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

[Phil Payne]

> 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.)