Night Vision question: "IR" illuminator


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Nightcrawler
August 16, 2005, 12:50 AM
I've read on here many references to "IR" illuminators in regards to night vision. In Qatar, we were issued night vision binoculars that had what was labeled as an "IR Illuminator". I've also read about IR Lasers, and IR adapters that can be fitted to flashlights.

This was essentially a very dim red light, that couldn't be seen by the naked eye unless you were looking right at it. HOwever, through the NVGs, it lit up the scene like a spotlight.

Now, I've also worked with true infra-red optics, that is, thermal vision. Infra red light is heat; it cannot be seen in the normal spectrum, and standard night vision is just normal-spectrum amplification.

So, a truly infra-red laser wouldn't be visible to light amplification goggles (and I think would be some kind of heat ray...cool). And, I don't think there is a filter you can put on a visible light to make it an infra red light, since IR light is heat.

Similarly, you can't see lasers or flashlights through a thermal scope, regardless of their filter. Different spectrum.

So what's the deal? Obviously it's not pure infra red light we're talking about, as IR light can't be seen with the human eye. Are IR lasers/lights, like, on the border of the IR spectrum, hence giving them that moniker?

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Flyboy
August 16, 2005, 01:22 AM
I'll probably be corrected by somebody who actually knows what he's talking about here in a minute, but I'll do the best I can from my high school science classes.

IR is not heat. Heat is just molecules moving. More motion (faster motion) = more heat/higher temperature.

IR is a byproduct of heat, and acts as a transmission mechanism. Objects give off IR of varying wavelengths (there's a formula for this, but I can't be arsed to look it up right now; look for "blackbody radiation") based upon temperature. When that IR hits something, it transfers that energy (remember, light is electromagnetic radiation, and thus energy) to the object it strikes, heating it (which is why we can use IR lights to warm food, or even McDonalds).

IR can be generated by means other than blackbody radiation (hot matter); atoms can be excited electrically (LEDs, lasers) to an energy state above "ground" (natural, minimum enegy state). When this happens, the electrons jump to higher energy levels (forced by the applied energy), then fall to their ground levels. Jumping from one energy level to another requires a specific amount of energy (depends on start and end levels); going up requires energy be added, and jumping down emits the same amount of energy. Some of those energy levels correspond to a photon in the visible spectrum, some in IR, and some in other EM wavelengths. IR-generating devices use materials that jump between IR-producing energy levels. Such devices actually tend to run fairly cool for their power level, much cooler than a blackbody at the same output. In any case, IR is not the same as heat, merely a byproduct that can be generated by heat.

Now, as for NVGs: depending upon the sensor technology, the instrument can be sensitive to IR. Most CCDs (digital and video cameras, for example) are most sensitive at longer wavelenths (IR), and least sensitive at shorter wavelenghts (blue, violet, UV). In fact, the effect is so pronounced that, in order to make the devices usable, IR filters have to be added.

If the sensor in your NVGs is sensitive to all wavelengths, which wouldn't surprise me, then any photon, regardless of "color," will be detected and amplified. That it's displayed in monochrome (green) doesn't matter--old black and white cameras and video cameras were sensitive to all colors, but rendered in B&W. If the sensor responds to IR, it'll indicate when it sees IR.

Also remember that IR is a broad band, like visible. There's "near IR," right below red, "far IR," way down the spectrum, and all kinds of stuff in between. Sensitivity varies based upon wavelength. If you're using near IR, and your sensor retains some response down into near IR, it'll show up on your NVGs without being seen by the naked eye. As for thermal scopes, they're just IR sensitive; hit them with the right wavelength, and the source doesn't matter. Lasers of the right frequency (IR lasers) should show up like a 747's landing lights, if pointed directly at the sensor (remember that lasers are, by definition, coherent; if it's not pointed at you, you're not going to see it). Flashlights, at least incandescants, ought to show up a little--incandescant bulbs glow because they get hot. LED flashlights probably won't show up. The "IR adapters" were probably just filters for incandescant flashlights: block the visible, pass the natural IR, and you have an IR flashlight. I've also seen little cards with a spot of some sort of photosensitive chemical that will glow when hit with (near) IR. Not entirely sure how those work.

Now that we've gone through all that rigamarole, and told you a bunch of things you probably don't care about, and given everybody lots of opportunities to correct my misunderstandings, let me answer your question. Odds are the illuminators are near-IR, just below red, and your NVGs retain a little sensitivity down there.

Gifted
August 16, 2005, 01:48 AM
What he said. It works.

Nightcrawler
August 16, 2005, 01:55 AM
Thank you! That makes sense. :)

Lancel
August 16, 2005, 02:23 AM
Shucks, before reading all of Flyboy's wonderful prose, I made the short answer:

Your premise is wrong as to the frequency range amplified by NVGs. Try the the logic and experience from the other direction:

Since you can see infrared light with NVGs then the circuity must amplify that also. After all what you're looking at is not really green monochrome but an electronic processed image.

Your question is similar to "How can my black & white TV show an image when the real scene is in color?" Doesn't matter, your eye is seeing the last step in the process.

Larry

Nightcrawler
August 16, 2005, 02:28 AM
But as I said, the "IR light" IS visible to the naked eye. It's just very dim red light, and gives off no appreciable heat like the heater lamps at a fast food joint. Hence my confusion.

homeka45
August 16, 2005, 02:31 AM
Wow, that's one hell of a high school education. Thanks guys for that question and answer.

