RGB and CMYK Printing Guidelines
1. RGB vs CMYK
RGB (Red, Green, Blue)
RGB is the most common mode used for digital image processing. It represents Red, Green, and Blue. Images in RGB mode are intended for light-emitting devices such as monitors and televisions. This mode covers a broad color gamut (containing over 16.7 million colors) and is considered a True Color mode.
CMYK (Cyan, Magenta, Yellow, Black)
CMYK is a subtractive color model used for printing. Images in this mode are only observed on printed materials. CMYK stands for the four primary inks used in printing: Cyan, Magenta, Yellow, and Black.
Note: The color gamut of CMYK is significantly smaller than that of RGB. Consequently, images seen on a monitor will appear more vibrant than those printed out.
Summary: Use CMYK for printing (four-color process) and RGB for digital screens. Converting RGB to CMYK often leads to color shifts; it is highly recommended to set the correct color mode when creating a new file.
2. Bleed and Safe Areas
Bleed Line
Objects positioned at the edge of the finished product must extend 3mm beyond the trim line. This 3mm extension is the "bleed." Without it, cutting inaccuracies at the print house may result in unsightly white edges along the product boundary.
Safe Distance
Objects not required to be at the edge must be kept at least 5mm away from the trim line. For binding sides (inner margins of facing pages), maintain a distance of at least 10mm (excluding saddle-stitched books). Otherwise, content might be lost in the spine gutter, hindering readability. Avoid spanning lines across facing pages; strict alignment is required for cross-page images (excluding the center spread of saddle-stitched books), as binding errors can prevent perfect alignment.
3. Preferred File Formats for Printing
It is standard practice to provide three files: the Ai or PSD source file (to allow the print house to make minor adjustments), a PDF for the actual printing, and a JPG for previewing purposes.
4. Outlining Fonts and Embedding Images
Create Outlines (Convert to Curves)
To prevent font missing issues and unnecessary delays at the print house, always "create outlines" for all text in the file. Select the text, right-click and choose "Create Outlines" (Shortcut: Ctrl+Shift+O).
Embed Images
To avoid missing images or broken links, ensure all images are embedded in the file. Select the image and choose the "Embed" option in the control panel. For high-quality printing, images must have a resolution of at least 300dpi.
5. Font Size and Line Spacing
Minimum Font Size
To ensure legibility after printing, the minimum font size should be at least 5pt.
Line Spacing
Generally, larger fonts require relatively smaller line spacing. For English text, standard line spacing is usually 1.2 to 1.5 times the font size.
6. Best Practices for Using Black
- Use Single Black (K=100) for small text, QR codes, and fine icons to avoid registration issues.
- Use Rich Black for large areas (K=100 with CMY set to 30 each).
- For deeper black tones, add 30 or 40 to specific CMY channels on top of K100.
7. Transparency Settings
Transparency for colors or images should not be lower than 10%, as very low opacity levels may fail to render correctly during the printing process.
