Gamma
The Gamma filter lets you reveal detail in a low-contrast image without significantly affecting the shadows or highlights. With this filter, the tonal correction of the image is based on the perception of tones relative to the surrounding area. For example, if you place a circle filled with 10 percent gray on a black background, and an identical gray circle on a white background, the circle surrounded by black appears lighter than the circle surrounded by white, even though the brightness values are identical. The Gamma filter affects all image values, but it is curve-based; consequently, changes are weighted toward the midtones.
You can access this adjustment from the Adjustments inspector or the Adjust menu.