LEDs do not have to flicker, but supplying low-ripple, constant-current DC power is not as cheap as ways of powering LEDs that do result in flicker.
However, once place where I do notice it and it's annoying is with taillights on some cars - sometimes when I sweep my eyes across the cars ahead, I see the strobe effect from LED taillights.
It doesn't seem to happen with all cars.
I've noticed it in a few bike taillights, but not nearly as much as in cars. I used to have an early LED bike headlight, and the strobing was very apparent when the light was dimmmed, I could only use it at its highest level.
I'm pretty sure the DOT requires that your headlights don't blink, so I wonder how low the PWM frequency can be until they consider it blinking?
I also wonder whether they do it on purpose. How hard would it be to run the PWM at a few kHz instead of the < 100 Hz frequency it seems like they're using?
That being said, obviously the fact we probably can't see 10KHz flicker, does not imply it has no effect on our well being.
Edit: if you turn your head at the same time, my numbers above are too conservative.
Consider a flickering streetlamp outside your house. Now imagine you have two sets of net curtains covering your window, 10cm apart. The net is very fine - say a 0.1mm mesh.
If you sit next to the window working, and move your head back and forth, you will see a moire effect. If you sit 10cm from the curtains and move your head at 1 m/s, the 'flashing' you see is 50 Khz. If the light source behind the curtains is flickering at 49,995 Hz, you will see the resulting 5Hz flicker when the flicker from the moire effect and the streetlamp get multiplied.
As a regular human, you have now made a 50kHz flicker detectable with no special equipment.
Problem is not everyone have this problem. I have this what I call latency intolerance syndrome.
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Let me know if you're still having trouble. Thanks!
Also, LED's unlike nearly every other electrical device are constant current devices (as opposed to constant voltage, which is what a USB port, battery, or nearly any other power supply offers for example).
Constant current devices must be wired in series rather than parallel, meaning that if you wanted more than one LED per transformer, you would need to break the existing circuit to add an extra LED. That isn't really compatible with standards around electrical plugs - you would need to short out all unused electrical outlets in a constant current system.
Constant Voltage vs Constant Current is another one of those 'DC vs AC' or 'electron has positive or negative charge' issues - We have chosen one standard, and everything is built on it. We could go back and redecide, but all infrastructure would need updating.
The question of supplying LEDs with CC or CV power supply is only the question of efficiency and cost.
Most LED strips are made as a serial/parallel combination of LEDs with an added resistor for each segment, to get the desired current through the LEDs in a segment from a ~12V CV power supply.
You don't really have that much choice when it comes to LED strips. CC power supply would need to generate thousands of volts or you'd need a bunch of them. And that would be costly.
If anything I simplify leds as maintaining a "constant voltage", no matter the current I send through them (in the right direction). Just like other diodes.
> it's easier to regulate the I component of P=I*V for consistency than it is the V portion.
I understand, and we're on the same page. But it doesn't change the fact that almost no led strips do this. You can iron out over individual LED differences by putting 3-4 in the series, and regulate the current using a resistor.
In fact, most of the LED indicators you'll see are driven like this. LED + resistor [+ optionally PWM or multiplexing].
Only for power LEDs you'll usually see anything more complicated invlving DC/DC CC switching power supplies.
kHz? Really? That seems very high to me.
In general, light is linear and time invariant. That means in a flickering light system, a flicker of blue light then of red light, or red and blue at the same time, are indistinguishable if the 'sampling' system doesn't have sufficient bandwidth to resolve the flicker at all.
There do exist nonlinear optical components[1], but none that exhibit nonlinearities at the light intensities you'll find at home!
Additionally, even if the above were not true, flickering LED sources tend to flicker at a multiple of the AC supply, so it's very unlikely you'd see one flickering at 1000 Hz and another at 1001 Hz for example. For battery powered devices, it could easily happen though.
https://en.wikipedia.org/wiki/Beat_(acoustics)
I don't see why the same effect would not happen with the light sources.