Range Gated Cameras
Range Gated cameras are very special. They operate on an entirely different concept. These cameras use “science” and even have Einstein helping them get the job done.
As you know, light travels at some known speed. Because it has a known speed it is possible to know how long a light beam will take to travel from one place to another. If we send a light beam out toward an object and then “look” only when light reflecting off that object has returned to us, we have a Range Gated camera.
Yes, we close our eyes and send out some light and then just when we think that light may have bounced off of something of interest to us and traveled back to where we are, we look.
The tricks are:
1.) Send out a pulse of light that is short enough that the “echo” won’t be lost in the size of the pulse we sent.
2.) Have a very accurate timer that can count in small increments and then give us a pulse when even just one millionth of a second has elapsed.
3.) Have a special light amplifier that can amplify any light received by several million times.
4.) Make sure you can turn that light amplifier on and off in less than one millionth of a second.
5.) Have a very sensitive video camera that looks at the image from that light amplifier and can see very faint amounts of light.
The simplest way to do all of this is to use a regular laser as the light source, a regular starlight scope tube as the amplifier and a regular low light level camera to look at the output of that starlight scope imager.
The laser is pulsed and it will output a burst of light far shorter than a millionth of a second long. This light then travels a mile in six millionths of a second. When the light hits something it is reflected just as if you had used a flashlight. The reflected light then travels back toward the camera at a speed of one mile every six millionths of a second.
If you use a timer and turn on the imager only when light from a certain distance has returned to the camera you can “see” things that reflected light at that distance and only from that distance. The actual low light level camera is not capable of being turned on and off that fast. It is the starlight scope imager tube that offers that ability.
If there is a heavy fog and there is a bad guy a mile away — and you set the range of the camera at one mile (a delay of six millionths of a second out and six millionths of a second back) all you will see on the screen is the guy a mile away.
There is a problem … If there is fog and you don’t know if there is anything out there in the fog then you have to step slowly from maybe 100 ft away to five miles away and see the blink of something of interest during one of those pulses.
The simplest way to do this is to “chirp” the camera so it zips at maybe 300 ft steps from your position to five miles. At 10 frames a second — so you can react — it can take ten seconds to scan the entire distance.
If the fog is so thick that you can’t see ten feet ahead of you then the Range Gated camera has to be set to shorter steps than 300 ft.
The second problem is that the laser used to send that brilliant a light is BRIGHT. This is solved in several ways. First, to keep people from seeing the brilliant flash they use an IR laser emits light that people cannot see. Second, to keep people from being blinded by the absolutely incredible blast of light, the amplitude of the light varies from low to high as the range is increased.
Yes, the bad news is that when you fire one of these things off, the WORLD comes alive (click on WORLD for videos). But the fact is, nothing else can do the job in fog or snow or some sandstorms.
Lastly, these range gated cameras are more reliable and robust and cheaper than many cameras of the “FLIR” type.
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