Using process black on small objects such as body copy text and fine keyline borders is OK. But if you have a large background of solid black or very large black type, then the solid black may look washed out when printed.
All process inks are translucent. They allow light to pass through them, much like a color filter. The light is then reflected back to our eyes and the result is a washed out looking black.
To increase the “richness” of the black it is necessary to add additional color to the black. In prepress this is called “Rich Black” or “Undercolor Black”.
Rich blacks are created by adding amounts of process cyan, magenta and yellow to the black. The amount of CMY added can vary, based on many things, such as the design of the project, press conditions, and the paper the project is to be printed on.
The Castle Press typical standard rich black is C=40%, M=40%, Y=40% and K=100%.
Now, a problem arises when you place an image over a Rich Black area in a page layout if the image has a black background. The black background of a scanned image will probably not be 40-40-40-100. It may look correct on your monitor but if it does not match the difference can be seen when printed.
An easy way to check is to open the image in Photoshop and check the color in the Photoshop Info palette. Position the cursor over the black area. In a CMYK image, the CMYK percentages will be in the upper left section of the Info palette. (If your image is RGB, convert it to CMYK.)
One way to make the necessary adjustments is to go to Image > Adjust > Selective Color. In the Black tab, use the sliders to adjust the cyan, magenta and yellow while watching the dot% in the Info palette. Continue to adjust until the scanned image background matches the rich black background.
You can also select the black area in Photoshop and fill the selection with matching Rich Black percentages. A Color Range selection with a fairly low fuzziness percentage often works well for isolating the black background.
Another option is to adjust the rich black in the page layout program to match the image.
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