Click 
. The
Gamma Correct window is displayed.
Enter the amount of gamma in the Value field, or use the scroll bar to adjust the gamma.
The results of your gamma value selection are displayed in the preview pane of the Gamma Correct window.
Enhancing Images > Using the Tool Box Toolbar > Enhancing Image Quality
The Gamma tool adjusts the overall intensity of an image.
To use this tool:
Click . The
Gamma Correct window is displayed.
Enter the amount of gamma in the Value field, or use the scroll bar to adjust the gamma.
The results of your gamma value selection are displayed in the preview pane of the Gamma Correct window.
Click OK. The image gamma is adjusted.
If the image needs further correction, use the brightness and contrast tools to make small adjustments.
To revert to the original image, click again.