optical sensor to differentiate between sample and background

I have an instrument that has a moving stage. The instrument takes readings in a fixed location and the stage moves to align the sample under the instrument. I would like to sprinkle sand on the stage and have an optical sensor tell me if there is a sample there or not (background). If there is a sample there, I would like to take a reading. If there is no sample there, I want the stage to move to the next area that has a sample.

I am not looking to find an optical sensor for the grains of sand. I would like to find an optical sensor to put on top of the eyepiece lens because you can easily see if there is a sample there or not. Background would be tape so I can choose whatever color that would be best. I am looking for the sensor to determine "yes there is a sample there" or "no sample".

Any suggestions?

I have an instrument that has a moving stage. The instrument takes readings in a fixed location and the stage moves to align the sample under the instrument. I would like to sprinkle sand on the stage and have an optical sensor tell me if there is a sample there or not (background). If there is a sample there, I would like to take a reading. If there is no sample there, I want the stage to move to the next area that has a sample.

I am not looking to find an optical sensor for the grains of sand. I would like to find an optical sensor to put on top of the eyepiece lens because you can easily see if there is a sample there or not. Background would be tape so I can choose whatever color that would be best. I am looking for the sensor to determine "yes there is a sample there" or "no sample".

Any suggestions?

I don't understand the "scale"...

If you are "looking" at one "spot" and you know exactly where to look you can check the amount of reflected light (with a photo transistor) or the color of the reflected light. You can check the color by illuminating it with an RGB LED (one color at a time) and mapping the result. If it's small, you'd need some lenses and in any case you have to eliminate/control ambient light.

If you're looking for a spot over a larger background area you'll need a camera, a computer (a lot more processing power than an Arduino) and some sophisticated software.

@Chemistry, please do not cross-post. Threads merged.

I missed a logical step in the description: the sand particles are the samples?

Are they significantly brighter or darker than the background?

What IS the sample?
What IS the instrument?
How big is it all?
How do you tell the difference between sample and sand?
Your post is full of ambiguity.

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