurements at the maximal speed allowed by the

instrument and immediately measure the tiles temperature again. The resultof three repetitions is shown in the table below.

The error due to thermochromism would be approximately 0.01 CIELAB

units. Given that in the certification procedures the tile is measured at most10 times and there is a 10 second wait between measurements, heating of the

ceramic reference tile by the instrument during a measure is not a problem.

6.4 ThermochromismThermochromism refers to a color change with temperature change. Among

others, it occurs with some tiles that contain selenium. Anne Compton

reported [6] that when a tile is heated from 25 to 35Ca typical temperaturechange in the instruments using prolonged polychromatic irradiaton

common at that timethe effect is a small reduction in reflectance with an

increase in temperature. She observed that in general thermochromism

occurs on steep spectral profiles. The largest value of E*ab occurred for the

dark green tile and was 0.9, a difference that is larger that the tolerances inour procedures. The instrument geometry was diffuse/0.

Mark Fairchild and Frank Grum [7] conducted a similar experiment* with an

instrument of 0/45 geometry. For the same temperature change they

observed a E*ab value of 0.7 for a green tile. However, they observed muchlarger changes for the Orange (1.6) and Red (1.5) BCRA Series II (CCS II)

tiles.

More recently the National Physical Laboratory (NPL) has released a comprehensive


collection of spectral reflectance and thermochromism data [8] on the

NPL Master Set of the BCRA CCS II tiles. The colorimetric data for the green

tile is shown in Table 15 for spex and spinc geometry; unfortunately no data isprovided for 0/45 geometry, but the interested reader can calculate it from

the data provided in the article. Some data for the other tiles is in Table 17.

In the last column is the deviation of Spectrolino S/N 10255 from the averagevalues obtained from 10 Spectrolinos and reported in Table 6 on page 21.Thermochromism is linear, so if we take from Table 13 on page 53 the maximum

temperature span of 7C, for the spinc measurement we would obtain a

difference of 0.48 E*ab. This value is almost twice the inter-instrumentagreement, i.e., if ambient temperature is ignored during measurements, asignificant error is introduced and the data is less valuable.

Remember also that the tolerance for the relative accuracy check in

Section 5.6 on page 50 is 0.3. Hence, a temperature difference of 4C is by

itself the limit to pass the validation, assuming the instrument is perfect.With the 2C temperature tolerance prescribed in that procedure, a 0.12 unit

error is already introduced by the temperature uncertainty, leaving 0.18 unitsfor drifts in the instrument. The Dublin factory, with a temperature excursionfrom 19C to 23C, will have a challenging time to pass the instruments quali-

fication. In reality, the procedure described in this report is more lenient,

because it is based on the CIE94 color difference, which is closer to CMC.

In time series analysis we examine a long series of measurements and try to

find trends. For highly significant assertions the analyst will always use the

same instrument. If the operation is very meticulous, then the repeatabilitywill be close to the short-term repeatability, which for Spectrolino S/N 10255 is

0.023 E*ab units. This number is an order of magnitude smaller than the

effect of thermochromism with the 2C tolerance, hence temperature variationswill limit the usability of the calibration data.This last qualification is important. Ink on paper most likely does not suffer

the thermochromism problem, so the measurements by themselves do not

limit the precision of time series analysis. The limitation is when the measurementsof the green tile are used to calibrate the data in the time series.

What we have seen is that this calibration is limited by the uncertainty of the

temperature.

If the temperature is stable for a period of 30 minutes (the time for a ceramictile to adapt to a small temperature change) or longer, and an accurate thermometer

is available, for example a mercury in glass thermometer graduated

in 0.1C intervals, then highly accurate measurements can be made by correctingthe spectral data for the temperature difference before the colorimetric

quantities are computed.

The correction data is listed in the following table listing the differences inreflectance caused by a 10C (25 to 35C) change in temperature for the Green

tile in the Master Set of the BCRA CCS II tiles:Wavelength [nm] difference380 0.0003

390 0.0002


400 0.0002

410 0.0001

420 0.0001

430 0.0002

440 0.0003

450 0.0005

460 0.0009

470 0.0017

480 0.0030

490 0.0049

500 0.0065

510 0.0064

520 0.0048

530 0.0033

540 0.0020

550 0.0009

560 0.0006

570 0.0002

5800.0001

5900.0004

6000.00056100.0006

6200.0006

6300.0007

6400.0007

6500.0007

6600.0007

6700.0007

6800.0006

6900.0005

7000.0003

710 0.0001

720 0.0002

730 0.0004Wavelength [nm] difference

The gray curve plots the NPL data for the difference in reflectance caused by a10C (25 to 35C) change in temperature (units on right axis) tabulated in

Table 16 above.

