Light Sensors

Light is a parameter that is just like salt in food. It is a mandatory component, yet people only notice it in its absence. Light makes it possible for humans and other living organisms to see what is what. This light also helps us in the sensing and monitoring process of various other parameters. The below article explains what is light how was it used in various sensors.


  1. What is Light?
  2. How is Light Measurement used in Sensing?
  3. Types of Light Sensors
  4. Most Commonly used Light Sensors
  5. Photo Resistors
  6. Photo Diodes
  7. Photo Transistors
  8. Light Emission Diodes

What is Light?

Light is a beam of electromagnetic radiation whose wavelength ranges between 400 nm to 750 nm. This wavelength allows the beam to be visible to the human retina. Most of the light particles that we see on the earth are caused by electromagnetic radiation from the sun. It is also possible to generate light using instruments like bulbs and lasers. Other main characteristics of light are listed below.

  • As stated above, light waves have a wavelength higher than 400 nm and 750 nm and they are visible to the human eye.
  • A light beam always travels in a straight line.
  • If interrupted by a reflective material, the light gets reflected and if interrupted by a non-reflective material, it gets absorbed.
  • The speed of light is higher than the speed of sound which is why we see an object before hearing from it.
  • Visible light consists of various colors. These colors include the colors of the rainbow, i.e., violet, indigo, blue, green, yellow, orange, and red.
  • A light wave also has various properties such as refraction, dispersion, interference, polarization etc.

How is Light Measurement used in Sensing?

Light energy has the property to be converted into various other energies easily. Hence light energy can be used to measure various parameters. Sensors that use light energy or some modified form of light is known as light sensors. Some of the main causes of using light for sensing are listed below.

  • Light sensors have the capacity to sense and monitor both natural light such as sunlight and artificial light like lights emitted from bulbs, lasers etc.
  • Light energy is made by a particle named photons and photons are versatile atoms that can be easily manipulated.
  • Generally, light sensors are analog devices that provide the output as a discrete value instead of a digital output.
  • A light sensor is a simple device that has no complex construction or advanced operating techniques so that it can be used in household measurement tasks.
  • A light sensor can be easily connected to a microprocessor or a microcontroller like Arduino, Internet of Things etc.


Types of Light Sensors

A light sensor can be classified into many categories based on different parameters just like any other sensor. Yet, one of the main classifications of the light sensors is based on the fact that whether the sensor emits or is absorbed. This pictorial representation of this classification and other insignificant classifications is shown below.

Classification of the Light Sensors
  • As shown in the above figure, light sensors are mainly classified as photo emissive cells and photo conductive cells.
  • Photo emissive cells emit energy in the form of light when energized. This is due to the emission of free electrons from the photo emissive material.
  • Photo conductive cells are just like a conductive component in a circuit such as a resistor and a capacitor.
  • They allow the light or just a certain part of the light to pass through them like a resistor and even sometimes blocks the light like a capacitor.
  • Photo junction diodes are again a light controlling devices that are manufactured from semiconducting materials.
  • They are used in light detecting and penetrating application as a controlling technique.

Most Commonly used Light Sensors

Though light sensor is widely classified into various types, in the end it is about the final device that people use in the household application and industrial application. The above-mentioned photo emissive cells and the photo conductive cells are one of the most widely used in many light sensors. Light sensors that are used in day-to-day applications are briefly explained below.

  • Photo resistors – Photo resistor uses a photo conductive cell to convert the light energy to resistance to make it easily measurable for the further analysis.
  • Photo diodes – A photo diode is a device that converts the light energy into an electrical current with the usage of semiconductors just like any other junction diode.
  • Photo transistor – A photo transistor is a device that absorbs light and produce electricity according the intensity of the light the transistor absorbed.
  • Light emitting diodes – The LEDs are one of the main applications of light sensors that are used in many applications. This is an example for a photo emissive material.

Photo Resistors

A photo resistor is the device that converts the light energy to a resistance value. As resistance is some of the parameters that can be easily measured, the photo resistors are used on various application. A Light Dependent Resistor or the LDR is the great example for a photo resistor. The construction and the working principle of a light dependent resistor is as follows:

  • An LDR consists of two electrodes and a photo sensitive material which is used to absorb the light.
  • This photo sensitive material is protected using a thin clear film which is attached above the material.
  • The entire construction is done on a ceramic block to prevent the current leakage as ceramic is an insulator.
  • The photosensitive material is connected to the external wire terminals that are used to connect the device to a circuit or a controller.
  • The LDR works on the principle that the free electrons in the valence band of the photosensitive material get excited when photons from the light energy collide with them.
  • This energy change results in the change of the resistance of the device which can be measured after.

Photo Diodes

A photodiode is similar to a semiconductor diode such as a PN junction diode and a Zener diode. But a photodiode is used to generate electricity based on the intensity of the light energy. A photodiode is a versatile device that can also detect electromagnetic rays with different wavelengths such as ultraviolet rays and X-rays along with visible light. The construction and working of a photodiode are explained below.

  • A photodiode consists of a glass lens that is used to absorb the incident light from the atmosphere.
  • The glass lens is placed on a semiconducting material with both p-type and n-type materials which are used to generate current by the movement of electrons between the two different materials.
  • A metal cathode is placed under the whole structure and two lead wires act as a terminal for external connection.
  • When light is detected in the glass lens, the free holes in the p-type semiconductor which is directly under the lens gets excited and starts to move towards the n-type semiconductor.
  • This also simulates the free electrons in the n-type semiconductor to move upwards towards the p-type semiconductor.
  • This movement of electrons and holes results in the generation of electricity across the photodiode.


Photo Transistors

A phototransistor is again a semiconductor device that is used to detect light and convert it into electrical value. The difference between the photodiode and a phototransistor is that the diode gives the output as a digital current instead of an analog value. Another major difference is that the gain value of phototransistors is relatively higher than that of the normal photodiodes. The main components and working of a phototransistor are as follows.

  • A phototransistor has a similar construction to that of a semiconductor transistor like a bipolar junction transistor.
  • A phototransistor consists of three blocks of semiconducting materials out of which two of them are of the same material, say n-type semiconductor and the other one is of another type, say p-type semiconductor.
  • When light is detected by the transistor, the free electrons and the holes in both the semiconductors starts to move.
  • This movement of electrons and holes results in the generation of electricity.

Light Emission Diodes

The LEDs or the light emitting diodes are one of the most commonly used photo sensors that emit light when energized. The LED’s are used for decoration purposes like different lightings, signboards etc. They are also used in the field of electronics for various applications like an output devices, LED screens etc. The construction and working of a LED are as follows:

  • An LED consists of a semiconducting material that emits light energy when an electric current is passed through it.
  • The anvil and a post have connected the semiconductor and they are used to conduct electricity to the semiconductor.
  • An anode and a cathode are also connected to the semiconductor to connect the semiconductor to an external circuit.
  • The whole construction is protected by an epoxy case which is a see-through material that allows the user to see the emission of light with naked eyes.

Key Points to Remember

Here are some key points you need to remember about the “Light Sensors”

  • Light is electromagnetic radiation of a certain wavelength range that makes it visible to the human eyes.
  • Light waves possess certain properties that enable them to be used in various sensors for measurement purposes.
  • Light sensors are widely classified into photo emitters, photoconductors and photodiodes.
  • Some of the most commonly used light sensors are photoresistors, phototransistors and light emission diodes.
  • The photoresistor is used to convert light energy into resistance for easier measurement purposes.
  • A phototransistor is used to convert the light energy into an electrical current by using a semiconducting material.
  • A Light emitting diode is used to emit light energy when energized and is used in various applications.

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Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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