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. (See About masks and alpha channels.)
Spot color channels specify additional plates for printing with spot color inks. (See About spot colors.)
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 right-most value in the status bar at the bottom of the window when you choose Document Sizes from the pop‑up menu.
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.
The Channels panel lists all channels in the image—composite channel first (for RGB, CMYK, and Lab images). A thumbnail of the channel’s contents appears to the left of the channel name; the thumbnail is automatically updated as you edit the channel.
A. Color channels B. Spot channels C. Alpha channels
Viewing thumbnails is a convenient way of tracking channel contents; however, turning off the display of thumbnails can improve performance.
You can use the Channels panel to view any combination of channels in the document window. For example, you can view an alpha channel and the composite channel together to see how changes made in the alpha channel relate to the entire image.
To show or hide multiple channels, drag through the eye column in the Channels panel.
Individual channels are displayed in grayscale. In RGB, CMYK, or Lab images, you can view the individual channels in color. (In Lab images, only the a and b channels appear in color.) If more than one channel is active, the channels always appear in color.
You can change the default to show the individual color channels in color. When a channel is visible in the image, an eye icon appears to its left in the panel.
In Windows, choose Edit > Preferences > Interface.
In Mac OS, choose Photoshop > Preferences > Interface.
Select Show Channels In Color, and click OK.
You can select one or more channels in the Channels panel. The names of all selected, or active, channels are highlighted.
A. Not visible or editable B. Visible but not selected for editing C. Selected for viewing and editing D. Selected for editing but not viewing
You can move alpha or spot channels above the default color channels only if the image is in Multichannel mode (Image > Mode > Multichannel). For information about that mode’s limitations, see Multichannel mode.
Spot colors are overprinted in the order of their appearance from top to bottom in the Channels panel.
For more information, see Create a new spot channel.
You may want to delete spot or alpha channels you no longer need before saving an image. Complex alpha channels can substantially increase the disk space required for an image.
Alt-click (Windows) or Option-click (Mac OS) the Delete icon .
Drag the channel name in the panel to the Delete icon.
Choose Delete Channel from the Channels panel menu.
Click the Delete icon at the bottom of the panel, and then click Yes.
When you delete a color channel from a file with layers, visible layers are flattened and hidden layers are discarded. This is done because removing a color channel converts the image to Multichannel mode, which does not support layers. An image isn’t flattened when you delete an alpha channel, a spot channel, or a quick mask.