Table 17 from reference [8] shows the colorimetric differences caused by a10C temperature change. From this data one might assume that the cyan or

even the deep blue tile would be a better choice for the problem at hand. However,

as we noted in the discussion of Figure 1, Reflectance of the green and

cyan master tiles,on page 15, the green tile with is sloped spectral distribution

curve is better suited for detecting the critical shift of the instrumentsscale.

The deep blue tile exhibits the lowest thermochromism, but with its flat spectral

distribution curve, by itself it is not useful at all to certify an instrumentscalibration. In summary, the green tile is the most suitable tile for certifying a

spectrophotometers calibration; due to its thermochromic properties, ambient

temperature must be controlled and recorded.Spectral reflectance of green tile GBB-1 measured with Spectrolino 10255 at 24.41C(left ordinate) and difference in reflectance caused by a 10 change in temperature


from 35C (right ordinate).Red 1.01 0.56

Orange 0.97 0.85

Bright yellow 0.76 0.44

Green 0.68 0.37

Difference green 0.72 0.39

Cyan 0.32 0.22

Deep blue 0.06 0.04

TIle E*ab E CMC (1:1)

IntroductionThere are many practices for making spectral measurements and performing colorimetriccomputations. The methods usedmay result in different numerical values for the same parameter for the same sample. Thus, onemay not be able to makevalid comparisons using data derived using different methodologies. The purpose of this standardis to specify amethodology for the measurement of graphic arts images that result in valid and comparabledata. While this standardreferences the standard established for graphic arts viewing conditions, it is not intended to

provide an absolute correlationwith visual color appearance.

1

Graphic technology Spectral measurement andcolorimetriccomputation for graphic arts images1 Scope and field of applicationThis standard establishes a methodology for reflection and transmission spectral measurement,and computation ofcolorimetric parameters for graphic arts images. Graphic arts includes, but is not l imited to, the

preparation of material for,and volume production by, production printing processes that include offset lithography,letterpress, flexography, gravureand screen printing.This standard also applies to images to be manufactured in limited quantities such as thoseproduced with photographic, ink

jet, thermal transfer, diffusion, electrophotography, mechanical transfer or toner technologies(e.g., off-press proofs) whenused for graphic arts applications.This standard does not address any requirements on the measurement of emission spectral datafrom video monitors nordoes it supersede the specification of other measurement geometries appropriate to specificapplication needs, such as theevaluation of materials (e.g., ink and paper) used in the graphic arts.NOTE Procedures for color measurement of spectral data from video monitors are included in ASTM E1336: "Standard test methodfor obtaining colorimetric data from a video display unit by spect roradiometry."[1] The use of integratingsphere geometry for paperevaluation is covered in ISO 2469, Paper, board and pulps Measurement of diffuse reflectance factor.[2]

2 Normative referencesThe following standards contain provisions that, through reference in this text, constituteprovisions of this standard. At the


time of publication, the editions indicated were valid. All standards are subject to revision, andparties to agreements basedon this standard are encouraged to investigate the possibility of applying the most recent editionsof the standards indicatedbelow.

ASTM E 308-99 "Standard Practice for Computing the Colors of Objects by Using the CIE

System," American Society forTesting and MaterialsCIE Publication No. 15, ColorimetryISO 5-2, PhotographyDensity measurements Part 2: Geometric conditions for transmissiondensityISO 5/3, PhotographyDensity measurements Part 3: Spectral conditionsISO 5-4, PhotographyDensity measurements Part 4: Geometric conditions for reflectiondensityISO 3664, Viewing conditions Graphic technology and photographyCGATS.17, Graphic technology Exchange format for spectral measurement, colorimetric, anddensitometric data inelectronic formNOTE See the Bibliography for further information listing other documents that are not necessary forcompliance with this standard

but may be of interest to the reader. It also contains contact information for sources of relevant documents.Referenced documents in thistext will be shown as [number] for reference.

3 Definitions, abbreviations, and symbolsFor the purposes of this standard, the following definitions, symbols and abbreviations apply:3.1calibrationset of operations that establish, under specified conditions, the relationship between values ofquantities indicated by ameasuring instrument or measuring system, or values represented by a material measure or areference material, and thecorresponding values realized by standards[ISO International Vocabulary of Basic and General Terms in Metrology]

NOTE Contrary to a common usage, calibration is not the process of adjusting a measurement system suchthat it produces valuesthat are believed to be correct. Calibration permits either the assignment of values of measurands to theindications (creating a referencetable) or the decision to reset or adjust the device. Following the resetting or adjusting of the device, acalibration should be repeated toensure that the new device setting(s) provide indications within the accepted values.3.2CIE standard illuminantsilluminants defined by the CIE in terms of relative spectral power distributions; e.g., A, C, variousD and F illuminants3.3illuminantradiation with a relative spectral power distribution defined over the wavelength range that

influences object color perception[CIE Publication 17]3.4measurement sourcecharacteristic of the radiant flux (light) incident on the specimen surface3.5opacity of substrate (0/45, D50, 2 degree, CIE Y)Osmeasure of the property that describes the ability of a sample to hide a surface behind and incontact with it


