In the US, and presumably most other countries, 'civilians' aren't allowed to own IR laser weapon-sights, presumably because of their utility in night-combat, and the State's desire to retain their advantage in such things.

So, how to go about bypassing such restrictions?

Well, you could make your own, either from scratch or by swapping out the visible laser diode from a commercial laser weapon-sight with an IR laser module.

But this is a hassle and and an unnecessary expense.

By using a simple polarizing filter, you can turn an ordinary visible-red laser weapon-sight into an invisible night-combat sight. :)

Since you have to have some sort of NVD to use IR lasers in the first place, you can use the NVD's light-amplification to your advantage, since it will amplify visible light, as well as invisible IR.

Placing a polarizing filter in front of the visible-red laser allows you to 'dim' its output to a level that is invisible to the naked eye, yet still clearly visible to a NVD, all while staying in conformance of the law, since the output is still in the visible spectrum, just too low to be eye-visible. :p

I developed this concept using a Night-Shot equipped camcorder, a $5 visible-red laser pointer, and a polarized sunglass lens.

The following experiment was performed in a dark room:

The polarized lens was placed in front of the activated laser and slowly rotated until the laser light was nearly extinguished. At this point, the light is barely visible against a white wall, and totally invisible against a dark towel.

Then the camcorder, in night-shot mode with supplemental IR light off, is used.

At this point, the laser dot becomes quite visible as a bright-white dot in the dark screen, when projected against the white wall, and faintly so against the dark towel.

Then the supplemental IR lighting is turned on.

Again, the dot is clearly visible against the white wall, but washed out on the dark towel.

I then repeated the experiment outdoors, during 41% moon, with a target 35 yards away, which was a white panel truck with reflectors on top, while videoing it (on FTP, in my folder). :)

The first half shows the laser undimmed, through the lens, and then fully dimmed, aimed at both the back of the truck, and at the reflectors. The slight glow present in the reflectors is the moonlight.

The second half shows the view at the target, with the very faint spot visible on the rear of the truck, with the shadow of the chainlink fence showing, and the shadow of my hand passing through the beam, then finally directly upon my hand.

The downside is that the laser source is still quite obvious, since it is operating in the visible spectra, but any designated targets would be unawares as long as they weren't looking straight at you.

I think this shows the potential is there, and I believe that proper NVD's, especially more powerful ones using GEN3 or higher, would have no problem seeing such faint (but visible frequency) laser light, even against dark backgrounds.

Also, the advantage is there in the simplicity of the device, and the ability to use the laser as either an eye-visible, or dimmed 'invisible' mode, all at the flick of a lever, and without the additional complexity of dual laser-diodes and having to sight-in two different lasers.

Not wanting to detract from yet another of NBK’s simple yet clever ideas, but I have to say just how this thread highlights the amazing and ridiculous void between the laws of the UK and the USA !

In the USA ‘Joe Average’ is allowed to own weapons that we in the UK can only dream of, unless that is we hold a Section5 F.A.C. which outside of the Armed Forces is all but impossible to get.

Yet on the flip side, unlike the USA, in the UK we can buy Rimfire and Centrefire Moderators / Silencers over the counter, as long as you state it’s to be used on an air weapon and not on a firearm etc.

No surprises where this one is going, here we can buy IR Laser weapon sights over the counter, for using legally on any type of weapon, be they Airsoft, Air Rifles, Shotguns or Firearms.

A good friend just purchased an excellent one recently for only GBP200 or about 400USD inc. delivery and insurance etc. (from www.nightvisiongear.co.uk) and I’ve seen examples for sale on eBay for as little as GBP120 (plus S+H and taxes).

This is indeed a clever idea.

Actually, I haven't heard of any specific legal restrictions in the US against selling IR lasers to the general public. My understanding (which may be mistaken) is that vendors typically choose not to sell to the general public because IR lasers are much less eye-safe than visible lasers. Of course the effect is the same regardless: we have trouble buying them. :mad:

Anyway, here's a simple formula that might be useful:

It = I0 * (cos[theta])^2 (only the cosine is squared, not I0)

It = intensity of light transmitted through the polarizer

I0 = maximum intensity of light transmitted (this happens when the polarization direction of the laser and that of the lens line up perfectly)

theta = angle of rotation of the polarizing lens around its own periphery, starting from the point where intensity is maximum

The downside is that the laser source is still quite obvious, since it is operating in the visible spectra, but any designated targets would be unawares as long as they weren't looking straight at you.A possible slight refinement might be to attach a relatively long, narrow tube to the output aperture of the laser. The longer the tube for a given diameter, the narrower the area of solid angle from which an opponent in front of you could see the laser source.

I find the suggestion that US vendors merely choose not to sell specific types of weapon sights, purely on the grounds that if incorrectly used, they can damage peoples eyes, to be highly unlikely in the extreme, because surely the possibility of damaging someone’s eyesight is the least of their problems when the said laser sight is fixed to a .50BMG rifle that the same vendor had no such qualms in supplying?

