Electronics > Projects, Designs, and Technical Stuff
Human Eye -- Peak or Average Response
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Benta:

--- Quote from: jweir43 on June 09, 2019, 09:14:36 pm ---
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.


--- End quote ---

Forget the 24 fps when talking lighting. The 100/120 Hz flicker from fluorescent lamps is absolutely perceptible in your peripheral vision, and there have been threads here on this forum with users who have problems with this in the direct vision field.
Absolute minimum is 200 Hz, and for a design I'd go for 500 Hz and above. Especially for the application you've mentioned.

bsdphk:
There is some text about this related to the the LED display in the HP35 somewhere, I remember the text being "non-stiff" so it is probably in one of the oral histories, but check also the HPJ issue.

The gist of it is that they saved battery-life by driving the LEDs with microsecond-width high-current pulses, based on the observation that it gave same visual response but used a lot less power.

That must presume some kind of peak-ish response-function to make sense.
TheUnnamedNewbie:

--- Quote from: bsdphk on June 12, 2019, 07:15:34 am ---There is some text about this related to the the LED display in the HP35 somewhere, I remember the text being "non-stiff" so it is probably in one of the oral histories, but check also the HPJ issue.

The gist of it is that they saved battery-life by driving the LEDs with microsecond-width high-current pulses, based on the observation that it gave same visual response but used a lot less power.

That must presume some kind of peak-ish response-function to make sense.

--- End quote ---

Likely also just the fact that driving with hard switching can be more efficient, both space-wise (on a chip) and energy-wise. Linear dimming of an LED requires you (unless you do fancy DC/DC converter stuff) to waste the excess power somewhere else (be it a resistor or a transistor). Switching an LED hard allows you to use more current (smaller resistor, if any resistor is used at all), for a shorter time, resulting is less wasted heat.

Another strange thing with PWM at low-ish frequencies is that you might not notice when it is still, but really can notice when it moves. This is, after all, what those rotating-stick-of-LED displays are based on. Easy way to notice: Just have an LED PWM at a few hundred Hz and then shake it violently, and now notice how you see dots instead of a line of light.



I have also been told that you often want to drive an LED for lighting (totally different field than what is discussed here) in PWM instead of constant-current for CRI reasons. The phosphors are non-linear in their response, and so will change color index slightly as you reduce current. In order to avoid this you want to drive the LED hard always, and just change dutycycle.


--- Quote from: Zero999 on June 11, 2019, 09:52:24 am ---
--- Quote from: jweir43 on June 10, 2019, 01:47:26 am ---
--- Quote from: Audioguru on June 10, 2019, 12:40:07 am ---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.

--- End quote ---

You were taught wrong.

Jim

--- End quote ---
No, he is right. The human eye acts like a low pass filter. Short pulses will appear dimmer, than long ones. Awhile ago, I did an experiment  which involved putting short pulses, of hundreds of Amps through high powered LEDs. The flashes appeared to be dim, but would have been bright enough to cause eye damage, had they been continuous, assuming the LED would have survived, which of course it wouldn't..

--- End quote ---

How much of them being less dim was simply because of the inductance in the leads and the capacitance in the LED, causing the actual current through the LED to be substantially lower? Not to mention, if you really start putting that much current in the LED you are going to significantly alter the bandgap, which will change color and efficiency both.
Zero999:

--- Quote from: TheUnnamedNewbie on June 12, 2019, 08:20:06 am ---
--- Quote from: bsdphk on June 12, 2019, 07:15:34 am ---There is some text about this related to the the LED display in the HP35 somewhere, I remember the text being "non-stiff" so it is probably in one of the oral histories, but check also the HPJ issue.

The gist of it is that they saved battery-life by driving the LEDs with microsecond-width high-current pulses, based on the observation that it gave same visual response but used a lot less power.

That must presume some kind of peak-ish response-function to make sense.

--- End quote ---

Likely also just the fact that driving with hard switching can be more efficient, both space-wise (on a chip) and energy-wise. Linear dimming of an LED requires you (unless you do fancy DC/DC converter stuff) to waste the excess power somewhere else (be it a resistor or a transistor). Switching an LED hard allows you to use more current (smaller resistor, if any resistor is used at all), for a shorter time, resulting is less wasted heat.
--- End quote ---
If the LED is being powered from a resistor or linear regulator, then PWM makes absolutely no difference to the efficiency. The main reason for PWM in LED displays is it's easier to implement with digital outputs, than a linear solution.


