When you select an uncorrected or georeferenced image, an option is provided to select one or three image planes. Three image planes are usually displayed as an RGB image with the first image plane used as the red component, the second as the green component, and the third as the blue component. If only one image channel is selected for tiedown, the RGB option menu is disabled (shaded out).
The RGB option menu allows the user to study one image plane at a time by selecting ``B&W 1'', ``B&W 2'', or ``B&W 3''. The ``RGB'' option can be selected to return to an RGB display of the image.
The fifth option (``PCT'') can be used to display the first image plane using it's default PCT table. This is the default mode when loading an image with a default PCT.
The RGB Display Control option menu is provided because certain types of image features are often clearer when a single image plane is viewed. It is also possible on 8-bit graphic systems to display much finer graduations of grey than is possible for colour images.