Accurately reproducing color from the digital artwork created by designers is an essential component to the success of any commercial or packaging print company. Within the print production process there are typically 3 different methods of communicating color. The first is the monitor as it is the medium by which designers create the original artwork. The second is the inkjet printer as it is typically used to produce proofs that represent the colors to be printed, and the third is the printing press that produces the final printed product. When trying to visually communicate color across these three mediums, it is important to take the following into consideration.
The monitor is used by designers to create the digital artwork in the origination phase of print. It uses an additive color process, meaning color is created by mixing light of two or more different colors. Red, green, and blue are the additive primary colors normally used in displaying color on the monitor. This additive color process offers designers an extensive color palette, so the color gamut is extremely large and often beyond the capability of most CMYK digital proofer’s or printing presses.
Digital proofs are the main mode of communication externally to the customer and internally during almost every phase of production for the commercial printer. The colors, images and graphic elements displayed in the digital proof are meant to provide an accurate representation of the printed end-result on the printing press. While designers are viewing additive color on their monitors, printers are viewing subtractive color on their inkjet proofs. Subtractive colors are created by subtracting (absorbing) parts of the spectrum of light present in ordinary white light, by means of colored pigments or dyes. The digital proofer typically uses dye based inks to reproduce color represented in the digital artwork supplied by the designer. The color gamut of a digital proofer is smaller than the monitor making accurate reproduction of many colors challenging.
Additionally, digital proofs are typically printed on inkjet paper. This creates a myriad of challenges because not only is the paper different from what will be used on press, but it often times has a semi-gloss coating and contains optical brighteners. Optical brighteners are chemically introduced into paper to make the paper appear bright white resulting in more vibrant printed colors. In reality these optical brighteners make the paper more blue and therefore colorimetrically inaccurate and not truly representative of what will, or even can be printed on press.
The two most common printing press technologies used by commercial printers are offset lithography and flexography. Both use a subtractive process (like the inkjet proofer), but unlike the digital proofer that sprays dye based ink onto an ink receptive material, it instead applies pressure to a print medium using a combination of oil based, pigmented ink and water to create the image. The color gamut of most printing presses is much smaller than that of a monitor or inkjet proofer. Many times commercial printers must have additional inks formulated in addition to the standard CMYK process inks in order to reproduce certain colors. These are called spot colors and usually cannot be reproduced by mixing a combination of the CMYK process colors on press.
Accurate and Consistent Color
The challenge is how to produce accurate and consistent color despite these 3 different methods of visually communicating it. The first step is to implement a color quality control process and adhere to standards like ISO 12647 and color standards like GRacol, SWOP3, SWOP5, and G7, which define how to achieve accurate and consistent color throughout the printing process. These standards define:
- color that is being used within a certain space
- type of paper (including the attributes of the paper)
- allowable dot gain, target values, and tolerances for each paper type
In the US, the most predominant color standard is the IDEAlliance color control strip. This confirms that a proof meets a specific ISO standard, ISO 12647-7 (which states that there is conformance to a known data set such as GRacol, SWOP3, SWOP5, G7, etc).
To determine if what is being produced by the proofer or the press is within the specified tolerances a color control media wedge or strip that contains all relevant color information must be printed and a spectrodensitometer must be used to measure the colors in that media wedge through all iterative stages of production. Using color quality control software in conjunction with the spectrodensitometer introduces an objective and empirical method for determining if colors, paper, dot gain, target values, and tolerances are within the documented specification.
A good example of ISO specified standard paper types for offset printing are:
- Standard papers types 1 and 2: Coated gloss and coated matte papers above 70 g/m2.
- Papers type 3: LWC (Light Weight Coated paper)
- Papers type 4: Uncoated white
- Papers type 5: Uncoated yellowish (recycled)
- SC papers: Super Calendared 45-60 g/m²
- MFC papers: Machine Finished Coating 48-80 g/m²
- SNP papers: Standard News Print (standard news paper for heatset web offset 40-52 g/m²)
- MF papers: Machine Finished (uncoated), 48-80 g/m²
- INP papers: Improved News Print (uncoated)