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 What is Infrared? |
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 Infrared has over 200 years of history |
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Nearly 20 decades, some advanced physical phenomenon can be used as the theoretical basis for modern thermal imaging technology. In 1800, Herschel used a prism to split up the sunlight, and he also directed the rays at the mercury bulb with a thermometer to test the energy represented by the rise in temperature.
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Nearly 200 years, some advanced physical phenomenon can
be used as the theoretical basis for modern thermal imaging
technology. In 1800, Herschel used a prism to split up the sunlight,
and he also directed the rays at the mercury bulb with a thermometer to test the energy
represented by the rise in temperature. Comparing to the temperature
of an unexposed thermometer, he discovered that violet light raised
the thermometer by 2℃ and the red light raised
the thermometer by 7℃, the light outside of the
red light, unseen by the naked eye, raised temperature by 2℃
even higher still, this is the experiment that discovered the
infrared light. Some people believe Herschel named this the infrared
light, but Hudson believes that at the time, it was called the
invisible rays, thermo-metrical spectrum, the ray that occasion heat,
dark heat, etc. but who dubbed the name infrared remains unknown. But
the term was widely available by the 1880’s. Brugel’s
book in 1951, separates infrared’s history into the “Dawning Period”
(1800~1880), “Exploration Period” (1881~1950), “Development Period”
(1951~today). There have been fast and colorful changes in the
development of infrared science since 1951. From 1960~1970 on ward,
infrared technology has developed applications for many different aspects, we can say this is the development
period. |
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Relationship between Thermal Radiation and Infrared |
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Objects at absolute zero (-273℃）will emit thermal energy from its surface via thermal radiation. Thermal radiation is one of the electromagnetic waves in the 0.1~100μm wavelength band, the emitted radiation is related directly with the temperature of the object, and is related indirectly with the material property of the object and the medium in the transmitting space.
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Objects at absolute zero (-273℃）will emit thermal energy from its surface via thermal radiation. Thermal radiation is one of the electromagnetic waves in the 0.1~100μm wavelength band, the emitted radiation is related directly with the temperature of the object, and is related indirectly with the material property of the object and the medium in the transmitting space. From the aspect of visualization, light can be divided into visible light and invisible light, but from the aspect of the spectrum, light can be divided from electric-wave, micro-wave, infrared-wave, visible light wave, ultraviolet wave all the way to X-ray. The visible spectrum to the human eye (about 0.2~1μm ) is a very small range within the spectrum, and the infrared in spectrum has a wavelength （0.78~1000μm）only larger than the visible light, therefore the light with the lowest frequency that is visible is the red light, the light with the highest frequency visible is violet light. This is the reason that frequencies below the red visible light is called the infrared light, the human eye cannot seen it, but it can be seen by infrared cameras.
Heat is transmitted from a heat source in three ways; conduction, convection and radiation. Infrared is one type of electromagnetic wave, Apparently the heat transmittance method is by radiation. The speed of infrared is same as light, and it can travel in a straight line directly as the light, and if using a smooth reflecting surface material that can change the infrared’s direction of transmittance.
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The separation of infrared frequency sections |
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Infrared is a type of electromagnetic wave, its wavelength is between 0.78~1000μm, its shorter waves can connect with visible light range, and the long wave overlap with the micro-wave range. In this range of wavelength that is covered by the infrared, different fields of application with its different separation in this spectrum. |
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Infrared is a type of electromagnetic wave, its wavelength is between 0.78~1000μm, its short waves can connect with visible light, and the long wave overlap with the micro-wave. In this range of wavelength that is covered by the infrared, different fields of application each has their different detailed separation. Normally, infrared can be separated by wavelength into near-infrared, mid-infrared, and far-infrared. (wavelengths are 0.78~2μm、3~5μm、7~14μm respectively) Industrial usages mostly utilize mid, and far-infrared, normally objects need to have very high temperatures before emitting visible light, for example, light bulb filaments. But all objects would only emit infrared energy in room temperature, the higher the temperature caused the energy radiated is higher. According to the law of conservation of energy, energy will not disappear, but will be transformed into other types of energy, so a part of the energy will be transformed into heat, the object needs to maintain balance, thus it emits heat, and emitting heat is a type of radiation, so in the process of radiating heat, a part of the heat forms infrared light. With the discovery of infrared light, more knowledge was gained about heating including heat transfer and their relationship with radiation.
