Radiation Heat Transfer

Heat transfer due to emission of electromagnetic waves is known as thermal radiation

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 reference to the 'black body'.

The Black Body

Radiation heat transfer - black body

The 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 hypothetical 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 black bodies heated to a given temperature emit thermal radiation.

The radiation energy per unit time from a black body 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 in Kelvin (K)

A = area of the emitting body (m2)

The Stefan-Boltzmann Constant in Imperial Units

σ = 5.6703 10-8 (W/m2K4)

    = 1.714 10-9 ( Btu/(h ft2 oR4) )

    =  1.19 10-11 ( Btu/(h in2 oR4) )

Heat transmission as radiation from a black body to surroundings with temperature absolute zero

Example - Heat 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 area can be expressed by modifying (1) to

 q / A = σ T4

    = (5.6703 10-8 W/m2K4) (5800 K)4

    = 6.42 107 (W/m2

    Gray Bodies and Emissivity Coefficients

    Heat transmission by radiation - gray body

    For objects other than ideal black bodies ('gray bodies') the Stefan-Boltzmann Law can be expressed as

    q = ε σ T4 A                                    (2)


    ε = emissivity coefficient of the object (one - 1 - for a black body)

    For the gray body the incident radiation (also called irradiation) is partly reflected, absorbed or transmitted.

    Heat radiation - incident reflected transmitted absorbed irradiation

    The emissivity coefficient is in the range 0 < ε < 1, depending on the type of material and the temperature of the surface.

    Net Radiation Loss Rate

    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 transfer - radiation from heated surfaces to unheated surroundings

    Lambert's cosine law

    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 

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