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image intensifier

2022-06-13
1817 次

Image intensifiers are vacuum phototubes that can transform low brightness optical images into sufficiently bright images. Due to their structural similarity to image tubes and their frequent use together, they are commonly referred to as image tubes. It is a type of low light detector, consisting of a photocathode installed inside a high vacuum tube shell, an electronic lens (with two types of electrostatic focusing and magnetic focusing), and a fluorescent screen. Its working principle is to transform the optical image projected on the photocathode into an electronic image. The electronic lens focuses and accelerates the electron image onto the fluorescent screen to produce an enhanced image, which is then recorded using photographic methods.


structure

Consisting of three basic components, as shown in Figure 1


One is the photoelectric conversion part, which is the photocathode, which can convert invisible light images or low brightness optical images into photoelectron emission images;


The second part is the electronic optics part, which is the electronic lens, which has two forms: electric focusing and magnetic focusing. It can accelerate the photoelectron image emitted by the photocathode while maintaining the relative distribution unchanged;


The third part is the electro-optic conversion part, namely the fluorescent screen, which can convert the electronic image hit on it into a visible light image.


图片1.png

Photocathode

A photocathode converts low brightness radiation images that are invisible into electronic images. There are four commonly used photocathodes in tubes: silver oxide cesium photocathodes, single base and multi base photocathodes, various ultraviolet photocathodes, and negative electron affinity (NEA) photocathodes with high sensitivity and wide response wavelength range.


Electronic optical system

The electronic optical system applies a strong electric field to electrons, allowing them to gain energy. Therefore, it can accelerate and focus the electron beam emitted by the photocathode on the fluorescent screen, thereby enhancing the brightness of the image and emitting much stronger light energy. There are two forms of electronic optical systems, namely electrostatic systems and electromagnetic composite systems. The former relies on the acceleration and focusing effects of the electrostatic field to complete, while the latter relies on the acceleration of the electrostatic field and the focusing effects of the magnetic field to jointly complete.


In composite focusing electron optical systems (i.e. electromagnetic focusing systems), there are both magnetic and electric fields. The magnetic field of the system is generated by a long solenoid outside the image tube through a constant current, while the electric field is generated by the high DC voltage applied between the photocathode and anode. Therefore, electrons emitted from the photocathode surface can move towards the anode with different helical lines under the combined effect of longitudinal electric and magnetic fields; Electrons emitted from the same point on the cathode surface, as long as they have the same initial velocity in the axial direction, can ensure that they converge at a point after one cycle, playing a focusing role. The advantages of magnetic focusing are strong focusing effect, easy adjustment, small edge aberration, high resolution, but the disadvantages are large volume and weight, and complex structure.


Fluorescent screen

The function of a fluorescent screen is to convert electronic images into visible light images under the bombardment of high-speed electrons. Generally, it is required that fluorescent screens not only have high conversion efficiency, but also that the emission spectrum of the screen should be consistent with the absorption spectrum of the human eye or the lower level photocathode coupled with it.


Usually, an aluminum layer is deposited on the side where electrons are incident. This can divert the accumulated negative charges on the fluorescent screen, while avoiding light feedback and increasing the output of emitted light.


Microchannel board

There are two structural forms of microchannel board image enhancers: dual proximity and inverted. Dual proximity image intensifier, using microchannel plates instead of electronic optical systems, to achieve electronic image enhancement. And its photocathode, microchannel plate, and fluorescent screen are very close to each other, so it is called double proximity. The photoelectrons emitted by the photocathode enter the input end of the microchannel plate under the action of an electric field, are multiplied and accelerated by MCP electrons, and then hit the fluorescent screen to output an optical image. This type of pipe is small in size, lightweight, and convenient to use, but has poor image quality and resolution. operational principle


The brightness gain of a single-stage image intensifier is 50-100 times. A multi-stage image intensifier composed of several single-stage transistors in series can achieve brightness gains of several thousand to several hundred thousand times. Taking photos of the Pleiades using a five level image intensifier shows that the exposure time is one thousandth of that of a regular photography method. Single stage is generally 80-100 pairs per millimeter, while multi-stage image tubes are 20-50 pairs per millimeter. Due to the extremely low sensitivity of ordinary photographic films in the infrared spectral region, using an image tube with a photocathode sensitive to infrared light can achieve significant gain.


Coupling operation


Coupling an image intensifier with a camera device to obtain a low light camera

The image enhancement tubes used in low light cameras can be cascaded tubes, inverted tubes, or close-up microchannel plate tubes. The gain of an amplifier can reach 10 ^ 4-10 ^ 5 times, so the coupled camera device can work under low light conditions, but it will degrade the output signal-to-noise ratio and decrease clarity.


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