The light pen itself had a small photoelectric cell in its tip.
The light would reflect back to not one, but a bank of photoelectric cells.
That had depended upon the photoelectric cell, set in the top of the desk.
Probably a photoelectric cell, with a dark beam, invisible to the eye.
The early photoelectric cells were not very sensitive, so three or four would be used with the subject in a darkened booth.
It had the round structure of an eye, or a photoelectric cell.
It is automatic and electric, its light controlled by photoelectric cells.
The best place for a photoelectric cell is a side of the building exposed to both the rising and the setting sun.
It was a photoelectric cell connected to a small tape recorder.
The best place for a photoelectric cell is at a spot that receives light from both the rising and setting sun.
