A photodiode is a kind of light detector, which involves the conversion of light into voltage or current, based on the mode of operation of the device. It consists of built-in lenses and optical filters, and has small or large surface areas. With an increase in their surface areas, photodiodes have a slower response time. Conventional solar cells, used for generating electric solar power, are a typical photodiode with a large surface area. A photodiode is a semi-conductor device, with a p-n junction and an intrinsic layer between p and n layers. It produces photocurrent by generating electron-hole pairs, due to the absorption of light in the intrinsic or depletion region. The photocurrent thus generated is proportional to the absorbed light intensity. The mechanism of the photodiode is like that of a (miniaturised) solar cell. Their response time is fast, on the order of nanoseconds. They are not as sensitive as a phototransistor, but their linearity can make them useful in simple light meters. Photodiodes - Communications, Bio-Sensings, Measurements and High-Energy Physics describes different kinds of photodiodes for applications in high-speed data communication, biomedical sensing, high-speed measurement, UV-light detection, and high energy physics. Photodiodes are used in consumer electronics devices such as compact disc players, smoke detectors, and the receivers for infrared remote control devices used to control equipment from televisions to air conditioners. For many applications either photodiodes or photoconductors may be used. Either type of photosensor may be used for light measurement, as in camera light meters, or to respond to light levels, as in switching on street lighting after dark. Photosensors of all types may be used to respond to incident light, or to a source of light which is part of the same circuit or system.