We need to define a physical_filter s and a band for when we have no chromatic element in the beam. This is the case for e.g. ComCam in its current configuration at the base centre, and for the LSSTCam when using the CBP.
I propose that the physical_filter be empty and the band be white.
The choice of physical_filter is not all that important, as it can be chosen per-instrument, but band should be common across all cameras.