# Laser beam Dots

1. Jul 17, 2005

### Lord Loh.

An Intresting thing about laser beams that I noticed is that you always find thousands of small dots in the spot that is formed by a laser beam on the surface? What could it be? Maximas of light beams caused by constructive interferences? I am yet to find out. Please let me know if you have any explanations about it.

You can see the picture at http://betaeta.blogspot.com

2. Jul 17, 2005

### Artermis

I read the rest of your blog, and can answer the power question. Why do you think it's not safe to look at the Sun directly with the naked eye? Think about it. The sun is XXX distance away from the earth, and the rays emitted by the sun have to travel all that distance and undergo all sorts of absorption through the atmosphere before it gets to your eyes. And yet it is still dangerous. Welcome to correction.

Also, I'll take a small jab at your first question, is it just because the laster is made of several independent beams of light? I mean, the laser is not just firing out 1 photon at a time, right? Welcome to correction.

3. Jul 17, 2005

### pallidin

Could it be that the laser light is "transmitting" the patterns and imperfections of the covering lens?

4. Jul 17, 2005

### Claude Bile

I think the phenomenon you are referring to is called laser speckle, and is caused due to random phase variations in the laser spot.

There are two types of laser speckle, objective speckle and subjective speckle. Objective speckle is caused by phase variations induced by a rough surface (by rough, I mean surface variations greater than $\lambda/4$). Subjective speckle is caused by random phase variations in the scattered light in the eye itself (Note I said scattered, never look directly into a laser beam, no matter how weak the beam seems to be).

The two types of speckle are easy to differentiate, subjective speckle never loses its focus as the random phase variations are occuring in the eye, whereas objective speckle varies in sharpness depending on how one images it with a lens.

The properties of laser speckle are exploited in correlation interferometry. I suggest googling this topic for more information on laser speckle.

P.S. The picture you supplied does not show up speckle very well at all (this is something best viewed with the naked eye). In fact the picture looks like a very nice Gaussian HE00 mode. The dark spots around the fringes in this case are probably caused by variations in the CCDs themselves.

Claude.

5. Jul 22, 2005

### Lord Loh.

So If I take a picture of a spot on a surface, the speckles may show up? I will ty it and post it here. And the speckles seem to move as I move the beam. They do not vhange their relative position.

Last edited: Jul 22, 2005
6. Jul 24, 2005

### Claude Bile

If you want a really good image, transmit the beam through a piece of frosted glass or perspex and image the spot on a wall. If the spot size is small, you will have trouble seeing the (objective) speckle, so try to make the spot as big as possible.

The spots that 'follow you around' is the subjective speckle. Notice how they never lose focus, even if you wear glasses (or take them off depending how good your eyesight is!). The subjective speckle will not show up on any image you take however.

Claude.