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Scantron readers use LED's as light sensors? I assumed they used regular photo sensors.

The older form readers used metal brushes that detected the conductivity of the graphite in pencil lead.

It's possible that the "newest" ones use photo sensors as they now also double as standard desktop scanners, but I'm not sure they would be marketed as OMR. I suspect Scantron and Pearson NCS won't be around much longer as standard scanners are much cheaper, can accomplish the same goals (minus printing results on the form, without a special attachment on expensive scanners), and embedded processing continues to grow.

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