The main purpose of the x-ray lens tube is to increase the brightness of the image so as to facilitate the television shooting. The ratio of the output screen image brightness to the input screen image brightness is defined as the brightness gain of the x-ray lens. The brightness gain depends on the following two factors.
1. Because the area of the output screen is smaller than that of the input screen, the electrons emitted by the photocathode will be concentrated on the output screen with smaller area through the electronic optics system. In this way, the number of electrons received per unit area of the output screen and the number of photons excited by them will increase, thereby increasing the brightness. This is called gain reduction. It is equal to the ratio of the input screen area to the output screen area. For a circular screen, it is also equal to the ratio of the square of the effective diameter of the input screen to the square of the effective diameter of the output screen. For example, the effective diameter of the input screen is 9 inches, the effective diameter of the output screen is 1 inches, and the narrowing gain is 92 = 81.
2. the electrons emitted from the photocathode energy is obtained during the anode positive potential acceleration. The higher the voltage between the photocathode and the anode, the greater the energy obtained by the electrons. The more photons are excited on the output screen, the stronger the brightness is. This is called energy gain, although only about one tenth of the electronic energy is converted into visible light by the output screen, due to energy amplification, the screen can still produce images of 50-100 fluorescent fluxes. From the above discussion, we can see that the brightness gain of the x-ray lens is equal to the product of the area reduction gain and the energy gain.