What is the taxiway optical beam?

The optical beam is the shape of light a fixture sends out: its intensity, in candela, at each angle above the horizontal and around the fixture. Taxiway lights are designed with a beam that matches how they are seen. Taxiway edge lights must show up to at least 75° above the horizontal and at every azimuth needed to guide a pilot taxiing in either direction. ICAO Annex 14 sets a minimum of 2 cd from 0° to 6° and 0.2 cd between 6° and 75°. The strongest light sits at low angles because that is where a pilot sees a light from a distance, close to the ground; the weaker upper fan is still enough when the aircraft is near.

Taxiway centreline lights use a different beam, dimensioned so the light is visible only from aircraft on or in the vicinity of the taxiway. That keeps a pilot on a parallel taxiway or a stand from picking up the wrong line. Annex 14 Appendix 2 gives separate intensity distributions for straight and curved sections, and the curved-section beam is wider so the light stays visible as the aircraft follows the bend (see Entry 26). Where edge lights sit at an intersection, exit or curve, they are shielded as far as practicable so they cannot be seen from angles where they might be mistaken for other lights.