Infrared light is light with a vacuum wavelength longer than ≈ 700–800 nm, the upper limit of the visible wavelength range. That limit is not well-defined, as the responsivity of the eye is reduced very gradually in that spectral region. Although the responsivity e.g. at 700 nm is already very low, even the light from some laser diodes at wavelengths beyond 750 nm can be seen if that light is sufficiently intense. Such light may be harmful for the eye even if it is not perceived as very bright. The upper limit of the infrared spectral region in terms of wavelength is also not precisely defined; it is usually understood to be roughly 1 mm.
Different definitions are used for distinguishing different infrared spectral regions:
- The near-infrared spectral region (also called IR-A) ranges from ≈ 700 to 1400 nm. Lasers emitting in this wavelength region are particularly hazardous for the eye, as near-infrared light is transmitted and focused to the sensitive retina in the same way as visible light, while not triggering the protective blink reflex. Adequate eye protection is then very important.
- The short-wavelength infrared (SWIR, IR-B) extends from 1.4 to 3 μm. This region is relatively eye-safe, since such light is absorbed in the eye before it can reach the retina. Erbium-doped fiber amplifiers for optical fiber communications, for example, operate in that region.
- The mid-infrared (mid-wavelength infrared, MWIR, IR-C) ranges from 3 to 8 μm. The atmosphere exhibits strong absorption in parts of that region; there are many absorption lines e.g. of carbon dioxide (CO2) and water vapor (H2O). Many gases exhibit strong and characteristic mid-IR absorption lines, which makes this spectral region interesting for highly sensitive trace gas spectroscopy.
- The long-wavelength infrared (LWIR, IR-C) ranges from 8 to 15 μm, followed by the far infrared (FIR), which ranges to 1 mm and is sometimes understood to start at 8 μm already. This spectral region is used for thermal imaging.
Note, however, that the definitions of these terms vary substantially in the literature.
Most glasses are transparent for near-infrared light but are strongly absorbing for longer wavelengths, where photons can be directly converted to phonons. For silica glass, as used e.g. for silica fibers, strong absorption occurs beyond ≈ 2 μm.
Infrared light is also called heat radiation, since thermal radiation from hot bodies is mostly within the infrared region. Even at room temperature and below, bodies emit significant amounts of mid- and far-infrared light, which can be used for thermal imaging. For example, infrared images of a heated house in winter can reveal leaks of heat (e.g. at windows, roofs, or poorly insulated walls behind radiators) and thus help to efficiently direct measures for improvement.
Sources for Infrared Radiation
Most lasers, for example Nd:YAG lasers, many fiber lasers and the most powerful laser diodes, emit near-infrared light. There are comparatively few laser sources for the mid- and far-infrared spectral regions. CO2 lasers can emit at 10.6 μm and some other wavelengths in that region. Typical problems with laser crystals for solid-state mid-IR lasers are the limited transparency range of the host crystal and the tendency for fast multi-phonon transitions bypassing the laser transition; crystal materials with very low phonon energies are required. Cryogenic lead-salt lasers were in earlier years often used for mid-infrared spectroscopy, but are now rivaled by quantum cascade lasers, which partly even achieve continuous-wave operation at room temperature. Free electron lasers can be used as broadly tunable and very powerful sources of infrared light.
Infrared light can also be generated via nonlinear frequency conversion. For example, mid-infrared light can be generated by difference frequency generation in nonlinear crystal materials, or with optical parametric oscillators. See also the article on mid-infrared laser sources.
Ordinary light bulbs emit substantially more infrared light than visible light; this is the essential reason for their very limited power conversion efficiency of the order of 5–10%. Sun light also has strong infrared components.
Detection of Infrared Light
Many types of photodetectors are suitable for detecting infrared light. For example, photodiodes based on semiconductors with a sufficiently small bandgap energy can be used. However, detectors for e.g. the mid-infrared region require such a small bandgap energy that carriers can be excited not only by light, but also via thermal energy, because the photon energy is not much larger than kBT at room temperature. Therefore, infrared detectors often have to be cooled to fairly low temperatures in order to increase their sensitivity. The same holds for infrared cameras.
Particularly for near-infrared light, there are infrared viewers, where infrared light from some scenery is imaged onto an infrared-sensitive photocathode, and generated photoelectrons are accelerated with a high voltage to a fluorescent screen, which then displays the image e.g. in green color. Such IR viewers are used e.g. in laser labs for tracking infrared laser beams.
Low-cost tools for visualizing infrared light are laser viewing cards which emit visible light (or change their color) when hits by infrared light.
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