What is the principle of infrared imaging?

"If you use infrared photography to image the human body and make a' heat map' of the body surface ..." You will have this understanding:

(1) The thermal image of an object photographed by infrared is its infrared image;

(2) Visible light can't make infrared film sensitive, only infrared light can make it sensitive;

(3) the infrared film records the infrared rays emitted by the target object;

(4) Ordinary cameras can also use infrared film for infrared photography.

In fact, these understandings are all wrong. The root of misunderstanding is that the difference between infrared image made by infrared photography and thermal map made by thermal imager is not explained, and the introduction of infrared film is not accurate enough. Let's elaborate on these two issues. Please correct me if there are any mistakes.

First, the discovery and classification of infrared rays

1800, when studying the temperature of monochromatic light, the British physicist Herschel found that the temperature of the thermometer outside the red light used for comparison was higher than that in the colored light, so he said that he had found an invisible "hotline" called infrared light.

In the electromagnetic spectrum, infrared ray is located outside the red end of visible spectrum, between visible light and microwave, and its wavelength is 0.76 ~ 1000 micron, which will not cause human vision. In practical application, it is often divided into three bands: near infrared with a wavelength range of 0.76 ~ 1.5 micron; Mid-infrared, the wavelength range is 1.5 ~ 5.6 microns; Far infrared, the wavelength range is 5.6 ~1000μ m. Their mechanisms are not consistent. We know that the molecules of objects whose temperature is higher than absolute zero are constantly doing irregular thermal motion and generating thermal radiation, so objects in nature can radiate infrared rays with different frequencies, such as cameras and infrared films themselves. At room temperature, the infrared ray radiated by an object is located in the middle and far infrared spectrum region, which is easy to cause the * * * vibration of the object molecules and has obvious thermal effect. So it is also called thermal infrared. When the temperature of an object rises, the electrons in the outer layer of the atom jump, which will radiate near infrared rays. For example, high-temperature objects such as the sun and infrared lamps all contain a lot of near infrared rays. Remote sensors with different functions can be made with the help of different physical properties of infrared rays in different bands.

Second, the principle and characteristics of infrared imaging in different bands

Infrared remote sensing refers to the science and technology that records the infrared radiation, reflection and scattering information of an object without direct contact with it, and reveals the characteristics and changes of the object through analysis. The instruments that can obtain image information in infrared remote sensing technology include: cameras using infrared film, digital cameras with infrared photography function, thermal cameras and so on. Although they all work by infrared rays, the imaging principle is quite different from the physical meaning of the image. Infrared photography usually refers to photography using infrared film and digital camera; The former belongs to optical photography and the latter belongs to photoelectric photography.

1. optical photography

Infrared film is a kind of film that can sense infrared rays, including black-and-white infrared film and color infrared film. Its imaging principle is similar to ordinary film: when exposed, silver halide changes chemically, and the electromagnetic wave information reflected by the scene is recorded on the film, and the scene image is obtained by developing and fixing technology. Ordinary film records visible light with wavelength of 0.4 ~ 0.76 micron; With the addition of infrared sensitizer, infrared film can record visible light and near infrared light with wavelength of 0.4 ~ 1.35 micron. In order to obtain a pure infrared image of the scene, it is necessary to install an infrared filter in front of the lens to filter out visible light and let only near infrared light pass through. So, is this part the near infrared light emitted by the scene? Obviously, the human body, trees and other scenery in daily photography can't reach the temperature that can radiate near infrared rays, and their thermal radiation can't make the film form a clear enough image, so it should be that the scenery reflects the near infrared rays in solar radiation. Therefore, near infrared is also called photographic infrared.

