Standard test flame calibration. Standard flame used at the UK's National Physical Laboratory (NPL). This flame provides a reproducible region of high-temperature gas of known temperature and composition, used in tests and experiments. It is used for the calibration of optical thermometry techniques commonly found in combustion research. The flame is initially calibrated using laser Rayleigh scattering, as seen here (green laser light). Photographed in 2015 at the National Physical Laboratory, Teddington, UK.