COLOR SYSTEMS - LITHOGRAPHY

ink rollers

- master
cylinder

impression
cylinder

offset cylinder

COLOR LITHOGRAPHY

WHAT IS IT?

Lithographic printing is used to print the black and white or color photos we see in many magazines, brochures, newspapers, and books. It is also called photo offset printing.

'LITHOGRAPHY': Lithography is a

A flat polished stone was used when lithography was first invented. Modern lithography uses a printing master instead of a stone. The printing master is similar to a printing plate except it is made of thin flexible sheets of metal or fiber. A finished printing master is flat in appearance. It does not have a raised image as would a rubber stamp. It is flexible so it can be wrapped around a cylinder or drum that rotates. (See the diagram below)

A printing master can be made by using photography. The photographic process can be chemical or electrostatic* (see definition below). The photo process creates a printing master where the image is divided up into two kinds of surfaces. One surface contains clear areas that attract water, but repel oil-based inks. The other surface contains areas that attract the oil-based ink, but repel water.

"LITHOGRAPHIC OFFSET" PRINTING PRESS:

1. First water, then the oil-based ink is applied to the surface of the printing master while it is rotating on a cylinder or drum.

2. The inked image is then transferred mechanically to a receiving surface. This receiving surface can be a flat sheet of rubber or other material that will hold the ink image. This is the "offset drum" or "cylinder". This is also called the "blanket".

3. From the offset drum, the image is then transferred onto the paper that glides along on a third drum, the paper impression cylinder.

DIAGRAM:

Two views are shown side by side below. The left diagram shows the three drums or cylinders, and how the middle drum produces a mirror-image. The image is then reversed again on the paper so that writing will appear normal. The right diagram shows the direction of the rotating cylinders.

With this system, the inked image is "offset". This means that the inked image from the printing master is transferred first to a rubber covered drum ("offset blanket cylinder") which then comes in contact with blank paper. This go-between step also saves wear and tear on the printing master which never touches the paper. The transferred oil-based ink portions will produce the printed image. The water portions of the image remain clear of ink. Just enough water is used so that the paper will dry quickly.

Presses may be designed to handle paper in single sheets or from continuous rolls as shown. Paper printed from continuous rolls is cut into sheets after printing.

DEFINITION:

*"Electrostatic" photocopy: Electrostatic photocopy is a process used by dry copier machines such as "Xerox^®" brand copiers. The method has been adapted to make lithographic printing masters in a similar way.

The electrostatic printing master is made using a special dry copier machine that uses a special powdered toner. The toner image is fused to the surface of a fiber-based sheet with heat. The printing master will then be wrapped around the master cylinder of a lithographic offset press in the same way as explained above. Any portions of the printing master that are covered with toner will pick up the oil-based ink, but will repel water.

How are different shades and colors printed?

The oil and water principle allows for only two possibilities: areas with ink, and areas without ink. So how are different shades of light and dark, as well as different colors printed? One answer is to break the picture into very tiny dots. Another answer is to print several color inks on the same piece of paper one on top of the other.

Each dot will print ink as dark as any other, but the size of the dots and the spaces between the dots will make a difference. It will make parts of the picture look lighter, or darker. The dots are too small to be easily noticed. Instead of dots, we see different shades. If you examine a newspaper photo closely, or use a magnifying glass, you can see the dots better. A picture that is broken up into small dots is called a "halftone" photo or "halftone" image.

"HALFTONE"

When greatly enlarged, the "halftone" can be seen as separate dots. It is the amount of white space between the ink that makes a difference. At a normal viewing size and distance, halftone photos are accepted as solid tones.

Color lithography does not need to have a different color ink for every single color in a picture. The color dots allow for several basic colors to appear as a whole range of different colors. When magnified, you can see the individual dots of color. From a normal distance, the color dots form different color mixtures. A change in the size of the colored dots at any one spot changes the color we see. Usually only four different color inks are used to make thousands of other colors.

YELLOW	CYAN



MAGENTA

BLACK

Composition of fruit, drawing inks, and candy

After the same paper goes through 4 separate printings, one for each color, a full color picture is produced.

If all colors can be made with cyan, yellow and magenta, then why is black also used?

There are a few reasons why black is used along with the primary colors. One reason is that is takes a lot of cyan, magenta, and yellow to make black. Too much ink in the same place may cause problems with the paper, and the printing press rollers. It might take longer to dry and the color may run or spread.

Also, due to other requirements of printing ink, the inks may lack color purity. This may produce weak or color- tinged blacks. It makes a lot more sense if only one layer of black ink is used instead of a mixture of three colors.

Another reason is that any writing that appears on the same page as the color pictures looks a lot better if printed with solid black ink. This is because alphabet characters printed with three colors to make black must be printed exactly one over the other. The smallest shifting of the color inks could cause color outlines around the letters. So black ink is used, and it looks neater and sharper.