Tone reproduction with electronically-cut stencils

The principle of cutting stencils by means of an electronically-controlled spark discharge was established about twenty years ago for black and white reproduction. A more recent development is the Roneo electronic stencil machine which also reproduces intermediate tone values shown by photographs. Various mechanical and electrical refinements are required to achieve satisfactory results. Tone reproduction is based on the fact that a linear increase of current across the electrodes produces a linear increase of density on the copies. It is shown how the reflected light signal received from the original has to be corrected in order to match the various densities of the copies with those of the original. Besides tone reproduction the machine can also cut stencils from line work originals which are not sufficiently ¿black¿ such as letter-heads in colour. Copies in full colour are obtained by use of the ¿contrast knob¿ which causes a deliberate distortion of the ¿tone setting¿.