4570Rick
August 16, 2005, 02:42 AM
I don't know how to explain the dif 'tween IR and Thermal Imiging but these guys do.

http://www.drs.com/products/index.cfm?gID=21&productID=280

Lancel
August 16, 2005, 02:44 AM
But as I said, the "IR light" IS visible to the naked eyeDoesn't matter, once the NVGs are on, the eye is no longer naked: apples and oranges.

Flyboy's answer is very complete. Now let's put on our NVG's and go count stars.:)

Larry

chris in va
August 16, 2005, 02:48 AM
I didn't know Yale has a high school... :cool:

JohnKSa
August 16, 2005, 03:01 AM
IR is not visible to the human eye, however, an IR source may also radiate energy in the visible spectrum. Like a fire that gives off both heat (IR) and light (visible energy.)

Like visible light has a broad range of frequencies (from red to violet) IR also has a broad range of frequencies. Medium and low frequency IR is basically heat, while high frequency IR acts more like visible light but is still below the range that the human eye can see.

NV (NOT thermal imaging) DOES convert energy over a broader frequency band than the human eye can see to visible light. It also amplifies it in the process. It can't see all the way down to heat (medium & low frequency IR) but it can see down to the high frequency IR--lower than what the eye can see.

That means that you can use a high frequency IR source (invisible to the naked eye) as an illuminator for non-thermal NV.

The amplification process converts the light into electrons which are then converted back to visible light by the phosphor on the viewer. Unfortunately the frequency information (color, if you will) is lost in the process so you only get a monochrome picture.

Nightcrawler
August 16, 2005, 03:06 AM
Doesn't matter, once the NVGs are on, the eye is no longer naked: apples and oranges.

No, apples and apples. As I said, the IR light was visible to the NAKED eye. If you took the NVGs, held them in your hand, and flicked on the IR light while looking at the front of them, you could see it. It appeared as a super-dim red light (not a laser, though).

But, JohnKSa summed up everything remaining nicely, I think.

brickeyee
August 16, 2005, 09:48 AM
In Desert Storm one of the 'markers' for friendly vehicles was a 12 V bulb inside a metal can on the antenna. Zero visual signal. Shows up purely in the IR.
The 'IR filter' for a typical incandescent bulb can leak some portion of visible light. More expensive filters leak zero visible light. The only way to tell if the thing is turned on is to remove the filter, or see if the bulb end is warm to the touch.

Zak Smith
August 16, 2005, 10:06 AM
Also note that a laser rangefinder such as the LRF1200-Scan is visible through NODs-- both what you're lasing, and looking into the LRF.

Harry Tuttle
August 16, 2005, 10:09 AM
IR LEDs can have different output wavelengths

the Radioshack 940nm LEDs i use for IR illuminators are totaly invisible to the human eye

the 830nm IR LEDs glow faint red

I re-rigged IR LEDS into solar powered garden lights

When the lights go out, they turn on and the IR sensitive B&W video camera sees the light

i also built an IR flashlight and you can not see the "light" it outputs

You can easily build IR lights that only the NV equipt doods can see.

boofus
August 16, 2005, 10:31 AM
IR is radiation just like any other kind of light. It's just light that has a longer wavelength than visible red light.

The IR illuminators shoot a 'spot light' of IR frequency light waves that reflect off objects. The reflections can be picked up by night vision optics and turned into images.

Lancel
August 16, 2005, 11:47 AM
No, apples and apples
Ok, didn't mean to poke you with the fruit.:)

After working with radar, night vision, and other military detection devices for so long, I mentally combine the device's viewpoint with mine and don't feel conflicted.

To add to what others have said:

Since things have broad signatures: acoustic, temperature differential, infrared, visible light, radio frequency, etc. Instead of replacing what I see with what the device detects, e.g. visible or IR or thermal, I combine the information: visible and IR and thermal to determine the source.

I'm sure you've done this yourself if you witnessed (using night vision) someone smoke a cigarette in the dark. You know that visible light is involved, see the infrared, and can almost feel the heat and hear the sizzle.
You know it's a cigarette.

In short,Are IR lasers/lights, like, on the border of the IR spectrum, hence giving them that moniker? Yes, they can be. you made a fair conclusion.

Larry

CJ
August 16, 2005, 02:56 PM
Go in a dark room, pick up your remote control, press a button and watch the bulb. See anything?

Put on your NVG (or, in my case, my $70 first generation Russian monacle, which, incidentally, includes an IR illuminator as well) and repeat with the remote. You'll see a pretty good flashing from the business end of the remote.

Infrared simply describes where on the spectrum of light that it falls. It happens to fall outside of where a normal human eye can see it.

For a complete (and more professional explanation), check out:
http://imagers.gsfc.nasa.gov/ems/infrared.html

chink
August 16, 2005, 03:22 PM
IR is not visible to the human eye.

A while back i went to Radio Shack got a bunch of IR LEDs and rigged it up to be used as a small light for my Camcorder. I never really knew when the rig was on. Because the LEDs were clear and didn't emit visible light. I kinda trusted my soldering skills it. It worked, but the lack of range made it not useful.

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