I picked this up at a thrift store yesterday, and am now trying to make mathematical sense of it. I think that I understand the original application of this device, in connection with stone-aged chemical-based photography, on an enlarger, to be to measure the color of the light being projected through a given point of a color negative, to allow one to figure out the color filter settings to get a properly white-balance print from that negative What I have figured out so far of its operation is this: It has four color “channels”—Cyan, Yellow, Magenta, and White. To take a measurement of each color, you expose the probe to the light to be measured, set the switch on the probe to that color, and then adjust the corresponding knob on the main unit until the needle is centered. You then read the measurements off of each knob, which is calibrated from 0 to 100. Higher levels of light cause the needle to go left, requiring a higher setting on the knob to center the needle. So, I created this image to display on my computer screen,and took measurements of each section, with the probe up against my screen. I also took measurements of my screen displaying solid white. I put these measurements into a Excel spreadsheet, and attempted two different methods of correcting the results. You can see that vertically, there are three sections. The upper section is the raw measurements from the PM2L; the middle segment was calculated by subtracting each channel from the white channel, and the bottom section was made by subtracting each measurement from the corresponding measurement of the white-screen sample. I was hoping, at this point, to have a much clearer view than I have, of how the response of this device relates to the color of the light that it is analyzing. I'm trying to shake off the fog of having taken a sleeping pill last night, which may be impairing my ability now to understand what I am seeing. Also confusing the issue, of course, is that most chemical photography is a negative process; exposing photographic material to a given color of light produces an image in the opposite color. For example, making the light hitting a print material more cyan in color would cause the print to come out more red. I'm getting confused trying to keep track of which direction to expect results to change with color. It does seem that any given color channel of this device gives a lower reading when seeing light of that color, as opposed to light of other colors. This is most prominent with the cyan channel, which gave a reading of 20 when measuring the cyan section of my image, while giving readings of 50 when measuring the magenta and yellow sections. It shows as less prominent with the magenta channel, and not at all with the yellow channel. It seems, also, that the channels are not consistent with one another in their sensitivity. I don't know if this is a failing of this instrument, or whether this is intentional, reflecting the color response of color print materials in common use in the 1970s-1980s, when this device was built. Honestly, I don't even know to what end I am trying to figure this out, beyond mere curiously about this device. I do imagine that perhaps I'll eventually be able to use this to measure light where I am taking pictures with my DSLR, and from it to calculate the correct white-balance settings to apply when those pictures are later post-processed on my computer.