Foglights [was: Cats and Audi's and rear fogs]

[Larry C Leung]

Touche!

On Fri, 28 Dec 2001 15:00:32 +0100 "Phil Payne"
<quattro at isham-research.com> writes:
>> Actually, front fogs project a flat topped (rather than spread)
>beam
>that
>> is wide which makes it's intensity lower due to the wide area of
>> dispersion.
>
>The main idea of fog lights is to illuminate the road surface while
>at
>the same time illuminating less of the fog between the road and the
>driver's eyes.  That is why they are mounted low down and have a very
>flat-topped beam.
>
>As they're lower than normal headlights they have to be angled higher
>to hit a useful part of the road - typically something like 0.3%
>instead of 1%.  This makes them proportionately brighter as percieved
>by an oncoming driver.
>
>The main advantage is utterly negated if the headlights are also on.
>Until recently, it was actually an offence to drive in the UK with
>both headlights and foglights switched on - it is still an offence to
>drive with foglights on if visibility is more than 50 meters.
>
>The perceived brightness of a light source is independent of
>distance.
>The amount of light reaching a distant point falls off with the
>inverse square law, but the apparent angle subtended by the source at
>the observer's eye is equally affected - thus the apparent brightness
>is independent of distance.  Bright lights a mile away are just as
>dazzling as bright lights a quarter of a mile away.
>
>(Any wanting to argue this point of elementary physics should
>remember
>the outcome of the 'hot water freezes first' debate or look up
>"Olbers' Paradox" in Google.)
>