Interesting Pacifica feature

Discussion in 'Pacifica' started by Art Begun, Dec 31, 2003.

  1. although I would nitpick a bit of it:
    As for instrument panels - try putting 1 mW into one of the better InGaN
    green or blue LEDs. That's 300 microamps. Light output of a good InGaN
    green at 300 microamps is about that of many cheap old-fashioned GaP green
    LEDs at 15-20 mA.

    - Don Klipstein ()
    Don Klipstein, Jan 9, 2004
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  2. Art Begun

    Steve Guest

    My bad, I was very unclear.

    My experience is that if driven at normal levels, any loss of output
    that would cause a noticeable reduction in the effectiveness of the
    signal will take YEARS. So long that many of the LEDS in the array will
    have begun to fail due to wire de-bonding and other failure modes.
    Steve, Jan 9, 2004
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  3. Art Begun

    ruud Guest

    More Detroit gimmickry. A useless "feature" (what else is new?).
    ruud, Jan 9, 2004
  4. I'm not sure what you mean by "normal" levels. Normal for what? Signal
    level LEDs or high brightness LEDs that are in competition to replace
    incandescent and fluorescent light sources for general lighting
    applications? You cannot compare your experience with signal level
    LEDs with the demonstrated performance degradation of high brightness
    LEDs, which you may consider "overdriven".

    And, let me preempt another discussion here. Sooner or later someone
    is going to say that light output from the LED die itself does not
    depreciate; all the depreciation in output is due to the package
    becoming less transparent or the phosphor used in white LEDs
    degrading. Research at Sandia and other places tends to support this
    argument, but I want to point out that the same is true for most
    "conventional" light sources.

    There is no decrease over life in the visible output of high pressure
    mercury, metal halide or high pressure sodium discharges. And there is
    very little to no decrease in visible light output from an
    incandescent filament over life. In all these light sources the
    measured lumen depreciation is due entirely or almost entirely to
    absorption of light the light generated by the basic "source" by the
    arc tube or bulb. These sources are then like high brightness LEDs
    that do not use phosphor.

    A somewhat similar situation occurs in fluorescent lamps. The UV and
    visible output of the low pressure mercury rare gas discharge remains
    constant over the life of the lamp. All the depreciation in output is
    due to degradation of the phosphor and loss of transparency of the
    phosphor and the lamp bulb. You can then compare the degradation
    problems of fluorescent lamps to white LEDs that use phosphor. They
    suffer degradation due to both darkening of the material used to
    encapsulate the LED plus degradation of the phosphor.
    Victor Roberts, Jan 9, 2004
  5. Come to think of it, I think I've seen this feature on some city busses, too...
    Scott in Aztlán, Jan 9, 2004
  6. Art Begun

    Bill Putney Guest

    Interesting, Don. Must be some recent development. At what voltage do
    those InGaN's run with 300µA?

    Bill Putney
    (to reply by e-mail, replace the last letter of the alphabet in my
    address with "x")
    Bill Putney, Jan 9, 2004
  7. Art Begun

    Martin Brown Guest

    A shade under 3v if the ones round here are any guide.

    More interesting is that you can have always on faint emergency lighting
    using a modern white LED drawing just 10uA and with 2.5v forward voltage
    (or 100uA and 2.8v). Enough light to see by when properly dark adapted.

    Martin Brown, Jan 9, 2004
  8. These have been around a couple years. Voltage at 300 uA is about 3

    - Don Klipstein ()
    Don Klipstein, Jan 9, 2004
  9. Blue and blue-green ones work even better for use that requires dark

    - Don Klipstein ()
    Don Klipstein, Jan 9, 2004
  10. White LEDs, which you keep bringing up, are a red herring. They
    aren't used in traffic signals.
    Incorrect in the case of metal halide. One failure mode which I've
    seen has the output decrease very significantly, apparently due to
    uneven redeposition of tungsten on the filament, causing thick spots
    which don't glow as brightly.

    In any case, all those lamps (as opposed to the arc discharges) do
    decrease in output, and discharge lamps aren't suitable for traffic
    signals anyway.
    Matthew Russotto, Jan 9, 2004
  11. Agreed - white LEDs are not used in traffic signals. However this
    discussion has shifted toward issues regarding lumen depreciation of
    It seems you are talking about tungsten halogen incandescent lamps
    instead of metal halide lamps - which are discharge lamps and do not
    use a filament.
    Victor Roberts, Jan 9, 2004
  12. Art Begun

    Steve Guest

    I have yet to see an LED for which a range of drive levels vs. lifetimes
    was not available from the manufacturer. There would be absolutely no
    need to drive traffic signal LED arrays at the ragged edge by any
    stretch of the imagination.
    Steve, Jan 9, 2004
  13. Art Begun

    Ken Guest

    Here in Sweden we use white LEDs for bus traffic signals
    (and for Trams).
    There are symbols and letters instead of colors (S for Stop).
    (I'm a bus driver since 3 years)
    Ken, Jan 9, 2004
  14. Seems to me they're used in a great many pedestrian signals (for the
    "WALK" part, or its "icon of a walking humanoid" pictogram equivalent)

    Daniel Stern Lighting, Jan 9, 2004
  15. We have some crosswalk signals that use them. I've also heard of their
    use in illuminated lane demarcation 'buttons'.
    Paul Hovnanian P.E., Jan 9, 2004
  16. OK, OK and OK - Thanks for the correction.

    I was just trying to be polite. I really don't know or care if white
    LEDs are used in traffic signals as it is not germane to the
    discussion :)
    Victor Roberts, Jan 9, 2004
  17. Art Begun

    Bill Putney Guest

    Ahh - the march of technology! 8^) Amazing developments.

    Bill Putney
    (to reply by e-mail, replace the last letter of the alphabet in my
    address with "x")
    Bill Putney, Jan 10, 2004
  18. Art Begun

    R.Lewis Guest

    If I understand a previous poster it appears that the color is not seen to
    be too important in LED traffic signals (which are designed with an
    increased unreliability to ensure that they fail catastrophically before any
    degradation would take them out of specification).
    I guess some of the Ambers will turn to Orangey-Red but I have never seen a
    'white' used for any of RYG. .
    R.Lewis, Jan 10, 2004
  19. Art Begun

    Bill Putney Guest

    I can't speak from intimate knowledge, but I would assume that the color
    degradation and life curves get approximately equally compressed when
    the LED's are overdriven. IOW, if you intentionally designed with that
    philosophy, it would be wishful thinking to believe that color
    degradation would not take place before the catastrophic failure. Yeah
    - the color degradation phase of the LED's life would be shorter, but
    it's complete life would be proportionally shortened (I'm guessing - it
    may not be exactly a linear relationship, but certainly as life is
    sacrificed, color accuracy would be also).

    Bill Putney
    (to reply by e-mail, replace the last letter of the alphabet in my
    address with "x")
    Bill Putney, Jan 10, 2004
  20. Art Begun

    R.Lewis Guest

    My comments were an attempt at sarcasm - I certainly hope so!
    R.Lewis, Jan 10, 2004
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