  We propose a practical method for radiometrically calibrating cameras using
widely available gaseous tritium light sources (betalights). Along with the
gain(conversion factor) and read noise level, the predictable photon flux of
the source allows us to gauge the quantum effciency. The design is easily
reproducible with a 3D printer and three inexpensive parts. Suitable for common
image sensors, we believe that the method has the potential to be a useful tool
in microscopy facilities and optical labs alike.