Far more likely is as NBK suggests, it’s probably yet another of the US establishment’s ill conceived ideas to try to maintain an advantage over the so called ‘opposition’, however way off the mark this may actually be and in spite of just how easily it can be circumvented, as proved in this instance by NBK’s idea !!

Woo Hoo! #1 hit in Google, baby, yeah!

'dimmable laser' or 'laser + dimmable'

It's a very narrow niche, but God damn it, I'm the King of this small pond! :D

What I don't get is the fucks that are using the text from this thread to load their key word indexs for non-blogs and other internet clutter. :rolleyes:

That's an interesting spin on an old trick,

I was going to bust into rhyme about a little used photography trick, where with a gelatin filter, IR film and a pre-focused camera one can photograph documents in the complete darkness.

If you use a SLR, pre focus it to desired length, add a length of string glued or taped to the camera at the correct focus length, gelatin filter the flash, tape the corners especially thoroughly of coarse, load the film in the COMPLETE dark, that is lights out in a dark room (if your lucky enough to have one) and giddy up.

You can use a stick, with a nice document holder to get the best results, but try hiding that down your pants....

It is a point to note, of coarse, that if your concealing the camera you run the risk of either knocking lens out of focus or the taped filter lifting from the flash...

both are pretty uncool, now that is old school, I haven't had to photograph paper in the dark lately, so I am sure that the technique has been updated.

A good photography store assistant should be able to turn up a gelatin filter for the LASER project, if your interested on another spin.

Lasers, by definition, emit a coherent beam of light of a single wavelength.

(Don't bring up the exceptions, they're not relevant to the discussion.)

Therefore, you can't use a gelatin filter to block all but the IR, because the visible lasers do not emit any IR to start with.

What you are doing with the polarizing filter is reducing the emission of visible laser light to such a level as to be invisible to the naked eye, but still detectable by the powerful light-amplifying ability of a GEN2+ NVD.

Now, you might be able to tear down a green laser to make an IR laser, as I remember that early green diode laser used a crystal to convert IR laser light into green.

Removing that crystal, if you can still find a laser that uses such a thing, would do the trick.

Now, you might be able to tear down a green laser to make an IR laser, as I remember that early green diode laser used a crystal to convert IR laser light into green.

Removing that crystal, if you can still find a laser that uses such a thing, would do the trick.Yup, you're referring to a process known as second harmonic generation (SHG) from the field of nonlinear optics.

For those who aren't familiar with this stuff, what happens in SHG is that a laser beam of a certain frequency enters a certain kind of crystal (e.g., potassium dihydrogen phosphate, a.k.a. KDP), and the interaction of the strong electric field from the laser with the electrons in the crystal causes the emission of a new laser with twice the frequency (half the wavelength) of the one entering the crystal. So, if you have a laser of IR wavelength (say, 1.064 microns), and you double its frequency via SHG, you end up with a visible laser of wavelength 532 nm. Remove the nonlinear crystal, and you still have the IR laser.

That gives me an idea: is it possible to do the reverse? In other words, are there any materials into which a visible laser could be shone that would cause the emission of an infrared laser?

At first thought, it doesn't seem likely. The reason is that nonlinear processes are based on positive integer powers of the electric field. So you get processes that can double, triple, and quadruple(?) a frequency, or processes that can add the frequencies of two lasers, etc., but I don't recall hearing of any that can increase wavelength. I'll research the subject a bit and add more info to this post if I find anything.

Generally, it is far harder to laze in the shorter wavelengths than the longer wavelengths. Hence the struggle to get blue diode lasers, compared with red lasers which have been around for years, and IR lasers, which is where it all started with IR diodes over 30 years ago.

If you want an IR laser, rip an old CD drive apart. DVD drives have red lasers, Blu-Ray have blue lasers. (Green is still stuck with DPSS crystals though, as no-one has found a green emission line worth a damn.)

There are people buying Blu-Ray machines purely to rip apart for the diode, to be the first on the block with a blue laser diode toy. There are also lots of people ripping CD burners apart for the high power IR heads in them. Take a look at www.candlepowerforums.com for details.

As regards the harmonics, second harmonic is rare in a crystal, triple is rare compared to second. I've never heard of a fourth harmonic crystal that is usable.

I found a graph showing the spectral pass characteristics of crossed glass polarizers:

http://www.anchoroptics.com/images/products/CommercialPolargraphweblarg.jpg

At the spectra typical of visible-red weapons lasers (635nm-680nm), emission is cut down to 0.15% of original, which would correspond to 0.0075mw from a 5mw diode laser. :)

This is for commercial grade glass polarizers, which would run about 20x as much as cheaper polaroid polarizing film.

With the film, this effect could be achieved for literal pennies per laser.

As a note, with the higher powered lasers (40mW and up) you have to take care with plastic optics, as it will melt them after a short time, especially things like the plastic polarisers and neutral density filters, as they absord a lot more of the radiation.