The application of CRF to office lighting design

Abstract

Veiling reflections reduce task visibility and may cause visual discomfort. They can be quantified by the Contrast Rendering Factor (CRF). The factors determining the magnitude of the CRF are each dealt with in turn using data derived from various published studies. The presence of a high luminance in the offending zone is shown to be the chief cause of low CRF. A minimum CRF standard of 0.7 for written tasks on matt or semi-matt paper is proposed for offices. The calculation and measurement of CRF is briefly discussed and a simple method of field measurement suggested. Some general guidelines for good CRF design are proposed. The use of a small number of efficient luminaires arranged in a regular linear array in a room with high reflectance surfaces, together with low specularity visual tasks, is advocated.