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| Re: Metallic vs more conventional paper |
All color printing is based on the concept overlapping dots of CYM semi-transparent inks subtracting color from the full spectrum illuminant. The illuminant passes through the ink layers, bounces off the paper, and through the ink again. Black is produced when all the illuminant is filtered out. That's the opposite of RGB where the three illuminants are added / overlapped to produce white.
Solid C+M+Y pigments don't filter perfectly because the cyan and magenta pigments are not theoretically pure. That explains why when you print a neutral looking image of a gray card that was R=G=B in Photoshop the CYM percentages are not equal. Part of what a printer profile does for any ink / paper combination is to determine the C+M+K "recipe" for all neutral tones darker than the paper base created with the ink dots. Because of the impurities 100% CYM creates a dirty brown not a neutral black. A "skeleton" K channel is needed (over the darkest neutral color the inks can produce) in darker shadows both for greater reflected density and neutral appearance.
The difference between offset printing and ink jet is how the overlapping dots are formed.
Traditional offset printing uses discrete dots typically spaced 1/133th" or 1/150th" apart (i.e. 133 DPI and 150 DPI screens). Offset screens are angled 45 degrees to each other for CMK and 15 degrees for yellow causing the the dots overlap is an rosette pattern. The eye can't resolve the pattern so the screened image is perceived as continuous tone. The discrete dots in offset printing in a magazine can be seen with a 10x loupe.
Ink jets and modern offset color separations use a process called Stochastic screening (a random pattern) and frequency modulation of the ink jets to form the dots but the cause and effect is the same. The inks are semi-transparent, overlap, and subtract wavelengths as the light bounces off the paper. The smaller the dots on the paper, the less filtering of the full spectrum "white" illuminant by the inks and the more the properties of the paper affect appearance.
Metallic substrates reflect the light passing through the ink differently than white paper. By way of analogy consider how light reflects off a wall vs. a mirror. The angle of illuminant can changes the appearance of the mirror but not the flat wall.
The ink will also react with the stock and dry differently. Ink absorbs and binds with the paper (matte finishes) or the paper coating (glossy stocks). That will not occur the same way on metallic stocks the same way. Having printed ink jet on metallic but when printing offset on them it could take several days to dry the sheets before finishing.