Infrared



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.

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.

"Heat"
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. 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.

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.

Thermography
Infrared thermography is a non-contact, non-destructive test method that utilizes a thermal imager to detect, display and record thermal patterns and temperatures across the surface of an object. Infrared thermography may be applied to any situation where knowledge of thermal profiles and temperatures will provide meaningful data about a system, object or process. Thermography is widely used in industry for predictive maintenance, condition assessment, quality assurance, and forensic investigations of electrical, mechanical and structural systems. Other applications include, but are not limited to: law enforcement, firefighting, search and rescue, and medical and veterinary sciences.

Aside from test equipment, training is the most important investment a company will make in an infrared inspection program. Advances in technology have provided infrared equipment that is user-friendly; however, infrared thermography is not a “simply point and shoot” technology. In addition to understanding the object or system being inspected, thermographers must also understand common error sources that can influence observed thermal data. Typically,infrared training courses should cover the topics of infrared theory, heat transfer concepts, equipment selection and operation, how to eliminate or overcome common error sources, and specific applications. Training courses from independent training companies are preferred since they are not biased toward a single brand or type of equipment.

Other imaging
In infrared photography, infrared filters are used to capture the near-infrared spectrum. Digital cameras often use infrared blockers. Cheaper digital cameras and some camera phones which do not have appropriate filters can "see" near-infrared, appearing as a bright white colour (try pointing a TV remote at your digital camera). This is especially pronounced when taking pictures of subjects near IR-bright areas (such as near a lamp), where the resulting infrared interference can wash out the image. There is also a technique called 'T-ray' imaging, which is imaging using far infrared or terahertz radiation. Lack of bright sources makes terahertz photography technically more challenging than most other infrared imaging techniques. Recently T-ray imaging has been of considerable interest due to a number of new developments such as terahertz time-domain spectroscopy.

Heating
Infrared radiation is used in infrared saunas to heat the occupants, and to remove ice from the wings of aircraft (de-icing). It is also gaining popularity as a method of heating asphalt pavements in place during new construction or in repair of damaged asphalt. Infrared can be used in cooking and heating food as it heats only opaque, absorbent objects and not the air around them, if there are no particles in it.

Infrared heating is also becoming more popular in industrial manufacturing processes, e.g. curing of coatings, forming of plastics, annealing, plastic welding, print drying. In these applications, infrared heaters replace convection ovens and contact heating. If the wavelength of the infrared heater is matched to the absorption characteristics of the material, significant gains in energy efficiency are possible.

Spectroscopy
Infrared radiation spectroscopy (see also near infrared spectroscopy) is the study of the composition of (usually) organic compounds, finding out a compound's structure and composition based on the percentage transmittance of IR radiation through a sample. Different frequencies are absorbed by different stretches and bends in the molecular bonds occurring inside the sample. Carbon dioxide, for example, has a strong absorption band at 4.2 µm.

Biological systems
The pit viper is known to have two infrared sensory pits on its head. There is controversy over the exact thermal sensitivity of this biological infrared detection system.

Other organisms that actively employ thermo-receptors are rattlesnakes (Crotalinae subfamily) and 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).

Web sites

 * List of infrared application examples broken down by industry from FLIR Systems
 * Infrared Spectroscopy NASA Open Spectrum wiki site.
 * Infrared WavesDetailed explanation of infrared light.