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Why are LED pulsed?

R

Ralph Mowery

Jan 1, 1970
0
John Popelish said:
I guess it is possible that some designers do believe that pulsing
LEDs makes raises the luminous efficiency and some apnote writers are,
apparently, trying to convince designers that it is the case.

I think they are mistaken.

I can also throw another one at you that you will not believe. The driver
chip can only dissapate so much current so the leds are pulsed for that
reason.

AS to the first part, the eye is tricked into believing the led is on all
the time at the same or near the same brightness if the led is pulsed. That
trickery lets you see a motion picture from film.
 
J

John Popelish

Jan 1, 1970
0
Ralph said:
I can also throw another one at you that you will not believe. The driver
chip can only dissapate so much current so the leds are pulsed for that
reason.

AS to the first part, the eye is tricked into believing the led is on all
the time at the same or near the same brightness if the led is pulsed.

Do you mean that the eye perceives a continuous brightness equal to
the pulsed brightness? If so, how do you know this to be true?
That trickery lets you see a motion picture from film.

The limited ability of the eye to perceive pulsations above some
frequency as separate events, is what allows a flashing sequence of
images to be perceived as a smooth variation in time is what makes
motion pictures possible, but that doesn't mean that the eye sees the
continuous time approximation at the brightness of the individual flashes.
 
H

Homer J Simpson

Jan 1, 1970
0
At 50 percent you have to doublee the current to
get the same brightness,so where is the advantage??
Or do you accept halve the lightlevel??
Because thats wat you get .

Assume you have 10 LEDs, all appearing to be on at once. But if you drive
them one at a time for < 10% of the time you have reduced the drain.
 
J

John Popelish

Jan 1, 1970
0
Homer said:
Assume you have 10 LEDs, all appearing to be on at once. But if you drive
them one at a time for < 10% of the time you have reduced the drain.

But each LED will then be (no more than) 1/10th as bright as if it was
driven at 100% duty cycle at the same current. To get them back up to
the same brightness as if each were driven at 100% duty cycle, you
have to hit them with 10 times as much current during their 10%.

Talking about electrical power without talking about light output is
pointless. If you ignore light output you could say that lowering the
current to zero would save all the power, but what is the point of
that observation.
 
T

The Phantom

Jan 1, 1970
0
This discussion has tweaked my memory. I remember the one and only time I
ever saw a suggestion that pulsing an LED was advantageous was in the HP Journal
article that discussed the design of the original HP35 calculator. They said
that pulsing the 7-segment displays had a bightness advantage, IIRC, but I don't
remember the details. I'll see if I still have that around, or if someone can
find it on the web, then I won't have to.
 
J

Jonathan Kirwan

Jan 1, 1970
0
<snip>
This discussion has tweaked my memory. I remember the one and only time I
ever saw a suggestion that pulsing an LED was advantageous was in the HP Journal
article that discussed the design of the original HP35 calculator. They said
that pulsing the 7-segment displays had a bightness advantage, IIRC, but I don't
remember the details. I'll see if I still have that around, or if someone can
find it on the web, then I won't have to.
<snip>

A while back (a couple of years ago) I bought up every copy of the HP
Optoelectronics Fiber-Optics Applications Manual, 2nd ed, that I could
find -- including other countries. Then shipped them to interested
parties. But you might look for another copy out there.

It does discuss the idea that LEDs will have a point of maximal
efficiency in pulsing, when considered from human perception (and not
from examining radiant power with instrumentation.) If you get a copy
in hand, look to page 5-20. They provide a curve of "relative
luminous efficiency" and it illustrates an example where you might see
about some improvement in brightness perception by multiplexing and
using higher currents. Not a lot, though.

And they only provide the one curve and it is probably for some old
red LED they had at the time. So this isn't anything but a data point
for a particular class of LEDs, I suspect. If that.

For example, assume your LED is spec'd for 5mcd @ 10mA. You choose to
pulse it at 40mA at 25% duty cycle, so the average current is the same
as the continuous would have been. They would show calculations like
this:

10mA 1.58
5mcd * ------ * ------ = 6.4mcd
10mA 1.24

So they say. The last fraction represents picking two different
numbers off of that chart, for pulsed versus continuous. By the way,
that curve pretty much flattens off at about 1.62 or so, no matter
what you pulse at as you go higher than about 50mA. So you get
nothing more after that, according to the chart. And remember they
are saying this is about perception, not instrumental radiance.

