What is the PAPI optical principle?
A lens and a colour filter edge at its focal plane turn one light source into a beam that is white above and red below.
A PAPI unit has no moving parts that change colour. It uses plain optics. A light source sits behind a projection lens, and a colour filter covers half of the source image in the lens’s focal plane. The edge of the filter lies exactly on the optical axis.
A lens turns every point in its focal plane into a parallel beam, aimed at an angle that depends on how far that point is from the axis. Points from the clear half leave the lens as white light travelling above the axis. Points from the filtered half leave as red light travelling below it. Because a single lens inverts the image, the red half of the filter sits above the axis inside the unit.
Since the filter edge is in the focal plane, the lens projects it as a very sharp change in angle: the pilot sees white, then red, with almost nothing in between. The axis is the transition, so tilting the unit sets the angle at which the pilot sees the change. Four units set at four angles form the PAPI signal.
Frequently Asked Questions
Q1What is the optical principle of a PAPI unit?
A light source sits at the focal plane of a projection lens. A colour filter covers half of the source image, with its edge on the optical axis. The lens turns each point of the source into a parallel beam, so one half of the filtered image becomes white light above the axis and the other half becomes red light below it.
Q2Why does the colour change so sharply?
Because the filter edge lies exactly in the focal plane of the lens. The lens projects that edge as a sharp line in angle, so the pilot sees the colour change within a very small vertical angle instead of a gradual blend. Anything that moves the filter or lens out of focus softens the transition.
Q3Why is the red filter on the upper half of the optical path?
A single lens inverts the image. The part of the source that lies above the axis is projected downward, so the red half of the filter sits above the axis inside the unit and appears as red light below the transition angle outside it.
Q4What sets the angle where red changes to white?
The elevation tilt of the whole unit. The transition lies along the optical axis, so tilting the unit up or down moves the angle at which the pilot sees the change. Each unit in the wing bar is tilted to its own setting angle.
Q5Does an LED PAPI work on the same principle?
Yes. LED units replace the lamp with an LED source but still form the red and white halves with a filter edge and a projection lens. The light source technology changes, the optical principle does not. Always follow the manufacturer's photometric data and the applicable standard.
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