Selective surface
（重定向自Selective absorber）
![Solar thermal collector system based on evacuated glass tubes. Sunlight is absorbed by a special material at the center of each tube that has a selective surface. The surface absorbs sunlight nearly completely, and emits very little of the solar heat as thermal radiation. Ordinary black surfaces are also efficient absorbers, but they emit thermal radiation copiously.](/Images/godic/202502/10/Col·lectors_solars_de_tub_de_buit2724.JPG")
In solar thermal collectors, a selective surface or selective absorber is a means of increasing its operation temperature and/or efficiency. The selectivity is defined as the ratio of solar radiation-absorption (alpha) - to thermal infrared radiation-emission (epsilon).