photocell

(¦fōd·ō·kən′dək·tiv ′sel)

(electronics) A device for detecting or measuring electromagnetic radiation by variation of the conductivity of a substance (called a photoconductor) upon absorption of the radiation by this substance. Also known as photoresistive cell; photoresistor.

---

A device for detecting electromagnetic radiation (photons) by variation of the electrical conductivity of a substance (a photoconductor) upon absorption of the radiation by this substance. During operation the cell is connected in series with an electrical source and current-sensitive meter, or in series with an electrical source and resistor. Current in the cell, as indicated by the meter, is a measure of the photon intensity, as is the voltage drop across the series resistor. Photoconductive cells are made from a variety of semiconducting materials in the single-crystal or polycrystalline form. See also Photoconductivity; Photoelectric devices.

---

A light sensor (photodetector) that varies its resistance between its two terminals based on the amount of photons (light) it receives. Used for photographic light meters, automatic on-at-dusk street lights and other light-sensitive applications, it is also called a "light dependent resistor" (LDR) and "photoresistor."

The photocell's semiconductor material is typically cadmium sulfide (CdS), but other elements are also used. Photocells and photodiodes are used for similar applications; however, the photocell passes current bi-directionally, whereas the photodiode is unidirectional. See photodiode.

Light Dependent Resistors

Photocells come in a variety of packages such as this assortment from PerkinElmer. As the photocell receives more photons, the resistance is lowered between the two terminals. (Image courtesy of PerkinElmer, Inc., www.perkinelmer.com)

Download Computer Desktop Encyclopedia to your iPhone/iTouch