What's with infrared photography?

People usually call the visible part of the spectrum light, while the invisible part is called "line". Infrared ray is generated by heating an object, and the wavelength of visible light is 0.38~0.78 micron, while the wavelength of infrared ray is 0.78~ 1000 micron. Because the wavelength of infrared light wave is longer than that of visible light (the figure below shows the position of visible light in the whole spectrum), it can help us see things that can't be observed by the naked eye.

In the AF function of the camera, the design of active focusing adopts infrared (another automatic ranging commonly used in SLR cameras is called passive focusing). The principle of infrared ranging is that when we press the shutter half-way, the infrared emitted by the camera is projected on the subject and then reflected back. Different infrared receiving points on the camera generate different voltages according to the distance of the target, and then the AF receiver converts the voltages into electrical signals to drive the lens to adjust the focal length.

Infrared ray has been widely used in scientific research, military and medical treatment. Because infrared ray has certain penetrability, infrared photography is beneficial to the research of scientific research projects, especially celestial photography in astronomy. As shown in the figure below, scientists use infrared photography technology to take photos of Jupiter satellites, which can help scientists get research conclusions through infrared development.

In medicine, far infrared rays, that is, infrared rays with a wavelength of 4- 1000 mm, are used in the treatment and adjuvant treatment of various diseases. Far infrared rays can be quickly absorbed by human body. After penetrating human body, it will cause atoms and molecules of human cells to vibrate, forming a thermal reaction, promoting the temperature rise of deep subcutaneous layer, dilating capillaries and promoting blood circulation. Therefore, far infrared rays can improve and prevent many diseases caused by blood circulation and microcirculation disorders. The most direct application is to receive infrared lamp radiation, which is beneficial to the healing of postoperative wounds and the treatment of rheumatism and other diseases.

Militarily, the role of infrared ray in geographical exploration is obvious, and at the same time, infrared ray also shows its talents in criminal investigation. Because of its strong penetrating power, the materials that can be penetrated by infrared rays are: thin chemical fiber fabric, ordinary paint pigment, non-insulated car film, non-insulated colored glass, water vapor, fog, silicon chip and resin material. Using infrared light and infrared sensitive materials, the criminal investigation department can see the altered handwriting through the ink, with a 74% probability of detecting the handwriting before deletion, with a 40% probability of detecting the handwriting after smearing, and can also find blood on clothes that are difficult to find, so that criminals can secretly track and monitor photography at night, and there is nowhere to hide.

In addition, infrared rays are widely used in our lives. For example, infrared thermometer was widely used during SARS. In addition, infrared identification system is also widely used in the current bank security system and personal computer file security system, such as fingerprint identification and personal information identification.