Abstract
Purpose: :
Visual accessibility refers to the use of vision to travel efficiently and safely through an environment. For people with low vision, the detection of steps and ramps is an important component of visual accessibility. Here, we report on the effects of the background contrast, arrangement of lighting, and viewing distance. We analyzed the effects of these environmental factors on the visibility of shape cues and contrast cues for detecting and recognizing steps and ramps.
Methods: :
Gray wooden staging was used to construct a sidewalk with one of five possible targets at a fixed location: a single step up or down (7 inch height), a ramp up or down (7 inch change of height over 8 ft), or flat. In a trial, subjects indicated which of the five targets was present. The responses were used to compute overall percent correct, and to construct confusion matrices yielding information transmitted (bits). Viewing distance from the target location was 5, 10 or 20 ft. subjects viewed the sidewalk monocularly through blur goggles that reduced Snellen acuity to ~20/135 and Pelli-Robson contrast sensitivity to ~0.82. There were three lighting conditions–overhead room lighting and side lighting from an artificial window placed either beyond the target or nearer than the target. 24 normally sighted young adults were randomly assigned to groups tested with either gray or black backgrounds surrounding the sidewalk.
Results: :
The group tested with the black background performed better (89% correct, 1.79 bits) than the group tested with the gray background (81%, 1.62 bits). The best lighting condition was the window beyond the target location (89% compared to 84% for the other two lighting conditions). Performance was better at the 10 ft viewing distance (87%) than 5 ft (83%) and 20 ft (81%). The step up was the most recognizable of the five targets (99%), while the least recognizable were ramp up and ramp down (78%).
Conclusions: :
The detection and recognition of surface irregularities (steps and ramps) with low-resolution vision are influenced by contrast features that depend critically on background color the arrangement of lighting, and geometrical (shape) cues that vary with viewing distance.
Keywords: low vision • detection • shape, form, contour, object perception