The evaluation of interior lighting on the basis of visual criteria

Contrast sensitivity and visual acuity data are used to derive a unified visual performance contour which describes the generalized improvement in visual performance resulting as task background luminance is increased. Factors which influence the difficulty of visual tasks are described, and examples are given of the degree of task background luminance, and hence illuminance, required to permit criterion levels of performance for sample tasks. Physical principles and devices are described which permit measurements of: (a) task object contrast, (b) the integral of ocular light scatter which reduces task image contrast, and (c) the joint effect of luminance differences in the environment which produce transitional adaptive effects on visual performance. These physical aspects of luminous environments are shown to influence visual performance in quantitative ways which may be assessed by reference to the standard performance contour. An over-all lighting performance index is derived which takes account of the ta background luminance and these three other measures of the effects physical aspects of luminous environments have upon visual performance. Lighting performance indices are presented for five sample lighting installations, which reveal the overriding importance of other aspects of luminous environments than the level of illuminance they provide. The problem of predicting the over-all visual performance to be expected from specific luminous environments in advance of construction is discussed, and empirically derived calculational methods are described.