# What causes the Laser "Speckled Light" appearance?



## LuxLuthor (Jan 2, 2006)

I was trying to figure out on another forum a simple explanation for what causes that granular "speckled" light appearance when you shine a laser on a wall. You can see in these pictures what I am asking about, but you see it in any type of laser photographs. 

Is it related to the coherent laser light being partially absorbed by a surface it strikes....or is it actually shining out of the laser as pin point beams? I really have a beginner's understanding, but it is unlike any appearance of a white flashlight.






http://home.earthlink.net/~waterhole/images/1.jpghttp://home.earthlink.net/~waterhole/images/1.jpg


----------



## nerdgineer (Jan 2, 2006)

I'm not sure but I think that when a coherent light beam like a laser hits any type of surface, the tiny imperfections on that surface each reflect the laser light like a coherent point light source. The net effect of a lot of coherent point sources is to create a complex diffraction pattern with many fine nulls and peaks. This diffraction pattern hits your retina and is perceived by your eye as a lot of tiny speckles. Each eye sees a different pattern which is part of what makes it look a little strange.


----------



## LuxLuthor (Jan 2, 2006)

Interesting....so you think in those pictures all the bright speckles of light are reflections from surface irregularities where the narrow beam is actually striking the surface material--and creating a kind of speckled shower of reflections around it?


----------



## abeland1 (Jan 2, 2006)

Some good info on this thread:

https://www.candlepowerforums.com/threads/97693&highlight=speckling


----------



## Databyter (Jan 2, 2006)

Because laser beams are a very intense focused and coherent light any side ways reflection of of say spackle or paint textures are much brighter and of course the least spread out and dissapated reflections are the ones that land actually on the _same_ surface a short distance away.

Picture a wall under a magnifying glass or a microscope and you will see lots of peaks and specks of dust that were caught in the paint as it dried. Almost any surface you can think of other than polished metals or glass have alot of imperfections. It doesn't take much of one to reflect a small amount of light a short distance away. And when it's super bright laser light as opposed to highly diffused regular light sources you can see it, which is sort of amazing.

Also your laser itself may have imperfections in the lenses or filters or they may be dirty, even on the inside where you can't see it.

The other day my red laser was really dim but still had a decent small dot with very little visable problems. 
I assumed it was batteries and changed them but still had the same problem. 
I was surprised to find a very large lint ball from my shirt pocket completely covering the aperature hole. 
The laser was so bright that it still produced a beam. Go figure. 
lasers have surprising and counter intuitive behaviors because we are used to judging light behaviors from sunlight or artificial incandescent or flourescent lights.

Sorry for being long winded..too much caffiene tonight..Ya think?


----------



## IsaacHayes (Jan 2, 2006)

Do green lasers have the "TV static" noise stuff like Red lasers have? I don't know if you see those with green lasers or not. I think the cause in red lasers is because of the light waves being aligned and in phase that some of them cancel each other out. Weird that 2 beams of light can cancel each other out and be come darkness.


----------



## HD58PHD (Jan 2, 2006)

WTH are you talking about??? LOL
I'm not sure where you live,but here on Earth,laser beams don't cancel each other out,in fact,if you were to TRY to align two beams directly and precisely into eachother,you will find that no matter how much you try,it cannot be done.
Only light posseses both both electromagnetic energy and MASS,IIRC,and two solids cannot occupy the same space at the same time.
Try to reflect a beam back into itself with a f.s. mirror,or the output mirror from a HeNe tube at about 4-6 inches,watch the beam be pushed aside by itself,it will dance around the original beam,and just when you get it almost centered,BAMM!!!without moving the mirror itwill jump to the other side of the beam.
All this was with a 1mw red HeNe,I know it sounds wierd,but it does.
You need a tight beam to demonstrate this,at a close range.


----------



## Databyter (Jan 2, 2006)

HD58PHD is right. Any Static noise is from a fluctuating current, crystal performance variations due to temperature etc. or just simple dust in the impure air we all breath. The light itself is very stable by it's very nature. Thats why the Phone Company and high tech mainframes love it.


----------



## comozo (Jan 2, 2006)

First off I don't know why you could not Google the answer.
Secondly everyone gave you a wrong answer and I don't know why they did not google for the right answer. Below you'll read the right answer
When the term 'laser Speckle' is mentioned it means only one thing. Laser speckle has nothing to do with the brightness of the light, how your eyes see light and stuff in the air or dirt on the lens.
It is simply do to the wave nature of light. If to peaks or valleys meet the light is reinforced bright area, if a valley and peak meet they cancel each other out creating a dark area. 

"As for the speckle pattern, that is usually called the interference pattern. It has nothing to do with your eyes and has no bearing on how well you can see as it is a real phenomenon. Laser light is completely monochromatic and is also in phase. When this light is scattered, it gets out of phase and the waves collide. When a wave at a low point and a wave at a high point collide, they cancel each other out (just like those noise-reduction machines that send out ambient sound 180 degrees out of phase, except this is with light). Where the light cancels itself out, there is a dark space, where it does not, there is a light space. This creates a three-dimensional lattice-work of light and dark spaces" 
link http://www.cat.gov.in/others/lasinfo/specle.html


----------



## nerdgineer (Jan 2, 2006)

comozo said:


> ...everyone gave you a wrong answer...


Check the first answer. The "interference pattern" Google refers to is also called a diffraction pattern.


----------



## LuxLuthor (Jan 3, 2006)

comozo said:


> First off I don't know why you could not Google the answer.
> Secondly everyone gave you a wrong answer and I don't know why they did not google for the right answer. Below you'll read the right answer
> When the term 'laser Speckle' is mentioned it means only one thing. Laser speckle has nothing to do with the brightness of the light, how your eyes see light and stuff in the air or dirt on the lens.
> It is simply do to the wave nature of light. If to peaks or valleys meet the light is reinforced bright area, if a valley and peak meet they cancel each other out creating a dark area.
> ...


