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High Range Illuminance Meter Calibration

Using Substitution Method

National Metrology Centre, Standards, Productivity and Innovation

Board

XU Gan, LIU Yuanjie, TAN Hwee Lang and WANG Xiaojuan

1 Science Park Drive, Singapore 118221

Tel: 65 67739874, Fax: 65 67739804

e-mail: yjliu@spring.gov.sg

Abstract

The measuring range of many commercial illuminance meters is well above 10,000 lx.

Calibration at such high illuminance levels cannot be done directly through the inverse

square law by using a normal luminous intensity standard lamp, which can only produce

an illuminance level up to a few thousand lx at the minimum acceptable distance (~ 0.5

metre). Some laboratories use lamps of very high power (up to 10 kW) for such

calibrations but this requires additional space and proper heat isolation, in addition to

the much larger power supply needed by such lamps.

A simple and low-cost method has been developed for high range illuminance meter

calibration based on a substitution-superposition technique. In this method, a luminous

intensity standard lamp is first set up at a known distance to calibrate the meter at a

point of low illuminance level. The light from the standard lamp is then blocked and the

meter is illuminated by an auxiliary lamp, the position and orientation of which are

adjusted so that the meter reading remains unchanged. The meter is then calibrated at

twice the initial level by using illumination from both the standard and auxiliary lamps

at the same time. Once this is done, the standard lamp is blocked and the auxiliary lamp

is again adjusted to give the same meter reading at twice the initial level, thereby

substituting the illuminance level of the standard lamp. By repeating the steps, the meter

can be calibrated at any desired level, limited only by the capacity of the auxiliary lamp

As the auxiliary lamp is used only as a substitute, similar to a capacitor in a holding

circuit in electronics, there is no special requirement on the type of lamp used except for

its stability. By this method, high illuminance can be achieved by using a focused beam

from a tungsten halogen lamp of only 50W at 12V dc with a collimator. Together with a

normal standard lamp, calibration of up to 50,000 lx can be performed. The relative

calibration uncertainty is similar to that of any low range illuminance meter calibration.

Introduction

Illuminance meters are normally calibrated using luminous intensity standard lamps

placed at several known distances for required illuminance levels according to inverse

square law. For inverse square law to be valid the standard lamps must fulfil the

conditions of a point source. i.e. the dimensions of the lamp filament shall be negligible

as compared with the distance between the illuminance meter and the standard lamp.

Such requirement sets a limit to the minimum distance (and hence the maximum

illuminance level) that can be used for illuminance meter calibration. For example, for a

1200 W tungsten halogen lamp of a filament size 10x10 mm running at 2856 K, the

maximum illuminance level it can produce is only about 7,000 lx at a minimum distance

of 0.5 m.

However the measuring range of many commercial illuminance meters is well above

10,000 lx. Calibration at such high illuminance levels cannot be done directly through

the inverse square law by using a normal luminous intensity standard lamp. To

overcome this problem, some laboratories have developed special standard lamps of

very high power (up to 10 kW), but this requires additional space and proper heat

isolation, in addition to the much larger power supply needed by such lamps. Others use

photometer-based method together with a special and complex source system that has a

spectral power distribution close to the CIE Illuminance A. Both methods require

substantial resources and the costs of establishment and maintenance of such calibration

facilities are not practical for most laboratories.

This paper presents a simple and low-cost method for high range illuminance meter

calibration based on a substitution-superposition technique. The calibration setup,

measurement procedure and working principle are illustrated. The additional

measurement uncertainties associated with this method are discussed.

Calibration setup

The setup for the proposed high range illuminance meter calibration is illustrated in

Figure 1.

Figure 1 Setup for high range illuminance meter calibration

The illuminance meter to be calibrated and a luminous intensity standard lamp (Model :

6895Z made by Philips, rated 1200W tungsten halogen lamp running at a correlated

colour temperature of 2856K) are set up on the main optical bench with a known

distance of 0.7 m between the measurement plane of the illuminance meter and the

reference plane of the standard lamp. This is exactly the same as one would use a

luminous intensity standard lamp to calibrate an illuminance meter by the inverse square

law.

