Human Eye -- Peak or Average Response

[jweir43]

Relatively dark room.  Plain jane 3mm red led.  1.5v fvd  13,5 volt DC supply. 

Two of these LEDs side by side.

One of them is connected to the DC supply through a 470 ohm series resistor.  If = 12/470 = 25 mA  Pd = 38 mW

One of them is driven by a square wave 13.5 volt peak, 240 ohm series resistor.  If (peak) = 12/240 = 50 mA   Pd (on the positive pulse) = 75 mW  Pd (on the negative pulse = 0 mW;   average Pd = 37.5 mW.

Neglecting the half a milliwatt difference, will one of them appear brighter to a human observer than the other?   And why?   


Jim

[wraper]

PWMed one should have lower brightness due to LEDs having lower efficiency at high currents.

[vk6zgo]

 Eyes, like most human sensors, have a logarithmic response, so I would guess, no difference.
You would need to swap the LEDs over to be sure.

Eyes can also modify their basic response by shrinking or enlarging the pupli, or even by blinking.(but that
should be the same for both cases)

The eye's spectral response is also important, as it means you can't directly compare the apparent brightness of, say, red & blue LEDs.(OK, I know both yours are red.)

[David Hess]

PWMed one should have lower brightness due to LEDs having lower efficiency at high currents.

This depends on the LED.  High brightness LEDs can have lower efficiency below 100s of milliamps which is one of the advantages of multiplexed operation.

As far as the test, LED efficiency can be verified with a photosensor.

[wraper]

PWMed one should have lower brightness due to LEDs having lower efficiency at high currents.

This depends on the LED.  High brightness LEDs can have lower efficiency below 100s of milliamps which is one of the advantages of multiplexed operation.

As far as the test, LED efficiency can be verified with a photosensor.

If LED can work at 100s of milliamps (even peak), and especially have high efficiency at that current, it's high power LED, not simply high brightness LED.

[Berni]

Lots of reasons why it might be diferent:
- LEDs have different efficiency curves at different currents.
- LEDs have capacitance that can alter the shape of a high frequency square wave after the resistor.
- The square wave driving it could have slow rise and fall times, giving it extra current outside of the 50% duty cycle

Only way to know for sure is to go trough and methodically test for each of these.

[Cerebus]

It's going to depend on the photochemistry of the eye, and how the optic nerves respond to that. The short answer is that the eye has responses on the order of several milliseconds and, in particular, the optic nerves rely on the concentration of metarhodopsin present. The production of metarhodopsin in the presence of light is quite quick (sub-millisecond response), but the varying concentration of it in a particular rod or cone is going to be the average of the last few milliseconds of activity. So there is, in effect, an averaging low pass filter in place as bursts of release of metarhodopsin average out into a concentration of metarhodopsin in solution. Quite where the roll-off frequency is, is a bit vague in the sources I have but one can get an estimate by considering that persistence of vision effects start to occur around 24 Hz or so.

More here: https://www.brainkart.com/article/Photochemistry-of-Eye-Vision_19676/

[jweir43]

Could you possibly translate that into a yes or a no instead of a maybe?

THanks,

Jim

[jweir43]

Lots of reasons why it might be diferent:
- LEDs have different efficiency curves at different currents.
That is exactly the point.


- LEDs have capacitance that can alter the shape of a high frequency square wave after the resistor.
Tenths of picofarads and hundreds of ohms translate into nanosecodn shapes, hardly a factor.

- The square wave driving it could have slow rise and fall times, giving it extra current outside of the 50% duty cycle
I postulated a square wave, not a modified triangle wave.  It will be a square as you wish.

Thanks,

Jim

Only way to know for sure is to go trough and methodically test for each of these.

[Benta]

Could you possibly translate that into a yes or a no instead of a maybe?

THanks,

Jim

You haven't mentioned the switching frequency. If it's below a few hundred Hertz it makes a difference to the eye response.

[David Hess]

PWMed one should have lower brightness due to LEDs having lower efficiency at high currents.

This depends on the LED.  High brightness LEDs can have lower efficiency below 100s of milliamps which is one of the advantages of multiplexed operation.

If LED can work at 100s of milliamps (even peak), and especially have high efficiency at that current, it's high power LED, not simply high brightness LED.

I am not going to waste time finding a specific example to refute that.  My point is that it can be more efficient to multiplex (or PWM) an LED at higher current than to drive it at a continuous lower current.  This depends on the specific LED characteristics.

