These images act as a first-order discriminant of surface composition based on overall albedo or brightness. The boundaries between the mare and highlands units are easily delimited. In contrast, multispectral images record brightness in three dimensions, the X and Y directions are the same as before, and the third dimension is band depth.
Reflectance is controlled by composition of the rock type and surface maturity.
Therefore, by measuring surface brightness over a variety of discrete wavelengths, it is possible to identify the presence, absence, and access qualitative composition of surface units. For example, iron preferentially absorbs light around 1000 nm, whereas titanium preferentially absorbs light near 415 nm. Furthermore, the strength of the absorption is proportional to the amount of that particular element. The image ratios reduce brightness variations due to albedo and topography, and the resulting image contains brightness variations due primarily to changes in lunar soil composition and maturity. The red channel represents areas that are low in titanium, or high in glass content, the green channel is sensitive to the amount of iron in the surface, and the blue channel reflects the surfaces with high titanium or bright slopes and albedos that are not compensated by using the image ratios.