Date: Wed, 08 Aug 2001 23:47:26 -0700
Author: Bernard Cleyet
Subject: Re: Blackbody Radiation Spectrum
I think such skepticism is admirable, and, as a result I now understand. You
are completely correct. With a 5200° K lamp, I think the eye would see the the
shift from green to red. A carbon arc is only about 4300° K; the peak being in
the red. Have you tried using a welding arc? Jenkins and White claims up to ~
12000° is possible with very high current densities.
I must have been stuck not on seeing the brightest part of the spectrum
shifting, but the very obvious progressive disappearanc
e of the blue end of the
spectrum, and visa versa. This does contradict your experience. I repeated
the experiment, Monday, with a 500 W. Si-halogen lamp (Torchiere with its
dimmer) in a dim room and a Project Star spectrometer. My rather poor
pyrometer suggests that at 1500° the intensity below ~ 0.530 is undetectable,
while at full power the violet is bright.
Next chance I get, I'm going to the Community college where I taught
Engineering Materials and get the welder to help me.
The MPT you used probably fails beyond ~ 0.8 micron. and that's the peak
at ~ 3700° K,!
You need something that has sensitivity from one to two microns. One can
obtain such PMTs, but probably must be cooled and expensive. As I suggested
before, try a PbS cell, or whatever they use in IR spectrophotometers (see your
friend in chemistry). A bolometer or thermopile will also work, and they have
the advantage of being easily made -- at the sensitivity you require. You can
make a PbS cell also.
Cliff Bettis wrote:
> When I use the grating, the filament, a variac and my eye, it looks very
> much like one just inserted a neutral density filter in front of the source,
> when I reduce the filament temperature. Enough like it to make me question
> whether what we claim to be the explanation for what we see is obvious.
> What I meant by a redward shift is that with the filament at its hottest,
> the peak of the BB spectrum should be around 800 nm; as the filament current
is turned down the peak shifts to a longer "redder" wavelength. I can see
> how my language was confusing.
> One could go to a better source within reason (clarity of the demo) or a
> more objective assessment of the spectrum (spectrometer or filters) if the
> clarity isn't lost. I've got a solid state detector in the shop and hope to
> try again with a flatter detector. With a PM tube and reasonable resolution
> in the spectrometer, the shift in the spectrum is not very dramatic as you
> turn dow
n the current.
> I didn't mean to raise a stink about this. I thought other people might have
> been a little uncomfortable too. My wife continually reminds me that I have
> a exaggerated sense of skepticism ;-)
> ----- Original Message -----
> From: "Bernard Cleyet"
> Sent: Monday, August 06, 2001 6:51 PM
> Subject: Re: Blackbody Radiation Spectrum
> > Initially I was going to tell CB he was crazy; that it was obvious
> > I thought: wait a min. the eye has wide gradual skirt selectivity -- as
> > intensity decreases, even if there is no temp. shift (e.g. with variable
> > filter), the blue will disappear, but no, so will the red, as the response
> > symmetrical. So the expt. is valid. (with trepidation, waiting for CB's
> > answer.)
> > initial aborted post:
> > I don't understand this -- I've been doing this for years with no
> > problem -- could i
> > t be that it
's mass hypnosis? About ten years ago I
> > gave Sandy Faber (A famous astronomer at UCSC) a bunch of plastic
> > gratings, a hundred watt clear bulb with a long straight filament
> > lamp, and a variac. (along with spectrum tubes, etc.). When she
> > returned them, she said it all worked quite well. True it must be done in
> > dark room, so the lamp is visible over a great temperature range.
> > Perhaps I don't understand what you mean by a redward shift. Here are
> > some caveats:
W is not a good black
> > body to get a good black body put
> > a tungsten heater in a Cu funnel (or horn, like a micro-wave one)
> > blanked off at the narrow end and insulated, naturlich. Obviously it won't
> > last long
> > above about 2000° K, the eye doesn't have a flat, extensive, linear (to
> > intensity) response, etc.
> > bc