Perfectly Even Copy Lighting - Page 6
Before using the Apply Image command, ensure your copy shot is open and is in Lab Color. Select the Lightness channel in the Channels Palette. In this example, this image is named "copyphoto.tif".
The "white reference-rgb.tif" file is the original white reference image, now converted to Greyscale and with the density adjustment performed.
Next, choose Image>Apply Image. The Source should be the white reference file, the destination will be the copy photo, the blending mode should be set to ADD and check the “Invert” checkbox. The settings are detailed in the screenshot below: