Optical Sensor Basics and Types Explained

This page covers optical sensor basics and optical sensor types, including their applications and working operation.

The optical sensor types include Point sensor, Distributed sensor, Extrinsic sensor, Intrinsic sensor, Through Beam Sensor, Diffuse reflective Sensor and Retro-reflective sensor. The optical sensor advantages and disadvantages are also covered.

Definition: The method of sensing light rays is known as optical sensing. The sensor type used for optical sensing is known as optical sensor. An Optical Sensor converts light rays into an electrical signal. This is similar to the function performed by a photoresistor.

Let’s understand the working operation of an optical sensor.

Optical Sensor Working Operation

Fig 1: Optical sensing

In general, there are two components in optical sensing:

1. Transmitter (i.e., optical source)
2. Receiver (optical detector)

The concept is depicted in the figure with the example of optical fiber. As shown, the light beam changes its parameters when any object comes between the transmitter and receiver.

There are five useful parameters of light which are being measured in optical sensing:

• Intensity
• Phase
• Wavelength
• Polarization
• Spectral distribution

Due to the advent of optical sensing technology, the following physical and chemical measurands can be measured:

• 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 optical sensor types 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, optical sensor types are divided into Point sensors and Distributed sensors.

In the Point sensor type, the sensor operates on a single point. In this type, transducers are placed at the end of the optical fiber. An example of this type is the fiber Bragg grating, which is spread across the optical fiber length. It is used to measure temperature or strain. This single-point method of optical sensing uses phase change for activation of the sensor.

In the Distributed sensor type, the sensor operates over a distribution of points. In this method, the sensor is reactive along a long series of sensors or an optical array.

Extrinsic sensor vs Intrinsic Sensor

There are two types of optical sensors based on where the light beam is changed for sensing.

If the light beam leaves the optical fiber cable or feed and it is changed before it continues on its path till the optical detector, then it is known as an extrinsic optical sensor.

If the light beam does not leave the optical fiber cable and it is changed inside the cable itself, then it is known as an intrinsic optical sensor.

An intensity-based fiber optic pressure sensor used to measure pressure between two plates is referred to as an intrinsic optical sensor.

Through Beam sensor vs Diffuse Reflective Sensor vs Retro Reflective Sensor

Based on the method of optical sensing and the placement of the optical transmitter and receiver, there are three optical sensor types: Through Beam, Reflective, and Retro-reflective.

• In the “Through Beam Sensor,” both the transmitter and receiver are placed pointing to each other so that they create a straight light beam path. When any object comes in between this path, the intensity of light changes, and accordingly, the object can be detected.

• In the “Reflective sensor,” both the transmitter and receiver are parallel to each other. The light transmitted by the transmitter is reflected by the object, and this reflection of light is measured by the receiver. This type of sensor has a drawback in differentiating between red and white light when a red color LED is used as an optical source. This is due to the fact that both red and white colors have the same amount of reflection.

• In the “Retro-reflective type,” both the transmitter and receiver are placed in one housing, and a reflector made of a special reflective material is used. The transmitter transmits a light beam which is reflected by the reflector and received by the receiver. If any object comes in between this light beam path, it breaks. Based on the difference between light beam intensity and other parameters, the 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 satellites.
• 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 a microwave environment.
• It has very high sensitivity, range, and resolution compared to non-optical sensors.
• It has a wider dynamic range.
• It is completely passive and hence used in explosive environments.
• It is small in size and light in weight.
• It is resistant to high temperatures and chemically reactive environments. Hence, optical sensors are ideal for hostile and harsh environments.
• It can be used to monitor a 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 a large number of distributed points.

Following are the disadvantages of optical sensors:

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