Color Printing Printing Knowledge

Before actually printing with color, it is necessary to understand some of the factors that influence the appearance of colors on a printed document. Listed below are some of the important points to consider:

Precise color matching can only occur when the preceding factors are considered. A color viewing booth can be used in order to accurately match the printed color with the proof. Lighting conditions, materials, and the techniques used, must be controlled in order to achieve the best results when printing with color.

Spot color is usually considered to be any color used on a document that is not a process color. It can be used simply as a secondary color or as a color to provide emphasis to an area of a printed piece. Spot colors can be used as an area of emphasis on a four-color process job with the spot color generally printed as a color on a separate plate and not as part of the four-color process.

Specialty inks can also be used as a spot color to provide even greater emphasis to an area. Specialty inks can range from fluorescent, fade resistant, opaque, and metallic inks.

Many of the specialty inks demand unique considerations for their proper use at the press. Metallic inks may require that the press run at a slower rate than usual and the preparation and clean up time may be longer. Fluorescent inks are very transparent and may require that the printed document be run a second time through the press to achieve the desired coating of ink. The challenges created by the specialty inks in terms of printing may increase the price of a printed document to a degree that it may not be cost effective to use the specialty ink for a particular application unless a large quantity is produced.

Spot Color - A single color applied to one area
or "spot" rather than layering colors on top of
each other to achieve a desired hue.

Printing an image in full-color is a complicated process involving a number of steps. A basic knowledge of these steps is not only essential for printers, but it is also important for designers in planning and creating the best layouts and for print buyers in understanding some of the issues that are raised when purchasing color work.

Color Separations

In order to reproduce or prepare a full color image for printing using four process colors, the image must be divided into the individual subtractive primary color components. The separation process can be accomplished photographically or electronically.

Photographic Separations: Using a large process camera, a full color image is converted into halftone negatives which, contain a series of dots of various sizes to represent shades of gray. The dots are achieved with the use of special screens placed over the negative material during exposure. When printed, the smaller dots create the lighter areas of the image, with the lightest appearing white, and the larger dots make up the darker areas of the image, with the darkest appearing black. This system of using the halftone screens directly with the film is known as the Direct Screen Photographic Color Separation method. A halftone negative is made for each of the separate subtractive color components of the image (cyan, magenta, yellow) and black. The four halftone negatives are records, in varying shades of gray dots, for each of the separate color components and are known as "Color Separations".

Another photographic technique known as the Indirect Photographic Color Separation method is also used. The original image is separated into the individual primary color components and continuous tone films are made for each of the separations. From the separated films, the halftone negatives or positives are created. The advantage in first creating continuous tone separations before creating the halftones is that the continuous tone separations can be reduced or enlarged and corrected before the actual halftone films are produced.

Color separations created from photographic methods are used to prepare analog plates for traditional printing methods. Each plate corresponds to one of the four colors, cyan, magenta, yellow, or black. The plates are coated with the corresponding ink colors during the printing process and the inked image from the plates is transferred to the substrate. The varying sizes of the halftone dots on the plates, determines the amount of ink that will be printed on the substrate. The dots from each of the different colored images overlap to produce the full color image.
Electronic Separations: The electronic separation of an image into its individual color components has become the most popular and labor saving method of producing color separations. The separations can be produced from an image produced with analog methods, such as photography, or from a digitally generated image.

When producing separations from an image produced with analog methods, the image is first placed in a scanner, which converts the image into a digital record. The digital image can then be imported into an image editing software program where the color separations can be produced with the click of a mouse. The scanned image and/or the color separations can be quickly manipulated and edited in an infinite number of ways with the tools provided with the software.

Color separations can also be produced from digitally produced images, such as from a digital camera or images composed entirely in design or illustration software on a computer. Digitally produced images eliminate the scanning step from the workflow.

Color separations created electronically can be output to film to prepare analog plates or they can be used for Computer-to-Plate applications and digital printing.

Screen Angles

The rows of dots on the screens used for each color separation must be positioned at a specific angle so that the printed dots do not form a distracting pattern. The angles are: 45 degrees for black, 75 degrees for magenta, 90 degrees for yellow, and 105 degrees for cyan. The angles form a rosette pattern, which is merged into one continuous tone by the human eye.

