At 11:47 AM 1/21/01 -0000, you wrote:
>Pulsed light is claimed to have a greater apparent brightness for the same
>average energy. I guess the human eye acts as a peak detector.

Much closer to an average detector. The apparent brightness only changes
slightly (usually improves) with shorter duty cycles and the same
average current. Losses increase, of course, as Vf increases.
With early (1970's) LED materials, there was a threshold of perhaps 1mA
below which not much light was emitted, but with modern materials it is
much more zero-based. Super-bright GaAlAs LEDs are usable and fairly
linear over a very wide range, from 10's of microamps to scores of mA.

It's easy to test this, all you need is a scope and a source of
square waves of different duty cycles (I'd expect you to use a PIC,
of course). You can scope the voltage across the dropping resistor to
see what the actual current and duty cycle is. Pick a couple of
matched LEDs and adjust them for equal brightness with different
duty cycles.

If the pulses are slow enough that the flicker is visible, then the
results will be different, favoring the short duty cycles if they
are long enough.

There are also some interesting shifts in the spectrum of the
emitted light with current, some of the effect due to heating of
the die, and some not.

Best regards,


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