I was just wondering why mercury vapor lamps get dimmer and dimmer as they age. They don't seem to "burn out" the way other HID lamps do, they just keep getting dimmer until they are practically a night light bulb.

Is this why MV has gone by the wayside? They do still seem popular in a lot of places, like those barn lights and for parking lot lights. The bulbs only cost about $10.

Metal halide, which is basically just an improved mercury vapor lamp, has the same lumen drop off characteristic as MV, though not as bad.

Peter

well the mercury vapour lumiaire system been around here for very long time i belive it started around mid to late 1930's until now.

ok the mercury vapour light bulb have pretty long arc tube inside of it and the color of the light it self is bleu-greenish color and they do have very long service life many do run 24,000 hours and plus but after about 18,000 hours the bulb start to go dim over the time because the arc tube get blacked out from tugsten filment [ hope spelling is correct] .

mercury vapour light bulbs have the lowest LPW [ lumines per watts ] in the hid family and the mercury vapour have typicaly about 58 to 65 lpw compared to metal haldie bulbs they do almost double over the mercury vapour ratings.

the cost of mercury vapour lumaires is kinda low but common used in sectery lighting and landscaping which they do bring out green out nice.

but many years ago it used to be very popuar and one catalog book i have old one the biggest one they have that time was 3,000 watts excat size i was not sure but feel bigger than bt56 bulb size wise

hope it will help some info here.

merci, marc

They do seem to last forever. I have one in front of my house and I change the lamp every 20 years whether it needs it or not! It's on all night and the lamp has never gone bad. Every other type of lamp need replacement at regular intervals.

-Hal

I think that the mechanism behind the dimming is the electrode metal evaporating and then condensing on the inside of the arc tube, creating a dark layer that blocks some of the light.

Thanks for the replies. The arc tube blackening makes sense, as I've noticed dead metal halide bulbs have similar darkening of the arc tube.

As I was driving home last night on the highway, I couldn't help but notice the harsh, poor quality light coming from the HPS lights. Makes me wish there was something better than what we have available. LEDs look promising.

Just a side note, the new makers of the new ceramic metal halide lamps claim to have the arc tube destruction problem pretty well conquered.

Peter

NJWirenut,
You are on the right track. What actually happens is Mercury Vapor lights have a vacumn in them and they suck the inky darkness out of the outsideness. The dark accumulation you notice is the sum total of the darkness the bulb has so far devoured.
When the bulb has absorbed as much darkness as it can hold, it stops working and is black inside.
~Peter

Oh, yeah. I forgot about the "dark sucker" hypothesis. More on this phenomenon at:
http://www.btinternet.com/~homepage/dark.htm