Re: [ATM] Bench star test

[Nils Olof Carlin <nilsolof.carlin@telia.com>]

I can't seem top find Jarvis' original message, sorry, so I have to go 
by the quoted parts:

-------

Jarvis,

>Laser diodes do not make good point sources because the diode 
emissions
>are generated in a short very thin line.

This is very true. And yet, region from which emission radiate
is very small.  Smaller than traditional point sources as described 
for 
Foucault
test  usage where holes are punched in a foil. It is difficult to make 
hole 
of the size
of laser diode face. Is the answer in the fact that holes, although 
bigger, 
are circular?


 >If the diode is not lasing,
>then the light source is excellent for use in the Foucault  or Ronchi
>test setup.

Why in this case diode should  not lase?


 >The diode must of course be rotated until the light source
>line is parallel to the knife edge or Ronchi screen.

This is analogy to the slit source usage.
Why this can't be done with lasing diode as well.
-----

Supposing for a simple model, the lasing diode performs as a 
rectangular aperture passing coherent light. It will emit a "line" of 
light, typically long (but typically truncated by the mechanical 
assembly) and perhaps 20 deg wide. This means that the aperture is much 
less than one wavelength of light (of some 630-650 nm) and a few 
wavelengths wide (say 3-4 micron). When not lasing, i.e. emitting 
incoherent light, this ought to be an extremely good point source 
(whether the "slit" is wider then, I don't know). When lasing, the 
"band" of light is perpendicular to the "slit", and reasonably, the 
diode should be oriented so that the band is perpendicular to the knife 
edge - the band falls along the measured diameter (but that's perhaps 
what was meant??)

Nils Olof 
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