@item screen-gamma
@cindex gamma correction
-If this is a number, Emacs performs ``gamma correction'' on colors. The
-value should be the screen gamma of your display, a floating point
-number. Usual PC monitors have a screen gamma of 2.2, so the default is
-to display for that gamma value. Specifying a smaller value results in
-darker colors, which is desirable for a monitor that tends to display
-colors too light. A screen gamma value of 1.5 may give good results for
-LCD color displays.
+If this is a number, Emacs performs ``gamma correction'' which adjusts
+the brightness of all colors. The value should be the screen gamma of
+your display, a floating point number.
+
+Usual PC monitors have a screen gamma of 2.2, so color values in
+Emacs, and in X windows generally, are calibrated to display properly
+on a monitor with that gamma value. If you specify 2.2 for
+@code{screen-gamma}, that means no correction is needed. Other values
+request correction, designed to make the corrected colors appear on
+your screen they way they would have appeared without correction on an
+ordinary monitor with a gamma value of 2.2.
+
+If your monitor displays colors too light, you should specify a
+@code{screen-gamma} value smaller than 2.2. This requests correction
+that makes colors darker. A screen gamma value of 1.5 may give good
+results for LCD color displays.
@item tool-bar-lines
The number of lines to use for the toolbar. A value of @code{nil} means