Prepress

Generally speaking, the term prepress includes all the steps required to transform an original into a state that is ready for reproduction by printing. Prepress includes the following steps: art and copy preparation (including typesetting), graphic arts photography (i.e., shooting negatives), image assembly and imposition (stripping), and platemaking. Depending on the nature of the original, other included aspects of prepress may also include halftone photography, color separation, or other procedures. Prepress should not be confused with makeready, which is the preparation of the printing press.

This article will begin with the traditional prepress methods, and will then cover newer digital prepress processes, which are replacing in large part the conventional ones. The article will conclude with the specific prepress considerations which vary according to the printing process to be utilized.

Strictly speaking, the term copy refers to text material only, while art refers to text as well as other page elements such as illustration matter (line art and photographs), decorative borders, etc.

'Manuscript'. Copy is supplied by the author (or other content provider) in manuscript form. (Although the term manuscript is used to refer to any original text matter, it technically refers only to handwritten copy, a no-no in modern copy preparation. The more correct term for a typed manuscript is typescript.) Since the setting of type needs to be done as quickly and as accurately as possible, there are certain manuscript conventions that should be followed, although the exact requirements can vary according to the temperament of the publisher and/or typesetter. Manuscript copy should always be typed (or output from a word processing program) and should be double-spaced. The pages should be consecutively numbered from the first page to the last page, never by chapter. Typing should only be on one side of the sheet, never on the reverse. Pages should always be 8H x 11 (in the United States), although A4-size paper is acceptable in many cases. (See A Series.) Copy produced on a typewriter should have uniformly-sized characters, so as to ensure an accurate character count. Copy prepared using a word processing program may not need to be; most such programs can automatically generate character and/or word counts. Line- and copy-editing alterations should be clearly indicated, preferably using standard proof marks. If the number of corrections on a certain page is excessive, the page should be retyped. As many corrections as possible should be made in the manuscript phase; after typesetting, it becomes increasingly expensive to make all but the most minor of changes. (Large-scale changes to typeset copy made beyond the simple correction of typographical errors are known as author alterations, and most publishers charge an author when the changes exceed a certain amount.)

'Type Specifications and Copyfitting'. It is often the job of a copy editor, designer, or other such person to indicate on the manuscript copy the typeface, point size, line length, leading, etc., in which the copy is to be set. This includes both the text type and the display type, if any. A prime determinant of the type specifications, aside from aesthetics and the nature of the copy itself, is the amount of space in which the type needs to fit. In book typography, the amount of space available is dependent upon the desired final page count of the book, which is determined by the length of the manuscript and by the desired cost of the book. Since books are printed in signatures of varying size, if is often desirable to allow the page count to come as close as possible to the end of a signature, so as to reduce the amount of blank pages at the end of the book. In magazine, newspaper, and other such typography, there are strict space requirements for a particular article, with little leeway. Additionally, most magazines also have pre-determined type specifications as aprt of the magazine's overall design, so cutting and/or adding of copy is often performed by an editor to fit the copy to the prescribed length. Other forms of typography, such as brochures, ads, etc., also have certain space requirements, but adhering to a certain design is not often a condieration in such "one-shot" jobs.

In all the cases mentioned above, the process of copyfitting is performed to fot the type in the alotted space as economically as possible. A character count is often performed to gauge the total number of characters in a particular manuscript. This number is often compared to the alphabet lengths of various typefaces, so as to determine how much space will be occupied by the type when set in a particular font. Depending upon the nature of the piece to be printed, this may be performed with more or less accuracy. (See Type and Typography, Copyfitting and Character Count.)

'Art Preparation and Layout'. In the simplest of cases, such as a novel, text can be set as straight type. In many cases, however, art needs to be included. Art preparation begins with a layout, or a blueprint that aims to show how all the disparate elements of a page will be organized. The specific layout of a particular piece is dependent upon the nature of that piece, and what the piece is supposed to actually do and for whom it is intended. The layout for a beer ad aimed at purportedly "hip" young adults would probably be very different than that for an academic book jacket aimed at Civil War historians. Regardless of the nature of the layout, the steps involved in the production of it are the same. At the outset, a sketch of the intended design is prepared, with varying degrees of completeness. The roughest sketch of a layout is a thumbnail, or a very crude pen or pencil diagram showing where text and images will be placed on a page. Thumbnails do not have to be the exact size as the ultimate page, can use wavy lines to indicate text, and can be prepared on any type of paper, and have even been prepared on cocktail napkins. A rough sketch is slightly more formalized than a thumbnail, and is closer to the actual size of the page. Thumbnails and roughs are often used during the brainstorming phase of design, and are used primarily to garner approval for a particular design idea. Once a rough has been approved, a comprehensive is produced, which is the most formalized of the early layout phases. It may be crudely drawn, but must be accurate with regard to page size, image size, and type size, as it is the primary guide that will be used to generate the camera-ready art.