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Infrared sensor technology |
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Sensor technology development and history |
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The development of infrared imaging technology started early, due to the key component (detector) was very expensive, had a short lifespan, or made unclear images and other limiting reasons, therefore it was limited mostly for military and aerospace applications. |
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The development of infrared imaging technology started early, due to the key component (detector) was very expensive, had a short lifespan, or made unclear images and other limiting reasons, therefore it was limited mostly in military and aerospace applications. The earliest use of infrared imaging for medical diagnostics is the U.S.A., the technology has always been kept close by the leading nation so that could not be widely utilized. When the U.S. Department of Defense firstly allowed in 1956 and applied for medical community to use infrared imaging and explain the effects of heat radiation from the human body. It was the first generation commercially used mechanical multi-prism scanning mechanism and storing infrared imaging instruments were made, and the usage gradually expanded to industrial and public use. Due to the slow speed and high amount of noise, the technology was not very widely used since 1982 to 1995. Starting from 1996 the second generation of cooled flat surface focus on FPA CCD design, but still having the problem of short lifespan of the cooling unit and bulky size, therefore the utility is also limited. At this stage, the application of infrared technology is becoming more and more recognized by each application field. Finally the room temperature FPA was born in 1997 and using Vanadium Oxide Microbolometer technology, breakthrough the need for cooling systems, and improved on the ability of Thermal Sensitivity, this is seen as the third generation product. The cost and pricing can be greatly reduced. The technology first breakthrough was to expand the area of application. In medicine, monitoring system is the application most directly benefited entity. When infrared imaging machine is paired with advanced computer technology to build a complete system, through software can simultaneously display clear image and temperature distribution diagram, allowing infrared imaging instruments to have the benefit of being faster, more precise, simpler, easier to operate, less intrusive. That is the greatest advantage in its application.
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Types of Sensors |
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Infrared sensor is categorized into two types according to its material and transmittance wavelength, one is for the photon sensor (capture infrared as photons and using electro-optics for processing), the other is heat sensor (utilizing the physical changes caused by the electromagnetic wave energy of the infrared light – mainly for heat effect).
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Infrared sensor is categorized into two types according to its material and transmittance wavelength, one is for the photon sensor (capture infrared as photons and using electro-optics for processing), the other is heat sensor (utilizing the physical changes caused by the electromagnetic wave energy of the infrared light – mainly for heat effect).
When photon sensor directly absorbed the infrared radiation, it was the effect of electrical charge. This type of sensor is very sensitive to temperature variations. Therefore it must be cooled with a coolant to maintain the high sensitivity of the sensor response. Infrared cameras using this type of sensor, usually uses a Stirling cycle unit or liquid nitrogen for cooling device, and thus is also called a non-room temperature infrared imaging detector, in the range of 3~5μm wavelength, PtSi, InSb, and MCT are often used, in the range of 8~12μm MCT and QWIP is often applied.
When heat sensors meet temperature changes, it will also absorb infrared radiation, according to different materials’ specific reaction to heat, it will produce an effect of electrical charge, this type of sensor is not as sensitive as photon sensors, but it can be operated in room temperature while maintaining a certain level of precision. Thus, in order to apply infrared imaging systems, the use of this type of sensor can avoid using coolants as cooling device. It is also called the room temperature infrared imaging sensor, usually used for wavelengths of 8~12μm, the common using sensors are the electrical resistance structure Vox Bolometer and other is the voltage sensing type called BST Pyroelectric.
