Laser pointer experiments

Pointers are now a cheap commodity item and, although the
quality is pretty variable, most can be persuaded to produce a
spot at least as good as the centre/edge finder versions.

The pointers incorporate three main items plus the
battery.

The laser diode itself which is a near "point" source and emits a
diverging beam.

A lens which changes this diverging beam to a collimated beam
(i.e. a parallel beam focussed at infinity)

A small bunch of electronics which monitors the beam intensity
and keeps it safely below laser diode burnout point independent
of battery voltage and laser temperature.

This is all organised to produce the brightest possible
beam. For center/edge finder work this is far too bright and
neutral density filters or crossed polaroids are commonly used to
reduce this to a manageable level. Without these, because the eye
saturates, the spot appears much larger

Another way of reducing brightness is to add a series
resistor to the battery to reduce the operating current
(typically 150mA). The amount required is dependent on the laser
characteristics - the useful range is about 50 to 500 ohms. In
adding wires to the pointer be sure to remember that the metal
case is POSITIVE.

A second change is to refocus the lens to produce its
focussed image at a more useful distance. The lens is normally in
a threaded metal mount which can be rotated to alter focus.
Because the initial beam diameter is small, the depth of focus is
large and not critically dependent on the distance to the image
point. The lens position needs to be typically 30% to 50% further
away from the laser diode.

Changing the lens position is the best option but it can
be a fiddly business. Much the same result can be obtained by
adding a second lens. Because the initial beam is parallel the
position of this lens is not critical - close to the existing
lens is convenient.

Ideally it should be plano convex with the convex side
facing the existing laser lens but the the biconvex lens fitted
many eye loupes performs pretty well. The focal distance depends
on the power of the eye loupe - a x5 loupe has a focal length of
2" and this is a convenient working distance. Higher power loupes
produce proportionately smaller working distance and spot size.

This reduction in spot size produces a substantial
increase in brightness so reduction of beam power is essential to
prevent eye saturation effects.

This spot is now a LOT smaller than the original short
range pointer collimated beam but, because it is now an image of
the actual emitting surface of the laser, the shape can be a bit
variable dependent on the particular laser diode. Two out of
three laser pointers produced acceptably shaped spots the third
produced an odd shaped elongated spot. Some improvement should be
possible with a strategically placed pinhole but I've not tried
this. The best pinhole position is likely to be very close the
the laser diode window. It's not easy to mount here and it would
require accurate alignment of the pinhole position.

It's pretty easy to check out the odd pointer so it
would be interesting to learn how well different pointers
performed and whether pinholes are a practicable improvement.


Jim

http://newsgroups.derkeiler.com/Archive/Uk/uk.rec.models.engineering/2008-06/msg00009.html

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