3.6reflectance factor (at a surface element, for the part of the reflected radiation contained ina given cone withapex at the surface element, and for incident radiation of given spectral composition,polarization andgeometric distribution)R

ratio of the radiant or luminous flux reflected in the directions delimited by the given cone to thatreflected in the samedirections by a perfect reflecting diffuser identically irradiated or illuminated[CIE Publication 17]NOTE The term commonly used in the industry and in this document for reflectance factor is reflectance.3.7sample backingmaterial placed behind the sample during measurement3.8self-backingsample backing composed of multiple sheets of substrate material identical to the substrate onwhich the image has beenplaced3.9transmittance/ (for incident radiation of a given spectral composition, polarization, andgeometricaldistribution)ratio of the transmitted radiant or luminous flux to the incident flux in the given conditions[CIE Publication 17]NOTE It is important to specify the geometry that establishes the given conditions of measurement.3.10transmittance factorratio of flux transmitted by a specimen in a given optical system to the flux transmitted when thespecimen is removed fromthe sampling aperture. This is the case when e.g. radiation penetrating a slide situated in aprojector and reaching a screen

is compared to the radiation when the slide is removed from a projector and only an empty slidemount is in the projector.4 Spectral measurement requirements4.1 Instrument calibrationThe measurement device or system shall be verified (calibrated and possibly adjusted) inaccordance with its manufacturer'sinstructions. See Annex A, Instrument calibration.NOTE CGATS.11/PIMA IT2.11 [5], defines the use of a certified reference material (CRM) to checkcalibration of a measurementsystem and provides additional information relating to the use of CRMs, the determination of combinedstandard uncertainty and datareporting requirements.Where multiple instruments are used for measurement there will be differences in the resultingdata due to the individualcharacteristics of the instruments and variations in measurement conditions. Annex B providesinformation on what could bedone to help multiple laboratories or areas obtain consistent measurement data.4.2 Measurement source for fluorescing materialsTo minimize the variations in measurement results between instruments due to fluorescence, themeasurement source fordetermination of spectral reflectance data for colorimetric calculations shall match the specifiedCIE illuminant for the


intended application over the wavelength range of potential energy absorption. Subclause 5.1specifies the use of CIEilluminant D50 for graphic arts colorimetric computations. Annex C provides further information onfluorescence andtechniques to test for its presence.NOTE It is recognized that many instruments presently do not have a measurement source tha t matchesthis illuminant. Theseinstruments are technically not in compliance with CGATS.5 and the user is cautioned when reporting datafor samples that fluoresce.4.3 Wavelength range and interval for measured valuesThe data should be measured from 340 nm to 780 nm at 10 nm intervals and shall be measuredfrom 400 nm to 700 nm,inclusive, at intervals of no more than 20 nm. The reference for spectral data shall be based oncomputed data at 10 nmintervals where the spectral response function is triangular with a 10 nm bandwidth at the halfpower point.

4NOTE Instrumentation with different intervals and response functions may produce different results. Thesedifferences may be

reduced by proper selection of bandpass characteristics (shape) for a given interval and applying the propermethod of calculation for thebandpass characteristic and interval selected.4.4 Reflectance factor measurement4.4.1 Sample backing materialThis standard makes provision for a black backing, or where appropriate a white backing or aself-backing (see 3.8)condition for use during measurement.Substrates with opacity of substrate as defined in 4.4.1.1, equal to or greater than 99, areconsidered opaque for thepurposes of this standard and the backing used for measurement is not relevant.The black backing in 4.4.1.2 shall be used where back printing is present (text or imagesprinted on both sides of the

substrate).The black backing in 4.4.1.2 should be used where the substrates opacity is 95 or above .The white backing in 4.4.1.3 or the self-backing in 4.4.1.4 should be used where thesubstrates opacity is below 95.NOTE 1 ISO 5-4 requires that black backing be used for spectral measurements from which ISO standardstatus densities are to becomputed. Therefore, densitometric data computed from measurements made using white backing does notcomply with the ISO 5-4standard. Further, colorimetry data computed with white backing may not be applicable for some processcontrol applications.NOTE 2 Users are cautioned that where self-backing is used issues of diffuse blurring need to be addressedand the accuracy oftypical instruments and targets used in graphic arts may need to be addressed for this issue.The sample backing chosen, shall be reported as in 6.2 i) and 6.3.3, Data reporting.NOTE 3 The choice of sample backing material appropriate to the application has become importantespecially in color managementdue to the differences in opacity of proofing system substrates and production printing paper materials.There may be no correct answer,only application specific compromises. When visual comparisons are to be related with measuredcolorimetric data, the backing usedduring viewing may strongly influence color and tone appearance. Similarly, the backing used duringmeasurement may have an impacton the data obtained depending on the opacity of the substrate materials. Therefore, the backing usedduring visual evaluation should be


