The optical system of the active infrared night vision device is roughly the same as that of the low-light night vision device. The objective lens is a transmission objective lens or a folding objective lens with strong light power; the eyepiece has a certain magnification and generally has a large exit pupil diameter. Since a variable tube is arranged between the objective lens and the eyepiece, the beam limitation of the objective lens and the eyepiece should be considered separately. When considering the beam structure of the objective lens system, the objective lens frame is regarded as the aperture diaphragm, and the effective range of the photocathode surface becomes the through hole of the field diaphragm. When considering the eyepiece system, the effective area of the fluorescent screen of the variable image tube determines the field of view of the eyepiece, and the pupil of the human eye is its exit pupil. It can be seen that due to the existence of the infrared imaging tube, the light rays directed to the objective lens and the light rays emitted from the eyepiece are no longer in one-to-one correspondence, and the structure of the imaging beam loses continuity, so the entire optical system cannot be integrated to consider the entrance pupil and the eyepiece. Conjugation of the exit pupil. This situation still exists in low-light night vision devices. This is all because the photoelectric imaging device destroys the continuity of the imaging beam.