Luminance is the intensity of light emitted from a area. The darkest luminance levels are known as the shadows, the brightest luminance levels are known as the highlights and then the luminance levels in between are known as midtones.
There are three automatic, and destructive, luminance adjustments, Auto tone, Auto Color and Auto Contrast.
The Auto Tone command makes the darkest pixels black and the brightest pixels white on a channel by channel basis, affecting each channel independently. This means the color cast of the image is changed. Thus if an image contains a color cast it can be removed.
The Auto Contrast command makes darkest pixels black and the brightest pixels white on a composite basis. So in other words, all three channels are affected in exactly the same way. That means darker shadows and brighter highlights, but the natural color cast of the image is not affected.
The Auto Color command makes darkest pixels black and the brightest pixels white, once again on a channel by channel basis, just as with the Auto Tone command, but it’s also neutralizing the midtones. This is a command that adjusts the midtones in order to neutralize them.
A fourth method to adjust luminance, also destructive, is the Brightness/Contrast Adjustment. But the nice thing about this adjustment is that it’s incapable of clipping luminance levels; it can’t take big shadow regions and make them black or big highlight regions and make them white.
This static adjustment has both sliders and and an auto button in its dialog box. The Auto button evaluates not only the shadows and the highlights, as with the Auto Tone and Auto Contrast commands, but it’s also evaluates the midtones, the way the Auto Color command does. The command does it on a composite basis, as a result there is no chance of a color cast being introduced.
There is method to use the Brightness/Contrast as a dynamic, non-destructive adjustment layer. At the bottom of the layers panel click the Create New adjustment Layer panel. Select Brightness/Contrast, this will create a Brightness/Contrast Adjustment Layer. The same functionally exists as the static Brightness/Contrast adjustment described above but it makes the changes on an adjustment layer and thus is non-destructive.