Quantitative and functional influence of surround luminance on the letter contrast sensitivity function.

AIM: To determine the influence of surround luminance on the letter contrast sensitivity function. METHOD: The binocular contrast sensitivities of 31 young and ocularly healthy individuals were measured with letters of sizes calculated to obtain the targeted fundamental frequencies of 3, 10, 20, and 30 c deg(-1), respectively; with surround luminances from 1 to 1000 cd m(-2), always with a test chart background luminance of 200 cd m(-2). RESULTS: The letter contrast sensitivity increased with surround luminance up to 100 cd m(-2) and decreased when surround luminance increased from 100 to 1000 cd m(-2). These increments are larger for higher fundamental spatial frequencies, while decrements are similar for all frequencies. To analyse pupil size influence, results were compared with theoretical predictions obtained by combining different ocular MTFs with a typical neural function, where pupil size decrease leads to letter contrast sensitivity increments and veiling luminance causes the observed decrements. Other possible optical or neural factors that influence these values have also been considered. CONCLUSIONS: Letter contrast sensitivity function depends on surround luminance and this influence should be considered in future standardized directives.

Influence of surround illumination on pupil size and contrast sensitivity.

Lighting plays a major role in contrast sensitivity (CS) measurements. Both the test and surround illumination influence the results although they are not usually considered in clinical practice. The effects of test luminance are well known, but the influence of surround luminance seems to be less investigated. This study aims to evaluate the differences in CS measured with two configurations of surround illumination typical of clinical practice; and to analyse the influence of the angular size of the target on pupil diameter for both surround luminances. An experimental arrangement was designed to measure CS with controlled illumination of both test and surround. An infrared pupillometer was also used to measure steady pupil size. A statistically significant increase of CS and a decrease of pupil size with higher surround illumination were found.