--- Quote ---Another strange thing with PWM at low-ish frequencies is that you might not notice when it is still, but really can notice when it moves. This is, after all, what those rotating-stick-of-LED displays are based on. Easy way to notice: Just have an LED PWM at a few hundred Hz and then shake it violently, and now notice how you see dots instead of a line of light.
--- End quote ---
Yes, that's another reason to keep the PWM frequency high.


--- Quote ---I have also been told that you often want to drive an LED for lighting (totally different field than what is discussed here) in PWM instead of constant-current for CRI reasons. The phosphors are non-linear in their response, and so will change color index slightly as you reduce current. In order to avoid this you want to drive the LED hard always, and just change dutycycle.
--- End quote ---
I think it depends on the LED. Green LEDs definitely do shift towards the blue end of the spectrum, especially when overdriven. I agree, it's better to keep the forward current to that recommended by the manufacturer to keep the colour and wavelength within specification.


--- Quote ---
--- Quote from: Zero999 on June 11, 2019, 09:52:24 am ---
--- Quote from: jweir43 on June 10, 2019, 01:47:26 am ---
--- Quote from: Audioguru on June 10, 2019, 12:40:07 am ---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.

--- End quote ---

You were taught wrong.

Jim

--- End quote ---
No, he is right. The human eye acts like a low pass filter. Short pulses will appear dimmer, than long ones. Awhile ago, I did an experiment  which involved putting short pulses, of hundreds of Amps through high powered LEDs. The flashes appeared to be dim, but would have been bright enough to cause eye damage, had they been continuous, assuming the LED would have survived, which of course it wouldn't..

--- End quote ---

How much of them being less dim was simply because of the inductance in the leads and the capacitance in the LED, causing the actual current through the LED to be substantially lower? Not to mention, if you really start putting that much current in the LED you are going to significantly alter the bandgap, which will change color and efficiency both.

--- End quote ---
The capacitance and inductance of the LEDs and wires are not a problem, as long as the layout is good enough. The current was measured using a current transformer and forward voltage monitored.

LED efficiency does drop, at higher currents, but this was taken into account, as the light output from the LED was measured using a photodiode. The purpose of the experiment was to see how much an LED can be overdriven and the reduction in efficiency, rather than the response of the human eye, but it was noted that longer pulses appear proportionally brighter, than short ones. For short pulses, the brightness is dependant on the energy, rather than the peak power or length, so a 2µs pulse will a appear to be the same brightness as a 1µs pulse, of double the peak intensity.
Siwastaja:
24Hz flickers massively, or almost "blinks". No one can stand it for long.

Cinematic projection has never worked at 24Hz. Depending on the actual projector shutter type used, they either doubled or tripled the film frame rate, so it's either 48Hz or 72Hz. TV was 50-60Hz depending on country, but CRT has some persistence (gradual fade during off-time) that LED doesn't have.

So now we are looking at a complex problem, which we can simplify:
1) After around 100Hz (there is some room for arguing), the eye integrates the light perfectly, and only the average matters,
2) Between about, say, 20-100Hz, it's a really fuzzy "depends who you ask" type of problem. Different people perceive quickly flickering lights in a different way.
3) Below about 20Hz, you have a clearly blinking light, and of course it appears brighter during the on-time, as your eyes recognize the on-time as a separate "event".

For 1), the only thing that matters is the light output vs. input current curve for the particular LED. Often, for modern LED types, this curve is quite linear for a big part. For a modern standard-power LED specified for 40mA absolute maximum, you would likely have the linear range somewhere between about 2-20 mA. If this is your case, then only the average current matters, and PWMing makes no difference. OTOH, if your PWM "on" current would be at the absolute maximum 40mA, the LED likely runs at lower efficiency, lowering the average light output for same average input current. But all this speculation is meaningless: you need to look at the curve of the actual LED you are using, and you have your answer!
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