This is very similar to the Harrison Light Corrector, albiet a little more complicated. You peer through it at a light source, and the field of view is split in half. One side is yellow, the other is red or green, depending on the position the of filter. You turn the knob until the color on either side of the line appears to be about the same, then read the number on the dial. Multiply it by 100 and that's the color temperature in degrees Kelvin. You can then figure out which camera filters to use to correct the light.