You can use the blending effects associated with layers to combine channels within and between images into new images. You can use either the Apply Image command (on single and composite channels) or the Calculations command (on single channels). These commands offer two additional blending modes not available in the Layers panel—Add and Subtract. Although it’s possible to create new combinations of channels by copying channels to layers in the Layers panel, you may find it quicker to use the calculation commands to blend channel information.
The calculation commands perform mathematical operations on the corresponding pixels of two channels (the pixels with identical locations in the image) and then combine the results in a single channel. Two concepts are fundamental to understanding how the calculation commands work: