ADR 0002: flux and detector normalization

Status: accepted

Optical engines return a non-negative photon-rate map in photons/s/native detector pixel. A configured direct rate is interpreted at the detector surface; magnitude normalization delegates to getframes radiometry. Fourier transforms are unitary, and finite detector cropping reports captured flux instead of renormalizing it. getframes.Camera.expose (or its optional expose_spectral cube variant) is the only boundary that applies QE, shot noise, read noise, gain, saturation, and digitization.