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.

The PAPI Optical System 2 lenses act effectively as one good lens Glass cover plate Red transparency plate Reflector throws light forward to produce brighter beam 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

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

Why 2 lenses in the PAPI? 2 lenses correct "spherical aberration", produces a sharper image with crisper transition from white to red in sky. Single lens will work OK – some manufacturers sell single-lens PAPI's: Flight Light Inc,

Condensation on glass cover plate strongly 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.

Ideal conditions for condensation: 100% humidity (Temperature = dew point) 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.