All objects, that have finite temperature emit energy, are in the form of electromagnetic radiation, which is arranged according to the wavelength or frequency of the electromagnetic spectrum. Depending on their wavelength, the electromagnetic spectrum can be divided into different regions which are ultraviolet, visible, infrared, and microwave. The Ultraviolet region occupies wavelength range of 0.1-0.4 μm while the visible region occupies wavelength range of 0.4-0.7 μm, which is the range that is detectable by a human eye. The infrared region is bounded by visible and microwave region and occupies nearly wavelength range of 0.7-1000 microns (μm). Because radiation in all heated objects is in the infrared range, the term heat and infrared are often used interchangeably. Infrared spectrum is in turn subdivided into five regions: the shortwave region from 0.7-3 μm, mid wave region from 3-5 μm, long wave region from 8-14 μm, and the Far/Extreme IR from 14-1000 μm . As an example, Infrared cameras for fever 6 screening work in the range of long wave infrared region . The aforementioned spectral regions are schematized in below image.
Thermal imaging devices are combined of optics, detectors and a signal processing unit. The signal processing unit has the function of detecting the emitted thermal energy of the target and the background presented within the field of interest
- The infrared radiations, which are received from the field of view, are collected, filtered
spectrally, and concentrated onto a multi-element detector array using an optical lens.
- Incident radiation heats the detector surface, heated surface of detector affects its material
properties such as electrical conductivity, which in turn interprets into variation of the
output signalSignal processing unit received output signal from the detectors and converts it to data for display.
- Depending on infrared emission intensity, the data produced by the signal processing unit
- is displayed with different colors. Figure shows the basic elements of the thermal imaging system