Optical Sensor Basics and Applications

An optical sensor converts light rays into an electronic signal. The purpose of an optical sensor is to measure a physical quantity of light and, depending on the type of sensor, then translates it into a form that is readable by an integrated measuring device. Optical Sensors are used for contact-less detection, counting or positioning of parts. Optical sensors can be either internal or external. External sensors gather and transmit a required quantity of light, while internal sensors are most often used to measure the bends and other small changes in direction.

The measurands possible by different optical sensors are Temperature, Velocity Liquid level, Pressure, Displacement (position), Vibrations, Chemical species, Force radiation, pH- value, Strain, Acoustic field and Electric field

Types of Optical Sensors

There are different kinds of optical sensors, the most common types which we have been using in our real world applications as given below.

  • Photoconductive devices used to measure the resistance by converting a change of incident light into a change of resistance.
  • The photovoltaic cell (solar cell) converts an amount of incident light into an output voltage.
  • The Photodiodes convert an amount of incident light into an output current.

Phototransistors are a type of bipolar transistor where the base-collector junction is exposed to light. This results in the same behavior of a photodiode, but with an internal gain.

The operating principle is the transmitting and receiving of light in an optical sensor, the object to be detected reflects or interrupts a light beam sent out by an emitting diode. Depending on the type of device, the interruption or reflection of the light beam is evaluated. This makes it possible to detect objects independently of the material they are constructed from (wood, metal, plastic or other). Special devices even allow for a detection of transparent objects or those with different colors or variations in contrast. Different types of optical sensors as explained below.

Different Types of Optical Sensors
Different Types of Optical Sensors

Through-Beam Sensors

The system consists of two separate components the transmitter and the receiver are placed opposite to each other. The transmitter projects a light beam onto the receiver. An interruption of the light beam is interpreted as a switch signal by the receiver. It is irrelevant where the interruption occurs.

Advantage: Large operating distances can be achieved and the recognition is independent of the object’s surface structure, color or reflectivity.

To guarantee a high operational dependability it must be assured that the object is sufficiently large to interrupt the light beam completely.

Retro-Reflective Sensors

Transmitter and receiver are both in the same house, through a reflector the emitted light beam is directed back to the receiver. An interruption of the light beam initiates a switching operation. Where the interruption occurs is of no importance.

Advantage: Retro-reflective sensors enable large operating distances with switching points, which are exactly reproducible requiring little mounting effort. All objects interrupting the light beam are accurately detected independently of their surface structure or color.

Diffuse Reflection Sensors

Both transmitter and receiver are in one housing. The transmitted light is reflected by the object to be detected.

Advantage: The diffused light intensity at the receiver serves as the switching condition. Regardless of the sensitivity setting the rear part always reflects better than the front part. This leads to the consequence to erroneous switching operations.

Different Light Sources For Optical Sensors

There are many types of light sources. The sun and light from burning torch flames were the first light sources used to study optics. As a matter of fact, light coming from certain (exited) matter (e.g., iodine, chlorine, and mercury ions) still provides the reference points in the optical spectrum. One of the key components in optical communication is the monochromatic light source. In optical communications, light sources must be monochromatic, compact, and long lasting. Here are two different types of light source.

1. LED (Light Emitting Diode)

During the recombination process of electrons with holes at the junctions of n-doped and p-doped semiconductors, energy is released in the form of light. The excitation takes place by applying an external voltage and the recombination may be taking place, or it may be stimulated as another photon. This facilitates coupling the LED light with an optical device.

A LED is a p-n semiconductor device that emits light when a voltage is applied across its two terminals
A LED is a p-n semiconductor device that emits light when a voltage is applied across its two terminals

2. LASER (Light Amplification by Stimulated Emission Radiation)

A laser is created, when the electrons in the atoms in special glasses, crystals, or gasses absorb energy from an electrical current they become excited. The excited electrons move from a lower-energy orbit to a higher-energy orbit around the atom’s nucleus. When they return to their normal or ground state this leads to the electrons emit photons (particles of light). These photons are all at the same wavelength and coherent. The ordinary visible light comprises multiple wavelengths and is not coherent.

LASAR Light Emission Process
LASAR Light Emission Process

Applications of Optical Sensors

Application of these optical sensors ranges from computers to motion detectors. For optical sensors to work effectively, they must be the correct type for the application, so that they maintain their sensitivity to the property they measure. Optical sensors are integral parts of many common devices, including computers, copy machines (xerox) and light fixtures that turn on automatically in the dark. And some of the common applications include alarm systems, synchros for photographic flashes and systems that can detect the presence of objects.

Ambient Light Sensors

mostly we have seen this sensor on our mobile handsets. It will extend the battery life and enables easy-to-view displays that are optimized for the environment.

Ambient Light Sensors
Ambient Light Sensors

Biomedical Applications

optical sensors have robust applications in the biomedical field. Some of the examples Breath analysis using tunable diode laser, Optical heart-rate monitors an optical heart-rate monitor measures your heart rate using light. A LED shines through the skin, and an optical sensor examines the light that reflected back. Since blood absorbs more light, fluctuations in light level can be translated into heart rate. This process is called as photoplethysmography.

Optical Sensor Based Liquid Level Indicator

Optical Sensor Based Liquid Level Indicator consist of two main parts an infrared LED coupled with a light transistor, and a transparent prism tip in the front. The LED projects an infrared light outward, when the sensor tip is surrounded by air the light reacts by bouncing back with-in the tip before returning to the transistor. When the sensor is dipped in liquid, the light disperses throughout and less is returned to the transistor. The amount of reflected light to the transistor affects output levels, making point level sensing possible

Optical Level Sensor
Optical Level Sensor

Have you got the basic information of an optical sensor? We acknowledge that the above-given information clarifies the basics of optical sensor concept with related images and various real-time applications. Furthermore, any doubts regarding this concept or to implement any sensor-based projects, please give your suggestions and comments on this article you can write in the comment section below. Here is a question for you, what are the different light sources of an optical sensor?

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