But this was all two decades ago. This is around the same time that
Edwin Land was reporting out a whole new perspective on color
perception in humans. There has been a lot of progress since this
time in the science side of things and I'd bet there is much better
information to be had, today. And more authoritative as well as being
less ad hoc and more theoretical.

Jon
 
J

Jonathan Kirwan

Jan 1, 1970
0
<snip>
For example, assume your LED is spec'd for 5mcd @ 10mA. You choose to
pulse it at 40mA at 25% duty cycle, so the average current is the same
as the continuous would have been. They would show calculations like
this:

10mA 1.58
5mcd * ------ * ------ = 6.4mcd
10mA 1.24

<snip>

I should have also mentioned (because I had written a note in the
margin about this) that the drive voltage also increases slightly with
increased current (v = (kT/q)*ln(1+I/Is), or something like that.)
Usually not much more than 5% or 10% more, though, for factors of 4X
or 5X.

What HP may have noticed could have been a due in some part to a shift
in emission wavelength distribution towards the shorter end where our
eyes are more sensitive (maximal somewhere in the 555nm area, I
think.) If not all of the slight increase in voltage is simply ohmic
overhead due to the larger current, and some of it might actually lead
to an even slighter increase in activation voltage/energy, then it's a
possible consideration. Can't say, though. HP doesn't spend much
time documenting their methods in understanding this phenomenon.

Jon
 
J

Jonathan Kirwan

Jan 1, 1970
0
due in some part to a shift
in emission wavelength distribution towards the shorter end where our
eyes are more sensitive

I should also note that I'm not suggesting much change. For certain
places in the spectrum there is quite a shift in sensitivities in the
red area with just a 1nm change. For example, there is about an 8%
improvement in sensitivity in just shifting from 690nm to 689nm.

That's the order of change I was wondering about.

Jon
 
J

John Popelish

Jan 1, 1970
0
The said:
This discussion has tweaked my memory. I remember the one and only time I
ever saw a suggestion that pulsing an LED was advantageous was in the HP Journal
article that discussed the design of the original HP35 calculator. They said
that pulsing the 7-segment displays had a bightness advantage, IIRC, but I don't
remember the details. I'll see if I still have that around, or if someone can
find it on the web, then I won't have to.

I also have a vague recollection that the early, deep red LED displays
had an energy loss mechanism that competed with photon production, but
the proportion of energy that went to photon production increased at
higher drive current. So multiplexing the LEDs improved their current
to photon ratio, slightly. But today's LEDs are quite linear, down to
unusable light levels so this effect is no longer operational.
 
J

John Popelish

Jan 1, 1970
0
Jonathan said:
I should also note that I'm not suggesting much change. For certain
places in the spectrum there is quite a shift in sensitivities in the
red area with just a 1nm change. For example, there is about an 8%
improvement in sensitivity in just shifting from 690nm to 689nm.

That's the order of change I was wondering about.

Many LEDs do have a dominant wavelength change with respect to
current, and, as you say, at some wavelengths, this can have a
significant effect on their visibility. Today, with so many
wavelengths to choose from, you might as well just select a device
that produces the desired wavelength. But this factor is something to
be aware of.
 
J

Jonathan Kirwan

Jan 1, 1970
0
For example, assume your LED is spec'd for 5mcd @ 10mA. You choose to
pulse it at 40mA at 25% duty cycle, so the average current is the same
as the continuous would have been. They would show calculations like
this:

10mA 1.58
5mcd * ------ * ------ = 6.4mcd
10mA 1.24

It would have helped some if I'd written this as:

(40mA*.25) 1.58
5mcd * ------------ * ------ = 6.4mcd
10mA 1.24

That way you can see better how they say to derive that top value.

Jon
 
J

Jonathan Kirwan

Jan 1, 1970
0
Many LEDs do have a dominant wavelength change with respect to
current, and, as you say, at some wavelengths, this can have a
significant effect on their visibility.

The main reason I mention this is that it's important to be aware of
possible ways by which an apparent brightness change might occur.
Otherwise, in ignorance, all manner of random guesses about it might
be supported by apocryphal observations. I'm in full agreement with
your general points here on the subject, but knew of a few details
that might explain an authoritative source for argument.

