Blinking LED and the human eye/perception

Thread Starter


Joined Jan 12, 2021
Hi Folks, hope all is well.

When an LED is flashing at 2 times a second, the human eye can perceive the ON and OFF state. We can clearly see when the ON state is and when the OFF state is.

When it is flashing 10,000 times per second, we see the LED in the ON state only.

Why is it that when the LED is flashing at 10,000 times per second, it is the ON state that is visible to the human eye but not the OFF state? Why does the ON state seem to have precedence over the OFF state?

Why don't we see the LED as OFF while it is flashing at 10,000 times per second? If the time periods of both states are the same, why does the LED seem to be ON all the time?

Why does the LED seem to be ON all the time? It might as well seem to be OFF all the time!

Assume that the duty cycle is 50%.

Thanks for the replies.
Last edited:

Audioguru again

Joined Oct 21, 2019
LEDs that are turning on and off at a high rate appear to be turned on but the brightness is adjusted with the pulse widths.
Narrow pulse widths appear dimmed but wide pulse widths appear bright.

The iris in our eyes adjusts for brightness so that wee can see things in very dim light but very bright light does not blind us.

Disregarding adjustment by the iris, half or double the amount of light appears to be only a little less or a little more because our vision's sensitivity to brightness is logarithmic.


Joined Mar 14, 2008
Why does the LED seem to be ON all the time? It might as well seem to be OFF all the time!
It doesn't see just the ON time, it responds to both.
But the eye's cone and rod light sensor respond relative slowing to changes in light intensity, so you see the average of the on and off light intensities.
That's why you can watch TV or movies without seeing the flickering as the picture changes from frame to frame at 24 to 60 times per second
So, for example, if the LED were blinking a 10,000 times per second with a 50% duty-cycle, the LED would appear approximately as bright as an LED that is on 100% of the time at 1/2 the current.


Joined Oct 2, 2009
The answer is not because of persistence of vision.

An LED flashing with 50% duty cycle would appear to be on.
Would it appear to be half as bright as one that is on for 100% of the time? No.
That is because our perception of intensity is not linear. I would appear almost as bright as the LED that is always on.


Joined Aug 21, 2008
The LED might appear to be on while being pulsed at thousands of times per second, however if your eyes move quickly or the LED moves quickly you may notice what is called "image breakup" when referring to displays. Your eyes do not smoothly scan the scene before you, even though we perceive it to be done that way. instead the center of our fields of view moves in tiny jerks called saccades, in which the view shifts by as much as 10,000 degrees per second. This is why color sequential displays are not popular - when the eye moves the image momentarily breaks up into its colored components, usually R, G, and B.

I remember about 25 years ago on a country road, seeing a car drive down the road that used pulse width modulation on its tail lights, but switched to 100% on when braking. It was a bumpy, winding road and I was watching the car at night from a distance. I distinctly remember seeing the car trace out a winding, bouncing dashed path of red tail lights as it drove. Moral of the story: You need a higher refresh rate than you think, and the brighter the light, the faster the refresh rate needs to be.