About channels

Channels are grayscale images that store different types of information:

Color information channels are created automatically when you open a new image. The image’s color mode determines the number of color channels created. For example, an RGB image has a channel for each color (red, green, and blue) plus a composite channel used for editing the image.
Alpha channels store selections as grayscale images. You can add alpha channels to create and store masks, which let you manipulate or protect parts of an image.
Spot color channels specify additional plates for printing with spot color inks.

An image can have up to 56 channels. All new channels have the same dimensions and number of pixels as the original image.

The file size required for a channel depends on the pixel information in the channel. Certain file formats, including TIFF and Photoshop formats, compress channel information and can save space. The size of an uncompressed file, including alpha channels and layers, appears as the rightmost value in the status bar at the bottom of the window when you choose Document Sizes from the pop‑up menu.

Note: As long as you save a file in a format supporting the image’s color mode, the color channels are preserved. Alpha channels are preserved only when you save a file in Photoshop, PDF, TIFF, PSB, or raw formats. DCS 2.0 format preserves only spot channels. Saving in other formats may cause channel information to be discarded.