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Daylight calculations

Kurt Bieri

Daylight calculations are generally more challenging because of the greater variance of the illumination (by comparison to artificially lit scenes). 

Another difficulty is that the sky hemisphere cannot be treated as a localized light emitter in the same way as a normal luminaire. 

For this reason, the sky portion of the illumination is evaluated using the indirect light process in Radiance. 

By contrast to the sun itself, which similarly cannot be localized because of its great distance, but which can easily be simulated as an angular light source 

with a source angle of approximately 0.5°.

To accurately simulate the illumination through daylight openings, it is recommended that a precalculation be run for all the openings (windows and skylights). 

Each daylight opening is treated separately, and a luminous intensity distribution is generated which describes the angular dependence of the incoming daylight, 

making allowance for the chosen sky model and all the external obstructions (if modelled). It is, of course, necessary for the level of the external interreflections 

to be correctly specified. The default (and minimum) value of 1 for the interreflection only takes the sky itself into consideration. 

All the geometry objects outside the room act as pure obstructions. With 2 or more interreflections, it is possible to additionally consider the amount of light 

(from the sun and the sky) which is reflected by external objects.

The precalculation routine thus converts daylight openings into localized light sources, making for a more accurate calculation by comparison to a simulation 

that uses the indirect algorithm for simulating the room as a whole. These localized sources can, however, produce disturbing highlights in conjunction with 

reflecting surfaces when images are generated. It is thus possible for this precalculation to be switched off. In this case, it is recommended that the number 

of interreflections be increased to 3-5 or more (both the interreflections within the room and those outside it must now be considered together). 

The number of indirect rays can perhaps also be increased (e.g. to 800-1000 or more). Switching off the window precalculation can also make sense for scenes 

with very large daylight openings, e.g. rooms with large window sections that cover entire walls from floor to ceiling.