Infrared spectroscopy examines absorption and transmission of photons in the infrared range. Satellites that can measure water vapor have sensors that are sensitive to infrared radiation. It will integrate radar observations with observations in the visible infrared and ultraviolet. Optics usually describes the behaviour of visible, ultraviolet, and infrared light. Clouds are excellent emitters of infrared radiation. Most of the emission at infrared wavelengths originates from interstellar dust. The VLT operates at visible and infrared wavelengths. This happens for infrared, microwave and radio wave radiation. Above infrared in frequency comes visible light. FTIR is a method of measuring infrared absorption and emission spectra. At infrared and optical frequencies, a common technique is ellipsometry. It is also used in infrared detectors. It is an ideal location for submillimeter, infrared and optical observations. Commercially, mercury cadmium telluride is of continuing interest for detection of infrared radiation. No energy is emitted from the sensor, thus the name "passive infrared".