There have been a lot of questions recently about how the color information in infrared images translates into NDVI values. So I finally started to pay attention to image histograms. These are part of the secret language of photographers. They are really just simple graphs, but photographers and photography programs always seem to display them with no labels on either axis, so they tend to be impenetrable to the uninitiated. Staring at image histograms has been helpful to me, and maybe it will help you too.