In national and international standards documents, the symbol M is recommended to denote radiant exitance ; a superscript circle (°) indicates a term relate to a black body. (A subscript "e" is added when it is important to distinguish the energetic (radiometric) quantity radiant exitance, M e , from the analogous human vision (photometric) quantity, luminous exitance, denoted M v .) In common usage, the symbol used for radiant exitance (often called radiant emittance) varies among different texts and in different fields.