Other important characteristics are those related to thermal radiation: transmissivity, absorptivity and reflectivity for long wave radiation. The characteristics are listed in Table 2.3. They are monochromatic and change according to radiation wavelength. Here, in the practical sense, total values, integrated over all wavelengths, are shown. This approach is common in engineering. The two important rules are 1) that the sum of the three thermal radiation properties is unity; and 2) that absorptivity is equal to emissivity. Therefore, transmissivity in this case can be indicated by one minus the other two: transmissivity = 1.0 -reflectivity - absorptivity.