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In the simplest case, a luminous surface will just emit light in all directions with equal intensity. This would be the case with a wax candle, an open incandescent bulb, or maybe with a light fixture made of frosted glass. In practise, there are only few sources with uniform distribution characteristics. Most luminaires will emit their light according to a very specific distribution. This prevents us from assigning a "light" type material to the opening surface of a typical luminaire, assuming that the simulation result would then resemble its real behaviour.
Fortunately, there are better ways. Virtually all vendors of light fixtures provide data sets for their products, that exactly describe their output characteristics. This data normally comes in one of several standard formats:
IESNA North american standard
Eulumdat European standard (supported)
CIBSE TM14 UK standard
The luminaire import dialog has three pages:
To use a luminaire definition in your project, you will have to create a special geometry object. This file must contain a small triangular polygon for each of the locations where you want to place a luminaire of one type. You can then have Rayfront replace those triangles automatically by the luminaire definition. This is explained in more detail in the documentation for the Object Properties dialog, which is reached from the material assignment and editing dialog.
The various luminaire formats are defined by different standard bodies in generally similar, but still sufficiently different ways to possibly cause confusion when they are used together.
Orientation of the C-Plane
For circular or square shaped luminaires, the original
C-0 plane can be used without further problems. For longitudinal
luminaires, it gets more complicated. The
Eulumdat format specifies that the C-0 plane is parallel to
the short axis of the luminaire, while the IESNA format has
it along the long axis. Rayfront will always rotate the
luminaire definition and it's distribution, so that
the C-0 plane is parallel to the long axis. This is
consistent with the fact that it will also be parallel
with the longer edge of each marker triangle in the scene
file which the luminaire definition is going to replace.
Of course, the angular relation between luminaire shape
and distribution data is not changed, we just "rename"
the C planes internally.
Some manufacturers even disregard the standard, and create files in IESNA format where the luminaire is wider than long. This is done by most of those european manufacturers who offer files in both (or all three) formats, probably as a consequence of using broken conversion software. The result with a lighting calculation program that doesn't do any normalization would be, that a luminaire gets a different orientation in the scene, depending on which file format its data has been read from. It seems that some US manufacturers use the same trick, in order to get the "most interesting" curve into the C0 plane, mainly for asymmetric fixtures.
Rayfront will always look at the actual orientation of a luminaire, and normalize the internal definition in such a way that the length is actually the longer axis, and the distribution data starts with the right values as the C-0 plane pointing the same way.
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