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Sensitometric Properties and Image Quality of Radiographic Film and Paper

Domanus, Joseph Czeslaw

Publication date:1985

Document VersionPublisher's PDF, also known as Version of record

Link back to DTU Orbit

Citation (APA):Domanus, J. C. (1985). Sensitometric Properties and Image Quality of Radiographic Film and Paper. Roskilde:Risø National Laboratory. Risø-M, No. 2500

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Sensitometric Properties and Image Quality of Radiographic Film and Paper J. C. Domanus

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SENSITOMETRIC PROPERTIES AND IMAGE QUALITY OF RADIOGRAPHIC FILM AND PAPER

J.C. Domanus

Atlas Advanced Engineering Division *)

Paper accepted for presentation at the Eleventh World conference on Non­

destructive Testing, Las Vegas, USA, November 3-8, 1985.

Abstract

When using X-ray film or radiographic paper for industrial applications

one is interestes in knowing not only their sensitometric properties (such

as speed and contrast) but also the image quality obtainable with a parti­

cular brand of film or paper. Although standard methods for testing both

properties separately are available it is desirable that the method permits

the assessment of all the relevant properties together. The sensitometric

properties are usually determined at constant kilovoltage and filtration

at the X-ray tube, whereas radiographic image thicknesses. The use of the

constant exposure technique could be used to compare both the sensito­

metric properties as well as the image quality for different

(Continued on next page)

*) Work performed under contract with Risø National Laboratory

Risø National Laboratory, PK-4000 Roskilde, Denmark

September 1985

radiographic materials. It consist of exposing different film or paper

brands at a chosen, constant mAmin exposure when testing radiographic

image quality for different thicknesses of a given material. From the

results obtained with the constant exposure technique conclusions are

drawn about its applicability as a standard method for assessing radio­

graphic film and paper.

INIS desriptors: INDUSTRIAL RADIOGRAPHY; PAPER; PHOTOGRAPHIC FILM;

QUALITY CONTROL; SENSITIVITY; X-RAY RADIOGRAPHY

ISBN 87-550-1100-4

ISSN 0418-6435

Grafisk Service Center, Risø 1985

- 1 -

CONTENTS Page

1. INTRODUCTION 1

2. SENSITOMETRIC STANDARDS 1

3- IMAGE QUALITY STANDARDS 2

4. CONSTANT EXPOSURE TECHNIQUE 3

5. EXPERIMENTAL PROCEDURE 4

6. RELATIVE SPEED AND REDUCTION IN KILOVLTAGE 4

7. CONTRAST 4

8. RADIOGRAPHIC IMAGE QUALITY 5

9. CONCLUSIONS 7

REFERENCES 8

SENSITOMETRIC PROPERTIES AND IMAGE QUALITY OF RADIOGRAPHIC FILM AND PAPER

J.C. Domanus Atlas Advanced Engineering Division*

DK-2750, Ballerup, Denmark

1. INTRODUCTION

When using X-ray film or radiographic paper for industrial applications one is interested in knowing not only their sensitometric properties (such as speed and contrast) but also the image quality obtainable with a particular brand of film or paper.

Although standard methods for testing sensitometric properties and image quality separately are available, it is desirable to find a method by the use of which all the relevant properties could be tested together.

The sensitometric properties are usually determined at constant kilovoltage and filtration at he X-ray tube, whereas the radiographic image quality is tested at dif­ferent kilovoltages and for different material thicknesses.

2. SENSITOMETRIC STANDARDS

In several countries standards have been published for the sensitometry of X-ray films used in medical and industrial radiography as well as dose monitoring. To those standards the ANSI standard for industrial 0 \ 0 and medical £"2.7 films as well as the German DIN standard £~3_7 for medical film.belong.

All those standards require the sensitometric testing to be performed by exposing the X-ray film to radiation of a well difined quality. The quality of the X-radiation is defined by the first half value layer (HVL) and initial filtration on the X-ray tube. The HVL method is commonly used to determine the effective energy of X-rays. Knowing the HVL one can calculate the attenuation coefficient of the monochromatic X-rays: ]i = 0.693/di/2- Then from adequate tables one can find the effective energy of the X-rays in keV.

A previous investigation in that field £"4J, presented at the 6th ICNT, has shown that the same effective energies (in keV) can be reached at different peak kilovolta­ges (kVp) of an X-ray machine and filtration at the X-ray tube. This is clearly illu­strated on fig. 1. where exposure factors (kVp, filtration through Al or Cu, keV) are given for different X-ray machines used throughout the investigation E*D'• This inve­stigation has proved that it is not enough to specify the quality of X-rays by the effective energy only as e.g. the speed of X-ray films vary considerably for the same keV but different kVp.