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Application of infrared technology |
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From the Hsung-Fong missile to disease detection – about the infrared imaging diagnostic system |
By Chang Tien Chun |
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Infrared radiation emitted from the body surface at a wavelength between to detect body temperature, and simultaneously draw out the body temperature distribution graph.
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Because the Hsung-Fong Missile (made in Taiwan) must be able to track the heat energy from a jet fighter, so it must have a guidance system to be able to detect heat sources from very far away and shot it down in time. This type of technology now is transferred to the field of medicine, promoting the improvement in medical technology.
Body temperature is an indicator of health, so when a person is ill to hospitals or clinics, the doctor or nurse will usually do is to measure his temperature, usually orally, rectally or in the ear.
Swelling is a symptom of inflammation, in order to detect the heat produced by the localized inflammation, besides using the feel of the hand, a more objective data is necessary, normal body thermometer is unsuitable at this time. Furthermore, with congested blood vessels, like diabetic foot, the temperature will be lower value. At this time detecting localized temperature change becomes an important technique.
Moreover, for breast cancer, using traditional photography, and biopsy when necessary are the standard methods for breast cancer diagnosis. But photographing the breast is painful and for young women to repeatedly take photographs of their breasts with x-ray, inadvertently becomes a cause of cancer.
Another example is during the SARS epidemic, when entering airports, examinations are required, and body temperature detection becomes very critical, but if everyone were to line-up to have their temperatures taken by an ear thermometer, it would take very long time cost manpower and money.
The above problems were solved quickly because of utilizing military technology, and further development from the infrared thermal imaging tracking technology.
Infrared radiation emitted from the body surface at a wavelength between to detect body temperature, and simultaneously draw out the body temperature distribution graph. But it is important to note that conventional near-infrared cameras are not to be used in conjunction. Near-infrared cameras are used for seeing objects at night, at this time, the machine will emit infrared light for light compensation, after reflecting from the object, then it is photographed, it has limited range and cannot detect temperature. Whereas the infrared thermal imaging diagnostic system only detects but do not emit radiation, thus it is harmless to the human body. It is not only radiation free, but also does not require contact with the human body. It’s like a camera without a flash bulb.
Currently this type of machine is used for body temperature detection at airport terminal, it is used for SARS examination or other infectious diseases that cause fever. In diabetic patients, it can be used to detect the blood vessel blockages in the patients’ foot, or temperature changes caused by the neurological diseases. In breast cancer, because of new born blood vessels, the blood flow is enriched and thus the heat becomes higher, this is detectable by the infrared thermal imaging system. Furthermore, in arthritis, temperature changes can be observed locally on the skin; sensory-motor nerves become damaged and related skin temperature changes can all be detected and diagnosed by this type of machine.
Furthermore, besides monitoring the body surface temperature, it can be used during heart surgery to observe the coronary arteries of the heart, after making a by-pass surgery, observe the status of the heart muscle circulation to evaluate whether the surgery had been successful.
In conclusion, infrared thermal imaging diagnostic system is a diagnostic tool that is completely non-intrusive but an excellent tool for detecting imbalances in the body’s biological function. Transferred from military application to medical application, it’s like turning a weapon into a helpful tool that benefits mankind, in the same spirit that Nobel invented the dynamite.
Recommended Reading:
1. Eg EY: Is thermal scanner
losing its bite in mass screening of
fever due to SARS? Med Phys 32:93,
2005.
2. Eg EY: Computerized detection
of breast cancer with artificial
intelligence and thermograms. J Med
Eng Technol 26:152, 2002.
3. Head JF et al: Infrared imaging: making progress in fulfilling its medical promise. IEEE Eng Med Biol Mag 21:80, 2002.
Article taken from the Journal of “Modern Medicine” June 2004, No. 380 Pages 437~438. Chang Tien Chun/ National Taiwan University College of Medicine, Professor of Internal Medicine |
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