Infrared Sensors or IR Sensors

Infrared radiation is the portion of electromagnetic spectrum having wavelengths longer than visible light wavelengths, but smaller than microwaves, i.e., the region roughly from 0.75µm to 1000 µm is the infrared region. Infrared waves are invisible to human eyes. The wavelength region of 0.75µm to 3 µm is called near infrared, the region from 3 µm to 6 µm is called mid infrared and the region higher than 6 µm is called far infrared. (The demarcations are not rigid; regions are defined differently by many). 

There are different types of IR sensors working in various regions of the IR spectrum but the physics behind "IR sensors" is governed by three laws:

1.         Planck’s radiation law:

Every object at a temperature T not equal to 0 K emits radiation. Infrared radiant energy is determined by the temperature and surface condition of an object. Human eyes cannot detect differences in infrared energy because they are primarily sensitive to visible light energy from 400 to 700 nm. Our eyes are not sensitive to the infrared energy.

2.         Stephan Boltzmann Law

The total energy emitted at all wavelengths by a black body is related to the absolute temperature as

3.         Wein’s Displacement Law

Wein’s Law tells that objects of different temperature emit spectra that peak at different wavelengths. It provides the wavelength for maximum spectral radiant emittance for a given temperature.

The relationship between the true temperature of the black body and its peak spectral exitance or dominant wavelength is described by this law

The world is not full of black bodies; rather it comprises of selectively radiating bodies like rocks, water, etc. and the relationship between the two is given by emissivity (E).

Emissivity depends on object color, surface roughness, moisture content, degree of compaction, field of view, viewing angle & wavelength.

ELEMENTS OF INFRARED DETECTION SYSTEM

A typical system for detecting infrared radiation is given in the following block diagram :

1.         Infrared Source

All objects above 0 K radiate infrared energy and hence are infrared sources. Infrared sources also include blackbody radiators, tungsten lamps, silicon carbide, and various others. For active IR sensors, infrared Lasers and LEDs of specific IR wavelengths are used as IR sources.

2.        Transmission Medium

Three main types of transmission medium used for Infrared transmission are vacuum, the atmosphere, and optical fibers.

The transmission of IR – radiation is affected by presence of CO2, water vapour and other elements in the atmosphere. Due to absorption by molecules of water carbon dioxide, ozone, etc. the atmosphere highly attenuates most IR wavelengths leaving some important IR windows in the electromagnetic spectrum; these are primarily utilized by thermal imaging/ remote sensing applications.

•           Medium wave IR (MWIR:3-5 µm)

•           Long wave IR (LWIR:8-14 µm)

Choice of IR band or a specific wavelength is dictated by the technical requirements of a specific application.

3.         Optical Components.

Often optical components are required to converge or focus infrared radiations, to limit spectral response, etc. To converge/focus radiations, optical lenses made of quartz, CaF2, Ge and Si, polyethylene Fresnel lenses, and mirrors made of Al, Au or a similar material are used. For limiting spectral responses, bandpass filters are used. Choppers are used to pass/ interrupt the IR beams.

4.         Infrared detectors.

Various types of detectors are used in IR sensors. Important specifications of detectors are

•           Photosensitivity or Responsivity

Responsivity is the Output Voltage/Current per watt of incident energy. Higher the better.

•           Noise Equivalent Power (NEP)

NEP represents detection ability of a detector and is the amount of incident light equal to intrinsic noise level of a detector.

•           Detectivity(D*: D-star)

D* is the photosensitivity per unit area of a detector. It is a measure of S/N ratio of a detector. D* is inversely proportional to NEP. Larger D* indicates better sensing element.

In addition, wavelength region or temperature to be measured, response time, cooling mechanism, active area, no of elements, package, linearity, stability, temperature characteristics, etc. are important parameters which need attention while selecting IR detectors.

5.         Signal Processing

Since detector outputs are typically very small, preamplifiers with associated circuitry are used to further process the received signals.

TYPES OF INFRARED SENSORS

1.         ACTIVE INFRARED SENSORS

Active infrared sensors employ both infrared source and infrared detectors. They operate by transmitting energy from either a light emitting diode (LED) or a laser diode. A LED is used for a non-imaging active IR detector, and a laser diode is used for an imaging active IR detector.

In this types of IR sensors, the LED or laser diode illuminates the target, and the reflected energy is focused onto a detector. Photoelectric cells, Photodiode or phototransistors are generally used as detectors. The measured data is then processed using various signal-processing algorithms to extract the desired information.

Active IR detectors provide count, presence, speed, and occupancy data in both night and day operation. The laser diode type can also be used for target classification because it provides target profile and shape data.

These sensors are used as reflective opto-sensors. Reflective opto-sensors are either intensity based or use modulated IR. Intensity based sensors are affected by ambient light. Modulated Infrared sensors wherein emitter is turned ON and OFF rapidly, are less susceptible to ambient light. Reflective opto-sensors are used in two configurations.

