It looks like a B&W transmission densitometer. You would need a calibration strip to calibrate it. Basically putting nothing and make a reading adjust for zero reading. Putting a known relatively high density 2.0 density or so and adjust the slope for the correct reading. Going back and forth until both readings are ok. Then reads several known reading at say .75 1.00 etc.. to see if it reads good. Then you can use it. That my guess by just looking at the unit.