Use the Process > Enhancements > Maximize Contrast... command to maximize the image contrast on the basis of the intensity histogram.
The dialog box that is opened when you use an image processing operation is made up in the same way for every operation. Click here, to switch to a description of this dialog box.
Exactly what happens?
With this command you carry out a linear spreading of the available value range of the intensity value to the maximum possible value range. Please note, however, that an image lacking in contrast will scarcely be improved if it contains single pixels that are either very bright or dark (originating, e.g. from noise or image faults). In this case, it is better to use the Process > Enhancements > Optimize Contrast... command.
Gray-value images: 8-bit gray-value images have a maximum value range of 0-255. They will therefore be spread in such a way that the darkest pixel will be set at the gray value 0, and the brightest at the gray value 255. 16-bit gray-value images will be respectively spread across the value range of 0-65535.
See also
1701