Once a layout has been approved, a mechanical is prepared. The mechanical is where all the actual elements are assembled for reproduction. The set and corrected type is pasted to the mechnaical in the proper position, and line art and other non-photographic elements are also pasted to the mechanical board. (Hence, a mechanical is also known as a paste-up.) In the case of photographs, which need to be converted to halftones, commonly rough copies (such as xeroxes) or strips of rubylith are pasted into the proper position, simply to indicate where they will be inserted (and are often marked FPO, or "for position only"). The actual halftone negatives will be inserted into the page during stripping.

'Line Art vs. Continuous-Tone'. There are two basic types of original art: line art and photographs. Line art consists of art that, as the name imples, are simply lines, shapes, or other similarly-rendered illustration. Type itself could be considered line art. Photographs are known as continuous-tone images, as they include gradations of color or gray, and are not simply black (or some other single color) or white. Most printing processes are incapable of reproducing continuous-tone images, so such images must be converted to a collection of tiny dots (called halftones). Each tiny dot is some level of gray (or color) and the packing together of these variably-shaded dots provides the illusion of continuous tones. Since negatives for halftone reproduction need to be photographed separately from line art and type, they cannot be added to the same mechanical. An FPO copy or strip of rubylith is used to create a hole or window in the negative, into which the halftone can be inserted. (See Haftone.) Other types of continuous-tone illustrations—such as paintings—also need to be converted to halftones.

'Image Acquisition'. Images can be generated or acquired in a variety of ways. Line art can be produced by hand, using pen and ink on paper, the only requirements being that the lines are clearly and solidly drawn, and that the paper be a background (such as white) that won't interfere with the image when it is photographed for a negative. Line art can also be produced in a computer drawing program (such as Adobe Illustrator, Corel Draw, etc.) and output from a laser printer, or higher resolution device. Continuous-tone images can either be reflection copy (such as prints) or transmission copy, such as slides or transparencies. The terms reflection and transmission refer to the manner in which they are photographed (for negatives) or scanned (in digital prepress): for reflection copy, light is bounced off the surface of the copy where it is captured by the camera lens or scanner optics, while for transmission copy, light is passed through the image. Line art can also be reflection or transmission copy, as well.

'Color Prepress'. The are two basic types of color reproduction: spot color and process color. Spot color is essentially an additional single color used to print specific page elements as a means of highlighting them. An example would be a black-and-white page of type that had a head printed in blue, or a company logo printed in red. Spot colors are physically separated from each other on the page, and are rarely overprinted. Each spot color will ultimately require its own plate, so it can be prpeared on a separate mechanical or, more commonly, on a transparent overlay attached to the mechanical. The mechanical itself should only contain the key color, or the color that comprises the bulk of the page (this is commonly black). Spot color overlays are taped into position over the key and can be exposed separately.

Process color is more complex, and is used to print "full color" or four-color illustrations. There are four basic process colors: cyan, magenta, yellow, and black. The inks used to print the process colors are somewhat transparent (unlike the opaque spot color inks), and it is the overprinting of halftone dots of these four colors in varying densities that produces the full range of reproducible colors. Depending on how tight the register of the colors needs to be, the different colors can be indicated on transparent overlays, with the color and screen percentage marked on each sheet, or combined on one layer of the mechanical board, the color breaks indicated on a tissue overlay. The actual color separation negatives, however, are always prepared separately and stripped in during flat assembly.

Process color images need to be color-separated during the prepress phase in order to produce a set of four different negatives so that four separate plates can be prepared. Although color separation has historically been performed photographically by exposing a full-color image to different color filters, most color separation is now performed digitally. (See Color Separation.)

'Image Sizing'. When assembling graphic images on a mechanical, it is rare that the size of the original image will correspond exactly to the apportioned size on the layout. Two means of resizing photographs can be used, either independently or in conjunction with one another. Scaling is the resizing of an image that changes the dimensions of the image without altering the ratio of the width to the height, which can be performed photographically or digitally. Cropping is the cutting of certain sections of an image, either to allow it to fit in a prescribed areas, or to eliminate unwanted and/or unnecessary portions of the image.

'The Final Mechanical'. The final mechanical that is sent for graphic arts reproduction contains the pasted-up type and usually any line art. Black-and-white halftones and color images are represented by black or red shapes that correspond eactly to the size and shape of the image(s) to be inserted. It is important that all images not supplied on the mechanical be clearly indentified and correlated with a marking on the mechanical (called keying) to ensure that the correct image is placed in the correct position. It is at this point that the mechanical is sent out to be converted to negatives.

