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.