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'[OT] Human visual response (was: anode drivers for'
2000\01\12@182024 by Ken Webster

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Robert Rolf wrote:

>You can also try strobing your LED's with a much higher (8x) peak
>current
>with a 10% duty cycle. Our vision system only sees peak brightness, not
>average
>so the LED's will appear MUCH brighter and yet you won't fry them. There
>should be a
>spec for "max peak current" for the devices.


Well, it must not really be pure peak brightness that we perceive because
PWM dimming of LEDs would not work in that case.  But I have noticed that a
PWM dimmed LED display at 50% duty definitely looks significantly brighter
than half of full brightness.

So, how does perceived brightness relate to actual brightness?  Is there a
difference between perceived brightness for a PWM source vs a steady source
if the average output is equal?  If so, how does the difference relate to
the frequency of the modulation?

Any vision experts out there?


Ken

2000\01\13@081918 by jkitchen

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I'd like to know the answers to your questions too.  I also suspect that a
flashing LED -- obviously, visibly flashing -- is "more visible" than a
non-flashing one at the same "brightness" level.  Comments?

Ken Webster wrote:

{Quote hidden}

2000\01\13@082953 by paulb

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Ken Webster wrote:

> Well, it must not really be pure peak brightness that we perceive
> because PWM dimming of LEDs would not work in that case.

 You're right.  The suggestion that the eye discerns peak brightness
is quite unsupported by its physiology.  It is a chemical process -
each photon causes the conversion of one (may perhaps be more) molecule
of receptor substance - if it happens to hit one.

 In anything other than darkness, since the replenishment rate of the
rhodopsin is limited, much or most of it will be in the "triggered"
state (it is a closed cycle) so many or most photons will miss sensitive
molecules and be lost.  This is the AGC mechanism of the retina - of
course the eye has an iris as well.

 I see no mechanism here to detect peaks.

>  But I have noticed that a PWM dimmed LED display at 50% duty
> definitely looks significantly brighter than half of full brightness.

 Do remember that LED brightness depends on current, not voltage, so
half brightness is at half of the original current.  And some poor-
quality LEDs appear to have a threshold current (around 1 mA or so) even
then which must indicate a charge leakage problem and non-linearity.

> So, how does perceived brightness relate to actual brightness?  Is
> there a difference between perceived brightness for a PWM source vs a
> steady source if the average output is equal?  If so, how does the
> difference relate to the frequency of the modulation?

 Perceived brightness uses an approximately logarithmic scale due to
the receptor dynamics mentioned.  I believe it is mostly said that above
the perceptible flicker frequency brightness is proportional to the
actual luminance, while below that, short pulses are discriminated and
seen for their brightness while on.

 In addition the eye is "programmed" to pick out movement, and a
flickering source is seen preferentially.
--
 Cheers,
       Paul B.

2000\01\13@122232 by ShadeDemon

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 It isn't the EYE that sees peek brightness in this manner,
it is your BRAIN that does this..  Just like the brain is
responsible for optical illusions, that line that is the
same length on your retina appears longer or shorter when
the arrows at the ends are pointing in or out, etc.  The
'flickering brightness preference' is an artifact of
preferred edge detection.  Your eye vibrates horizontally
slightly, think the freq was 70 Hz or so for most ppl, to
catch edges.  Think of a white index card on a black
background.  This vibration makes the vertical black/white
edges move across many cells, making them get a 'flickering'
image.  This is the main info sent to the brain.  (You have
a little lower res, and noticably less 'movement' perception
in the vertical plane due to this.)  The cells are seeing
all white 'center of the card' or all black 'background'
very rapidly stop sending any info till they change.  Your
brain has worked out how to keep track of the gaps, and
fills in that the middle of the card is white, and rest of
background is black.  Built in video compression.  With the
terribly high resolution of the eye, mainly sending edges
and remembering the rest cuts the image processing bandwidth
tremendously, can't imagine too many scenes in the natural
world where you'd be turning even 1/10th of them on from
frame to frame.  (In the natural world, most important info
is in mainly horizontal plane, since most of planet is in
'mainly horizontal' orientation.  On an empty road, turn
your head sideways parallel to ground and see how difficult
it is to drive this way.  (That is not only an EYE thing,
but will give some idea of how terribly horizontally
oriented our entire vision system is, one of the bigger
problems of working in space ))
 This is what leads to the difference in brightness
perception.  Take a very low amount of light energy.  Steady
on over 1 sec or so it may not even trip some cells to send
a signal.  In a much shorter flash, that same number of
photons will trip some cells, and your brain will hold that
perception for a noticable amount of time.  Same energy,
very different perception.  PWM dimming does work, but if
you stay well above the eye refresh rate, you can get by
with noticably less than 50% on for the same apparent
brightness as the same LED at 50% constant on..
 Hmm you've got me thinking now..  :)

