σ is called the Stefan-Boltzmann constant
This is the Stefan-Boltzmann law relating the total output to temperature.
If Me(T) is in W m-2, and T in kelvins, then σ is 5.67 x 10-8 Wm-2 K-4.
At room temperature a 1 mm2 blackbody emits about 0.5 mW into a hemisphere. At 3200 K, the temperature of the hottest tungsten filaments, the 1 mm2, emits 6 W.
This law relates the wavelength of peak exitance, λm, and blackbody temperature, T:
λmT = 2898 where T is in kelvins and λm is in micrometers.
The peak of the spectral distribution curve is at 9.8 µm for a blackbody at room temperature. As the source temperature gets higher, the wavelength of peak exitance moves towards shorter wavelengths. The temperature of the sun’s surface is around 5800K. The peak of a 6000 blackbody curve is at 0.48 µm, as shown in Fig. 3.
The radiation from real sources is always less than that from a blackbody. Emissivity (ε) is a measure of how a real source compares with a blackbody. It is defined as the ratio of the radiant power emitted per area to the radiant power emitted by a blackbody per area. (A more rigorous definition defines directional spectral emissivity ε(θ,φ,λ,T). Emissivity can be wavelength and temperature dependent (See Figure 2).