A thought question

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gar

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161228-1120 EST

Using a single beam obscuration type photo-sensor how can you get long term stability (no adjustment required) with a threshold sensitivity of better than 0.1" over a beam length of 3 feet where the object is approximately midway between the light source and sensor with nominal variations in voltage (95 to 135 V), temperature (40 to 110 F), and ambient light (10 to 200 FC)?

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gar said:
161228-1905 EST

What happens when the light source is 6" in diameter?

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It doesn't matter, as long as the beam is well collimated and the detector aperture is small. The two parameters that matter here are the "beam parameter product" or M2 factor (effectively the product of beam diameter and beam divergence - how "straight" or "beam-like" the light is), and the diameter of the sensor's entrance pupil.

Think about it. You want the sensor to go from fully lit to fully dark when an object moves 1/8" halfway between the source and the sensor. What must happen for that to occur? The object must either block the entire beam leaving the source, or the entire portion of the beam entering the sensor. If your source is 6" in diameter then your only option is to block the entire portion of it which enters the sensor aperture. This requires that the aperture be less than 1/8" and that the light beam be well enough collimated that the object forms a crisp shadow. Realistically, you want the aperture to be much less than 1/8".
 
And if you want to reduce the overall ambient light at the detector relative to the light from the source you can make the aperture smaller, move it farther from the detector, or both.

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