I believe that the discussion "pulsed is brighter than continuous" does not
refer to "pulsed fast enough to appear as if continuously on" LEDs. Maybe some
authors misquote things out of context.

Human brains and eyes are very good at seeing single events in contrast, for
example a led flashing for only 100usec once a second can be seen fairly well in
a normally lit room even if the pulse current is only 5mA. That's equivalent to
a duty factor of 1/10000 or to a ridiculously low average LED current of 0.5uA,
visible in a normally lit room! Having the LED run at 0.5uA DC under the same
conditions would likely cause the viewer not to be able to tell whether it is
on, even in a darkened room.

I could recently prove myself this fact with a USB to RS232 adapter which has a
TX LED. The cable appeared to be dead due to interface incompatibility but then
i opened a terminal and sent spaces or other characters and could see the LED
blip once per character, very faintly. I later realized that the baud rate was
256kBauds or higher and that the LED had been on for a ridiculously short amount
of time. I confirmed this switching to 300Bauds and there the LED made fat wide
pulses.

Some smoke alarms and the like use the pulse-led-once-in-X-seconds method as a
'heartbeat' and battery life monitor. The average current consumed for this is
very low and affordable. For example pulsing a LED on with 5mA every 10 seconds
for 1msec would lead to the same 1:10000 duty cycle or 0.5uA average current
consumption. This 0.5uA is low enough to be used for a clock that is meant to
run for a year on a single cell, and for similar purposes.

So, I think it's more about contrast in slow pulsing and human ability to detect
it, and not about fast pulsing at all. Also, there are special slow pulsing
frequencies which are particularly well noticed. I am not sure if they are in
the same range with those which cause seizures and must be avoided.

  Peter


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