Emissivity, symbol ε, is the ratio of energy radiated by a particular material to energy radiated by a black body at the same temperature, which corresponds to a value of 1. Surfaces with a modest emissivity emit little, those with high emissivity radiate much energy. Emission levels of common construction materials vary from 0.80 to 0.95. An exception are coated glasses and polished metal surfaces, whose values may be considerably lower. If the emission level of a building material is unknown, 0.90 can be assumed.