2000\01\13@165217 by paulb

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Alan King wrote:

>   It isn't the EYE that sees peek brightness in this manner,
> it is your BRAIN that does this..

 Yeah, OK.  This applies then only at or below the 70 Hz flicker
frequency.  Above that, the eye sees it as continuous and jsut averages
the pulses.
--
 Cheers,
       Paul B.

2000\01\15@170618 by andy howard

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> Well, it must not really be pure peak brightness that we perceive
because
> PWM dimming of LEDs would not work in that case.  But I have noticed
that a
> PWM dimmed LED display at 50% duty definitely looks significantly
brighter
> than half of full brightness.

> So, how does perceived brightness relate to actual brightness?  Is
there a
> difference between perceived brightness for a PWM source vs a steady
source
> if the average output is equal?  If so, how does the difference relate
to
> the frequency of the modulation?
> Any vision experts out there?

> Ken


The following all seem relevant:

   Talbot's law. Named after the English physicist and photographer,
William Henry Fox Talbot     (1800-1877), who discovered that when the
cycle of a flickering light reaches so high a rate     that it perceived
as being continuous, its apparent brightness is equal to the mean of the
brightness of the complete flicker cycle. Also known as the
Talbot-Plateau law, after Talbot     and the Belgian physicist Joseph
Antoine Ferdinand Plateau (1801-1883).

   Weber's law. Named after the German psychophysicist, Ernst Heinrich
Weber (1795-1878), who     discovered that the just-noticeable
differences in the intensity of various stimuli are         proportional
to the intensity of the original stimulus. For instance, if a 100 watt
light         must be increased by 5 watts in order for the difference
to be perceptible, then an increase     of 50 watts would be necessary
for the change in a light of 1000 watts to be perceptible.

   Purkinje phenomena. Named after the Czech physiologist, Jan
Evengelista Pukinje (1787-1869),     who discovered that as multicolored
displays decrease in brightness, those colors at the         "cool" end
of the spectrum lose their brightness less rapidly than those at the
"warm" end.     It was later discovered that this due to the
photosensitive cells in the retina responsible     for vision in dim
light the rods) being more sensitive to light shorter wavelength (which
corresponds to "cool" colors) than cells responsible for vision in
bright light (the cones).


...from
http://www.yorku.ca/dept/psych/classics/Watson/glossary.htm#absolute

2000\01\16@224515 by paulb

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andy howard wrote:

>     Talbot's law. Named after the English physicist and photographer,
> William Henry Fox Talbot (1800-1877), who discovered that when the
> cycle of a flickering light reaches so high a rate that it perceived
> as being continuous, its apparent brightness is equal to the mean of
> the brightness of the complete flicker cycle.  Also known as the
> Talbot-Plateau law, after Talbot     and the Belgian physicist Joseph
> Antoine Ferdinand Plateau (1801-1883).

 What you perceive is the average for frequencies above teh "flicker"
or "fusion" frequency.

