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'[OT] Measuring led intensity in ambient light'
1998\11\21@122235 by Craig Lee

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Gentlemen,

I have yet another perplexor to present to the piclist.

I have a circuit board where the customer requires
testing of the light level emitted from each led.  The
test will be pass/fail, but will require some parameter
resolution and immunity to ambient light.

The ambient light will be indoor florescents, but there
is continuous movement, and thus changes in reflected
light.

For practical purposes, I would like to measure this light
3/4 or 1" away from each led.

Are there photodiodes or phototransistors that only react
to specific wavelengths of light?
Are they sensitive enough to detect minute changes?
Are part to part variances low?
Would a coloured filter be effective?
Is a CDS cell more prudent?
Do the detector sensitivities change over time?
Has anyone created a calibration procedure?
Are pre-calibrated parts available?
Would a current mirror improve sensor drift?
Has anyone done something similar?

Thanks,

Craig

1998\11\21@133935 by Gabriel Gonzalez

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Maybe it would not be practical, but using a dark enclosure during test is
the fastest, cheapest and easiest way of conducting this kind of test.

LEDs emmit light in certain wavelenghts, and you can get sensors and filters
to improve rejection of unwanted light, but even this does not mean that
there won't be spurious light in the same wavelenght as the LED.

Just 0.02

Calvin


{Original Message removed}

1998\11\21@214516 by Reginald Neale

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Craig:

Can you control the current to the LED? Your job is much easier if you can
modulate the current and only detect light output at the modulation
frequency. Also, as you suspect, a filter over the detector to restrict
wavelength range will help a lot. CdS cells are often very slow to respond,
which will make it more difficult to detect a modulation frequency. Si
would probably be better. TV and VCR remotes typically use a modulation
frequency of about 40kHz. Prefabricated detector modules for this purpose
are readily available.

Reg Neale

1998\11\22@150859 by paulb

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Craig Lee wrote:

> For practical purposes, I would like to measure this light
> 3/4 or 1" away from each led.

 Another question short on details of the test situation but long on
wild, expensive ideas.

 Is this test automated or manual?  Why do you think 1/2 or 3/4" away
from the LED is good?  What mandates this?

 The easiest and most reliable method is going to be to place a hood
(cone - like teh space shuttle) directly over the LED.  This can be
performed quite well by a mindless minion or a pick-and-place type
robot, or by a single-axis locator if the PCB is jigged.  Single-axis
drives with a throw of about 1" and accuracy of 1/96" are usually junked
for $2 or so and are *very* common, complete with interface electronics.

 Even at 1" distance, a 6" diameter black vinyl "skirt" will remove
most ambient light.

 If the LED is modulated (either inherently, or by an induced test
pattern in the microcontroller) it makes things much easier, as has been
said.

 I gather you refer to multiple LEDs.  Either you use pick-and-place or
a multiple test jig.  BPW30-style photodiode sensors have a fairly
large receptive area so that lateral and angular alignment is less
critical.

 We're all 'dying' to know the next kink in the situation?
--
 Cheers,
       Paul B.

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