原创 InfraRed

http://encyclopedia.jrank.org/articles/pages/1148/Infrared-Photography.html

ANDY FINNEY

Atchison Topeka and Santa Fe Limited

Light that is detectable by the human visual system occupies a tiny part of the electromagnetic spectrum, which extends from the long wavelengths of low-energy radio radiation at one end to very short wavelength and high-energy gamma rays at the other. The visible spectrum has a range of wavelengths from 400 to 700nm. Infrared radiation lies between visible red light and microwave radio waves and, for the purposes of this article, is divided into the near-infrared (NIR; from 700-1200nm, 0.7-1.2 μm) and the far-infrared (FIR) from about 2-30 μm. This also includes the region sometimes described as the mid-infrared.

The existence of radiation outside the visible spectrum was first discovered in 1800 by Sir William Herschel. He was experimenting with thermometers to measure the temperature of the bands of colored light produced by a prism, and placed thermometers outside either end of the visible spectrum as controls. To his surprise, the thermometer beyond the red end of the spectrum warmed up more than the rest, and he realized that there was radiation there that we could not see.

NIR is very similar to “ordinary” light in that it can be focused by normal lenses and recorded on special films or captured by imaging chips. The most common sources of NIR are those that also emit visible light and might include the sun, incandescent lamps, and flash guns. The making of images with NIR and traditional camera equipment can correctly be referred to as photography.

FIR is the radiated heat of an object detected by special electronic detectors and is sensed by its effect on the skin. FIR sources are usually the objects themselves, and images are made by recording the emitted thermal radiation. FIR imaging is thus known as thermal imaging or thermography and is described later.

Expressions such as white-hot and red-hot have real meanings in science, although these expressions are often used in every day language. The hotter an object is the more energy at all wavelengths each unit area of its surface radiates. An object (such as a human body) with a temperature of 310K (about 40°C) will be radiating at wavelengths of around 8000 to 9000 nm (8-9 μm) in the FIR. A soldering iron can reach a temperature of 1000K (about 700°C) and will be radiating strongly at 2 μm and weakly in the red part of the spectrum, so will glow a dull red. By contrast the sun has a surface temperature of about 5500 K (about 5200°C) and appears white. Physicists use an idealized concept known as a “black body” in order to describe how the wavelength distribution of emitted radiation varies with temperature. In the visible part of the spectrum the relationship between temperature and radiated light gives us the idea of color-temperature.

The peak wavelength of energy radiated by an object is given by a simple empirical formula known as Wein’s Law: 2898 μm/T where This in Kelvins. This wavelength is governed only by temperature: The nature of the object has no effect.

Near-Infrared: Light We Cannot See

The boundary between visible red light and infrared (NIR) is approximately at 700 nm wavelength but is not a well-defined location. Between 700 nm and about 1200 nm, NIR can be photographed with photographic film or captured using a charge-coupled device (CCD) or CMOS sensor of the kind used in digital cameras and camcorders. In these situations NIR generally behaves like light. In particular it passes through glass and is reflected and refracted by conventional optics, but some image differences will be evident as a result of imaging with the longer wavelengths.

Blue and red visible light will behave differently in a glass lens, but the effects are often overlooked because they are minimal. Blue and red light will be refracted differently and lens designers must compensate to avoid chromatic aberration. Additionally, the color energy will be scattered to different degrees as the energy passes through the atmosphere. Rayleigh (small particle) scattering is why the sky is blue and distant mountains are hazy. However, the large particles of dust storms, industrial pollution, and the water vapor in clouds are opaque to NIR. If we assume that whatever happens with red light (as opposed to blue) will be more pronounced with NIR (and extreme with FIR), it is possible to deduce some of the characteristics of an infrared photograph.

The lens focus when using NIR will be different than when photographing visible light. Since IR energy is refracted differently, a scene focused at infinity will have to be focused as if it were closer. On many lenses originally used with film cameras, there was often a red mark on the barrel to indicate the corrected infrared focus. This location may be different for the wide-angle and telephoto ends of a zoom lens. Using a small aperture to increase the depth of field will reduce the focus error potential. Autofocus systems used on many digital cameras that sense the sharpness of the image as seen by the sensor may compensate automatically for focus error.

