**1. Introduction**

Radiation has been defined as "the emission or transmission of energy in the form of waves through space or through a material medium" [1]. Electromagnetic waves are different from sound waves because they do not require a medium for propagation. As they spread in the air and solid materials, the spread in the empty space does not contain substances [2]. It has been reported that the term radiation includes electromagnetic radiation (e.g., radio waves, visible light, x-rays and gamma radiation), acoustic radiation (e.g., ultrasound, sound and seismic waves) and particle radiation (e.g., alpha radiation, beta radiation and neutron radiation) [1, 3]. The electromagnetic spectrum is a concept that comprises all possible electromagnetic radiation based on the rules of physics anywhere in the universe and the relative locations of this spectrum according to the wavelength or frequency of different radiation derivatives [4, 5].

Infrared (IR) radiation comprises a beam located in the electromagnetic radiation family that arises from the thermal vibrations of radiation with longer wavelengths than visible light, but shorter wavelengths than microwave radiation **Figure 1**. Its wavelength is between 750 nm and 1 mm, and its frequency is between 300 GHz and 400 THz [5]. The amount of thermal IR radiation emitted by an object is associated with the object temperature, and IR may be used to remotely determine the temperature of an object [6]. IR radiation is also referred to as thermal radiation and is defined by three groups. Far IR radiation is between 300 GHz and 30 THz frequency and between 1 mm (=1000 μm) and 10 μm wavelength. Mid-IR radiation is between 30 and 120 THz frequency and between 10 and 2.5 μm wavelength. Near-IR (NIR) radiation is between 120 and 400 THz frequency and between 2.5 μm (=2500 nm) and 750 nm wavelength [2, 5].

**Figure 1.** Location of infrared radiation in the electromagnetic spectrum.
