Jim Wilson wrote:
> How do they work in real life? It seems that animation shouldn't be
> necessary.
I got to see a 2-light PAPI up close at Fishers Island*, NY once.
It's an astonishingly simple device. Basically, it's a box with a
bright white interior. At one end is a lens. At the other (on the
focal plane of the lens) is a white card painted red on the top and
white on the bottom. Along the sides of the box are flourescent light
tubes.
+----------------------------------------------------+
Red# /\
# | | Lens
~ | |
Wht~ \/
+---------- --------+
==============light===============
So when you look through the lens, you see a singularity image
(i.e. infinite magnification) of whatever you're looking at on the
back card. If you're below the centerline, you see the red side. If
you're above, you see the white.
The box is mounted on a pair of sturdy concrete poles, and is tilted
up at the glideslope angle.
Basically, Curt's approach seems exactly right. The image of the
thing depends entirely on direction to the viewer. There are no
moving parts at all.
Andy
* A really cute airport for anyone in the northeast. Fishers Island
is a tiny spot off the tip of Long Island, just south of New London,
CT. The demographic is similar to Martha's Vinyard or Nantucket --
a small contingent of locals and a larger group of very wealthy
vacation home owners. My sister-in-law's parents were in the former
group. I got there on a ferry, not an airplane. :)
--
Andrew J. Ross NextBus Information Systems
Senior Software Engineer Emeryville, CA
[EMAIL PROTECTED] http://www.nextbus.com
"Men go crazy in conflagrations. They only get better one by one."
- Sting (misquoted)
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