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A false-color image of two people taken in long-wavelength infrared (body-temperature thermal) radiation This pseudocolor infrared space telescope image has blue, green, and red corresponding to wavelengths of 3.4, 4.6, and 12 μm, respectively.Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with waves that are just longer than those of red light (the longest waves in the visible spectrum), so IR is invisible to the human eye. IR is generally understood to include wavelengths from around 750 nm (400 THz) to 1 mm (300 GHz).[1][2][3] IR is commonly divided between longer-wavelength thermal IR, emitted from terrestrial sources, and shorter-wavelength IR or near-IR, part of the solar spectrum.[4] Longer IR wavelengths (30&#;100 μm) are sometimes included as part of the terahertz radiation band.[5] Almost all black-body radiation from objects near room temperature is in the IR band. As a form of electromagnetic radiation, IR carries energy and momentum, exerts radiation pressure, and has properties corresponding to both those of a wave and of a particle, the photon.[6][5]
Infrared gets its name from the Latin word "infra", meaning "below", and the English word red. It falls just beyond the red portion of the visible spectrum, hence its name meaning &#;below red.&#;[7]
It was long known that fires emit invisible heat; in the pioneering experimenter Edme Mariotte showed that glass, though transparent to sunlight, obstructed radiant heat.[8][9] In the astronomer Sir William Herschel discovered that infrared radiation is a type of invisible radiation in the spectrum lower in energy than red light, by means of its effect on a thermometer.[10] Slightly more than half of the energy from the Sun was eventually found, through Herschel's studies, to arrive on Earth in the form of infrared. The balance between absorbed and emitted infrared radiation has an important effect on Earth's climate.[7]
Infrared radiation is emitted or absorbed by molecules when changing rotational-vibrational movements. It excites vibrational modes in a molecule through a change in the dipole moment, making it a useful frequency range for study of these energy states for molecules of the proper symmetry. Infrared spectroscopy examines absorption and transmission of photons in the infrared range.[11]
Infrared radiation is used in industrial, scientific, military, commercial, and medical applications. Night-vision devices using active near-infrared illumination allow people or animals to be observed without the observer being detected. Infrared astronomy uses sensor-equipped telescopes to penetrate dusty regions of space such as molecular clouds, to detect objects such as planets, and to view highly red-shifted objects from the early days of the universe.[12] Infrared thermal-imaging cameras are used to detect heat loss in insulated systems, to observe changing blood flow in the skin, to assist firefighting, and to detect the overheating of electrical components.[13] Military and civilian applications include target acquisition, surveillance, night vision, homing, and tracking. Humans at normal body temperature radiate chiefly at wavelengths around 10 μm. Non-military uses include thermal efficiency analysis, environmental monitoring, industrial facility inspections, detection of grow-ops, remote temperature sensing, short-range wireless communication, spectroscopy, and weather forecasting.
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There is no universally accepted definition of the range of infrared radiation. Typically, it is taken to extend from the nominal red edge of the visible spectrum at 700 nm to 1 mm. This range of wavelengths corresponds to a frequency range of approximately 430 THz down to 300 GHz. Beyond infrared is the microwave portion of the electromagnetic spectrum. Increasingly, terahertz radiation is counted as part of the microwave band, not infrared, moving the band edge of infrared to 0.1 mm (3 THz).
Position in the electromagnetic spectrum[
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Name Wavelength Frequency (Hz) Photon energy (eV) Gamma ray less than 10 pm more than 30 EHz more than 124 keV X-ray 10 pm &#; 10 nm 30 PHz &#; 30 EHz 124 keV &#; 124 eV Ultraviolet 10 nm &#; 400 nm 750 THz &#; 30 PHz 124 eV &#; 3.3 eV Visible 400 nm &#; 700 nm 430 THz &#; 750 THz 3.3 eV &#; 1.7 eV Infrared 700 nm &#; 1 mm 300 GHz &#; 430 THz 1.7 eV &#; 1.24 meV Microwave 1 mm &#; 1 meter 300 MHz &#; 300 GHz 1.24 meV &#; 1.24 μeV Radio 1 meter and more 300 MHz and below 1.24 μeV and below[
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Sunlight, at an effective temperature of 5,780 K (5,510 °C, 9,940 °F), is composed of near-thermal-spectrum radiation that is slightly more than half infrared. At zenith, sunlight provides an irradiance of just over 1 kW per square meter at sea level. Of this energy, 527 W is infrared radiation, 445 W is visible light, and 32 W is ultraviolet radiation.[15] Nearly all the infrared radiation in sunlight is near infrared, shorter than 4 μm.
On the surface of Earth, at far lower temperatures than the surface of the Sun, some thermal radiation consists of infrared in the mid-infrared region, much longer than in sunlight. Black-body, or thermal, radiation is continuous: it radiates at all wavelengths. Of these natural thermal radiation processes, only lightning and natural fires are hot enough to produce much visible energy, and fires produce far more infrared than visible-light energy.[16]
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In general, objects emit infrared radiation across a spectrum of wavelengths, but sometimes only a limited region of the spectrum is of interest because sensors usually collect radiation only within a specific bandwidth. Thermal infrared radiation also has a maximum emission wavelength, which is inversely proportional to the absolute temperature of object, in accordance with Wien's displacement law. The infrared band is often subdivided into smaller sections, although how the IR spectrum is thereby divided varies between different areas in which IR is employed.
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Infrared radiation is generally considered to begin with wavelengths longer than visible by the human eye. There is no hard wavelength limit to what is visible, as the eye's sensitivity decreases rapidly but smoothly, for wavelengths exceeding about 700 nm. Therefore wavelengths just longer than that can be seen if they are sufficiently bright, though they may still be classified as infrared according to usual definitions. Light from a near-IR laser may thus appear dim red and can present a hazard since it may actually be quite bright. Even IR at wavelengths up to 1,050 nm from pulsed lasers can be seen by humans under certain conditions.[17][18][19]
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A commonly used subdivision scheme is:[20][21]
A comparison of a thermal image (top) and an ordinary photograph (bottom). The plastic bag is mostly transparent to long-wavelength infrared, but the man's glasses are opaque.
NIR and SWIR together is sometimes called "reflected infrared", whereas MWIR and LWIR is sometimes referred to as "thermal infrared".