•           Break Beam Sensors

This type of sensors consists of a pair of light emitting and light detecting elements. Infrared source transmits a beam of light towards a remote IR receiver creating an “electronic fence”. Once a beam is broken/interrupted due to some opaque object, output of detector changes and associated electronic circuitry takes appropriate actions.

Typical applications of such sensors are intrusion detection, shaft encoder (for measurement of rotation angle/rate of rotation) 

•           Reflectance Sensors

This type of sensors house both an IR source and an IR detector in a single housing in such a way that light from emitter LED bounces off an external object and is reflected into a detector. Amount of light reflected into the detector depends upon the reflectivity of the surface.

This principle is used in intrusion detection, object detection (measure the presence of an object in the sensor’s FOV), barcode decoding, and surface feature detection (detecting features painted, taped, or otherwise marked onto the floor), wall tracking (detecting distance from the wall), etc. 

It can also be used to scan a defined area; the transmitter emits a beam of light into the scan zone, the reflected light is used to detect a change in the reflected light thereby scanning the desired zone.

2.         PASSIVE INFRARED SENSORS

These are basically IR detectors; they don’t use any IR source. These form the major class of IR sensors/detectors.

A passive infrared system detects energy emitted by objects in the field of view and may use signal-processing algorithms to extract the desired information. It does not emit any energy of its own for the purposes of detection. Passive infrared systems can detect presence, occupancy, and count.

Passive Infrared Sensors are of two types: Thermal & Quantum.

Thermal type sensors have no wavelength dependence. They use the infrared energy as heat and their photosensitivity is independent of wavelength. Thermal detectors don’t require cooling but have disadvantages that response time is slow & detection time is low.

Common types of thermal type IR detectors are

•           Thermocouple-Thermopile

A detector that converts temperature into an electrical signal is commonly known as a thermocouple. The junction of dissimilar metals generates a voltage potential, which is directly proportional to the temperature. This junction can be made into multiple junctions to improve sensitivity. Such a configuration is called a thermopile.

The active or ‘Hot’ junctions are blackened to efficiently absorb radiation. The reference or ‘Cold’ junctions are maintained at the ambient temperature of the detector. The absorption of radiation by the blackened area causes a rise in temperature in the ‘hot’ junctions as compared to the ‘cold’ junctions of the thermopile. This difference in temperature across the thermocouple junction causes the detector to generate a positive voltage. If the active or ‘hot’ junction were to cool to a temperature less than the reference or ‘cold’ junction the voltage output would be negative.

These detectors has a relatively slow response time, but offers the advantages of DC stability, requiring no bias, and responding to all wavelengths.

•           Bolometer

A bolometer is a simple thermal or total power detector. A bolometer changes resistance when incident infrared radiation interacts with the detector. This thermally sensitive semiconductor is made of a sintered metal oxide material. It has a high temperature coefficient of resistance

It essentially consists of two main elements: a sensitive thermometer and an absorptive element and a heat sink. Absorber is connected by a weak thermal link to a heat sink (at temperature T0). Incoming energy increases the temperature of the absorptive element above that of a heat sink and rise in temperature is measured by a thermometer.

Delta T = T - T0 = E/C

Bolometers use metals or semiconductor/superconductors as absorptive elements.

•           Pyroelectric detector

Pyroelectric detectors use PZT having pyroelectic effect, a high resistor and a low noise FET, hermetically sealed in a package. Pyroelectric materials are crystals, such as lithium tantalate, which exhibit spontaneous polarization, or a concentrated electric charge that is temperature dependent. PZT is spontaneously polarized in dark state. As infrared radiation strikes the detector surface, the change in temperature causes a current to flow. This results in change of polarization state which is reflected in terms of voltage change at the output.

This detector exhibits good sensitivity and good response to a wide range of wavelengths, and does not require cooling of the detector. It is the most commonly used detector for gas monitors.

Quantum type offer higher detection performance and a faster response speed although their photosensitivity is wavelength dependant. Quantum type detectors require cooling for accurate measurements (except for those in near IR region).Quantum type detectors are further classified into two categories

•           Intrinsic type

i)       Photoconductive

Photoconductive type of IR detectors makes use of photoconductive effect. This effect causes change in resistance when IR radiation falls upon detecting elements.

Examples are PbS, PbSe, MCT (HgCdTe)

Bandgap of PbS, PbSe have negative temperature coefficient and hence their spectral response characteristics shift to long wavelength region when cooled. However, bandgap of HgCdTe depends upon the composition and therefore, spectral response characteristics can be tailored to suit the requirements.

ii)      Photovoltaic

Photoconductive type of IR detectors makes use of photovoltaic effect. Incident IR light cause increase in voltage output of these detectors.

Examples are InGaAs PIN photodiodes, InAs, InSb

•           Extrinsic type

Various types of detectors like Ge:Au, Ge:Hg, Ge:Cu, Ge:Zn, Si:Ga, Si:As and are used depending upon the requirements of the application- spectral response, D*, etc.

