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Image of two girls in mid-infrared ("thermal") light (false-color)

Infrared (IR) radiation is electromagnetic radiation of a wavelength longer than that of visible light, but shorter than that of radio waves. The name means "below red" (from the Latin infra, "below"), red being the color of visible light of longest wavelength. Infrared radiation spans three orders of magnitude and has wavelengths between approximately 750 nm and 1 mm.[1]

At the atomic level, infrared energy elicits vibrational modes in a molecule through a change in the dipole moment, making it a useful frequency range for study of these energy states. Infrared spectroscopy is the examination of absorption and transmission of photons in the infrared energy range, based on their frequency and intensity.[2]


Main article: Thermal radiation

Infrared radiation is popularly known as "heat" or sometimes "heat radiation," since many people attribute all radiant heating to infrared light. This is a widespread misconception, since light and electromagnetic waves of any frequency will heat surfaces that absorb them. Infrared light from the Sun only accounts for 50% of the heating of the Earth, the rest being caused by visible light.[How to reference and link to summary or text] Visible light or ultraviolet-emitting lasers can char paper and incandescently hot objects emit visible radiation. It is true that objects at room temperature will emit radiation mostly concentrated in the 8–12 micron band, but this is not distinct from the emission of visible light by incandescent objects and ultraviolet by even hotter objects.

Heat is energy in transient form that flows due to temperature difference. Unlike heat transmitted by thermal conduction or thermal convection, radiation can propagate through a vacuum.


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Night vision

Infrared is used in night-vision equipment when there is insufficient visible light to see an object. The radiation is detected and turned into an image on a screen, hotter objects showing up in different shades than cooler objects, enabling the police and military to distinguish warm targets, such as human beings and automobiles. Also see Forward looking infrared. IR radiation is a secondary effect of heat; it is not heat itself. Heat itself is a measure of the translational energy of an amount of matter. "Thermal" detectors do not actually detect heat directly but the difference in IR radiation from objects. The device itself that detects the radiation is known as a photocathode. Military gunnery ranges sometimes use special materials that reflect IR radiation to simulate enemy vehicles with running engines. The targets can be at the exact same temperature as the surrounding terrain, but they emit (reflect) much more IR radiation. Different materials emit more or less IR radiation as temperature increases or decreases, depending on the composition of the material. Infrared imagery is usually formed as a result of the integrated inband intensity of the radiation, based on temperate and emissivity.

Simple infrared sensors were used by British, American and German forces in the Second World War as night vision aids for snipers.

Smoke is more transparent to infrared than to visible light, so firefighters use infrared imaging equipment when working in smoke-filled areas.

Infrared vision

Thermographic image of a snake eating a mouse

This is a form of vision found in all snakes of the Crotalinae family (including pit vipers, rattlesnakes and pythons). They have two infrared sensory pits on either side of the head, below the eyes. They use these organs to detect prey, which emit infrared radiation with the heat of their bodies.

There is controversy over the exact thermal sensitivity of this biological infrared detection system.[3][4]

Other organisms that actively employ thermo-receptors are boas (Boidae family), the Common Vampire Bat (Desmodus rotundus), a variety of jewel beetles (Melanophila acuminata), darkly pigmented butterflies (Pachliopta aristolochiae and Troides rhadamathus plateni), and possibly blood-sucking bugs (Triatoma infestans).[5]

See also

  • Night vision
  • Infrared spectroscopy
  • Infrared thermometer
  • Infrared homing


  1. Dr. S. C. Liew. Electromagnetic Waves. Centre for Remote Imaging, Sensing and Processing. URL accessed on 2006-10-27.
  2. Reusch, William (1999). Infrared Spectroscopy. Michigan State University. URL accessed on 2006-10-27.
  3. B. S. Jones; W. F. Lynn; M. O. Stone (2001). Thermal Modeling of Snake Infrared Reception: Evidence for Limited Detection Range. Journal of Theoretical Biology 209 (2): 201-211. DOI:10.1006/jtbi.2000.2256.
  4. V. Gorbunov; N. Fuchigami; M. Stone; M. Grace; V. V. Tsukruk (2002). Biological Thermal Detection: Micromechanical and Microthermal Properties of Biological Infrared Receptors. Biomacromolecules 3 (1): 106-115. DOI:10.1021/bm015591f.
  5. A.L. Campbell, A.L. Naik, L. Sowards, M.O. Stone (2002). Biological infrared imaging and sensing. Micron 33 (2): 211-225.

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