Photolithography

“ Four baths are used, each of different strengths, the strongest is used first, the weakest last. I quote from the catalogue of the Boston Art Museum, of the exhibition illustrating the technical methods of the Reproductive Arts and Photo-Mechanical Processes, held January 8, 1892: "Photo-aquatint (photogravure) for the production of half-tone intaglio plates from photographs from nature, paintings, etc. A dry aquatint ground is laid on a metal plate, and over this is mounted a gelatine negative film, made by the pigment printing process. ”

“ Many workers have tried, with varying success to devise a means whereby photography could be made to reproduce not only the outlines and gradations of natural objects but the colors as well, and there is now available a method of great worth for this purpose. In brief, it consists in making, by one exposure in an ordinary camera, a set of three-color negatives, each of which represents that portion of one of the primary colors—violet, green and red—which was reflected from the subject. ”

“ The use of this process appears specially important for those cases in which an ordinary photographic negative is not sufficient, {25} for example where a printing plate cannot be prepared from the negative. It ought also to be useful when the light could not be controlled in making the negative, and thus incorrect effects appear in the reproduction, which often occurs in negatives from nature and with polished plastic metal objects. The enlargements from the original are then used because the details can be better seen, and the artistic character can be more easily seen. ”

“ Float the paper upon the following solution for three minutes and hang it up to dry:
39 The sensitiveness increases in proportion to the quantity of uranium nitrate. With the above formula the paper can be exposed in the camera, or, for printing, under a negative cliché.
In printing an exposure of five seconds in diffused light gives an image perfectly visible, and a grayish black tone; ten seconds gives a vigorous image almost of a black color; in from fifteen to twenty seconds the image is very strong, with the color of an engraving. In sunshine the action is neces­sarily much more rapid. ”

“ Reproduction photography has to reckon on this disadvantage, for with a side illumination the raised up lines cast shadows, and a correct negative does not follow.
By the third method, lithography, of which we have already spoken, the impression lies generally flat on the paper like a drawing, if a very strong deep etching was not used, in which case it is also slightly raised. In printing from the graver or from stone etching a faint relief is generally noticeable, which is never so high as with copperplate printing, and which is no important obstacle to making a photographic negative. ”

“ Good opacity in the ground and clear glass in the lines of the drawing are the fundamental requirements of a negative for photo-lithography. But drawings are not always prepared in such a manner that this can be completely obtained. With flat drawings which are drawn in grey instead of black the first intensification is only continued till the finest lines appear clear. For further intensification the negative after drying is given a coating of a light sensitive solution of—
Ammonia is now added till a light colour is formed. ”

“ Transparencies can be made same size of negative by contact and exposure to artificial light, or enlarged or reduced in the camera by daylight, with equal perfection in result. To make transparencies by contact place one of the Keystone thin crystal glass transparency plates over the negative in printing-frame, lay piece of dark soft material over it, close down the back, and expose to the light of the lantern or to a gas flame or other artificial light, for 10 to 30 seconds, according to density of negative, at a distance of 20 inches from the flame. ”

“ The half-tone process is used almost exclusively for reproducing photographs and wash drawings, though it will produce a facsimile of any kind of copy, such as impressions from type, old manuscripts, or typewriting, but a shade composed of minute black dots will appear over the entire print and there will be no absolutely whits areas unless they are produced by routing the plate or cutting out the high lights. (See p. 74.) The reproduction of an ordinary outdoor photograph requires very little handwork, except for re-etching, burnishing, and cutting the borders. ”

“ All that is, therefore, required to be done, in using the camera obscura for photographic drawing, is to place upon the opening at the top of the box the prepared paper, and immediately to cover it with the lid, C, so that it may not be acted upon by any other light than that reflected from the mirror. The time required for producing the necessary effect will depend on several circumstances, such as the preparation of the paper and the intensity of the light when the experiment is made ”

“ The practice of photography depends primarily on the fact that certain chemical compounds are changed into other compounds by the action of light. Another fact is closely allied with this, namely, that a suitably constructed lens of glass or other transparent material, or a fine needle-hole used instead of a lens, will project the image of objects placed in front of it. ”

“ Drawings made on scraper boards, grained papers, or by the aid of shading mediums, are photographed and reproduced by the ordinary line method. In making sketches for “half-tone” photo-mechanical reproduction it must be remembered that, as far as photo-lithography is concerned, it is impossible to accentuate effects by what is known as “fine etching” when applied to photo-engraving. A negative for photo-process work of any description should be absolutely perfect in every respect. ”

“ The art of producing images on prepared surfaces by means of the actinic or chemical rays of the sun’s light. The principal photographic processes now in use are the positive-, negative-, and dry-collodion processes, in each of which a thin film or skin of iodised collodion forms the sensitive surface, a plate of glass being generally used as the foundation of the film. In a positive picture the ‘lights’ are silvery white by reflected light, and the ‘shades’ are produced by a ‘backing’ of black varnish or velvet, showing through the glass plate. ”

“ And in the near future it will be a great acquisition when it is once generally recognized that colour plates can be prepared by photographic means without any considerable amount of manual or artistic help. It is the more to be wondered at that photo-lithography has not yet found that extension and general use which it in so high a degree deserves. ”

