|Year : 1965 | Volume
| Issue : 3 | Page : 83-87
Brightness contrast as source of error in the ishihara test for colour blindness
Vasant G Joshi
153 "C", Matunga, Bombay 19, India
|Date of Web Publication||22-Feb-2008|
Vasant G Joshi
153 "C", Matunga, Bombay 19
Source of Support: None, Conflict of Interest: None
|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) ,,,, . 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|| |
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|| |
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|| |
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|| |
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.
| Acknowledgments|| |
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|| |
Wright W. (1959) Personal Communication.
Wright W. (1946) Researches on Normal and Defective colour vision: H. Kimpton, London.
Wright W. (1957) Annals Roy. Coll. Surg. Eng., 20, 177-191.
Wright W., Belcher S., et al (1958) Br. J1. Ophthal., 42, 355.
Ishihara S. (1959) Personal Communication.
Ishihara S. (1954 & 1958) Tests for Colour blindness. 1 1 th & 13th Eds. Tokyo, Japan; Shuppan.
Pickford R. W. (1959) Personal Communication.
Pickford R. W. (1950) Br. J. Psychol, (Gen. Sect.) 41, 52.
Fickford R. W. (1949) Individual Differences in Colour vision: Rout- ledge and K. Paul, London.
Katz D. (1939) The World of Colour: London.
Duke-Elder S. (1960) Personal Communication.
Hardy Le Grande, Rand H., Rittler M. (1945): J. Opt. Soc. Am. 35, Nos. 4 and 5.
Grunewald K., Hapten K. (1954) Acta Ophthal., 32, 425.
Grunewald K., (1959) Personal Communication.
McKellar P. (1952) Text-book of Human Psychology: London.
Selwyn E. (1959) Personal Communication.
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