Photosensors or photodetectors are sensors of light or other electromagnetic energy. A photo detector has a p–n junction that converts light photons into current. The junction is covered by an illumination window, usually having an anti-reflective coating. The absorbed photons make electron-hole pairs in the depletion region. Photodiodes and photo transistors are a few examples of photo detectors. Solar cells convert some of the light energy absorbed into electrical energy.