Michael Rigby-Jones <Michael.Rigby-Jones <at> oclaro.com> writes:
> How well do white LEDs work in applications where you want a very short
> pulse of?  Intuition is suggesting that anything using phosphors is
> going to significantly limit the minimum pulse width.  The datasheets I
> have found so far from CREE etc. don't have any data on this.

I have tried to measure pulse width output from white LEDs and the results are
mixed. Basically each component phosphor has a different afterglow time, the
longest being green. This results in a 'rainbow' type of streak with a long
green tail that does not seem to want to go off at all (I used streak
photography to get the pulse shape). The green afterglow is so strong one can
see it on some LEDs with bare eyes in a dark room after the LED has been turned
off for a second or longer.

After this I switched to super-bright orange LEDs which do not have this problem
and have an acceptable color for strobe effects (almost the same color as Na
lights and fairly visible in daylight).

I think that to drive high efficiency LEDs fast 'off' it is necessary to use a
pulse shaper or push-pull drive to drain the charge from the device as they seem
to emit 'a little' 'forever' without this. I used hints from fiber laser diode
driver circuits to develop my own for that reason.

-- Peter


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