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Optical Sensing and Optical Sensor basics

This page covers basics of optical sensing. It mentions optical sensor working operation, optical sensor types, optical sensor applications and optical sensor advantages and disadvantages.

Definition: The method of sensing light rays is known as optical sensing. The sensor type used for optical sensing is known as optical sensor.

Optical Sensor converts light rays into electrical signal. This is similar to the function performed by photoresistor. Let us understand working operation of optical sensor.

Optical Sensor Working Operation

optical sensing using optical sensor
fig:1 Optical sensing

In general, there are two components in optical sensing viz. transmitter (i.e. optical source) and receiver (optical detector). The concept is depicted in the figure with the example of optical fiber. As shown light beam changes its parameters when any object comes in between transmitter and receiver. There are five useful parameters of light which are being measured in optical sensing viz. intensity, phase, wavelength, polarization, spectral distribution.

Due to advent of optical sensing technology, following physical and chemical measurands can be measured. They are • Temperature, • flow, • pressure, • displacement, • liquid level, •  vibration, • rotation, • acceleration, • magnetic fields, •  force, • Ph, • radiation, • chemical species, •  humidity, • strain, • electric fields, • velocity, • acoustic field etc.

Optical Sensor Types

Following are the types of optical sensor based on different characteristics.
• Point sensor, Distributed sensor
• Extrinsic sensor, Intrinsic sensor
• Through Beam Sensor, Diffuse reflective Sensor, Retro-reflective sensor

Point sensor vs Distributed Sensor

Based on working operation they are divided into Point sensor and distributed sensor. In Point sensor type, sensor operates on single point. In point sensor type, transducers are placed at the end of optical fiber. Example of this type is fiber Bragg grating which is spread across optical fiber length. It is used to measure temperature or strain. This single point method of optical sensing uses phase change for activation of sensor. In distributed sensor type, sensor operates over distribution of points. In this method sensor is reactive along long series of sensors or optical array.

Extrinsic sensor vs Intrinsic Sensor

There are two types of fiber optic sensors based on where light beam is changed for sensing. If light beam leaves the optical fiber cable or feed and it is changed before it continues on its path till optical detector, then it is known as extrinsic optical sensor. If light beam does not leave the optical fiber cable and it is changed inside the cable itself, then it is known as intrinsic optical sensor. Intensity based fiber optic pressure sensor used to measure pressure between two plates is referred as intrinsic optical sensor.

Through Beam sensor vs Diffuse Reflective Sensor vs Retro Reflective Sensor

Through beam vs diffuse Reflective vs retro reflective sensor types

Based on method of optical sensing and placement of optical transmitter and receiver there are three types viz. Through Beam, Reflective and Retro-reflective.
In "Through Beam Sensor" , both transmitter and receiver are placed pointing to each other so that they create straight light beam path. When any object comes in between this path, intensity of light changes and accordingly object can be detected.
In "Reflective sensor" , both transmitter and receiver are parallel to each other. The light transmitted by transmitter is reflected by the object and this reflection of light is measured by the receiver. This type of sensor has drawback to differentiate between red and white light when red color LED is used as optical source. This is due to the fact that both red and white color has same amount of reflection.
In "Retro-reflective type", both transmitter and receiver are placed in one housing and reflector made of special reflective material is used. Transmitter transmits light beam which is reflected by reflector and received by the receiver. If any object comes in between this light beam path, it breaks. Based on difference between light beam intensity and other parameters object can be detected or sensed at the receiver.

Optical Sensing Applications

Following are the applications of optical sensors:
• It is used in remote sensing satellite.
• Used in imaging
• Quality and Process Control applications
• Metrology
• Medical instruments

Optical Sensor Advantages and Disadvantages

Following are the advantages of optical sensors:
• The optical signal is immune to electro-magnetic interference and hence it is ideal to be employed in microwave environment.
• It has very high sensitivity , range and resolution compare to non optical sensors.
• It has wider dynamic range.
• It is completely passive and hence used in explosive environment.
• It is small in size and light in weight.
• It is resistant to high temperatures and chemically reactive environment. Hence optical sensors are ideal for hostile and harsh environment.
• It can be used to monitor wide range of physical as well as chemical parameters as listed above.
• It provides complete electrical insulation from high electrostatic potential.
• Multiplexed/distributed sensors provide measurements at large number of distributed points.

Following are the disadvantages of optical sensors:
• The only problem with both optical sensor and non-optical sensor is interference from multiple effects. For example, sensor used for pressure/strain measurement is very sensitive to temperature variation. Research is in progress to study and find out ways to have distinction between these different effects to have accurate measurement in optical sensing.
• The other concern different types of losses involved in optical domain. The common among them are absorption loss, scattering loss, coupler loss, insertion loss, reflection loss, impurity loss etc.

Sensors and Transducers Related Links

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