Re: Wavelength and power measurement of laser pointer

"Ioannis" <morpheus@xxxxxxxxxxxx> writes:

"Sam Goldwasser" <sam@xxxxxxxxxxxxxxxxxx> wrote in message
news:6w3belkrgp.fsf@xxxxxxxxxxxxxxxxxxxxx
[snip]

Most CDs are pretty close to 1.6 um pitch so only a single reference
is really needed unless you really want super accuracy.

The curved tracks really don't matter for a narrow beam like that from a
HeNe laser or laser pointer.

You are indeed right that the CD curved tracks won't matter for a narrow
beam like that from a common laser. In fact they don't even matter with
extended light sources, provided the capturing angle is small, or that the
camera has zoomed deep into the spectrum.

What DOES matter however, is that CD's make really lousy gratings because
they are not optical quality. As a result they introduce a plethora of
artifacts which really screw up the image. In the following DVD spectroscope
demonstration, barring occasional bad focus by the camera (which cannot
sense the correct focus because of the extreme zooming in) you can see all
sorts of ghosts, from multiple resonant back and forth reflections between
the front and back surfaces, to ghosts from higher order spectra which
interfere with the actual lines of the light source.

http://ioannis.virtualcomposer2000.com/spectroscope/toyspectroscope.html

Even in the best spectrum pictures taken with CD's and DVD's like that of
Jerry Xiaojin Zhu,

http://www.cs.cmu.edu/~zhuxj/astro/html/spectrometer.html

you can still see ghosts around very bright lines.

Sure, but here we're talking about resolving what is essentially a single
line so it's just a matter of finding the brightest peak. I just tried
it with a $1 laser pointer and a CD. The first order spot is very narrow
and distinct.

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