similar to the backing present during measurement. See the Committee for Graphic Arts TechnologiesStandards Recommended IndustryPractice: Developing a color characterization data set Analysis and reporting (currently underdevelopment) for further information.NOTE 4 At the time of issue CGATS has not identified commercial sources of the black or white backingmaterials.See Annex D for more detailed discussion on the issues related to sample backing during

measurement.4.4.1.1 Determination of opacity of substrate, Os (0/45, D50, 2 degree, CIE Y)The following procedure shall be used to determine the opacity of substrate (0/45, D50, 2 degree,CIE Y).Two measurements shall be made of the material, one backed with a black backing material(4.4.1.2) and the secondbacked with a white backing material (4.4.1.3).Compute the opacity of a substrate (0/45, D50, 2 degree, Y), using the following:Os = (Yb/Yw) x 100 (1)whereOs is opacity (0/45, D50, 2 degree, Y)Yb is the Y tristimulus value computed from measurements made using black backing

5Yw is the Y tristimulus value computed from measurements made using white backingNOTE Although the symbol 0/45 is used to designate the measurement geometry, 0/45 and 45/0 geometryare assumed to beequivalent and either may be used.4.4.1.2 Black backing

A black sample backing material, as defined in ISO 5-4 shall be used.NOTE ISO 5-4 defines the following requirements:spectrally non-selective, i.e., the total range of spectral diffuse reflection density throughout thewavelength interval from 400 nm to700 nm shall not exceed 5% of the average density obtained over the same interval;diffuse-reflecting (i.e., no perceptible specular reflection when viewed at any angle under typical officeroom illumination conditions);and

an ISO visual reflection density of 1.50 0.20. (This is approximately equivalent to a CIE L* value rangeof 15 - 27 or a reflectancerange of 2% - 5%).4.4.1.3 White backing

A white sample backing material shall have the following characteristics:shall be known to be opaque (e.g., ceramic or other coated materials);diffuse-reflecting (i.e., no perceptible specular reflection when viewed at any angle undertypical office room illuminationconditions);CIELAB C* less than or equal to 3.0;non-fluorescing (no emission in the response band of interest when excited by the instrumentsource); andspectral reflectance values greater than those shown in Table 1 in the range 400700 nm.Table 1 Spectral reflectance values

Wavelength Spectral reflectance400 0.30410 0.30420 0.75450 0.75460 0.80670 0.80680 0.75700 0.75


6

NOTE This corresponds to a CIE L* greater than 92.4.4.1.4 Self-backingWhen measurements are made using self-backing, multiple layers of material, identical to theimaged substrate, shall be

used such that the opacity of the total pad is greater than 99 when determined as defined in4.4.1.1. To be consideredidentical the backing material should be from the same lot/batch/roll as the imaged substrateand not exhibit visible colorappearance differences. The CIE color difference values between the individual sheets makingup the stack of backingmaterial, and between the individual sheets and an unprinted area of the imaged substrate shouldvary no more than 1.0*

ab E .4.4.2 Measurement geometryMeasurement geometry shall be 45/0 or 0/45 and conform to the geometric conditions defined inISO 5-4. The sample shalllie on a flat surface while being measured. The instrument base and the sample shall lie in the

same plane.NOTE 1 The use of 45/0 or 0/45 geometry will not adequately address variations in all surfacecharacteristics. Other instrumentationcan be used to detect specific characteristics such as "bronzing." See Annex E.NOTE 2 It is recognized that many instruments do not conform to the ISO 5-4 requirement for a 2 mmboundary beyond the samplingaperture due to the physical size of the control elements which are normally measured. Annex F providesfurther information on aperturesize.4.4.3 ResolutionReflectance factor shall be measured to the nearest 0.0001 relative to a perfect reflecting diffuserhaving 1.0 reflectancefactor at all wavelengths. This data may be reported as either reflectance factor or percentreflectance (i.e., reflectance

factor multiplied by 100).4.5 Transmittance factor measurement4.5.1 Measurement geometryMeasurement geometry shall be normal/diffuse (0/d) or diffuse/normal (d/0) and conform either tothe geometric conditionsdefined in ISO 5-2 or CIE 15. The diffuse geometry used, sphere or opal, shall be reported. (See

Annex E.3.)4.5.2 ResolutionTransmittance factor shall be measured to the nearest 0.0001 relative to a perfect transmittingdiffuser having atransmittance factor of 1.0 at all wavelengths. This data may be reported as either transmittancefactor or percenttransmittance (i.e., transmittance factor multiplied by 100). See Annex E.3.

