定义：
波长在750nm和1mm之间的不可见光。

红外光的波长大于700-800nm，即可见光的波长上限。二者的界限并不是很明确，在该区域眼睛的响应度非常缓慢的减小。尽管在700nm时响应度已经很低，但是当激光二极管的光足够强，波长为750nm时也是可以看到的。尽管这些光并不亮，但是对眼睛是有伤害的。红外光谱的上限也是没有明确的定义的，通常认为约等于1mm。 
不同的红外光谱区域的定义为： 

• 近红外区域（也称为IR-A），波长从约700 nm到1400 nm。在这一波长区域工作的激光器对眼睛伤害很大，因为近红外光同可见光一样可以穿透并且聚焦于视网膜，但是不会引起起保护作用的眨眼反射。因此需要进行眼睛防护。 
• 中红外光（MWIR）波长从3到8微米。大气对该波段的光有强烈的吸收，具有许多吸收线，例如二氧化碳（CO2）和水蒸气（H2O）。许多气体具有很强的中红外吸收线，因此这一光谱区域的光可用于气体光谱学。 
• 长波红外光（LWIR）波长从8到15微米，后面为远红外（FIR），波长范围到1 mm，有时认为从8微米开始。这一光谱区域可用于热成像。 

一定要注意的是，实际中这些区域的定义有些变化。 
许多玻璃对于近红外光是透明的，但是在更长波长时吸收很强，因为这时光子可以直接转化为声子。对于二氧化硅玻璃，强吸收发生在波长约为2微米时。 
红外光也称为热辐射，因为热体的热辐射通常在红外波长范围内。即使在室温或者低于室温时，热体也会辐射很强的中红外和远红外光，可以用于热成像。例如，冬季供暖房子的红外成像可以显示屋内存在热泄漏（例如，在窗户、屋顶等），因此可以提高直接探测的效率。 

红外辐射源 
大多数激光器，例如Nd:YAG 激光器，光纤激光器以及高功率激光二极管都会辐射近红外光。而产生中红外和远红外光谱的激光器则相对较少。
二氧化碳激光器辐射的光为1060 nm和该区域内其它波长的光。

固体中红外激光器的激光晶体存在的普遍问题是宿主介质有限的透明度，以及存在快速多光子跃迁过程取代激光跃迁过程；
需要声子能量很低的晶体材料。低温铅盐激光器是早期经常采用的中红外光谱学光源，但是现在被量子级联激光器取代，该激光器甚至在室温时都能实现连续波工作。

自由电子激光器可以用做宽带调谐的红外光光源。 
红外光也可由非线性频率转换过程产生。例如，中红外光可由非线性晶体材料的差频产生得到，或者由光学参量振荡器得到。可以参阅词条中红外激光器光源。 
普通的电灯泡辐射的红外光比可见光多，因此其功率转化效率只有5-10%。太阳光也包含很强的红外成分。 

红外光探测 
可以采用很多类型的光电探测器探测红外光。例如，采用半导体的光电二极管带隙能量足够小，这样载流子不仅能被光激光还能被热能量激发，因为在室温下光子能量比kBT大不了很多。

因此，红外探测器需要冷却到足够低的温度来提高其灵敏度。红外相机也需要如此。 
尤其是近红外光，存在红外线探测器，一些风景中辐射的红外线在红外敏感的光阴极上成像，并且显示出来，例如绿色。这些红外线探测器通常用于激光器实验室中用来追踪红外激光光束。

Acronym: IR light

Definition: invisible light with wavelengths roughly between 750 nm and 1 mm

More general terms: light

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 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.

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 in order 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%. Sun light 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_BT 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.

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.