A Guide for overprint in Photoshop
One technique used to prevent noticeable registration errors is the overprint command. This command is not activated by default and must be applied manually in graphic design software like Illustrator and CorelDRAW. Overprinting can serve multiple purposes, including masking registration errors and creating a deeper black, which significantly improves the quality and readability of printed materials, especially in packaging.
While overprinting is commonly implemented in vector-based software, there are situations where it's necessary to apply this command in Photoshop. Reasons for this include: lack of access to vector software, system limitations, specific printing requirements, or simply user familiarity.
The process of creating an overprint in Photoshop involves using blending modes. These modes can only be selected and cannot be customized. The Multiply blending mode is often used as a substitute for overprint, but technically, it can lead to color inconsistencies during the printing process. This is because overprinting involves combining the colors of two overlapping objects, and in specific cases, the color codes must be precise to ensure proper control by the print operator. The Multiply mode cannot always provide these accurate color codes.
Blending Modes
The general rule for overprinting in vector software is that the background color completely covers the underlying object to prevent any white areas. For example, if the background is 20% cyan and the object is 30% yellow, the overlapping area will contain both cyan and yellow. This means that in the overlapping area, both the background and foreground colors contribute to the overall color.
While this concept is straightforward in vector graphics, it becomes more complex in Photoshop due to the use of blending modes. The Multiply blending mode is one option, but it is primarily effective for black text on a colored background. However, when two colored objects need to be overprinted, the Multiply mode may produce unexpected results.
Overprint Alternative
When overprinting colored objects, it's recommended to use the Darken blending mode. This mode follows the correct blending rules and produces results that closely match expectations. Consequently, controlling the combined colors with Darken is more accurate during printing.
It's important to note that when previewing an overprinted area using software like Esko Deskpack or Adobe Acrobat Pro, you may see a new combined color with different CMYK values. This can sometimes confuse print operators and quality control personnel. However, these values are correct and do not indicate a four-color black. The initial black was for the solid text, and the new color is a result of the overprint process.
Another crucial point is that the higher percentage of any CMYK value in the background or foreground will determine the final value in the overprinted area. For example, if the background is 20% cyan and the foreground is 35% cyan, the resulting cyan value in the overprinted area will be 35%. If the overall ink coverage becomes excessive, it can cause printing issues. This is one reason why overprinting is often avoided in web offset printing. In such cases, trapping is a more suitable technique.
Remember that...
overprinting is a valuable technique for improving print quality and addressing registration errors. While vector software provides more direct control over overprinting, Photoshop offers blending modes like Multiply and Darken as alternatives. Understanding the principles of color blending and the limitations of each blending mode is essential for achieving desired results.