Resolution

The typical resolution of a digital press is lower than a traditional imagesetter or platesetter used for offset printing. With an average print resolution of 600 dpi, the presses must use some type of screening enhancement software to simulate the typical line screen of 150 lines per inch achieved by offset equipment.

Digital presses with a resolution of 600 dpi or better do a good job in reproducing images that are clear and sharp. Applications printed on digital presses with lower resolutions (400 dpi) will not reproduce images with the same level of quality. Printed text may look a bit fuzzy and it may be possible to see the actual printed dots that make up the text.

Scanning

It is advisable to scan images as close to the finished size as possible. Images should be scanned in CMYK rather than in RGB mode. RGB is meant for images that will remain in an electronic form, such as for viewing over the Internet.

Because digital presses print at at fairly low resolution, they do not use a fixed line screen, but instead they simulate the average screen ruling used for offset printing which is usually around 150 lines per inch. Images below 250 ppi should not be used for digital printing applications. Since a resolution of 300 pixels per inch is standard for applications printed with offset technology, the same resolution should be used with digital printing. If the application is printed conventionally on some occasions and digitally on others, it is helpful to use the same resolution for both.

RIPs

Digital presses (along with other types of digital output equipment) generate images by interpreting Adobe PostScript files. The digital press has a powerful computer called a raster image processor (RIP) which interprets the PostScript files as raster images (patterns of spots arranged as a bitmap) to allow them to be output on the digital press.

Vignettes

Gradations should be of a wide range, such as 15% to 85% to reduce the banding of colors that may occur if the tint range is narrower. Banding is more likely to occur on some digital presses than on offset presses because the lower resolution may not accurately reproduce a narrow tint range as a smoothly graduated vignette, but will instead create subtle, but noticeable blocks or bands of colors.

File Formats

Images should be saved in the proper image formats that are compatible with the RIP. Any images that must be enlarged, reduced, rotated, or cropped should be edited in a program like PhotoShop first and then saved, otherwise the RIP takes much longer to process the information, which in turn reduces the speed of the print engine of the digital press.

Color Gamut

The color gamut refers to the range of colors that can be viewed, displayed, or printed. Digital printing equipment is able to reproduce a greater range of color than traditional offset technology which gives it a distinct advantage over offset. 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:

1. Human Eye
2. Photographic Film
3. Television or Computer Monitor
4. Digital Printing Equipment
5. 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.

Color Models

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. This process is known as the "Additive Color Process", because when the colors of light are added to one another the result is an increase in light intensity. An object appears to be white when all portions of the white light illuminating the object are reflected back to the eye. 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. These secondary colors of light are Cyan, Magenta, And Yellow (CMY). The secondary light colors are also the primary colors of the "Subtractive Color Process". This process is used to reproduce color for printed media.

Subtractive Color

Red, Green and Blue (RGB), are the primary colors of white light and are called the Additive Primary Colors. We are able to see color because different objects reflect and absorb, or subtract, the primary colors of light differently. For example, we see an object as yellow because it absorbs (or subtracts) blue light from white light. Since the blue light has been absorbed, the red and green light are leftover and are reflected back to our eyes. The red and green light combine to make yellow and so we see the color of the object as yellow. This is known as the Subtractive Color Process because portions of the visible light spectrum are subtracted from white light to reveal color. If 100% of the red light is subtracted from white light the resulting color is cyan. (The green and blue light are the remaining primary light colors after the red is removed and the green and blue combine to form cyan). If 100% of the green light is subtracted from white light the result is magenta (red and blue light form magenta) and when 100% of the blue light is subtracted from white light the result is yellow (red and green light form yellow). Cyan, Magenta, and Yellow (CMY) are the Subtractive Primary Colors, and are used as the primary colors for color reproduction in process color printing.

The combination of the primary colors in varying degrees is what creates the illusion of a full color printed image. In the lightest and whitest areas of the printed image, very little of the primary colors are present so these areas appear very light. In the darkest areas of the printed image, much more of each color is present making the areas appear darker. The combination of the three subtractive primaries, each at 100% strength, should create black, but since cyan, magenta, and yellow color pigments are not perfect, there is no way of creating a true black color when they are combined. If full strength cyan, magenta, and yellow, are combined, the resulting color is actually a dark, muddy brown color. Even if it were possible to combine all three primaries at full strength to create black, printing 100% of any or all of the subtractive primaries can cause a saturation of color on the printed surface which can result in very poor print quality. Because of this problem, the addition of black is necessary to assist the cyan, magenta, and yellow in creating the darkest hues, to deepen shadows, and to produce a true black text. Cyan, Magenta, Yellow, and Black, (CMYK), form the basis of the four color printing process. The "K" is used to signify black to avoid confusion with blue.

