LED Headlights

[Jim/GA]

Originally Posted by 406CID

I was concerned about this early in the design too. It was pointed out to me that the 21/21 - 32/50 bulbs emit like an isotropic emitter (light in all directions). It is brighter up and down than side to side, but light is emitted in all directions from the original bulbs. This means on your incandescents and original reflectors, light goes down to the bottom of the parabolic reflector and reflects up somewhat. When you add the high beam, you simply add more light as the two filaments emit from the same location (not two very different locations like a modern day halogen). Most of the bulbs investigated even had a common point on one side, so the filaments were colocated.

I'm sorry, but the above is not true for an original Model A Two-Light headlight reflector and original bulb. You need to do some more research on original bulbs and reflectors, not aftermarket replacement junk that was produced in the '50s and '60s.

Yes, the light is emitted from each filament in all directions. It approximates a point source. That is how a parabolic reflector works. No, a Model A High beam does not have both filaments lit to "add more light from the same location". It is one or the other filament (High or Low), not both.

On an original headlight assembly, when focussed, the High beam filament is exactly centered at the focal point of the parabolic reflector. The placement of the bulb socket is NOT exactly on the center axis; it is a bit high of center. But the High beam filament is also off center (low) inside the bulb by the same amount. So this places the High beam filament exactly on the center line axis of the reflector. You use the focus adjusting screw to move the filament along this axis until you exactly hit the focal point.

Then you adjust the headlight bucket itself to aim this beam straight out from the car (on a wall 25 feet from the headlights). When the High beam is aimed in this manner, the Low beam is automatically correct, by design.

That's because the Low beam filament, inside the original Model A 20 cp/20 cp bulb, is a separate filament that is higher than the High beam filament inside the bulb. When it lights, because it is now high to the center line and focal point of the reflector, the beam is slightly dipped down at the road (not straight out into the other driver's eyes).

This is all well documented in original drawings that you can get from the Benson Ford archives.

Note, the above description does not apply to the 1928 and early 1929 headlights, with the fluted lenses. They only had 20 cp Driving lights (similar to High beams) and 3 cp parking lights in the headlight buckets. They did not have a bright beam that was aimed down at the road. They did not have the lens of the later Model A; their headlight lenses looked exactly like the fluted Model T lenses.

It sounds like you need to go back to the drawing board because this design is based on faulty assumptions and measurements. Otherwise oncoming drivers will be blinded by the light put out.