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Thread: LOW BEAM only headlights

  1. #1

    Default LOW BEAM only headlights

    Why do LOW BEAM only headlight bulbs not have a reflector cup inside the bulb like an H4 Bulb ?

    I would think all low beam only headlight bulbs would have a reflector cup inside to reduce light glare ?

  2. #2
    Moderator Alaric Darconville's Avatar
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    Sep 2001
    Stillwater, America

    Default Re: LOW BEAM only bulbs

    Quote Originally Posted by RandyMan79 View Post
    Why do LOW BEAM only headlight bulbs not have a reflector cup inside the bulb like an H4 Bulb ?
    There isn't any "low beam only" bulb; there are bulbs that either have a major and minor filament to provide both a high and a low beam, and there are single-filament bulbs, which might be used in a high beam or might be used a in a low beam. For example, the H11 might be used as a high beam bulb for certain vehicles, although it is used more often as a low beam bulb.

    By "reflector cup", do you mean the small internal shield for the H4's minor filament? It reserves the lower portion of the reflector for the major filament-- otherwise the lower portion would reflect light outwards from the minor filament, but not in any useful pattern because of the minor filament being at a different focal point from what is required to work as a high beam.

    For those composite headlamps using a separate compartment for the high and low functions, the individual compartment is optimized for the beam pattern desired.

    Essentially, the only need for a shielded low beam filament is when it's in a dual-filament bulb a single-compartment dual-beam lamp.

    I would think all low beam only headlight bulbs would have a reflector cup inside to reduce light glare ?
    Given that there's no such thing as a "low beam only" bulb, there's no need to think that. There are low beam only *lamps*, but those are designed for their selected bulb.

    This is your second post where you talk about "low beam only" bulbs, only to be told there's no such thing. Again: There's no such thing.
    Last edited by Alaric Darconville; 12-22-2017 at 10:47 PM.

  3. #3

    Default Re: LOW BEAM only headlights

    Yeah, the filament shield -- it's not a reflector -- under the low beam filament of bulb types H4 and HB2 (9003) (and motorcycle-only headlight bulb HS1, and nonstandard variants of it) was originally designed to allow the production of a strong, symmetrical high beam as well as a low beam with a sharp light/dark cut-off at the top of the beam, as required in Europe, using the simple parabolic reflectors available at the time these bulbs were developed. The shield blocks off the filament through a 165-degree arc of the bottom of the reflector. The projection of the edges of this shield is what creates the cut-off at the top of the low beam pattern. There is no shield on the high beam filament, so its light strikes the whole reflector including that bottom section where the lens/reflector optics are optimized for high beam focus (just as the upper part, the only part used by the low beam filament, is optimized for low beam focus). This is how lamps equipped with these bulbs produce a low beam and a high beam that are fundamentally different from each other in terms of the beam shape. As the years went on, the H4 bulb was used for US-spec headlamps, too, with different optical techniques used to soften and reshape the cut-off.

    The US low beam specifications don't require a cut-off, and so low beam and high beam patterns tended to be more or less the same shape -- just focused straight ahead for the high beam setting or downward and to the right for the low beam setting. The way this is done is to put the high beam filament at the focal point of the reflector, and the low beam filament a little bit above and a litle bit to the left (as viewed from the driver's seat) of the focal point. Filament a little higher = beam a little lower. Filament a little left-shifted = beam a little right-shifted. Neither of two filaments has an H4-style shield, and the whole area of the reflector is used for both low beam and high beam in this system. That makes the American system (as seen in bulbs like HB1/9004, HB5/9007, and H13) more efficient, in that the beam contains more light because of the larger effective reflector area on low beam. But this means we can't have areas of the reflector/lens that are optimized for one beam or the other, so the low and high beam can't really be different shapes. These are some of the tradeoffs faced when deciding what kind of light source a new headlamp will be engineered to use.

    Single-filament bulbs (like H1, H3, H7, H9, H11, HIR2, and a bunch of others) do not have filament shields. The oldest way of creating a low beam with a bulb like this was to have a filament shield built into the headlamp unit, blocking off the lower part of the reflector/lens from the filament, same technique as in H4, just done with an external shield. That was in Europe where the sharp cut-off was required. In the US, again the whole reflector/lens was used for low beam.

    Nowadays we have much more sophisticated reflectors than the old simple parabola. A complex-shape reflector can create pretty much any beam shape we want, using the whole reflector area and without blocking off any part of it, even if a sharp cutoff is required. This is why that thing you said was wrong the other day about H11 bulbs radiating light only upward, any downward light being a waste, etc. And it's also the answer to your question today about why single-filament bulbs used in low beam headlamps do not have filament shields.

    The newest 2-filament bulbs, H19 and its motorcycle variant H17, do have a filament shield, but it's a smaller/narrower one than on the H4. We no longer project the edges of the shield to create the cutoff, instead we use complex-reflector technology to create the cutoff, just like a modern reflector with a single-filament bulb. That means greater system efficiency, since a larger portion of the reflector is used for low beam. The filament shield's only job in these new bulbs is to reserve a portion of the reflector for optimization of the high beam.

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