Infrared Radiation and the Electromagnetic Spectrum

Human vision is limited to detecting visible light, which forms only a small fraction of the electromagnetic spectrum. Beyond this range, most of the spectrum consists of invisible light carrying additional information. Infrared radiation is a crucial part of this spectrum, alongside radio waves, microwaves, ultraviolet rays, X-rays, and gamma rays. These different forms of electromagnetic energy are classified based on their wavelength. Infrared radiation spans approximately 0.7 µm to 1000 µm within the electromagnetic spectrum, positioned between visible light and microwaves. For temperature measurement purposes, the range of interest is typically between 0.5 and 14 µm.

According to the sensor response division scheme, this infrared band scheme divides up the band based on the response of various detectors and the atmospheric windows. The infrared wavelength band for non-contact temperature measurement is split into the near-infrared (NIR), short wavelength infrared range (SWIR), mid-wavelength infrared range (MWIR), and long wavelength infrared range (LWIR) range. Near-infrared is the region closest in wavelength to the radiation the human eye detects. Mid-and far-infrared are progressively further from the visible spectrum. The mid and long wavelength range is often called „Thermal Infrared,“ while NIR and SWIR together are sometimes called „reflected infrared.“ The following table represents the wavelength ranges of these spectral bands.

Figure 1 illustrates the atmospheric infrared window. The infrared atmospheric window is a spectral region where Earth’s atmosphere absorbs minimal thermal radiation, allowing infrared energy to escape into space. Water vapor (H₂O), carbon dioxide (CO₂), and ozone (O₃) define this window’s boundaries. CO₂ absorption at 14.7 µm sets the long-wavelength limit, while water vapor’s vibrational bands set the short-wavelength edge. Ozone strongly absorbs at 9.6 µm, partially blocking transmission. Water molecule concentration can narrow or close the window, affecting IR measurements.

The infrared spectrum visually represents how electromagnetic wave intensities are distributed across different wavelengths. Although they vary, all electromagnetic waves follow the same core principles: diffraction, refraction, reflection, and polarization. Under normal conditions, they move at light speed. In linear media, a wave pattern can be expressed as the distinct propagation of sinusoidal components. The consistent relationship between wavelength λ and frequency f is captured in the equation where v denotes the speed of light in the medium. This concept is fundamental to the behavior of all types of electromagnetic radiation [1,2,3]:

[math]\upsilon=\lambda \cdot f[/math]