Image enhancement by spectral rationing

In a reflectivity image, contrast produced by differences in surface material is often of more interest than that produced by differences in surface orientation or illumination. The latter factors may be difficult to predict or to allow for in practice. We show that, in certain cases, contrast caused by differences in surface orientation and illumination may be reduced or eliminated by dividing an image obtained in one spectral band by one obtained in a different spectral band. We illustrate this technique by enhancing the images of military vehicles recorded on color film, and by enhancing man-made objects in an aerial multispectral scanner image relative to vegetation.