Clinical testing of contrast thresholds using a commercial television monitor system

Background: The Medmont AT-20 has incorporated a contrast threshold test using a predetermined letter size that can be applied in clinical settings. This paper describes a pilot study that evaluates this technology and the effects of certain parameters on test outcomes. Methods: A photometric calibration of the test was performed to define the relationship between the AT-20 scale and Weber contrast (W%). We determined the effects of repeated measures (precision), target size (6/6 to 6/96), viewing duration (50 to 1,000 msec), defocus (+0.50 to +1.50 DS) and a macula scotoma on thresholds. The accuracy of the staircase (PEST) procedure was evaluated with and without false-negative responses. Results: The AT-20 scale has an almost linear relationship to a logarithmic transformation of W% and provides a suitable measure of contrast threshold. In the absence of monitor calibration, threshold uncertainty could be as great as 0.22 log units (W%) compared with published norms. We found that threshold variability averaged +/- 7.1 AT-20 scale units (95 per cent limits of agreement) and was proportional to threshold magnitude. One dioptre of defocus decreased thresholds by about one log unit (W%) for a 6/24 target. We propose that a 6/24 letter shown for 500 msec should provide a useful target for most clinical settings. The PEST procedure can yield endpoints in 47 (+/-12) seconds, is robust to false negative (FN) responses and gives abnormal thresholds in the presence of a macula scotoma. Conclusions: The Medmont AT-20 contrast test provides a useful clinical measure of contrast threshold. With calibration, the test could also be applied to research projects.