Re: Highwaycode and pedals question

David Martin wrote: 
Matt B wrote:



If the whole sphere of light emitted
from a point source is concentrated uniformly into a cone occupying 1/10th
of the sphere 1000/d**2 will be the formula?



No. It won't.

If you take a beam from a headlight and measure the intensity at a
distance d, this will eb equivalent to a beam from a point source at an
apparent distance p.

so the ratio between teh intensity at 1 unit distance , and the
intensity at n units distance is

p**2/(p+n-1)**2

As n becomes large, p becomes negligible. You can measure this
effective distance by looking at how rapidly the beam on your lights
spreads with respect to distance. It is still obeying the inverse
square law, but from a virtual point light source, rather than the
reflector.

The properties of the reflector are best modelled as a gain in the direction of the observer, thus a 100W isotropic light source and a reflector with a gain of 10 appears as a 1000W light source to the observer.

Whatever the distance to the observer, the source will always be 10x isotropic.

This is quite a useful effect in the Radar world, where the gain works on both transmission and reception. See Skolnik et al for the maths.

Al.
.

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