Image > Adjustments > Equalize
The Equalize command redistributes the brightness values of the pixels in an image so that they more evenly represent the entire range of brightness levels. Equalize remaps pixel values in the composite image so that the brightest value represents white, the darkest value represents black, and intermediate values are evenly distributed throughout the grayscale.
If you selected an area of the image, select what to equalize in the dialog box, and click OK:
Equalize Selected Area Only Evenly distributes only the selection’s pixels.
Equalize Entire Image Based On Selected Area Evenly distributes all image pixels based on those in the selection.
Experiment with the following before applying the Equalize command to get best results: