Theoretical Basis for Radiation Measurements

Blackbody Radiation

The thermal energy radiated by an object is expressed in relation to the energy radiated at the same temperature by a perfect radiator, traditionally called a black body. A blackbody absorbs all the radiation it receives and radiates more thermal radiation for all wavelength intervals than any other mass of the same area and temperature.

Though the blackbody is an ideal, and no perfect blackbody exists, specially constructed laboratory sources emit radiation with an efficiency compared to a blackbody of 98% or higher. Laboratory sources with 99.98% efficiency compared to a blackbody have been constructed. The most common approach to realizing a blackbody is to use a spherical cavity with a small hole in the surface or a closed-end tube that is longer than its diameter. The opaque walls of the sphere or tube are held at uniform temperature.

As shown in Figure 1, these constructions provide for multiple reflections of any radiation entering the opening. Thus, though the sphere or tube walls are slightly reflective, after many reflections all the energy is absorbed, i.e., at room temperature the aperture in the sphere or tube appears to be black in the visible part of the spectrum and is also nearly totally absorbing in other regions of the spectrum. At any given temperature the aperture radiates energy at nearly the same rate as a blackbody of the same size and temperature.

As shown in Figure 1, these constructions provide for multiple reflections of any radiation entering the opening. Thus, though the sphere or tube walls are slightly reflective, after many reflections all the energy is absorbed, i.e., at room temperature the aperture in the sphere or tube appears to be black in the visible part of the spectrum and is also nearly totally absorbing in other regions of the spectrum. At any given temperature the aperture radiates energy at nearly the same rate as a blackbody of the same size and temperature.