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By default, the apparent brightness of rectangular lights changes with their size - larger rectangles emit more light. It can be convenient to instead adjust the brightness so that changing the lights size does not affect the brightness by normalising the light intensity. This function computes a factor that normalises to a 100x100 unit light source (1m^2 in Enlighten units).
Returns
Will return -1 for invalid input (nans, infs, and tiny unitScales) and 0 for zero area lights.
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