Abstract: A Ross filter spectrometer, consisting of 7 energy channels and covering 18-88 keV energy range, was designed by calculation with the optimization goal of band-pass flat response, in which the residual response, relative error outside the energy channels and flatness inside energy channels were used to optimize the thickness of the filters. The energy channel width of the spectrometer was between 2 keV and 20 keV. The relative error and flatness were lower than 10% and 20% respectively for most energy channels. This spectrometer was applied to experiments performed on a microfocus X-ray tube in which high-energy X-ray was used to backlit spherical shell samples made of high-Z metals. The results show that, under different experimental conditions, spectra measured by Ross filter spectrometer were in agreement with theory models. The measured spectra interpreted the images quite well and could be used to obtain areal density from the images.