Virtually no NIR is scattered by the atmosphere when the atmosphere is clear, so a clear sky appears black or dark in a monochrome infrared photograph. As a result there is less scattered light reaching the shadows so they also appear darker. Bodies of water under an open sky will similarly look dark because there is no sky to reflect.

The effect of natural atmospheric haze is reduced or removed. The longer the wavelength, the less the scattering effect, so at NIR wavelengths the haze will often disappear. This is a mixed benefit for landscape photography as haze provides depth cues in an image. Haze particles in the air measure less than 1000 nm (1 μm) and are of the same order of size as NIR wavelengths. Fog, clouds, and dust, on the other hand, have particles ten times larger and so remain opaque until the imaging wavelengths approach the size of the particles, when it is possible to “see” through them.

In visualizing how a scene might look in an infrared photograph, it is helpful to think of NIR as an invisible color since color is an indication of which wavelengths of light an object absorbs, scatters, or reflects. This means that something that is dark in visible light may become light when viewed in NIR; in particular, artificial fibers and dyes used in clothing can reflect NIR strongly. Infrared photographs are popular at weddings, and the photos often show up the lapels and piping on the groom’s dark tuxedo quite dramatically. In addition, NIR penetrates a few millimeters into skin, often giving a milky and flattering appearance to portraits. There have even been embarrassing instances of flimsy clothing being transparent in the NIR. Some visually opaque substances, such as certain woods and plastics, also transmit some NIR, so photographic darkslides and processing tanks have been known to allow fogging of infrared film.

The high reflectance from green plants between 700 and 1300 nm results in green foliage appearing white in an infrared photograph, particularly when the leaves are in direct sunlight. The effect is so distinctive that NIR images are often mistaken for snowy scenes. This is because NIR is strongly scattered by the cell walls of leaves in the way that visible light is scattered by small crystals of transparent water-ice. However, some of the light passes through the leaves, so there is more NIR under a canopy of foliage than green light, which is mostly reflected. Chlorophyll is transparent to NIR and plays no part in this process: Any infrared fluorescence is of such low intensity as to have no photographic effect.

The white foliage phenomenon (see Figure 48) is known as the Wood Effect, named for Professor Robert Williams Wood. He was the first person to publish infrared (and ultraviolet) photographs and discussed both in his benchmark presentation to the Royal Photographic Society in London in 1910.

Shooting the Invisible

Both film and electronic cameras can be used for NIR photography, and the light sources involved are much the same as for conventional photography. The sun, flash guns, or tungsten lights work well, but some visible light sources, such as television screens, fluorescent lights, and narrowband lighting used for streets and buildings, produce little or no NIR.

Digital still and video cameras have either CCD or CMOS imaging sensor chips that are as sensitive to NIR as to visible light (see Figure 50). Ultraviolet exposure may be evidenced as a blue cast to images shot at high altitudes; NIR interferes with correct color reproduction. In the case of UV contamination, a UV-blocking filter is used over the lens. To filter out the NIR, an infrared-blocking filter, also known as a hot mirror, is usually built into the camera or its sensor.

Using a digital camera to make infrared photographs requires either removing the IR blocking filter or taking long exposures in bright sunlight. In either of these techniques visible light is removed by using a visually opaque filter that passes NIR. It should be noted that removing the blocking filter can be difficult and not always possible, since it is often an integral part of the imaging chip assembly. In some of the early digital camera models, the IR cut-off filter was coupled to an anti-aliasing filter. Similarly, early generation digital cameras are also susceptible to noise and other artifacts when taking long exposures.

Digital photographs taken through an infrared filter can appear to be colored, even though NIR is essentially monochromatic. The colors range from subtle to bizarre depending on the imaging chip used (see Figure 50). They are due to the different NIR responses of the color filtering and processing in the red, green, and blue channels of the camera and are not
necessarily representative of anything in the scene itself. The digital result is completely different to the result produced by color-infrared film, which is described below. A digital infrared photograph is likely to need some adjustment in a computer, especially to sort out black and white levels and compensate for any unwanted coloration.

One key benefit of using a digital camera to shoot NIR images is that the results can be seen immediately. If it is possible to see “live” images using the camera’s screen or electronic viewfinder then the infrared effects can be seen as the shot is composed. However, the opaque filter makes it impossible to see anything through the viewfinder of an SLR, and in this case the shot must be composed before the infrared filter is fitted.

