Blend channels with the Calculations command

The Calculations command lets you blend two individual channels from one or more source images. You can then apply the results to a new image or to a new channel or selection in the active image. You cannot apply the Calculations command to composite channels.

Open the source image or images.
Note: If you are using more than one source image, the images must have the same pixel dimensions.
Choose Image > Calculations.
To preview the results in the image window, select Preview.
Choose the first source image, layer, and channel. To use all the layers in the source image, choose Merged For Layer.
To use the negative of the channel contents in the calculation, select Invert. For Channel, choose Gray if you want to duplicate the effect of converting the image to grayscale.
Choose the second source image, layer, and channel, and specify options.
For Blending, choose a blending mode.
For information on the Add and Subtract options, see Add and Subtract blending modes. For information on other blending options, see List of blending modes.
Enter an Opacity value to specify the effect’s strength.
If you want to apply the blending through a mask, select Mask. Then choose the image and layer containing the mask. For Channel, you can choose any color or alpha channel to use as the mask. You can also use a mask based on the active selection or the boundaries of the chosen layer (Transparency). Select Invert to reverse the masked and unmasked areas of the channel.
For Result, specify whether to place the blending results in a new document or in a new channel or selection in the active image.