5 Colorimetric computation5.1 Calculation of tristimulus valuesTo provide consistency with graphic arts viewing conditions, defined in IS0 3664, calculatedtristimulus values shall be basedon CIE illuminant D50 and the CIE 1931 standard colorimetric observer (often referred to as the2 standard observer) asdefined in CIE 15. Computation shall be either at 10 nm or 20 nm intervals. Factors representingthe product of CIEilluminant D50 and the 2 standard observer data, to be used for weighting spectral reflectanceand transmittance data shall


7

be those given in Table 2 for 10 nm intervals and Table 3 for 20 nm intervals, as taken fromASTM E 308. The user isstrongly encouraged to use data at 10 nm intervals to improve the accuracy of the results.NOTE 1 The 2 standard observer was selected rather than the 10 standard observer, because it more

closely matches the size ofimage detail found in printed material.NOTE 2 ASTM has introduced an additional set of weighting functions for the computation of tristimulusvalues from spectral data inthe most recent versions of ASTM E 308. These new tables are known as Tables 6 (of ASTM E 308) andare intended for use wheremeasurement data have not been corrected for spectral bandpass dependence. The weighting functionsused here and in the 1995edition of CGATS.5 are based on Tables 5 of ASTM E 308, which are intended for use where measurementdata have been correctedwithin the measuring instrument for spectral bandpass dependence.Since currently available instruments do not provide information regarding the inclusion of corrections forspectral bandpass dependence,CGATS recommends that no change be made at this time. If instruments include bandpass correction intheir reported spectral data then

Tables 5 are appropriate for use. Where they do not, Tables 6 are more appropriate for use. The smalldifferences in results betweenusing Tables 5 versus Tables 6 do not justify the confusion that would be introduced into the graphic artsindustry by a change at thistime. For typical graphic arts images (IT8.7/1 [6] scanner targets or IT8.7/3 press characterization targets[8]) the CIE *abE colordifference between the use of these tables is an average of approximately 0.2 and a maximum of 0.7.If the measured spectral data begin at a wavelength greater than 360 nm, then all the weightingvalues in Table 1 or Table 2for wavelengths less than the first measured wavelength shall be summed and added to theweighting value for the firstwavelength measured. If the last measured spectral data point is at a wavelength less than 780nm, then all the weighting

values in Table 1 or Table 2 for wavelengths greater than the last measured wavelength shall besummed and added to theweighting value for the last measured wavelength.NOTE 3 This procedure is consistent with ASTM E 308-99.The general form of these computations is:=780 =780X = (R x W(X)) X = (T x W(X)) (2)=360 =360=780 =780Y = (R x W(Y)) Y = (T x W(Y)) (3)=360 =360=780 =780Z = (R x W(Z)) Z = (T x W(Z)) (4)=360 =360whereR is the reflectance value at wavelength ;T is the transmittance value at wavelength ;W(X) is the weighting factor at wavelength for tristimulus value X;W(Y) is the weighting factor at wavelength for tristimulus value Y;W(Z) is the weighting factor at wavelength for tristimulus value Z.If measured data is at intervals and bandpass smaller than 10 nm, the method of Annex G shallbe used to widen thebandpass of the data.


NOTE 4 The weights given in Table 1 or Table 2 are based on triangular bandpass characteristics asreferred to in 4.3.The values of Xn=96.422, Yn=100.000 and Zn=82.521 shall be used to do colorimetriccalculations.

8NOTE 5 Adding the values of the weights from 360 nm to 780 nm in Table 2 or in Table 3 does not give asum equal to the values forXn, Yn and Zn. This is because the writers of ASTM E 308 computed Xn, Yn and Zn to greater precision thangiven by the summation ofthe table values. The sums for X, Y and Z in the tables are of value as a data entry check of the tables.

9Table 2 Spectral weights (W) for illuminant D50 and 2 observerfor calculating tristimulus values at 10 nm intervals Wavelength (nm)W(X)1W(Y)1W(Z)1360 0.000 0.000 0.001

370 0.001 0.000 0.005380 0.003 0.000 0.013390 0.012 0.000 0.057400 0.060 0.002 0.285410 0.234 0.006 1.113420 0.775 0.023 3.723430 1.610 0.066 7.862440 2.453 0.162 12.309450 2.777 0.313 14.647460 2.500 0.514 14.346470 1.717 0.798 11.299480 0.861 1.239 7.309490 0.283 1.839 4.128

500 0.040 2.948 2.466510 0.088 4.632 1.447520 0.593 6.587 0.736530 1.590 8.308 0.401540 2.799 9.197 0.196550 4.207 9.650 0.085560 5.657 9.471 0.037570 7.132 8.902 0.020580 8.540 8.112 0.015590 9.255 6.829 0.010600 9.835 5.838 0.007610 9.469 4.753 0.004620 8.009 3.573 0.002

