The PAPI Lighting System

1. The PAPI Lighting System A PAPI unit is a simple optical system. It is a big slide projector. The image of a red/white slide is projected onto the sky.

2. The PAPI Optical System Glass cover plate Red transparency plate Reflector throws light forward to produce brighter beam 2 lenses act effectively as one good lens Empty space below red plate acts as white part of "slide" We have verified correct operation of our reconstructed PAPI system: • brightness • angular width of beam • sharpness of white/red transition • aviation red color

3. The PAPI light path Rays passing through points on red slide produce red beam into lower part of sky. White beam Rays passing through points below red slide produce bright white beam into upper part of sky. Red beam Single effective lens produced by 2 PAPI lenses

4. Why 2 lenses in the PAPI? • 2 lenses correct "spherical aberration", producing a sharper image with crisper transition from white to red in the sky.

5. Condensation on glass cover plate scatters light clear glass fogged glass • Small droplets scatter much more strongly than large droplets: • You can see through glass with rain drops on it, but not through fogged glass. • You can see through falling rain, but not through clouds or fog.

6. Ideal conditions for condensation: • Close to 100% humidity (temperature = dew point) • Mostly clear sky: The clear sky is cold. Objects that can see the sky cool by radiation. • Time is just before dawn: PAPI has had all night to cool.