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Why were 5 different exposures needed?

My reading of the diagram says that 5 different exposures were needed to establish a 5+1 level gray scale.

Note that each picture is scanned simultaneously, and contributes to whether 0 - 5 holes are made in the tape.

At the receiving end, the number of holes controls the brightness of the beam reconstructing the image.

This makes sense, but I guess you could still achieve the same effect with only one exposure + a configurable readout offset with a variable resistor or something along these lines?

I think the tape just had 5 channels and each channel related to one level of grayscale.

If the tape had more or less channels, there would be more/less exposures.

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