So back in the Bronze Age my teacher would collect all the scantron cards -- which by the way can already be automatically graded; that is the entire point of the scantron -- and put them on the overhead projector with the correct answers masked out. Overhead projector was more than bright enough to shine through the paper card. The teacher could easily grade the entire class in a minute or two, no cameras or computers needed.
The light would shine through each student's answer card and key, like when you press two sheets of sketch paper together. So any mismatches would be easy to see and mark as you scan down the card. It would only take a few seconds to score each card and there would be no cognitive load at all, as in you wouldn't have to evaluate any answers.
