A Dobson Unit (DU) is a measurement used to quantify the concentration of ozone in the Earth's atmosphere. One Dobson Unit corresponds to a layer of ozone that would be 0.01 millimeters thick at standard temperature and pressure. This unit helps scientists monitor and assess the health of the ozone layer, which protects life on Earth from harmful ultraviolet (UV) radiation. The Dobson Spectrophotometer, an instrument developed by G.M.B. Dobson in the 1920s, is commonly used to measure ozone levels in the atmosphere. By analyzing sunlight that passes through the atmosphere, this device can determine the amount of ozone present. Monitoring ozone levels is crucial for understanding environmental changes and the effects of pollutants.