630 5.926 2.443 0.001640 4.171 1.629 0.000650 2.609 0.984 0.000660 1.541 0.570 0.000670 0.855 0.313 0.000680 0.434 0.158 0.000690 0.194 0.070 0.000700 0.097 0.035 0.000710 0.050 0.018 0.000720 0.022 0.008 0.000


730 0.012 0.004 0.000740 0.006 0.002 0.000750 0.002 0.001 0.000760 0.001 0.000 0.000770 0.001 0.000 0.000780 0.000 0.000 0.000

Check SumsWhite pointX = 96.421Xn= 96.422Y = 99.997Yn= 100.000Z = 82.524Zn= 82.521NOTE These weighting functions are extracted with permission from Table 5.9 of

ASTM E 308-99.

10Table 3 Spectral weights (W) for illuminant D50 and 2 observerfor calculating tristimulus values at 20 nm intervalsWavelength (nm) W(X)1 W(Y)1 W(Z)1360 -0.001 0.000 -0.003380 -0.007 0.000 -0.034400 0.100 0.001 0.459420 1.651 0.044 7.914440 4.787 0.325 24.153460 4.897 1.018 28.125480 1.815 2.413 15.027500 0.044 6.037 4.887520 1.263 13.141 1.507540 5.608 18.442 0.375560 11.361 18.960 0.069580 16.904 16.060 0.026

600 19.537 11.646 0.014620 15.917 7.132 0.003640 8.342 3.245 0.000660 3.112 1.143 0.000680 0.857 0.310 0.000700 0.178 0.064 0.000720 0.044 0.016 0.000740 0.011 0.004 0.000760 0.002 0.001 0.000780 0.001 0.000 0.000Check SumsWhite pointX = 96.423

Xn= 96.422Y = 100.002Yn= 100.000Z = 82.522Zn= 82.521NOTE These weighting functions are extracted with permission from Table 5.10 of

ASTM E 308-99.

115.2 Calculation of other colorimetric values


Colorimetric values and color differences shall be calculated using the equations of CIEPublication 15. These equations, aswell as equations used to compute color differences, are included in Annex H for information.5.3 Calculation of densitometric valuesThe computation of densitometric values from spectral data is defined in ISO 5/3.NOTE The ISO 5 series of documents is currently being revised and is expected to be available at the endof 2002. ISO 5/3 willcontain the agreed upon procedures to compute the various status densities from spectral data. For furtherinformation the reader isdirected to contact TC42 Secretariat at http://www.pima.net/standards/iso/tc42/WG21/WG21_pow.htm.

6 Data reporting6.1 GeneralThere is no requirement that data be reported electronically. Where data is reportedelectronically, the provisions of 6.3shall be followed.6.2 Required informationWhen data generated in accordance with this standard are reported, they shall be accompaniedby the following information:a) a statement that the measurements and computations are in conformance with CGATS.5;b) the originator of the data;c) the creation date of the data;d) a description of the purpose or contents of the data being exchanged;e) a description of the instrumentation used, including, but not limited to, the brand, modelnumber and sampling apertureinformation;f) the measurement source (light source and filter conditions used);NOTE Examples of filters to report include uv, polarisation, light balancing, etc.g) the wavelength interval (10 or 20 nm) used for calculation;h) the spectral products weighting function (Status or Type response) when density data isreported; andi) the sample backing (black, white, self-backing or na). The spectral reflectance of the whitebacking or the stack of selfbackingmaterial shall be reported.

6.3 Electronic data reporting6.3.1 GeneralWhen data is reported in electronic form it shall be in compliance with one of the data formats(ASCII and XML) ofCGATS.17.

12The first 7 characters in the file shall be CGATS.5The primary definition of keyword is contained in CGATS.17 and repeated here for information.Where a conflict existsbetween the definitions of keywords in this standard and those contained in CGATS.17,CGATS.17 shall take precedence.6.3.2 Required keywords or corresponding XML tags

The following keywords (or corresponding XML tags) as defined in CGATS.17 shall be used:ORIGINATOR Identifies the specific system, organization or individual that created the data file.DESCRIPTOR Describes the purpose or contents of the data file.CREATED Indicates date of creation of the data file.INSTRUMENTATION Used to report the specific instrumentation used (manufacturer and modelnumber) togenerate the data reported. This data will often provide more information about theparticular data collected than an extensive list of specific details. This is particularlyimportant for spectral data or data derived from spectrophotometry.


MEASUREMENT_SOURCE Illumination source used for spectral measurements. This data helpsprovide a guide tothe potential for issues of paper fluorescence, etc.NUMBER_OF_FIELDS Number of fields (data format identifiers) that are included in the dataformat definition thatfollows. The value associated with this entry is an integer.