An auxiliary tungsten halogen lamp is set up off-axis on a mini-bench at a distance of

about 0.5 m away from the head of the illuminance meter to be calibrated. This

auxiliary lamp has a rated power of 50W at 12V dc (Model : DECOSTAR 51, 41870 SP

made by OSRAM) and has a dichroic reflector which collimates its light beam to a

divergence angle of about 10 degree. The dichroic reflector reflects visible light but

allows most of the infrared radiation to pass. This reduces the heating effect of the

tungsten halogen lamp on the head of the illuminance meter to be calibrated.

The auxiliary lamp is mounted on a rotating holder with which the direction of the

collimated light beam can be adjusted freely. With the collimating reflector the 50 W

auxiliary lamp is capable of producing a high illuminace reading of more than 50,000 lx

when its light beam is pointed directly at the head of the illuminance meter.

Calibration principle and procedure

Set the calibration system up as illustrated in Figure 1. Arrange the auxiliary lamp and

its baffle properly so that they do not block the light from the standard lamp reaching

the head of the illuminance meter during the calibration. The principle and detailed

calibration procedure of substitution-superposition technique is explained in Figure 2.

The first step is to calibrate the illuminance meter at low illuminance level produced by

the standard lamp. This is done by block the direct illumination of the auxiliary lamp

and illuminate the head of the illuminance meter with the luminous intensity standard

lamp alone, and take the meter reading Y1. Y1 is the illuminance meter reading for

illuminance level of x + D, where x is the net direct illuminance value from the

standard lamp calculated based on inverse square law and D is the dark signal

(including stray light) when the direct illuminance from both lamps are blocked.

Next is to substitute the illuminance from the standard lamp with an equivalent

illuminance from the auxiliary lamp. This is done by block the standard lamp and

illuminate the meter only with the auxiliary lamp, and adjust the angle of the light beam

of the auxiliary lamp so that the illuminance meter reading exactly equals Y1, indicating

that the auxiliary lamp now provides a substitute illuminance equal to the illuminance

produced by the standard lamp (in terms of the illuminance meter response).

Figure 2 Calibration procedure using substitution-superposition technique

Strictly speaking this equivalence of substitution is valid only if the stray light level

remains the same after the adjustment of the pointing angle of the auxiliary lamp. This

condition can be verified by blocking both the lamps and measure the dark signal

again after the adjustment of the auxiliary lamp. Experiments showed that the dark

signal varies less than 0.1% of x under the calibration setup described in the last section.

This variation can be treated as an uncertainty source in the calibration uncertainty

budget.

Once the substitution is established, let both the standard lamp and the auxiliary lamp

illuminate the head of the illuminance meter simultaneously and take the meter reading

Y2. Y2 is the illuminance meter reading for illuminance level of 2x + D. This is the

superposition process.

Repeat these steps n times will increase the illuminance to the desired value:

nx + D, n =1,2,..

The validity of this substitution-superposition step requires that the dark signal

remains constant and the output of the auxiliary lamp is stable. Under the calibration

setup, a 1200 W standard lamp with a luminous intensity of 1715 cd placed at a distance

of 0.7 m away from the illuminance meter, x will be 3500 lx, and the calibration level of

nx + D will reach 50,000 lx with a n of less than 15.

Uncertainty analysis

As can be seen from the calibration setup, the auxiliary lamp has a collimating reflector

and is placed off-axis. The illuminance it produces on the detector head of the

illuminance meter is therefore not uniform. And because of the dichroic reflector the

spectral power distribution of the illumination from the auxiliary lamp cannot be treated

as CIE Illuminant A. These are two important factors to be considered for normal

illuminance meter calibration. However they are not critical in the proposed method

because the illumination from the auxiliary lamp is used only to produce a response on

the illuminance meter that is equal to the response of the same illuminance meter to the

known illuminace produced by the luminous intensity standard lamp. Although the

actual average illuminance produced by the auxiliary lamp might be different from the

illuminance produced by the standard lamp, they still can be used to substitute to each

other as long as the responses produced by them on the illuminance meter to be

calibrated are the same.

There are two factors affecting the validity of this substitution-superposition technique.

The first is the variation of the stray light level when the angle of the auxiliary lamp is

altered. This variation can happen at random directions or at the same direction during

the process of the substitution-superposition technique. In the worse case if all the steps

the variations are at the same direction, they will be accumulated. E.g. if the stray light

increases by 0.1%x at each step of substitution-superposition, then the total increase of

stray light after n steps will be 0.1%x multiplied by n. Since the calibrated illuminance

level is also increased to nx after n steps, the relative increases in stray light will still be

0.1%.

