Interesting Pacifica feature

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.

Come to think of it, I think I've seen this feature on some city busses, too...

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")

In message , Bill Putney writes

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.

Regards,

These have been around a couple years. Voltage at 300 uA is about 3 volts.

- Don Klipstein ( snipped-for-privacy@misty.com)

Blue and blue-green ones work even better for use that requires dark adaptation.

- Don Klipstein ( snipped-for-privacy@misty.com)

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.

Agreed - white LEDs are not used in traffic signals. However this discussion has shifted toward issues regarding lumen depreciation of LEDs.

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.

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.

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)

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)

DS

We have some crosswalk signals that use them. I've also heard of their use in illuminated lane demarcation 'buttons'.

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 :-)

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")

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. .

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")

My comments were an attempt at sarcasm - I certainly hope so!

Which I obviously missed. 8^)

Bill Putney (to reply by e-mail, replace the last letter of the alphabet in my address with "x")