Success in sending pictures by radio from flat photographs and receiving them on flat photo negative plates (and subsequently of radio vision), really began with the perfection of automatic machines for the making of these prismatic rings, for by means of these prisms and a light sensitive cell at the sending station the light values which make up the picture are converted into electrical values, and broadcast.
So to put this picture on a radio carrier wave we simply slice up the picture (figuratively) into slices one-hundredth of an inch in width, in the best pictures, by sweeping the picture across the light sensitive cell by means of these rotating prismatic rings. With each downward sweep the picture is moved one-hundredth of an inch to the right until the whole picture has crossed the cell, the cell converting the light strengths of the different parts of each such slice into corresponding electrical values.
The process very much resembles a bacon slicer in the market, each slice showing fat and lean. Similarly these imaginary slices of our picture show light and dark parts, and these lights and shadows moving across the sensitive cell produce corresponding strength of electric current, modulating the radio carrier wave of the broadcasting set accordingly.
Further, of course, it is immaterial whether the current modulation is taken directly from a flat photograph, from a solid object, or from an outdoor scene at which the transmitter is pointed.
RECEIVING METHODS:
To put these light values back together again at the distant receiving station to make up a negative of the picture being broadcast from the sending station, it is only necessary to reverse the process; first, with a point of light to draw lines across a photographic plate, which the rotating prismatic rings do; and, second, to vary the density of the different parts of the successive lines corresponding to lights and shadows of the picture at the sending station, and this the varying strength of the incoming radio signal does by varying the intensity of the light.
Dense areas in the negative are built up where the light is successively very bright at the same place in adjacent lines; halftones where the light is less intense; while where the light is very faint, little or no exposure occurs, and shadows will result.
It is thus the lights and shadows which make the picture are built up, line by line, for when this negative is developed, and paper prints made therefrom, the dense areas produce high-lights in the picture; the less dense areas the halftones; and the thin areas the shadows of the picture, person or scene broadcast at the sending station. It is simply that a photographic negative has been made of what the lens at the sending station is looking at.
So, then, to receive pictures by radio, it is only necessary (1) to cover a photographic plate in parallel adjacent lines, and (2) to vary the density of the lines, to build up the shadows, the halftones, and the high-lights of the picture.
If one puts a nickel under a piece of paper and draws straight lines across it with a dull pencil, a picture of the Indian appears. And that is exactly the way photographs by radio are received, except that a photographic plate is used instead of a piece of white paper, and a pencil of light instead of the pencil of lead, the light pencil changing the exposure in various parts of the successive adjacent parallel lines by reason of the variation of the incoming radio signals.