Offset Printing
Offset printing is the technique used for most books, brochures, magazines, and newspapers. It is plate-based printing. The image is rasterized as described under “�asic Printing Techniques” – for the print and transferred onto a printing plate by projection. This projection currently is done with lasers or LED arrays. First, a plate is coated with a light-sensitive layer. Thelaser (or LED array) inscribes the image pattern onto this layer. The printing plate is chemically developed, then wrapped around the printing cylinder Those parts of the plate not printed are smooth and do not pick up water when passing the wetting roller parts of the plate to be printed are rougher and pick up ink when passing an ink-soaked roller. ink-pattern is transferred (by offset) to a rubber-coated rotating cylinder (this is why the printing technique is called “offset printing”). The paper to be printed passes between this rubber cylinder and the paper roller 5 pressing the paper against the rubber cylinder. Thus, ink is transferred onto the paper and the image is transferred. In color printing, this process is repeated in additional printing units – one for each primary color (normally C, M, Y, and K).
The gamut of an offset print may be enhanced if six rather than the normal four (CMYK) inks are used. Printing with six inks is also called hexachrome printing (in addition to CMYK, green and orange are used). However, this requires special printing presses having additional print units. Hexachrome printing is far more expensive than CMYK (4C or 4-color) printing, and also requires a special color separation process and special plug-ins for Photoshop to prepare images or DTP documents).
Traditional offset printing might be considered if you intend to make a print run of 1,000 or more copies. As few, if any, home-users or small offices can justify printing equipment for offset printing, we will not discuss it any further (additionally, we have a very limited knowledge of the techniques involved).
In recent years, digital offset printing has come onto the market (e . g . HP Indigo press). These systems work with printing techniques similar to that of laser printers. These digital offset printers are mainly used for smaller print runs (typically 50–1,000). With most models, the maximum print size is restricted to A4 or A3. The resolution of digital offset printing is greatly
inferior to that of analog offset printing (e.g. HP Indigo press 5000 has a resolution of 812 × 812 dpi, while analog offset printers work with typically 2,400 × 2,400 or even 3,200 × 3,200 dpi). Printing photos, this leads to a visible reduction of image quality. As for color gamut, some digital offset printers exceed the gamut of traditional CYMK offset presses. They, how-ever, are still inadequate for high-quality fine art prints.Traditional offset printers, as well as digital offset printers are quite fast. For example, HP gives a printing rate of about 4,000 A4 pages perhour for its HP Indigo press 5000. The speed of traditional offset printing presses may exceed 100,000 pages per hour.The costs of digital offset printers starts at about $ 50,000 US and up.
The price of a traditional offset press may easily exceed one million dollars.
Laser Printers
Laser printers are well established, reasonably fast (from four pages per minute up to 100 pages per minute) and reasonably inexpensive for the cost per page (typically about 4–6 cents per Letter/A4 page in black-and-white at an ink coverage of 5 % per page and about 16–20 cents at an ink coverage of 90 % per page). While color laser printers were quite expensive formerly, in 2004 and 2005 their price dropped dramatically. You can buy a color laser printer for less than $ 500 US currently. With low-priced color laser print-ers, manufacturers use a business model similar to low-cost inkjet printers:
they sell inexpensive printers and earn their return via rather expensive toner units. A color toner set – lasting for about 3,000–5,000 pages at 5 % medium ink page coverage – costs about the same as the basic laser printer unit ($ 300–$ 400 US), resulting in a cost per page (Letter/A4) of roughly $0.10 US with a 5 % medium ink coverage per page and about $ 1.60 US when printing full page size colored images. Nevertheless, printing of text and graphic pages with a color laser printer is much faster and somewhat cheaper than using an inkjet printer.
Laser printers use very much the same technique (see figure 1-7) used
by modern photocopy systems (some models even combine both func-tions: scanning and printing). CMYK, U. S. Web coated, SWOP Epson R2400, Premium Semigloss
The gamut of a photo inkjet printer
(light-gray area) is larger than the gamut with traditional analog offset printing (colored area).
A photo drum is charged positively by a charging. The image of the print is rasterized by the printer’s RIP (Raster Image Processor), and his raster is applied onto a drum using a laser beam and rotating mirror (or alternatively by an array of LEDs). Where the light hits the drum, the positive charge is erased. Then, the drum passes the toner unit. Those parts that saw light pick up the positively charged toner, while those parts hat bear a positive charge reject the toner. Further on, the toner is trans-used onto the paper and burned in by a heated roller (fuser).
Color laser printers use four inks – cyan, magenta, yellow, and black (CMYK). They either use four drums or a transfer belt that picks up toner from four separate drum rotations and four toner units and transfers the complete color image onto the paper with one rotation.As for image quality, the limiting factor of today’s color laser printers is the resolution used (600–2,400 dpi, usually just 600 or 1,200 dpi) and the number of colors they use – which is only four (CMYK). A further limita-tion stems from the size of the toner particles, much larger than those of dye-based or even pigment-based inks on inkjet printers.Most laser printers have problems producing homogeneously colored areas or fine tonal gradients (you usually see smaller blotches of unevenly printed colors). With several of the color laser printers, the image shows a gloss, which may impair viewing with some images. You cannot avoid the gloss even when using matte papers. This is especially true for solid inks used by some XEROX color laser printers.
