Kerry Wong wrote an article on reverse engineering a Beseler PM2L Color Analyzer:
I recently acquired an old Beseler PM2L color analyzer. This kind of color analyzer was designed to analyze the color or exposure of film negatives at a certain location by comparing the intensity of the filtered light of each color channel (CMY and white). The PM2L model was made in the 70’s through early 80’s and has long become obsolete. But since it has a 931A photomultiplier tube (PMT) inside, you can easily repurpose it for other uses. And the good news is, these old color analyzers are cheap on eBay. You can usually grab one for a price cheaper than a bare PMT itself.
Here is a picture of the PM2L color analyzer I got. The cap for the magenta adjustment knob is missing. The black box in the front is the light sensor which houses the photomultiplier tube.
Check out the video after the break.