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Seeing Infrared

January 19, 2015

Infrared light has been technologically useful for many things. Short-range data communication by infrared light in the 850–900 nanometer range is used in many devices, including the ubiquitous remote control. As in many consumer technologies, advances in functionality were obtained though standardization; in this case, the Infrared Data Association (IrDA) standard.

Remote controlsA collection of infrared remote control devices in my house. The Panasonic remote for our legacy videocassette recorder is seldom used.

(Photo by author).

A typical human eye will respond to wavelengths from about 390 to 720 nm, so we can't see into the infrared. The lens of the eye blocks ultraviolet light, and when the lens is absent, vision to wavelengths as low as 300 nm has been reported.

Although infrared light is useful for many applications, it's often converted to visible light by optical frequency doubling using the optical non-linearity of some materials. Doubling of frequency is equivalent to reduction of the wavelength by half. I wrote about frequency doubling in two previous articles (Second Harmonic Light Generation, September 30, 2011 and Thin Optical Doubler, July 23, 2014).

Efficient optical doubling can be achieved in crystals of lithium niobate (LiNbO3), lithium tantalate (LiTaO3), potassium titanyl phosphate (KTP, KTiOPO4), lithium triborate (LBO, LiB3O5) and β-barium borate (BBO, β-BaB2O4). These non-linear optical effects only occur at high light intensities, where the electric field component of the light is correspondingly high, so it only works for laser light sources. Efficient conversion occurs only for a long optical path through the doubling crystal. As an example, the solid state laser, Neodymium-YAG (Nd:YAG), emits light at the infrared wavelength of 1.064 μm. A nonlinear optical crystal will double this 1.064 μm light to 532 nm, which is green light.

Lasers and frequency-doublers aren't the only way to convert infrared to visible light. Another method is the use of an upconversion phosphor. The simplest such process is excited state absorption in which an electron, already excited to a higher energy level by a photon, is excited to a still higher level by a second photon. When the electron again seeks its ground state, it emits visible light.

Energy level diagram for ESA upconversionEnergy level diagram for upconversion via excited state absorption.

An electron at a ground state (0) is excited to a first (1) energy level by an infrared photon, and a higher, second (2) energy level by another photon. When it reverts back to its ground state, visible light is emitted.

(Illustration by the author, rendered with Inkscape.)

A more useful form of upconversion is energy transfer upconversion in which infrared photons are absorbed by two different atom types, an "activator" ion and a "sensitizer" ion. As shown in the figure, non-radiative transfer of energy from the sensitizer atom to the activator electron allows it to achieve a higher energy state than it could have through absorption of a single infrared photon. One useful activator-sensitizer combination is Er3+ and Yb3+.[1-2]

Energy level diagram for ETU upconversion

Energy level diagram for upconversion via energy transfer. An electron at a ground state (0) in an activator ion is excited to a first (1) energy level by infrared light, and a second (2) energy level with help from non-radiative energy transfer from a sensitizer ion. When it reverts back to its ground state, visible light is emitted. (Illustration by the author, rendered with Inkscape.)

Since ultraviolet light is useful as an antimicrobial agent, there has also been research done on upconversion of visible light to ultraviolet light.[3] Since an upconversion phosphor converts two infrared photons to a single visible photon, it can have an apparent efficiency greater than one. Upconversion phosphors will saturate when their excited state is filled because of its long lifetime.

The human eye is a sensitive photodetector, but it's always been thought that it was not possible for the eye to detect infrared. A quick glance at the response of the retinal cones to light of various wavelengths (see figure) shows that there is no vision above about 720 nanometers. However, a large international team of scientists from Polgenix, Inc. (Cleveland, Ohio), the Washington University School of Medicine (St. Louis, Missouri), the Nicolaus Copernicus University (Torun, Poland), the University of Bern (Bern, Switzerland), Case Western Reserve University (Cleveland, Ohio), and the University of Oslo (Oslo, Norway), has just published evidence of human vision at wavelengths beyond 1,000 nm.[4-5]

Visual response of retinal S,M and L conesVisual response of the retinal S, M and L cone cells that define color vision.

(Modified Wikimedia Commons image.)

There have been anecdotal reports of scientists seeing flashes of green light while working with infrared lasers, an effect that would indicate frequency doubling but had not been explained.[4-5] Says Frans Vinberg, a postdoc in the Department of Ophthalmology and Visual Sciences at Washington University and lead author of the study,
"They were able to see the laser light, which was outside of the normal visible range, and we really wanted to figure out how they were able to sense light that was supposed to be invisible."[5]

Vinberg and his colleagues combed the scientific literature to see what conditions resulted in human observation of infrared, and they conducted psychophysical experiments that proved the effect.[4-5] In their experiments, the research team beamed pulses of infrared light into the eyes of their subjects. By varying the pulse width, but keeping the total number of photons in each pulse constant, they found that a rapid influx of photons resulted in two infrared photons teaming to produce a green light photon.[5]

The sensitivity increased at wavelengths above 900 nm. Doubled 900 nm light would be at 450 nm, which is at the lower range of optical sensitivity. The effect displayed a quadratic dependence on laser power, an indicator of a nonlinear optical process.[4] Other experiments showed that infrared light causes a photoisomerization of a model chromophore compound, which indicates that human perception of infrared light occurs by such an isomerization of visual pigments.[4]

This strange visual phenomenon might have diagnostic value for diseases such as macular degeneration, since it would allow physicians to stimulate the retina in a new way.[5] This research was funded by the National Institutes of Health (NIH) and other agencies.[5]


  1. N. Menyuk, K. Dwight and J.W. Pierce, "NaYF4 : Yb,Er—an efficient upconversion phosphor," Appl. Phys. Lett., vol. 21, no. 4 (August 15, 1972), pp. 159 ff. http://dx.doi.org/10.1063/1.1654325
  2. Yanmin Yang, Chao Mi, Fuyun Jiao, Xianyuan Su, Xiaodong Li, Linlin Liu, Jiao Zhang, Fang Yu, Yanzhou Liu, and Yaohua Mai, "A Novel Multifunctional Upconversion Phosphor: Yb3+/Er3+ Codoped La2S3," Journal of the American Ceramic Society, vol. 97, no. 6 (June, 2014), pp. 1769–1775.
  3. Ezra Lucas Hoyt Cates, Dissertation: Development of Visible-to-Ultraviolet Upconversion Phosphors for Light-Activated Antimicrobial Technology, Georgia Institute of Technology, May, 2013.
  4. Grazyna Palczewska, Frans Vinberg, Patrycjusz Stremplewski, Martin P. Bircher, David Salom, Katarzyna Komar, Jianye Zhang, Michele Cascella, Maciej Wojtkowski, Vladimir J. Kefalov, and Krzysztof Palczewski, "Human infrared vision is triggered by two-photon chromophore isomerization," Proc. Natl. Acad. Sci., (Online Early Edition, December 1, 2014), doi:10.1073/pnas.1410162111.
  5. Jim Dryden, "The human eye can see 'invisible' infrared light," Washington University School of Medicine Press Release, December 1, 2014.

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