The infrared image tube is the core of the active infrared night vision device. It is a high-vacuum image conversion device. Its function is to complete the conversion from near-infrared images to visible light images and to enhance the image. Infrared image tube includes four parts: photocathode, electron optical system, phosphor screen and high vacuum tube shell.

(1) The classification of infrared imaging tubes is divided into structural materials. Infrared imaging tubes are divided into metal structure type and glass structure type. According to the working method, they are further divided into continuous working mode and gated working mode. The gated image tube is mainly used for gated imaging and ranging.

(2) The working process of the infrared camera tube is composed of three basic parts, namely, the silver oxide cesium (Ag-0-Cs) photocathode, the electronic optical system and the phosphor screen. The near-infrared radiation image is imaged on the photocathode surface, and the photocathode produces electron emission proportional to the intensity of human radiation at each point to form an electronic image corresponding to the electron current density and the intensity of infrared radiation; the electronic image is imaged on the fluorescent screen by the electron optical system ; Under the bombardment of high-energy electrons, the fluorescent screen emits a visible light image proportional to the electron density; thus completing the conversion process from near-infrared to visible radiation images.

 

The photocathode of the infrared imaging tube is a silver oxide cesium photosensitive layer sensitive to the near infrared. The spectral response range is 0.3~1.2um, the peak sensitivity is around 08um, the long wavelength limit is 12μm, the light sensitivity is 30~40uAlm, and the quantum efficiency is<1 %. Its thermal emission current is large, and the thermal emission current density can reach 10-2A/cm at room temperature. The additional background caused by thermal emission reduces the contrast of the image, but is a practical photocathode due to its sensitivity to infrared radiation.

The electronic optical system of the infrared camera transmits the electronic image on the photocathode to the phosphor screen, and in the process of transmission, the enhancement of the electron energy and the scaling of the image size are accomplished. It generally uses electrostatic focusing (there are also a few that use electromagnetic focusing). Usually the glass type image tube uses a binocular classical electron focusing lens, and the metal structure image tube uses a collimator ball to form an electrostatic focusing system.

 

The fluorescent screen of the infrared picture tube is the unit that completes the electro-optical conversion. It is made by coating a phosphor on a substrate. The phosphors used now are generally silver-activated zinc sulfide cadmium sulfide (ZnS·CdS:Ag), copper-activated zinc sulfide, zinc selenide (ZnS·ZnSeCu) (No. P-20) and copper-activated zinc sulfide (ZnSCu) ) and other substances. Among them, the luminous color of P-20 is yellow-green, the peak wavelength is p=056um, the corresponding time of 10% afterglow is 0.05~2ms, and the particle diameter is about 3.5μm (controlled in the range of 1~6) to ensure the resolution. . In order to improve the quality of the screen and prevent the light from being fed back towards the anode, an aluminum layer should be evaporated on the powder layer, that is, the so-called aluminization of the fluorescent screen. Aluminization improves the brightness and contrast of the screen. The optimal thickness of the aluminum layer can ensure the effect of aluminization while minimizing the energy loss of the electron beam when passing through the aluminum layer.