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According to a physicist friend, it ain't the infra red getting in - it's
the higher energy radiation (read: most of the visible light plus the stuff
up at the blue end of the spectrum) that gets frequency down-shifted when it
passes thru the canopy plastic. Once shifted down to the lower energy area,
i.e., infra-red (noticeable as heat), the resulting longer wavelength won't
penetrate the plastic so it can't get out. Ergo, the interior gets hot.

If this scenario is accurate, the need is to block out the spectrum all the
way from the visible up to the ultra violet. The impinging infra red can't
get in in the first place, so it's not the problem. Means that if you want
to be comfortable and cool, all flights must be IFR since the result is that
all light penetrating the canopy must be blocked.

Dan Schaefer


[Thank your friend, the physicist, for the concise explanation.  It is
precisely this "down-shift" which he mentions that brought about the
implementation of low-E coatings to the window business back in the mid-
80's.  The trick with these coatings is that they are transparent to the
higher wavelengths but reflective to the lower ones.  This is the reason
that if you are trying to conserve heat in a house the coating goes on
the airspace side of the inner pane of the insulating unit... when
the light enters it passes all the high wavelength energy but then reflects
the IR that's trying to escape through the window back into the house.
Besides sending back some of that otherwise lost heat into the house it
also heats the inner pane slightly and removes some of the cold spots
around the windows.  OTOH, if you're fighting a cooling battle, the
coating is normally placed on the airspace side of the outer pane, to
reflect the lower wavelengths which are generated mostly by reflection from
the surrounding area.  I do believe that there is some direct IR
attenuation by the coating, but the effect is not as great as what is
experienced by the down-shifted energy from the inside.  That being said,
by placing the coating on the inside of a tinted canopy you get to attenuate
the IR wavelengths before they get into the cockpit, and cut back on the
visible light which will ulltimately downshift into additional IR on
its reflected way out.
    <Marv>       ]
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