Using photometric lights
Because Iray uses physically-based algorithms, if you work with realistic photometric input values in SI units, like luminous intensity values in candela, you will get realistic output results in SI units. Using physical units makes tweaking light parameters also more intuitive.
Real-world lights don't shine with the same brightness in all directions but brightness changes in different directions. Such distributions are stored in light profile files. One example are IES light profiles that store a 3d representation of the light intensity distribution. You can load those light profiles within a photometric light in Iray to render with light distributions from real-world lights. To obtain IES light profiles, visit the web page of the light manufacturer. Most professional light manufacturers provide downloadable IES files for their lights.
Lights using measured light profiles are a good way of achieving convincing lighting effects. Note, however, that such measurements assume a point light shape, which tends to yield unnaturally hard shadows. This can be ameliorated by using small area light shapes like disc and rectangle. Note the global_distribution parameter when using area light shapes and that the shape can actually support the emission characteristics, for example, a rectangle cannot emit light in tangent directions.
Iray supports the following types of Emission Distribution Functions (EDFs):
- Light profile (df::measured_edf)
- Spotlight (df::spot_edf)
- Uniform diffuse (df::diffuse_edf)
and light shapes:
In addition, any geometry in the scene can be turned into a light source using the emission property of MDL materials.