ALFA extends the Radiance lighting engine to conduct simulations in 81 color channels. Viewing the world in high spectral resolution allows ALFA to accurately account for the action of each of the five photoreceptors in the human eye – including the melanopsin-containing cells that help regulate alertness and sleep.
To carry out 81-channel renderings, ray trees produced by Radiance are parsed by a program called spectree. Spectree inserts a source spectrum at the terminus of each ray path (e.g. at a luminaire or the sky), and then moves backward toward the ray origin, accounting for spectral reflectances and/or transmittances at each material interaction.
The resulting radiance spectra are identical to those produced by a brute force N-step approach (which involves repeating a 3-channel run 27 times), but the spectree approach is 10-15 times more CPU efficient.
Renderings and sensor-grid simulations are both carried out progressively, meaning noisy results are delivered quickly and then smooth out over time. For most lighting scenes, accurate lux values are achieved after merely a few passes, often in a matter of seconds.