If the screens are positioned at the wrong angles and then printed, the rosette pattern will not be correct and a moiré pattern appears, which results in an image that no longer has a smooth gradation of color. An example of a moiré pattern is shown below.

Dot Gain

Dot gain occurs when the size of the halftone dot increases due to several variables. If the size of the dot increases too much, then the image and colors print darker than what was originally intended. Some of the variables that affect dot gain are:

Prepress - if the negatives from different sources are duplicated to produce the final film, the dots may increase in size. Printing plates produced from the duplicated negatives may be incorrect which may result in a change in the dot size.
Printing Equipment - the type of press, the age and condition of the equipment, and press settings such as plate and blanket packing, printing cylinder settings, and ink and water balance, may determine the degree of dot gain on a printed sheet.
Ink - ink is not the same from all manufacturers. Inexpensive or low-grade ink is more likely to cause dot gain problems than a higher priced, high-quality ink.
Paper - it is normal for printed dots to increase in size slightly as wet ink spreads and is absorbed by the paper. Different papers cause ink to absorb and spread differently at different rates. An uncoated paper stock may not be as suitable for color printing as a coated stock, because it absorbs ink at a faster rate. It is always best to consult the print provider when determining which paper is best suited for a particular application.

Registration

Several factors can cause the individual colors of a four-color image to become misaligned, which results in gaps and color shifts between objects. Some of these factors include:

Prepress - negatives may be produced correctly or they may become stretched, resulting in the separate colors not registering correctly when printed. Also, if printing plates are produced incorrectly, the image on the plate may become crooked, resulting in poor registration of the individual colors when printed.
Plates - improper plate and/or blanket packing on the press can affect the image on the printing plates. The plates can also be improperly installed on the press resulting in poorly registered colors.
Paper - occasionally, paper stretches and distorts when it absorbs moisture as it passes through a printing press. The stretching and distortion of the paper may result in poor registration of the individual colors.

Trapping

Trapping can help compensate for registration problems by slightly expanding one color into another and eliminating the gaps and color shifts between colors. For trapping to be effective, it must compensate for less than perfect registration without forming noticeable lines of unrelated color around objects caused by the overprint. Process colors that share a sufficient percentage of common colors do not require trapping, because any registration problems will reveal a color that is similar to each of the adjacent colors. An example of where trapping would not be necessary would be a printed item containing mostly reds and oranges. If the object on such a page were not in perfect register, the result would not be visually distracting because the colors in the gaps and overprinted areas would be similar to any adjacent colors.

High-fidelity is a method of color printing that utilizes additional process inks to produce a greater range of color on the printed piece. There are many different versions of high-fidelity printing, but they all use conventional or enhanced versions of CMYK inks plus additional colors to increase the color gamut.

Like four-color printing, a halftone negative is made for each of the individual color components. Instead of four separate negatives that are required for CMYK color separations, five or more separations may be required, depending on the high-fidelity process used. Plates are then exposed using the halftone negatives. The exposed plates are then ready for printing the application using the appropriate ink colors. Listed below are several options that can be used to achieve high-fidelity printing:

An example of a high-fidelity color system is the PANTONE Hexachrome® system which uses orange and green in addition to enhanced versions of cyan, magenta, yellow, and black. The use of the six high-fidelity colors increases the range of color by 20% and provides improvement in transitions between colors. The high-fidelity color system can reproduce more than 90% of the Pantone Matching System® colors.

An important point when considering the use of high-fidelity printing for an application is the fact that the process requires a press with five or more printing units in order to print an application in one pass on one side. Without the proper quantity of printing units on the press, the application would have to be run through the press more than one time, making the job more expensive than if it had been printed in one pass. Although conventional offset presses as well as digital six-color presses make the process more efficient, they are very expensive and require additional skills to operate, which also adds to the cost of printing a high-fidelity application.

High-fidelity color printing is also more expensive than traditional four-color printing because of the extra prepress work involved. For a six-color high-fidelity application, two additional color separations are required as well as two additional plates. When compared with four-color jobs that are printed with additional spot colors, there is not a great difference in the cost.

Printing a high-fidelity application digitally can be less expensive than with conventional printing depending on the quantity. Very small quantities are often more economical when printed with digital equipment because there are fewer steps involved with the process. The price per sheet is the same regardless of the quantity, which differs from conventional printing in that the smaller the quantity, the greater the price per sheet.

Color Printing - PrintingTips.com