Computing Visual Comfort Ratings for a Specific Interior Lighting Installation

The evaluation of discomfort glare is an important design criterion in illuminating engineering. Extensive data and analyses have provided the basis for the development of a new comprehensive visual comfort rating system which includes a method for preparing general visual comfort probability (VCP) tables for standard conditions of room characteristics, luminaire arrangements and mounting heights, and illumination. It is expected that such tables for specific or typical luminaires will satisfy most of the requirements of the lighting designer by enabling him to select units which will provide a visually acceptable environment. However, there will be an occasional need for evaluating discomfort glare for luminaires whose luminance distribution or arrangement in a room do not correspond to those for which VCP tables are available. Furthermore, designers of lighting equipment may be interested in obtaining a rating for an experimental unit which is to be used in a special installation. In such cases it may be desirable to make individual computations to insure that the desired degree of visual comfort will be achieved.

The procedure for determining specific ratings as described in this paper parallels the one used for preparing general VCP tables. The primary differences are that the standard uniform arrangement of luminaires is replaced by a specific layout, and the level of illumination will be that actually obtained. These will require the determination of certain values needed for obtaining solid angles, position indices and luminances for which predetermined factors are provided when preparing the tables. Nevertheless, the entire procedure is simple and straightforward, and can be performed with a slide rule or desk calculator, standard tables of squares and cubes of numbers and trigonometric functions, and graph paper. Charts and tables have been prepared in order to facilitate the determination of certain values.