Presented at a seminar course on "History of Printing" Columbia
School of Library Science, New York.
The reproduction of pictures and photographs in quantity has long been a technical problem seeking solution, and is yet to find a perfect solution. It is true that we now have amazing automatic techniques for reproducing photographs in quantity, but from the point of view of the printer the half-tone is still the only reasonable approach for continuous tone materials. Half-tone techniques were already in successful use when electronics was still in its infancy. However, electronics has since made its mark in every industry including printing.
It was inevitable that printers would apply electronics to such printing problems as half-tones, not to mention composition and press work. This paper shall merely try to indicate some principles which are common to the half-tone process as well as the transmission of pictures through electronics, whether it be to a distant receiver or to a nearby plastic half-tone plate.
One could say it is all a matter of resolution. Lockrey would have us believe erroneously that the human eye is so constructed that it ‘translates’ a mass of fine, evenly spaced dots into an even tone. Actually the case is that we are not able to resolve the spaces between dots, so that if we can place several dots close together, the eye, beyond its threshold of resolution, will not be able to resolve the dots, and will then see a ‘blurred’ condition or an even tone. In any case, putting the psychological problem aside, it is known in practice that it is possible to obtain tonal effects through the use of dots on paper or any other medium including the television screen. Thus, several hundred small dots in a small area will be seen as black or dark gray, while a much smaller number will obviously be seen as light gray or white. Any photograph or document can be made into such a pattern of dots through the use of the commonly known half-tone screen. However, it should be remembered that in a certain sense even the original photograph or printed page of a book is also a pattern of ‘dots’ which we are not able to resolve because the dots are so small. Actually they are particles of photographic emulsion or printers’ ink. If there is any doubt about this then examine any book under a strong magnifying glass and the seemingly black type will suddenly show white spots. Through the screening process every tone on the picture is translated into a pattern of dots, the size of the dots varying with the amount of light transmitted through the openings in the screen.
Mason, Hiller, Flader, add Lockrey cover the theoretical add practical aspects of the half-tone process most adequately. If the reader is in doubt about any aspect of the standard half-tone processes these works may be consulted. For the purposes of this discussion it is only necessary to recall that the standard half-tone is made by placing a fine screen in front of a picture, another picture is taken, producing a new picture which is a half-tone. The resulting photographic negative is used to prepare either a metallic or plastic plate (sometimes paper is used by burning or etching. Many people are not aware that half-tones are commonly used for ‘offset’ duplicators found in many small offices Thus, the attached sample was reproduced from a large colored wall display by taking a photograph, preparing a half-tone from the photograph, and a zinc plate from the half-tone. The zinc plate was then used to run off copies on a standard office multilith machine.
Electronic techniques do away with many of these costly steps. If a beam of light is focused a page the light will be reflected in proportion to the blackness or whiteness of the page. The very fact that you can see a light spot on a page is an indication it is being reflected, otherwise you would not see it. By taking such reflected light to the receiving end of a photocell, it is possible to translate this reflected light into a source of electric impulses. Photocells have a light-sensitive filament, which cause a flow of electrons, i.e. a flow of electricity in the output of the tube. Thus, if our light source is permitted to scan a picture or printed page, light will be reflected in continuously varying strengths depending the degree of blackness or whiteness. In the Fairchild automatic engraving machine a positive print is placed on a revolving cylinder, which is scanned by such a beam of light, the light being picked up by a photocell On an accompanying cylinder, a sheet of plastic is similarly wrapped. A pyramidal stylus penetrates the plastic, by heat, in response to the output of the photocell. The stylus engraves 380 holes per second and later models will do better. The important point here is that the stylus can be controlled so that when the scanning unit is directed at a light area the stylus cuts deeply. In this sense the Fairchild engraver duplicates electronically the standard half—tone process because it produces dots of varying size.
Stenafax works on a similar principle. The scanning system is essentially the same, but the stylus cuts a hole in a stencil rather than burning or melting away the plastic as the Fairchild engraver does. With the Stenafax one obtains uniform holes, and yet the manufacturers claim they can sometime get the quality of 144 line half-tones. Stenafax is used mainly for cutting mimeograph stencils, but the same technique could be used for preparing printing plates.
In a certain sense the Stenafax is more truly electronic, in that it gives the all or nothing effect. Either a hole is punched or it isn’t. This binary principle is
constantly at work in electronic systems, particularly in facsimile transmission.
In recent years there has been developed by WU a revolutionary type of facsimile paper known as Teledeltos paper. This electro-sensitive paper is able to respond to an electric current and turn from light gray or blue to dark black. It has, at present, a limit in regard to resolution. Although it is possible to produce variations in the size of dots it is not feasible to make a dot smaller than one hundreth of an inch, which is the equivalent of 100 line half-tones. Using the paper at its minimum dot size one then is using the yes or no principle. Either a dot is made or it is not. A sample of Teledeltos paper has been attached. By using scanning unit similar to the Fairchild engraver it is possible to transmit the output signal of the photocell over a wire or over radio waves and cause an electrical impulse in a "hot" stylus which "burns" a black dot in the paper. Actually the electric current burns away the coating and reveals a layer of carbon black.
Let us finally consider a similar problem and one which is close to all of us today — television. Without going into all the technical problems of television it is interesting to note the similarity of problems between transmitting a picture and. the reproduction of pictures for printing purposes. Television men also have a "screen" to work with. If the area of a screen is divided into hundreds of little squares as we do on a half-tone screen, it can be seen that a picture can be reproduced by making a square white, when the original picture is white or close to it, and black, when black is the shade observed. Tones are produced as in half-tone by groups of squares. That is why we get such sharp contrasts on the television screens, especially the earlier makes. The "screen" used was 525 lines per second, you examine the face of a television tube very closely when it is in operation, these lines are plainly visible and hence the picture is not as clear close up. The dots on a television screen are made, however, not by a stylus but rather by a beam of electrons hitting against an electron-sensitive material, with which all tubes are coated. This very same principle is now being applied to the printing problem in what are called cathode-ray printers and it will hot be surprising if the principle is later applied to the preparation of half-tones. Essentially on the cathode-ray printer the electrically sensitive paper will somehow be enclosed in the tube and the dots will be produced as a result of the presence of the electron bean.
We have seen how the half-tone is quite similar in principle to the binary principle common to all electronic systems, how electronics will now permit us to make standard half-tones more economically, and how ultimately entirely new types of half-tones will be produced with completely electronic systems.
Lockrey, A. J. 2nd ed..J.J. Tepper
Lockrey, A. J. Halftone processes. 2d. rev. ed. N.Y., J.J.Tepper, 1941. 61 p.
Mason, Edward F. How half-tone dots make pictures. Iowa City, Ed. F. Mason, c1949. 89 p.
Flader, Louis, and Mertle, J. S..Modern photoengraving. Chicago, Modern Photoengraving publishers, c1948, 294p.
Hiller, Theodore S..Half-tone photography for lithographic offset. N.Y.,Lithographic Technical Foundation 1945. 63p.
Melcher, Daniel, and Larrick, Nancy. Printing and Promotion Handbook. N. Y., McGraw-Hil 1949. 386p.