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Comment by hunter2_ | Hacker News Reader

How could the same modulation achieve "dimming" and "fully bright" simultaneously?

For traditional lightbulbs, a PWM signal actually makes the light noticeably dimmer, and has a very simmilar effect to simply reducing the current. This is because the mechanism for them is heating up the filement, which happens on a much slower time scale then the PWM duty cycle.

In practice, traditional dimmers are not quite PWM as they do not generate a square wave. Instead they generate a sin wave with portions of each cycle clamped to 0.

LEDs already need driver circutry to condition the relativly high AC voltage into a stable lower voltage DC. Dimmable LEDs create a stable DC power supply from the chopped up AC power, then use the width of the active portions of the AC as a signal to drive their own dimmer logic.

That's a good PWM explanation but I think GP was asking a different question. Walter claimed that if the PWM was high enough frequency you would perceive it as full brightness while using less power.

It obviously wouldn't work that way when lighting a room, but what about for an LED indicator light that you look at directly? I don't know enough to form an opinion.

Without any more information on frequencies and measurements it's impossible to know. My guess is either A) They did not perceive the dimming, and/or B) The frequency was high enough that the inductive and capacitive effects of the circuitry became relevant and was filtering the PWM signal to DC signal that still drew the same amount of power as it did when fed the full voltage DC signal

The measurable brightness of the LED is a straightforward sum of the times it's on and the times it's off. Shift the ratio so it's off more than it's on and it gets dimmer.

The "fully bright" part is a consequence of human vision. Your brain is making sense of limited, noisy information coming from your eyes. A flickering light appears brighter than the light in steady state, lots of still images shown in rapid succession look like they're moving, as far as you can tell you can perceive the full range of colour out of the corner of your eyes, and the dress could be either colour.

Also (and I think more important, but I might be wrong), we don't perceive light intensity linear but logarithmic. A 50% duty cycle does appear brighter than half as bright. A 90% duty cycle might be only barely perceptible.

The apparent brightness is caused by the ratio of on/off and is called the duty cycle. 50% brightness would mean that half the time the light is on, and half the time the light is off.

If the cycling of the light on and off is done at say 10kHz it's perceived as a dim light.

you dont. But your eyes perceive brightness non-linear, which means to your eye, 80% of brightness is very close to 100%

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