According to ANSI ClJ for the determination of the speed and average gradient (contrast) of industrial X-ray films, four radiation qualities are specified, as shown in table 1. For X-ray films characteristic curves ought to be produced in the nett density range from 0.2 to 4.0. From those curves the speed of the X-ray film is defined as the reciprocal of exposure, which produces the film density of 2.0.

The average gradient (contrast) is the slope of the straight line drawn on the density - log exposure curve between the points on the characteristic curve at nett densities of 1.5 and 3.5 for exposures with or without lead screens and 0.5 and 2.5 for exposures with fluorescent screens. x Work performed under contract with Risø National Laboratory

- 1 -

Table 1. Radiation quality according to ANSI

X-or gamma rays

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The ANSI method of determining speed and contrast of X-ray film from characteristic curves produced by ra-. diation of the quality specified in table 1 is rarely used. The reason for that is, that it is rather difficult and cumbersome method to produce ra­diation of the specific quality men­tioned above and therefore the user of the X-ray film will rarely use it. ; It could be, however, used by the X-: ray film manufacturers, thus giving ' absolute and comparable data about film speed and contrast. Unfortunately, it is not done and although film manu-. facturers give relative speed and con­trast of their different film brands

Fig. 1. Effective X-ray energy reached at they do not specify the radiation qua-different kVp and filtration lity at which those data were obtained.

In the numerous pamphlets, cataloques and handbooks on radiography, publish­

ed by X-ray film manufacturers, the standard ANSI method is not even mentioned. One can also wonder why in the NDT handbooks l_ 57, when properties of X-ray films are de­scribed, no mention is given about standardized sensitometric procedures.

An even worse situation exists in the field of radiography on radiographic paper. Here no standards have been published and the problem is not treated in any publica­tion, either from paper manufacturers nor in radiography handbooks. One must remember that the X-ray film and paper sensitivity depends strongly on the effective energy of X-rays. As was e.g. shown in C*J, for almost all film brands the maximum film sen­sitivity occured at about 45 keV and for other energies could be decreased as much as by a factor of 7.

Having in mind the existing situation in that field it was tried to find an easy and simple method for determining the sensitometric properties of X-ray film in every­day use of a particular radiographic laboratory. The results of this effort are de­scribed below.

3. IMAGE QUALITY STANDARDS

The situation i s quite different in the field of radiographic image quality stan­dards.. Here one can find both internation (ISO) standards £~6J7 as well as many natio­nal ones (e.g. the ASTM standards /T7,B_7). One can also find ample information on that subject both in radiographic handbooks as well as in publications issued by X-ray film manufacturers.

The most widely used test pieces to control the quality of the radiographic image are the wire image quality indicators (IQI) £"6,8.7 and the so called hole penetrame-ters Z~7 J'. The first are being most commonly used in Europe and the second in North America. The relative merits and drawbacks of use of the IQI's and penetrameters were discussed for many years. They were summarized e.g. in E§J-

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Both the wire IQI's as well as the hole penetratneters were used throughout the present investigation.

4. CONSTANT EXPOSURE TECHNIQUE

The constant exposure technique, described in 1976 by P.A. Ruault £10.7 was used in the assessment of the properties of the radiographic paper /THJ7- The principle of this technique was further explained in the following way £"12_7. From a densito­metry scan of a defect (e.g.a hole) represented on a radiograph (see fig. 2) one can determine both the contrast and gradient of the radiographic signal and the quality of information obtained from a radiograph can be defined as the relation between the am­plitude of the signal (h) and the noise (h').

VMAÆf hi signal amplitude h'. amplitude of background noise I: length of signal at half amplitude

c = -j7 s contrast

G= -f = gradient

Fig. 2. Densitometric scan characterizing the radiographic signal

Fig. 3. Comparison between the constant kilovoltage and constant exposure technique

Looking on fig. 3, where ordinary technique (constant kilovoltage} used in radio­graphy is compared with the constant exposure technique (for a fast and slow radiogra­phic system) one can see that:

1) For the constant kilovoltage technique the signal from the defect has a constant amplitude, but the amplitude of the background noise depends on the speed of the system (it is lower for the slower system). Thus for the constant kilovoltage technique the quality of information obtained decreases with the increasing speed of the system.

2) If, on the other hand, one will maintain the same exposure for the faster system, then the kilovoltage can be lowered because a lower radiation dose is necessary for the faster system. The lowering of the kilovoltage increases the amplitude of the sig­nal . Thus the ratio for the signal to background noise can be relatively improved for the faster system.