The high brightness AlGaAs red LED for displays shown below is more efficient if multiplexed at 10 milliamps instead of being driven at 1 milliamp or lower for the same brightness.  Some (newer) LEDs have a very linear intensity curve at low currents so this does not apply to them.

[wraper]

The high brightness AlGaAs red LED for displays shown below is more efficient if multiplexed at 10 milliamps instead of being driven at 1 milliamp or lower for the same brightness.  Some (newer) LEDs have a very linear intensity curve at low currents so this does not apply to them.

See the difference between 10mA and 100s of mA? PWMing is certainly not a way to achieve high LED efficiency. You would need to drive powerful and expensive LEDs at fraction of their rated current where efficiency peaks and then reduce power even further by PWM.

My point is that it can be more efficient to multiplex (or PWM) an LED at higher current than to drive it at a continuous lower current.

Only if you use them at small fraction of rated power when PWMed compared with tiny fraction at constant current. You would be much better by just selecting weaker LED.

[Zero999]

The high brightness AlGaAs red LED for displays shown below is more efficient if multiplexed at 10 milliamps instead of being driven at 1 milliamp or lower for the same brightness.  Some (newer) LEDs have a very linear intensity curve at low currents so this does not apply to them.

See the difference between 10mA and 100s of mA? PWMing is certainly not a way to achieve high LED efficiency. You would need to drive powerful and expensive LEDs at fraction of their rated current where efficiency peaks and then reduce power even further by PWM.

Some LED types also exhibit a shift to shorter wavelengths, as higher currents. Some of the high brightness green LEDs appear almost cyan when overdriven. The human eye is also less sensitive to shorter wavelengths, than green, especially at high intensities, so efficiency will drop more.

[Someone]

Could you possibly translate that into a yes or a no instead of a maybe?

THanks,

Jim

Perhaps you would like to pay a professional for a report on the matter? Expecting an exact answer for free seems a bit presumptuous.

[jweir43]

Don't know why?  I'm a pro at what I do, but I certainly share my knowledge when asked.

Jim

[Berni]

Because asking tips, opinions, information on a topic, etc... is different than asking someone for what sounded like "doing all the research, perform testing and then presenting a report on it"

Additionally you haven't really provided all the information to produce such an solid yes/no answer in the first place. We pointed out all the factors that could contribute to a difference in apparent brightness of a LED. So you pretty much got the best possible answer to your provided input.

Most people on this forum are more than glad to help out fellow engineers, but they can only really help if you work with them.

[Nusa]

Not enough information given in the original question.

We need to know the frequency of the square wave. For instance, if the frequency were 1 Hz, it's obvious that the second LED would be much brighter half the time and much darker half the time. At least until the LED failed from overcurrent, then it would be much darker all the time.

We also need to know viewing angle. You'll find that your peripheral vision performs differently than your direct vision, due to the different distribution of rods and cones. Rods react much faster than cones and are more light sensitive, while cones are slower and see color, so that ties into the frequency question again. Ever notice that you can sometimes detect monitor flicker when you aren't looking right at the monitor?

In the general sense, at a high enough frequency to achieve fusion, I'd say the PWM LED would appear brighter. Reducing the ON time of an LED is a common technique to save power. In the extreme case, it can save components as well by eliminating the resistor.

But a little empirical testing wouldn't hurt you. It's a pretty trivial experiment after all.

[jweir43]

Not enough information given in the original question.

We need to know the frequency of the square wave. For instance, if the frequency were 1 Hz, it's obvious that the second LED would be much brighter half the time and much darker half the time. At least until the LED failed from overcurrent, then it would be much darker all the time.

We also need to know viewing angle. You'll find that your peripheral vision performs differently than your direct vision, due to the different distribution of rods and cones. Rods react much faster than cones and are more light sensitive, while cones are slower and see color, so that ties into the frequency question again. Ever notice that you can sometimes detect monitor flicker when you aren't looking right at the monitor?

In the general sense, at a high enough frequency to achieve fusion, I'd say the PWM LED would appear brighter. Reducing the ON time of an LED is a common technique to save power. In the extreme case, it can save components as well by eliminating the resistor.

But a little empirical testing wouldn't hurt you. It's a pretty trivial experiment after all.

Sorry, I though it obvious that I would have to get over the 24 Hz. flicker frequency to avoid the persistence of vision obstacle.  Let's say that you have your choice of any frequency from 25 Hz. to daylight.

