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

Abstract

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.

A New Photographic‐based System for the Measurement of Contrast Sensitivity

“…(iv) Another important factor which must be considered is the choice of sinusoidal or square waveform grating patterns. Because of the inherent nonlinearities present in the photographic system, pure sinusoidal luminance profiles caused by appropriate variation in print density are difficult to achieve in practice, particularly at high contrast levels (see Verbaken and Jacobs, 1985). On the other hand square waveform gratings can be reproduced accurately using relatively simple methods.…”

Assessment of contrast sensitivity of patients with macular disease using reduced contrast near visual acuity charts

“…Thirty letter charts were produced by the photographic technique of Verbaken and Jacobs (1985). They all had the same average reflectance and the contrast of the charts ranged from 0.40 dB to 58.0 dB* (C = 0.950 to 0.0013) in an average of 2 dB steps.…”

“…Letter charts having a contrast greater than 30 dB were calibrated with a Spectra Pritchard 1980B spectroradiometer in its photometric mode. Contrast could not be measured with sufficient accuracy for contrasts lower than 30 dB and the contrast of these plates was calculated from photographic intensity and exposure data using formulae devised by Verbaken and Jacobs (1985).…”

“…It presents an array of edge stimuli of varying contrast and orientations and can be used to estimate contrast sensitivity for the peak spatial frequencies. The photographic technique used to develop this test has been validated (Verbaken and Jacobs, 1985) and the test has been shown to have good test-retest reliability and to correlate well with corresponding measures using a CRT display (Greeves, Cole and Jacobs, 1987 macula, grating or edge targets may not be suitable for measurement of contrast sensitivity since gratings and edges are extended stimuli that may fall outside the affected part of the macula. Letter charts can avoid this problem since letter stimuli are usualiy small enough to be confined within the fovea.…”

“…Unlike a grating stimulus, reduced contrast letter charts have the advantage of a form familiar to the patient and also allow easy scoring. Verbaken and Jacobs (1985) developed a photographic technique to produce sets of near logMAR letter charts that varied in contrast from high to low and could be used clinically to measure foveal contrast sensitivity. This study examines the test-retest reliability of these varying contrast near logMAR letter charts as well as their validity with respect to conventional measures of grating CSF for patients with macular disease.…”