The effect of observation time and contrast on visual acuity

Field studies of the duration of fixation during daytime driving find that it is about 0.2 seconds on average. Thus, the time available to read traffic signs is often limited. There are other circumstances where time to read alphanumeric messages is limited. Data relating time of observation are needed for purposes of ergonomic design as the data available in the literature are limited and uncertain.

Visual acuity was measured using Landolt rings in negative contrast, which were presented on a computer screen with a background luminance of 32 cd/m(2) for durations that varied between two and 0.02 seconds at three different contrasts.

Visual acuity depends on the log of the light dose reduced by a factor that may be related to absorption of the light by the eye media. For young observers, it was found that: VA = 0.57 (log C.t) + 1.705, where C is the contrast and is expressed as DL/L, where DL is the difference between target and background luminances in cd/m(2) and t is the observation time in seconds.

This formula or the functions fitted to the data obtained can be used to calculate the increase in target size needed to allow for short observation times.