Infrared Film

Infrared photographs can be taken using film with most 35 mm cameras if the lens can be focused manually. There are a few cameras that use infrared sensors for film loading and transport. These sensors can fog the film and so should be avoided. Possible leakage of infrared through seals, bellows, the film edges, and even some plastics used in camera bodies
should also be checked. An emulsion specific to infrared photography has been made by several manufacturers since the 1930s, but currently only Kodak in the United States and Hans O Mahn and Co. in Germany continue to produce IR films.

Kodak’s high-speed infrared film, known as HIE, is a monochrome negative film and is notable for having its own idiosyncrasies that add to those of NIR itself. There is no anti-halation layer in this film, consequently, the highlights in the print will have a distinctive “glow.” This is an artifact of the film and is accentuated in the NIR because silver halide emulsions are quite transparent at these wavelengths. It is also possible for patterns caused by reflections of IR from the camera’s film pressure plate to be recorded in the film. The lack of an anti-halation layer also means that light can enter the film through an outside edge of the film and then be piped through the substrate and fog the whole roll even through the tail of film juts out from a “daylight-loading” film cassette. HIE must always be loaded and unloaded in complete darkness. The Kodak film is panchromatic, with a “dip” around 500nm and an NIR sensitivity extending to just under 900nm. It is also grainy, but the halation and grain are often considered to be part of the character of infrared photographs taken with this material.

The Mahn film (branded Maco), is also a black and white, panchromatic negative film and has been available both with and without an anti-halation layer. The NIR sensitivity of the film is slightly narrower than the Kodak and its spectral sensitivity curve is relatively flat.

These films are sensitive to visible and ultraviolet light, so a filter must be used to make an infrared image. A visually opaque NIR-pass filter produces the strongest NIR effects, but a deep red filter is almost as effective and allows composition of the shot through the viewfinder of an SLR. Alternatively a
twin-lens or range-finder camera can be used or an infrared filter can be mounted between the film rails inside the camera, in which case the viewfinder is unobstructed.

Color infrared film is a false-color material made exclusively by Kodak as Ektachrome Professional Infrared (EIR) reversal film. When the film was introduced in 1943 it was intended for camouflage detection and other military use. The way the color dyes are coupled to the sensitized layers results in a shift of colors such that near-infrared in the scene is reproduced as red, red appears as green, and green as blue. Blue and UV are removed from the scene by shooting through a yellow filter as all layers are blue- and UV-sensitive. The resulting images feature red or magenta foliage and deep blue skies (see Figure 51). EIR is less prone to the light-piping effect of HIE so it can be loaded in subdued light.

The NIR sensitivity film deteriorates relatively quickly at room temperature and shelf life is greatly prolonged if the films are stored in a fridge or freezer. They should be allowed to warm to room temperature before use to avoid condensation.

The ratio of NIR to visible light varies with the time of day, the light source, and the filtration among other factors, so a normal exposure meter provides only a rough guide. Information on filtration, exposure, and development of these films can be found in the manufacturer’s data sheets; however, it is important to avoid any sources of infrared light during handling and processing. Leakage through the walls of some plastic development tanks and infrared sensors used in commercial processors have caused problems.

Some Applications of NIR

Art

It is possible to use infrared photography to great artistic effect looking for the qualities that are unique to photographing in this spectrum. The glowing foliage and black skies can add drama to landscape scenes and the unusual appearance of skin and eyes in infrared makes for novel portraits. Prints of infrared landscapes also respond well to hand-coloring. Color infrared film is less used artistically but can produce startling results with appropriate subjects.

Art restoration

The ability of NIR to penetrate more deeply through some materials—such as vellum—and the different reflectance characteristics of inks and pigments give infrared imaging a place in the diagnostic stages of art restoration. NIR photography has been used to see through the back of works of art when the base material is thin enough and transparent at NIR wavelengths. Longer NIR wavelengths, for example, 2-3 μm, can be used to reveal even more information in works of art by “seeing” further through the paint materials from the front. In this way, many over-painted preliminary drawings and other hitherto hidden works have been discovered. This technique is called infrared reflectography. NIR is non-invasive and, by definition, is a lower energy radiation than visible light. This naturally adds to its suitability for use with rare and delicate works of art.

