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ARTICLE
Year : 1965  |  Volume : 13  |  Issue : 3  |  Page : 83-87

Brightness contrast as source of error in the ishihara test for colour blindness


153 "C", Matunga, Bombay 19, India

Date of Web Publication22-Feb-2008

Correspondence Address:
Vasant G Joshi
153 "C", Matunga, Bombay 19
India
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Source of Support: None, Conflict of Interest: None


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How to cite this article:
Joshi VG. Brightness contrast as source of error in the ishihara test for colour blindness. Indian J Ophthalmol 1965;13:83-7

How to cite this URL:
Joshi VG. Brightness contrast as source of error in the ishihara test for colour blindness. Indian J Ophthalmol [serial online] 1965 [cited 2020 Aug 13];13:83-7. Available from: http://www.ijo.in/text.asp?1965/13/3/83/39223

Defective colour vision is clinically seen as a variable inability to match colours correctly; spectral sensitivity and wavelength descrimination may also be abnormal Wright (1946, and 1957), Pickford (1949 and 1950), Duke- Elder (1960) [2],[3],[8],[9],[11] . The Japanese Ishihara test is one of the oldest and most popular tests employed to assess colour vision. All the above authori­ties are agreed that it is one of the best tests available. Currently, there are no universally accepted criteria or standardisation, either for administra­tion and scoring or for assessing the type and degree of colour vision defect on the Ishihara.

During an investigation of defective colour vision in psychiatric patients the author confirmed some previous observations that individual plates in the test varied widely in their effi­ciency of detecting defective colour vision. In fact, some of the plates were most unreliable in this respect. The older the edition the greater the unreliability. Moreover, it was noted that the unreliable plates showed their hidden numerical when photographed on black and white panchromatic film emulsion in ordinary daylight. The plates were then re-photographed on a fast, panchromatic, black and white film emulsion (Kodak Plus X) under tungsten illumination with the use of a special correction filter (Kodak Wratten Eleven). This modified technique assures that the colour sensiti­vity of the emulsion is uniform throughout the range of the visible spectrum and is comparable to that of the human retina-Selwyn (1959). By this method, some of the plates still showed their hidden numericals. It thus becomes obvious that the colour differ­ences on the plates' mosaics is not the sole factor by which the 'hidden nume­rical' is visible to the human eye, but in some plates at least, relative differences in the reflected luminance (herein call­ed brightness contrast) also plays an important role. Since the author did not find any mention of this observa­tion in the past, and since it is a very interesting and perhaps a potentially important observation, it merits re­porting.


  Method Top


Plates 1 to 25 inclusive of the 11 th (1954) and 13th (1958) editions of the Ishihara test for colour blindness were photographed on a fast, panchroma­tic, black and white film emulsion under tungsten illumination using a special correction filter so as to make the colour sensitivity of this emulsion linear and uniform to the whole range of the visible spectrum. Results are shown in figures.


  Results Top


The results can be seen in the plates on pages 84, 85.

These results are comparable to those of past workers. They assessed efficiency by comparison with other re­liable tests, etc. Pickford (1950) states 7 plates as very liable in which he in­cludes 14, 15 and 23. Wright Belcher, et al (1958) include the following plates in descending order of reliability: 6, 14, 8, 15, 25, 23 and 24. Ishihara himself recommended a short revision of his test (5th edution) in which he recom­mends plates 6, 9, 14, 15, 22 and 23.


  Discussion Top


The test is based on the so-called principle of 'pseudo-isochromatopsy' (confusion colour pattern) in psycho- physics. Incidentally, this principle is an extremely good example of visual perception by 'Gestalt', here applied strictly in reference to colour vision. The 'figure', i.e., the 'hidden numeri­cal', is perceived from the 'ground', i.e. the multi-coloured dot-mosaic. This fact has not been appreciated in the past.

The confusion colour pattern prin­ciple is itself based on empirical ob­servations but nonetheless presupposes `constancy' of all other variables ex­cept 'qualitative perceptual differences' of surface colours. Since the colour defective specifically lacks this ability in comparison to the colour normal he is detected on the test. The assump­tion of constancy of other factors ap­pears to be gravely erroneous as shown here. The very important factor of re­lative difference of reflected luminance or brightness contrast can, by itself, sometimes dominate the factor of colour contrast as demonstrates here. It is known that the colour defective can often compensate his anomaly for distinguishing between colours by dis­tinguishing between differences in brightness of colours.-Wright (1946, 1958); Pickford (1958); Duke-Elder (1960). Hence, the crucial importance of strictly equalising this factor of brightness contrast in any test designed for detecting the defect of colour vision per se.


