Select the light in either the Scene browser or the Viewer window, and select either the RGB or Alpha options in the Light preview area to preview either the RGB or Alpha channels of a loaded image.

Select RGB to display a full-color version of the image in the Image Preview area, and select Alpha to display the Alpha image.

Light Texture preview A. RGB view B. Alpha view

The Alpha image displays the areas that are not projected in black, like a gobo, which is a filter placed over a light that projects light patterns.