Light sensors are among the simplest and cheapest sensors, allowing their inclusion into numerous consumer products ranging from night lights to cameras. The simplest optical sensor is a photoresistor, and can be identified by the meandering channel between two terminals. The small bandgap of GaAs allows the low-energy photons in infrared light to free electrons into the conduction band. Some applications of light sensors use reflected light with optical detection for position sensing, including bar code readers, laser printers, and autofocusing microscopes. Ambient light sensors are typically placed next to the speakers in laptop design, where the case has an opening for light. Another common application is an ambient light sensor used in a cell phone, where every mA-hour saved translates into longer battery life and satisfied customers. There is a wide variety of optical sensors available in small packages in the market.
