piclist 1998\10\12\021047a >
Thread: how to filter surounding light
face BY : paulb@midcoast.com.au

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!
       Paul B.

<36218B43.57B@midcoast.com.au> 7bit

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