Photodiodes are semiconductor diodes which take in light and use the photoelectric effect to convert it into electric current. Three different operating modes should be differentiated here: In forward direction, the direction of flow of the current, the diode is used as a solar cell for power generation, whilst in short circuit mode it is used for brightness measurement. If on the other hand it is used in the reverse direction, a very small current, called the blocking current, flows in linear proportion to the light. This operating mode is used above all for integral CMOS sensors. Photoresistors reduce their electrical resistance the more light falls on them. Therefore amongst other applications they are used as light meters in cameras or by means of their to their inertia to reduce mains hum. Phototransistors on the other hand act similarly to photodiodes, but are significantly more sensitive. They are used in some light barriers or dimmer switches.