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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
Comments: To: Neil N <musomuso@GMAIL.COM>
In-Reply-To:  <CAB2Rwfj+Srnt2umN0d0iOyN+KH1s=opP8e1kfNctDd2g9MsKTQ@mail.g
              mail.com>
Content-Type: text/plain; charset="us-ascii"; format=flowed

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.


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