The second factor affecting the calibration uncertainty of the substitution-superposition

technique is the stability of the output of the auxiliary lamp. The extent of this factor

depends on the time required for the whole calibration process. Experiments shown that

the total time required for a 15-step calibration is less than 30 minutes. The maximum

drift of the auxiliary lamp during a 30 minutes time period is assessed to be less than

0.1%.

Most of the uncertainty components associated with the first-step calibration, i.e. the

direct illuminance calibration by the standard lamp based on reverse square law, will be

accumulated in absolute values. But their relative values against the total illuminance

level will remain unchanged just as the worse case of the stray light variations

discussed.

Table 1 shows the uncertainty budget of the high range illuminance meter calibration

using the substitution-superposition technique under the calibration setup shown in

Figure 1. Among the 12 uncertainty components, u(1) to u(10) are uncertainty

components associated with the first-step calibration, u(11) and u(12) are additional

uncertainty components due to the substitution-superposition process. The relative

expanded uncertainty for the direct illuminance meter calibration at low range using the

standard lamp is 1.11%. The relative expanded uncertainty for the high range

illuminance meter calibration using the substitution-superposition technique is 1.13%.

Conclusion

A new approach based on a substitution-superposition technique is proposed to

calibrated illuminance meter at levels much higher than what can be achieved by

commonly available luminous intensity standard lamps. The advantages of this

approach are its simplicity and low cost. No specially designed, complex and high

power source is required. By using a 50W tungsten halogen lamp as an auxiliary source,

which is commercially available at very low cost, illuminance meters can be calibrated

up to 50,000 lx illuminance level. The relative calibration uncertainty is similar to that

of any low range illuminance meter calibration. More than one auxiliary lamp can be

used if even higher range is required, and the same principle still applies.

References 1. CIE Publication No. 53, Methods of Characterizing the Performance of Radiometers and

Photometers (1982)

2. CIE Publication No. 69, Methods of Characterizing Illuminance Meters and Luminance

Meters (1987)

3. Ohno, Y., High Illuminance Calibration Facility and Procedures, Paper for IESNA Annual Conference (1997)

Uncertainty components

u(xi)

Description Value of u(xi)

(%)

Sensitivity factor Ci

Probability Distribution

Multiplier k

Standard uncertainty ui(y) (%)

Degree of freedom

u(1) Calibration uncertainty of the luminous intensity standard lamp (type B)

0.8 1 normal 2 0.40

u(2) Long-term stability of the standard lamp (B)

0.5 1 rectangular 1.73 0.29

u(3) Calibration uncertainty of multimeter for lamp current measurement (B)

0.005 6.25 rectangular 1.73 0.02

u(4) Calibration uncertainty of standard shunt resistor for lamp current (B)

0.005 6.25 rectangular 1.73 0.02

u(5) Uncertainty due to temperature change of standard shunt resistor (B)

0.01 6.25 rectangular 1.73 0.04

u(6) Uncertainty of distance measurement, 1mm for a distance of 0.7 m (B)

0.14 2 rectangular 1.73 0.16

u(7) Alignment (cosine) error, 2 degree from the normal (B)

0.06 1 rectangular 1.73 0.03

u(8) Uncertainty due to the resolution of the meter under test

0.1 1 rectangular 1.73 0.06

u(9) Repeatability of illuminance meter readings (A)

0.1 1 normal 1 0.10 n-1

u(10) Uncertainty due to departure from inverse square law, 1mm for 0.7m (B)

0.14 2 rectangular 1.73 0.16

u(11) Uncertainty due to extra stray light 0.1 1 rectangular 1.73 0.06

u(12) Uncertainty due to short-term drift of the auxiliary lamp

0.1 1 rectangular 1.73 0.06

u(y) Combined standard uncertainty 0.565

U(Y) Expanded uncertainty 2 1.13

Table 1 Uncertainty budget of high range illuminance meter calibration. u(1) to u(10) are uncertainty components associated with

the first-step calibration, u(11) and u(12) are additional uncertainty components due to the substitution-superposition process.