What are Light sensors？

Light sensors are meteorological instruments that detect light and converts light energy into electrical output. Once converted into electricity, the radiant energy within the infrared to ultraviolet spectrum sources can be measured.

One recent but now almost overriding application of light sensors is in LCD Backlight Control. Here, ambient-light sensor ICs are designed to detect brightness over a wide range, from darkness to direct sunlight, and output the data in order to adjust LCD brightness for optimum visibility and lower power consumption.

Another variety of light sensor, photoelectric sensors, consists of devices used to detect the distance, absence, or presence of an object by using a light transmitter (usually infrared) and a photoelectric receiver. Photoelectric sensors are employed in commercial and residential lighting, as well as in security applications.

These and other light sensors fall into two main categories. The first group generates electricity when illuminated and is known as either photovoltaic, converting solar radiation into direct-current electricity, or photo-emissive, which releases free electrons from a light sensitive material such as cesium when struck by a photon of sufficient energy. The second grouping includes those that change their electrical properties in some way, such as a photoresistor (whose resistance decreases with increasing incident light intensity) or a photo-conductor (which varies electrical resistance when subjected to light).

Photoconductive sensor

In a photoconductive light sensor, material becomes more electrically conductive due to the absorption of electromagnetic radiation such as visible light, ultraviolet light, infrared light, or gamma radiation.

Photoconductive cells consist of a thin single-crystal or polycrystalline filmof compound semiconductor substances. Most commercially available photoconductive cells are manufactured from cadmium sulfide (CdS), which is sensitive to light in the visible spectrum. Other examples of photoconductive materials include the conductive polymer polyvinylcarbazole, used extensively in photocopying (xerography), lead sulfide (PbS), used in infrared detection applications such as heat-seeking missiles, lead selenide (PbSe) and lead telluride (PbTe).

The photoresistor, a semiconductor device that uses light energy to control the flow of electrons as well the current flowing through them, is the most common photoconductive device. CdS photoconductive cells are often referred to as light-dependent resistors (LDR). They function within the same general spectral range as the human eye and are therefore widely used in applications where this type of spectral response is required. The resistance of the cell when not illuminated is very high (about 10 MΩ), but drops to about 100 Ω when fully illuminated.