- #1
joema
- 106
- 3
Assuming a typical white-light focusing flashlight (e.g, Mag-Lite) with a parabolic reflector focused to produce a convergent/divergent beam, how do I calculate the smallest achievable spot size (i.e, smallest beam cross sectional area) at a given distance?
How does this vary with reflector diameter, source size, focal length, distance to target, and beam focus (whether source is inside/outside the focal point)?
I can't just use angular field = arctan (source dia. / focal length), as the beam can be initially convergent then divergent.
Observationally it seems to involve mirror diameter since a large WWII searchlight beam converges for a long distance before it diverges.
Looking for the right formula.
How does this vary with reflector diameter, source size, focal length, distance to target, and beam focus (whether source is inside/outside the focal point)?
I can't just use angular field = arctan (source dia. / focal length), as the beam can be initially convergent then divergent.
Observationally it seems to involve mirror diameter since a large WWII searchlight beam converges for a long distance before it diverges.
Looking for the right formula.