I have a question regarding the options for spectral resampling in ENVI and I would like to better understand when ENVI does in the background. For an image with defined sensor type and defined wavelengths per band, I can perform a spectral resampling to a 'pre defined filter function'. These filter functions can be found in the envi install directory and seem to be identical to the sensor specific spectral response functions. What happens exactly computationally when I perform spectral resampling of e.g. a Landsat 7 ETM+ image using the Spot4 spectral response function? The result is an image with only the corresponding number of channels. Obviously the images only provide a single value for each band and nothing is known how the reflectance is distributed over the wavelength that is covered by a given band... I suspect that the overlap between both spectral response functions is used as a filter - but I would appreciate if someone could provide some additional details on what happens here. How could I recreate this process computationally in IDL (..without the Envi doit options)? Thanks, Patrick