I. AbstractFor the determination of the solar absorptance αs and the thermal emittance ε preferably optical methods are utilized. To these methods belong spectral reflectance αs and ε measurements and / or emission measurements (only ε), whereby for αs must be measured in the common range 0,3 μm to 2,5 μm and ε in the range 5 μm to 35 μm with an essentially better resolution than 10 nm. Besides so-called integrated reflectance measurements are known, for which special detectors are used, in the above indicated spectral ranges. These optical measuring procedures are relatively exact. They deliver also information about a perhaps spectral selective behavior of surfaces, however, special measuring equipment is required.The known thermal (or calorimetrical) methods  require, that the test surfaces are in the thermal balance with their environment and at least for the αs determination a suited solar simulator is required with high constance of the irradiation intensity during the entire measuring phase. Since these measuring phases up to the reaching of the thermal equilibrium are very long, this is a fundamental disadvantage of the thermal method. The work presented in this paper demonstrates possibilities, to neglect for the determination of ε completely and to shorten the irradiation times for the αs determination drastically whereby the requirement of the long time constance of the solar irradiation intensity does no longer play a dominant role.