The Image:Filters:Illumination Gamma and Image:Filters:Illumination Curves menu selections provide additional methods of modifying the illumination field.
The Illumination Curves selection brings up the dialog box (A). The user can modify the spectral character of the image by modifying the curves that map the current image values to new image values. By selecting the channel, different curves can be applied to the R, G, and B channels independently. Manipulating the illumination curves provides fine control over the modifying function. To add control points, click on the curve. Control points can be added, moved, or removed, as necessary.
The Illumination Gamma selection brings up dialog box (B), which permits a simple gamma compression of the illumination field. Gamma correction of the illumination field is a simple and effective method of compressing the overall dynamic range of an image without modifying the contrast that exists in the reflectance field. Adjusting the illumination gamma while viewing the product image provides immediate feedback regarding the effect of the adjustment.