Perfectly Even Copy Lighting - Page 4
The white reference file should be converted to greyscale. Using Image>Mode>Greyscale is sufficient. Using the Color Sampler Tool , place a sampler in the brightest and darkest regions of the white reference. Change the readout in the Info Palette to provide Greyscale values.
Read the two areas and subtract them to find the difference. The density difference in this particular setup is 6%.