Infrared bands (NIR, SWIR, MWIR, LWIR)
The divisions of the infrared from 0.78 µm to 1 mm. Engineering usage splits it by detector and atmospheric window: near infrared to about 1 µm, short-wave to about 3 µm, mid-wave 3–5 µm, long-wave 8–14 µm and far infrared beyond. The boundaries differ between standards and industries.
Infrared radiation runs from the red end of the visible, conventionally 780 nm, to about 1 mm, where it meets the terahertz and microwave region. Across that range the photon energy falls from 1.59 eV to about a millielectronvolt, and the detectors, optical materials and sources that work change several times, so the region is divided into bands. There is no single set of boundaries; the names below are the common engineering ones, and a specification should state the wavelengths it means.
Engineering bands
| Band | Typical range | What defines it |
|---|---|---|
| Near infrared (NIR) | 0.78–1.0 µm (often to 1.4 µm) | Silicon detectors still respond, to their 1.1 µm cutoff |
| Short-wave infrared (SWIR) | 1.0–3 µm (often 0.9–1.7 µm in camera usage) | InGaAs detectors; the telecom bands at 1.26–1.675 µm |
| Mid-wave infrared (MWIR) | 3–5 µm | Atmospheric window; InSb and HgCdTe detectors; thermal imaging of hot objects |
| Long-wave infrared (LWIR) | 8–14 µm | Atmospheric window; peak of room-temperature thermal emission; microbolometers |
| Far infrared (FIR) | 15 µm to 1 mm | Absorbed by the atmosphere; merges with terahertz |
The gaps between 5 and 8 µm and above 14 µm are where water vapour and carbon dioxide absorb strongly. The limits of each band follow detector materials as much as physics: an InGaAs photodetector lattice-matched to InP cuts off at 1.7 µm, which is why "SWIR camera" usually means 0.9 to 1.7 µm, while extended-wavelength InGaAs reaches about 2.6 µm.
Standards
ISO 20473 divides the infrared into near (0.78–3 µm), mid (3–50 µm) and far (50–1000 µm). The CIE uses IR-A (0.78–1.4 µm), IR-B (1.4–3 µm) and IR-C (3 µm–1 mm), the division laser safety and photobiology use, because the eye's transmission changes at 1.4 µm: shorter wavelengths reach the retina, longer ones are absorbed in the cornea and lens. That boundary is why lasers beyond 1.4 µm are sometimes called eye-safe, which refers to their higher ocular damage thresholds; such lasers can still injure the eye.
Converting
A wavelength in micrometres converts to photon energy in electronvolts as , and to wavenumber in cm⁻¹ as : 1.55 µm is 0.800 eV and 6452 cm⁻¹, 10.6 µm (the CO₂ laser line) is 0.117 eV and 943 cm⁻¹. Spectroscopists usually work in wavenumbers in the mid infrared, where molecular vibrations fall between about 400 and 4000 cm⁻¹. The wavelength, frequency and wavenumber entry covers the conversions.
References: ISO 20473:2007, Optics and photonics: Spectral bands; CIE S 017:2020, ILV: International Lighting Vocabulary; ITU-T G.Sup39 (telecom bands).