School color: red, green, blue, yellow and black (middle)

Increased tone value

When considering the increase in the hue value (the dot gain), the difference between the CMYK images prepared for various print copying systems is increased. Scanners and printer operators understand that ink dots printed on substrates produce images that are much darker than the original digital data—an effect known as dot gain.

In addition to factors such as the surface of the paper and the viscosity of the ink, each printer also plays a role in determining the dot enlargement of the printed image. Compensating for dot gains during color separation means that the darkening that occurs during printing can be offset, making the image brighter when converted to CMYK.

Moving an image from one printing state to another without compensating for changes in the tonal value will make the image too dark or too bright, which will cause color shifts because the gray balance of highlights, midtones, and shadows increases dot gain. Big plays a different role.

Use RGB and CMYK image data

Few modern prepress departments are aware of the importance of RGB image data. These imaging professionals recognized that scanning and digital photography should be saved in RGB mode throughout the color correction and revision process, and after all adjustments were completed, the conversion to CMYK. Because of these corrected and corrected RGB data, the professional prepress department can store files for a long period of time. This allows images retrieved from archive storage to be used on a printer (or other replication system) different from the original output device. This emphasis on RGB image data has had a good impact in many publishing workflows, whether the color separation method is a system-level color management method or an image batch conversion method in Photoshop using predetermined Ac tions.

The most important thing is that the effect of copying the same image on various printing presses, digital proofing devices, or computer monitors should be strictly the same. This is possible when separate separations are made for each device. Because each replication system requires slightly different mixes of cyan, magenta, yellow, and black to produce a similar appearance, separate color separations make the images look the same on different devices.

The way to observe (and measure) the color differences replicated by these devices is to measure the amount of cyan, magenta, and yellow needed to produce neutral ash—a kind of gray balance that we call the replication system.

If the image has been color corrected or corrected after conversion to CMYK, reusing the last image on a different output device requires adjusting the highlight, midtone, and dark tone points of the CMYK image and changing the overall gray balance and color saturation. It is difficult to change the amount of black in the image without impairing the image quality, but printing an image without correcting the black data may cause undesirable results.

For example, CMYK images originally color-separated on a high-quality, on-line, dry sheet-fed press can cause smearing if printed on a cold-fixed web press. The compromise is to correct any CMYK image used in web pages or CD-ROM electronic publications. RGB images can use a larger RGB tone range to reproduce brighter, more saturated colors. However, after the image is separated into CMYK, all the pixels in the image are within the CMYK tone range.

The development trend of the entire printing industry filing RGB images has encountered some resistance from experienced scanner operators and color separation specialists. These veteran professionals learned color separation techniques when scanners using arranging rows of knobs and RGB image data were only able to drive the output drum's laser beam. But they did not hear RGB image files used for prepress until the customer began scanning on their cheap desktop CCD scanner. For departments with high-end color devices, RGB images begin to symbolize desktop scanners as a threat. As a result, some prepress technicians linked RGB color correction to low quality image capture.

Almost a decade ago, Lino type Hell (now Heidelberg Prepress) published its first LinoColor. This software program supports the color correction of image data before the image data is converted to CMYK.