The technical problems of producing photographic prints for the measurement of human contrast thresholds

Whilst most optometrists today are aware of the need for a practical method of measuring the contrast sensitivity function (CSF), cost and length of testing have usually precluded this measurement. As the Arden Grating Test is a screening test for normal CSF, and since many patients with low vision cannot see any of its gratings, there is a need for a test with an extended contrast range which is calibrated to allow quantitative measurement of contrast thresholds in the clinic. This paper discusses the problems associated with the use of the photographic process to manufacture such a test. A series of stimuli covering a wide range of contrasts were manufactured photographically and evaluated photometrically and psychophysically to validate the photographic process. The results show that the photographic techniques can be controlled sufficiently to enable the manufacture of stimuli of the required contrasts and that, at low contrasts where calibration is inaccurate or impossible, the contrasts produced are linearly related to exposure. The results also show that the illumination and observation conditions need to be accurately controlled for the plate contrast to be unaffected by veiling glare.