Heat Loss

[Andy Finney <andy@xxxxxxxxxxxxxxxx>]

Bob Long wrote:

Before this list started, Willem and I had a conversation on the
Usenet about a photography I'd seen a number of years earlier that
purported to show heat loss from a house.  It had been taken at night
and looked pretty unreal, with "ectoplasm" oozing from the doors and
windows.  Willem said it could not have been photographed with regular
IR film, because film is sensitive to near-IR only, and heat is on the
far end of the spectrum.  Maybe he's right; maybe the picture was made
through a night-vision scope.  I just don't remember.  On the other
hand, there are the iron pictures made on IR film that do show the
heat.  And, as discussed here earlier, whether a given wavelength
should be considered as heat depends on what materials we're talking
about.  Anyone have any further thoughts on whether it might be
possible--using a long exposure at night, no doubt--to actually
photograph heat loss on HIE?


.. and I philosophised:

I think we're back to the question of how hot is hot?  An iron can get very
hot.  You would certainly blister your skin if you touched it and you can
follow the heat versus colour curve down from white heat down to red (like
an electric fire) and if you drop the temperature a bit more the metal is
radiating near infrared simply because it isn't hot enough to radiate
visible red.  I think it's something to do with the energy of the photons
or some such thing I never learned at school.

So the radiated wavelength is related to the absolute temperature ... hence
'colour temperature'.  This would mean that nomatter how long you exposed
your HIE outside a house leaking heat at room temperature you would never
get an exposure because a photon with the right energy to activate the
emulsion would never be produced.  (I use the word 'never' in a non-Quantum
way here).  Your house would have to be spilling air that is as hot as that
iron for it to show up.  Another way of looking at it ... if you expose a
red light for long enough on colour film, will it ever turn blue?  The
ectoplasm look to the photo would be because it is the hot/warm air that is
emitting the photons that the far-IR detector 'sees' and that air will be
flowing and convecting and stuff like that.

So here's one for the Quantum physicists.  If the probability in quantum
mechanics says that a photon of the requisite energy could be produced ...
it's just very unlikely ... how long would you have to wait for enough to
hit the film and expose it.  I suspect we're talking years rather than
hours.  And if the emulsion needs a critical 'mass' to even be exposed at
all then it may never happen.

But I haven't got the foggiest idea about Bob's laquer and subway light.  I
don't see how a light can change colour depending on how far you are from
it unless the angle of your view of the light changes.  That's how some
runway lights work ... the angle at which you view the light through a
multi-coloured filter changes depending on how high (as an angle) you are
in the beam and the filter is coloured accordingly.  If the filter is red
at top, green in the middle and blue at the bottom then you will see green
if you are flying towards the light at the correct angle and red if you are
too high or blue if you are too low.

But Bob ... what I wasn't sure of was whether you were looking through the
laquer at a light source or looking at it and the red and blue colouration
was reflected rather than transmitted.  The colour change could be an
interference phenomenon (depending on the laquer thickness and how it was
laid down).  Because disc cutting laquers need to have a very homogenous
structure to avoid noise and have to provide a very flat surface to
minimise swarf, maybe they were spluttered in distinct layers for better
control of the hardening.  In this case the individual layers would be very
thin and so subject to interference.  I don't know, it's years since I was
involved in disc cutting, so that's definately a guess.




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Topic No. 7