infrared
Infrared (English:, referred to as IR) is an electromagnetic wave with a wavelength between microwave and visible light , with a wavelength between 760 nanometers (nm) and 1 millimeter (mm), [1] is an invisible light with a wavelength longer than red light , the corresponding frequency is about in the range of 430 THz to 300 GHz [2] . Most of the thermal radiation emitted by objects at room temperature is in this band. Infrared was first described by William Herschel in 1800 . The earth's absorption and emission of infrared radiation has an impact on the climate . Nowadays, infrared rays are also used in different technological fields.
Infrared photo of a dog
False-color infrared telescopic image, where blue, green, and red correspond to wavelengths of 3.4, 4.6, and 12 µm .
Discovery and Features
Infrared was discovered in 1800 by the astronomer William Herschel, who, by placing a thermometer outside the red region of the solar spectrum and finding that the temperature rose, pointed out that there is a radiation with a frequency lower than red light: invisible to the naked eye, but still It can increase the temperature of the surface of the irradiated object. More than half of the energy that the earth receives from the sun is in the form of infrared absorption. The balance of Earth's absorption and emission of infrared radiation has a critical impact on its climate .
Infrared light is absorbed or emitted when molecules change their rotational or vibratory motion. From the energy of infrared rays, the vibrational modes of molecules and the changes of their dipole moments can be found . Therefore, infrared rays are the ideal frequency range when studying molecular symmetry and energy states. Infrared spectroscopy studies the absorption and emission of photons in the infrared range [3] .
relationship with light
Light is a radiated electromagnetic wave with wavelengths ranging from 300 nm ( ultraviolet ) to 14,000 nm ( far infrared ). However, in terms of human experience, "light domain" usually refers to the light wave domain visible to the naked eye, that is, the range from 400nm (purple) to 700nm (red) that can be perceived by human eyes, and is generally called "visible light domain". "(Visible). Due to the development of modern science and technology, human beings use various "medium" (sensors of special materials) to expand the range of perception from the "visible light" part to both ends, the lowest can reach 0.08~0.1nm (X-ray, 0.8~1Å ) , Up to 10,000nm (far infrared, thermal imaging range).
Infrared rays in nature
The equivalent temperature of sunlight is 5,780 Kelvin, and half of the spectrum of its thermal radiation is infrared. At sea level, the irradiance of sunlight is 1 kilowatt per square meter . Among them, 527 watts of energy are infrared rays, 445 watts are visible light, and 32 watts are ultraviolet rays [4] .
At the surface of the Earth, whose temperature is much lower than that of the sun, almost all thermal radiation consists of different frequencies of infrared rays. Of these natural sources of heat radiation, only lightning and fire are hot enough to produce some visible light, and fire produces more infrared than visible light.
Infrared rays in different fields
Objects often radiate infrared light across different wavelengths, but detectors are usually designed to only receive radiation within the specific spectral width of interest. As a result, infrared light is usually divided into smaller segments of different wavelengths.
general classification
The classification of general users is [5] :
- Near-infrared (NIR, IR-A DIN ): The wavelength is between 0.75-1.4 microns , defined by the absorption of water. Due to the low attenuation rate in silica glass, it is usually used in optical fiber communication. Wavelengths in this region are very sensitive to image enhancement. For example, night vision devices such as night vision goggles are included.
- Short Wavelength Infrared (SWIR, IR-B DIN ): 1.4-3 microns, with a significant increase in water absorption at 1,450 nm. 1,530 to 1,560 nm is the main spectral region that dominates long-distance communications.
- Mid-wavelength infrared (MWIR, IR-C DIN ) is also called mid-infrared: the wavelength is 3-8 microns. Passive infrared heat-seeking missile technology is designed to use the atmospheric window in the 3-5 micron band to work, and the homing of the infrared marking of the aircraft is usually aimed at the plume emitted by the aircraft engine.
- Long wavelength infrared (LWIR, IR-C DIN ): 8-15 microns. This is the area of "thermal imaging", where sensors at this wavelength do not require additional light or external heat sources such as the sun, moon or infrared lights to obtain a complete passive image of heat emissions. Forward-Looking Infrared (FLIR) systems use this region of the spectrum, which is sometimes classified as "far infrared."
