<P> Here, α (\ displaystyle \ alpha \,) represents the spectral absorption component, ρ (\ displaystyle \ rho \,) spectral reflection component and τ (\ displaystyle \ tau \,) the spectral transmission component . These elements are a function of the wavelength (λ (\ displaystyle \ lambda \,)) of the electromagnetic radiation . The spectral absorption is equal to the emissivity ε (\ displaystyle \ epsilon \,); this relation is known as Kirchhoff's law of thermal radiation . An object is called a black body if, for all frequencies, the following formula applies: </P> <Dl> <Dd> α = ε = 1 . (\ displaystyle \ alpha = \ epsilon = 1. \,) </Dd> </Dl> <Dd> α = ε = 1 . (\ displaystyle \ alpha = \ epsilon = 1. \,) </Dd> <P> In a practical situation and room - temperature setting, humans lose considerable energy due to thermal radiation . The energy lost by emitting infrared radiation is partially regained by absorbing the heat flow due to conduction from surrounding objects, and the remainder resulting from generated heat through metabolism . Human skin has an emissivity of very close to 1.0 . Using the formulas below shows a human, having roughly 2 square meter in surface area, and a temperature of about 307 K, continuously radiates approximately 1000 watts . If people are indoors, surrounded by surfaces at 296 K, they receive back about 900 watts from the wall, ceiling, and other surroundings, so the net loss is only about 100 watts . These heat transfer estimates are highly dependent on extrinsic variables, such as wearing clothes, i.e. decreasing total thermal circuit conductivity, therefore reducing total output heat flux . Only truly gray systems (relative equivalent emissivity / absorptivity and no directional transmissivity dependence in all control volume bodies considered) can achieve reasonable steady - state heat flux estimates through the Stefan - Boltzmann law . Encountering this "ideally calculable" situation is almost impossible (although common engineering procedures surrender the dependency of these unknown variables and "assume" this to be the case). Optimistically, these "gray" approximations will get close to real solutions, as most divergence from Stefan - Boltzmann solutions is very small (especially in most STP lab controlled environments). </P>

As the temperature of an object increases the rate of radiation emitted