Date: Fri, 3 Jul 2015 04:15:40 -0400
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: LED Headlight bulbs
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At 09:50 PM 7/2/2015, Neil N wrote:
>I haven't owned Vanagons THAT long!
>And yes. "Bacon" is a reference to Canada.
However long you've been on the list there have been people
advocating for H-4 E-code lighting, I among them. I'm somewhat
distinguished from the herd by not pushing for high-wattage lamps --
but I had H-4s on my Sunbeam Alpine and on every vehicle since that
didn't have those nasty custom headlamps. It's a perennial subject
on the list and with me.
>I think this was mentioned, but I'd
>be dubious of long term reliability.
They are a stable and well-understood technology, and the art of
making incandescent lamps with precise control of filament placement
and dimensions and light output existed long before halogen lighting
came along to make everything better (until one explodes in your face).
>But then OTOH, LED festoons
>might withstand vibration better
>than a piece of tungsten?
Oh, wait -- you were wondering about LED durability? The devices
themselves are extremely reliable and with extraordinarily long
lifetimes so long as you don't overdrive them or let them get
hot. The last is a problem; compared to incandescents they put out
very little heat; but they also die and go to heaven hundreds of
degrees before even a hint of a glow appears. They like room
temperature just fine, and I would expect that not too far above that
you'd run into a regime where lifetime is cut in half for every 10C
rise in temp (rule of thumb for chemical reactions around room temp
is double/half the reaction rate per 10C change). So you have to
pull the heat away from the semiconductors and get rid of it somehow,
and that's one of the major challenges of power LEDs.
The rest of the asseembly will have to work to keep up with a halogen
lamp grouted into a metal base with all connections welded. Solder
joints and all, not to mention environmental protection for the
actual semiconductor and its bond wires. However I see a ton of
traffic lights with sections of the surface that flicker or don't
light -- don't think I've ever seen one with a single element not emitting.
Optically they're utterly different from incandescents, Headlights
and such are designed to work with short line sources that radiate
pretty uniformly into a sphere that looks more and more like a
cylinder as they get longer. The only way you can simulate that with
an LED is to map a spheroidal surface some distance away and cover it
with emitters -- but those emitters aren't unidirectional and shining
only outward, they're themselves radiating into some portion of a
hemisphere, so there's a bunch of stray light coming to the
reflectors from the wrong direction. You can make wonderful LED
lights but you have to design for how they work, not use them as
pretend incandescents in existing fixtures. Not a problem for van
cabin lights and such but no fixture with precise optical
characteristics can fail to be degraded. You may well get light
where you want it, or mostly where you want it; but you will also get
light in places where you specifically do not want it. And getting
it where you do want it, uniformly, means mapping onto a larger
surface; where keeping it from where you don't implies going back to
your point/line source.
Yrs,
d
>Neil.