>     Weber's law. Named after the German psychophysicist, Ernst
> Heinrich Weber (1795-1878), who discovered that the just-noticeable
> differences in the intensity of various stimuli are proportional to
> the intensity of the original stimulus.  For instance, if a 100 watt
> light  must be increased by 5 watts in order for the difference to be
> perceptible, then an increase of 50 watts would be necessary for the
> change in a light of 1000 watts to be perceptible.

 The eye's perception of brightness is logarithmic.

 That's as I said, but it's a real neat reference, thanks.
--
 Cheers,
       Paul B.

2000\01\16@233019 by William Chops Westfield

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   >     Talbot's law. Named after the English physicist and photographer,
   > William Henry Fox Talbot (1800-1877), who discovered that when the
   > cycle of a flickering light reaches so high a rate that it perceived
   > as being continuous, its apparent brightness is equal to the mean of
   > the brightness of the complete flicker cycle.  Also known as the
   > Talbot-Plateau law, after Talbot     and the Belgian physicist Joseph
   > Antoine Ferdinand Plateau (1801-1883).

     What you perceive is the average for frequencies above teh "flicker"
   or "fusion" frequency.

Whereas others have said that apparent brightness is based on peak
brightness, for a flashing LED.

If both are correct, then a blinking LED at a particular pulse width
(percentage) should suddenly appear to get dimmer when you exceed the
flicker frequency...  Has anybody acctually observed this?

BillW

2000\01\17@003459 by Adam Mead

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----Original Message-----
From: William Chops Westfield <spam_OUTbillwTakeThisOuTspamCISCO.COM>
To: .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU <PICLISTspamKILLspamMITVMA.MIT.EDU>
Date: Monday, 17 January 2000 12:32
Subject: Re: [OT] Human visual response (was: anode drivers for LED)



I'm Doing up some graphs that should explain all of this...

I'll have them in a moment.

Regards
Adam Mead

2000\01\17@024454 by Adam Mead

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part 0 1376 bytes
Here are some points we have learned so far

1) Yes, we can drive an LED at a much higher current if we pulse it. And
therefore attain higher levels
    of brightness

2) From what I gather, the Human optic system has a certain level of
persistance (similar to a CRT)
    by which a flash of light may remain visible to a witness for a
slightly longer period than that of the
   flash. ( I belive CRT Monitors and TVs rely on this theory )

Therefore this graph should clarify why an LED appears to be brighter than
half brightness when pulsed at 50% duty cycle, and should enable us to
conclude whether we can infact brighten an LED using this method.

The graph attached Illustrates a scenario that may be similar to that of a
rear bike light (to warn Motorists of a Cyclist).
Here, the LED is pulsed at approx 15% duty cycle at about 5 times its rated
current.
We can see here that the retina reacts immediately to the LED pulse. However
its persistance prevents it from reacting immediately to the falling edge of
the LED pulse.
>From here we can assume (from previous information on this thread) that the
brain perceives an average of the retina's reaction to the light pulse. If
we time the pulses correctly, we can create a much brighter looking output
than the rated output for any given LED.



Attachment converted: wonderland:Graph.GIF (GIFf/JVWR) (0001517F)

2000\01\17@064836 by paulb

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William Chops Westfield wrote:

> If both are correct, then a blinking LED at a particular pulse width
> (percentage) should suddenly appear to get dimmer when you exceed the
> flicker frequency...  Has anybody acctually observed this?

 Should be fairly easy to try, but I haven't done this (so far).  Could
use a PIC with an A/D input or - a LCD display and "up" and "down"
buttons.  That [OT] was supposed to be on the *end*, guys, so it can be
added and deleted without affecting the mail sort too much.

 Delete the word "suddenly" from the above.  Fusion is a gradual
process, depends (refer back to the reference given) on how close the
flicker source or its image is to visual centre (fovea).

