Incident photons strike a photocathode material that is present as a thin deposit on the entry window of the device.
They detect light and multiply the current produced by the incident light by as much as 100 million times (160 dB), enabling individual photons to be detected when the incident flux of light is very low. Photomultipliers are vacuum tubes that are extremely sensitive to light. At the end of the line, the charges from all the different picture elements (pixels) can be converted to electrical signals ( 230–232). The packets of charge are not initially converted to an electrical signal, but rather moved from cell to cell by the coupling and decoupling of potential wells within the semiconductor that makes up the CCD. A CCD generally has an array of cells to capture a light image via the photoelectric effect. 13.03.12.2 Charge-Coupled DeviceĪ charge-coupled device (CCD) is a metal oxide semiconductor chip sensor that transports electrically charged signals. Cadmium sulfoselenide (CdS) is a photoconductive material commonly used in photoresistors ( 226–229). A high intensity of light incident on the surface will cause a lower resistance, whereas a lower intensity of light will cause higher resistance. The resistance generated varies depending on the light striking at his surface. 13.03.12.1 Photocell or PhotoresistorĪ photocell or photoresistor is a sensor that changes its resistance when light shines on it. There are many ways to detect light, and based on the working principle, light sensors can be of different types.
They are very simple and inexpensive, allowing their inclusion in a multitude of consumer products, including night lights, cell phones, burglar alarms, garage door openers, bar code readers, etc. Devices that include these sensors have many uses in scientific applications, but they are also found in items that people encounter each day. Spain, in Comprehensive Materials Processing, 2014 13.03.12 Light Sensing MaterialsĪ light sensor, as its name suggests, is a device that is used to detect light. The main classes of photocells are photoresistors, photovoltaic materials, and photoemitters.Ī. A few devices, notably some silicon photodiodes, have their peak sensitivity for the same colour as the peak sensitivity of the human eye. For many types of sensors, the peak sensitivity may be at either the red or the violet end of the visible spectrum, and some sensors will have their peak response for invisible radiation either in the infrared or the ultraviolet. For the range of wavelengths used in photocells, however, you will often see the approximate figure of 1 mW/cm 2 = 200 lux used.Īnother important point relating to the use of photocells is that they are not uniformly sensitive at all visible colours. The use of milliwatts per square centimetre looks more comprehensible to anyone brought up with electronics, but there is no simple direct conversion between power per square centimetre and lux unless other quantities such as spectral composition (colour balance) of light are maintained constant. A value of 1000 lux is the level of illumination required for close inspection work and the reading of fine print on this scale, direct sunlight registers at about 100 000 lux.
The lux figures for illumination are those obtained by using photometers, and a figure of 50 lux corresponds to a ‘normal’ domestic lighting level good enough for reading a newspaper. The sensitivity of photocells can be quoted in either of two ways, either as the electrical output at a given illumination, using illumination figures in units of lux, often 50 lux and 1000 lux, or as a figure of power falling on the cell per square centimetre of sensitive area, a quantity known as irradiance. In addition, the unit energy depends on the frequency of the wave.
Light radiation carries energy, and the amount of energy carried depends on the square of the amplitude of the wave. Light is an electromagnetic radiation of the same kind as radio waves, but with a very much shorter wavelength and hence a much higher frequency. Ian Sinclair, in Passive Components for Circuit Design, 2001 PhotocellsĪ photocell is a light-to-electrical transducer, and there are many different types available.