Light sensors or photosensors, designed to measure light intensity, are one of the most commonly used sensors in electronic applications. Light intensity is one of the seven basic physical quantities. Measuring light intensity is useful in many consumer, industrial, and security applications.
What is a light sensor?
A light sensor is a photoelectric device that converts light energy into electrical energy. These sensors are designed to be sensitive to visible, infrared, or ultraviolet light, meaning they are sensitive to a narrow range of the electromagnetic spectrum.
Light sensors are constructed from selective materials that generate electricity when exposed to a specific part of the spectrum. The amount of electricity is proportional to the intensity of the incident light.
Light Intensity Units
Light intensity is one of the seven basic physical quantities. Its “SI” unit is candela. A candela is the luminous intensity in a given direction from a source, which emits monochromatic green light of 540×1012 hertz and has a radiant intensity of 1/683 Watt per steradian in the same direction.
Candela is often used to indicate the intensity of artificial lights. Other units include lumen and lux. Lumen is the unit of luminous flux and measures the total amount of light emitted by a source. It is defined as the amount of light emitted per second at a solid angle of one steradian from a uniform source of one candela. Lumen is often used to indicate the brightness of light sources.
While lumen is a unit used to express the total amount of light from a source, lux is the total amount of light from a source incident on a given surface area. One lux is equal to one lumen of incident light per square meter.
Types of light sensors
Light sensors are mainly passive devices. They are categorized into two classes:
1. Generates electricity when exposed to light (i.e. photoemissive and photovoltaic devices)
2. Conducts electricity when exposed to light (i.e. photoconductive/photoresistive and photojunction devices)
One of the best examples of a photovoltaic device is a solar cell. A phototube is a photoemissive device. A light-dependent resistor is a photoconductive/photoresistive device. Photodiode and phototransistor are popular photojunction devices. However, it is important to note the differences between these mechanisms.
Photoemissive devices are constructed from photosensitive materials such as cesium, which generates free electrons when exposed to photons. These devices generate current when exposed to light. The higher the frequency of the incident light, the greater the energy of the incident photons and the greater the amount of electrical current generated.
In photovoltaic devices, the difference between two semiconductor materials is generated in response to incident light energy. Due to these potential differences, current flows between the two semiconductor layers.
Photoconductive devices are constructed from semiconductor materials that undergo changes in conductivity based on exposure to light. Due to the energy absorbed from the incident light, more free electrons are generated and the conductivity of these materials increases. The most common photoconductive material used in LDR cells is cadmium sulfide.
Photojunction devices are constructed from typical semiconductor materials such as silicon or germanium. They operate like any normal diode or transistor, except their PN junction is exposed to light and conducts when subjected to light. The response of a photodiode or phototransistor is tuned to a specific range of the electromagnetic spectrum.
LDRs
A light dependent resistor (LDR) or photoresistor is made of a photosensitive semiconductor whose conductivity changes when exposed to light.
The resistance of the material is several thousand ohms or mega ohms in the dark and drops to a few hundred ohms when subjected to light. Semiconductor material is often placed in a zigzag pattern on a ceramic substrate to increase dark resistance.
The semiconductor materials typically used for constructing photoresistors are lead sulfide (PbS), indium antimonide (InSb), lead selenide (PbSe), and cadmium sulfide (CdS).
Cadmium sulfide is the most common material used in the construction of LDRs. It is a low-cost semiconductor with a response curve that approaches that of the human eye. The peak sensitivity wavelength of cadmium sulfide is 560 nm to 600 nm.
Generally, LDR is used for detecting light or dark. It can be connected into a voltage divider network with a transistor circuit or a microcontroller/microprocessor. It can also be connected to a Wheatstone bridge with an operational amplifier circuit.
Photodiodes
A photodiode is a photojunction device. It is a normal diode with its PN junction exposed to light through a transparent case or transparent lens. These diodes have the same voltage-current characteristics as any other junction diodes. But they have higher conductivity than conventional diodes because their junction is open to exposure to light.
The photodiodes are connected in a reverse bias configuration, which conducts a reverse leakage current in the dark. When the photodiode is subjected to light, the reverse leakage current increases several times.
The reverse leakage current of a silicon diode in the dark is 1 uA. That of a germanium diode is 10 uA. On exposure to light, the reverse leakage current can reach up to 300 uA. The greater the intensity of the incident light, the greater the reverse leakage current.
LDRs or photoresistors have a long response time. It may take several seconds to change conductivity after exposure to light. Photodiodes, on the other hand, have an instantaneous response.
Although an LDR is tuned to the visible spectrum of light, photodiodes are sensitive to visible and infrared light. The biggest disadvantage of photodiodes is that their reverse leakage current is still in the microamp range – even when subjected to light. Therefore, they require an operational amplifier circuit for light detection.
Photodiodes have a response time in nanoseconds. They are used in sophisticated applications including cameras, imaging and scanning devices, CD and DVD players, fiber optic communication, motion detection and positioning sensors.
Phototransistors
Phototransistors are similar to photodiodes except they provide current amplification. They are usually designed using normal NPN transistors with their collector base PN junction exposed to light through a transparent case or transparent lens. Due to current amplification, its output current is 50 to 100 times greater than that of photodiodes. The base region is electrically isolated or has sensitivity control.
As the phototransistor already provides current amplification, unlike a photodiode, it does not require an external amplifier for its operation. A phototransistor is simply a typical transistor with a base collector exposed to light.
NPN phototransistors are connected in a circuit with their base collector in reverse bias configuration. In the dark, there is a small leakage current from the emitter. When exposed to light, the base current increases and is amplified by the transistor. The sensitivity of a phototransistor depends on the DC gain of the transistor. The output current can be controlled by the resistance between the base and emitter of the phototransistor.
For higher sensitivity applications such as optocouplers, Darlington phototransistors are used. In Photodarlington transistors, two NPN type phototransistors are connected as a Darlington pair. The output current amplification is the product of the current amplification of the two phototransistors. Photodarlington transistors have a longer response time compared to phototransistors but offer greater sensitivity.
Phototransistors are typically used as optical switches, optical isolators, or infrared filters, and in IR remote controls and fiber optic communication.
Solar cells
Solar cells or photovoltaic cells are not sensors. They are mainly used to generate solar power and are made of monocrystalline silicon PN junctions, similar photodiodes but with a broader response curve.
Unlike photodiodes connected in a reverse bias configuration, solar cells are connected in a forward bias configuration, much like typical diodes. These cells are designed to be sensitive to sunlight rather than a narrow band of the electromagnetic spectrum. When exposed to solar radiation, a cell generates a potential difference of 0.58V.
Typically, multiple solar cells are connected in series on a panel to produce a higher voltage. This DC voltage can drive a resistive load or be converted to AC for transmission.
Light sensor applications
LDRs, photodiodes and phototransistors are commonly used as light sensors in various applications. Examples include: brightness adjustment on mobile devices, automatic lights, automatic irrigation, optical isolation, fiber optic communication, motion detection, IR remote controls, position detection, optical data, and optical imaging.
Light sensors are also used for security and home automation applications. For example, they are often used in cargo shipments to detect what times the container was opened to track lost goods. Some light sensors are also used for motion detection in many smart home security applications.