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White LED spectral wavelength spead

J

Joe G \(Home\)

Jan 1, 1970
0
Hi All,

Is there any specification on the spectral spread of white LEDs?

I have a few LEDs from a batch that have slightly different colour when put
through a defuser.
(3mm LED). The difuser seems to filter out certain wavelengths and the
colour difference
can be seen.

Any pointers would be appreciated.

Thanks in advace
Joe
 
W

Wolfgang Mahringer

Jan 1, 1970
0
Hi Joe,
Is there any specification on the spectral spread of white LEDs?

I have a few LEDs from a batch that have slightly different colour when put
through a defuser.
(3mm LED). The difuser seems to filter out certain wavelengths and the
colour difference
can be seen.

What about having a look on the LEDs datasheet?

greetz
Wolfgang
 
This is pretty typical. Usually you'll see some that are greenish,
yellowish or blueish in a particular batch. If you're using this for
anything requiring precision, you may be able to ask your supplier to
provide you with more consistent batches or to sort them for you (if
you're using them in high volumes).

What the spec sheet usually tells you is the "typical" spectrum curve,
which is usually indicative of an average of a wide distribution.

If you're sorting them yourself, lower the current through them or use
the diffuser to dim the light enough to distinguish the different
colours.

Chris
 
S

Spehro Pefhany

Jan 1, 1970
0
This is pretty typical. Usually you'll see some that are greenish,
yellowish or blueish in a particular batch. If you're using this for
anything requiring precision, you may be able to ask your supplier to
provide you with more consistent batches or to sort them for you (if
you're using them in high volumes).

What the spec sheet usually tells you is the "typical" spectrum curve,
which is usually indicative of an average of a wide distribution.

If you're sorting them yourself, lower the current through them or use
the diffuser to dim the light enough to distinguish the different
colours.

Chris


Another possibility might be to use RGB LEDs and adjust the color
balance yourself. Probably more stable over time (both brightness and
color balance) than phosphor-type white LEDs too.


Best regards,
Spehro Pefhany
 
J

Jonathan Kirwan

Jan 1, 1970
0
Another possibility might be to use RGB LEDs and adjust the color
balance yourself. Probably more stable over time (both brightness and
color balance) than phosphor-type white LEDs too.

But that calibration takes effort to do well, if you are trying to
make one "look like" another one next to it. I've designed both the
software and (using a CIE method as a base) mathematical portions to
allow a technician to "tweak" three different trim pots, one each for
red, green, and blue dacs that controlled the current through each.

Before using the tool I developed, the technicians had used a standard
piece of equipment (not cheap) that provided a calibrated (x,y) CIE
color curve coordinate value and the technicians had a particular
(x,y) they needed to "hit." But in tweaking three pots to get there,
it was a rather frustrating process because adjusting the green, for
example, would impact both CIE coordinates in rather complex ways. It
could honestly take them a significant time (and some experience, to
get better at it) to center the (x,y) where it was needed to be.

What I did was to develop instead a convolving matrix that teased out
the various impacts of each of the three pots, using a single "snap
shot" with a spectrophotometer to calibrate it accurately, using any
initial setting of the pots (didn't matter, so long as the LEDs each
put out something measurable -- rough mid-point settings was good) and
that was all that was needed to exactly then provide specific slider
bar displays the technicians needed to set each pot individually,
without worrying about the effects each had on the (x,y) coordinate.
When they got each slider centered on the 100% tick mark, whether by
left or right turns, they were done with that pot. And when all three
were done, the (x,y) was always exactly on target.

So it can be a little bit of fun to calibrate a good white point. Not
to mention that the three dies also need to be "close" in order for
them to appear to mix.

Jon

P.S. Another complicating problem, if you are being really peevish
about high stability in the intensity output of any LED, can be the
erratic drift some LEDs have even while holding them temperature-
stable in an oven and controlling their current drive, precisely. A
lot of them are poor at holding the same optical output, even under
these circumstances. However, if you monitor their outputs and run a
bake cycle for a few hundred hours, you can separate out those which
do not appear to stabilize from those which do appear to "anneal" into
a very slow drift over time. Those which are stable after a few
hundred hours make for good optical standard candles, given that you
stabilize their temperature and current drive, of course.
 
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