24. The method of claim 23 wherein the separating step includes isolating the energy at wavelengths higher than the highest said frequency bandwidth and separating that energy into a plurality of high frequency bandwidths, and the energy spectrum deriving step includes incorporating the energy from the high frequency bandwidths into the energy spectrum without passing through the summing amplifiers.
23. The method of claim 20 wherein the energy spectrum deriving step includes passing the output of adjacent sets of difference amplifiers through a plurality of summing amplifiers, and summing the outputs of the summing amplifiers.