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For the purpose of optical simulation, a plethora of formats exist to describe the properties of a light source.
Except for the EULUMDAT and IES formats which describe sources in terms of aperture area and far field
intensity, all these formats are vendor specific, and no generally accepted standard exists. Most illumination
simulation software vendors use their own format for ray sets, which describe sources in terms of many rays.
Some of them keep their format definition proprietary. Thus, software packages typically can read or write only
their own specific format, although the actual data content is not so different. Typically, they describe origin
and direction of each ray in 3D vectors, and use one more single number for magnitude, where magnitude may
denote radiant flux, luminous flux (equivalently tristimulus Y), or tristimulus X and Z. Sometimes each ray also
carries its wavelength, while other formats allow to specify an overall spectrum for the whole source. In addition,
in at least one format, polarization properties are also included for each ray. This situation makes it inefficient
and potentially error prone for light source manufacturers to provide ray data sets for their sources in many
different formats. Furthermore, near field goniometer vendors again use their proprietary formats to store the
source description in terms of luminance data, and offer their proprietary software to generate ray sets from this
data base. Again, the plethora of ray sets make the ray set production inefficient and potentially error prone. In
this paper, we propose to describe ray data sets in terms of phase space, as a step towards a standardized ray set
format. It is well known that luminance and radiance can be defined as flux density in phase space: luminance
is flux divided by etendue. Therefore, single rays can be thought of as center points of phase space cells, where each cell possesses its volume (i.e. etendue), its flux, and therefore its luminance. In addition, each phase space cell possesses its spectrum, and its polarization properties. We show how this approach leads to a unification of the EULUMDAT/IES, ray set and near field goniometer formats, making possible the generation of arbitrarily many additional rays by luminance interpolation. We also show how the EULUMDAT/IES and individual ray set formats can be derived from the proposed general format, making software using a possible standard format downward compatible.
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