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The International Commission on Illumination (CIE) recommended the division of infrared radiation into the following three bands:[24][25]
Abbreviation Wavelength Frequency IR-A 780&#; nm 215&#;384 THz IR-B &#; nm 100&#;215 THz IR-C 3&#; μm 0.3&#;100 THz[
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ISO specifies the following scheme:[26]
Designation Abbreviation Wavelength Near-infrared NIR 0.78&#;3 μm Mid-infrared MIR 3&#;50 μm Far-infrared FIR 50&#;1,000 μm[
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Astronomers typically divide the infrared spectrum as follows:[27]
Designation Abbreviation Wavelength Near-infrared NIR 0.7&#;2.5 μm Mid-infrared MIR 3&#;25 μm Far-infrared FIR above 25 μmThese divisions are not precise and can vary depending on the publication. The three regions are used for observation of different temperature ranges,[28] and hence different environments in space.
The most common photometric system used in astronomy allocates capital letters to different spectral regions according to filters used; I, J, H, and K cover the near-infrared wavelengths; L, M, N, and Q refer to the mid-infrared region. These letters are commonly understood in reference to atmospheric windows and appear, for instance, in the titles of many papers.
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Plot of atmospheric transmittance in part of the infrared regionA third scheme divides up the band based on the response of various detectors:[29]
μm.
Near-infrared is the region closest in wavelength to the radiation detectable by the human eye. mid- and far-infrared are progressively further from the visible spectrum. Other definitions follow different physical mechanisms (emission peaks, vs. bands, water absorption) and the newest follow technical reasons (the common silicon detectors are sensitive to about 1,050 nm, while InGaAs's sensitivity starts around 950 nm and ends between 1,700 and 2,600 nm, depending on the specific configuration). No international standards for these specifications are currently available.
The onset of infrared is defined (according to different standards) at various values typically between 700 nm and 800 nm, but the boundary between visible and infrared light is not precisely defined. The human eye is markedly less sensitive to light above 700 nm wavelength, so longer wavelengths make insignificant contributions to scenes illuminated by common light sources. Particularly intense near-IR light (e.g., from lasers, LEDs or bright daylight with the visible light filtered out) can be detected up to approximately 780 nm, and will be perceived as red light. Intense light sources providing wavelengths as long as 1,050 nm can be seen as a dull red glow, causing some difficulty in near-IR illumination of scenes in the dark (usually this practical problem is solved by indirect illumination). Leaves are particularly bright in the near IR, and if all visible light leaks from around an IR-filter are blocked, and the eye is given a moment to adjust to the extremely dim image coming through a visually opaque IR-passing photographic filter, it is possible to see the Wood effect that consists of IR-glowing foliage.[30]
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In optical communications, the part of the infrared spectrum that is used is divided into seven bands based on availability of light sources, transmitting/absorbing materials (fibers), and detectors:[31]
Band Descriptor Wavelength range O band Original 1,260&#;1,360 nm E band Extended 1,360&#;1,460 nm S band Short wavelength 1,460&#;1,530 nm C band Conventional 1,530&#;1,565 nm L band Long wavelength 1,565&#;1,625 nm U band Ultralong wavelength 1,625&#;1,675 nmThe C-band is the dominant band for long-distance telecommunications networks. The S and L bands are based on less well established technology, and are not as widely deployed.
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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",[32] but light and electromagnetic waves of any frequency will heat surfaces that absorb them. Infrared light from the Sun accounts for 49%[33] 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 emit radiation concentrated mostly in the 8 to 25 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).[34]
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.
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Active-infrared night vision: the camera illuminates the scene at infrared wavelengths invisible to the human eye. Despite a dark back-lit scene, active-infrared night vision delivers identifying details, as seen on the display monitor.Infrared is used in night vision equipment when there is insufficient visible light to see.[35] Night vision devices operate through a process involving the conversion of ambient light photons into electrons that are then amplified by a chemical and electrical process and then converted back into visible light.[35] Infrared light sources can be used to augment the available ambient light for conversion by night vision devices, increasing in-the-dark visibility without actually using a visible light source.[35][1]
The use of infrared light and night vision devices should not be confused with thermal imaging, which creates images based on differences in surface temperature by detecting infrared radiation (heat) that emanates from objects and their surrounding environment.[36][10]
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Thermography helped to determine the temperature profile of the Space Shuttle thermal protection system during re-entry.Infrared radiation can be used to remotely determine the temperature of objects (if the emissivity is known). This is termed thermography, or in the case of very hot objects in the NIR or visible it is termed pyrometry. Thermography (thermal imaging) is mainly used in military and industrial applications but the technology is reaching the public market in the form of infrared cameras on cars due to greatly reduced production costs.[2]
Thermographic cameras detect radiation in the infrared range of the electromagnetic spectrum (roughly 9,000&#;14,000 nm or 9&#;14 μm) and produce images of that radiation. Since infrared radiation is emitted by all objects based on their temperatures, according to the black-body radiation law, thermography makes it possible to "see" one's environment with or without visible illumination. The amount of radiation emitted by an object increases with temperature, therefore thermography allows one to see variations in temperature (hence the name).
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Hyperspectral thermal infrared emission measurement, an outdoor scan in winter conditions, ambient temperature &#;15 °C, image produced with a Specim LWIR hyperspectral imager. Relative radiance spectra from various targets in the image are shown with arrows. The infrared spectra of the different objects such as the watch clasp have clearly distinctive characteristics. The contrast level indicates the temperature of the object.[
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Infrared light from the LED of a remote control as recorded by a digital cameraA hyperspectral image is a "picture" containing continuous spectrum through a wide spectral range at each pixel. Hyperspectral imaging is gaining importance in the field of applied spectroscopy particularly with NIR, SWIR, MWIR, and LWIR spectral regions. Typical applications include biological, mineralogical, defence, and industrial measurements.
Thermal infrared hyperspectral imaging can be similarly performed using a thermographic camera, with the fundamental difference that each pixel contains a full LWIR spectrum. Consequently, chemical identification of the object can be performed without a need for an external light source such as the Sun or the Moon. Such cameras are typically applied for geological measurements, outdoor surveillance and UAV applications.[38]
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In infrared photography, infrared filters are used to capture the near-infrared spectrum. Digital cameras often use infrared blockers. Cheaper digital cameras and camera phones have less effective filters and can view intense near-infrared, appearing as a bright purple-white color. 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 can make terahertz photography 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.