For this reason, image quality with color laser printers is clearly infe- to all other printing methods described here and can’t touch that of photo inkjet printers. If, however, you have a color laser printer, we recommend using it for fast index printing. The color and detail quality, in most cases, is good enough for a first, fast inspection.
Image longevity very much depends on the type of paper and inks (toner) used and ranges from about 10 to 20 years. The permanence of black-and-white prints is much better, and may be used for archiving doc-uments (provided you use the appropriate paper).
Dye-Sublimation Printers
Thermo-sublimation printers are frequently used for the fast and simple production of photographic prints, often directly from the digital camera via a USB cable using the DPOF (Digital Print Order Format) or PictBridge protocol supported by today’s digital cameras – even the cheaper consumer models. Alternatively, you may plug your camera’s memory card into a card-slot the printer provides.
With thermo-sublimation printing (also called dye-sublimation or dye-sub for short), color is transferred from a color-coated ribbon (foil) onto the paper. The transfer is done by an array of tiny heating elements (integrated into the print head). Where the ribbon is heated, the color on the foil evaporates (sublimates) and enters the paper where it cools down. CMY as well as CMYK ribbons are used. Three or four passes (or sections) of colored ribbons are needed to produce a complete image on the paper. Usually the ribbon consists of sections with the alternating basic colors (three for CMY or four for CMYK). The used portion of a ribbon becomes unusable, is rolled up and finally discarded. The paper must make three (CMY) or four (CMYK) passes under the print head. Individual color intensity is determined by the amount of heat. When the next primary color is added (to those colors previously composed of several primary col-ors), their colors merge into a combined color due to the heat. The three or four colors merge in the paper and form an (almost) continuously toned color.
LightJet® Printing (Digital Photo Print)
There are various names for this technique: digital photo print or LightJet® printing or direct digital printing or direct photo printing. Here, in essence, the image is imposed onto conventional photo graphic material by lasers. To produce an RGB print, three lasers are used. Th e material may be photo- RGB print, three lasers are used. Th e material may be photo- print, three lasers are used. The material may be photo-graphic paper used for traditional color photos or may be photographic film for translucent prints. The exposed material is then developed in a traditional wet process. The resolution used by most printers is either 300 dpi (or ppi) or 400 dpi. Lower resolutions may be used, as well, and will be interpolated to the printer’s native resolution. This seems very low, compared to the resolu-tion of inkjet printers or offset presses. With direct photo printing, however, no dithering is required to produce halftones, and every exposed dot on the paper combines red, green, and blue, thus resulting in (almost) continuous tone dots. For this reason, 400 dpi or even 300 dpi will produce a very fine image quality As direct photo printers are quite expensive,
This technique is almost exclusively used by service shops – often the very largest ones. There are two kinds of photographic print shops:
1. Consumer-oriented photographic print shops
They produce a very large quantity of prints per day at very low prices (typically from 15 cents for a 4 × 6 inch print to about $ 5 US for a Letter-sized print). The processing is done fully automatically and in large quantities. Special requests are usually not handled by these shops. In most cases, an automatic image optimization is performed. This may be disabled at most shops when you place an order, which you should do if you have done your own optimization.
The quality of their prints is usually quite reasonable and uniform, in most cases.
Currently, ICC profiles are ignored by these printers; all images are assumed to be in sRGB. If you send an image to them for processing, you should convert it to sRGB (if not already in this format).
Most of these shops offer only standard image formats. If your image format differs from those supported, you have the option of using either the full width with some parts of the image being trimmed off or receive an image with white borders – which, for fine art prints, may be what you want, anyway. In most cases, it is preferable to set your image to one of their standard formats and decide where and what kind of white frame (or other colored frame) you wish to use.
2. Professional photographic service bureaus
They specialize in high quality prints (usually in smaller quantities), also accommodating special requests. They may even offer to optimize your image for printing, which may or may not be appropriate. These bureaus should provide you with a printer’s ICC profile (often, you may download this from their Web home-page). These profiles may be used for two purposes:
Use as soft-proofing to assess how your image will appear when A) printed.
To convert your image to another profile. When you send your B) images to a service bureau, the image should be converted to the correct profile as other embedded profiles are ignored by the print-ing process. This hopefully will change in the future!
Inkjet Printing
Having taken a glance at other printing techniques, we want to dig deeper into inkjet printing. While some techniques mentioned are rather old, ink-jet printing is rather young. The first color inkjet printers came to the mar-ket in about 1985. Compared to today’s inkjet printers, they were very slow and showed extremely poor image quality. They were used largely to render simple color plots and production of transparencies for presentations. Print permanence of that first generation of color inkjet printers was quite poor.Soon, however, some specialized high-end printers came on the mar-ket. The IRIS printer – made by IRIS Graphics of Bedford, Massa chusetts (later acquired by Scitex) – was one such machine. The IRIS printer, at an early stage of inkjet history, provided a reasonably high print speed and considerable resolution and image quality, while print permanence and maintenance were problems. Prints produced by IRIS printers are some-times called Giclée prints.
Along with the growth of the PC and Macintosh markets, the need for inkjet printers grew, and today you scarcely find a home PC without an inkjet printer, their cost dropping from several thousands of dollars US to about 150–800 dollars (depending on the maximum print size) for quite acceptable desktop photo inkjet printers. For large-format inkjet printers (those beyond a print size of A3+/Super B), you will have to spend several thousand dollars.
source by:
http://www.dpunkt.de/
