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'how to filter surounding light'
1998\10\11@181403 by marbel

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Hello everybody
I want to measure reflected light from an object with an photo diode
but i don't want surrounding light to effect measurement .
I thought of lighting the object with infrared light pulsed at a certain
frequentie and detect the signal back and read the detected level.
My fear is this way is sensitive for noise and i wand a high level
resolution.
Are there good filters that pass light at an certain wave length?
an other way i thought of is to first measure surrounding light then light the
object with an additional light source measure again and subtract the
surrounding light from measurement with additional light
this method has also risks like is the surrounding light changes
Any body with suggestions or idea to this matter?




Marcel
Electonic Wokshop Scoop
Amsterdam

spam_OUTmarbelTakeThisOuTspamxs4all.nl

1998\10\11@223913 by Mark Willis

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If you use a lens to focus just the light from the area you want to
examine, and use matte black paint inside the focusing assembly (and
keep the lens clean), that should pretty much do it (look into how
astronomers get telescopes to reject surrounding light, basically this
is the same problem on a *very much* smaller scale <G>)

 Mark, .....mwillisKILLspamspam@spam@nwlink.com

M van der Bilt wrote:
{Quote hidden}

1998\10\12@021047 by paulb

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M van der Bilt wrote:

> I want to measure reflected light from an object with an photo diode
> but I don't want surrounding light to effect measurement.
> I thought of lighting the object with infrared light pulsed at a
> certain frequentie and detect the signal back and read the detected
> level.  My fear is this way is sensitive for noise and I want a high
> resolution.

 Why would it be particularly noise sensitive?  Hint:  Integrate over
time.  It is in fact the standard approach to the problem, the way most
of us would suggest.

> Another way I thought of is to first measure surrounding light then
> light the object with an additional light source measure again and
> subtract the surrounding light from measurement with additional light

 That is simply restating the first method.  The slower you do it, the
more likely the ambient light it to change in the meantime, so this is
*less* accurate.

> this method has also risks like is the surrounding light changes

 The only particular problem (presuming again the first method) is
linearity of your detector.  You *might* want to measure background
light level and compensate, which is implied in your second.  And you
may want to add a path switch to a known target for self-calibration.

 This is all about time filtering, Mark has mentioned spatial filtering
so take note and start prototyping!
--
 Cheers,
       Paul B.

1998\10\12@021053 by tjaart

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M van der Bilt wrote:

> Hello everybody
> I want to measure reflected light from an object with an photo diode
> but i don't want surrounding light to effect measurement .
> I thought of lighting the object with infrared light pulsed at a certain
> frequentie and detect the signal back and read the detected level.
> My fear is this way is sensitive for noise and i wand a high level
> resolution.
> Are there good filters that pass light at an certain wave length?
> an other way i thought of is to first measure surrounding light then light the
> object with an additional light source measure again and subtract the
> surrounding light from measurement with additional light
> this method has also risks like is the surrounding light changes
> Any body with suggestions or idea to this matter?

A very effective way around this is to pulse your TX with a say, 25% duty
cycle, and to average the RX side. Use this averaged version to set the
gain and/or the threshold for the RX detect. This method is used in spread
spectrum receivers.

--
Friendly Regards

Tjaart van der Walt
.....tjaartKILLspamspam.....wasp.co.za

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1998\10\12@121921 by Gabriel Gonzalez

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I've used something similar, a short piece of rubber hose (the automotive
kind), they are pretty matte and black. The only problem would be if you use
a long section, it will bend easy, but on short lenghts, it is straight
enough.
The other advantage is that you can find it in a lot of diameters, and they
are real cheap and easy to find.

Just my 0.02

Calvin

{Original Message removed}

1998\10\12@141353 by Eric Naus

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What you want to do is modulate the light at a certain freq. and then
 look for a response at that freq. That way ambient light will have little
effect.
There are many projects on the net that show how this is done.


Good luck and have fun :-)


Eric

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