Principles and advantages of infrared photography

One of the film trick photography methods, that is, a photosensitive film sensitized by cyanine dye absorbing infrared wavelength, takes infrared light as the light source. The infrared spectrum for photography is in the range of 700 ~ 1300 nm. When shooting, put a dark red to black color filter in front of the lens to filter out the ultraviolet and visible light in the sun, or illuminate the subject with an infrared light source. The photosensitive film is only exposed to infrared light and a small amount of red light, and then the positive film is obtained through general development. Because infrared rays can penetrate smoke and mist, clear scenery images can be obtained in distant view and high altitude photography, and TV is mostly used to simulate night scenes under the sun.

According to the test, the infrared ray in natural light radiation can reach more than 40%. In black-and-white photography, special filters can be used to block visible light from red to dark red, so that infrared images can be sensitized on film. For example, the "Leiden 87C" filter can block almost all visible light and produce pure infrared photography effect. This filter is almost opaque to the naked eye, so you should adjust the focal length before adding the filter, and you need to shoot to get normal exposure. Generally speaking, "Leiden 25" red mirror can be used for infrared photography and can also produce obvious infrared effect. So what is the reason for infrared photography? The brightest objects seen by human eyes, such as blue water and sky, can't reflect more infrared light, so that although imaging in ordinary black and white film is normal, the color of infrared film is dark. The reason why trees and grasslands turn pale is that chlorophyll can reflect a lot of infrared rays, thus achieving an unrealistic artistic conception. Infrared ray not only provides a special creative technique for photography, but also can take clear photos in fog and smoke due to its high transmittance. It is often used in exploration and military reconnaissance in scientific research. Using the characteristics of infrared rays penetrating the surface of the picture and pigment, we can also judge the authenticity of the famous paintings of masters. In fact, the camera also receives infrared light, but the general negative film can't sense it. Traditional infrared photography relies on the negative film dedicated to infrared light, while the CCD or CMOS used in digital cameras can sense infrared light, but in general, the infrared effect cannot be seen because the amount of visible light is much larger than infrared light (another reason is that many DCs are equipped with IR cut filters).

The function of infrared filter is to block visible light and let infrared light pass through smoothly, except infrared cut-off filter, which is used to block infrared light. After the infrared filter is installed in front of the lens, the negative or CCD can only see infrared light, which is the simple principle of infrared photography. Compared with traditional infrared photography, the advantages of digital infrared photography are that traditional infrared film is expensive, and it is troublesome to keep (it needs refrigeration) and needs to be developed in a specific place. Digital camera can see the results on the spot, which is helpful to correct the parameters and can meet the interactive application of general infrared photography without two cameras!