Photometry at wavelengths shorter than 300 nm, to which the Earth’s atmosphere is totally opaque. The observations therefore have to be made from space. Many ultraviolet spectrophotometry observations have been made with the International Ultraviolet Explorer. The Hubble Space Telescope has ultraviolet filters well suited to photometry, and can reach much fainter objects than earlier satellites. Hot stars emit most of their radiation at wavelengths shorter than 300 nm, as do accretion disks in cataclysmic variables. See also U magnitude.