Agriculture

The relative amounts of green and NIR reflected by foliage are a sensitive indicator of its condition. This is best seen using false-color infrared photography where a shift from red toward magenta in the image indicates that the plants are under some kind of stress from factors such as disease, infestation, or lack of water which, in turn, can also be an indicator of localized external factors, such as obstructions just below the surface or changes in the water table.

Remote sensing using NIR is particularly useful in forestry. The foliage of different kinds of trees has different infrared reflectivity, but not enough to differentiate species. In general, hardwoods look darker in black and white infrared than softwoods, and there is some infrared darkening of conifers as they age even though their visible appearance remains the same. This has the artistic effect of giving mixed woodland a more varied tonal range than is seen in visible light, especially if hardwood and softwoods are mixed together.

Astronomy

In this most remote of remote sensing applications, infrared imaging is invaluable. The better-resourced professionals tend to use FIR wavelengths, where the ability to peer through fine dust to see the first stages of star formation is of enormous value. However, at great distances, the expansion of the universe shifts visible light and even UV light into the infrared, so IR detectors are essential for studying the most distant objects. FIR observations are made from earth-orbiting satellites to avoid atmospheric absorption. Amateur astronomers also use the infrared for imagery, and this field is becoming increasingly popular.

Cinematography

In the heyday of black and white motion pictures, infrared film was used to simulate night-time shooting during the day, known as “day-for-night.”. It was also one of the ways that a traveling matte could be produced so that the background of a scene could be changed. Neither of these techniques is used today.

Photography in the dark

NIR can be used to take photographs in total darkness. Normally the light source (flash or lamps) is filtered to remove any visible component and will not be seen unless the subject looks directly at it. By this means images have been taken of audiences in cinemas, to study their reactions, and to record people sleeping. Infrared cameras are also present in the bedrooms of the Big Brother television program and are used for animal behavior studies as well as wildlife television. NIR is widely used for surveillance, where a combination of infrared floodlights and infrared video cameras allows security personnel to observe activity around buildings at night without needing to illuminate them with visible light. Some industrial processes that have to take place in darkness, such as photographic development, can be observed in this way.

Forensics

NIR will often show up features that are invisible to the naked eye. An example of this is modification to a document as a result of fraud or forgery. This might be detectable using reflective infrared photography or by changes in infrared fluorescence, which can also be photographed. Similarly, faded or writing on charred paper may become visible using NIR techniques. Some inks are transparent to NIR and this can provide a way of examining otherwise obliterated portions of documents.

Medicine

Since NIR penetrates a few millimeters into the skin and may be reflected differently by skin pigmentation, thus it can be used as a diagnostic aid. Patterns of blood vessels near the surface can be seen. Infrared photos have proved useful in showing changes in blood vessels caused by a variety of conditions including breast cancer, cirrhosis of the liver, and varicose veins. NIR can pass through eye defects such as some cataracts and cloudy corneas. Because the pupil does not react to NIR, studies of pupil dilation in very low light can be carried out.

Zoology and botany

The penetrative powers of NIR can aid photomicrography of specimens such as small insects by allowing a view through the insect’s chitin, or hard external skeleton. On a larger scale, some insects and even larger animals that are difficult to see because they blend into their surroundings can become easily visible using NIR.

Thermal imaging

The essential thing about thermal imaging in the FIR is that objects are detected by means of the radiation that they emit because they are warm. FIR images of a scene may share some features with a visible or NIR image but this is superficial. The way in which objects—solids, liquids, dust, and gas—absorb, reflect, refract, or scatter visible light has little influence on the formation of a thermal image, so thermal infrared images do not look like normal photographs. However, many regions of the FIR spectrum are strongly absorbed by the atmosphere and are affected by humidity, so the full FIR range is only accessible from space.

All objects that are warmer than absolute zero (0K, -273 °C) emit some radiation because of the thermal motion of their constituent atoms. The warmer an object is, the more vigorous the thermal vibration and the more radiation is emitted, over an increasing range of wavelengths. As the temperature increases it will eventually extend into the visible region, where it is first seen as dull red (red-hot). Long before red heat is reached, warm objects emit radiation in the FIR part of the spectrum which extends from about two to over 30 μm (microns, millionths of a meter).

