Hey, just look at this nifty invention from Mullard in 1951. Using flying spot scanning, the time bases of two cathode ray tubes, one a transmitter and the other a receiver are synchronised. In the transmitter, a raster is projected onto a glass overscreen. In the absence of any writing on the overscreen, the light beam passes through the glass. If however, a mark is made on the overscreen then some light is reflected then detected by a photocell. The electrical currents produced in this way are amplified and used to modulate the beam in the receiving cathode ray tube which results in a facsimile reproduction of the original work.
This was hailed as a very clever piece of kit in 1951 - no PDA, tablets or iPAD then you know and these pieces of kit were so big that no-one could steal them easily....... unlike a PDA, tablet or iPAD!!!!!!