IES Luminaires and Spectral Radiance

The IES file format[1] for the transfer of photometric data defines the distribution of light from a luminaire. The distribution is defined for a point light source even though most luminaires have a luminous aperture of some area. The file format specifies luminous intensities,

, for a set of directions, thus allowing for non uniform distributions. In order to compute spectral radiance from this information we must make two assumptions: that the photometry can be spread uniformly across the luminous apperature and that a spectrum exists which defines the color of the luminaire. If the luminous apperature is polygonal, we can make the first assumption understanding that the photometry will be correct for distances in which the luminaire subtends a "small" solid angle of the hemisphere above the illuminated point. The photometry will be incorrect if the luminaire subtends a "large" solid angle with respect to the illuminated point, however, in most cases the results will look reasonable.

Usually the type of lamp used in the luminaire will be defined in the IES file. By maintaining a database of spectra that correspond to particular lamp types, the second assumption can be made. In the case that no lamp type is specified a default spectrum can be used. Some of these spectra can be found in the IES Lighting Handbook[3] others are distributed by the lighting manufacturers. Fortunately, these spectra are defined in terms of or spectral radiant flux . This will allow us to derive a function of spectral radiance . The derivation is as follows:

The known quantities are luminous intensity , spectral radiant flux , the maximum spectral luminous efficacy , and the photopic luminous efficiency curve . The goal is spectral radiance . However, based on our assumption that the photometry can be spread uniformly across the surface of the luminaire, the following relationship holds: