Re: How to calculate achievable flashlight beam divergence

"Jim Klein" <jameseklein@xxxxxxxxxxxxx> wrote in message
news:2260s1d4g53ke0c0tihts1gmuk8p5l8i2k@xxxxxxxxxx
>
> Measure the diameter of the bulb, call it D
>
> The full angle divergence angle will be close to the arctan(D/FL).
>
> This is approximate but ought to be close.
>
Jim, thanks for the reply. I understand the beam angle will be
given by that equation.

But a parabolic light source (like a flashlight) can be focused
to have a convergent beam -- a negative beam angle. At a close
distance you can focus the convergent beam to a small spot.

As distance to target increases, you can change the D/FL ratio
all you want, but you lose the ability to achieve a small spot.
Note also this is white incoherent light, not laser light.

I'm trying to figure out what equation governs this. Given
D (source dia.), FL (focal length), mirror diameter, and wavelength,
how do you calculate that?

Observationally it seems to involve mirror diameter, since the beam of
a large searchlight visible through fog converges over a long distance
before it diverges. By contrast the beam of a small focusing flashlight
converges over a short distance before it diverges, no matter how you
try and focus it.

I know about the Rayleigh criterion formula for spatial resolution,
but am not sure whether it applies in this case.

Would appreciate any advice.

-- Joe D.









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