Re: laser through pinhole

In <6wr6npr8i1.fsf@xxxxxxxxxxxxxxxxxxx>, Sam Goldwasser wrote in part:
don@xxxxxxxxxxxxxx (Don Klipstein) writes:
<I edit for space>

I think more like using an ocular with focal length in/near the 10-16 mm
range, or one of the small lenses used in oculars and lower magnification
microscope objectives. I think that gets to be a good time to hack a
cheap toy quality telescope or microscope. Otherwise borrow an ocular
with focal length near/around 10-16 mm. Other than that, Anchor Optics
(probably about $6 for a small lens, plus shipping charge that I have
found to be on the high side and appearing to me to have added to it some
percentage of the merchandise price, and maybe a minimum order).

Probably needs to be a shorter focal length.

4 * wavelength * (lens focal length)
Spot Diameter = --------------------------------------
pi * (beam diameter)

or

(spot Diameter) * pi * (beam diameter)
focal length = ----------------------------------------
4 * wavelength

The typical green laser pointer has a 1 mm diameter beam. To obtain a
beam spot diameter less than 5 um requires an f/ number of about 7 mm.
And, the focused spot diameter should be less than 5 um to get most of
it through the 5 um pinhole so a 3 or 4 mm lens would be better.

I was figuring cheap red laser pointer with wavelength 650 nm and beam
width about 2.5 mm.
I was also figuring on pushing for higher f ratio even if a little light
misses the pinhole, to make it less critical how far the pinhole is from
the lens, given a mild requirement of throughput and my past experience of
having difficulty getting a target longitudinally centered in the waist of
a focused laser beam. :)

Now I check my red laser pointers again, and one (nowadays very common)
has a beam at the muzzle being close to a 3 mm circle in overall
dimensions. However, it is more of an oblong pattern, with most of the
light being in a pattern roughly 3 by 1.4 mm.

Plug 5 um spot size and 650 nm into the above, and f/ratio is 6.04.

Now that I am considering an initial beam width of 1.4 mm, I need to
revise the focal length figures that I had in mind. This works out to 8.4
mm. I think around 8.4-10 mm if you want a wider longitudinal range of
beam diameter close to its minimum or want minimum divergence, and less
than 8 mm if you want to try for a smaller waist to get essentially all of
the light through the pinhole.

- Don Klipstein (don@xxxxxxxxx)
.

Relevant Pages

  • Re: laser through pinhole
    ... I got a pinhole of 5 microns and need at least 1 mW ... Use a convex lens to focus down the beam. ... the hole with a commercial laser pointer. ...
    (sci.optics)
  • Re: laser through pinhole
    ... I got a pinhole of 5 microns and need at least 1 mW ... Use a convex lens to focus down the beam. ... the hole with a commercial laser pointer. ...
    (sci.optics)
  • Re: laser through pinhole
    ... I got a pinhole of 5 microns and need at least 1 mW ... Use a convex lens to focus down the beam. ... the hole with a commercial laser pointer. ...
    (sci.optics)
  • Re: laser through pinhole
    ... I got a pinhole of 5 microns and need at least 1 mW ... Use a convex lens to focus down the beam. ... the hole with a commercial laser pointer. ...
    (sci.optics)
  • Re: Digital cameras & depth of field (my experience)
    ... The diameter of the beam is determined by the ... diameter of the pinhole...so, a really big pinhole will make a less ... sharp image and a very small pinhole will make a very sharp; ...
    (rec.models.railroad)