BEGIN_DATA_FORMAT Marks the beginning of a definition of a data set. Data format identifiersindicatethe relationship between fields, and their value. Data format may have an arbitrarynumber of entries, and is closed by END_DATA_FORMAT. The values of the data formatmust be either data format identifiers or keywords.END_DATA_FORMAT Marks the end of a data format definition. END_DATA_FORMAT must bepreceded byBEGIN_DATA_FORMAT, and must be followed by white space.NUMBER_OF_SETS Number of repetitions or sets of data corresponding to the data formatfields that areincluded in the data to follow. The associated value is an integer.BEGIN_DATA Marks the beginning of a stream of data.END_DATA Marks the end of a stream of data.6.3.3 Data type dependant keywords

Where data is based on reflectance measurements the following keywords shall be used:SAMPLE_BACKING Identifies the backing material used behind a hard copy sample duringreflectancemeasurement. Allowed values are black, white, "self-backing", or na.NOTE When sample substrates are opaque (i.e., Os is greater than 99% according to 4.4.1.1) the samplebacking is not an issueand should be reported as na.Where data is based on transmittance measurements the following keywords shall be used:DIFFUSE_GEOMETRY The diffuse geometry used. Allowed values are sphere or opal

136.3.4 Optional keywordsOptional Keywords for use in reporting data are defined in CGATS.17.

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Annex A(informative)Instrument calibrationA.1 IntroductionWebster's Dictionary defines calibration as, "to standardize by determining the deviation from astandard so as to ascertainthe proper correction factors." It also defines adjustment as, "a correction or modification to reflectactual conditions." Bothare used with measuring instrumentation. The definitions indicate a means of normalization andboth, if used correctly, willyield accurate values of a measured sample.

According to the ISO International Vocabulary of Basic and General Terms in Metrology,calibration is a set of operationsthat establish, under specified conditions, the relationship between values of quantities indicatedby a measuring instrumentor measuring system, or values represented by a material measure or a reference material, andthe corresponding valuesrealized by standards.


Contrary to a common usage, calibration is not the process of adjusting a measurement systemsuch that it produces valuesthat are believed to be correct. Calibration permits either the assignment of values of measurandsto the indications(creating a reference table) or the decision to reset or adjust the device. Following the resetting oradjusting of the device, a

calibration should be repeated to ensure that the new device setting(s) provide indications withinthe accepted values.The user should be aware that even though they have calibrated their instrument using themanufacturers calibrationprocess, the instrument may have only been verified relative to the calibration reference.A.2 Calibration, adjustment and standardization of measuring systemsWhen trying to determine the procedure for calibrating a measuring device, one can sometimesbe confused as to what theprocedure actually does. If one picks up a meter stick, he would assume that the meter stick hadbeen properlymanufactured and is therefore calibrated. Yet, with very precise measurements, technically awarepeople know that themeter stick will only be accurate within a certain temperature range. If it is too cold or too hot, thematerial may havechanged its physical dimensions thereby throwing it out of "calibration".Even so, the meter stick can be compared to another meter stick "calibrated" at the highertemperature. Once the bias forthe original meter stick is known, the original meter stick can then be used to make very accuratemeasurements if acorrection factor for calibration adjustment is used. This concept is not often used but is actuallytechnically acceptable inmost circumstances. Once the correction factor has been established, the user can say the meterstick has been calibratedfor the higher temperature, in which case a description of the conversion factor must include itsintended target temperatureand temperature range.Many see the act of adjusting the instrument to a predetermined value or values as calibration. As

described above, this isnot necessary to calibrate a device. Additionally, it may do nothing to calibrate a device. A case inpoint is an analog voltagemeter. With this type of device, the meter's indicator needle can be adjusted to properly align withzero. Is this calibrating thedevice? Certainly the "Zero" value has been set, but the up-scale values have not beennormalized. In this instance, wehave adjusted the device, but have not calibrated it.Finally, there is "standardization". This is what many people are most familiar with and maysimply call calibration. In thisinstance, a physical reference is used to normalize the device. The reference should be for aspecific instrument orinstrument type and must have values traceable to a standards laboratory, such as NIST.

Calibration adjustment requires aninstrument to be set to known values as indicated on the physical reference. In the case of areflection instrument the

15instrument is used to measure a low density or white reference. The instrument is then adjustedso the device displays thevalue for that reference, as listed on the reference itself or on its accompanying documentation.Likewise, the high density or


slope of the instrument is checked with a physical reference and if the instrument does notdisplay the appropriate value orvalues, then the instrument is adjusted to display the proper value(s).Older instruments actually required calibration-adjustment along the entire measurement rangeas the electronics were notcompletely linear. With the advent of higher quality components, instrument linearity is now

ensured. Those instruments thatmay not be inherently linear now provide linearity adjustments to help compensate for anydifferences.