Thermal imaging in the FIR thus detects and displays patterns of heat radiated by objects and the environment. This means that if an object is at a different temperature when compared to its surroundings it can be “observed” and its temperature measured by means of its radiated energy. In general, the cooler an object, the longer the wavelength it emits, and the more complex is the equipment required to acquire an image. This becomes obvious when it is realized that the imaging optics and camera itself may be warm enough to be FIR radiators themselves.

For all these reasons a good deal of FIR imaging is confined to specialist technical applications, where the complexities of cooling an imaging system with liquid nitrogen or even liquid helium are acceptable. However, not all FIR imaging requires cryogenic cooling and many manufacturers offer cameras for less demanding applications. Many commercial uncooled IR sensors operate in the 8 to 12 μm range, where source energy is high and atmospheric absorption is low. With a strong signal, it is possible to discriminate quite small temperature differences, such as heat leaks from buildings and changes in skin temperature due to poor blood circulation. These imagers are sturdy and versatile enough to be used by firefighters searching for people who are alive but unconscious in smoke-filled buildings, and to detect intruders at night. Even a hand placed briefly on a surface will leave a heat trace, briefly visible as a hand-print in the FIR.

Thermal imaging cameras are still expensive items but their price is falling. However, all of them capture an essentially monochrome image. The camera displays can be in false color or black and white, and many models allow the user to choose the mode of display. In false-color modes the temperature is represented by color, often blue for “cold” and red for “hot” (see Figure 53). In black and white mode, the brightness is proportional to temperature. Color modes are often used for industrial survey and medical applications, while black and white is used for surveillance. Since the imaging wavelengths are relatively long, and the dynamic range often truncated, it is not possible to achieve the imaging resolution possible in visible or NIR images. The images generated by thermal cameras are usually of standard definition television quality or less.

Some Thermal Applications

Astronomy

Beyond NIR wavelengths astronomical imaging is mostly done from above the atmosphere due to atmospheric absorption. Astronomers usually refer to NIR as extending to 5000 nm (5 μm) and refer to the band from 5 μm to 25/40 μm as mid-infrared with far-infrared extending from here to 200/350 μm. The split between these bands derives from the kinds of detectors required.

At a wavelength of 2 μm, particles of dust that might obscure the center of our galaxy become transparent. As the temperature and energy of the objects of interest reduces imaging moves to longer wavelengths, so that objects as cold as 140 K are visible and the stars seem to be absent. However, the earliest stages of star formation, occurring in cold, opaque molecular clouds, can be seen, as well as the central regions of galaxies such as the Milky Way, normally shrouded in dust.

Preventative maintenance

It is a common misconception that NIR photographs can be used to show how well a building is insulated; however, this is a common application for FIR and thermal imaging, where areas of heat leakage are easily detected. FIR sensors also can remotely monitor the temperature of components in machinery to detect friction or electrical overheating, and a thermal image of industrial plant can allow an engineer to detect potential trouble spots at a glance.

Medical

One medical application of FIR is as a remote-sensing thermometer. Thermal imaging cameras are marketed specifically for this purpose, and one application is to quickly scan arriving international passengers for an elevated head temperature, which might indicate an infectious illness such as SARS or Avian Flu. Thermal imaging can similarly show patterns of temperature on the surface of human and animal bodies. This is a painless and non-invasive technique that is useful for showing patterns of blood flow, such as changes due to disease and subtle changes in skin temperature caused by tumors.

Night vision/remote surveillance

The human body radiates strongly at a wavelength around 9.3 (μm, corresponding to a temperature of 37°C (310K), whereas open ground at, say, 15°C (288 K) will radiate at 10 (μm. Thus it is possible to differentiate between body heat and the ambient temperature, particularly at night. Law enforcement and military personnel use FIR cameras mounted on helicopters to follow action on the ground when visible light cannot be used. It is also easy to detect recently used automobiles by the FIR glow from a still-warm engine and hot spots in forest or building fires that are otherwise masked by smoke.

Many remote applications involve detecting small temperature differences. This aids in searching for people lost in landscapes, the sea, or trapped under buildings. The body heat will be detectable at a distance and a hidden body may warm the local environment enough for detection if the camera has sufficient temperature resolution. Work has been done in using FIR to detect clear-air turbulence or volcanic ash as an aid to aircraft safety. Vulcanologists can use FIR to map lava flows and temperature changes on the ground that result from hidden volcanic activity. Of course, lava temperatures are such that it often registers in NIR and even visible light images.