Near-infrared

Near-infrared (near-IR) is the region of the electromagnetic spectrum that is just lower in energy than the red region of the visible spectrum.

Infrared radiation (IR) is electromagnetic radiation of a wavelength longer than that of visible light, but shorter than that of microwave radiation. The name means "below red" (from the Latin infra, "below"), red being the color of visible light of longest wavelength. Infrared radiation spans three orders of magnitude and has wavelengths between approximately 750 nm and 1 mm.

Infrared is often subdivided into:

• near-infrared NIR, IR-A DIN, 0.75–1.4 µm in wavelength, defined by the water absorption, and commonly used in fiber optic telecommunication because of low attenuation losses in the SiO₂ glass (silica) medium;

• short wavelength (shortwave) IR SWIR, IR-B DIN, 1.4–3 µm, water absorption increases significantly at 1450 nm;

• mid wavelength IR MWIR, IR-C DIN, also intermediate-IR (IIR), 3–8 µm;

• long wavelength IR LWIR, IR-C DIN, 8–15 µm);

• far infrared FIR, 15–1000 µm.

However, these terms are not precise, and are used differently in various studies i.e. near (0.75–5 µm) / mid (5–30 µm) / long (30–1000 µm). Especially at the telecom-wavelengths the spectrum is further subdivided into individual bands, due to limitations of detectors, amplifiers and sources. Infrared radiation is often linked to heat, since objects at room temperature or above will emit radiation mostly concentrated in the mid-infrared band.

The common nomenclature is justified by the different human response to this radiation (near-infrared = the red you just cannot see, far IR = thermal radiation), other definitions follow different physical mechanisms (emission peaks, vs. bands, water absorption) and the newest follow technical reasons (The common silicon detectors are sensitive to about 1050 nm, while InGaAs sensitivity starts around 950 nm and ends between 1700 and 2200 nm, depending on the specific configuration). Unfortunately the international standards for these specifications are not currently available.

The boundary between visible and infrared light is not precisely defined. The human eye is markedly less sensitivity to red light above 700 nm wavelength, but particularly intense light (e.g., from lasers) can be detected up to approximately 780 nm. The onset of infrared is defined (according to different standards) at various values between these two wavelengths, typically at 750 nm.

Optical telecommunication in the near-infrared is technically often separated to different frequency bands because of availability of light sources, transmitting/absorbing materials (fibers) and detectors.

• O-band 1260–1360 nm
• E-band 1360–1460 nm
• S-band 1460–1530 nm
• C-band 1530–1565 nm
• L-band 1565–1625 nm
• U-band 1625–1675 nm