- Far Infrared (FIR): 50-1,000 microns (see Far Infrared Laser).
NIR and SWIR are sometimes referred to as "reflected infrared", while MWIR and LWIR are sometimes referred to as "thermal infrared", this is based on the properties of the blackbody radiation curve, typical of 'hot' objects, like exhaust pipes, as well Objects are usually brighter in the MW band than in the LW band.
Classification system of the International Commission on Illumination
The International Commission on Illumination recommends that infrared rays be divided into the following three categories [6] :
- Infrared-A (IR-A): 700 nm - 1,400 nm (0.7 microns - 1.4 microns)
- Infrared-B (IR-B): 1,400 nm - 3,000 nm (1.4 microns - 3 microns)
- Infrared-C (IR-C): 3,000 nm - 1 mm (3 microns - 1,000 microns)
ISO 20473 classification
The classification of ISO 20473 is as follows:
| name | abbreviation | wavelength |
|---|---|---|
| near infrared | NIR | 0.78-3 microns |
| mid-infrared | MIR | 3-50 microns |
| far infrared | FIR | 50 – 1,000 microns |
Astronomical Classification Scheme
Astronomers usually distinguish the infrared range by the following bands [7] :
| name | abbreviation | wavelength |
|---|---|---|
| near infrared | NIR | (0.7-1) to 5 microns |
| mid-infrared | MIR | 5 to (25-40) microns |
| far infrared | FIR | (25-40) to (200-350) microns |
This classification is imprecise and dependent on the unit being issued. These three areas are used to observe different temperature ranges and spaces under different environments.
Sensor Response Classification Scheme
Atmospheric penetration map of the partial infrared region.
It can be classified according to the detectable range of different sensors [8] :
- Near Infrared: The wavelength range is 0.7 to 1.0 µm (from the range that cannot be detected by the human eye to the range that silicon can respond to)
- Short-wave infrared: the wavelength range is 1.0 to 3 µm (from the cut-off frequency of silicon to the cut-off frequency of the atmospheric infrared window), InGaAs can range to 1.8 µm, and some less sensitive lead salts can also detect this range.
- Mid-Wave Infrared: Wavelength range from 3 to 5 µm (defined by the atmospheric infrared window, which is also the range covered by InSb, HgCdTe, and sometimes PbSe)
- Long wave infrared: wavelength range of 8 to 12 or 7 to 14 µm (the range covered by HgCdTe and microbolometers)
- Far Infrared (VLWIR): The wavelength range is 12 to 30 µm, which is the range covered by doped silicon
Near-infrared rays are closest to the wavelength range that can be seen by the human eye , while mid-wave infrared rays and long-wave infrared rays are gradually farther away from the visible spectrum. Other definitions will be based on different physical mechanisms (frequency or frequency band of maximum emission, whether it will be absorbed by water, etc.), the latest definition is based on new technology (common silicon detectors can sense below 1,050 nm, and InGaAs can be sensed in the range of 950 nm to 1,7002,600 nm.
Depending on the reference standard, the shortest wavelength of infrared light is approximately between 700 nm and 800 nm, but there is no clearly defined boundary between visible light and infrared light. The human eye is less sensitive to light with a wavelength above 700 nm, so if a light source of normal intensity emits light with a longer wavelength, the human eye cannot see it. However, with some high-intensity near-infrared light sources (such as infrared lasers , infrared LEDs, or sunlight after removing visible light), infrared rays around 780 nm can be detected and will be regarded as red light. An infrared light source with a higher intensity can detect infrared rays with a wavelength of 1050 nm by the human eye, and it will be regarded as a dark red beam. Therefore, it will cause the problem that near-infrared rays can be seen with the human eye when the surroundings are completely dark (generally, indirect lighting is used to improve this problem). The leaves will be extraordinarily bright under the near-field outside line. If you use an infrared filter to filter out visible light, and allow your eyes to adapt to the particularly dark image after passing the infrared filter for a period of time, the human eye may be able to see the glowing under the infrared. Leaves, that is, the Robert Wood effect [9] .
