LED lighting, persistance effect and lumens...

[Externet]

Hi all.

 

LEDs operated in pulses over ~30 Hz are not perceived by the eye as pulsing but continuous emission of light; is the lumens emitted same in pulsing effect than if ran on DC at full power/constant current ? (pulses being also at full peak power)

 

Operating LEDs in AC, the emission of light occurs only when the LED voltage Vf threshold is surpassed, and at the rest of the waveform, they are 'off'

If the frequency is high enough, the 'fast intermittance' is not perceived.

Do sensors to determine/measure luminosity of a light source respond with different lumen figures in pulsed and DC current ?

 

In other words, constantly lit LEDs produce more lumens than pulsed ones ?

To the eye, will a room feel brighter if not pulsed ?

 

[John Cuthber]

"In other words, constantly lit LEDs produce more lumens than pulsed ones ?

To the eye, will a room feel brighter if not pulsed ? "

It depends.

The eye will roughly respond to the average power.

You can pulse a LED at higher power than you can run it continuously.

LEDs don't have a real threshold- the brightness varies with the current i.e. roughly exponentially with the voltage drop.

[Externet]

Thanks, John.

Then, I take constant DC lit having more average power than pulsed, the pulsed LED room will be perceived as dimmer. As expected.

[StringJunky]

The visual system can unconsciously register the difference between frequencies in the range 100 - 150Hz, even though we can only see about up to 75Hz in a consciously differential manner... any higher and just appears continuous.

Characterizing and Minimizing LED Flicker in Lighting Applications

[Janus]

Thanks, John.

Then, I take constant DC lit having more average power than pulsed, the pulsed LED room will be perceived as dimmer. As expected.

Well, that depends on the nature of the pulses, and how you are measuring the AC voltage. Let's say you run 12v AC through a full wave rectifier before feeding the resulting pulses through your LED. Your LED should glow just as bright as if you put 12V DC through it.

This is because AC voltage is measured as its RMS value and not peak value.(if your house current is rated at 120V, that's 120V RMS. The peak voltage is 169.7V) RMS voltage is the DC equivalent for an AC voltage.(Running 12 volts RMS AC through resistor delivers as much power as 12V DC.

 

Of course, if you just run AC directly to your LED, you will only get 1/2 wave rectification and half the effective brightness.

 

This also assumes sinusoidal pulses. If you are using square-wave pulses, then the brightness will depend on the duty cycle.( In fact, SCR (silicon controlled rectifier) dimmer circuits use a varying duty cycle to control the brightness of incandescent lights)