A close friend emailed me a commercial printing blog comment asking whether any proofing device could really match ink colors on press. Not just close, but dead-on.
I thought about this and did some research. This raises a number of interesting issues.
How Many Colors Can Be Reproduced?
All offset printing presses create color with cyan, magenta, yellow, and black ink. Because of this, only certain colors can be reproduced on press, far fewer colors than can be perceived by the eye or reproduced on a color monitor in the RGB (red-green-blue) color space.
That said, some commercial printers augment this color set with additional inks. These can include a “touch-plate,” a single extra PMS color in a fifth ink unit used to improve the fidelity of greens or purples in a large format print poster, for example. Other options can include “hexachrome” or similar color ink sets that employ CMYK inks plus orange and green (or two other colors). In these cases, the custom printing vendor seeks to expand the color range on press to match more colors visible to the eye (the vibrant colors in nature, for instance).
Colors on a computer monitor are created with red, green, and blue light and therefore (without adjustment) do not match CMYK ink colors on press.
Colors on proofing devices are also created with cyan, magenta, yellow, and black ink, but manufacturers have augmented this ink set with such colors as light magenta, light cyan, various black inks, sometimes orange and green, and sometimes even red, green, and blue (but not all at once). The goal is to reproduce more colors (again, such as the vibrant colors in nature) and, more specifically, to match press output more accurately.
Color Management Improves Color Matching
All of these devices treat color somewhat differently, so to make color more consistent across the various devices, color management was born. Essentially, using various light, ink density, and color reading instrumentation, a commercial printer can measure the output of the inkjet and color laser proofing devices as well as the monitors and the offset presses.
The next step is to create color curves that “map” the color of one device to that of another. Think of this as a universal translator that can avoid a “tower of babble” situation. The color management device curves make it possible to consistently predict and control the translation of color information as it travels from the monitors in the client’s office (if they have been calibrated) to the monitors in the commercial printer’s prepress department, to the inkjet proofing device, to the offset press.
If all of the equipment is dutifully maintained and recalibrated regularly, this works in theory. This makes it possible to produce a proof and assure a client that the proof will match the final output on press.
Fingerprinting the Proof to the Press
Many custom printers fingerprint the proof to the press. That is, since they have set up the press to produce colors optimally with four-color process inks (and accounting for such offset-printing challenges as press dot gain, which can affect colors), the commercial printers then adjust their digital proofing devices to match their presses as closely as possible.
(So in theory, at this point, the proof matches the offset press product. However, the press sheet must also match the substrate on which the proof has been produced. Ink on a cream press sheet will not match inkjet ink on a white proofing stock.)
Better Yet, Depend on Color Management Standards
A better (just my opinion) option would be to match both the offset press color and the inkjet proofer color to an objective color standard, such as G7 or GRACoL. While it could take a book to explain either of these standards, the important point is that if a commercial printer’s inkjet proofer and offset press match an objective standard, then the color within his custom printing work will match that of other commercial printers’ work.
Screening Patterns Are Different from Proof to Print
Inkjet proofing output is composed of minuscule spots of color. Offset printing images are built with halftone screens of the process colors tilted slightly relative to one another, thus creating “rosette” dot patterns. These screening processes are different. This will affect the color match. Only by using true halftone dot proofing (like the Kodak Approval) can you match proof to press exactly.
Flaws in Certain Proofing Devices
Some proofing devices are less accurate in yellow/orange and some in violet. Ask your commercial printer about the limits of his color proofer.
Pleasing Color vs. Critical Color
There are two mutually exclusive objectives in achieving color on press: “critical color” and “pleasing color.”
Critical color implies an absolute color match. For example, if you need to match a fabric color for a clothing catalog, you would bring a sample of the cloth to a press inspection and ask the pressman to match the color. Critical color would be required for photos of automotive, food, and fashion products.
Pleasing color has a little more latitude. This is when the color has to be good, but not dead on.
In-Line Conflicts
In addition, keep in mind that any pages that are “in-line” (above or below one another on a press sheet) will be affected by any changes to in-line color information on press. Changing an ad on one page will affect a photo or background tint on the page immediately above or below it on the press sheet.
Are InkJet Proofs Accurate?
So where does this huge amount of information leave us? For an inkjet proof to be accurate:
- A printer’s inkjet proofing device, press, and monitors must be consistently monitored and calibrated to a standard like G7 or GRACoL (or the proofing device must be regularly fingerprinted to the press).
- Dot gain on press will need to be taken into account.
- The proofing and printing paper stocks will need to match.
- The limits of the color gamut (CMYK vs. other expanded ink sets in many proofing devices) must be understood.
When In Doubt, Nothing Beats a Press Check
When in doubt, nothing is as good as a press check. Only ink on paper can give you an absolutely color-faithful idea of how the final output will look.
(Then again, even that will look different under different kinds of light: fluorescent, incandescent, sunlight.)
So the safest bet is to say that a commercial printer will come as close as possible to matching proof to print, based on his equipment and color management, but that no proof is perfect and that most printing involves a certain level of compromise.
This entry was posted
on Thursday, February 23rd, 2012 at 7:14 pm and is filed under Offset Printing, Prepress, Printing, Proofing.
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