Additive Color

Color reproduction on the computer monitor (and on television) is achieved by combining Red, Green, and Blue light (RGB) in varying levels to produce a full color image. Red, green, and blue are the primary colors of white light and they cannot be produced by the combination of any other colors. When they are combined full strength, they will produce white light. The combining of red, blue and green colors is known as the "Additive Color Process", because when the colors of light are combined with one another, or added together, the result is an increase in light intensity. If none of the additive primaries are present, the color is perceived as black.
When two primary colors of light are combined, a secondary color is produced. The secondary colors of light are Cyan, Magenta, and Yellow (CMY). The secondary light colors are also the primary colors of the "Subtractive Color Process". The process of combining cyan, magenta and yellow colors is used to reproduce color for printed media

Color Gamut
The color gamut refers to the range of colors that can be viewed, displayed, or printed. The items in the list below are arranged from the greatest to the least in terms of the range of colors that each is able to reproduce:

• Human Eye
• Photographic Film
• Television or Computer Monitor
• Digital
• Printing Equipment
• Offset Printing

Defining Color
The following are the properties that are used to define color:

• Hue: is used to describe the name of a color such as red, orange, violet, or blue.
• Value: describes the lightness or darkness of the color. Value is also known as tone, tone value, or lightness.
• Saturation: describes the intensity of the color such as bright or dull. Saturation is also known as intensity or chroma. It is important to remember that a color that has a very intense or bright level of saturation may not necessarily be very light in value just as a very dull color may not necessarily be very dark in tone or value.

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Color

Color Conversion
The color gamut of RGB is broader than CMYK and the file size is smaller as well, but CMYK is the better choice for documents that will be used on the Web and for print. CMYK documents work well enough in the RGB environment, but RGB doesn't work as well in the CMYK environment.

Trapping
Trapping can help compensate for registration problems by slightly expanding one color into another and eliminating the gaps and color shifts between colors. PDF stores the trapping instructions in such a way that the trapped images will look the same on any computer and also when the images are output.

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.

The sequence in which the process colors are printed also influences ink trapping. For standard four color printing, the correct sequence to print the process colors are black, cyan, magenta, and finally yellow. Printing in the correct sequence will help ensure the best results. A change in the standard sequence will result in a change in the ink trapping characteristics.

Color Management
Device color spaces refer to the color space of the input device such as a scanner and the color space of the output device such as a printer. Data that is scanned may not display the same colors on a monitor and a printer, so a color management system is necessary. There are many differences between color profiles, so there is no guarantee that colors will be consistent throughout the workflow.

The ICC (International Color Consortium) was developed to set standards for device independent color. ICC profiles are electronic files that describe all of the color characteristics of a device. They can be either generic for a certain device such as a scanner or a monitor or they can be customized with profile generation tools. Color profiles are important because the color spaces of devices such as scanners, monitors, printers, presses, and digital cameras vary greatly.

PDF is able to support objects that are specified in different device dependent, device independent, or special color spaces. Acrobat supports Adobe Color Engine (ACE) and ICC Color Management that are in Photoshop and Illustrator. Displaying and printing color documents is more consistent. InDesign does not support this.

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Color Bar

Color bars are also referred to as color control bars, color control strips, or proofing bars. Color bars are rows of different colored patches printed in the trim area of the press sheet. They are used by proofers and press operators to control the trapping, ink density, dot gain, and print contrast of the proof or the printed sheet. They usually consist of solid and tint blocks of cyan, magenta, yellow, and black; two and three color solids and tints; and additional elements and patterns such as resolution targets and dot gain scales.

Solid process colors ofBlack, Yellow, Cyan, andMagenta. A color bar are used to measure ink densities.

Solid overprint colors include blue (C&M), green (C&Y), and red (M&Y)

Screen tint patches usually 25%, 50%, and 75% of the process colors. A color bar are used to check for dot gain.

A color patch made up of 50C/30M/30Y. A color cast in the patches will show an imbalance in one of the colors, either from incorrect ink density or from dot gain.

There are several different types of color bars and which one to use depends on the type of printing being used. Prepress and print suppliers purchase original film or the rights to digital files of color bars and other test images. The film and files can then serve as constant reference points throughout the proofing, platemaking, and print processes. They are available from GATF (Graphic Arts Technical Foundation), DuPont, RIT (Rochester Institute of Technology), and others.

Color bars are an essential troubleshooting tool. They are easy to analyze, both visually and with a densitometer, which is an instrument used for measuring the relative density of any part of an image.

Color bars created for computer-to-plate (CTP) systems may require additional features than color bars generated for film.

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Spot Color

The colors used on spot color proofs are also known as "matched" colors, special colors, or by one of the brand names "Pantone" or "PMS" colors. Spot colors are printed from a single ink on a separate printing unit of the press. Spot colors can be standard, pastel, fluorescent or metallic colors. Several colors can be printed in select areas to create two, three or more color jobs. Spot colors can be used on may types of jobs, such as business cards, letterheads, forms, brochures and direct mail pieces.

