Photometry in the wavelength range 1.22–21 μ‎m, that is, in the near- and mid-infrared parts of the spectrum. Wavelengths longer than 2.5 µm are called the thermal infrared, because thermal emission (i.e. heat) from the telescope and the atmosphere starts to cause considerable background noise. Detectors that observe in the thermal infrared are cooled with liquid nitrogen or helium to reduce noise. It is usual to chop back and forth between the object and a neighbouring patch of sky to subtract the strong background signal from the telescope, its surroundings, and the sky itself (see Chopper). Further, this part of the spectrum contains many strong bands of water vapour and other molecules in the Earth’s atmosphere, so filters must be used to select the windows between them. Observations free from this interference have been made from satellites such as the Infrared Astronomical Satellite (IRAS), the Infrared Space Observatory (ISO), the Spitzer Space Telescope, and the Herschel Space Observatory.