  Summary and Conclusions Top


When plates of the Ishihara test for colour blindness were photographed in black and white by so modifying the technique of exposure that the colour sensitivity of the film emulsion was made uniformly constant to all spec­tral colours and hence comparable to the human retina, the hidden numeri­cals from some of the plates became visible. The older the edition the greater was the visibility of the numeri­cals.

This special technique assures a re­cord sensitive to 'brightness contrast' only; still some of the numericals were visible. Hence factors of 'colour con­trast' and 'brightness contrast' appear to be of comparative importance in the Ishihara test.

Details of this simple and reliable special photographic method are described.

Importance and urgency for inter­nationally standardising the time- honoured and popular Ishihara test and allied tests are emphasized.[16]


  Acknowledgments Top


Grateful acknowledgement is due to the following for advice and helpful criticism :­Sir Stewart Duke-Elder, G.C.V.O., Director,

Institute of Opthalmology, University of London.

Professor W. Wright, Imperial College of Science and Technology, University of London.

Professor R. W. Pickford, Psychology De­partment, Glasgow University.

Mr. E. Selwyn, Physics Research Labora­tory, Kodak Ltd., England.

Messrs. R. Horner and F. Wall, Physics Re­search Laboratory, Ilford Ltd., Brent­wood, Essex UK.

Dr. D. W. Liddell, MRCP, DPM, Physi­cian Superintendent, St. Francis Hospital, Haywards Heath, Sussex.

Special acknowledgement with sincere thanks to

Dr. Bryan Broom, M.B., Ch.B., Dept. of Neuro-surgery, Atkinson Morley's Hos­pital, Wimbledon, London, for editing the paper.

Dr. S. G. Bayliss, M.B., Ch.B., Director of Clinical Research, Roche Products Ltd., Welwyn Garden City, Herts, Eng. for de­fraying part of the expenses for photo­graphy.

Dr. Peter Hansell, Dept. of Medical Photo­graphy, Westminster Hosp. Medical School, London, for taking the photo­graphs.

 
  References Top

1.
Wright W. (1959) Personal Communi­cation.  Back to cited text no. 1
    
2.
Wright W. (1946) Researches on Nor­mal and Defective colour vision: H. Kimpton, London.  Back to cited text no. 2
    
3.
Wright W. (1957) Annals Roy. Coll. Surg. Eng., 20, 177-191.  Back to cited text no. 3
    
4.
Wright W., Belcher S., et al (1958) Br. J1. Ophthal., 42, 355.  Back to cited text no. 4
    
5.
Ishihara S. (1959) Personal Communi­cation.  Back to cited text no. 5
    
6.
Ishihara S. (1954 & 1958) Tests for Colour blindness. 1 1 th & 13th Eds. Tokyo, Japan; Shuppan.  Back to cited text no. 6
    
7.
Pickford R. W. (1959) Personal Com­munication.  Back to cited text no. 7
    
8.
Pickford R. W. (1950) Br. J. Psychol, (Gen. Sect.) 41, 52.  Back to cited text no. 8
    
9.
Fickford R. W. (1949) Individual Differences in Colour vision: Rout- ledge and K. Paul, London.  Back to cited text no. 9
    
10.
Katz D. (1939) The World of Colour: London.  Back to cited text no. 10
    
11.
Duke-Elder S. (1960) Personal Com­munication.  Back to cited text no. 11
    
12.
Hardy Le Grande, Rand H., Rittler M. (1945): J. Opt. Soc. Am. 35, Nos. 4 and 5.  Back to cited text no. 12
    
13.
Grunewald K., Hapten K. (1954) Acta Ophthal., 32, 425.  Back to cited text no. 13
    
14.
Grunewald K., (1959) Personal Com­munication.  Back to cited text no. 14
    
15.
McKellar P. (1952) Text-book of Hu­man Psychology: London.  Back to cited text no. 15
    
16.
Selwyn E. (1959) Personal Communi­cation.  Back to cited text no. 16
    


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