There are several variables involved with spot colors. A few manufacturers create color guides such as binders, chips, and fan books to show the spot colors and the equivalent process colors. It is important to realize that the color guides fade over time and the paper may change colors so check the date of your guide and replace it periodically.

When you choose the spot color on the monitor, remember that it is simulated in RGB, and not as the spot color or CMYK. Be sure to tear off a chip from the guide and include it with the job.

Remember that if you are trying to match a spot color with process colors, very few colors will match. Many spot colors are outside of the color gamut of process inks. The paper stock that is used for the job will also provide different results than expected. The use of varnishes or coatings can also change the appearance of the color.

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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 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 continuous tone 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 original image is placed in a scanner which stores the image as a digital record. The digital image can be imported into a software program such as Adobe PhotoShop™ 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 created electronically can be output to film to prepare analog plates or they can be used for Computer-to-Plate applications and digital printing.

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Common problems encountered in Offset printing

Here is a list of problems that need to be paid attention to during the process of offset printing:

Uneven colour: Always make sure that the colour is the same on both sides of the sheet by rolling the sheet up so that you can look at opposite edges of the sheet side by side.

Ghosts: These are either

• Images formed in areas where they should not appear, or
• Formation of lighter areas where there should be even inking.

Hickies (aka Fish-Eyes): These arenon-inked circles created by lint on the plate or blanket. They are generally something that can be cleaned up when appearing on several sheets in a row. However, chances are you will never be able to eliminate every single hickie, especially if you have large solids in your printing piece and have to print thousands of sheets.

Picking: These are the non-inked areas that look like snow created in areas where the ink and some of the paper has been lifted off the sheet when the ink is too tacky.

Note : Picking can occur due to a paper related problem.

Muddy or Uneven Screens: These occur as a result of dot gain (see SCREENS.) In the worst case scenario caused by a muddy or uneven screen, you may need to remake a plate using a coarser screen. However, some dot gain can be alleviated by

• Running the ink a little lighter and
• By ensuring that the gradients on your piece, if any, look even. (Refer to GRADIENTS to see how to avoid banding in the design and prepress stages of the job.)

Offsetting: This is the appearance created on the back of a sheet by an image present on the sheet touching it. Offsetting happens as a result of

• Too much ink
• Insufficient driers in the ink or
• Irimming of the job done when the ink is too wet

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Three Color Printing

Theoretically it is possible to produce an adequate range of colours using just Cyan, Magenta and Yellow. However, three colour printing may also refer to the use of either.

• Three special inks or
• Black combined with two special inks

The Four Colour Printing Process, which is a system rarely used today, uses a black plate to add shade and depth reducing the amount of ink required.

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Four Color Process Printing

Four colour process printing is the most common system for producing full colour print. In fact, the vast majority of magazines and colour books are produced using the four-colour process.

Originally the artwork and originals had been separated using filters and four printing plates were produced.

The four colour process printing consists of four ink colours namely, Cyan (Blue), Magenta (Red), Yellow and Black and is often referred to as CMYK. The inks used for four colour process printing are translucent due to which they can be overprinted and combined in a variety of different proportions to produce a wide range of colours.

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Advantages of Offset Printing

There are many advantages to offset printing namely:

• Economical-It is less expensive, especially when there are large quantities involved.
• Widespread Usability – It can be used on the widest range of printing surfaces, including paper, wood, metal, even leather.
• Ease of creating the printing plates.
• The paper does not have to be rated for the high temperatures and stress of either the copiers or laser printers.

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Offset Printing Process

The process of offset printing:

Offset printing has been the most common form of commercial printing for the last sixty years. The most important element of the offset printing process is a plate manufactured from metalor polyster. This plate carries the copy of the image to be printed. The process of printing takes place as follows:

• The plate is chemically treated so that water may be absorbed only by the non-image areas of the plate.
• The plate is next attached to a plate cylinder, which rolls past the water roller. At this stage, the water on the water rollers is absorbed by the non-image areas of the plate, i.e. the areas not requiring ink.
• The cylinder next rolls over the ink roller. At this stage, the ink on the ink rollers sticks to the areas of the plate that are free of water.
• The rubber surface on the offset cylinder takes the ink from the plate cylinder and presses it onto the paper, as it is being rolled between the offset cylinder and the impression cylinder.

The above steps are repeated for each color of the ink present in the publication, to achieve a crisp and professional print. Though the system of offset printing involves a complex process, the entire system of is extremely efficient and robust in its overall performance and output.

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What is Offset Printing

Offset printing is a printing process in which the inked image is transferred (ie, "offseted") from the plate to an intermediate blanket before being printed on the substrate. Following are the common terms you come across in offset printing.

• Opacity - This refers to the quality of being impenetrable by liquids or light. With paper, it refers to the ability to keep print from showing through to the other side.
• Opaque - In production it refers to a solution or paint used to block out areas on a film negative to keep them from being reproduced on the plate.
• Overprinting - It generally refers to printing an image over another impression. It also refers to printing a secondary color over a primary color in graphic arts work.

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