Directed band ratioing for the retention of perceptually-independent topographic expression in chromaticity-enhanced imagery

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Abstract
Enhancements of multispectral imagery, intended for visual interpretation of geological information, are typically designed to increase the discrimina-bility of band-variant surface reflectance features. Band ratioing has been commonly used for this purpose with various degrees of success. A disadvantage of band ratioing has been the loss of topographic expression and surface albedo information or, in some cases, the perceptual convolution of both topographic expression and albedo information with band-variant reflectance information. Topographic expression is a valuable indicator of structural and lithomorphic information and is typically the primary provider of reference landmarks. Albedo information is commonly critical in discriminating between some rock types. We have developed a method of band ratioing that directs the retention of topographic expression and albedo information so that they remain depicted as prominent variations in image intensity. The method involves adjusting the band data so that ratio values for each surface material coherently increase with increasing pixel bispectral radiance for all three ratios of the colour composite. Because hue is almost perceptually independent of intensity, the retained topographic expression and albedo information do not significantly distort the ratio-enhanced band-variant reflectance information, which remains depicted as colour. Our resultant images are similar to chromaticity-enhanced band composite images but have the advantage of requiring only simple arithmetic processing steps instead of forward and reverse principal components analysis or intensity-hue-saturation transformations.