"Jim Wilson" wrote in message
link.net...
Peter Reilley wrote...

Those cheap lasers are surprisingly good, between 1 to 2 mrad
divergence. This is the same for ordinary HeNe gas lasers.

That may be, but I have a few of these things and some of them diverge
noticeably more than others. Perhaps it's in the optics.

You cannot narrow the beam by passing it through a slit.

I disagree and offer the following for consideration:

http://www.paragoncode.com/temp/laser_beam.jpg
http://www.paragoncode.com/temp/laser_setup.jpg

I didn't place a ruler in the setup, but I believe the effect is clear.

A slit will cause the beam divergence to increase.

This is true, but like I said, within reason, the beam width itself can
nonetheless be decreased by a slit. To be fair, the definition of "beam
width" is debatable, but for practical purposes -- as in the OP's
application, it can be narrowed.

Jim

I have seen those devices. The plastic lens that produces the divergence
in one axis that gets you the line also increases the divergence somewhat
in the other axis. You cannot get better than the 1 to 2 mrad divergence
with a lens or a slit or the expensive lasers would do it.
Have a look at the spec's for lasers in the Edmund's Scientific catalog.
They are all in that range for red lasers no matter what the price.

Pete.