Computer controlled marble sorting machine

Take a look at this video of a wood and brass marble sorting machine. The machine is monitored and controlled by a computer. As the marbles start down the track, their color is detected and certain gates are opened by servos to sort them. From there, they pass through a series of steps until they are finally placed as sets into waiting holders. I wonder what type of sensor is used to determine the color of the marble.

I love the grooved wood Archimedes screw marble lift!

[ Thanks Christoph! ]

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