You should get an optics textbook. The surfaces ARE parallel due to the
manufacturing process, and as Robert stated, if there were optical
distortion, you wouldn't be able to look through properly.

On an untreated surface, you have about 25-30% of the incident light which
is reflected.

--
Bert Willing

ASW20 "TW"


"Willy VINKEN" a écrit dans le message de
...
On Wed, 05 May 2004 18:15:06 +0000, Robert Ehrlich
wrote:

Finbar wrote:

I think I can see how this would happen with the canopy open, and the
sunlight hitting it from "inside," reflecting off the canopy surface
onto the focal point of the almost-parabolic canopy. I'm still
holding onto the idea, though, that this can't happen in flight
because the sun can't hit the canopy from the inside. The thought of
an in-flight cockpit fire is really not comforting. Has anyone had
solar/canopy - related charring happen in flight?

I completely agree, focusing solar light can only occur by reflection,
by transmission, as the canopy has parallel inner and outer surfaces, it
cannot change the direction of light rays, only slightly offset them.
Hey, this is just why we can see through it as if it were not there !

I don't agree.
I've seen it happen at EBSH on the rear headrest of a Ka21 with open
canopy. The sunrays incidence was almost tangential to the uprised
dome and definitly crossed the plexiglass from the outside.
Even if outer and inner surfaces are parallel (which has not been
proved), near-tangential rays have to cross more material where they
hit the vertex of the curved dome, which acts as a biconvex lens,
with in this case, a bright focal point on top of the headrest.
I don't believe in reflection-related damage. The sun would have to
be too low on the horizon for this to be possible with open canopies
and due to the see-through qualities of plexiglass, at best only a
very small part of the energy would be reflected.
The transmission-type damage I witnessed could not happen in flight
either: the sun would also have to be too low -thus weak-, even the
slightest movement of the glider would prevent the same spot to be
focused continuously, and above all, the lens-effect of closed
canopies would focus well above anything in the cockpit.
Willy