APPLICATIONS OF INFRA RED RADIATION

1.         Radiation thermometers

Compared to various other methods of temperature measurement, radiation thermometers have following features

a) Measurement without direct contact with the object

b) Faster response

c) Easy pattern measurements.

Detectors used for radiation thermometers depend upon the temperature and material of the object to be measured. For example, glass have peak wavelength near 5 µm and hence, detectors sensitive to these wavelengths are used.

2.         Flame monitors

Flame monitors are used for detecting light emitted from the flames and to monitor how the flames are burning. Light emitted from flames extend from UV to IR region. PbS, PbSe, Two-color detector, pyroelectric detector, etc. are some of the commonly employed detector used in flame monitors.

3.         Moisture analyzers

These analyzers use those wavelengths which are absorbed by moisture in IR region, i.e., 1.1 µm, 1.4 µm, 1.9 µm, and 2.7 µm. Objects are irradiated with light having these wavelengths and also with reference wavelengths. Lights reflected from the objects depend upon the moisture content and is detected by analyzer to measure moisture (ratio of reflected light at these wavelengths to the reflected light at reference wavelength). InGaAs PIN photodiodes, PbS photoconductive detectors are employed in moisture analyzers.

4.         Gas Analyzers

Gas analyzers use absorption characteristics of gases in IR region to measure gas density. Two types of methods are used

a)         Dispersive: Emitted light is spectroscopically divided and their absorption characteristics are used to analyze the gas ingredients and the sample quantity.

b)         Nondispersive: It is more commonly used; it uses absorption characteristics without dividing the emitted light. Nondispersive types use discrete optical bandpass filters, similar to sunglasses that are used for eye protection to filter out unwanted UV radiation. This type of configuration is commonly referred to as nondispersive infrared (NDIR).

Dispersive or ingredient analyzer is used for carbonated drinks, whereas nondispersive analyzer is used in most of the commercial IR instruments, for automobile exhaust gases fuel leakages, etc.

5.         IR Imaging devices

This is one of the prime applications of IR waves, primarily by virtue of its property that it is not visible. It is used for thermal imagers, night vision devices, etc.

Water, rocks, soil, vegetation, the atmosphere, and human tissue all features emit IR radiation. Thermal infrared detectors measure these radiations in IR range and map the spatial temperature distributions of the object/area on an image. Thermal imagers usually composed of In:Sb (indium antimonide), Gd:Hg (mercury-doped germanium), Hg:Cd:Te (mercury-cadmium-telluride)

The detectors are cooled to low temperatures using liquid helium or liquid nitrogen. Cooling the detectors insures that the radiant energy (photons) recorded by the detectors comes from the terrain and not from the ambient temperature of objects within the scanner itself.

6.         Remote sensing

As all objects emit light, the measurement of each wavelength of this emitted light provides lots of specific information about the object. This is precisely what is done in remote sensing to obtain information like temperature of land and sea water, gas concentration of atmosphere, etc.

  

7.         Missile Guidance

Missiles use passive infrared guidance system wherein Infrared energy emitted by a target is used for homing. Infrared seekers are used in missiles for this purpose.

8.         Optical Power meters

For long distance optical communication systems, IR beams in the wavelength region from 1.3 to 1.5 are employed. Optical power meters are used to measure light intensity, optical fiber transmission loss, laser power, etc. in applications like optical fiber communications, lasers, etc. They use InGaAs PIN photodiodes, etc. for optical power measurement.

9.         Sorting devices

These devices use the inherent property of absorption of some IR wavelengths to sort agricultural crops from stones, clods, etc. Such devices use InGaAs PIN photodiodes, PbS detectors.

10.       Human body detection

Such devices are used for detection of a person. Typical applications are intrusion detection, autolight switches, etc. Intrusion alarm devices sense the temperature of a person and rings alarm if sensed temperature crosses some threshold. Such devices also employ optical filters to make use of a specific window (appropriate for human body) of electromagnetic spectrum and to protect it from external disturbances.

11.       Spectrophotometers(FT-IR)

Infrared detectors are at the heart of Fourier Transform IR. In FTIR spectrophotometry, interference signal from double beam interferometer undergoes Fourier transformation by which signal is decomposed into a spectrum.

12.       LD Monitors

The output level and emission wavelength of a Laser Diode varies with the temperature. For the purpose of automatic stabilization of Laser Diode emission wavelength, Laser diode power is monitored using InGaAs PIN photodiodes, InAs, InSb detectors, etc

PROS AND CONS

Advantages:

1.         Low power requirements: therefore ideal for laptops, telephones, PDAs

2.         Low coding/decoding, simple circuitry.

3.         Beam directionality ensures that data isn't leaked or spilled to nearby devices during transmission.

4.         Few international regulatory constraints.

5.         Relatively high noise immunity.

Disadvantages:

1.         Line of sight requirement.

2.         Blocked by common objects

3.         Short range

4.         Direct sunlight, rain, fog, dust, pollution can affect transmission

5.         Lower data rate