Reflected light photograph in various infrared spectra to illustrate the appearance as the wavelength of light changes.[
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Infrared tracking, also known as infrared homing, refers to a passive missile guidance system, which uses the emission from a target of electromagnetic radiation in the infrared part of the spectrum to track it. Missiles that use infrared seeking are often referred to as "heat-seekers" since infrared (IR) is just below the visible spectrum of light in frequency and is radiated strongly by hot bodies. Many objects such as people, vehicle engines, and aircraft generate and retain heat, and as such, are especially visible in the infrared wavelengths of light compared to objects in the background.[39]
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Infrared radiation can be used as a deliberate heating source. For example, it is used in infrared saunas to heat the occupants. It may also be used in other heating applications, such as to remove ice from the wings of aircraft (de-icing).[40] Infrared radiation is used in cooking, known as broiling or grilling. One energy advantage is that the IR energy heats only opaque objects, such as food, rather than the air around them.[26]
Infrared heating is also becoming more popular in industrial manufacturing processes, e.g. curing of coatings, forming of plastics, annealing, plastic welding, and print drying. In these applications, infrared heaters replace convection ovens and contact heating.
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A variety of technologies or proposed technologies take advantage of infrared emissions to cool buildings or other systems. The LWIR (8&#;15 μm) region is especially useful since some radiation at these wavelengths can escape into space through the atmosphere's infrared window. This is how passive daytime radiative cooling (PDRC) surfaces are able to achieve sub-ambient cooling temperatures under direct solar intensity, enhancing terrestrial heat flow to outer space with zero energy consumption or pollution.[41][42] PDRC surfaces maximize shortwave solar reflectance to lessen heat gain while maintaining strong longwave infrared (LWIR) thermal radiation heat transfer.[43][44] When imagined on a worldwide scale, this cooling method has been proposed as a way to slow and even reverse global warming, with some estimates proposing a global surface area coverage of 1-2% to balance global heat fluxes.[45][46]
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IR data transmission is also employed in short-range communication among computer peripherals and personal digital assistants. These devices usually conform to standards published by IrDA, the Infrared Data Association. Remote controls and IrDA devices use infrared light-emitting diodes (LEDs) to emit infrared radiation that may be concentrated by a lens into a beam that the user aims at the detector. The beam is modulated, i.e. switched on and off, according to a code which the receiver interprets. Usually very near-IR is used (below 800 nm) for practical reasons. This wavelength is efficiently detected by inexpensive silicon photodiodes, which the receiver uses to convert the detected radiation to an electric current. That electrical signal is passed through a high-pass filter which retains the rapid pulsations due to the IR transmitter but filters out slowly changing infrared radiation from ambient light. Infrared communications are useful for indoor use in areas of high population density. IR does not penetrate walls and so does not interfere with other devices in adjoining rooms. Infrared is the most common way for remote controls to command appliances. Infrared remote control protocols like RC-5, SIRC, are used to communicate with infrared.
Free space optical communication using infrared lasers can be a relatively inexpensive way to install a communications link in an urban area operating at up to 4 gigabit/s, compared to the cost of burying fiber optic cable, except for the radiation damage. "Since the eye cannot detect IR, blinking or closing the eyes to help prevent or reduce damage may not happen."[47]
Infrared lasers are used to provide the light for optical fiber communications systems. Infrared light with a wavelength around 1,330 nm (least dispersion) or 1,550 nm (best transmission) are the best choices for standard silica fibers.
IR data transmission of encoded audio versions of printed signs is being researched as an aid for visually impaired people through the RIAS (Remote Infrared Audible Signage) project. Transmitting IR data from one device to another is sometimes referred to as beaming.
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Infrared vibrational spectroscopy (see also near-infrared spectroscopy) is a technique that can be used to identify molecules by analysis of their constituent bonds. Each chemical bond in a molecule vibrates at a frequency characteristic of that bond. A group of atoms in a molecule (e.g., CH2) may have multiple modes of oscillation caused by the stretching and bending motions of the group as a whole. If an oscillation leads to a change in dipole in the molecule then it will absorb a photon that has the same frequency. The vibrational frequencies of most molecules correspond to the frequencies of infrared light. Typically, the technique is used to study organic compounds using light radiation from the mid-infrared, 4,000&#;400 cm&#;1. A spectrum of all the frequencies of absorption in a sample is recorded. This can be used to gain information about the sample composition in terms of chemical groups present and also its purity (for example, a wet sample will show a broad O-H absorption around cm&#;1). The unit for expressing radiation in this application, cm&#;1, is the spectroscopic wavenumber. It is the frequency divided by the speed of light in vacuum.
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In the semiconductor industry, infrared light can be used to characterize materials such as thin films and periodic trench structures. By measuring the reflectance of light from the surface of a semiconductor wafer, the index of refraction (n) and the extinction Coefficient (k) can be determined via the Forouhi&#;Bloomer dispersion equations. The reflectance from the infrared light can also be used to determine the critical dimension, depth, and sidewall angle of high aspect ratio trench structures.
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IR satellite picture of cumulonimbus clouds over the Great Plains of the United States.Weather satellites equipped with scanning radiometers produce thermal or infrared images, which can then enable a trained analyst to determine cloud heights and types, to calculate land and surface water temperatures, and to locate ocean surface features. The scanning is typically in the range 10.3&#;12.5 μm (IR4 and IR5 channels).
Clouds with high and cold tops, such as cyclones or cumulonimbus clouds, are often displayed as red or black, lower warmer clouds such as stratus or stratocumulus are displayed as blue or grey, with intermediate clouds shaded accordingly. Hot land surfaces are shown as dark-grey or black. One disadvantage of infrared imagery is that low clouds such as stratus or fog can have a temperature similar to the surrounding land or sea surface and do not show up. However, using the difference in brightness of the IR4 channel (10.3&#;11.5 μm) and the near-infrared channel (1.58&#;1.64 μm), low clouds can be distinguished, producing a fog satellite picture. The main advantage of infrared is that images can be produced at night, allowing a continuous sequence of weather to be studied.
