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Filter that makes laser beam visible?
March 6, 2012 6:13 PM   Subscribe

Are there types of laser light, color or wavelength, whole beam can be seen in daylight conditions with a special eyeglass filter?

I'm looking for a way for a person to see a series of beams without turning the lights off or introducing steam or smoke or haze in the air. Is there a color or wavelength or type of laser that could be seen in daylight conditions using a pair or special eyeglass lenses?
posted by pallen123 to Technology (11 answers total) 1 user marked this as a favorite
 
Laser beams are visible precisely because they're hitting things in the air, regardless of the wavelength or power.
posted by odinsdream at 6:33 PM on March 6, 2012 [1 favorite]


I would have said, "Not a chance," but apparently I was wrong. I guess with enough power, a laser can energize (poor terminology, sorry) the actual molecules of air rather than relying on reflecting off dust or fog or vapor. So they wouldn't work in a vacuum. I'm a little skeptical, but it makes sense in theory. I'm just surprised something like that can run off a couple C batteries.
posted by supercres at 6:40 PM on March 6, 2012


Could you please clarify what you're looking for? Do you want the beam to be hidden unless an observer is wearing special glasses? Because that's not that easy to do. Your eyes can observe only a certain range of frequencies, and a simple filter wouldn't be able to shift an infrared beam into the visible spectrum. Night vision goggles, on the other hand, would do very well at spotting an infrared beam, assuming they could handle being used in daylight.
posted by wnissen at 8:12 PM on March 6, 2012


There's Laser Induced Plasma emission. A Japanese group has been working on it for more than 5 years. They use an IR laser to stimulate a plasma at a point in space, which then emits a visible light. They demonstrated it at CES this year. More of a research project than a real thing though.
posted by bonehead at 8:44 PM on March 6, 2012


Yes. A few dozen, maybe hundreds of Watts of IR light, say above 850 nm but below 1200 nm should be invisible to the naked eye, but visible through Rayleigh scattering when viewed with active silicon-based IR viewers. This level of laser power is beyond very dangerous. Any wall the beam hits will emit a certain amount of visible light, because it will be on fire. Trace amounts of scatter will be blinding. You were warned.
posted by fatllama at 10:34 PM on March 6, 2012 [2 favorites]


I'm looking for a way for a person to see a series of beams without turning the lights off or introducing steam or smoke or haze in the air.

The way we see a beam of laser light (or anything) is by having light coming from the point in space (middle of hte beam) directly toward the eye. A laser is special because all the photons are going almost exactly along a single line. The only way light will leave that line and come toward our eye is if it hits something. This can be an air molecule (but that doesn't work very well) humidity in the air (but it has to be pretty humid, i.e. steam) or some suspended particulate (smoke, dust). The higher-power the laser is, the less particulate matter needs to be present to get sufficient number of photons scattered to the eye, so lab-power laser beams are visible without doing anything to the air, but usually only if you turn off the lights.
In short, no, there's not a way to get laser light to scatter visibly without introducing scatterers, or having a LOT of light to start with.
posted by aimedwander at 6:28 AM on March 7, 2012 [2 favorites]


I have seen 100mw green lasers in action. You can kinda see the beam in normal room ambient light, but not daylight, and not very well. I have also seen a 1 watt blue laser--again, you can see a weak beam, but you really have to look pretty hard. Note these are not the cheap ones from the drug store, but $200 or so special-ordered items.
posted by MrMoonPie at 7:19 AM on March 7, 2012


I had another idea while sleeping last night. If the laser beam is linearly polarized, so is the Rayleigh scattering. So, if one were wearing polarizing sunglasses with the opposite sense of polarization, the beam would be invisible. Rotating the glasses by 90 degrees (or switching the polarization of the laser beam) would make the beams visible. This will work even for visible wavelengths.

A corollary is that polarized Rayleigh scattering has so-called dipole directional dependence. This is similar to the radiation pattern one gets from a radio antenna because really those air molecules are being driven like little dipoles by the intense laser field. If a horizontally travelling beam is polarized vertically (that is, along the direction of gravity), one will only see the laser if one is looking at the beam from the side, and not from above. (Never mind about staring directly into the beam!) In other words, the Rayleigh scattering from this circulating dye laser laser beam (self link) would be invisible if I had taken that photograph from above rather than to the side of this laser. Just for reference, there is perhaps 30W in that pictured "intracavity" blue beam.

A third thought: may consumer digital cameras can see near-infrared light that the human eye can't, although often the color is rendered as white. To pick up the Rayleigh scattering from air (or even dust) on a camera, one would still need many many Watts of laser power.

Again: don't do any of this. I'm fairly liberal with laser safety ("If I can see it, I can dodge it"), but I run for the laser goggles when any uncontrolled, free-space beam exceeds many tens of mW, regardless of whether it is a color I can see or the more insidious colors I cannot. IR and UV demand extra respect. You really shouldn't screw around with beams powerful enough to light up the air, no matter who's selling what laser pointer shaped like a lightsaber on the internet.
posted by fatllama at 7:40 AM on March 7, 2012


You can kinda see the beam in normal room ambient light, but not daylight, and not very well.

Any ambient air will have a low-level amount of suspended particulates in it. Higher-power lasers (100 mW+) are intense enough to scatter a lot of light, and thus become visible, even when the air is relatively clean.

As fatlama says though, those are getting in the range where intensities can cause lots of damage. A few hundred mW red laser can set things on fire, but they're plenty bright enough to be visible even in a normal room.
posted by bonehead at 9:10 AM on March 7, 2012


Cell phone cams can sometimes see IR beams. If I point my TV remote at my camera. It's like shining a flashlight into your eyes.
But you don't really see it from the side.
posted by hot_monster at 10:03 AM on March 7, 2012


It occurred to me - if your goal is to have visible beams, and the part about the filter or glasses or viewer was just groping for a way that might make beams visible, then no, it's not going to work like that, the reason we can't see the beams without scatterers is because the light isn't coming toward our eyes, and a viewer won't change which way the light is going.
On the other hand, if your goal was to have a physical layout where beams were invisible and then hand someone a thing that they can use to see the beams, maybe a virtual reality thing would solve the problem. This is probably fancier than your project can stand, but I've definitely seen an iPad interface that uses the camera to look at a scene, and overlays the mechanisms on the camera image. The thing I saw was an eco-building demo, where you point the camera around the room and it overlays info on all the fancy systems and features that are saving energy (look at the HVAC circulation behind this wall); yours would be virtual laser beams. Of course, if your point is to have people interact with the beams, this might go quickly toward impossible, or at least impossible without a dedicated team of very smart programmers.
posted by aimedwander at 6:24 AM on March 8, 2012


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