It should be pointed out at this point that much of the foregoing began—in the mid-1980s—to be replaced by digital means, all type and art being assembled in page makeup programs such as QuarkXPress and PageMaker. Mechanicals, negatives, and color separations can be output directly from the computer. Increasingly, too, the printing plates themselves are imaged directly from digital data. (See Digital Prepress below.)

Although this article has talked primarily about producing negatives for graphic reproduction, this is only true in the case of some of the major printing processes. Some require film positives in order to properly prepare the final image carrier, while others require negatives. Some processes can utilize either negatives or positives. The most common of the printing processes, offset lithography, more commonly images plates (called negative-working plates) from negatives, but can also image plates (called positive-working plates) from film positives. Each type of plate has somewhat different performance characteristics, but the preparation of the flat for platemaking is essentially the same. Thus, the term "negative" will be used throughout this article, but bear in mind that the term "positive" may also be appropriate. Where the processes of preparing the two types of film differ, distinctions will be made.

'Graphic Arts Cameras and Films'. Unlike portable (or even not-so-portable) cameras used for creative photography, photojournalism, or holiday snapshots, graphic arts cameras are very large devices, which, once installed, are rarely moved and are commonly attached to the darkroom in which the films are developed. There are several different configurations of graphic arts cameras, but they all share certain common features. The copyboard is where the mechanical is attached for exposure, and many cameras also include transparency holders, allowing for the imaging of both relection and transmission copy. (In color separation and halftone photography, higher image quality is achieved with transmission copy.) Lenses used in graphic arts cameras differ from other types of camera lenses. Graphic arts camera lenses commonly have f/numbers between f/8 and f/11 (the higher the f/number, the smaller the lens aperture and the longer the exposure time required) and focal lengths that can range from eight inches to forty-eight inches, depending on the size of the camera. (In contrast, a typical 35mm camera has a two-inch focal length.) Lenses for graphic arts cameras are also prepeared to generate as few aberrations and distortions as possible, and are also prepared to reproduce the colors of the visible spectrum as accurately as possible. The exposure times for graphic arts films are comparatively long, thus the lighting needs to be of much higher intensity than that required for other types of photography. Many graphic arts cameras also include suspension systems to minimize vibrations of the camera during exposure, and some have computer-controlled exposure systems which measure the intensity of the light source and vary the exposure time accordingly. Some varieties of cameras are vertical models, on which the copyboard, lens, and lensboard are aligned in a vertical plane, rather than the more typical horizontal system. The chief advantage of vertical cameras is their economical use of space.

Films used in graphic arts photography are similar to those used in other types of photography: an emulsion consisting of silver halides suspended in a gelatin is applied to a plastic or paper base. After exposure to an image, the light areas of the original are represented (on a photographic negative) by heavy deposits of silver, while the dark areas of the original are very light or transparent. (On a film or paper positive, the reverse is the case.) A photographic negative is used to prepare either film positives or prints. Different types of films are required for different types of originals, be they halftones, color separations, or line art.

Photographic films after exposure contain latent images, which only become visible after immersion in a developing solution, which transforms the silver halide to metallic silver in proportion to the amount of light to which the emulsion was exposed. Thus, in less exposed areas of a photographic negative (the image areas), less silver halide is converted to silver, while in the more highly-exposed areas, more silver halide is converted to metallic silver. Fixing solutions after developing convert the unexposed silver halide in image areas to a soluble form, which can then be washed away. (Film positives operate in an opposite manner.) The chemical composition of the developing solution, the temperature at which the film needs to be developed, and even the color of the darkroom's safelight all vary according to the type of film used. (See Photography: Graphic Arts Photography. See also Photography: Halftone Photography and Color Separation.)

'Image Assembly'. The process of image assembly is known by a variety of terms, including imposition and stripping, although stripping is a term considered by some to be technically exclusive to offset lithography. All the terms, however, refer essentially to the assembly of negatives (or positives) into pages and signatures for platemaking and printing.

