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Old Nov 20th 2017, 11:31 PM   #1
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Optics for dipped headlight

I'd like to turn a bike light into a dipped headlight. I.e. it should spread down and sideways, but not up, above eye-level. Can this be achieved with a lens? If so, what shape would this lens be? Assume typical bike-light lens material.
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Optics for dipped headlight-7153yujya1l._sl1500_.jpg   Optics for dipped headlight-7quot-round-dot-approved-led-headlights-conversion.jpg  

Last edited by IcedChill; Nov 21st 2017 at 09:06 AM.
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Old Nov 21st 2017, 02:07 AM   #2
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My understanding is a car headlight is dipped by switching off the main beam filament and powering a secondary light source which is off set from the focal point of the parabolic reflector ... it's not done by the lens ...

If you only want a light in dipped mode , it's very easy , just adjust it so it's aimed where you want it ...

If you want to be able to switch from main beam to dipped , then the most sensible way is to have two separate lights aimed in the way you want , only powering one at a time , have two switches on the handlebars .
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Old Nov 21st 2017, 07:06 AM   #3
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I'd like the projection just below eye level to be very bright. Simply re-angling the light won't achieve that. Please see attached photo for what I'd like to achieve.

Last edited by IcedChill; Nov 21st 2017 at 09:10 AM.
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Old Nov 21st 2017, 07:22 AM   #4
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If you can fit a dual filament main/dipped beam bulb into the headlight shell,
then I think that you should be able to achieve the effect you are looking for.

The two filaments in this type of bulb are separated by a reflective plate
thus the dipped filament only illuminates the lower portion of the parabolic reflector, while the main filament illuminates the upper part of the parabolic reflector.

The lens modifies the beam further, in particular directing the beam laterally away from the oncoming traffic.

While this modification may be feasible,
be aware that there may be legal requirements for vehicle headlights that you might have to meet.
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Old Nov 21st 2017, 07:59 AM   #5
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Ah useful to know how car headlights work!

Unfortunately I can only add something in-front of the headlight shell.

I was wondering about adding a lens cut in half to form a semi-circle, leaving the lower part of the light with no lens at all.
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Old Nov 21st 2017, 08:38 AM   #6
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An alternative you could consider might be a physical shade to restrict the beam for dipped headlights.
Arrange this shade so that it covers the top halt of the light, and this will greatly reduce the top half of the beam.

I was thinking of something a bit like the old war-time headlight shields, designed to stop lights being visible to enemy aircraft.

It could work, but how practical would it be for day-to-day use?

I don't think the half lens would give you the effect you want.
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Old Nov 21st 2017, 09:09 AM   #7
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Ah actually I have already built a little baseball cap for my light. But it still throws a lot of light upwards reflecting from the bottom of the reflector, and making the peak longer to fix this will catch the wind. That's why I was hoping to use a lens.
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Last edited by IcedChill; Nov 21st 2017 at 09:14 AM.
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Old Nov 21st 2017, 11:17 AM   #8
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Silly Me, I was thinking you were talking motor bike.

A lens could be designed to redirect the beam,
but its shape would not be immediately obvious.

The current shape is essentially based on a section of a sphere and is designed to focus the diverging light from the bulb into almost, but not quite a beam, so it still diverges slightly.

I guess that the new shape would have to be somewhat non-symmetrical so that the top part of the beam is bent down more than the bottom half.

If you can find a clear plastic rod with a diameter about half that of the current lens,
try placing that rod in front of the lens, across the beam, and seeing what it does.
You might find it better with a rod which has a D cross section.

I don't know if this will work, but it might be worth some experimentation.
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Last edited by Woody; Nov 22nd 2017 at 04:39 AM. Reason: spelling
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Old Nov 21st 2017, 07:51 PM   #9
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The lamp in your picture does not appear to have a zoom ....

This is essential for what you want ... there are many extremely cheap ones on ebay search " bicycle light zoom "

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