In any case, if those disagreeing with you intend to say that the
human eye is some kind of peak detector, they need to show this
through both theory and experimental result. On theory, I don't know
of a mechanism by which 'peak detection' would be selected in human
perception -- and given that what would in fact _be_ selected for is
the ability to see over a very wide range of lighting situations, from
no more than star light to broad daylight and specular reflections,
it's easy to understand why there is a rough log behavior in response
-- a linear system would be overwhelmed and useless or underwhelmed
and useless, most of the time. I see no reason for peak detection in
photopic vision. And given that some animals have acquired a tapetum
in their retina in order to enhance scopotic vision, I don't find much
argument for a mechanism there, either. Finally, I _have_ spent time
playing with various pulsing schemes and have found very little upon
which to base such 'peak detection' arguments, despite the perennial
rumors about it. It just doesn't seem to help one way or another.

Which made me wonder a little about wave-shifting effects, because it
doesn't take a lot of that in some cases to have an apparent change in
intensity perception. But I'm sure all this has been researched and
is available somewhere in the literature. I just haven't cared enough
about it in practice to go look it up. If anyone has citations to
some peer-reviewed science papers on the subject, I'd be interested.
Today, with so many
wavelengths to choose from, you might as well just select a device
that produces the desired wavelength.

And with the desired efficiency.
But this factor is something to be aware of.

Mostly in just being aware that there may be more than one source for
finding an apparent shift in brightness without changing the averaged
drive current. I don't think pushing wavelengths slightly factors
into much of anyone's design.

Jon
 
S

Sjouke Burry

Jan 1, 1970
0
Homer said:
Assume you have 10 LEDs, all appearing to be on at once. But if you drive
them one at a time for < 10% of the time you have reduced the drain.
Yes ,and the light level is also 10 percent.
As luck would have it,your eye does not interpreet
in a linear way, so even a reduction to 10 percent
might not look like much if you judge by memory.
If you test 2 however with the same resistance
and pulse one ,you will see the difference.
 
D

Don Klipstein

Jan 1, 1970
0
----- Original Message -----
From: "Mr. C" <[email protected]>
Newsgroups: sci.electronics.basics
Sent: Friday, January 13, 2006 1:19 PM
Subject: Why are LED pulsed?


Many people believe that the eye does act as a peak detector, and therefore
a pulsed LED will use less power than a continuously lighted one off the
same apparent brightness.

This is largely a myth.

Many LEDs and most (not all) LED digital displays appear brighter for
equal average current (of no more than a few milliamps per LED chip) when
pulsed than when fed continuous current. However, the relevant
nonlinearity is in the LEDs and not in human vision. Human vision has a
significant nonlinarity, but that is after an "averager" that works
impressively well when the pulse rate is fast enough to avoid visible
flicker.

I have a web page on this - http://www.misty.com/~don/ledp.html

- Don Klipstein ([email protected])
 
D

Don Klipstein

Jan 1, 1970
0
If we start listing the things that many people believe (in the absence of
demonstrated facts), it is going to make a long list.

Not having any personal experience in the field, I'll stick to the "many
people believe" line. But the idea does have some credence among the
experts.

The following is an excerpt from On Semi ap note AND8067D

"There are two main reasons why LEDs are brighter when pulsed. First, the
human eye functions as both a peak detector and an integrator; therefore,
the eye perceives a pulsed LED's brightness somewhere between the peak and
the average brightness [4]. Thus, an LED driven by a high intensity low duty
cycle light looks brighter in a pulsed circuit compared to a DC drive
circuit that is equal to the average of the pulsed signal. The second factor
controlling the improved brightness is shown in the relative efficiency
versus peak current curves of an LED."

I did experiments in this area. The second explanation is nearly enough
the whole story.

I know a few LEDs that when pulsed look brighter to then the human eye
than they do than when fed steady current for same reading by a solar cell
- but only when higher instantaneous current shifts their spectrum towards
wavelengths that the human eye is more sensitive to. And most LEDs that
do this noticeably are less efficient at higher currents.

- Don Klipstein ([email protected])
 
D

Don Klipstein

Jan 1, 1970
0
A while back (a couple of years ago) I bought up every copy of the HP
Optoelectronics Fiber-Optics Applications Manual, 2nd ed, that I could
find -- including other countries. Then shipped them to interested
parties. But you might look for another copy out there.

It does discuss the idea that LEDs will have a point of maximal
efficiency in pulsing, when considered from human perception (and not
from examining radiant power with instrumentation.)

With LEDs that appear to get brighter by increasing instantaneous
current without increasing average current, I have found radiometric
increase also. The LEDs get more efficient.
If you get a copy in hand, look to page 5-20. They provide a curve of
"relative luminous efficiency" and it illustrates an example where you
might see about some improvement in brightness perception by multiplexing
and using higher currents. Not a lot, though.

