Alpha Channels

Stores selections so they can be used more than once
Stores pixel opacity information

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Alpha Channels for Opacity Information

We can change the opacity of all the pixels in an image layer by changing the Opacity value on the Layers palette.  This opacity information is stored in an alpha channel.  However, opacity alpha channels are not what we see in the Photoshop Channels palette.  The alpha channels we see in the Channels Palette are used to store masks and selections.  Therefore, for purposes of the scope of this web site, we will focus on using alpha channels for the storing of selections.
 

Alpha Channels for Selections

Color Channels

Figure 1 is an example of the Photoshop Channels palette.  For an RGB image, we will see four layers.  The topmost layer is the composite color channel.  The next three layers are the individual color channels.  The topmost layer is Photoshop's composite rendering of the three individual color channels.  The composite channel is not physically stored in the image file.  The individual color channels are.

Alpha Channels

We can also see two other channels in Figure 1.  The fifth layer is a typical alpha channel.  We can have more than one alpha channel.  The bottommost layer is a special Photoshop alpha channel called Quick Mask.  We can have only one Quick Mask layer.

Alpha channels are used to store selections.  A selection is stored in an alpha channel as a grayscale mask.  By storing the selection, we can recall the selection as many times as we need.  Because it is stored with the image, the selection will not disappear when the image file is closed.
 

Quick Mask

Quick Mask is a convenient feature for creating a selection in a special alpha channel.  However, even though an image can have many alpha channels, it can only have one Quick Mask layer.