Infrared Light

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

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) is normally understood to range from ≈ 750 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-wave infrared (MWIR) 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 laser absorption spectroscopy.
• The long-wavelength infrared (LWIR) ranges from 8 to 15 μm.
• This is followed by the far infrared (FIR), which ranges from 15 μm to 1 mm (but starts only at 50 μm according to ISO 20473). This spectral region is used for thermal imaging.

Note that the mid-infrared according to ISO 20473:2007 is not equivalent to the mid-wave infrared (see above), but spans the whole wavelength region from 3 μm to 50 μm, where the far infrared starts according to ISO 20473. IR-C according to DIN spans the range from 3 μm to 15 μm.

Unfortunately, considerable confusion arises from conflicting definitions used in the literature.

Most glasses are transparent to 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.

Infrared Optics

Many optical elements working in the visible spectral range can also work well in the near infrared, possibly with modified dielectric coatings. For longer wavelengths (mid and far infrared), one requires special infrared optics, which are often based on optical materials with low phonon energies to obtain a long infrared absorption edge. See the article on infrared optics for more details.

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.

Outside the area of lasers, there are other kinds of infrared emitters, particularly infrared light emitting diodes (LEDs) and thermal emitters, the latter generating thermal radiation.

Ordinary light bulbs (incandescent lamps), generating thermal radiation, 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%. Sunlight also has strong infrared components.

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.

Detection of Infrared Light

Many types of photodetectors are suitable for detecting infrared light. For example, photodiodes based on semiconductors with a sufficiently small band gap 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 <$k_\textrm{B}T$> at room temperature. Therefore, infrared detectors often have to be cooled to fairly low temperatures 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.

There are also photocathode materials which allow the operation of photomultipliers in the infrared.

Low-cost tools for visualizing infrared light are laser viewing cards which emit visible light (or change their color) when hits by infrared light.

Quite high detection sensitivities are possible with upconversion of infrared light into the visible.

Removal of Infrared Light

In illumination systems based on incandescent lamps, e.g. in high-power image projectors, the produced infrared radiation is useless and can cause detrimental effects. Therefore, certain heat control filters have been developed for removing infrared light. They can be realized in the form of cold mirrors or hot mirrors.

More to Learn

Encyclopedia articles:

• infrared optics
• infrared emitters
• infrared detectors
• infrared viewers
• laser viewing cards

Suppliers

The RP Photonics Buyer's Guide contains 136 suppliers for infrared optics. Among them:

Vortex Optical Coatings

Vortex Optical Coatings design and manufacture infrared filters for the most demanding and complex applications. We cover the band from 1–6 microns.

Avantier

Avantier produces a wide range of high quality infrared optics, including infrared lenses, prisms, windows, mirrors, and laser and imaging assemblies. Our state-of-the-art equipment allows us to achieve unparalleled precision and surface quality, and every piece we manufacture is subject to stringent quality controls.

UltraFast Innovations

UltraFast Innovations (UFI®) offers a varied selection of broadband infrared mirrors designed for ultrafast laser systems. For example, we provide mirrors for thulium- and holmium-based systems operating in the 2-μm spectral region, Cr:ZnS systems around 2.4 μm and Cr:ZnSe for 3.2 μm. Such mirrors can be provided with precise control of chromatic dispersion.

EKSMA OPTICS

EKSMA Optics has substrates and finished optical components – windows, lenses and mirrors made from lithium fluoride (LiF), calcium fluoride (CaF₂), barium fluoride (BaF₂), sapphire (Al₂O₃), zinc selenide (ZnSe) and germanium (Ge) for laser and optical instruments applications in the infrared wavelength range.

Edmund Optics

Edmund Optics offers a wide range of infrared optics, using materials like aluminum, calcium fluoride, fused silica, germanium, magnesium fluoride, sapphire, silicon, zinc selenide, zinc sulfide and other infrared materials.

OPTOMAN

OPTOMAN employs IBS technology to manufacture laser optics made for mid-IR. Broadband dispersive and low GDD mirrors for mid-IR range can reduce or even completely eliminate the need to use combinations of various bulk materials to compensate dispersion.