Adam Mead wrote:

> 1) Yes, we can drive an LED at a much higher current if we pulse it.

 FAQ here.  Yes, but *how* much higher, and is it efficient to do so?
Remember that the light intensity is, unlike an incandescent bulb,
proportional to current as the light producing mechanism corresponds to
the threshold voltage of the LED.

 The effective series resistance of the LED however, the part that
increases the voltage drop in proportion to current, leads to a heat
source which dissipates according to the square of the current.  AFAIK,
the maximum rating of the LED is that at which heating will destroy the
die.

 When you double the current and halve the duty cycle, you obtain twice
the light for half the time.  The voltage drop across the LED however
increases during the "on" time, so the power dumped is *more* than twice
the original.  Consequently, at double the rated If, you must use *less*
than 50% duty cycle and unless you are using a "flicker" effect as
discussed, the average intensity will actually be less than for the
continuous operation.

 If you use PWM to facilitate operation from 5V without a resistor
(which is not kosher directly from the PIC), the overall *efficiency* of
conversion to light will not necessarily be lost, as the LED is simply
incorporating the function of the series resistor.  Unless de-rated
however, it will in fact be less efficient due to *temperature rise*
(remember that?).

> And therefore attain higher levels of brightness.

 *Only* if the flicker effect is in your favour.

> 2) From what I gather, the Human optic system has a certain level of
> persistance (similar to a CRT) by which a flash of light may remain
> visible to a witness for a slightly longer period than that of the
> flash. ( I belive CRT Monitors and TVs rely on this theory )

 You mean they rely on phosphor or visual persistence or both? ;-)

> Therefore this graph should clarify why an LED appears to be brighter
> than half brightness when pulsed at 50% duty cycle, and should enable
> us to conclude whether we can infact brighten an LED using this
> method.

 Actually, it doesn't, because you have the facts backwards.
Persistence is what *stops* you seeing the LED as brighter!  You have
omitted the "up" ramp in the persistence.  Just as it takes time for the
perception to decay, it also takes time for it to "charge", and that's
why the effect is lost above the "flicker" frequency.  Shorter pulses
simply won't "charge up" fully.

 The apparent enhancement is largely due to a high-pass filter function
later in the system.  If the pulses are *slow* enough, the eye can in
fact discern that whilst on, they *are* brighter.  Overall, it's a band-
pass effect.  Continuous, it's just so bright.  Pulsed slowly with the
same *average* power, the individual pulses are seen to be brighter
because they *are* brighter, and they *also* attract more attention,
whilst above the flicker frequency, the average brightness is correctly
discerned.

> The graph attached Illustrates a scenario that may be similar to that
> of a rear bike light (to warn Motorists of a Cyclist).  Here, the LED
> is pulsed at approx 15% duty cycle at about 5 times its rated current.

 Note that for this to be useful, the 15% "on" time has to be the same
or slower than half the period of the "flicker" frequency.  Shorter than
this and you will just *lose* brightness.

> We can see here that the retina reacts immediately to the LED pulse.

 Nope.  It ramps up.  You have it wrong.

> However its persistance prevents it from reacting immediately to the
> falling edge of the LED pulse.

 True enough.

>> From here we can assume (from previous information on this thread)
>> that the brain perceives an average of the retina's reaction to the
>> light pulse.

 Nope, it's post-processed with a high-pass filter.

> If we time the pulses correctly, we can create a much brighter looking
> output than the rated output for any given LED.

 Insofar as the eye is correctly determining that whilst on, the LED
*is* brighter than its nominal continuous output.  But that only works
if it can distinguish off and on.
--
 Cheers,
       Paul B.

2000\01\17@173913 by Wagner Lipnharski

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I could be wrong, but when working with 7-seg LED display in
multiplexing mode, looks like that near the flickering frequency the
LEDs are brighter than when using a multiplexing higher frequency.

Wagner

William Chops Westfield wrote:
{Quote hidden}

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