These infrared pictures can depict ocean eddies or vortices and map currents such as the Gulf Stream, which are valuable to the shipping industry. Fishermen and farmers are interested in knowing land and water temperatures to protect their crops against frost or increase their catch from the sea. Even El Niño phenomena can be spotted. Using color-digitized techniques, the gray-shaded thermal images can be converted to color for easier identification of desired information.
The main water vapour channel at 6.40 to 7.08 μm can be imaged by some weather satellites and shows the amount of moisture in the atmosphere.
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The greenhouse effect with molecules of methane, water, and carbon dioxide re-radiating solar heatIn the field of climatology, atmospheric infrared radiation is monitored to detect trends in the energy exchange between the Earth and the atmosphere. These trends provide information on long-term changes in Earth's climate. It is one of the primary parameters studied in research into global warming, together with solar radiation.
A pyrgeometer is utilized in this field of research to perform continuous outdoor measurements. This is a broadband infrared radiometer with sensitivity for infrared radiation between approximately 4.5 μm and 50 μm.
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Beta Pictoris with its planet Beta Pictoris b, the light-blue dot off-center, as seen in infrared. It combines two images, the inner disc is at 3.6 μm.Astronomers observe objects in the infrared portion of the electromagnetic spectrum using optical components, including mirrors, lenses and solid state digital detectors. For this reason it is classified as part of optical astronomy. To form an image, the components of an infrared telescope need to be carefully shielded from heat sources, and the detectors are chilled using liquid helium.
The sensitivity of Earth-based infrared telescopes is significantly limited by water vapor in the atmosphere, which absorbs a portion of the infrared radiation arriving from space outside of selected atmospheric windows. This limitation can be partially alleviated by placing the telescope observatory at a high altitude, or by carrying the telescope aloft with a balloon or an aircraft. Space telescopes do not suffer from this handicap, and so outer space is considered the ideal location for infrared astronomy.
The infrared portion of the spectrum has several useful benefits for astronomers. Cold, dark molecular clouds of gas and dust in our galaxy will glow with radiated heat as they are irradiated by imbedded stars. Infrared can also be used to detect protostars before they begin to emit visible light. Stars emit a smaller portion of their energy in the infrared spectrum, so nearby cool objects such as planets can be more readily detected. (In the visible light spectrum, the glare from the star will drown out the reflected light from a planet.)
Infrared light is also useful for observing the cores of active galaxies, which are often cloaked in gas and dust. Distant galaxies with a high redshift will have the peak portion of their spectrum shifted toward longer wavelengths, so they are more readily observed in the infrared.[12]
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Infrared cleaning is a technique used by some motion picture film scanners, film scanners and flatbed scanners to reduce or remove the effect of dust and scratches upon the finished scan. It works by collecting an additional infrared channel from the scan at the same position and resolution as the three visible color channels (red, green, and blue). The infrared channel, in combination with the other channels, is used to detect the location of scratches and dust. Once located, those defects can be corrected by scaling or replaced by inpainting.[48]
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Infrared reflectography[49] can be applied to paintings to reveal underlying layers in a non-destructive manner, in particular the artist's underdrawing or outline drawn as a guide. Art conservators use the technique to examine how the visible layers of paint differ from the underdrawing or layers in between (such alterations are called pentimenti when made by the original artist). This is very useful information in deciding whether a painting is the prime version by the original artist or a copy, and whether it has been altered by over-enthusiastic restoration work. In general, the more pentimenti, the more likely a painting is to be the prime version. It also gives useful insights into working practices.[50] Reflectography often reveals the artist's use of carbon black, which shows up well in reflectograms, as long as it has not also been used in the ground underlying the whole painting.
Recent progress in the design of infrared-sensitive cameras makes it possible to discover and depict not only underpaintings and pentimenti, but entire paintings that were later overpainted by the artist.[51] Notable examples are Picasso's Woman Ironing and Blue Room, where in both cases a portrait of a man has been made visible under the painting as it is known today.
Similar uses of infrared are made by conservators and scientists on various types of objects, especially very old written documents such as the Dead Sea Scrolls, the Roman works in the Villa of the Papyri, and the Silk Road texts found in the Dunhuang Caves.[52] Carbon black used in ink can show up extremely well.
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Thermographic image of a snake eating a mouseThe pit viper has a pair of infrared sensory pits on its head. There is uncertainty regarding the exact thermal sensitivity of this biological infrared detection system.[53][54]
Other organisms that have thermoreceptive organs are pythons (family Pythonidae), some boas (family Boidae), the Common Vampire Bat (Desmodus rotundus), a variety of jewel beetles (Melanophila acuminata),[55] darkly pigmented butterflies (Pachliopta aristolochiae and Troides rhadamantus plateni), and possibly blood-sucking bugs (Triatoma infestans).[56] By detecting the heat that their prey emits, crotaline and boid snakes identify and capture their prey using their IR-sensitive pit organs. Comparably, IR-sensitive pits on the Common Vampire Bat (Desmodus rotundus) aid in the identification of blood-rich regions on its warm-blooded victim. The jewel beetle, Melanophila acuminata, locates forest fires via infrared pit organs, where on recently burnt trees, they deposit their eggs. Thermoreceptors on the wings and antennae of butterflies with dark pigmentation, such Pachliopta aristolochiae and Troides rhadamantus plateni, shield them from heat damage as they sunbathe in the sun. Additionally, it's hypothesised that thermoreceptors let bloodsucking bugs (Triatoma infestans) locate their warm-blooded victims by sensing their body heat.[56]
Some fungi like Venturia inaequalis require near-infrared light for ejection.[57]
Although near-infrared vision (780&#;1,000 nm) has long been deemed impossible due to noise in visual pigments,[58] sensation of near-infrared light was reported in the common carp and in three cichlid species.[58][59][60][61][62] Fish use NIR to capture prey[58] and for phototactic swimming orientation.[62] NIR sensation in fish may be relevant under poor lighting conditions during twilight[58] and in turbid surface waters.[62]
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Near-infrared light, or photobiomodulation, is used for treatment of chemotherapy-induced oral ulceration as well as wound healing. There is some work relating to anti-herpes virus treatment.[63] Research projects include work on central nervous system healing effects via cytochrome c oxidase upregulation and other possible mechanisms.[64]
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Strong infrared radiation in certain industry high-heat settings may be hazardous to the eyes, resulting in damage or blindness to the user. Since the radiation is invisible, special IR-proof goggles must be worn in such places.[65]
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The discovery of infrared radiation is ascribed to William Herschel, the astronomer, in the early 19th century. Herschel published his results in before the Royal Society of London. Herschel used a prism to refract light from the sun and detected the infrared, beyond the red part of the spectrum, through an increase in the temperature recorded on a thermometer. He was surprised at the result and called them "Calorific Rays".[66][67] The term "infrared" did not appear until late 19th century.[68] An earlier experiment in by Marc-Auguste Pictet demonstrated the reflection and focusing of radiant heat via mirrors in the absence of visible light.[69]
Other important dates include:[29]
Infrared radiation was discovered in by William Herschel.[
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Temperatures of black bodies for which spectral peaks fall at the given wavelengths, according to the wavelength form of Wien's displacement law
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Energy conservation is one of the factors that determine the usefulness and success of the operation of any food industry unit. Heat is transmitted by conduction, convection, and radiation. The goal of heating food is to increase the shelf life and improve the taste of foods [2]. Temperature is a measure of thermal motion at the molecular level. When the temperature of the material increases, the molecular motion gains more energy, and when it increases more, it causes physical and chemical changes in the heated material. In conventional heating, which comes from the combustion of fuel or electric heaters, heat is transferred to the material from the outside by convection by hot air or by thermal conduction. The process of transferring energy from source to food depends on the type of cooking. For example, in the case of the baking process, the energy is transmitted through convection, while frying and boiling are through conduction. Energy will be very close to the surface of the food and then heat food gradually from the hot surface towards the inside. Heat is transferred to the food through conduction only and this requires continuous processing of heat. The high temperature and time required for food depend on the thermal and engineering properties of the food [3].
