How much do bulbs dim over 20 years?

I just replaced the headlight bulbs from a 1994 Integra, made in March 94, after one of them burnt out.They seem to have been the original bulbs that came with the car, being marked Koito and made in Japan. The brown base also had significant fading (which was only obvious after taking them out of the connector).

This got me thinking, how much have they dimmed? Of course just eyeballing it won't detect anything, and both bulbs were replaced at the same time.

IIRC, the new bulbs in the car now are the Amazon fire-sale Nighthawk Platinum, which definitely won't last 20 years!

Unless the gastight seal has been compromised, the light emitted by the filament will not be any different. If the glass has yellowed, it might reduce the amount of light that actually gets into the beam.

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How much dendritic growth has there been?
I pulled out a Toshiba HIR2 (the other expired), and it looks fuzzy. There is even dendritic growth on the return lead.
The halogen cycle doesn't return the tungsten exactly as if found it, so the surface area:volume changes, and you emit less visible light.

I had a sealed beam headlamp lose most of the reflector. That sure hurt output.

MichaelW said:

How much dendritic growth has there been?
I pulled out a Toshiba HIR2 (the other expired), and it looks fuzzy. There is even dendritic growth on the return lead.
The halogen cycle doesn't return the tungsten exactly as if found it, so the surface area:volume changes, and you emit less visible light.

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I have also noticed that if you use a halogen cycle light very long at low power with a dimmer you can end up with a very short life. If the inside of the envelope does not reach the required temperature the halogen cycle cannot return the tungsten. So even with a reduced evaporation rate the filament soon gets thin in one place.

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inetdog said:

Unless the gastight seal has been compromised, the light emitted by the filament will not be any different.

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Over time, part of a bulb's filament vaporizes and gets deposited on the inside of the glass. This of course makes the filament thinner. Which changes its electrical characteristics, and therefore its power consumption, filament temperature and color temperature (not by much though, I'd guess). Ultimately that contributes to an electric bulb failing, of course.

For a halogen bulb the story is more complicated as (afaik) filament material is also lost, but re-deposited somehow. Which of course changes shape / structure of the filament over time.

Basically the evaporated tungsten combines chemically with the halogen. That keeps it from depositing on the inside of the envelope or on the supports. When the molecules finally touch the hot filament the compound breaks down, redepositing the tungsten on the filament. To some extent it will deposit faster on the hottest parts of the filament faster, minimizing the selective thinning of parts of the filament.

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inetdog said:

If the glass has yellowed, it might reduce the amount of light that actually gets into the beam.

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Glass doesn't yellow. If anything, a non-halogen bulb that has lasted through 20 years of normal use will usually have a nice black pearlescent look to it.

A halogen bulb may still exhibit about 90% of its original luminance near the end of the rated lifespan. If it goes well beyond the rated lifespan, then it may have been a "Long Life" variety which outlived its usefulness long before burning out.

See this thread.

The reason your light lasted so long is because your car's wiring is crap. (This is prevalent on most if not all cars). Your bulb isn't receiving the amount of voltage it's rated to, meaning your headlights are only producing half of what they could be. Think about it, the wires have to go to the headlight switch on your dash all the way to your headlights. By the time the current reaches the headlights, the current probably has dropped from 12v to 10v, and the output to half.
Just FYI.


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TotalDbag said:

The reason your light lasted so long is because your car's wiring is crap. (This is prevalent on most if not all cars).

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Headlamp wiring quality can vary between makes and models, but there's one sure way to see if you're losing voltage, following the instructions on this page.

Your bulb isn't receiving the amount of voltage it's rated to, meaning your headlights are only producing half of what they could be.

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It will produce half the light it could be producing only if it's getting about 10.5V of the 12.8V it should be getting. At 10V, it'll be closer to 43% of the original ouput (the light color temp may also be visibly 'off' by that point, looking yellowish or brownish).

Think about it, the wires have to go to the headlight switch on your dash all the way to your headlights.

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Most vehicles on the road today still use a relayed system-- you might notice the stalk or knob isn't getting hot when you're driving-- and you may hear the relays switching, as well. Still, the runs are long and the wires are not an extremely heavy gauge.

By the time the current reaches the headlights, the current probably has dropped from 12v to 10v, and the output to half.

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*voltage

Also, at 12V, a bulb designed for 12.8V will already be at 80% output. Hope your bulbs are getting better than 12V.