The constant exposure textyuq'ie was succesful ly used at Risø to assess various ra­diographic systems (such as >acH*>^aphic paper with fluorescent and fluorometallic screens £"13J^ X-ray film and >j^*r fast systems Z~14,15.7/and was recently described in ZT16.7- As a general conclusion one can say that by using the constant exposure technique the use of fast radiographic systems is possible and advisable as the loss of image quality is very small or nil. One can point out the main advantages of using fast radiographic systems as follows:

-3 -

- Shorter exposure times due to the higher speed of the system. - Lower material cost for paper radiography. - Shorter processing times, especially with automatic processors. - Lower labor costs due to shorter exposure and processing times. - Lower equipment costs due to lower kilovoltages in use. - Easier radiation protection due to lower kilovoltages in use.

5. EXPERIMANTAL PROCEDURE

In the situation described above, where no absolute data about sensitdnetric pro­perties of X-ray film or paper can be found, it is necessary to produce those data ir. ones own radiographic laboratory for the exposure conditions and materials routinely radiographed.

Below, examples will be given how this was done at Rise for aluminium (30 mm thick) and steel (10 mm) plates exposed by self-rectified X-ray machines using diffe­rent X-ray film and radiographic paper brands.

Those test plates were exposed together with the ISO wire IQI's and ASTM hole pe­netrans ter s both at constant kilovoltage (and varying exposure) as well as at constant exposure (and varying kilovoltage) to give the same film density of 2.5 or paper den­sity of 1.0. From eacn radiograph the smallest percent ISO wire IQI diameter and the best ASTM percent penetrameter level were determined by three observers.

The slowestX-ray film (Kodak Industrex single coated SR) was choosen as reference with a relative speed of 1.0. For this film to reach the density of 2.5 for 30 mm Al an exposure of 25 mAmin at 170 kV and for 10 mm Fe an exposure of 100 mAmin at 215 kV were necessary (at 1 m FFD).

Altogether 12 Kodak Industrex and 10 Agfa-Gevaert Structurix film, paper and screen combinations were tested (FM means fluorometallic screens).

6. RELATIVE SPEED AND REDUCTION IN KILOVOLTAGE

The results of the investigation described above are given in fig. 4. Here on the upper part results obtained for Kodak Industrex systems are represented whereas on the lower part results for Agfa-Gavaert Structurix systems are given. For the constant kilovoltage (170 kV for 30 mm Al and 215 kV for 10 mm Fe) exposures (in mAmin) neces­sary to obtain densities of 2.5 or 1.0 are shown on the vertical scale in fig. 4. By dividing the exposure (25 mAmin for 30 mm Al and 100 mAmin for 10 mm Fe) necessary for the slowest system (Kodak single coated Industrex SR film) by the exposure neces­sary for other systems one can calculate the relative speed of the system (shown in the tables in fig. 4.).

On the other hand, for the constant exposure (25 mAmin for 30 mm Al and 100 mAmin for 10 mm Fe) kilovoltages necessary to obtain respectively densities of 2.5 and 1.0 are shown on the horizontal scale in fig. 4. In the tables given in this figure abso­lute and relative kilovoltage reductions are given, possible when using faster systems (than Kodak SR) with the constant exposure technique.

As can be seen the relative speed factor can be as high as 625 and relative reduc­tion in kilovoltage - 36%.

7. CONTRAST

Unlike thf speed, the contrast of X-ray film and paper does not depend much on ra­diation quality. Therefore one can calculate X-ray film contrast from characteristic curves given by film manufacturers. While calculating contrast of X-ray film the cri­teria given in the ANSI standard C^J can *>« u8ed (as quoted in 2 above).

Very often the contrast of the X-ray film is given together with the characteristic curve by the X-ray film manufacturer.

Unfortunately, there are no sensitometric standards for the radiographic paper. As is well known the paper density does not increase in such a way as that of the X-ray film (while exposure is increased) and reaches a saturation level. Therefore for the radiographic paper not the average gradient (contrast) is calculated but the maximum

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gradient (calculated from the characteristic curve). The relation between the radio­graphic paper contrast and its optical reflection density was studied previously Z"17J7 and it was prooved that the maximum contrast occurs at the paper density of about 1.0 (see also fl&J).

8. RADIOGRAPHIC IMAGE QUALITY

As explained in 5 above the radiographic image quality was tested by the use of 30 mm Al and 10 mm Fe plates on which ISO wire IQI's and ASTM hole penetrameters were placed. Thus all combinations of X-ray film and radiographic paper and intensifying screens were tested using both the constant kilovoltage as well as the constant expo­sure technique. The results of this investigation are shown on fig. 5 for Kodak, and on fig. 6 for Agfa-Gevaert systems obtained by the constant exposure technique.

