Heat transfer through radiation takes place in form of electromagnetic waves mainly in the infrared region. Radiation emitted by a body is a consequence of thermal agitation of its composing molecules. Radiation heat transfer can be described by a reference to the so-called 'black body'.
A black body is defined as a body that absorbs all radiation that falls on its surface. Actual black bodies don't exist in nature - though its characteristics are approximated by a hole in a box filled with highly absorptive material. The emission spectrum of such a black body was first fully described by Max Planck.
A black body is a hypothetic body that completely absorbs all wavelengths of thermal radiation incident on it. Such bodies do not reflect light, and therefore appear black if their temperatures are low enough so as not to be self-luminous. All blackbodies heated to a given temperature emit thermal radiation.
The radiation energy per unit time from a blackbody is proportional to the fourth power of the absolute temperature and can be expressed with Stefan-Boltzmann Law as
q = σ T4 A (1)
q = heat transfer per unit time (W)
σ = 5.6703 10-8 (W/m2K4) - The Stefan-Boltzmann Constant
T = absolute temperature Kelvin (K)
A = area of the emitting body (m2)
σ = 5.6703 10-8 (W/m2K4)
= 0.1714 10-8 ( Btu/(h ft2 oR4) )
= 0.119 10-10 ( Btu/(h in2 oR4) )
Example - Radiation from the surface of the Sun
If the surface temperature of the sun is 5800 K and if we assume that the sun can be regarded as a black body the radiation energy per unit time can be expressed by modifying (1) like
q / A = σ T4
= 5.6703 10-8 (W/m2K4) (5800 (K))4
= 6.42 107 (W/m2)
For objects other than ideal blackbodies ('gray bodies') the Stefan-Boltzmann Law can be expressed as
q = ε σ T4 A (2)
ε = emissivity of the object (one for a black body)
The emissivity coefficient lies in the range 0 < tyle: italic;">ε < 1 depending on the type of material and the temperature of the surface. The emissivity of some common materials
If an hot object is radiating energy to its cooler surroundings the net radiation heat loss rate can be expressed as
q = ε σ (Th4 - Tc4) Ac (3)
Th = hot body absolute temperature (K)
Tc = cold surroundings absolute temperature (K)
Ac = area of the object (m2)
Heat loss from a heated surface to unheated surroundings with mean radiant temperatures are indicated in the chart below.
Heat emission from a surface in an angle β can be expressed with Lambert's cosine law as
qβ = q cos β (4)
qβ = heat emission in angle β
q = heat emission from the surface
β = angle