 This form of the Stefan-Bolzmann law is the one that we will use the most. Note that for a perfect emitter, epsilon equals to 1, the total irradiance submitted into a hemisphere equals the product of the Stefan-Bolzmann constant, sigma, and the temperature to the fourth power. However, the irradiance is modified by the emissivity, which equals the absorptivity. Note that this emissivity here is some sort of average over all emissivities for different wavelengths, and we have seen that emissivity can vary a lot with wavelengths. Water vapor, for example, has a very low emissivity in the visible, but a very strong one in the infrared. The absorptivity depends on the composition of matter, but it also depends on the number concentration of gaseous matter, and the pass length through that matter. Go back and look at Beer's law of absorption to see this dependence. With this form of the Stefan-Bolzmann law, we can compare the irradiances of two different bodies of matter at different temperatures or different emissivities.