Color Neutralization

Strong highlight areas (pure white) and strong shadow areas (pure black) should be neutralized with scanning software or in PhotoShop. Pure white, which is 100% red, 100% green, and 100% blue, and represented as the value R255, G255, B255, can be backed off to R247, G247, and B247. Pure black, which is 0% red, 0% green, and 0% blue, and represented as the value R0, G0, B0, can be advanced to R5, G5, and B5. Software programs will convert the RGB values into the correct CMYK values for output. Color neutralization will provide more pleasing highlights and shadows as well as a better gray balance for the entire image.

Dot Size

In order to achieve the best results, it is important to know the minimum and maximum dot areas that can be reproduced by the digital equipment. With offset printing, dot sizes between 5% and 95% can be accurately printed and with digital equipment the range is usually between 3% and 97%.

Color Management Systems (CMS)

Color Management systems provide a profile of the color characteristics for each device used in the print workflow, such as a scanner or a digital press. Color characteristics profiles are incorporated into the print workflow in order to provide consistent color reproduction throughout the process. The most popular color management systems are Apple ColorSync and Microsoft ICM.

Cielab

Cielab is an acronym for Commission Internationale de l'Eclairage Lab, with "L" representing lightness, "a" representing the re/green axis, and "b" representing the blue/yellow axis. Cielab allows easy communication concerning colors because it is an international system based on numerical values, which takes the guess work out of color correction. Cielab is the worldwide standard for color measurement.

Cielab models can be used to check if the colors of an image fall within the color range of the print process that will be used or the piece of equipment that will be used to print the image. Cielab is also used to check the color-carrying capabilities of various papers, to illustrate to clients the types of papers that are best suited for their application. As most printers are aware, coated stock is capable of producing a wider color gamut than uncoated paper, so using Cielab to illustrate the color gamut of various papers can justify the use of a more costly stock to achieve better results.

Color Matching

Sometimes it may be impossible to match a spot color exactly. All color digital presses can print four-color process, but not all models can be configured to print any additional colors when spot colors are needed. Most of the time, spot colors are converted to their equivalent values in CMYK in order for the spot color to be simulated with the process colors. The results may vary widely depending on which color is being simulated. It is helpful to have a swatchbook, such as the "Solid-to-Process" swatchbook offered by PANTONE, which shows the solid colors that can best be reproduced using process colors.

Coverage

When using digital equipment, the maximum coverage of the toner on the substrate must be limited. The combination of all the process colors (Cyan, Yellow, Magenta, and Black) at full strength would be 400% coverage (100% for each process color) resulting in an oversaturation of toner, creating a thick muddy appearance on the substrate. The combination of the process colors into various quantities should not exceed a total of 320% to prevent oversaturation from occurring.

QuarkXPress

QuarkXPress, developed by Quark, Inc., is one of the most popular software programs for desktop publishing. You can create almost any type of document for print publishing such as business cards, brochures, newspapers, internal company forms, and many others. Create a simple black and white document or full color pages with multiple graphics and stylized text. You can output your document directly to a digital printing device or it can be converted to a format which will enable your document to be output through offset printing. Regardless of the method of output, QuarkXPress will give you control over your design and speed up the process of producing your printed materials.

Adobe InDesign

When Adobe's InDesign was released, it was one of the most anticipated desktop publishing tools, mainly because of the reputation of Adobe's other popular and user friendly software programs. InDesign incorporates an easy to use format and many tools, filters, and settings contained in movable palettes, much like Adobe's Photoshop and Illustrator. When you have an application that requires additional design features, you can customize any part of InDesign using Adobe plug-in software. InDesign works easily with other Adobe software programs and there are versions for both Macintosh and Windows operating systems.

Adobe Illustrator

Adobe Illustrator is a graphics program that allows you to create new images for the Web or for print publishing. Unlike Photoshop, which is a program for the modification of photographs and other bitmapped images, Illustrator is a drawing program where illustrations, objects, line art, and graphics can be created and modified. Illustrator offers compatibility with other Adobe applications such as Photoshop and InDesign and it is available for both Macintosh and Windows platforms.

Macromedia FreeHand

Macromedia FreeHand is an object oriented drawing program (like Adobe Illustrator) used for creating illustrations and graphics for print publishing and the Web. A project in FreeHand is made up of strokes and fills rather than bitmap images in which pixels are edited. The resolution of a printed FreeHand document is device dependent, meaning that the resolution is determined by the device on which the document is output. FreeHand is available for both Macintosh and Windows operating systems.

Photoshop

Photoshop allows you to scan and edit continuous tone images to achieve any desired effect or you can create new bitmap images. Photoshop integrates the Photoshop software with that of ImageReady which is no longer sold as a separate software package. The integration of ImageReady with Photoshop makes it possible to produce images for the Web as well as for printed media. You can go back and forth between traditional Photoshop features and ImageReady features without losing any settings or properties you have established in either one. Photoshop works easily with other Adobe programs such as Illustrator or InDesign and there are packages available for Macintosh or Windows platforms.