Two-dimensional photometry for interior surround lighting

Photopic photometry is often found to be limited when applied to interior lighting applications. Some of these limitations arise from the large field-of-view which is typical of most lighting conditions, as well as from the various consequences of the scotopic sensitivity of the eye. We present here a new graphing procedure which makes these limitations readily apparent, and provides a clear visual means to make predictions that can be tested in lighting practice or experimental laboratories. This graphing method is intended to be used to compare lamps, the comparison being with respect to multiple variables: 1) disability glare, 2) visual effort, 3) room brightness, and 4) relative energy cost. To provide these four output measures, the graphing method uses two independent photometric parameters as inputs: 1) vertical photopic illuminance, and 2) vertical scotopic illuminance. Hence, the photometric parameters important for interior surround lighting applications are two-dimensional. Lamp efficacy is an additional constant input parameter for each lamp. This paper describes the relevant variables, the way to graphically interpret the measures, examples in the use of the graphs, and the limitations of the method.