Re: Bears and Butterflies

[Wayde Allen <allen@xxxxxxxxxxxxxxxx>]

On Sat, 30 Nov 1996, Robert Long wrote:

> On Wed, 27 Nov 1996 13:26:24 +0000 (GMT), Andy Finney wrote:
> 
> |Nice to have a discussion on the group that didn't involve chemistry :-)=
>  As
> 
> 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.

This sounds like the image may be of air turbulence caused by the hot air
rising from the house?  I'm not sure how this is done, but am wondering if
perhaps a schlieren (sp?) couldn't be used to produce this effect.  (I
think there is a description of this kind of photography on the photoforum
web site.)

> 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.

He is correct!

> Maybe he's right; maybe the picture was made
> through a night-vision scope.

Not likely, night-vision scopes are also primarilly sensitive to near IR.
They only amplify existing near IR and visible light.

> there are the iron pictures made on IR film that do show the
> heat.

Sure it you have a hot enough object the radiation spectrum can get high
enough to record on film.  (Sunlight is a pretty good example.)  See my
other posts.

There are thermal recording systems, but these make use of special sensors
that need to be cooled below ambient to get the noise level down.  They
don't use film.  Keep in mind that if the film were sensitive to room or
body temperature radiation levels it would be pretty useless.  The film
would radiate enough energy to fog itself in this case!

>  And, as discussed here earlier, whether a given wavelength
> should be considered as heat depends on what materials we're talking
> about.

NO!  Heat is energy and doesn't depend on wavelength at all.  It also
doesn't have anything to do with materials.  The TEMPERATURE that a
given amount of heat will create in an object will depend on the material
however.

> More recently I raised a question (and got one specific wrong) on a
> peculiarity of certain pigments that I don't understand.  I once had a
> lot of old lacquer recording blanks that I had to strip to send the
> aluminum substrates off for what today we'd call recycling.  I
> discovered that when you looked through a single layer of the lacquer,
> it looked sort of a characterless blue color.  But when I doubled the
> thickness to two layers it looked deep red (very much like a Wratten
> 29), and when I increased the number of layers above two, the red got
> progressively deeper, but only gradually.

The color shift is due to the interference between light reflected from
the top of the film and the bottom of the film.  The color you see is
related in wavelength to (twice?) the film thickness.

>  Similarly, there are bulbs
> in the New York subway tunnels that play a similar trick.  At a
> distance, you see a ruby red glint, but when you get close it turns
> out to be a blue bulb.  (That's what I got wrong--I had near and far
> conditions reversed.  And I don't know what the bulbs are there for.)
> I'd be interested in any explanation of these two phenomena.

Don't know about this.  The eye doesn't see blue very well so perhaps
the red is an optical illusion?

- Wayde Allen
  (allen@xxxxxxxxxxxxxxxx)


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Topic No. 6