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Fig, 8. Percent radiographic quality for Agfa-Gevaert systems obtained by the constant kilovoltage technique

relative speed and image quality can in general only be done for those materials and their thicknesses which are generally used in a particular radiographic laboratory.

2) The contrast of the X-ray film or radiographic paper can easily be determined from characteristic curves published by film and paper manufacturers.

3) By using the constant exposure technique one can easily find faster radiogra­phic system and at the same time have the information wheather the radiographic image quality of this system satisfies the particular quality requirements.

REFERENCES

£T--7 ANSI PH2.8-1975. American National Standard method for the sensitometry of industrial X-ray films for energies up to 3 million electron volts.

C2j ANSI PH2.9-1974. American National Standard method for the sensitometry of medical and dental X-ray films.

£~3j7 DIN 6829.1959. Rontgenfilme zur Vervendung ohne Verstarkerfolien in der medi-zinischen Diagnostik. Bestimmung von Empfindlichkeit (withdrawn in Febr. 1976).

£&ZI J- Domanus. Determination of quality of X-rays used for the sensitometric in­vestigation of X-ray films. Report No D5 to the Sixth International Conference on Non-destructive Testing. Hannover, 1-5.6.1970.

£"5J7 L-E. Bryant, P. Mc.Intire (editors). Non-destructive Testing Handbook. Ameri­can Society for Non-destructive Testing 1985.

£"6 J ISO Recommendation R 1027. Radiographic image quality indicators. Principles and identification.

£"7, 7 ANSI/ASTM E142-77. Standard method for controlling quality of radiographic testing.

£8j ASTM E747-80. Standard method for controlling quality of radiographic testing using wire penetrameters.

C^D J-C. Domanus, H:M. El Fouly. ISO wire IQI's vs. ASTM penetrameters in paper radiography. Paper B-6. Second European Conference on Non-Destructive Testing. Vienna, 14-16.9.1981.

/10J7 P.A. Ruault. La technique radiographique a exposition constante. 8th World Conference on Non-destructive Testing. Cannes, 6-11.9.1976, paper 3E1.

£l\ZI J.C. Domanus, P.A. Ruault. Industrial application of radiographic paper. Materialprufung 22 (1980), No 3, March 1980, 111-116.

/I2J7 J'C. Domanus. Constant exposure technique in industrial radiography. Risø-M-2398, August 1983.

/13J7 J-C. Domanus. Application of fluorometallic screens for paper radiography. Risø-M-2395. July 1983.

/I4J7 J-C. Domanus. Assessment of fast radiographic systems by the constant exposure technique. Risø-M-2440, August 1984.

/15I7 J.C. Domanus. Fast radiographic systems. Proceedings of the 3rd European Con­ference on Non-destructive Testing. Florence, 15-18.10.1984, Vol. 3, 288-299 and Risø-M-2453, August 1984.

/16J7 J'C. Domanus. Radiographic constant exposure technique. Materialprufung. 27 (1985) No. 1/2, Jan./Febr. 1985, 15-18.

£.1 £1 J.C. Domanus. Industrial radiography on radiographic paper. Risø Report No. __ 371. November 1977.

/l8_/ R. (Juinn, J.C. Domanus. film and paper radiography. Section 5 in /~5_7.

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Risø National Laboratory Riso-M-EioZ]

o o tr CM

I

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Title and author(s)

SENSITOMETRIC PROPERTIES AND IMAGE QUALITY OF RADIOGRAPHIC FILM AND PAPER

J.C. Domanus

Paper accepted for presenta t ion a t the Eleventh World Conference on Nondestructive Test ing, Las Vegas USA, November 3-8, 1985-

Group's own r e g i s t r a t i o n number(s)

8 pages + l t ab les + 8 i l l u s t r a t i o n s

Date September 1985

Department or group

Metallurgy

Abstract

When using X-ray film or radiographic paper for indu­

strial applications one is interested in knowing not

only their sensitometric properties (such as speed

and contrast) but also the image quality obtainable

with a particular brand of film or paper. Although

standard methods for testing both properties separately

are available it is desirable that the method permits

the assessment of all the relevant properties together.

The sensitometric properties are usually determined

at constant kilo\vltage and filtration at the X-ray

tube, whereas radiographic image quality is tested

at different kilovoltages and for different material

thicknesses. The use of the constant exposure technique

could be used to compare both the sensitometric proper­

ties as well as the image quality for different radio­

graphic materials. It consists of exposing different

film or paper brands at a choosen, constant mAmin expo­

sure when testing radiographic image quality for diffe­

rent thicknesses of a given material. From the results

obtained with the constant exposure technique conclusi­

ons are drawn about its applicability as a standard

method for assessing radiographic film and paper.

Copies to

* • )

Work performed under contract with Risø National Laboratory.

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