Application Distance Photometry
Due to the relatively low ceiling heights existing in many office buildings, indirect luminaires must often be mounted close to the ceiling. This need for close mounting distances (short suspension lengths) has led to the development of indirect fluorescent luminaires having highly efficient, inverted, batwing distributions. Specular 9mirrorlike) reflectors and specially designed lenses are important elements of such luminaires. The unique nature of such optical systems reduces the accuracy of discretizing techniques normally used by computer programs when calculating illuminance close to the luminaire. Because far-field photometry provides no information about luminaire luminances, discretizing techniques must assume that the luminaire is planer and of perfectly uniform luminance. They also assume that each element of the luminaire has the same luminous intensity distribution as each other element. These assumption usually work for luminaires located several feet from the surface they are lighting. However it will be shown here that discretizing techniques coupled with far field photometric data do not yield sufficiently accurate results when indirect luminaires are mounted close (less than 30 inches) to the ceiling.
This paper was initialed because on of the authors (Brass) noticed that mirror type optical systems deviated significantly from the usual discretizing assumptions. This occurs because virtual images produced by reflectors or lenses are greatly displaced from the assumed light center and even the physical extents of the luminaire. Furthermore, these image appear and disappear in a way that defies simplifying assumptions.
Application Distance Photometry is proposed as a solution to these problems. Equivalent luminous intensity data obtained by application distance photometry produce accurate results when used by any computer program (or hand method) capable of solving the inverse square law.