INTRODUCTION TO OPTICAL SENSOR

Using Light as a Measurement Tool
Optical sensors detect the presence or behavior of light waves. This could include light in the visible spectrum, or outside the visible spectrum.
Optical sensors are a class of sensors that use light waves as an input. These light waves can be used directly to determine the proximity of an object, or indirectly to measure other properties. There are several different types of optical sensors:

· Photodetectors – Photodetectors, also known as proximity sensors, are used to determine if a moving object enters the range of a sensor. The most commonly found photodetector is the “electric eye”. This type of sensor works by projecting a beam of light from a transmitter to a receiver across a specific distance. As long as the beam of light maintains a connection with the receiver, the circuit remains closed. If an object passes through the beam of light, the continuity of the circuit is lost, and the circuit opens. An example of this type of sensor is a garage door opener safety sensor that will halt the closing of the door if an object breaks the beam.
· Infrared sensors – Active infrared sensors project a beam of light in the infrared spectrum and receive the returning reflection from objects in the sensor’s range. Infrared sensors can be used as proximity sensors, such as in automatic doors. Passive infrared sensors are used to measure the radiation of heat within its range. Examples of passive infrared sensors include “heat-seeking” missile guidance systems and infrared thermography systems.
· Fiberoptic sensors – Fiberoptic sensors can be used to measure a wide range of physical phenomena, depending on the configuration of the sensor. Optical fibers can be coated with materials that respond to changes in strain, temperature, or humidity. Optical gratings can be etched into the fiber at specific intervals to reflect specific frequencies of light. As the fiber is strained, the distances between the gratings change, allowing the physical strain to be measured.
· Interferometers – An interferometer is a device that adds waves from two light sources, generating an interference pattern. The pattern can be used to determine properties of the waves. Interferometry is used in many industries, but its most visible use is in astronomy. Two small telescopes mounted a fixed distance apart can achieve the resolution of a single telescope with a diameter equal to the distance between the two telescopes. Early interferometers were limited to large wavelength radio telescopes, but are now applied to shorter wavelengths of light. This method allows astronomers to measure the diameters of stars, and future projects using interferometers will help astronomers detect and perhaps measure extrasolar planets.
Optical sensors can in many cases provide non-contact measurement in environments where direct contact of electrical circuitry is not possible, such as in high-voltage applications. Optical sensors are used for both very low-tech and very-high tech applications.