There are different ways to classify sensors used in remote sensing. One of them is the division into imaging and non-imaging sensors. Imaging sensors typically employ optical imaging systems (from VIS to TIR). They operate primarily at window frequencies, where atmospheric absorption is low and surface features can be imaged or measured. Non-imaging sensors include microwave radiometers, microwave altimeters, magnetic sensors, gravimeters, Fourier spectrometers, laser rangefinders, and laser altimeters.