When heating is done by radiation, the heat is transferred by convection and conduction. The broiling process takes place due to thermal radiation. Electromagnetic radiation causes thermal movements of the molecules, but conversion efficiency is highly dependent on the frequency (energy) of the radiation. Radiation-transmitted energy at shorter wavelengths than infrared causes electron-chemical changes in radiation-absorbing molecules, such as chemical bonding, electronic excitation, and dissipation of absorbed energy in the form of less heat. The efficiency of converting absorbed energy into heat is great at high wavelengths in infrared radiation, so the electromagnetic radiation produced by infrared radiation deepens the food by a few millimeters. Infrared radiation is absorbed by organic matter at separate frequencies that correspond to the transport of internal molecules between energy levels. This transition within the range of infrared energy is expressed regarding the rotational movement and the vibrational (stretching) movement of internal atomic bonds. The rotational frequencies range from to Hz with a wavelength of 30 µm&#;1 mm. The energy transfer during the separation of liquids is very small, and therefore, infrared absorption is continuous. Infrared absorption bands associated with food heating are shown in .
shows that there is a strong absorption due to longitudinal vibrations. The absorption of the material to the radiation does not make it saturated with infrared radiation because the molecules excited by the vibratory movement continuously lose energy in random directions as a result of collisions between the molecules, which transfer energy to the surrounding environment in the form of heat. Wavelengths ranging within 1.4&#;5 µm are considered more effective in cooking food because of their ability to penetrate the steam layer surrounding the food as well as within the food a few millimeters deep. Most infrared radiation is absorbed by a thin layer of organic matter and water, so heating is superficial. The process of infrared heating is faster because the energy is transferred from the heating source to the food simultaneously. Therefore, there is no need for another method to transfer energy, for example, the use of hot air. The heat from infrared heating is produced on the surface of the infrared treated material, so the inside of the material is heated by the connection between the food molecules, thus the temperature is graded from the surface to the center. The air in contact with the surface of the food is heated indirectly, but it is not as hot as it occurs in heating by convection and conduction. The infrared absorption ranges by food components are shown in , which shows that the food components interfere with each other in the absorption of different infrared spectra. Water mainly affects the absorption of incident radiation at all wavelengths, while the absorption of proteins by infrared radiation is at wavelengths 3&#;4 and 6&#;9 µm. Fat absorption is at wavelengths 3&#;4, 6 and 9&#;10 µm, and sugars are 3 and 7&#;10 µm. The water absorption beams are 3, 4.7, 6, and 15.3 µm [13]. In addition, when the thickness of the food increases, the absorption increases.
Combining infrared and hot air is more efficient than if it was used individually as a result of their collaborative effect. Afzal et al. [ 11 ] found that when infrared and hot air were combined to dry barley, the energy consumption was reduced with good quality of barley. The use of infrared radiation with hot air reduces the total energy requirement by 245% compared to hot air alone.
Laohavanich and Wongpichet [ 41 ] stated that the drying curve of rice at a wavelength of 2.7 µm is a function of drying time at initial moisture contents of 0.22, 0.27, 0.32, and 0.37 based on solid db weight, while moisture content 0.37 is a function of drying time at wavelengths of 2.47, 2.58, and 2.7 mµ. The moisture content decreases exponentially with the drying time and also shows that there is a significant effect of wavelengths on the drying rate of rice. The drying rate increases with increasing infrared wavelength. Drying time decreases with increasing wavelength.
The natural vibration of the water molecule is in two cases, namely, symmetrical stretching vibration and symmetrical deformation vibration. Infrared energy relative to those frequencies is efficiently absorbed by the body. Therefore, the food absorbs infrared radiation efficiently at wavelengths greater than 2.5 µm through the change in the vibration state of the mechanism of vibration, which causes its temperature rise (heating) [ 39 ]. Richardson [ 40 ] noted that there are two basic vibrations: Stretching and bending, and expansion means increasing or decreasing the distance between the atoms and bending means the movement of atoms. When infrared radiation strikes molecules, energy will be absorbed and the vibration changes.
