Editing Infrared (section)

== Heat ==

{{Main|Thermal radiation}}

[[File:Effect of emissivity on apparent temperature.jpg|thumb|Materials with higher [[emissivity]] appear closer to their true temperature than materials that reflect more of their different-temperature surroundings. In this thermal image, the more reflective ceramic cylinder, reflecting the cooler surroundings, appears to be colder than its cubic container (made of more emissive silicon carbide), while in fact, they have the same temperature.]]

Infrared radiation is popularly known as "heat radiation",<ref>{{Cite book |title=Infrared Radiation. Van Nostrand's Scientific Encyclopedia |publisher=John Wiley & Sons, Inc. |year=2007 |isbn=978-0-471-74398-9 |chapter=Infrared Radiation |doi=10.1002/0471743984.vse4181.pub2}}</ref> but light and electromagnetic waves of any frequency will heat surfaces that absorb them. Infrared light from the Sun accounts for 49%<ref>{{Cite web |year=1980 |title=Introduction to Solar Energy |url=http://www.azsolarcenter.com/design/documents/passive.DOC |url-status=dead |archive-url=https://web.archive.org/web/20090318200719/http://www.azsolarcenter.com/design/documents/passive.DOC |archive-date=2009-03-18 |access-date=2007-08-12 |website=Passive Solar Heating & Cooling Manual |publisher=Rodale Press, Inc. |format=[[DOC (computing)|DOC]]}}</ref> of the heating of Earth, with the rest being caused by visible light that is absorbed then re-radiated at longer wavelengths. Visible light or ultraviolet-emitting lasers can char paper and incandescently hot objects emit visible radiation. Objects at room [[temperature]] will [[spontaneous emission|emit]] [[Thermal radiation|radiation]] concentrated mostly in the 8 to 25&nbsp;μm band, but this is not distinct from the emission of visible light by incandescent objects and ultraviolet by even hotter objects (see [[black body]] and [[Wien's displacement law]]).<ref>{{Cite web |last=McCreary |first=Jeremy |date=October 30, 2004 |title=Infrared (IR) basics for digital photographers-capturing the unseen (Sidebar: Black Body Radiation) |url=http://dpfwiw.com/ir.htm |url-status=dead |archive-url=https://web.archive.org/web/20081218010429/http://dpfwiw.com/ir.htm |archive-date=2008-12-18 |access-date=2006-11-07 |publisher=Digital Photography For What It's Worth}}</ref>

[[Heat]] is energy in transit that flows due to a temperature difference. Unlike heat transmitted by [[thermal conduction]] or [[thermal convection]], thermal radiation can propagate through a [[vacuum]]. Thermal radiation is characterized by a particular spectrum of many wavelengths that are associated with emission from an object, due to the vibration of its molecules at a given temperature. Thermal radiation can be emitted from objects at any wavelength, and at very high temperatures such radiation is associated with spectra far above the infrared, extending into visible, ultraviolet, and even X-ray regions (e.g. the [[solar corona]]). Thus, the popular association of infrared radiation with thermal radiation is only a coincidence based on typical (comparatively low) temperatures often found near the surface of planet Earth.

The concept of [[emissivity]] is important in understanding the infrared emissions of objects. This is a property of a surface that describes how its thermal emissions deviate from the ideal of a [[black body]]. To further explain, two objects at the same physical temperature may not show the same infrared image if they have differing emissivity. For example, for any pre-set emissivity value, objects with higher emissivity will appear hotter, and those with a lower emissivity will appear cooler (assuming, as is often the case, that the surrounding environment is cooler than the objects being viewed). When an object has less than perfect emissivity, it obtains properties of reflectivity and/or transparency, and so the temperature of the surrounding environment is partially reflected by and/or transmitted through the object. If the object were in a hotter environment, then a lower emissivity object at the same temperature would likely appear to be hotter than a more emissive one. For that reason, incorrect selection of emissivity and not accounting for environmental temperatures will give inaccurate results when using infrared cameras and pyrometers.