Enhanced Luminance Improves Salience of Objects if it also Enhances Contrast

Purpose: Understanding how illumination and luminance contrast affect the salience of key environmental features for people with low vision is vital for designing visually accessible spaces. An experiment was conducted to examine the relative contributions of illumination and contrast to the probability of detecting environmental objects with severely reduced acuity. We hypothesized that spotlighting objects to increase their luminance would enhance their salience when it also served to increase contrast, but would be less effective when it reduced or had little effect on contrast. Methods and Results: 30 normally sighted subjects, all wearing blur goggles which reduced acuity to 20/1250, completed a visual search task. Under typical ambient classroom lighting, subjects walked along a path while visually searching for target objects adjacent to it. Performance was quantified with “detection rate” - the proportion of targets subjects reported seeing. Targets were white or grey 2 ft tall Styrofoam boxes on the floor, 9 ft away from the path, in a room with white walls and black doors. Spotlights enhanced the luminance of a subset of targets to three-fold their ambient luminance. This increased contrast for some targets (white targets against white walls: from 15% to 60%, grey targets against black doors: from 72% to 89%), but had little impact on or decreased contrast for others (white targets against black walls: from 90% to 97%, grey targets against white walls: from 43% to 10%). Enhanced illumination improved detection when it also increased contrast, with detection rate of white targets against white walls increased by 21% (p = 0.002) and grey targets against black doors increased by 32% (p < .001), with no other significant differences. Conclusions: These results suggest a guideline for designing visually accessible spaces: enhanced illumination most effectively increases the salience of objects when it increases both luminance and contrast.