P3D light/heat/projection

[John Toeppen <toeppen@xxxxxxxx>]

When I worked for Bell and Howell designing microfilm equipment we were
well aware that all radiant energy can be transformed into specific
heat.  Specific heat is the amount of energy that is stored in a mass
that has absorbed energy.  This energy is usually conducted to a point
where convection can carry it away.  Re-emmission is not an effective
way to remove heat from objects unless they are glowing orange with
heat. The matter was of concern to us because microfilm distorts when
held at temperature.  It can also photochemically bleach into
transparency.  Absorbed light energy can shake molecules apart.

We found many methods to improve our thermal problems.  IR absorbing
glass was effective to some extent, but the blue clipping from the green
glass was objectionable.  Dichroic cold mirrors reflected 92%  visible
light from 400 out to 600 with a sharp fall-off leading to a half power
point at 650nm and a residual 10% fresnel reflection 700nm on out. 
Pyrex condensers were typically used before the cold mirror and Bk7
could be used after it. Springs were used to hold the condensers and
other glass to avoid cracking caused by thermal expansion in a rigid
mount.

I found that crystal quartz had 80 times the thermal conductivity of
glass. This dropped the film plane temperature 10 degrees F.  I was also
able to use a zinc alloy to hold the projection lens (25 degrees F
drop). Keeping the lens cool was also important to protect the cement
from melting in the doublets.

Some dyes are simply not photochemically stable. Dye molecules are
antennae with a physical size similar to the wavelength that they
absorb.  Usually a couple of ring structures hanging out like butterfly
wings are joined at a hub of nitrogen where the whole thing bends,
allowing the "wings to "flap".  If you bend it too hard or too often it
will break there, and the material will bleach.  One of the problems is
that to write an image the system needs to be photosensitive, and to
read it the system needs to be photostable (nonvolatile memory).

I work on a daily basis with lasers running from UV to IR.  The words
that we select are chosen to convey a point, and we often have to
clearly define our words.  Usually we specify a line like 351, 532,
1064, 1053, 990.  Sometimes we define our line width or the frequency
chirp that we impose on a beam.  Often we just say "near IR" for an 850
diode or "mercury green" instead of 546.  Correctness is in the clarity
of the message rather than the specific "correct" verbage. The word
"light" is broadly and loosely used.

Relativity shows that the color of waves of light are relative to your
point of observation, and is also a matter of realtive velocities.  Thus
nature does not ever really give an absolute wavelength to any ray.
Homodyne and hetrodyne techniques applied in radar theory, also revealed
in music theory, are sometimes applied in optics.  So doppler radar and
sum frequency optical mixing have a common thread of physics connecting
them.  It is important to have a wholistic understanding of this kind of
stuff if you want to make effective use of it.  Otherwise, we degenerate
into bickering fools and become obsessed with seeking specific
definitions for intrinsicly fuzzy terms.  Language can never accuratly
embrace reality, it is only a symbolic code.

Well, I am really way off topic now.  But I have built and repaired more
than one 3D projection system where light and heat mattered. Maybe I
should stick to the .jps topic.

John Toeppen


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