A unit of luminous intensity expressed in lumen per steradian (lm sr−1). The candela was first defined as 1/60 the luminous intensity, in the perpendicular direction, of a 1 cm2 blackbody radiator at the freezing temperature of platinum (about 2042 K) and a pressure of 1 atmosphere. It is now defined as the luminous intensity of a light source producing single-frequency light at a frequency of 540 terahertz with a power of 1/683 watt per steradian. It is the standard unit of measure for luminous intensity adopted by the International System of Units (SI). In some texts, it is referred to as the international standard candle.