🚀 go-pugleaf

In article <h6h528drc8h9e7772prit4j5r3drmrlc1p@4ax.com>, 
jonk@infinitefactors.org says...
> 
> On Wed, 8 Aug 2012 05:03:53 -0700 (PDT), George Herold
> <gherold@teachspin.com> wrote:
> 
> >><snip>
> >> >Re spectrometer calibration: �We had this "black body" source. (from
> >> >Ortec?) �That could be used for intensity calibration. �PITA though.
> >>
> >> The problem for students is that they cannot afford the $800
> >> it takes to get a $8 tungsten bulb calibrated. (It can cost a
> >> lot more than that, too.) Then they need to set up a careful
> >> station for making the calibration. It's just plain too
> >> expensive to do. And I don't know a way around it. Wavelength
> >> calibration is CHEAP!! $12 buys a merc-argon calibration
> >> lamp, which fires up with the Argon first (for about a minute
> >> or two where you get some very nice Argon lines in the longer
> >> wavelengths) and then the Mercury kicks in and dominates with
> >> it's lines. A couple of well-timed snapshots and you've got
> >> enough (with software) to completely calibrate your pixel
> >> positions in a cheap ($10) megapixel camera.
> >>
> >> I'd LOVE to figure out a cheap intensity calibration setup,
> >> though. So would a lot of other people, I suppose.
> >>
> >> >But maybe you could do something on the cheap with a tungsten light
> >> >bulb.
> >>
> >> But that's the problem. I can get a cheap tungsten lamp. They
> >> are VERY CHEAP. In fact, the EXACT SAME ONE I would buy,
> >> calibrated, is dirt cheap uncalibrated. It is the calibration
> >> data that comes with a newly calibrated lamp that costs so
> >> darned much. It needs to be traceable to standards. And you
> >> pay for that. Plus, you need an expensive power supply to
> >> operate it (0.1% current control is standard.) And it ages,
> >> too. Within 100 hours or so, it's drifted enough that it
> >> needs recalibration.
> >
> >OK first a disclaimer, I know squat about the details of spectrometer
> >calibration.
> >
> >It would seem to break down into two pieces.  Absolute sensitivity (at
> >one wavelength) and then relative wavelength sensitivity.
> 
> The two calibrations I need are:
> 
> (1) Wavelength calibration which assigns a specific center
> wavelength to each pixel in the array (or matrix in the case
> of a camera.) Wavelength span is another issue, but it can be
> finessed a bit with several different alignments of the
> grating, if details are needed.
> 
> (2) Intensity calibration which assigns sensitivities to each
> pixel (no two pixels have the same sensitivities and there
> are other losses elsewhere, as well.) The whole system as a
> unit needs to be calibrated for intensity on a per pixel
> basis so that you know how much you are getting when you get
> it. This is an absolute level calibration, not a relative
> one.
> 
> Between the above two, you get accurate wavelength and
> accurate intensity. Getting sources to help calibrate
> wavelength is cheap. Getting sources (and setups) to help
> calibrate intensity is not cheap.
> 
> >For the first part you could maybe do something with a laser diode
> >into photodiode and then the same laser (though a bunch of
> >attenuators) into the spectrometer.
> 
> I'd like to hear more about this and how I might make that
> work for a megapixel camera over the entire CCD matrix. I'm
> at a loss regarding absolute calibration here. And keep in
> mind that it needs to account for the entire system's losses.
> So I can't go tickering around with the optical path to move
> the laser beam around because it will never be the same
> again. (I don't think so, anyway.) Maybe that would work with
> a monochromator? I don't have experience with those, though
> in any case it's not what I'm caring about right now.
> 
> >For the second you have the students use the black body curve and the
> >tungsten lamp.  Operate the lamp at two different temperatures.
> >(Maybe build a nice stable DC supply for the lamp.)  It seems like you
> >should be able to extract an approximate sensitvity curve from two
> >black body curves, even if you don't know the exact temperatures.
> >(But I'll leave that as an excersise for the students :^)
> >
> >How good a calibration do you need?
> 
> Good enough to do real work that is traceable to standards.
> Otherwise, it's not repeatable across instruments.
> 
> For now, the students get something useable on wavelength
> alone. And there is a lot of "science" that can get done
> without more than that. Examining fluorescence wavelengths of
> plant materials, for example. Stuff like that. They don't
> have to have intensity to replicate some of the science
> results they see in books.
> 
> But it would be very interesting to come up with a scheme
> that is cheap for intensity calibration. But like temperature
> calibration (freeze points, etc) it's not so cheap to do.
> 
Would good ol' Sol be an adequate calibration source?  You can
probably look up the daily solar intensity somewhere on the net.
Then you correct for local atmospheric effects.   Of course,
if this project has to work in Oregon, it will probably have
to be used only during summer school!   ;-)

Oh,  and the spectral output is pretty well known and even
includes a few good marker lines.


Mark Borgerson