At 09:46 PM 11/30/99, Alex wrote:
The height thing I think is important and I believe that, in the US, putting
driving lights low down allows them to give a cool appearance, yet be low
enough to avoid blinding oncoming drivers like high beams.
Driving lights should always be placed as high as practical so that they
are able to extend their illumination as far ahead as possible. Driving
lights are made for normal/high speed motoring. People who place driving
lights very low are possibly after the cool (questionable) effect, or they
do not understand the principles at play.
…I don’t personally
think this achieves what driving lights should, as when mounted higher.
You’ve got it right to here…
…The
idea of direct reflections back at the driver is, though, what we want, if
there’s something out there to see. If there’s some mist, then, as on freight
trains, the light should be aimed from the viewer’s eyeball height for maximum
effect. If they are lower, or higher, then an angled light path will appear,
in any mist, that we then can’t quite see through.
Well, um, no. Most of the time, yes, having the light aimed from near eye
level is what you want. The rub, however, is in the mist.
A driving light typically has a “pencil” pattern or similar. While it is
fairly narrow, it still has a considerable amount of dispersion as the
light travels outward from the lens. It is this spread that causes the
problem in the fog (or mist).
When light travels through mist/fog, a considerable portion of it is
scattered off-axis and/or reflected back toward the source. As a result,
when you shine a light along your line of sight through fog or mist, its
path is marked by a bright spot (the reflected portion) surrounded by a
diffuse halo (the scattered portion which is eventually reflected back
toward the observer by a complex path through several droplets). When
driving lights are mounted high and used in fog/mist, you are seeing
illuminated targets (cars, trees, moose, etc.) through this glow and after
the light has traveled the wet path twice (once out, once back, diminished
and scattered each way). Not ideal for discrimination.
Fog lights, on the other hand – and as the name suggests – are made
specifically for this situation. They are designed to provide a broad
“fan” pattern as low as possible to the road. Why low? Two reasons.
First, by placing the lights low, the diffuse glow described above is
placed far below the driver’s line of sight – he sees considerably more
light reflected from the target ahead than reflected from the water in the
air. Second, a characteristic of fog is that it is frequently less dense
quite near the ground. At the level of properly mounted fog lights, there
is normally much less fog to penetrate so there is a higher proportion of
the original output of the lamp returning to the driver’s eyes (having been
reflected from the car, tree, moose, etc.).
With their low mountings, there will be, of course, a loss in ultimate
illumination distance due to intervening rises in the road. However, it is
assumed that fog lights will be used in low speed situations where the
importance of seeing those last N meters is less important than seeing,
say, the interface between road and shoulder (or ravine).
Which, BTW, is why I don’t think the analogy of the locomotive is
especially applicable. Unless he does something really wrong, the
engineer can’t run off the road. Therefore, because if the superior clear
air performance, the high mounting of his lights is best for him. He does,
however, suffer from the same fog blindness in poor weather.
I suppose that in a world with little regard for aesthetics, we’d have a
pair of fog lights hanging under the bumper and a pair of driving lights on
stalks above the bumper. Ugly, but optimal.
Cheers.
Burgess
86VdP(Federal)