Why do LED lights seem "digital" when you take your eyes off them?

That’s because LEDs flash at the frequency of electricity (60Hz) which you can’t normally see, but can see if you swish your head from side to side quickly.

Whereas, incandescent bulbs have a filament that’s heated by the electricity. The filament doesn’t get cool enough during that 60Hz cycle to stop glowing (a glowing hot filament takes a few milliseconds to cool enough to stop glowing when the electric is shut off), so it stays hot and glows continuously.

Most LED lamps are set up so the LEDs are not full-on 100% of the time because they would run too hot and toast themselves to a puddle.
To give them less power especially if they are dimmed the juice from the wall socket is reduced in voltage to the proper voltage for the particular parts, and spit out in a square wave with the up portion of the square being ‘on’ and the down being ‘off’.

The square wave in a lot of LED applications tracks with the wall juice at 60 Hz, so you can see the LEDs going on and off 60 times a second.

More sophisticated LED lamps, especially dimmable, might up the frequency usually to 120 Hz, so there is less flicker, but that is still visible to the fast head shake.

Why are you swinging your head around like that in the first place? XD

@HungryGuy is right. it’s just “faster”

“it’s just faster”
No, HungryGuy is right that the incandescents don’t go all the way off and the LEDs do flash on and off. They both vary at the same rate.

@dabbler well technically it’s “faster”, because its going on and off rapidly. while incadesents sort of fade, so the fading is not really a full cycle of on/off

LEDs, especially in electronic displays, are sometimes driven at a blink rate of several kilohertz to eliminate flicker and allow better PWM control of brightness.

I’m starting to see cars on the road with LED taillights that blink too slowly—they cause distracting flicker when the vehicle is moving. I wish the car makers would speed up the refresh rate.

I’m not as familiar with using LED’s for light replacements but I’m familiar with how LED’s work. An LED is a type of diode (light emmiting diode) which has an anode and a cathode. The polarity in an AC power supply is always switching from positive to negative at a certain rate depending on the frequency, and in most residential/industrial applications this frequency (hertz) is around 60 cycles a second. A diode will only conduct current when the proper polarity reaches the proper side of a diode. The anode side will conduct the positive phase of the current while the cathode will conduct the negative phase of a current. This is how a typical fullwave bridge rectifier (consisting of 4 diodes in a certain order) converts AC into DC.

The LED is oscillating on/off very fast at about 60 times a second (unless you deliberately change the frequency through another means). The incandescent bulb never really oscillates between on and off because the filament is a straight resistive load that is always conductive regardless of the polarity of the cycle. The filament is also protected and sealed from the air while generating much more heat than the LED. More heat means the element will stay lit longer along with the fact that the element is always conductive to current regardless of the frequency cycle. If you had a straight pure (or near pure) DC power souce energizing both of your incandescents or LED’s I’m guessing that you would likely not notice this visual effect. Note: I’m not familiar with these new LED replacements and whether or not they have their own internal circuitry involved here. (spelling edit)

Do LEDs that are used for general illumination (replacing incandescents) actually blink synchronously with the 60 Hz line frequency? (or 120 Hz for full-wave bridge rectification?) You’d think they would use a DC power supply to continuously light the diodes.

@gasman Most LEDs use PWM (pulse-width modulation) to allow heat to dissipate.
They don’t generate a lot of heat for the amount of light produced but it is generated in a very small area/volume (the diode junction) and they will literally meltdown if put at full duty cycle. You’ll see heat-sink fins and metal parts on most LED bulbs that replace regular ones to draw the heat away from the tiny devices. The super bright LEDs on cars are all on metal substrates with fins to help them last a long time.

Good observation that some signs and other more sophisticated applications will crank up the frequency of the modulation to make them appear very steady. That’s the classy way to do it but it requires a circuit to generate the higher chop rate. Simple LED applications will modulate at 60hz or 120hz because that’s doesn’t require any additional circuitry, you can get those free off the powerline.

@dabbler Thanks. As a hobbyist I’m used to working with regular LEDs that take about 20 mA continuous current—didn’t realize that higher-power devices had heat dissipation issues, but makes sense.

indeed ! Low power indicators and battery-op illumination like these throwies

@gasman In an AC system running at 60 hertz (or switching polarities every 1/60 of a second) the current would turn off twice in a 1/60 sec cycle or 120 times a second, obviously much too fast for the eye to notice or to affect most electrical loads. This is because there is a zero threshold (polarity potential) that is reached twice when the maximum negative sine wave peak moves to the maximum positive sine wave peak. Technically the LEDs (or other loads) are turning on/off 120 times a second. Note: You can see what I’m talking about if you plug the oscilloscope’s leads into a wall outlet that uses ac voltage and observe the sine waves.