Principal components analysis of multitemporal image pairs

Principal components analysis of eight-channel data sets consisting of multitemporal LANDSAT MSS image pairs often generates higher-order principal components that are related to changes in 'brightness' and 'greenness'. This is the expected result of such analysis in a wide variety of biological and geological environments where the original imagery is intrinsically two dimensional; the two dimensions are 'brightness' and 'greenness'; and the change in land cover between images exceeds some threshold value.