Viewing angle is dead on boresight.

Empirical testing to follow.  It is a trivial experiment and one that I probably should have done instead of asking the question if anybody has done the experiment and how did it turn out?

Let me pose the question for what I **REALLY** want to know.

I've got a white landing light of about twenty watts (DC power).  I shine the light at the ground when I'm about 350' AGL.  It lights a rough ellipse on the ground and lets me see whether or not there are animals on the runway before I collide with them.

I run the light on DC and get a particular illumination.  If I run the same exact device on AC of any frequency you want above the flicker level and I adjust the current so that the pulse width times the current gives me the same power into the light, will the light appear brighter to me as reflected from the SAME ground at the SAME altitude at the SAME attitude, at the SAME ....   In other words, the only differences is pulsed power versus DC.  Does the pulse width affect the result (right up to the maximum pulse current of the device)?  Is it any brighter at any combination of pulse width and current than at DC?

THat should take care of any test conditions, I hope.

Jim

[jmelson]

I've got a white landing light of about twenty watts (DC power).  I shine the light at the ground when I'm about 350' AGL.  It lights a rough ellipse on the ground and lets me see whether or not there are animals on the runway before I collide with them.

Landing light?  Single engine aircraft?  You will be looking through the prop?  Hmmm, I'm not sure you are going to like this strobe effect at all, no matter what your PWM frequency is.

As to the original question, I think the eye responds to PEAK brightness, so the pulsed LED should appear much brighter than the CW, at matched average power input.

Jon

[Circlotron]

The difference in pulsed vs continuous *perception* of brightness for the same power input might not apply with white LEDs, depending on how fast the phosphor responds.

[Audioguru]

I was taught that a fast pulse or pulses of light for durations less than 30ms appear to be dimmed. I am not talking about a slow incandescent light bulb.

[Nusa]

Sorry, I though it obvious that I would have to get over the 24 Hz. flicker frequency to avoid the persistence of vision obstacle.  Let's say that you have your choice of any frequency from 25 Hz. to daylight.

Viewing angle is dead on boresight.

Empirical testing to follow.  It is a trivial experiment and one that I probably should have done instead of asking the question if anybody has done the experiment and how did it turn out?

Let me pose the question for what I **REALLY** want to know.

I've got a white landing light of about twenty watts (DC power).  I shine the light at the ground when I'm about 350' AGL.  It lights a rough ellipse on the ground and lets me see whether or not there are animals on the runway before I collide with them.

I run the light on DC and get a particular illumination.  If I run the same exact device on AC of any frequency you want above the flicker level and I adjust the current so that the pulse width times the current gives me the same power into the light, will the light appear brighter to me as reflected from the SAME ground at the SAME altitude at the SAME attitude, at the SAME ....   In other words, the only differences is pulsed power versus DC.  Does the pulse width affect the result (right up to the maximum pulse current of the device)?  Is it any brighter at any combination of pulse width and current than at DC?

THat should take care of any test conditions, I hope.

Jim

24 Hz may give you persistence of vision, but flicker is easy perceived by just about everyone. Still, even that was pretty amazing to people early on in the moving pictures industry. These days it's upsampled for display, unless you want the old-time effect on purpose.

RE 20 watt white light. If it can run on AC or DC, we're not talking LED's anymore, are we? Most likely we're talking incandescent and the theory of operation is completely different. The light comes from the filament being HOT enough to glow, which is directly related to power applied. It takes a LOT of time (relative to an LED) to cycle between ON (HOT) and OFF (COOL), so the filament itself has persistence of temperature that will largely mask frequency effects.

Also, this is a completely different question. The original question was how bright does the LED appear. The new question is how effective is a light being cast while moving at high speed. I can't really answer that, other than to imagine that any sort of perceptible strobe effect could be very bad indeed, both because it's annoying and because you might miss something on the ground.

[jweir43]

Oh, dear God.

I  can buy 50 watt LEDs if I can figure out how to heat sink them.

Your stuff about moving light sources and propeller sync have NOTHING to do with the question.  The LED is WELL outboard of the propeller.  PLEASE do NOT introduce extraneous problems as there are none.

You want a clean airframe?  Same problem; consider a glider.

Sheesh

Jim

[jweir43]

Please, just GO AWAY.

Thanks,

Jim

[jweir43]

I was taught that a fast pulse or pulses of light for durations less than 30ms appear to be dimmed. I am not talking about a slow incandescent light bulb.

You were taught wrong.

Jim