Light Sensors:Principle,Types, 5 Applications

What is a Light Sensor?

A light sensor is a photoelectric instrument that converts light energy or photons (that can range from the infra-red to ultraviolet spectrum) into an electrical (electrons) signal. The light sensor generates an electrical output signal having an energy corresponding to the input light energy. Light Sensors are also termed as photo sensors or photoelectric sensors. 


What is the principle of light sensors?

The working principle of the light sensor is based on internal photoelectric effect, which states that when light energy or photons are bombarded on a metal surface than it can cause the free electrons from the metal to excite and jump out resulting in electron flow or electric current. The amount of current produced depends on the energy of the photon (i.e. wavelength of light). The emission of electrons from the metal surface occurs only after the light reaches a certain threshold frequency that corresponds with the minimum energy required by the electrons to break the metal bonds.

light sensor
Representation of photon bombardment resulting in emission of electron off a metal surface. Image source: PonorPhotoelectric effect in a solid – diagramCC BY-SA 4.0

How does a light sensor work?

The light sensor consists of a very high precision photoelectric tube. Inside the photoelectric tube, a small flat metal plate is present that composes of “two needle-type tubes”. The impact of light on the ends of the photoelectric tube when a reverse fixed pressure is applied results in the release of electrons or electric current. The generation of electric current or the variation in the electric current demonstrates the presence of light energy i.e. it senses light.

What are the types of light sensors?

Light sensors can be of several types. These sensors can either generate energy in presence of light or various other electrical properties. The most common types of light sensors are photovoltaic cells, photodiodes, photo-resistors, and photo-transistors.

Photovoltaic cells:

Photovoltaic cells, like the name suggests, follows the principle of the photovoltaic effect for converting light energy directly into electrical energy. These cells produce an electromotive force proportional to the radiant energy received. The most popular single-junction silicon cells generate a maximum open-circuit voltage of about 0.5 to 0.6 Volts. Solar cells use selenium as a photovoltaic material.

Apart from generating and storing electrical energy, photovoltaic cells also act as photodetectors. Photo-conductive cells and photo-emissive cells also provide a similar function.

A crystalline silicon solar photovoltaic cell. Image source: Unknown author, Solar cell, marked as public domain, more details on Wikimedia Commons


Photodiode light sensors are diodes that change light energy into a flow of electrons. Photodiodes are comparable to the common P-N junction diodes, but instead of an opaque casing, these diodes have a transparent lens for focusing light on the P-N junction. These diodes are more sensitive towards the light with longer wavelengths i.e. light belonging to the red and infra-red spectrum than the visible or ultraviolet spectrum. Silicon and germanium are the most commonly used materials in such diodes. Since photodiodes are sensitive to infrared light, they offer a lot of applications in medicine.

Photodiodes Top (Germanium), bottom three (silicon). Image source: http://Ulfbastel (, „Fotodio“,


Photo-resistors are also known as light-dependent resistors or LDR. Photo-resistors are devices that vary its resistance based on the amount of light energy it receives. The lower the intensity of light, the higher is the resistance. This is because more light (intensity) ensures more flow of electrons and hence, less is the resistance. Cadmium sulfide cells (a high resistance semiconductor material) that is sensitive towards infrared light, are mostly used in photo-resistors. Sometimes, materials like indium antimonide (InSb), lead selenide (PbSe), and lead sulfide (PbS) are also used. Photo-resistors take a considerably longer time (of about a few seconds) to respond to the exposed light.


Photo-transistors can be referred to as photodiodes with amplification. Photo-transistors have a reverse biased collector-base P-N junction that is exposed to the radiant light energy source. Photo-transistors are much more (about 50 to 100 times) sensitive compared to photo-diodes because of amplification. Photo-transistors have bipolar NPN transistors with the base region electrically unconnected. Here, the radiant light energy is focused on the base junction by a transparent lens. Photo-transistors are widely used in mobile phones and automobiles.

A NPN phototransistor symbol. image source: myself, PhototransistorSymbolCC BY 3.0

What are the applications of a light sensor?

Over the years, light sensors have been used for a variety of applications like:

Solar cells:

Photovoltaic cells are commonly used as solar cells for generating electricity. Over the years, as the usage of renewable sources of energy has become highly popular, solar cells offer an extremely important role. With the invention of solar cells, it has been possible to provide electrical energy to remote places.

Consumer Electronics:

Light sensors are used for performing a wide range of functions in smartphones and tablets. The motion sensors and auto-brightness sensors present in smartphones use light detectors like photo-transistors. Remote control devices that operate in infrared light also use photodiodes for conducting its functions.


Light sensors or light detectors are used in automobiles for detecting the surrounding ambient light. These detectors automatically switch on the automobile lights when it gets dark. Nowadays, light detectors are also used for ensuring safe driving and parking in several car models.

Security devices:

Light sensors are commonly used for processing shipment cargos in order to ensure whether the boxes are properly sealed or not. Several types of movement sensors also use light detectors that sense the variation in the exposure of light. Photodiodes are also used in smoke detectors present in offices, airports, trains, etc.

Agricultural devices:

With the development in technology, light sensors have contributed to the field of agriculture also. These sensors detect the amount of surrounding ambient light to activate the sprinkler irrigation system. Light sensors activate the sprinklers only when the intensity of sunlight is less for ensuring adequate hydration of crops.

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