The active infrared night vision device with gated working mode uses a pulsed mid-infrared laser to irradiate the target, replaces the ordinary infrared image tube with a variable image tube with a gated electrode, and discriminates the target beam and scattered beam according to the time of reaching the optical system. , and rejects the scattered beam, allowing only the target beam to reach the image plane. This is an effective measure for the backscattering influence of the moderate atmosphere.

 

In this way, the gating camera becomes the key. It adds a gate electrode between the photocathode and the tapered anode. When the gate electrode is applied with a lower potential than the cathode, the electron emission of the photocathode is blocked, and the picture tube is "off". Only when a proper focusing potential is applied to the gate electrode, the photoelectrons emitted by the cathode can reach the phosphor screen, and the picture tube is in the "on" state. Using the "gating" performance, a special circuit is designed to control the potential of the gating electrode, so that when the beam returning from the target reaches the observation system, the image tube is in the "on" state. closed" state.

 

The pulsed laser source of infrared searchlight (such as array GaAs laser, etc.) should have high enough energy and enough pulse time. For example, when the pulse duration Δt=100ns, the propagation distance of light in one pulse interval is about 30m. At this time, the atmospheric backscattering within 15m before and after the target will enter the system to form additional background on the image surface. Backscatter outside this range has no effect on imaging. It is generally believed that t=100~200ns is feasible, and good image effects can be obtained.

 

Due to the lag and afterglow of the phosphor screen, the human eye cannot perceive the temporal discontinuity of the image.

 

From the above, it can be seen that to achieve a reasonable "", it is necessary to strictly coordinate and cooperate with the laser and the selection of the image tube in the timing sequence. For example, to observe the target at 1200m ahead, consider that the round-trip time of the laser beam is about t=8ps, so the strobe time of the video tube is delayed by 8ps from the time when the laser pulse is emitted. If the duration of the laser pulse is Δ=200, the result of gating is to enable the observer to see the scene within the range of 1200m±30m in front; if t=100ns, only the scene within the range of 1200m±15m can be seen. Of course, from the perspective of excluding the influence of large backscatter, it is hoped that the duration Δt of the laser pulse is as small as possible; and when considering ensuring a sufficient "depth of field" for observation, it is hoped that Δt should be as large as possible. This creates a paradox, exposing the limitations of gating techniques. Fortunately, the strobe time interval of the imaging tube, the duration of the laser pulse, and the pulse repetition frequency have enough adjustment ranges to make up for this defect.

 

The application of gating technology enables the active infrared night vision device to have both ranging function, so that we can not only achieve effective observation, but also measure the distance of the target. At the same time, due to the use of pulsed laser to irradiate the target, the probability of exposing oneself is also reduced, which to a certain extent makes up for the shortcoming that the active infrared night vision device is easy to be found by the enemy.