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Offset Lithography (Printing)
Offset lithography is basically the modern printing process. If you regularly do any printing beyond the laser printer or inkjet, you need to know a little about this process.
 Camera to Plate. Modern printing still begins with a printing plate. This used to be done through a complex photochemical process where a photo was taken of a document, then the developed negative and more chemicals were used to etch a printing plate. It took a few hours. Now this is all done in a closed box with lasers and some bottles of goo in like 10 seconds.
Oldstyle versus offset. The old "Gutenburg" style of printing actually used ink transferred to raised letters on a printing plate and then pressed onto a piece of paper (hence "printing press"). Today, a printing plate is basically flat. The difference is that chemicals etch areas of the plate so oil-based ink will stick to it. The rest of the plate is designed to repel the ink and accept a cleaning solution that is primarily water. Because oil and water naturally separate from each other, this process works quite well.
Why is it called offset lithography? "Lithography" refers to the "oil and water" printing method and was invented by Alois Senefelder in Germany in 1798. "Offset" refers to a type of press which uses multiple rollers to keep the printing plate from actually touching the paper. The ink transfers to a rubber "blanket" (more commonly called "offset") roller before it goes on the page and a much cleaner print is the result. This basic process is the heart of all printing presses, from the mid-1800s to today.
Only Solids Leads to Screens. Unlike a computer monitor, a printing press cannot produce "shades" or variations of ink color. The photochemical process will only produce a solid image on the printing plate. To make variations possible for printing photographs or graphics, the photo or graphic is divided into millions of tiny solid dots. This used to be done photographically by placing a fine mesh screen in front of the photo during the camera process. Now it's of course done in the computer.
The finer the screen, the smaller the dots, and the clearer the picture. An 85 lpi (line per inch) screen is typical for a newspaper, while a 300 lpi screen is what you'd find in National Geographic. Look at either of these under a magnifying glass and you'll notice that photos are really just a collection of very small dots of different sizes. Although we no longer use screens to make photos and graphics printable, we still use the term "line screen" to refer to the desity of these tiny dots, and hence measure the clarity of the resulting printing piece.
Spot Color Printing. Printing in a single color is called "spot color printing." These colors are usually selected from a library of standard choices. The Pantone company maintains a standard of color used in the U.S. Truematch and DIC are other color standards. When you specify Pantone 208, you can be sure it's the same burgundy whether you print it in Florida or California.
Process Color Printing. If you want to have more than one or two colors on your document, you'll switch to "process color printing," or full-color printing. This method uses just four ink colors--cyan, magenta, yellow, and black--to produce nearly any color in the rainbow. Four colors of ink means four individual printing plates, which in this case must be carefully aligned and calibrated to produce a clear and correctly colored image. Again, this used to be extraordinarily complicated and expensive and today is child's play. In fact, process color printing is the same method your inkjet printer uses to make color prints in your home.
Printing presses are usually either designed to print one or two colors (spot colors) or full color (process color). Some printing presses can print more than four-color process, even printing both sides at the same time, or adding a varnish or spot color in addition to process color.
The shift to digital. The advantage of offset lithography, even in the age of color copiers and laser printers, has been efficiency. It's simply cheaper to print 100,000 color cards on a printing press than on a color copier. For a while, the quality of a printing press was also much higher than what was available through a digital copier. This is changing rapidly, and within the next generation we will likely witness the demise of the traditional printing press altogether, in favor of digital color copiers and printers.
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