Laser "TV static" effect?

[Battlefrank]

I've always noticed that laser light has a strange property to it.
To see what I mean, hit a ice cube with a laser
the ice cube should light up. notice how along with the light, a field of extremely tiny dots shows up. looks like tv static.
I've always wondered what causes this effect, it seems to be universal with lasers.
If you don't understand what I'm talking about, wait for me to hit ten posts so I can illustrate it for you

 

[pseudolobster]

You mean this?
http://en.wikipedia.org/wiki/Speckle_pattern

 

[Dutch_Man_Dan]

pseudolobster that is seriously the funniest, most awesome avatar I have ever seen.

 

[Switch]

pseudolobster that is seriously the funniest, most awesome avatar I have ever seen.

I was gonna comment about his sig but was thinking it's offtopic.I guess you didn't

Anyway, the speckle pattern was one of the many things that made me consider my first 1-2mW red laser pointer as the coolest thing I've ever seen about 10 years ago.It was a complete mistery to me. ;D

 

[Deleted member 8382]

I actually have no idea what you talking about. SO i must shine a laser in an ice cube? Gona tesst and i say you

 

[rog8811]

I first saw this effect when I lit up a chip type laser stripped out of a $1 store pointer, the wierd part is that the dots move with you when you move your head.

Regards rog8811

 

[SuicideKing]

hmmmm...

If the speckle pattern is completely random, but dependent, to a large degree, by the lens and or surface texture the laser is shined upon, then one should be able to create an unbreakable cypher using the speckle pattern, with the key being a particular surface the laser is shined upon.

For example, say I wanted to encode some information that only I or someone I wanted to have the information could decode it. -a little bit of programing, a laser and a USB webcam should let me use any repeatable surface pattern as the coding and decoding key.

I encode the information using the random speckles produced from shining the laser on the fake wood grained plastic on my 1975 Panasonic clock radio. Then the only way to decode the information, would be for me or some else, to shine their laser on another 1975 Panasonic clock radio with the same type of plastic and fake wood grain.

Just a thought.

 

[kendon]

Then the only way to decode the information, would be for me or some else, to shine their laser on another 1975 Panasonic clock radio with the same type of plastic and fake wood grain.

and the exact same spot within a few nm, from the same position, distance and angle and capture the speckles also in the exact same way. that's what i would think, because if you are trying to use the image as a key you'd need to create that exact same image (or at least an image that gives you the same information) for deciphering.

if there was a way to encrypt using the "kind" of speckle from different surfaces, i could not imagine this to be secure, i.e. impossible to recreate that pattern artificially.

but interesting idea

 

[SuicideKing]

and the exact same spot within a few nm, from the same position, distance and angle and capture the speckles also in the exact same way. that's what i would think, because if you are trying to use the image as a key you'd need to create that exact same image (or at least an image that gives you the same information) for deciphering.

if there was a way to encrypt using the "kind" of speckle from different surfaces, i could not imagine this to be secure, i.e. impossible to recreate that pattern artificially.

but interesting idea  

Oh, I don't know, how about the picture of George Washington on the $1 dollar bill.
The U.S. Mint seems to be able to reproduce that surface pretty consistently...

As to it's security, the key speckle pattern might be reproducible artificially, but getting to that correct pattern, out of all possible correct patterns,  would take more processing power than the IRS uses in a year.

I'm certainly no expert, but from what I do know about cryptography, is that the two main things are having an available random generator and being able to have a key that can take that random hashing and turn it back into real data.

 

[691175002]

Interference patterns will be completely different if the surface varies by even a fraction of the wavelength of the laser. We are talking nanometers here.

 

[ElektroFreak]

Speckle is just too random.. Your cipher idea, while good, would require the deciphering laser to be oriented in the exact same position in 3 dimensional space as the encrypting laser down to the nanometer, as 691175002 just said.. At the same height, angle, and distance from the surface. And it would have to be the exact same surface, not just one made of the same material.

 

[GooeyGus]

speckle can also be interference happening on the cells in your retina. Is it really even there? That will blow your mind.

 

[ElektroFreak]

speckle can also be interference happening on the cells in your retina. Is it really even there? That will blow your mind.

Never thought of that, but I imagine you're right.. light can be pretty weird. It's been messing with people's minds since the dawn of time...

 

[flogged]

the 'TV static' is created because the laser beam is coherent, and interferes (http://en.wikipedia.org/wiki/Interference) with itself. See the 'double slit' experiment also .. something you can recreate easily using a laser.. http://en.wikipedia.org/wiki/Double-slit_experiment. Interference is an example of the wave nature of light in action. when talking about lasers one commonly refers to 'photons', or particles of light. actually light is neither a particle or a wave, tho many common models assume so. interference is also the principal employed to create holograms.

SAM's laser FAQ has more info on this:
http://www.repairfaq.org/sam/laserioi.htm#ioiscs

it's easier to see the speckle if you unfocus your eyes, or if you wear glasses to take them off. since you can't focus on the static it's always sharp.

my avatar was created by bouncing the laser spot off an incandescent light bulb and taking a picture of the reflected spot. another example of interference.