Custom mid-IR optics can be designed for spectral range 1 – 5 µm, using standard or CaF₂, MgF₂, YAG, sapphire or silicon substrates.

UM Optics

UM Optics is the biggest optics supplier in China. We supply CaF₂, BaF₂, MgF₂, LiF material covering the VUV to IR spectrum. We can also deliver cut blanks, polished lens, drilled windows, spherical lenses, aspheric lenses, galvo scanning mirrors, prisms, cylindrical lenses and mirrors in very large quantities at best price. UM Optics also grows silicon material in optical grade and supplies optics like silicon wafers, mirrors, AR-coated lenses, and prisms. We are also good at ZnSe/ZnS/Ge IR material optics.

IRD Glass

IRD Ceramics manufactures precision infrared optical components which are essential to infrared cameras and sensors used by homeland security, border patrol, defense and security companies. We perform all fabrication in house, allowing us to produce low-cost IR mirrors, lenses and windows for commercial applications as well as customized lenses to meet the exact demands of defense and security companies. IRD Ceramics works with sapphire, silicon, chalcogenides, germanium, BaF₂, CaF₂, zinc selenide and more.

LightMachinery

LightMachinery has extensive expertise in the manufacturing and testing of infrared optics based on zinc selenide, zinc sulfide and germanium optics for CO₂ lasers, e.g. in the form of mirrors, lenses and rhomb retarders. In addition, we have a thorough understanding of the importance of high damage threshold coatings for our laser customers.

Sinoptix

We offer IR optics in germanium, silicon, ZnSe, ZnS, BaF₂, MgF₂ or chalcogenide glass – lenses, prisms and windows. We do on demand manufacturing.

Ecoptik

Ecoptik manufactures precision infrared optical devices based on germanium, silicon, zinc selenide, zinc sulfide and other materials. Customized high-precision and complex optical devices, including short-wave infrared optical devices of many kinds can be made. Feel free to contact us!

Shalom EO

Shalom EO offers a wide range of infrared optics – not only singlet IR optical lenses, IR optical windows and IR domes, but also lens modules designed for MWIR (3–5 μm) and LWIR (8–12 μm) thermal imaging cameras. A variety of specific infrared optical materials are available: germanium, zinc selenide (ZnSe), zinc sulfide (ZnS), chalcogenide glass, silicon, sapphire and fluoride (CaF₂, BaF₂, MgF₂ and LiF). Our fabrication techniques include conventional polishing and diamond turning. Moreover, multiple types of modules with flat, spherical and aspherical optical surfaces are optional for different requirements. Referring to the thermal imaging lenses, Hangzhou Shalom EO offers standard lenses and hundreds of custom free-designed lens modules (e.g. athermalized lenses, fisheye lenses, single FOV, dual FOV, zoom lenses).

Further, our infrared windows can be used at 0.75 μm to 20 μm and our infrared lenses are suitable in a wide spectral range from 700 nm to 20000 nm. Infrared domes are suitable for 3 μm to 12 μm. AR coating options include broadband anti-reflection coatings (BBAR), long-pass anti-reflection coatings and hard diamond-like carbon (DLC) coating for application in harsh environments.

Knight Optical

Knight Optical can offer a variety of stock and custom infrared optics, such as lenses, windows, prisms and filters. We can provide these in a wide range of different materials including germanium, silicon, zinc selenide, calcium fluoride, sapphire, magnesium fluoride, zinc sulphide. Our custom infrared optics include aspheric lenses, filters working in the IR wavelengths, and coatings optimised for the different thermal wavebands, as well as diamond-like carbon (DLC) coatings for extra durability of a surface.

Shanghai Optics

At Shanghai Optics we design and manufacture a wide variety of custom optical IR components. Our state of the art equipment allows us to achieve unparalleled precision and surface quality, and every piece we manufacture is subject to stringent quality controls.

We offer a wide range of IR substrates, including Ge, Si, ZnSe, ZnS, ZnS , CaF₂, BaF₂ and GaAs. Our lenses and windows are available with multiple anti-reflection coating options that can increase durability and improve performance.

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