While working on fog issues, I noted that the spectrum keyword (which allows you to pair lights to materials
based on spectrum number value) does not have an entity arg.
I was considering adding one to make the spectrum feature a little more useful but it struck
me that the main use case for spectrum outside of "hidden writing" is trying to keep lights
from illuminating unintended surfaces. In that context, we would not want to manually specify the matching
spectrum number for everything