The efficiency gain was at least mostly in the LED and not in human
vision.
And they only provide the one curve and it is probably for some old
red LED they had at the time. So this isn't anything but a data point
for a particular class of LEDs, I suspect. If that.

- Don Klipstein ([email protected])
 
J

Jonathan Kirwan

Jan 1, 1970
0
With LEDs that appear to get brighter by increasing instantaneous
current without increasing average current, I have found radiometric
increase also. The LEDs get more efficient.

I suspect that manufacturers have developed standard drive levels that
they spec things at, on the basis of this kind of knowledge.

But the HP book I'm citing from specifically points out that the curve
they present is __NOT__ congruent with radiometric changes. So that
wasn't their point. (And I'm not defending what they said then about
parts available now [or then], either. Just noting it.)
The efficiency gain was at least mostly in the LED and not in human
vision.

They weren't precise about what they meant, Don. It's possible that
there was a slight wavelength shift attending the change and that this
coupled with human perception -- and that might fit within what they
were saying. Here's the quote: "Equation 5.2.4-1 applies to luminous
intensity as perceived by the eye and is not applicable to radiant
power output."

Frankly, I think this is much-ado-about-nothing-useful, though.

Do you have the book I mentioned?

Jon
 
T

The Phantom

Jan 1, 1970
0
This discussion has tweaked my memory. I remember the one and only time I
ever saw a suggestion that pulsing an LED was advantageous was in the HP Journal
article that discussed the design of the original HP35 calculator. They said
that pulsing the 7-segment displays had a bightness advantage, IIRC, but I don't
remember the details. I'll see if I still have that around, or if someone can
find it on the web, then I won't have to.

I dug out the article in question; it was published in the HP Journal in
June 1972. Here's a quote:

"It was apparent early in the HP-35 planning that new display techniques
would be required. Existing light-emitting-diode products used too much
power and cost too much. HP Associates developed a magnified five-digit
cluster which saves both power and cost and is packaged in a convenient
14-pin package (Fig. 5). Each digit has a spherical lens molded in the
plastic over it. A slight reduction in viewing angle results, but for the
handheld calculator this is not a problem.

LED's are more efficient if they are pulsed at a low duty cycle rather
than driven by a DC source. In the HP-35, energy is stored in inductors
and dumped into the light-emitting diodes. This technique allows a high
degree of multiplexing..."

Of course, this is for LED's of the 1970 era, and there was no further
explanation than the statement you see in the quote. Nothing about just
how much more efficient pulsed drive might be.
 
J

John Popelish

Jan 1, 1970
0
The said:
I dug out the article in question; it was published in the HP Journal in
June 1972. Here's a quote:

"It was apparent early in the HP-35 planning that new display techniques
would be required. Existing light-emitting-diode products used too much
power and cost too much. HP Associates developed a magnified five-digit
cluster which saves both power and cost and is packaged in a convenient
14-pin package (Fig. 5). Each digit has a spherical lens molded in the
plastic over it. A slight reduction in viewing angle results, but for the
handheld calculator this is not a problem.

LED's are more efficient if they are pulsed at a low duty cycle rather
than driven by a DC source. In the HP-35, energy is stored in inductors
and dumped into the light-emitting diodes. This technique allows a high
degree of multiplexing..."

Of course, this is for LED's of the 1970 era, and there was no further
explanation than the statement you see in the quote. Nothing about just
how much more efficient pulsed drive might be.

I think LED technology has changed a lot since the 70's. I have been
searching LED data sheets for this nonlinear effect that shows
increasing luminous efficiency with increasing current, and I haven't
found an example for any LED produced in the last 20 years or so. At
high enough current, all LEDs have nonlinearity that goes the other
way (less luminous efficiency at higher current). I suspect this
whole myth revolves around a particular short period in LED history
that is equivalent to the point contact era of transistor technology.
Things have changed.
 
T

The Phantom

Jan 1, 1970
0
I think LED technology has changed a lot since the 70's. I have been
searching LED data sheets for this nonlinear effect that shows
increasing luminous efficiency with increasing current, and I haven't
found an example for any LED produced in the last 20 years or so. At
high enough current, all LEDs have nonlinearity that goes the other
way (less luminous efficiency at higher current).

That's been my experience.
I suspect this
whole myth revolves around a particular short period in LED history
that is equivalent to the point contact era of transistor technology.

The quote I provided is the only time I ever saw mention of such an
effect.
 
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