The energy that dehydrates food is radiant energy. The infrared source used in food drying is infrared lamps and ceramic heaters by electricity or gas. Infrared rays do not need a medium to transmit radiation energy from the source to the surface of the food. This is an excellent feature, as the food is considered to absorb the infrared radiation and dry itself directly. Therefore, in order to improve the drying efficiency, the absorption and dispersion of the incident radiation should be lower and food should contain water. The infrared source must be in a closed room and its surface should be highly reflective for the purpose of maximizing the multiple reflections to increase energy efficiency [ 9 ]. Infrared absorption in food is differentiated regarding protein, fat, carbohydrates, and water. The direction of incident radiation, the properties of the food surface, and the spectral structure also determine the absorption of infrared radiation. One of the determinants of the use of infrared radiation in food is the heterogeneity of its shape and size, so the intensity of radiation falling on the material is different from one place to another. shows the transformation of fell IR on rice grains into different components [ 38 ]. The walls and bottom of the plate should be coated with aluminum foil in order to reduce heat loss and to reflect the falling rays on them and be radioactive walls. The increase of reflected and emitted radiation, heat transfer by convection and heat of evaporation is different depending on the surface characteristics and the water condition in the rice [ 36 , 38 ].
Infrared wavelengths range from 2.5 to 200 µm and are often used in food drying processes. Water is strongly absorbed by infrared energy at wavelengths 3, 6, 12, and 15 µm [ 36 , 37 ]. Ceramic heaters are often used for drying processes because their emission is up to 3 µm. The reason why water absorbs infrared radiation strongly is the presence of O-H bonds in water, thus it begins to circulate at the same frequency of radiation. The process of converting infrared radiation into circulation energy causes water to evaporate. When infrared radiation hits the surface, part of it is absorbed, reflected, and transmitted. If the permeability is too small, the material reflects or absorbs infrared radiation depending on the nature of the radiation and the properties of the surface of the material and this is called emissivity (ε).
Phenolic compounds are antioxidants extracted from plants [42]. They have the ability to donate hydrogen or electrons as well as make the free radicals more stable [43,44]. The external peels of plants contain a large amount of phenolic compounds for the purpose of protecting their internal parts. shows the effect of infrared radiation at different temperatures on the total phenol content of orange peel and orange leaves. Fresh orange peel had a higher phenolic content compared to leaves. Infrared radiation has a significant effect on the peel and leaf content of total phenols. Plant cell components in the desiccant materials adhere to each other, and thus the possibility of solvent to extract the bioactive compounds will be more difficult [45]. When infrared treatment at high temperatures (60 and 70 °C) at a short period of time, the peel and leaf content of total phenols were higher because phenolic compounds resist thermal breakdown, as shown in . The long drying time at low temperatures (40 and 50 °C) leads to the destruction of some phenols [46]. Anagnostopoulou et al. () found that total phenols in infrared-dried orange peels were higher than in hot-air-dried [12]. Infrared rays can reactivate the low molecular weight antioxidants because heating the materials will be without damaging the underlying molecules of the heated surface and also contribute to heat transfer to the center of the heated material [47]. The effectiveness of the phenolic content increased after exposure of rice husks to FIR [48,49]. Lee et al. [50] found that exposure of rice husks to infrared radiation for two hours increased the content of phenolic compounds. When the rice husks are exposed to infrared radiation, the covalently linked phenol compounds that have antioxidant activity are released and activated.
Open in a separate windowLee, et al., [2] showed that the total phenol content of an aqueous extract of peanut shells increased significantly when infrared exposure time and thermal treatment time were increased ( ). Total phenols increase from 72.9 µM for standard treatment (0) to 141.6 µM for infrared and 90.3 µM for conventional heating at 150 °C for 60 min. Infrared FIR is, therefore, more efficient in increasing phenol content in peanut shells compared with conventional heat treatment. Infrared radiation is biologically active [51], and heat is transferred evenly to the center of matter without breaking down surface-forming molecules [47]. Infrared may be able to access covalent bonds and release antioxidants [47,48]. On the other hand, simple heat treatment has increased the phenol content in the defat sesame, as well as citrus peel [52]. This shows that the association of phenolic compounds in plants is different depending on the type of plant. Effective manufacturing steps to release antioxidants from different plants may not be the same.
When exposing the aqueous extract of peanut husks to FIR for 60 min, the percentage of free radical capture increased from 2.34% to 48.33%. In contrast, simple heat treatment increased to 23.69%. The increase depends on the time of exposure to both infrared and conventional heating [48,51].
The effectiveness of antioxidants was higher using infrared radiation with the initial treatment (pre-treatment with 5% potassium carbonate and 0.5% olive oil for 2 min at 20 °C) compared with standard treatment (infrared only) at 62 and 88 W ( ). The antioxidant efficacy of standard treatment at 125 W was higher than that of infrared treatment with the initial treatment. Therefore, in order to increase the effectiveness of antioxidants, the infrared capacity during drying should be reduced [53].
The value of peroxide increases rapidly when only infrared and infrared with hot air are treated together as a result of higher temperatures. The value of peroxide after three months was 1.59, 12.10, and 36.07 meq/kg at temperatures of 130, 140, and 150 °C, respectively ( ). Infrared roasting at 150 °C gives a significant increase in peroxide value and higher oxidation rates than other treatments. The reason for this was that the infrared rays penetrate the almonds quickly and cause the fat to move to the surface exposed to high temperature, thus causing rapid oxidation. The best conditions for roasting almonds and ensuring that the peroxide number of almonds within the permissible limits of 5 meq/kg are the use of infrared and hot air together and hot air only at temperatures of 130&#;150 °C, and the use of infrared radiation at 130 °C prolong the duration storage from four to five months at 37 °C, while hot-air roasting prolongs the storage period even longer [54]. Infrared roasting of cashew nuts improves the oxidative stability of its oil [55]. This may be the result of the formation of the products of Millard reactions that have antioxidant effects.
Open in a separate windowTuncel et al., [56] showed that the flaxseed content of γ and δ-tocopherol (flax does not contain α and β-tocopherol) for fresh and roasted infrared seeds was 146.57&#;193.14 and 2.91&#;3.23 mg/100 g, respectively. The effect of infrared on δ tocopherol was not significant, while the amount of γ tocopherol was high compared to fresh. The reason for obtaining the highest contents of γ tocopherol by infrared heating was the rupture of the cell walls by heat treatment, which led to increased extraction of tocopherol from oil. Rim et al., [57] demonstrated that exposing peanut shells to infrared rays gives the highest antioxidant efficacy compared to conventional heating treatment. The antioxidant efficacy increases with infrared exposure time. In addition, Seok et al. [58] showed that when thermal processing of grapes using infrared was performed, the levels of antioxidants and phenolic compounds were increased.
