Photoshop Tutorials - Equalize
This command lets you equalize the brightness of the image automatically. This command redistributes the brightness values of the image areas so that they would evenly present the overall brightness range. The darkest color becomes black, the brightest gets turned to white, while the other brightness levels are evenly spread over the range so that the image tones would look more balanced. This command works similar to Auto Levels, but it is not the same though.
If you apply the Equalize command to the selection the dialog menu opens with the radio button that lets you select one of the options: Equalize Selected Area Only (the brightness is equalized within the selection) and Equalize Entire Image Based On Selected Area (the image brightness is equalized based on the brightness range of the selection). The last option is useful when you need to set the brightness of the mage based on the brightness of the certain image area.