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glowing around support wires in MH

Discussion in 'Lighting' started by Pawel Paron, Mar 13, 2007.

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  1. Pawel Paron

    Pawel Paron Guest


    Here is my another experiment with overpowering an MH bulb: Osram HQI-T 250
    W/D powered with a 400W coil ballast (using its 240V terminal instead of
    230V, so lamp power is slightly less). I run this test yesterday, all was
    fine for couple of hours (it's in my DIY projector), nice bright picture and
    colours, then I closely inspected the bulb after cooling and didn't notice
    any signs of visible wear or damage. But this morning I tried again, and
    there was trouble starting the lamp, I saw a weird blue glowing through the
    projection lens, so I took a look at the bulb, and this glowing was not
    inside the arc tube, but around those wires, that support the arc tube. Like
    in the neon tubes, a steady blue light along the wires. I've never seen it
    before, although I had some problem lamps, that didn't start correctly, the
    striking was always inside the arc tube. But here there is no abrupt
    striking, but a steady discharge, a blue light about 1mm thick along all the
    lenth of those wires inside the outer envelope. Finaly this lamp started
    after a while, and I had no time to test it again, but I wonder if it might
    be a sign of a lamp damage, or maybe ignitor? I expect, that ignitor would
    generate short pulses, so what would be this steady neon-like glowing coming


    ps. yes, I know all the warnings about overpowering gas discharge bulbs
  2. JB

    JB Guest

    Sounds like you've got an arctube leak, probably caused by overpowering the
    lamp. Not surprising really. The ignitor does produce pulses, but at a very
    high repetition rate.

  3. Pawel Paron

    Pawel Paron Guest

    I don't want just to vandalize it :) But it's a pretty cheap lamp, so I
    decided to risk it for my experiment. It has a very good colour spectrum, as
    I see that colour rendering is noticeably better than I had with previous
    Venture bulb.

    Here is this weird phenomenon I observe:

    Today I checked three times, and the lamp still works, it just takes longer
    to start than the first time, but eventually it starts every time, after
    maybe 15-20 seconds, and this startup time did not extend, at least for this
    few trials. So I doubt it's a leak, as the gases would finally spread evenly
    inside the whole envelope, and the lamp would probably stop working, or at
    least work improperly. Or maybe this process of leaking is so slow, that it
    will take days or weeks? I'll see.
    I don't know what ignitor it is, some electronic one, and there is a label
    saying 4.5V max pulse voltage. And I don't try to hot restrike this lamp,
    this blue glowing happens in a cold lamp. Pulsing frequency is rather slow,
    I can hear it, and it's visible as little strikes inside the arc tube, but
    this outer glowing is steady, so I guess it might be powered just with some
    capacitancies (wires, and lamp itself), charged to high pulse voltage? It's
    definitely not a blinking with each pulse, but steady glowing, which
    disappears when a solid arc in the tube is ignited. Just interesting, as
    I've never seen it before.

    Thanks everybody for responses.

  4. Pawel Paron

    Pawel Paron Guest

    I just posted link to a photo. I don't know the lamp technology, is it
    supposed to be a vacuum around the arc tube, or some gas? I know people on
    DIY sites sometimes strip such bulbs of this outer shell, to fit the arc
    tube into the optics, and it works, at least for some reasonable time.

  5. I wonder if the inner discharge tube has fractured and leaked mercury
    vapour into the outer tube or if the high voltage is stimulating
    discharge in the outer vacuum.
  6. The way that glow looks to me does not look like that of getting a glow
    in a practical vacuum. The way the glow looks to me, I suspect the
    pressure within the bulb is in/near the 1 torr ballpark. I suspect
    leakage - not sure whether from the arc tube or if the outer bulb leaked
    in some air, although I think more likely the arc tube.

    If the arc tube is leaky, the lamp's electrical characteristics will
    change as the arc tube loses mercury.

    Meanwhile, I am surprised that a metal halide lamp would have had a
    vacuum. I thought they had a fill gas between the arc tube and the outer

    - Don Klipstein ()
  7. Is this one of the home-made video projectors that uses a standard metal
    halide discharge lamp and a full size LCD monitor for the image?

    If it is then it's a pretty neat idea. Bulky, but very cheap and
    serviceable. The LCD monitor has the back and light panel removed so it
    can be used as a huge slide. The large image area makes the optics less
    critical and it's also easier to keep the LCD panel cool.
  8. Pawel Paron

    Pawel Paron Guest

    Yes, it is:

  9. Pawel Paron

    Pawel Paron Guest

    I can hardly imagine such a tiny leak, that wouldn't quickly damage lamp
    performance, especially that pressure must be significantly different during
    on and off conditions. And, if there is really a vacuum in the outer shell,
    any leak would quickly suck out the gas content of the arc tube, leaving the
    lamp dead. Maybe under extremely high temperatures, the glass starts some
    tiny leaks, like developing some microscopic pores, that will slowly let the
    gas out? Or there was a temporary leak during first hours of "overpowered"
    usage, that magically sealed itself after a while? Just an idea, but I've no
    clue about physics inside MH lamp, so please don't laugh at me. Yesterday in
    the evening this lamp was still fine, I didn't notice any visible change in
    brightness or colour, just the same trouble with the ignition.

  10. Pawel Paron

    Pawel Paron Guest

    Or another idea (can be also stupid): maybe due to this high temperature,
    some substance evaporated or reacted in other ways inside the outer glass,
    and created this "glowing gas"? Something from metal wires, or from the
    glass, or some chemical deposits, left inside during the manufacturing

  11. JB

    JB Guest

    Very ,very unlikely in an Osram lamp. There is also a reactive metal 'flag'
    in the lamp called a 'getter' to absorb any microscopic traces of water
    vapour and oxygen which may be left after sealing the outer envelope.

  12. Looks good. One thing to be careful of is the lamps reflector
    arrangement. If the reflector focusses the light in a manner that causes
    a hot-spot it can cause seal failure.
  13. Pawel Paron

    Pawel Paron Guest

    Unfortunately it's the only reasonable solution for a DIY projector. There
    are only spherical reflectors easily available, as parabolic or elliptical
    one would be enormously huge and expensive. And a spherical reflector has to
    be focused on the light source, thus rising heat in the lamp. At least I
    found a dichroic one, so some infrared is not reflected back on the arc

    Anyway, this is proven to work by many people, at least with not overpowered
    lamps, and I also tested it for months with a 400W lamp, overpowered to
    about 550W, no problems. By the way, yesterday my 250W lamp overpowered to
    400W was still fine.

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