The guidelines for assembling page negatives are governed by the number of pages that will comprise a signature which is in turn determined by the size of the printing press and the paper to be used. In printing, it is desirable (both in terms of time and cost) to minimize the number of individual pressruns. Thus, a number of pages is printed on one side of a large press sheet, backed up, then the completed sheet is folded and cut into individual pages. As an example, say you want to print an eight-page newsletter, each page being 8H x 11 inches. You could of course run four sheets of 8H x 11-inch paper through the press twice, but that amounts to eight separate pressruns, and eight separate plates, which is scarcely time- or cost-effective. A more efficient solution is to make only two pressruns. Using paper that is 22 x 17 inches, four pages can be printed at a time. The imposition would look like this for one pressrun:

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while the printing on the reverse would look like this:

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Folding and cutting yields the proper number of pages. (And on a perfecting press, which prints both sides of the sheet simultaneously, only one pressrun is required.) This one 22 x 17-inch sheet is known as a signature. (More specifically, it is known as a quarto, or a single printed sheet of paper that is folded twice to yield eight pages.) Book printers can create signatures of up to sixty-four pages. (See Book Typography.) Consequently, the size of the signature that can be produced needs to be known prior to beginning image assembly.

The process of image assembly involves the attaching of a photographic negative of a page to a carrier sheet. For single-color printing, this carrier is most often a sheet of opaque goldenrod paper. The negative is attached to the back of the sheet, and holes are cut in the front over the image areas. (The entire assembly of negatives attached to a carrier sheet is called a flat.) This will allow light from the platemaking apparatus to pass through the transparent image areas, exposing the plate only in those areas. After the holes have been cut, the process of opaquing involves placing the negative on a light table and using a thick fluid or opaque marking pen to cover up any extraneous pinholes or other undesirable transparent regions of the negative, thus ensuring that unwanted spots, speckles, or marks will not be imaged on the plate. During opaquing and stripping, the stripper may also add crop marks, register marks, or other guides for the printer. Additionally, pin register systems—used throughout the prepress processes—use devices to punch holes or slots in non-image areas of film and copy, while pins are fitted into the holes to ensure that successive pieces of film and/or copy are positioned correctly with respect to each other. Pin register systems are most often used whrn making multiple images on plates or films, especially in multi-color production.

The size of the flat is at least equal to the size of the press sheet which will be utilized. The negative for each page to be printed on that sheet is stripped into the proper position on the flat, with certain allowances for trimming and binding considerations. (In particular, shingling, or the progressive increase of the size of the gutter margin of folded sheets to make allowances for the increase in thickness of a document that is bound by saddle-stitching, is set at this point.) It is also at this point that individual negatives that have been prepared separetely (such as halftones) are inserted, making use of the transparent window that strips of rubylith have provided.

The final process in prepress is platemaking. Platemaking is a general term which refers to the act of exposing the image carrier for the printing process to be utilized, and not all printing processes use plates specifically; gravure prints from an engraved gravure cylinder while screen printing utilizes a handcut or photographically-prepared stencil. Each speicfic printing process—or application within that process—requires different plates or platemaking considerations. See Plate: Offset Lithography, Plate: Flexography, Gravure Cylinder and Stencil. See also the prepress considerations for each of the major printing processes mentioned below.

The prepress processes outlined above are increasingly being replaced by digital methods. The history of digital prepress is one of evolution and revolution.

Before the 1950s, when letterpress was still the dominant form of printing, type was set using linecasters by trade typesetters and color separations were made by engravers or other specialized trade shops. The advent of offset lithography and the photographic means of preparing plates were applied to the photographing of a letterpress-printed sheet. In the 1960s, phototypesetting eliminated this step in the process. Color separations were eventually made using electronic color scanners which could output the separations directly to film. In the late 1970s, the invention of the raster image processor and the raster-based imagesetter allowed the output of high-quality and high-resolution images and type directly on film, eliminating much of the need for graphic arts camera work. In the mid-1980s, the introduction of the Apple Macintosh personal computer, page makeup software (such as Aldus PageMaker [now Adobe PageMaker]), and the Adobe PostScript page description language allowed the output of digital pages either as proofs on low-resolution laser printers or as film on high-resolution imagesetters. Additionally, these systems (and later versions of these systems) also devised means of making color separations on the desktop, as well.

Essentially, advances in digital prepress have served to bring the front end (i.e., the acquisition of text and images) closer to the back end (i.e., the printed page). Some of the newest systems can image plates directly on press, while some can generate high-quality output directly on paper. Prepress systems range from low-end desktop systems which use a personal computer, off-the-shelf page makeup software, desktop scanners, and perhaps an imagesetter to generate digital pages which can be output to film or plate to high-end color electronic prepress systems (CEPS) which use high-quality drum scanners, large computer systems often including many workstations connected together in a network, proprietary page makeup software and color separation programs, and high-end imagesetters. The difference between the systems—aside from price—is the quality of the output, especially halftone images and color separations. However, it should be pointed out that the gulf in output quality is rapidly closing, as low-end desktop systems increasingly generate quality on a par with much more expensive proprietary systems.