 
Great link. That finally explains it. Just so you know, I spent about 2 hours with google, and posting on another laser forum trying to find a clear explanation, and never saw that link, nor did anyone else give this explanation. Thanks!


----------



## bertik (Jan 4, 2006)

comozo said:


> First off I don't know why you could not Google the answer.
> Secondly everyone gave you a wrong answer and I don't know why they did not google for the right answer....



Indeed all those answers were wrong. I'd like to add two things:

1) the best general link for laymen and laser hobbyists is
http://www.repairfaq.org/sam/laserfaq.htm
and the speckle phenomenon is described in detail there.

2) The statement that two beams cannot be aligned is not correct, and of course one can aim a beam back into a HeNe. The thing may become unstable and fluctuate strongly, but this is precisely the sign that the beam is nicely aligned.

Also, one can indeed cancel out a beam by using a Fabry-Perot interferometer (essentially consisting of two transparent parallel mirrors). When properly aligned, then a laser beam may pass through the configuration, or not at all, depending on the precise distance of the mirrors; by slightly translating one of them (eg by a piezoelectric ceramic), one may continuously switch off or on the beam. Of course, when the beam does not pass through, it is not so that the energy just dissapears into nowhere; rather, the incident light is fully reflected back, so energy is conserved.


----------



## comozo (Jan 4, 2006)

BTW the info the link provided can also be found at Sams Laser FAQ


----------



## Lunarmodule (Jan 4, 2006)

To add just another layer of fun regarding the wave nature of light and phase cancellation, there was an article in Scientific American about cutting edge physics research where a group of researchers was able to demonstrate the ability to render certain solid objects transparent, i.e. the hypothetical Klingon cloaking device, by way of bombarding the target with a scanning laser array creating complex large scale phase cancellation of reflection. The net effect was jaw dropping, there were even outdoor pics of the system used on a tree trunk allowing one to see through it to the other side. Short of it, phase cancellation of light, particularly coherent light, is a very likely cause of this speckle phenomenon. 

There are a lot of folks that have posted things about "burning" objects with their green lasers. While setting up a projector system, I had used a small piece of angled steel covered in vinyl electrical tape as a temporary beam stop. When I first tried the system, I noticed the obvious melting/bubbling of the tape, but FAR more interesting was the pattern it generated in the huge diffuse reflection on an adjacent wall. It looked like a huge monochromatic projection of a microscopic area of the beam's contact area, like a very large scale photomicrograph. It was mesmerizing to watch and imagine that what was displayed on the wall was a magnified view of the tiny area of vinyl tape being transformed as it went from solid to liquid state. Just another neat laser phenomenon.


----------



## Kiessling (Jan 4, 2006)

Lunarmodule ... got a link to that article about making objects transparent? oo:
bernie


----------



## Databyter (Jan 9, 2006)

> lasers have surprising and counter intuitive behaviors because we are used to judging light behaviors from sunlight or artificial incandescent or flourescent lights.



Well, at least I got that part of my explanation right. Interesting and counterintuitive explanation that certainly would not have occured to me.

I still think that alot of what people call Speckle with diode lasers is actually reflection. I believe this because with my own laser I can almost eliminate the speckle appearance by shining it onto a perfectly flat surface.

Not to take away from the true explanation that someone else provided but alot of what people see around the dot is not caused by that phenomenon in my opinion. Even the article mentions the effect is not as apparent in diode lasers.


----------



## leobaby (Jan 13, 2006)

Is it possible that there are two phenomena at play here? Sparkle around the dot created by the laser and projected onto the surface; and sparkle reflected from the dot alone but perceived by the eye to be on the surface. I think an easy way to test this is to 
project the dot onto a few different surfaces and 
a. with another object block your view of the sparkle but not the dot - and if the sparkle still exists (in front of your blocking object) it must come from the dots reflection on an irregular surface 
b. block your view of just the dot - any sparkle remains is projected onto the surface by the laser.


----------



## kushy04 (Jan 13, 2006)

Dirty lens! Do I win?


----------



## Databyter (Jan 13, 2006)

I think the real explanation lies here;

The creatures that live in the sixth dimension are trying to communicate with us and can't understand why we can't figure out their very clear message.

Vote for me!:rock::wave:

Do I win?


----------



## comozo (Jan 13, 2006)

Databyter said:


> Well, at least I got that part of my explanation right. Interesting and counterintuitive explanation that certainly would not have occured to me.
> 
> I still think that alot of what people call Speckle with diode lasers is actually reflection. I believe this because with my own laser I can almost eliminate the speckle appearance by shining it onto a perfectly flat surface.
> 
> Not to take away from the true explanation that someone else provided but alot of what people see around the dot is not caused by that phenomenon in my opinion. Even the article mentions the effect is not as apparent in diode lasers.



I've not checked my facts but if memory serves a perfectly or nearly flat front surface mirror will not cause laser speckle. You are right it is caused by surface uneveness of reflected light.


----------



## AJ_Dual (Jan 13, 2006)

Constructive/Destructive interference would actualy make a good lighting effect for a haunted house or a club.

I was messing with my 20mW Leadlight in a room with gloss paint and textured walls. The rough plaster combined with the glossy paint surface on the ceiling lit the walls of the whole rom with green static that was constantly shifting and moving with the movment of the beam across the imperfections in the ceiling texture.

An audio soundtrack of looped static would make it like being "Trapped in TV" or whatnot.


----------

