🚀 go-pugleaf

In article <34je285htmmfscg6643s4qqlqdfjbchhbs@4ax.com>, 
jonk@infinitefactors.org says...
> 
> On Sat, 11 Aug 2012 23:22:35 -0700, I wrote:
> 
> >On Sat, 11 Aug 2012 22:56:20 -0700, Mark Borgerson
> ><mborgerson@comcast.net> wrote:
> >
> >>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.
> >
> >yeah. See:
> >  http://www.google.com/patents/US5835267
> >
> >My intent is to only use parts that a student can afford to
> >buy and OWN at home. Not school equipment. Not access to
> >expensive tools. Plain, simple, stuff. I have worked out a
> >folding piece of cardstock paper that makes the box and has
> >all the right angles set up for them, with baffles (also out
> >of paper.) The DVD-RW is cheap and is a precision diffraction
> >grating. The camera is cheap (blister-pack, $10 or less, at
> >common grocery stores.) The merc-argon bulb is $8 to $12,
> >depending. That's the ballpark I'm talking about. They build
> >everything themselves, take it home, and own all of it.
> >
> >If you come up with a scheme that can align with the sun in a
> >consistent fashion (without burning their poor eyes out),
> >get's good data on the sky filtering and effects due to angle
> >of incidence, uses a stop and baffles in the optical path
> >that children make by hand using glue and scissors and a
> >ruler, let me know.
> >
> >Think 12-year old and up.
> 
> What I'm looking for would be analogous to using an ice bath
> for temperature calibration. In almost every way, temperature
> calibration is difficult. That is, everywhere except for ONE
> temperature. And there, it's easy and cheap and any idiot can
> do a decent job no matter how clumsy they may be. And single
> point temperature calibration is better than nothing at all.
> 
> Kind of like that. It needs to be cheap and idiot proof. I'm
> not holding my breath. But that doesn't mean I've stopped
> looking, either. I just don't think it is in the cards, yet.
> 
OK.  What is the final goal as a teaching experience and how will
the difference between 0.1%,  1%, and 10% calibration  affect
the learning experience?

1%  solar calibration should not be too difficult.  Set up
a couple of baffles so that direct sunlight falls on a circle
of known diameter on a piece of white card stock visible to
the spectrometer.  The spectrometer looks at this circle of
light and reports a result.

The alternative is to use your own light source and provide
a sample of known reflectance and spectral characteristics. 
After calibration with that standard, the student can examine
leaves, fabrics, minerals, etc, and get a pretty good estimate
of their spectral reflectivity.

Mark Borgerson




Mark Borgerson