The image formed by infrared film is very different from that formed by ordinary film. Human body and grassland have strong infrared reflection, so their black-and-white infrared images are whiter; Rivers and the sky reflect infrared rays weakly, and black-and-white infrared images are dark. Because the photosensitive spectrum and coupler of color infrared film are different from ordinary color film, the color in color infrared photo is not the reflection of the real color of the scene, so it is also called false color infrared film. For example, healthy green plants reflect near infrared rays, and their infrared images are red, while the infrared images of clear rivers are dark blue. Although the diseased plants and healthy plants are green to the naked eye, there is no difference in ink marks before and after document alteration, but their reflection intensity to infrared rays is different, and the infrared images formed are obviously different. Therefore, it is often used in criminal investigation, land and resources investigation, environmental protection and other fields.

The characteristics of strong infrared penetration and the susceptibility of infrared film to thermal radiation determine that shooting with infrared film requires higher operation. The infrared film has the best photosensitivity to the near infrared ray with the wavelength of 0.76 ~ 0.9 μ m. With the increase of the perceivable wavelength, the influence of temperature on the photosensitizer becomes more and more obvious, and the chemical stability of the photosensitizer also decreases. For example, infrared films with the upper limit of photosensitive wavelength of 1. 1μm can be stored for three months, and only for eight days when the upper limit of photosensitive wavelength reaches1.35 μ m. Therefore, both storage and carrying need to be refrigerated, and loading and unloading of films need to be carried out in a darkroom or a special anti-infrared dark bag. Due to the long exposure time of infrared film, there is no standard sensitivity when leaving the factory, so it is necessary to manually adjust the sensitivity according to experience, and the infrared light emitted by the infrared counter of the automatic camera can be exposed; So it's best to use a camera with a manual metal body. Attention should be paid to focusing in infrared photography. Some cameras have an infrared focusing index on the objective lens, which is marked as "R"; If there is no such mark, focus on the visible light first, and then move the lens forward about 1/250 of the focal length of the visible light.

2. Photoelectric photography

Some substances in nature will change their electrochemical properties after irradiation. For example, when the temperature rises, the resistance becomes smaller and voltage is generated. Using their physical properties, they can be made into photoelectric detectors, and the radiation energy collected by the optical system of remote sensing instruments can be converted into photoelectricity through the detectors. According to the different mechanism of electromagnetic wave and detector, it can be divided into photon detector and thermoelectric detector.

Photon detector uses the photoelectric effect of photosensitive materials to convert electromagnetic wave signals with a certain wavelength into electrical signals for output. For example, the photoelectric coupler (CCD) of some digital cameras with infrared photography function can respond to the spectrum of 0.4 ~ 1. 1 micron, and infrared filters should also be installed during infrared photography. What CCD feels is the reflection of the scene or the near infrared emitted by the camera's own infrared lamp.

Thermoelectric detectors work by using the thermal effect of target radiation on the electrical characteristics of thermistors. For example, the thermal infrared imaging device passively receives the thermal radiation of the target, through which the optical imaging system focuses on the detection element for photoelectric conversion, signal amplification and digitization, and after being processed by multimedia image technology, the temperature field of the target-thermal infrared image (thermal map, thermal image) is displayed in false color on the screen. The hue of thermal image depends on the surface temperature and emissivity of the object. It reflects the infrared radiation energy distribution of the target, but it cannot represent the real shape of the target. For example, after the plane takes off, it can get the thermal image of the plane at its original parking position. The working band of the detector is usually 3 ~ 5 microns and 8 ~14 μ m. In order to obtain sufficient sensitivity, it is necessary to cool the detector. The second generation thermoelectric detector is equipped with thermal infrared imaging device with temperature measurement function, also known as thermal imager, which is widely used in medical treatment, fire fighting, aerial remote sensing, military and other fields.

To sum up, the infrared image formed by infrared photography uses the near infrared reflected by the scene and reflects the geometric shape of the scene; The thermal map of human body made by thermal imager is an image showing the temperature distribution on the surface of human body by using the thermal radiation of human body itself. Are two different concepts. The photosensitive substance in infrared film is silver halide, and visible light can also make it photosensitive. (