Does anyone know how to calibrate a photomultiplier? I need to measure very, very low levels of light (almost no light) and am considering using a photomultiplier. However, I don't know how easy (or difficult) it would be to calibrate a photomultiplier to be sensitive to such a low level of light. If a photomultiplier won't work, are there other options? I've looked at lux meters but could not find any that would measure below 10^-2 lux, which is too much light. Thank you for any help.