Regardless of whether prepress is digital or "analog," the same basic processes need to be performed.

'Copy and Art Preparation'. In digital prepress, text material can be obtained in a variety of ways. It can either be typed directly into a program such as QuarkXPress or PageMaker while the page is being prepared, or it can be imported from a word processing, spreadsheet, database, or other program. The ability of these page makeup programs to allow the importing of text from other programs means that authors or other content providers do not necessarily need to have the same program as the typographer. Imported text can be edited and formatted with ease directly on the page (or "mechanical"), prior to film or paper output, unlike conventional typesetting which involved a good deal of literal cutting and pasting. Text can also be sent over the Internet (or tranmsmitted from system to system by modem or by means of networking and/or telecommunications), where it is transferred as ASCII text and can be imported into the page makeup program. The drawback to ASCII text, however, is that it is incapable of representing formatting commands, such as italics, bold, etc.

Line art can be obtained in a variety of ways. Often, drawing programs are used by illustrators and artists to create digital line drawings, using sither a conventional computer mouse to move a digital "pen" around the screen or other types of input devices such as digitizing tablets. These drawings can be saved in a PostScript file format (["encapsulated PostScript [EPS]"]) and input directly into a page makeup program in the proper position, and can be resized and cropped if necessary. They can also be output with the rest of the page or separately, if need be. Line art can also be drawn by hand on paper, digitized using a scanner, and converted to EPS format. There are many commercially-available drawings programs, the most popular of which include Adobe Illustrator, Corel Draw, and Macromedia Freehand.

The acquisition of photographic images is undergoing some turbulent changes. The earliest (and still common) means of obtaining photographic images is to use a scanner to digitize a transparency or print, edit, fix, color correct, or otherwise manipulate it using image-processing software (such as Adobe Photoshop), insert a low-resolution version of it in the page (comparable to the low-quality FPO image in mechanical paste-up), and generate a separate halftone negative or color separation negatives using the digital data. Newer means involve the use of electronic cameras and digital cameras to take photographs directly as digital information. These cameras can be plugged into a computer system and the images are already in digital form, eliminating the need for film developing and scanning. These cameras are still in their infancy, however, but an intermediate technology devised by Kodak (called the PhotoCD) is increasing in popularity. The PhotoCD is a technology which allows photographic images shot with conventional film cameras to be digitized by the developer from the original film and supplied to the customer on a CD-ROM, eliminating the need for scanning. Regardless of the means of digitizing an image, attention needs to be paid to the file format in which an image has been saved. For importing and exporting to and from page makeup programs, certain file formats produce the best quality or have the fewest compatibility issues. The most common formats for photographic images are TIFF and EPS. Images sent via the Internet or World Wide Web are often in the GIF, JPEG, or other format, and have often had some means of data compression performed on them, so as to reduce the size of the file that is transferred.

'Bit Maps vs. Vectors'. All of the types of copy and art mentioned above (and this includes text type) fall into one of two forms: bit maps or vectors. A bit map is essentially a computer display or output in which a screen object is described by a series of dots called pixels. If you look closely at your computer monitor, you'll notice that each character comprises a series of dots. (Computer output—such as from a laer printer or even an imagesetter is also made of pixels, but these latter dots—technically called spots to distinguish them from halftone dots—are so much smaller than screen pixels that they are barely perceptible.) When a computer system or program describes a bit map, it essentially saves in its memory a grid corresponding to the display area of the screen, and the status of each pixel is noted. Most paint programs and image-processing software make use of bitmapped images, which is the only practical means of handling photographic or other continuous-tone images. Type and line art, however, when described as bit maps, tend to exhibit the stair-step pattern known as aliasing (or jaggies). This is because straight lines, for example, are made up of discrete dots which become visible at certain angles. Bitmapped type and line art tends to result in output that is not smooth. A solution to this problem is the use of vector graphics, in which type characters and line illustrations can be represented by mathematical formulas or descriptions of lines, angles, and curves. This results in much smoother output, and also requires less processing power than bit maps. Vector graphics, however, cannot be manipulated on a pixel-by-pixel basis. (See Bit Map and Vector Graphics.)

'PostScript'. In the early days of personal computing, all output was bitmapped, which was fine when dot matrix printers were the most common output device. In order for personal computers to make any headway into the world of prepress, however, some means of generating high-quality, high-resolution output needed to be devised. Enter PostScript, a page description language introduced in 1985. The important advantage to PostScript was its ability to describe typographic characters as vectors or outlines, rather than as bit maps (although computers do need bit maps to display the font properly on the screen). Essentially, the PostScript language is a text-based descriptio