“ The basis for the preparation of a photo-litho is as a rule a photographic negative on glass or gelatine, which, as the word negative implies, when examined by transmitted light, must have all its tones reversed, that is to say, the drawing or the lines, strokes or points which must in the print be black, must look transparent, whilst the other part of the negative which forms the groundwork must be covered or opaque. ”

“ The operations of photography serve to make us understand how it is that the sun presides over chemical action in the vegetable world. What is photography? What does that curious phenomenon which fixes a drawing formed by light upon a sheet of paper, consist of? A paper steeped in chloride or iodide of silver is placed in the focus of the lens of a dark camera, and the image formed by the lens is made to fall upon paper sprinkled with water. ”

“ Unfortunately, no pigments have been found that can reproduce in purity the colors of the spectrum, and to this fact is due the failure of the process to reproduce exactly all the colors, tints, and shades of an original. When a drawing in black on white paper is photographed only the white paper affects the negative film. The transparent parts of the developed negative thus represent the black, and the opaque parts, which have been acted upon by light, represent the whits. ”

“ But no sketching or drawing can make records of cirrus or alto clouds with the speed and accuracy necessary for careful study. Photography is really the only way in which the amazing wealth of detail can be truthfully portrayed. Yet even the camera has its limitations. It does not record colour, and completely fails to delineate the forms of alto-stratus, stratus, or nimbus, if they are present in the most typical condition, that is to say, when they cover the whole sky with a uniform tint. ”

“ PHOTO-ENGRAVING, ETCHING, AND LITHOGRAPHY. PART I. PHOTO-ENGRAVING IN LINE. CHAPTER I. APPLIANCES FOR MAKING PHOTOGRAPHIC NEGATIVES.
For all methods of heliographic printing a suitable photographic negative is absolutely necessary, and to produce that negative good apparatus and pure chemicals, used with skill and intelligence, are indispensable.
The studio must be well lighted, with a perfectly firm floor. The dark-room should be kept for negative work only (the room where the zinc printing is carried on being a separate room) . ”

“ Every light spot in the camera-picture becomes dark on the sensitive coating of the glass-plate. But where the shadows or dark parts of the camera-picture fall, the sensitive coating is less darkened, or not at all, if the shadows are very deep, and so these shadows of the camera-picture become the lights of the glass-picture, as the lights become the shadows. Again, the picture is reversed, just as in every camera-obscura where the image is received on a screen direct from the lens. ”

“ It will be evident from the nature of the process, that the lights and shadows of an object are reversed. That which is originally opaque will intercept the light, and consequently those parts of the photogenic paper will be least influenced by light, while any part of the object which is transparent, by admitting the light through it, will suffer the effect to be greater or less in exact proportion to its degree of transparency. The object wholly intercepting the light will show a white impression ”

“ For this reason photography is used not only in astronomy but in many other branches of science, among its most important uses being the making of records of microscopic objects.
A device carrying a photographic plate is attached to the eye-piece of a microscope; the plate being exposed affords, on development, a precise record of the subject under observation. It may be noted that in this case, as in astronomical photography, no camera lens is required; the microscope, like the telescope, projects an aërial image which is impressed on the plate. ”

“ It is not at all necessary to know the process of making a sensitive plate, but it is quite necessary to know something of the action of light on the plate in order to make a good picture. A photographic negative is formed by the chemical action of light admitted through a lens or even a tiny hole, into an otherwise light tight box, and striking a glass plate, film, paper, or celluloid, coated with sensitive silver salts. The part of the light that affects this coating is the blue rays. ”

“ Holography involves a completely different approach to photography. In addition to more immediate applications in microscopy, information storage and retrieval, and interferometry, it promises such bonuses as 3-dimensional color movies and TV someday. ”

“ If we take, for the sake of simplicity, not a picture, but a scale with four or five-tone gradations from light to deep black, the light tones will act more or less on the sensitive photographic plate according to their degree of brightness, but as the rays of light have to pass through the cross-lined screen interposed between the lens and sensitive plate, and as the rays can only pass through the transparent parts and not through the opaque, no homogeneous surface is obtained on the negative, but a tone produced by the cross-lines of the screen. ”

“ In support of this may be urged the fact that the interposition of glass prevents the making of dark pictures. But on the other hand it must be remembered that celluloid and sheet-gelatine, also air-tight substances, are able to store up light and to give it out again. It is well known among photographers that to allow sunlight to fall on the inside of a camera is apt to have a “fogging” effect on a plate that is exposed in the camera afterwards, though the greatest care be taken to keep all external light from the plate. ”

“ The exposure to light in the printing frame will vary according to the density of the negative, and the strength of the light, and should be continued until the image can be seen well defined upon the gelatine film when examined in the dark-room, by opening one-half of the back of the frame. ”

“ The effect of the camera obscura may be produced without a lens, merely by admitting light through a small aperture in a shutter into a darkened room, for the rays from each point of an external object being allowed to illuminate only a very small surface, will produce a tolerably distinct image without being made to converge, provided this image be received on a white surface, placed near the hole in the shutter. ”

“ As everyone acquainted with photography knows, a picture is made ordinarily by exposing a sensitized plate or film, developing the latter to make a negative, then exposing sensitive print paper to the light that comes through the negative, thus reversing the lights and shadows and creating a positive in the exact semblance of the subject photographed. A concern in Cleveland, Ohio, the Positype Co., produced Bromotype paper which could be exposed directly in the camera, coming out of the developing process as a positive without the intervention of a film or plate negative. ”