Jim Edwards said:"Tim McNamara" <[email hidden]> wrote in message "]news:[email hidden]...
Quoted message said:At PBP this year I saw many people using LED lights, most of which cast a very bright and very
narrow beam. Many people also had *no* clue about how to mount and aim a headlight, putting it
above the bars and pointing it at the ground right in front of the wheel. The light appeared
adequate in brightness but too narrow in focus for my tastes.
I have a question about the following statement: "Many people also had *no* clue about how to
mount and aim a headlight, putting it above the bars and pointing it at the ground right in front
of the wheel"
It seems you would out-ride your headlight at a very slow speed. I don't hammer on my early
morning commutes, but I need to see at least 5 feet in front of me to be able to dodge road
debris. It seems like pointing the light in front of the front wheel would allow you to see what
you are running over, not avoid it. How do the rest of you aim your lights?
OK, let's start with a brief discussion of perception. Why are we lighting up the road? To make it
possible to obtain information. What information do we need? We need to be able to see the edges of
the road, the texture of the road surface, objects on the road surface, and objects that might
project into the space above the road from the side. We also need to be able to see our own
movement. As diurnal organisms, humans are visually oriented to the horizon and the position of the
sun, and as we move we see a sort of "optical flow" coming at us from a point on the horizon and
receding behind us to a point on the opposite horizon- visual texture that diverges as we approach
and converges as we recede from any point in the environment (see James J. Gibson, _The Senses
Considered as Perceptual Systems_ and _The Ecological Approach to Visual Perception_).
A light mounted high, such as above the handlebars, has to be pointed downwardly at a more acute
angle in order to illuminate the road in front of the wheel. The road is lit brightly, with a "pool"
of light extending from a foot or two in front of the wheel to maybe 15 to 20 ahead. There is not
much illumination of the road ahead, therefore, including road signs, lane stripes, and objects that
might project into the space above the road. We also don't get much sense of movement because we
cannot properly see the divergence of visual texture ahead of us (and hence we tend to overestimate
our speed when riding at night).
The lower the light is, the longer the illuminated "pool" because the beam is more horizontal and
closer to the road surface. The pool of light can still start a foot or so in front of the wheel and
extend maybe 50 feet. You can see the texture of the road better because this type of lighting
creates shadows better and there is less bright "scatter" coming back at you which permits better
dark adaptation (this is why fog lights are mounted low, usually below the front bumper). This gives
you a more accurate sense of your speed.
My headlight is mounted moderately low, just in front of the fork crown. The (old?) British standard
is to mount the headlight on one of the fork blades, an even lower placement. The French tended to
mount the light at the front of the front fender or on one side of the front rack, lower than my
placement and higher than the British. I have seen pictures of the light being mounted at or below
the fork end. There's a tradeoff with such mounting- the wheel throws a shadow to one side, and one
will tend to be turning into the dark when turning to the side away from the light. My headlight
location casts a shadow directly in front of the bike.
I would also recommend staying away from the clear or translucent lights, which Cateye in
particular sells. The idea is to provide a 360 degree immumination so cars and such can see you-
but this of course will shine in your eyes and prevent good dark adaptation. All the light should
go forward; you should wear reflective clothing and use a taillight to provide visibility from the
side and rear.
Head mounted or helmet mounted lights are an interesting case, tending to produce a "flat" visual
field with inadequate depth cues and a lack of shadows and texture. On the other hand, wherever you
are looking is illuminated, which is not the case with a fixed-mount light on the bike. I have a
helmet mounted light and don't care for it very much because of the drawbacks, but if I was doing an
off-road night ride I'd want both a fixed light and a helmet-mounted one. I have also found it
helpful in attracting the attention of drivers.
