In physics, **radiant energy** is the energy of **electromagnetic** and **gravitational radiation**. The term “**radiant energy**” is most commonly used in radiometry, solar energy, heating, and lighting. As energy, its SI unit is the **joule (J)**. The quantity of radiant energy may be calculated by integrating radiant flux for time. **Radiant heat transfer** is very important in the power industry because it is one of the most important ways to transfer thermal energy. It does not need a medium, such as air or metal, to take place. Any material that has a temperature above absolute zero gives off some radiant energy. Most energy of this type is in the infra-red region of the electromagnetic spectrum, although some of it is in the visible region.

**The radiant heat transfer** rate from a body (e.g.,, a black body) to its surroundings is proportional to the **fourth power** of the absolute temperature. It can be expressed by the following equation:

*q = εσT ^{4}*

where** σ** is a fundamental physical constant called the **Stefan–Boltzmann constant**, equal to** 5.6697×10 ^{-8} W/m^{2}K^{4}**. This relationship is called the

**Stefan–Boltzmann law**. The

**emissivity, ε**, of the surface of a material is its effectiveness in emitting energy as thermal radiation and varies between 0.0 and 1.0. By definition, a black body in thermal equilibrium has an emissivity of

*ε*= 1.0. It can be seen and radiation heat transfer is important at very high temperatures and in a vacuum.

Two bodies that radiate toward each other have a net heat flux between them. The net flow rate of heat between them is given by:

**Q = εσA _{1-2}(T^{4}_{1} −T^{4}_{2}) [J/s]**

**q = εσ(T ^{4}_{1} −T^{4}_{2}) [J/m^{2}s]**

The area factor A_{1-2}, is the area viewed by body 2 of body 1, and can become fairly difficult to calculate.