Infrared, invisible light spectrum, heat radiation...
WHAT IS INFRARED?
Infrared was discovered in 1800 by British astronomer William Herschel, who discovered a type of invisible radiation in the light spectrum lower; in energy, than red light. And that became called “InfraRed”. Infrared waves is longer than those of visible light, so it goes unnoticed to human eyes.
Infrared is a type of electromagnetic waves. Hot objects radiate electromagnetic waves; when it travels and absorbed by a surface of bodies and objects, energy is transferred and the surface heat up. And that is all we can sense and feel it, as heat.
Unlike thermal conduction or thermal convection which primarily works on heating the circulating air, electromagnetic waves, Infrared radiation, do not need a medium to travel through. They can travel through the vacuum of space until they find bodies and objects which can absorb and re-emit the energy.
Simply says, Infrared is invisible heat radiation that we encounter most in everyday life and experience its warmth on our skin.
HOW INFRARED WORKS?
Infrared radiation, the same as the Sun’s rays in everyday life, heats bodies and objects directly without heating the air. We feel the warmth on our skin, and the heat is then passed by the blood circulation around the body. Because people (and other animals) are most sensitive to radiant effects, this leads to an extremely comfortable feeling of warmth at lower air temperatures.
Please see the following link to learn more on this topic: "Infrared heating for humans“. by Simon Lea
TYPES OF INFRARED
Infrared radiation is one of the three ways that heat is transferred from one place to another, the other two being convection and conduction.
The International Commission on Illumination (CIE) recommended the division of infrared radiation into the following three bands considering on its wave length and frequency.
Wave length between 700 nm–1400 nm (0.7 µm – 1.4 µm, 215 THz - 430 THz)
IR-B (Short wavelength-Infrared)
Wave length between 1400 nm–3000 nm (1.4 µm – 3 µm, 100 THz - 215 THz)
IR-C (Mid – wavelength and Long-wavelength)
Wave length between 3000 nm–1 mm (3 µm – 1000 µm, 300 GHz - 100 THz)
Near-infrared is the region closest in wavelength to the radiation detectable by the human eye. Mid- and far-infrared are progressively further from the visible spectrum.