Ok, I KNOW this has been asked i am sure, but i cannot find it.

So I want to know how to creat dust in a light beam, BUT... not canned stuff. I want like when you look up close at a beam of sunlight you can see the tiny particles floating around. Not consentric dots, but particles. Looking for a tutorial. Hard to find NON phycadelic srtuff on the net LOL

I want "realism" hopefully :)

I have always liked Mystical Lighting for what you are looking for, lots of variations with light streaks and other effects as well.

can it be mapped to a window ?

Like with a selection or anything to that effect ?

No. It is an indpendent software but works as if it was attached to PShop. Not sure if they have free trials Check but their site. http://www.autofx.com/  and it appears that they do become a PS filter. When I purchased mine that did not add to CS3 or later.

I had a look over at the site, but I didnt find anything like what iam looking for :(

Some really awesome stuff though ;)

I can get a similar look in my 3D package, but it is very heavy and very not so real LOL

Couldn't you do a selection add noise?

Filter > Noise > Add Noise

You could also use or make a grunge brush.

Are you trying to get the volumetric light effect from millions of dust particles, giving the smokey beam of light effect or just a random method of painting lots of dots at once?

If it is the latter then try a google search for for free starfield brushes.  These might give you the effect you are after.

Breaking this image down into the various elements you have the particles themselves, depth of field and motion blur.  To replicate this you would need to build up your image in layers.  Paint some random dots of various sizes onto a new layer.  Duplicate, rotate and move these layers to speed up the process.  Apply gaussian blur to some layers and motion blur to others, then erase anything that you need to in order to get the result you want.  Change the colour shading and opacity of the layers containing the particles furthest from the camera.  Doing this will create an image very similar to the one you posted.

I think he wants the little squiggly dust-mote fiber shapes visible, not the out of focus circles.

Which makes me wonder if there's some random brush that would have wiry hair-like shapes. Using a brush like that to paint a mask on a gradient layer and screen setting or similar as a layer mode may produce some interesting results.

I haven't tried it, but I think you can figure out what I'm saying.

You could try and make your own, if you have a paint prog, that allows for layers. Make one layer blurred, another gausian blur, and another layer with minimal noise. Save as a PSD or PNG format image.

Deviant has some dust brushes you could use.

http://browse.deviantart.com/resources/?q=dust

Your problem is more that you're imagining something than describing it.  People have "an idea" of what volumetric light looks like, but what it actually looks like in any given setting is very dependant on the particulars. With these kinds of volume effects, you have to be very precise about particle size . . . the effect that produces a good volumetric fog won't necessarily give you a good dust cloud.

In 3D applications, the general term for this is "participating media" (meaning that the "empty space" that the light travels through actually has some optical properties) or "volumetric lighting". Google "Vray volumetric lighting" or "mental ray volumetric lighting" and you'll find a lot discussion. You'll find a thread discussing dust in volume lights in Blender here: http://blenderartists.org/forum/showthread.php?t=164715&page=1

In 2D applications, the effect can be faked at much lower compute cost with particle systems, these work well for the bright discrete dust flakes that you seem be trying to achieve. In Photoshop, I think I'd just make a brush with a bunch of fleck samples, assign a layer, and start painting.

The big thing is that volumetric lighting is usually very contrasty-- a shaft of sunlight coming into a dark basement, for example-- and things reflecting in the beam of light shold be brighter than you might think.