A method to merge images from different sensing modalities for visual display was introduced by Toet, van Ruyvein, and Valeton in 1989 that produces a fused image by nonlinear recombination of the ratio of low-pass (RoLP) pyramidal decompositions of the original images. The appearance of merged images that are produced by this scheme is highly dependent on the contrast and mean gray level of the input images. That nonlinear multiplication of the successive layers of a ratio of low-pass pyramid results in a contrast- enhanced image representation that is highly invariant for changes in the global gray-level characteristics of the original image is shown. Application of this nonlinear multiplication procedure in image fusion process results in composite images that appear highly independent of changes in lighting and gray-level gradients in the input images. The method is tested by merging different degraded versions of parallel registered thermal (FLIR) and visual (CCD) images.