Bicycle Headlight Optics Info

A post from the randon@topica mailing list, with some useful info on bike light optics...
Date: Tue, 12 Sep 2006 07:42:21 -0400
From: "Ingle, Bruce" 
Subject: RE: EverLED Model ?

Bob asked:

> Which EverLED model are you using with the HL500,
> the EverLED Classic (wide pattern),
> or EverLED Lambertian (narrow pattern)?

I've only ever used the EverLED Classic, which has a
side-emitting Luxeon.

The long-winded explanation, if anybody's interested:

The majority of the focused light seen in a tightly-
focused beam from a well-designed halogen headlamp
is focused (collimated) by the reflector to a point,
then diffused horizontally by a cylindrical fresnel lens
into a line.  When this is projected onto the road
at a narrow angle, the result is a trapezoid of light.

(There were some new optical designs a few years ago
where the collimating and horizontal diffusion were
incorporated into the reflector alone; these designs
are marked by a clear front lens.  They boast increased
efficiency, but have largely been rejected by cyclists.
A prime example is the Cateye MC-200, which has since
disappeared in favor of the older HL-500II design.)

The remainder of the light outside the main beam is
that which emanates from the bulb but is not collimated
by the reflector -- the light that you see from the
bulb when viewing the light from outside the focused
beam.  It doesn't contribute to the focused beam, but
it does improve visibility as well as providing enough
stray light to detect movement (e.g. skunks, deer)
outside the focused beam.

So, if you want an LED to closely replicate the beam
pattern of the halogen lamp it's replacing, it needs
to put the majority of its light into the reflector.
The side-emitting Luxeon of the EverLED Classic is
designed to do this.

A lambertian Luxeon focuses the light from the die
into a somewhat tight spot by itself, similar to that
seen from lower-power discrete LEDs like those used
in the Petzl Tikka, etc.  However, when used in a
headlamp designed for a halogen lamp, the spot is not
as small as that which is focused by the reflector, and
it may be diffused somewhat unpredictably by the
forward lens.  For example, the lens of the Cateye HL500
incorporates a spherical element in the center of the
pattern designed to focus some of the radial forward
emanations of the bulb into the main beam; shining a
Lambertian Luxeon directly into this should only
diffuse the Luxeon's beam into a yet larger spot.

I'm not sure what application the EverLED Lambertian
is specifically intended for.  I've seen some homebrew
designs that combine a lambertian Luxeon with a focusing
headlamp (e.g. Maglite or Vistalite VL400) with decent
results, but these generally aren't well-suited to road