On 2009-12-26, Richard J Kinch wrote:
DoN. Nichols writes:

Then select
a pinhole based on the tradeoff between perceived spot size and
brightness (you need sufficient brightness so you don't lose the spot.

I don't think you get the point.

Making a pinhole shadow of a big blob doesn't make the blob any more
accurate. The blob moves around and the pinhole stays illuminated.

It makes the blob (Airy disk) dimmer, and thus makes the part
*visible* to the user smaller. If the disk is full brightness, you see
the peak, and several rings of minor peaks surrounding it. If you make
it dimmer, you limit what you can *see* to the central peak only.

And -- if the pinhole is mounted rigidly to the front of the
laser, you are selecting the same portion of the blob as the laser and
pinhole move as a unit.

the whole approach just defies elementary principles. You can't improve on
the beam divergence angle, and that is many thousandths of an inch at the
spindle distance scale.

The beam, through a pinhole, is producing an Airy disk -- a
central peak surrounded by rings of diminishing brightnesses. Make the
pinhole small enough, and you can see only the central peak.

Have you *tried* the experiment, or are you working purely from
your own understanding of the underlying optics laws? Remember -- those
laws are not taking into account what is visible to a human eye, just
the overall distribution of light.

I tried it with a series of pinholes mounted in a turret
(designed for adjusting the energy content of IR from a black body
source while retaining the same color temperature). Granted, these were
quality pinholes, made in very thin metal, blackened, and with absolute
minimum reflection through the bore (because of the thin construction).
The pinhole was mounted about an inch in front of the laser pointer, and
was projected on a wall about five feet away.

Down to a certain size, the visible dot got smaller, then the
diffraction started making things worse and spreading it out. FWIW,
this was a red laser pointer. A green one would probably behave
somewhat differently.

Enjoy,
DoN.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---