Beams
The landing and taxi lights, on the ground and in the air.
A lamp's beam is taken as a Gaussian: its intensity falls to a tenth of the peak at half the catalogue's spread, each way, so a 50° × 10° taxi light is wide and flat.
On the ground
The beam lights the scene as a three spot light of the lamp's peak, with the Gaussian laid over it. A point on the ground gets the intensity the lamp sends its way over its distance squared, in lux, and three's materials reflect it.
In the air
Looking through a beam, the eye sees the light the air scatters towards it from every point along the ray: the lamp's intensity that way over the distance squared, times the air's scattering at that angle, less what the air takes on the way in and out. It is summed with a few dozen samples, set closest where the ray passes nearest the lamp, where the light changes fastest.
The cone is drawn only as far as that light can show. A ray passing a lamp collects at most its intensity, times the air's scattering, times π over how near it passes, and none passes nearer than the lens: past where that falls below what the screen shows, along the beam or across it, there is nothing to draw.