> Russell McMahon wrote:
> > The modern "white" phosphor" LED, invented by Nichia,  mixes only two
> > colours in its vector sum. The LED proper generates a deep blue, and a
> > proportion of the blue is absorbed by a phosphor and re-emitted in the
> > yellow part of the spectrum so that the vector sum of the two lies
> > somewhere in the above region. Note that while the yellow is a single
> > re-emission wavelength it in turn can be thought of in terms of a mix
> > of the RBG primary colours of this chart. In this case it is NOT made
> > that way, but could be arrived at in that manner - with possibly
> > different second order effects - see below.

> Actually, the emission spectrum diagrams that I've seen show a narrow spike
> for the blue, but a rather broad peak for the yellow.

Yes. My comment was misleading to the point of being able to be
construed as being wrong :-). What I was trying to convey is that the
phosphor emits yellow light and not say red and green or other mixes
which sum to yellow. The yellow peak is very broad and varies greatly
with the LED shade of white.

Here is the excellent Cree XR-E.
http://www.cree.com/products/pdf/XLampXP-E.pdf
It is available in a number of shades or 'colour temperatures" of
white and the means of accomplishing this is well demonstrated by the
graph at the top of page 6. The relative intensity of the remaining
output form the blue emitter and the amplitude and distribution
patterns for the "yellow" phosphor output can be clearly seen.

The 3700 K and 5000 K colour temperature LEDs have almost identical
blue output magnitude but the 5000 k yellow peaks at 550 nm while the
3700 k yellow peaks at about 580 nm and is about 60% higher in
magnitude. (Looking at the graph is far easier than trying to make
sense of the above).

The 2600 k white has a lower amplitude of blue and stronger yellow at
about 600 nm.

Note that when naming LED colours the names used are usually somewhat
arbitrary. One man's reddy-orange may be another woman's orange. A
difference of 20 nm can be enough to result in a new colour name and I
imagine  that some people have no trouble distinguishing colours 5 nm
apart. (I haven't actually tried that - must do so some time).

Amplitudes tend to be far less noticeable. Two white LEDs with no
common reference surface to illuminate can be 2:3 in amplitude before
many people notice. Even 1:2 can seem not overly large. When "wall
washing" with LEDs much smaller amplitude differences can be noticed.
Generally a variation in the 10%-20% range is noticeable from adjacent
LEDs. This can lead to problems with 'lumen maintenance' if a failing
emitter is replaced in isolation.


                        Russell McMahon
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