The total ozone content of the atmosphere can be measured from satellites or by ground based instruments. For ground based observation UV-spectrophotometers are most commonly used. Their measurement principle is based on the differential absorption of solar light by ozone: the intensities of the solar light arriving at the Earth's surface at two (or more) wavelengths in the UV (between 280 and 330 nm) are measured and compared. Ozone is mostly absorbed by light at short wavelength. From the knowledge of the absorption spectrum of ozone and the calibration of the instrument, the ozone content of the atmosphere is derived. There are two main types of these instruments: the Dobson and the Brewer spectrophotometers.