becks wrote:brett1868 wrote:The NightSite uses an IR illuminator in the 850nm spectrum which is classed as visible so it may spook certain species. If you want true invisibility then the 915nm / 940nm illuminators should be used but the camera must be capable of "seeing" them. The joy of thermal scopes is that they emit nothing but you're paying a lot more for them.
Would it be right to say then that once you're into the invisible range that IR gives better visibility?
I mean if thermal emits nothing, and can only see what is already there, but IR emits an invisible (to us) light to reflect and amplify what it's seeing does that mean it can do it better?
His unit uses a near-IR spotlight because it doesn;t do a great job of seeing in the dark. It doesn't suck for the technology, I'm sure it is nice. It is just based on technology that is inferior to Image Intensification ("Tube" Night Vision) and Thermal. Broadly speaking. It does have some major advantages under certain circumstances, which I will get to later.
Tube night vision (Gen II or above) and Thermal do not usually need a spotlight under most conditions. They are passive, as in they can see what is there without sending out a light source. Single stage Gen I night Vision and CCD (digital) simply don't have the photoresponse and signal to noise ratio to see very much in the dark. So they usually add a Near IR light source. It's basically the same thing as hunting with a flashlight, but you can't see MOST of the wavelength of the flashlight, so you have a device added to your scope to convert the light and amplify a little visible light as well.
There are downsides to this. First, the light source is usually visible to some limited extent. It's a mixed bag with animals, it's dangerous with humans, and it's a downright death sentence to humans with night vision. You're the biggest target on the field. Range is limited to what you can illuminate with the "flashlight." With tube night vision and thermal range is unlimited. You can see things dozens of miles, or even lightyears away, I use them for stargazing.
There are great advantages to the spotlight approach. It is cheaper. You can usually also get VERY high resolution and signal to noise ratio at close ranges under dark conditions. It will also work when there is no light at all. This is the reason even military (real) night vision usually has a little lamp as well, to see in complete darkness when necessary. The military diode lamps emit NO visible light, have a narrow wavelength band, and only work at very short range. Like for reading a map or walking through a cave.
I think you sort of stumbled upon something with regard to Near IR light. Only the US produces Gen III night vision. Europe produces a "Supergen" Gen II that meets or exceeds US night vision under 90% of circumstances. But there is that 10%. The main distinction between Gen II and Gen III is that Gen III uses a gallium arsendide photocathode that has its highest sensitivity in the Near IR range. Hmm... Is that so it can be used with a near IR spotlight? No. It's for two other reasons. As you sort of suggested, Near IR light produces higher contrast than visible light. If there's only the tiniest bit of light, and you have to amplify it thousands of times to see anything at all, you are going to want something that is sensitive to the Near IR because it is the best light you are going to get. Second, if there is ZERO visible ambient light from the earth, no skyglow, and complete cloud cover on a moonless night, all night vision (except thermal) will basically fail. Except for Gen III. The Earth is constantly bombarded with Near IR from space and it penetrates cloud cover. This is the condition under which Gen III excels beyond modern Gen II. It actually also applies to jungle warfare.
If this all didn't actually bore you to death and you made it this far, there is an interesting conclusion. Night Vision (tube, not thermal) is generally described as "gathering" and amplifying visible light to see in the dark. American Gen III does not attempt to do this. Recent American Gen III FILTERS OUT VISIBLE LIGHT before it gets to the photocathode (the sensor at the front). Not all of it, mind you, but most of it. Now Gen III is good enough that even filtering out 90% of visible light it could use the last 10% under nearly the darkest conditions. But why filter it out? It filters out visible light so it can crank up the gain and get more of that yummy Near IR. If you crank the gain when the signal is too high, you get too much noise. So we throw out most of the visible light signal so we can crank it up 10x more and get more yummy Near IR.