In the "experimental" methods, one actually cools a surface on which gas condenses and then measures the temperature at which the condensation takes place. The experimental methods can be divided into manual and automated systems. Manual systems, such as the Bureau of Mines dewpoint tester, depend on an operator to manually cool the chilled mirror slowly and to visually detect the onset of condensation. The automated methods use automatic mirror chilling controls and sensors to detect the amount of light reflected by the mirror and detect when condensation occurs through changes in the reflected light. The chilled mirror technique is a first principle measurement. Depending on the specific method used to establish the dew point temperature, some correction calculations may be necessary. As condensation must necessarily have already occurred for it to be detected, the reported temperature is lower than when using theoretical methods.