Sir William Herschel discovered the relative energy of sunlight by using a prism and passing sunlight through it. The rainbow colours appeared, and he found that the temperature was rising when he used a thermometer to measure the temperature at the end of the visible red light.
He concluded that the spectrum continued beyond the visible red light, which he called Infrared. Infrared radiation is part of the electromagnetic spectrum along with visible light, UV, X-rays, radio waves, etc.
Thermal imagers work in the thermal infrared range of 0.7-15 μm and part of the entire infrared spectrum (0.7-1000 μm).
Different subranges are used for temperature measuring due to different spectral responses. "Longer wavelength" instruments are primarily used for lower temperatures, while "shorter wavelength" instruments are used for higher temperatures.
The length of the electromagnetic waves also affects the use of thermal imagers. This is because longer wavelengths have larger waves. For example, visible light has different wavelengths ranging from 400 nm (blue light) to 700 nm (red light), with blue having a shorter wavelength and higher frequency than red.
The phenomenon of blue light being scattered more by particles in the atmosphere and appearing blue can also be applied in industrial applications for temperature measurement.
In atmospheres with hot smoke and particles, such as in biomass combustion furnaces, the presence of hot furnaces, process, and combustion gases, along with H2O and CO2, also affects temperature readings in different ways. For example, the LWIR range (8-14 μm) has significant and changing absorptions, while the MWIR range (3.9 μm) has nearly much fewer absorptions.
Using NIR/SWIR or MWIR instead of LWIR also negatively influences temperature reading accuracy. The temperature reading error is lower at the shorter MWIR and NIR/WIR wavelengths compared to the LWIR range.
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