The discovery of infrared rays
In 1666, Newton discovered the spectrum and measured that 3,900 angstroms to 7,600 angstroms (400nm to 700nm) are the wavelengths of visible light. On April 24, 1800, William Herschel of the Royal Society of London announced that sunlight has an invisible extended spectrum besides the red light of the visible spectrum , which has a thermal effect. The method he used was very simple. He used a thermometer to measure the temperature of the various colors of light after the spectrophotometry , from purple to red, and found that the temperature gradually increased. However, when the thermometer was placed in a part other than the red light, the temperature continued to rise, so Determine the presence of infrared rays. The same test was done on the ultraviolet part, but the temperature did not increase. Ultraviolet light was discovered by RITTER in 1801 with silver chloride sensitizer.
The near-infrared wavelengths that the film can sense are twice the wavelength of light that can be seen by the naked eye. The upper limit of the wavelength that can be recorded with the film is 13,500 angstroms . If other special equipment is added, the maximum can reach 20,000 angstroms , and then go up It must be detected by physical instruments.
Infrared Radiation Source Distinguishing
Infrared radiation sources can be divided into four parts:
- Actinic range: or "photochemical reaction zone", the rays produced by incandescent objects range from the visible light range to the infrared range. Such as light bulbs (tungsten filament lamps, TUNGSTEN FILAMENT LAMP), the sun .
- Hot-object range: heat rays generated by non-incandescent objects, such as electric irons and other electric heaters, with an average temperature of about 400°C.
- Calorific range Heat rays generated by boiling hot water or hot steam pipes. The average temperature is lower than 200°C, and this area is also called "Non-actinic".
- Warm range: heat rays produced by human body , animals or geothermal heat , with an average temperature of about 40°C.
- From the point of view of photography and photography technology, look at the photosensitive characteristics: the energy of light waves and the sensitivity of photosensitive materials are the most important factors that cause photosensitivity. The longer the wavelength , the weaker the energy, that is, the energy of infrared rays is lower than that of visible light and lower than that of ultraviolet rays . But another problem that high-energy waves have to face is: the higher the energy, the stronger the penetrating power , and it is impossible to form reflected waves to make the photosensitive material capture images, such as X-rays , which must be taken behind the illuminated object. Therefore, photography must develop in the direction of long wavelengths - "near-infrared rays". With the development of chemical and electronic technology, near-infrared photography with the goal of imaging has evolved into the following three directions:
- Near-infrared negative film: The main sensing range is near-infrared rays with a wavelength of 700nm to 900nm. The emulsion with special dyes is used to produce photochemical reactions, so that the light changes in this wave range are converted into chemical changes to form images .
- Near-infrared electronic photosensitive material: The main sensing range is near-infrared with a wavelength of 700nm to 2,000nm. It uses silicon- based compound crystals to generate photoelectric reactions and form electronic images.
- Mid- and far-infrared thermal image sensing materials: The main sensing range is mid-infrared and far-infrared rays with a wavelength of 3,000nm to 14,000nm, and electronic images are formed by using special sensors and cooling technologies.
Infrared and the Greenhouse Effect
Articles: Greenhouse effect and Greenhouse gases
Simplified diagram of the greenhouse effect
Visible light and radiation emitted by the sun are absorbed by the earth's surface and clouds , and then emitted into the atmosphere in the form of infrared rays. Specific substances in the atmosphere (such as water droplets and water vapor in clouds, as well as carbon dioxide , methane , nitrogen monoxide , sulfur hexafluoride , and chlorofluorocarbons [10] ) absorb infrared rays and emit them back to the earth. The greenhouse effect can increase the temperature of the atmosphere and the surface of the earth [11] .
Topics and applications covered include blackbody radiation , solar cells , and infrared communication techniques .
technology application
Infrared light can be used in military, industrial, scientific and medical applications. Infrared night vision devices use real-time near-infrared images to observe people or animals at night without being detected. Infrared astronomy uses telescopes with sensors to penetrate stardust in space (such as molecular clouds ), detect stars such as planets , and detect redshifted stars left over from the early universe [12] . Infrared thermal imaging cameras can detect heat loss from insulation systems, watch for changes in blood flow in the skin, and overheating of electronic equipment. The ability of infrared rays to penetrate clouds and fog is stronger than visible light. For example, infrared guidance is commonly used in missile navigation, thermal imagers and night vision goggles can be used in different applications. Infrared astronomy and far-infrared astronomy can apply infrared technology in astronomy .