Re: Laser Centre Finder / Edge Finder


<pentagrid@xxxxxxxxx> wrote in message news:50lf34honjhlno3ch3t4vna99k0uq8b6c5@xxxxxxxxxx
On Fri, 23 May 2008 18:29:41 +0100, Chris Edwards
<Mustardmender-one@xxxxxxxxxxx> wrote:

On Fri, 23 May 2008 14:54:15 +0100, "Steve" <steve.withnell@xxxxxxxxxxxxxx>
wrote:

I treated myself to one of these last week, but I'm not "getting it".

From the web materials, I expected it to produce one very small red dot,
ie
about 0.05mm ish. What I actually got when I switched it on was quite a
large "splodge" which was made up of some laser sparkle and what appeared to
be about five small red dots. Pointing at a matt black finish made the five
dots pretty clear. I called the supplier to see if this was a colimation
problem and they volunteered to check one from stock and send that out pdq,
which they did. Just tried the "checked" one and it's the same as the
first.

So not at all sure whats going on here, do I need new eyes?

Steve

You will get a much more sharply defined pinpoint if you add the
optional polarizing attachment - see
http://www.chronos.ltd.uk/cgi-local/ss000001.pl?RANDOM=NETQUOTEVAR%3ARANDOM&PAGE=SEARCH&SS=laser+centre+finder&TB=A&GB=A&ACTION=Search


With most simple laser pointers, at short range, the image is so
bright that the eye saturates on the central spot. This makes the
spot appear much larger than the main high intensity point.

The usual way of dealing with is a neutral density light
attenuating filter or crossed polaroids between the pointer and
the target. Crossed polaroids have the advantage that the
brightness can be adjusted by changing the relative rotation
between the two polarising films.

This is just a matter of brightness attenuation. It
really doesn't matter where the attenuation is placed - between
the laser and the target, or between the target and the eye.

If you've got an old pair of polarising sunglasses the two
halves can be crossed at a suitable angle to give exactly the
same effect.

Jim
Thanks Jim. I have some lenses in the scrap box somewhere! One other effect that I didn't appreciate was the appearance of "columns" of light in the laser spot. The columns don't go away when you attenuate the light, though the spot does reduce in apparent size. Still not confident that a piece of ground silver steel and a cigarette paper stuck to the workpiece isn't more accurate for edgefinding though.


.

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