Phototypesetting disc. As the Fototronic disc spun at a high speed, a light would flash at the correct moment to expose the desired character onto photopaper. The two rows of thin radial lines identify the characters. There is space for seven lines between every pair of longer lines (possibly corresponding to the encoding on the 7-hole punched tape used as input). The radial column of squares for each character defines the character spacing (O'Brian 1965).