A Light Sensor generates an output signal indicating the intensity of light by measuring the radiant energy that exists in a very narrow range of frequencies basically called "light", and which ranges in frequency from "Infrared" to "Visible" up to "Ultraviolet" light spectrum. The light sensor is a passive devices that convert this "light energy" whether visible or in the infrared parts of the spectrum into an electrical signal output. Light sensors are more commonly known as "Photoelectric Devices" or "Photo Sensors" becuse the convert light energy (photons) into electricity (electrons).
Photoelectric devices can be grouped into two main categories, those which generate electricity when illuminated, such as Photo-voltaics or Photo-emissives etc, and those which change their electrical properties in some way such as Photo-resistors or Photo-conductors. This leads to the following classification of devices.
• Photo-emissive Cells - These are photodevices which release free electrons from a light sensitive material such as caesium when struck by a photon of sufficient energy. The amount of energy the photons have depends on the frequency of the light and the higher the frequency, the more energy the photons have converting light energy into electrical energy.
• Photo-conductive Cells - These photodevices vary their electrical resistance when subjected to light. Photoconductivity results from light hitting a semiconductor material which controls the current flow through it. Thus, more light increase the current for a given applied voltage. The most common photoconductive material is Cadmium Sulphide used in LDR photocells.
• Photo-voltaic Cells - These photodevices generate an emf in proportion to the radiant light energy received and is similar in effect to photoconductivity. Light energy falls on to two semiconductor materials sandwiched together creating a voltage of approximately 0.5V. The most common photovoltaic material is Selenium used in solar cells.
• Photo-junction Devices - These photodevices are mainly true semiconductor devices such as the photodiode or phototransistor which use light to control the flow of electrons and holes across their PN-junction. Photojunction devices are specifically designed for detector application and light penetration with their spectral response tuned to the wavelength of incident light.
A light sensor, as its name suggests, is a device that is used to detect light. There are many different types of light sensors, each of which works in a slightly different way. A photocell or photoresistor, for example, is a small sensor that changes its resistance when light shines on it; they are used in many consumer products to determine the intensity of light. A charged coupled device(CCD) transports electrically charged signals, and is used as a light sensor in digital cameras and night-vision devices. Photomultipliers detect light and multiply it.Devices that include these sensors have many uses in scientific applications, but are also found in items that people encounter each day. A simple light sensor may be part of a security or safety device, such as a burglar alarm or garage door opener. These types of devices often work by shining a beam of light from one sensor to another; if the light is interrupted, an alarm sounds or the garage door won't close.Many modern electronics, such as computers, wireless phones, and televisions, use ambient light sensors to automatically control the brightness of a screen, especially in low-light or high-light situations. They can detect how much light is in a room and raise or lower the brightness to a more comfortable level for the user. Light sensors also may be used to automatically turn on lights inside or outside a home or business at dark.Barcodescanners found in most retailer locations work using light sensor technology. The light emitted from the scanner illuminates the barcode, which is read and decoded by a sensor. Quick Response (QR) codes operate in much the same way, though they contain more information and typically can be read using a smartphone if the user has downloaded a code reader.While some products that use light sensors have been around for a number of years, these sensors continue to become more important, especially through infrared technology. Warm-blooded animals, including humans, emit heat, which can be seen as infrared light. This energy can be detected using infrared light sensors to tell when a person is walking by, as opposed to being activated by another, non-human movement. Motion-activated light sensors that recognize infrared can be found in grocery stores, for example. When a shopper walks by, the sensor in a display case recognizes that a person is passing, and the lights are turned on. The lights dim when no customer is in front of the case, and the store saves on energy costs. Many retailers and businesses use similar technology to control lights in rooms that are not in constant use, such as conference rooms or restrooms. Light sensors continue to have many uses in science, from the simplest science fair experiments to the latest breakthroughs in space, medicine, and robotics. Robots, for example, can use light sensors to "see" and navigate around a room, detecting objects by sensing how light bounces off them. Improvements in fiber-optics technology will likely bring even more breakthroughs, as this technology can measure light and send signals in extreme conditions or remote locations where electricity isn't available.