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" Cc: randon@topica.com 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 cycling.