Infrared radiation can be used to inhibit bacteria, spores, yeasts and mold in liquid and solid foods. The effectiveness of infrared inhibition depends on the amount of infrared energy, food temperature, wavelength, wave width, food depth, microorganism type, moisture content, and food material type. Increasing the capacity of the infrared source needed for heating produces more energy. Therefore, the total energy absorbed by microorganisms increases and thus increases microbial inhibition.
Hamanaka et al. [29] used infrared radiation to sterilize the grain surface of wheat and found that the surface temperature of wheat increased rapidly when infrared radiation fell on them without the need for conductors. When the radiation power was 0.5, 1, 1.5, and 2 kW, the temperatures within the device were 45, 65, 95, and 120 °C. As a result, the microbial content was 0.83, 1.14, 1.18, and 1.90 CFU/g after 60 s of exposure to infrared heating. Molin and Ostlund [59] studied the effect of infrared temperature on the inhibition of microorganisms. D values of Basillus subtilis were 26, 6.6, 9.3, and 3.2 s at 120, 140, 160, and 180 °C, respectively, while the z-value was 23 °C. The low treatment time at high temperatures was sufficient to eliminate pathogenic microorganisms. The logarithmic numbers of E. coli bacteria decreased to 0.76, 0.90 and 0.98 CFU/g after 2 min of infrared exposure [60].
Jun and Irudayaraj [61] used infrared within a wavelength of 5.88&#;6.66 µm using optical band bass filters to inhibit Aspergillus niger and Fusarium proliferatum in corn flour. The specific wavelength denatures the protein in microorganisms and results in a 40% increase in inhibition compared to the use of infrared radiation without determining a specific wavelength. If the wavelength was determined and not specified, the decrease in the logarithmic numbers of A. niger was 2.3 and 1.8 CFU/g, respectively, after five minutes of infrared radiation exposure. In contrast, the logarithmic numbers of F. proliferatum were 1.95 and 1.4 CFU/g, respectively, at infrared radiation exposure. The reason was that the energy absorption by the innate spores was greater at the elected wavelength and consequently lead to a higher mortality rate [61].
Thermal inhibition works by damaging DNA, RNA, ribosome, cell cover, and proteins in bacterial cells. Sawai et al. [62] studied the mechanics of the microbiological inhibitor of infrared radiation against E. coli bacteria in saline phosphate fever. The obtained results suggested that partially damaged cells would become more sensitive to antibiotics that have an inhibitory action on the damaged part of the cell. RNA, proteins, and cell walls are more vulnerable to infrared heating than conductive heating. The order of magnitude of infrared damage is as follows:
Protein > RNA > Cell wall > DNA
Using infrared heating at 3.22 kW/m2 for 8 min resulted in a reduction of 1.8, 1.9, 2.7, and 3.2 log of E. coli when the Agar was rich in nalidixic, penicillin (PCG), rifampicin (RFG), and chloramphenicol (CP). However, the reduction rate of E. coli was 1.8 log without using any of above-mentioned antibiotics. This means that the effect of inhibitory factors led to a decrease of 0.1, 0.9 and 1.4 log due to PCG, RFP, and CP, respectively. The infrared penetration depth is a low. The surface temperature of the food materials increases rapidly and the heat is transferred to the food through thermal conduction.
The thermal conductivity of solid foods is lower than liquid foods. In the case of liquid foods, heat transfer occurs by convection using infrared heating, thereby increasing the microbial mortality [2]. Hamanaka et al. [28] studied the inhibition efficiency of B. subtilis treated with three infrared heaters of different wavelengths (950, , and nm). The results found that inhibition of pathogenic microorganisms at 950 nm was higher than other wavelengths at the same temperature. The decimal time at water activity of 0.7 and wavelengths of 950, , and nm was 4, 12, and 22 min, respectively. The obtained results indicated that the inhibition efficiency was depended on the radiation spectrum, as shown in . The effect of infrared radiation on microbial inhibition was decreased with increasing the food depth because infrared penetration depth is low, therefore, infrared can be used to sterilize food surfaces only. Rosenthal et al. [63] showed that infrared heating was efficient in reducing the growth of yeasts and molds on the surface of cheese at 70 °C for 5 min without affecting the quality of the cheese.
Open in a separate windowInfrared lamps used in hatching poultry eggs and pest control. According to Kirkpatrick [64], infrared rays led to the elimination of insects by 99% of Sitophilus oryzae and 93% of Rhyzopertha dominica and wheat temperature increased to 48.6 °C during treatment.
Infrared radiation can be effectively used to inhibit enzymes. Lipooxygenase enzyme responsible for soybean damage is inhibited by 95.5% using infrared radiation [15]. Lipase and α amylases are strongly influenced by infrared radiation at a temperature of 30&#;40 °C [64,65]. Lipase activity decreases by 60% after infrared treatment for 6 min while it decreases by 70% after using thermal conductivity. The inhibition of the polyphenol oxidase enzyme in treated potato chips using infrared heating starts when the temperature of the center of the slice reaches 65 °C and the inhibition cannot reach 100% in the center of the slice. This requires the first area of the device to provide a higher capacity to ensure the inhibition of higher efficiency and reduce the thickness of the chips [62].
Yi et al. [66] found that the best pre-treatment for apple cubes was dipping for 5 min in calcium chloride and ascorbic acid 0.5% for inhibition of brown coloration. Infrared heating at the intensity of W/m2 may inhibit the enzymatic polyphenol oxidase and peroxidase much faster than the intensity of W/m2. The enzymes polyphenol oxidase and peroxidase possessed high heat resistance and their inhibition process occurred by following the first-order kinetics and fractional-conversion models, respectively. Quick-boiling by using infrared drying is characterized by its rapid inhibition of complex enzymes that cause quality damage and no loss or very simple loss of vitamins, flavors, dyes, carbohydrates, and some water-soluble components. The reaction rate during infrared dry boiling is very low. The inhibition of phosphatase in infrared apple slices depends on the thickness of the chip and the intensity of the radiation. Infrared boiled peas retain more ascorbic acid and flavor than boiling with hot water. Infrared radiation can be used to inhibit enzymes effectively. The infrared boiling time of carrot slices requires a time of 10&#;15 min compared to the boiling steam and hot water methods, which requires a time of 5&#;10 min ( ). This may be attributed to the gradual increase in the temperature of the product as a result of intermittent infrared heating and movement of air on the surface of the product. This led to the stability of the temperature of the product and improved the quality, where the amount of vitamin C was higher compared to the steam and hot water methods [67].
