Orientation invariant light source parameters

At present there is no common language for comparing two different light sources. Trying to quantify (or even to explain qualitatively) the difference in performance terms, for example, of an axial and a transverse filament, or a gas discharge tube and a light-emitting diode, or a gas lamp mantle and a laser, is like comparing apples and oranges. We have developed a concept for a common representation of these diverse kinds of light sources, without reference to any coordinate system or light generation process. Based on an expansion in spherical harmonics of an intensity data set over a sphere, we characterize the angular light distribution of a source by providing a prescription for calculating the orientation-independent parameters d(l) that represent the proportion of rms power distributed in each angular mode. (C) 1996 Society of Photo-Optical instrumentation Engineers.