When we started working with photometric data for version 21, the purpose was to enhance the realism of the visualization in general and the footprints of beams in particular. The beams we had up until version 20 had essentially looked the same for quite some time and with all other live shading enhancements in place, they simply weren't cutting it any more. So the plan was that once AtlaBase started supplying us with photometric profiles (typically the raw data loaded from IES files) we would simply apply them to the beams pretty much like gobos. However, we quickly ran into a somewhat unexpected issue: spill lighting.
The "full angle"
Lighting designers learn that the beam angle is the angle at which a beam's intensity is down to 50% and the field angle is where the intensity is down to 10%. We had always rendered beams up to the field angle, but when we loaded IES files and simply cut the photometric profiles at the field angle, it looked terrible - the edges of soft edge fixtures were brutal. (The image below shows PAR, fresnel, PC, profile, symmetric, asymmetric and LED wash footprints.)
It was clear then that we would need to apply some kind of filtering, a soft fade-out at the edge of the beam. After some experimenting we simply weren't able to pull it off at the field angle, so we needed a new angle. Enter the "full angle", the angle at which it is realistically possible to fade out the edge of the beam and get away with it. It varies slightly for different optics types, but is normally 2% of the peak intensity of the beam. It felt like a Pyrrhic victory - we were finally able to do proper realistc beam rendering, but had to mutilate the photometric profiles (for performance reasons, as increasing the size of the beams has a dramatic impact on performance).
The "spill angle"
Thanks to further R&D and performance enhancements we will now be able to offer users of version 22 the option of visualizing the spill lighting, the part of the beam that ranges from the "full angle" to what we call the "spill angle" where the intensity is down to almost zero. The impact on scenes that use a lot of soft lighting, such as from moving head "wash" lights and LED fixtures can be dramatic!