Interesting Pacifica feature

[Don Klipstein]

(Stuff that largely apprears to me true and making sense),

although I would nitpick a bit of it:

Hmmmm - 1mW is next to nothing for a *single* LED die, much less an
array. 1mW on a single die probably would be barely visible even close
up on a simple instrument panel - by any standard, not useful for any
practical application, much less to be seen from a distance on
vehicles.

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 ([email protected])

 

[Steve]

I am not clear which part is contrary to your experience. The fact
that most LEDs will operate virtually forever or the fact that their
output will decrease with time?

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.

 

[ruud]

It's not only not standard, it's not legal and not safe.

Under Federal and Canadian Motor Vehicle Safety Standard 108 (Lamps and
Reflective Devices), all brake lamps including the center high mount lamp
are required to burn steadily whenever the brakes are applied. They are
NOT permitted to flash. The only exception to this does not apply to the
center brake light: The left and right brake lights may be flashed to
serve the rear turn signal and rear hazard warning functions. The center
brake lamp may never legally be wired to flash.

But there are several CHMSL flasher kits on the aftermarket sold by those
who know nothing of proper signal design or legality, and figure a light
that flashes just *has* to be better than a light that doesn't. They make
all kinds of unsupported and unsupportable claims for improved safety, and
there are people who believe 'em.

There's been a proposal in Europe (from Germany) to flash the CHMSL under
extremely hard braking as a means of indicating "I'm not just slowing
down; I'm braking and I REALLY MEAN IT!" signal to following traffic. The
US and Canadian regulatory bodies have rejected it out of hand for exactly
the same reason the aftermarket CHMSL flashers are unsafe: We already have
flashing red lights on the backs of cars in North America. They mean the
driver is pumping his brakes OR signalling for a turn OR has activated the
hazard flashers. Adding yet another possible meaning for following drivers
to try and decode on the fly does NOT improve safety.

DS

More Detroit gimmickry. A useless "feature" (what else is new?).

 

[Victor Roberts]

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.

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.

 

[Scott in Aztlán]

I had the"opportunity" to follow a fire truck yesterday - a brand new
pumper recently aquired by our municipality, and it has LED rear
lights. The brake lights "strobe" 3 or 4 times before coming on
steady. It really catches your attention.

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

 

[Bill Putney]

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 ([email protected])

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")

 

[Martin Brown]

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

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,

 

[Don Klipstein]

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

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

- Don Klipstein ([email protected])

 

[Don Klipstein]

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.

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

- Don Klipstein ([email protected])

 

[Matthew Russotto]

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.

White LEDs, which you keep bringing up, are a red herring. They
aren't used in traffic signals.

There is no decrease over life in the visible output of high pressure
mercury, metal halide or high pressure sodium discharges.

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.

 

[Victor Roberts]

White LEDs, which you keep bringing up, are a red herring. They
aren't used in traffic signals.

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

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.

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.

 

[Steve]

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".

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.

 

[Ken]

Agreed - white LEDs are not used in traffic signals.

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)

 

[Daniel Stern Lighting]

Agreed - white LEDs are not used in traffic signals.

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

 

[Paul Hovnanian P.E.]

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

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

 

[Victor Roberts]

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

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

 

[Bill Putney]

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

- Don Klipstein ([email protected])

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")

 

[R.Lewis]

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

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. .

 

[Bill Putney]

...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 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")

 

[R.Lewis]

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!