Radiation heat transfer, heat radiation or simply called as thermal radiation occurs without the necessity of any medium even in vacuum space. Or, when a medium is available, the transfer process does not depend on the medium itself. Limitless transfer of heat radiation will occur if there is no obstruction. A classic example is heat transmission from the sun to the Earth. How exactly can the heat transferred without any medium? The answer is that the heat is transferred as an electromagnetic wave which is similar to light.
Factor Affecting Heat Radiation
After understanding the definition and how the transmission of heat radiation described above, we can discuss further regarding several factors that affect the amount of heat transferred:

Temperature difference
For radiation, temperature shall be power by four and subtract. Thus, radiation heat transfer is more significant when heat source has a very high temperature. As the opposite, radiation heat transfer can be omitted when the temperature difference is small.

Emissivity
Emissivity is the effectiveness of a surface to emit thermal radiation. The value is a ratio of thermal radiation from a surface to thermal radiation from an ideal black body. The maximum value for emissivity is 1 and the minimum value is 0. A gray body is defined as any material that has emissivity value in between.

View factor
View factor is a factor that characterized arrangement of each related surface. It is the proportion of the radiation which leaves one surface that strikes surface B. Difference in the view factor can occurs when the surface is parallel or not. The surface shape also affects this factor.
Heat Radiation Equation
From the above factors, thermal radiation rate can be expressed by following equation:
With:
q_{r }= Heat transfer rate (Watt, Btu/h)
s = Boltzman constant (5,669 x 10^{8} W/m^{2}.K^{4}, 0.1714 x 10^{8 }Btu/h.ft^{2}.^{0}R^{4} )
T = Radiation temperature (^{o}C, K)
hr = Radiation heat transfer coefficient (W/m^{2} K, Btu/h ft^{2 0}F)
F12 = View factor between surfaces