Yes, that is true. Blue LED's are GaN, or InGaN, and are direct
emitters. UV LED's are AlGaN, or AlGaInN, and also are direct emitters.
However, most white LED's are a phosphorescent (fluorescent) material that
is pumped with a blue or UV LED, and glows white.
There are also some white LED's that are a combination of red, blue and
green LED's, but they are expensive, and require special drivers as they
have 4 leads... one for each color, and a common anode (cathode).
-Chuck Harris
David I. Emery wrote:
On Wed, Sep 10, 2014 at 08:34:11AM -0400, Chuck Harris wrote:
The white LED's I have worked with, though tiny, glow for a long time
after being run at full brightness. That persistence should make them
poorly suited for use as a pulse modulated white light source.
The old GaAs LED's used to modulate well into the GHz region. I doubt
that you can modulate the white light output of a white LED with much
depth into the 60Hz region.
My understanding is that most "white" LEDs contain phosphors
excited by actual UV LED UV light drive (or at least used to) and so this
is not surprising at all.
A blue LED might, on the other hand, be mostly LED light directly.
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