Open in a separate windowBaking bread is a complex process that involve a combination of physical, chemical and biochemical changes in foods such as gelatinization of starch, protein denaturation, release of carbon dioxide due to the addition of yeast, water evaporation, baking crust formation and brown reactions as a result of heat and mass transfer through the product and space inside the oven. Heat is transmitted to the dough by radiation, convection and conduction. Pei [68] classifies traditional bread into four phases: Crusty white bread, transfer heat from inside to crust, cooking or gelatinization and browning. The alternative technology for traditional bread is short-wave infrared [68,69,70].
In , Ginzburg used infrared radiation as an oven to bake bread. At the time, this technique was not developed because of the lack of information about this technology. In , researchers used infrared radiation as a means of heating food, especially for frying meat products [10,71]. Then, this technique was applied to baking bread [72]. Infrared biscuit bread was applied by Wade [70], and it was found that there is a wide range of biscuits that can be baked with an infrared wavelength of 1.2 µm and require half the time compared to the conventional method.
The benefit of using infrared heating in an oven for baking bread is to transfer the heat rapidly to the bread. The property of the bread allows a good penetration up to 2&#;3 mm and speed of heating. The reason why infrared ovens are better than conventional ovens is that this technique is more efficient in heating surfaces and central parts of food at a short baking time due to efficient heat transfer to the surface. This results in higher water content in the center of food during baking. Therefore, the shelf-life of the product will be better and longer [16].
Heist and Cremer [73] studied the effect of infrared bread on the sensory qualities and energy use of cakes made from white and bleached and non-white flour and compared it to a traditional oven. Lee [74] merged between the microwave oven and the halogen lamp. Ninety percent of radiation energy within wavelength was less than 1 mµ and used as an infrared source. Two of them were used above and two at the bottom so that there was no interference between them in the microwave and this method gives more homogeneity in cooking. In this design, there were two mechanisms: Microwave heats food quickly, and infrared activates the reactions of tanning and crisping, and this method eliminates the problem of poor quality of baking using microwave [75]. The microwave has halogen lamps that emit infrared rays, which are divided into two parts, one part placed up and another down and there is a rotating base for the purpose of homogenization. The halogen lamps are 15 cm away from the material to be baked while the other halogen lamps are placed under the rotating plate ( ). The results of the experiment are that the cake size increased with increasing baking time and the color and hardness of the cake were similar to the conventional oven [76].
Open in a separate windowAghajanzadeh et al. [18] developed an infrared heating system for lime juice as shown in . It consists of an infrared heating chamber of W. The distance between the infrared source and the surface of the juice is 8.5 cm and the system is equipped with a temperature control system. In addition, the system is equipped with a sample stirring system every 15 s for uniform heating. shows that the required time to reach temperature was lower using infrared radiation compared to conventional heating. This has positive effects on the nutritional quality of the juice and reduces the energy consumption and color of the juice. When the manufacturing temperature increases, the value of D (the time required to destroy 90% of ascorbic acid) decreases [32,77]. The temperature and heating time have a significant effect on the loss of ascorbic acid from the juice. Ascorbic acid is reduced by any heat treatment, whether infrared or conventional heating, and the process of crash of ascorbic acid follows the reaction kinetics during the process of juice production with a large correlation coefficient [18]. When the manufacturing temperature increases, the value of D (the time required to destroy 90% of ascorbic acid) decreases [32,77].
Open in a separate windowOpen in a separate windowThe retaining amount of ascorbic acid was higher using infrared heating compared to conventional heating, indicating that infrared heating is more effective in keeping juice during manufacturing [18].
In recent years, infrared drying technology has been successfully applied to fruits and vegetables, such as potato drying [78,79] sweet potatoes [80], onions [81,82], and apples [7,83]. Drying of seaweed, vegetables, fish flakes, and pasta was also examined using infrared tunnel dryers [84]. Bejar et al. [27] showed that the temperature of infrared drying had no significant effect on the surface, thickness, and size of the orange peel. It does not shrink when its moisture content drops to 0.1 kg water/kg d.b. However, a very simple contraction occurs when the temperature increases from 40 to 70 °C. The thickness of shrinkage was greater at 70 °C and lower at 40 °C. The volume of shrinkage was observed to be lower at 60 °C and higher at 50 °C due to the thickness of shrinkage. Shrinkage of infrared dried orange peels was the result of the amount of moisture evaporated.
Bejar et al. [27] also studied the effect of infrared drying temperatures on the color characteristics of orange peel (L *, a *, b *, C, and ΔE). There were significant differences in the color of dried orange peel compared to fresh samples. Infrared drying had a significant effect on a and b as the values of a, b and c decreased. Temperatures 50&#;60 had a significant effect on c and there was no significant effect of temperature 70 °C. The b value has decreased rapidly at 40, 50, and 60 °C and there was no significant effect at 70 °C. The L value was increased significantly using infrared drying. The color variation was the result of the breakdown of flavonoids and carotenoids, which were responsible for orange and yellow in crusts [85]. The lowest value of ΔE is obtained at the highest temperature. Infrared processing was applied to dry two varieties of strawberries. Two factors were used to find the optimized condition of infrared drying. The infrared time of Camarosa variety was 508, 280, and 246 min, while the infrared time of festival varieties was 536, 304, and 290 min at drying temperatures of 60, 70, and 80 °C, respectively. The results showed that infrared time was totally affected by the drying temperature. The drying time of Cama-rosa variety was longer than festival variety.
An et al. [86] reported the cost of using infrared heating compared to the diesel-burning air heater for the cultivation of strawberry. The average night air temperature was 6.6 °C in the infrared heater treatment and 7.1 °C in the air heater treatment. The results revealed that the heating cost of using the air heater system was $537.35 based on 543 L tax-free diesel, while the cost of using the infrared system was $203.05 by consuming kWh of electricity. Therefore, the infrared heater system was able to save approximately 62.2% of heating costs. The cost of different heating modes was calculated and summarized that the main cost of IR drying was the radiators. This study also presented a significant relationship between the costs of different types of emitters [87].
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