Contract proofing for printing has been traditionally done by press proofing. This is costly and wasteful, not just in terms of equipment and labor but also in terms of expendables. The advent of off-press proofing was greeted with some degree of uncertainty by the printing industry. With press proofing, the proof was literally a preview of what would happen on a press. The press proof, although often printed on a different press, generally used the same type of inks, plates and substrate that would characterize the final print. With offpress proofing, printers were comparing apples to oranges; instead of comparing a press sheet to a press sheet they were comparing a press sheet to an approximation of a press sheet. However, over time as printers learned to read off-press proofs, they became accepted as contract proofs. The same situation has now befallen digital proofs. In the particular case of flexography, the proofing problem is a bit different. Off-press analog proofs were designed with lithography in mind, they were characterized to simulate lithographic dot gain. In order to make a proof that looks like a flexographic press sheet, two sets of films are required; one which compensates for flexographic dot gain (this is the set from which the job would be printed) and one which has extra dot gain built into the highlight and quarter- tones (this is the set that the proof would be made from). This extra set of films is wasteful and time consuming to generate. Digital proofing seems to be well suited for flexography, because the dot gain can be built into the proofing system and no extra film is required to create the proof. At the most basic level there are two types of digital proofers available; those which simulate halftone dots and those that do not. Whether or not the dots are necessary is open to discussion, however, in the case of flexography the dots appear to be crucial. For this research document it was decided that the halftone dots were preferred. The reason for this is that at about 133 lpi the rosette patterns formed by halftone dots are at the threshold of resolution by the human eye. For more course screen rulings this is even more critical. Much of flexography is printed at screen rulings of 133 lpi or lower, so very often the dots can be resolved by the eye. Therefore, the mindset at the beginning of the research was that if the dots can be resolved on the press sheet then the dots should be resolved on the proof. The major thrust of this research was to observe whether or not a halftone-based digital proofer can simulate the appearance of a flexographic press sheet. A flexographic test form was created and printed on a film based substrate. A press sheet was sent to two vendors who manufacture halftone proofers. The proofing systems are not mentioned by name; they are instead referred to as Digital Proof A and B. They then attempted to match the press sheet as closely as possible. Thus, through reverse engineering, the vendors created a device profile for this set of printing conditions. Upon receipt of the proofs, they were compared to the press sheet in terms of optical density, hue (AE) and halftone dot size. Later, a visual assessment was executed to observe how closely the digital proofs matched the press sheet using a 3M Matchprint, that had been altered to approximate flexography, as the reference or control proof. The results showed that there were significant differences between proof and press sheet in some instances and insignificant differences in others. In terms of the physical structure of the halftone dots, the 3M Matchprint had the closest match to the press sheet dot structure. In terms of physical dot size; digital proof A best matched the 50 and 75% dots and the Matchprint matched the 5% dots the best. In terms of optical density; digital proof A best matched the density of the 25% dots, digital proof B best matched the density of the 5% dots and the Matchprint best matched the 50 and 75% dot patches. In terms of AE values (color or hue difference); the Matchprint most closely matched the press sheet, digital proof B was next, and digital proof A was last. In terms of a visual match, the three proofs were found to be statistically equal in their ability to visually match the press sheet. The visual match being the most powerful of the criteria; shows that the measurable differences in the proofs did not directly affect their ability to match the press sheet. The results show that either of the two halftone digital proofs could have been used in place of the 3M Matchprint. The results also question the need for halftone dots in a proof. This is primarily because the two halftone digital proofs utilized a different RIP than the Agfa generated films for the 3M Matchprint and flexographic press sheet. Yet the visual observations made by the judges could not, at a normal viewing distance, discern this difference. The conclusion is that there is no visual difference between the halftone digital proofs and the 3M Matchprint proof in terms of visually matching the press sheet.
Library of Congress Subject Headings
Color printing; Digital printing; Proofs (Printing)--Evaluation; Flexography
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Hanna, William, "A Study utilizing halftone based digital proofing systems in the flexographic printing process" (1996). Thesis. Rochester Institute of Technology. Accessed from
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