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Annex B(informative)Improving inter-instrument agreementThere are a few items to consider to obtain agreement between different devices of likeconfiguration (e.g., two 0/45spectrophotometers).B.1 Instrument geometry

Optical measurement instrumentation has been designed for various applications. Spheregeometry spectrophotometerswere developed for the measurement of colorants (dyes and pigments). The use of 0/45instrumentation is preferred whenthe purpose of the measurement is to simulate how the human eye perceives light and color. Thisstandard specifies the useof 0/45 instrumentation because of its applications in the graphic arts, the industrys use ofreflection densitometers (alsohaving 0/45 instrument geometry) and the need to have data that at least in some way cancorrelate to color as perceived bythe human eye. Whichever is used, when comparing data, geometries should be the equivalent.Refer to Annex E for furtherdiscussion on instrument geometries.

NOTE It has been found that instrument geometries can be reversed with differences found to be negligibleor non-existent. As anexample, although the 0/45 geometry is indicated here, 45/0 geometry instruments would be deemedequivalent in geometry.

B.2 Black backingISO and ANSI presently require, for most situations, that materials to be measured be backedwith a flat, black material witha density of 1.50 0.20D (see Annex D for more detailed information on this topic). This helpsreduce the variation of themeasurement when images are printed on the reverse of the area to be measured. It alsoprovides a consistent backingsituation. If a sample is backed with a white backing and measured and then backed with a blackmaterial and measured,given the same area was measured, the difference in the backing may affect density on thin

substrates by 0.02 or more andsimilar unwanted variations on color measurements will occur. Unless the samples substrate isopaque, the sample backingmust be at the very least consistent.B.3 CalibrationMeasurement devices should each be calibrated using its own, up-to-date, calibration reference.The reference should bethe type and kind supplied by the manufacturer of the instrument or a substitute as specified bythe manufacturer. It is good


practice to include verifying the reference certification date, ensuring the white referencereflection values in the instrumentare correct, and then making both a white reflection reference, and a zero reflectancemeasurement (made in a black trapapparatus, or in a dark, non-reflecting location) as part of the full calibration procedure. In allcases follow the procedures

specified by the manufacturer of the instrumentation and calibration reference. In most moderninstruments, many of thecalibration procedures have been automated to require little human intervention. It is always agood idea to verify acalibration once completed by measuring the calibration reference and comparing themeasurements to the target values.B.4 Screen ruling

Areas with tone value should not be measured with an aperture that is too small to effectivelyevaluate the sample. ANSIand ISO standards have recommended minimum aperture sizes for use when measuring tonevalues with various screenrulings. See Annex F, Table F-1 for more detailed information

17B.5 Common aperture configurationDifferent apertures measure different areas of a sample. Therefore, one should always use acommon aperture size andshape for comparison measurements or measurements from multiple locations. If you aremeasuring tone values (dot area,for instance), refer to the chart in Annex F to ensure that the aperture is large enough to assurevalid measurements at theselected screen ruling.B.6 Assure common measurement conditionsSome cases of measurement disagreement can be traced to the comparison of data measuredunder different conditions.For density values, different Status responses are used throughout the graphic arts and

associated industries. Status T,Status E and Status I responses are all used within the graphic arts, and Status A is usedprimarily in photo processingapplications. In addition instruments may have specialized, non-standard responses that weredeveloped prior to theadoption of national standards or for customer specific applications. In the case of thesespectrophotometers, an operatorusually has a choice of options that may be set for the series of measurements that are to bemade. Such choices includethe Illuminant, Standard Observer, UV source component included or excluded, polarizing filterselected or not, and specularcomponent included or excluded (this condition would be used with sphere spectrophotometersonly). Make certain the

measurement conditions of each instrument are identical or equivalent.B.7 Optional filtersFor some applications, supplemental filters are used. Polarization filters or UV cut-off filters arenot uncommon oninstruments manufactured within the past decade. Measurements from models with such filterscannot be compared tomeasurements from models without these filters. If you are unsure whether your instrument hasthese filters, please contactthe manufacturer of the instrument for clarification.B.8 Common channel


Most densitometers measure samples using all four channels (cyan, magenta, yellow and visual)simultaneously. Likewise,spectrophotometric data can be weighted to give densitometric data for all four filter channels foreach Status response.These devices may be configured to display one or more of the density channels. It is importantto ensure that comparisons

are made between common channels of density measurements. Without care, one mightincorrectly compare different colorchannels with one another.B.9 Common model and manufacturerThe best agreement is achieved when devices from a single manufacturer, and better still, fromthe same product family areemployed. If instruments have not already been purchased, serious consideration should begiven to the acquisition ofinstruments from a single manufacturer and of a common model type.B.10 Substrate considerationsMany instruments feature the option to measure the absolute value of a sample, or alternatively,to measure the substrate

Documents

Calibration Certification of Spectrophotometers