Luminance contrast preferences of people with a vision impairment for elements in the built environment.

BACKGROUND: Standards for building elements recommend a minimum luminance contrast of 30%. The basis of this value and the metric originally used is not known. OBJECTIVE: To begin to provide an evidence base for the specification of minimum contrast in building elements. METHODS: Subjects with and without a vision impairment were characterized by visual acuity, contrast sensitivity, visual fields and color vision. On an iPad they rated the visibility, as a function of contrast, of simulated door frames, door handles, light switches and stair nosings as "not visible at all", "poorly visible", "easily visible" and "extremely easily visible". RESULTS: The contrasts for each level of visibility were highly correlated with visual acuity and contrast sensitivity. A principal component analysis also verified the importance of visual acuity, contrast sensitivity and visual fields in rating visibility of simulations of building elements. The required contrast for door handles, light switches and stair nosings to attain the same ratings of visibility were very similar but less contrast was required for door frames. CONCLUSIONS: 30% Michelson contrast for building elements renders building elements only poorly visible for those with severe vision impairments. 65% luminance contrast is necessary for all elements to be "easily visible". Some increase (not a decrease) on the present 30% requirement and encouragement to exceed this requirement would seem appropriate. The use of simulated objects facilitates a systematic examination of the effect of contrast, but the applicability of the results to real-life remains to be demonstrated.

Issues in specifying contrast in building elements for people with a visual disability.

BACKGROUND: Standards writers, national and international, have used different contrast calculations to set requirements in building elements for people with visual impairments. On the other hand, they have typically set a single requirement (30%) for specifying the minimum contrast. The systems are not linearly related and 30% means something rather different in each system. OBJECTIVE: To provide a comparison of the various scales in order to illustrate the differences caused by multiple scales with a single compliance value, recommend a single scale for universal adoption and, if a new measure is problematic for implementation, to recommend the most perceptually uniform of the present methods. METHODS: We use the contrast between combinations of 205 paint colours to illustrate the relationships between the measures. We use an internationally accepted scale, with equal perceptual steps, as a "gold standard" to identify the most perceptually uniform measurement scale in the existing methods. RESULTS: We show that Michelson contrast is the most perceptually uniform of the existing measurement scales. We show the contrasts in the proposed method that equate to the various current requirements. CONCLUSIONS: We propose that CIE Metric Lightness could be used as the contrast measure. Alternatively, Michelson contrast is the most perceptually linear of the current measurement scales.