What are the differences between halftone dot digital proofs and continuous tone digital proofs

Answered by Eric Neumann, Assistant Product Manager, Color Management Products, Enovation Graphic Systems, Inc.

What are the differences and pros/cons of halftone dot digital proofs and continuous tone digital proofs?

Halftone Dot Digital Proofs
Originally all proofs were halftone-based. Before "proofing" systems evolved, the press-proof was the only way to verify the printed sheet. Then analog or laminate-based proofing systems evolved and these allowed printers to make a color proof from the exact same (halftone) separation films that would be used to make the plates that would run on press. Advances in color technology eventually made these proofs fairly accurate to the press sheet.

Enter digital. "Contone" digital proofs actually evolved first, because of the complex requirements for accurate screening and color management in halftone digital proofing. Digital halftone proofers need to take a digital file, screen it with a halftone screen similar to the one used on the CTP device, add an appropriate level of dot gain and finally color manage it so the lay down of the dots yields a color that accurately matches the target press sheet. For this reason, other than a press-proof, there is really no other proofing method as accurate as halftone digital proofing. However, as a result, the cost of equipment and materials far exceeds just about every contone proofing system available.

The main reason that many customers demand halftone digital proofing (in addition to being color accurate) is because it accurately represents the halftone screening as it will appear on the printed sheet. This allows the customer to detect screening issues such as moiré, caused by some screen combinations or even certain subject matters (i.e. tweed or mesh materials in the image). These are issues that would typically not be seen in a contone proof. Halftone proofers require a RIP that can screen and typically must handle very high resolutions (equal to CTP, i.e. 2400 dpi).

Continuous Tone Digital Proofs
The first digital proofers, while quite expensive, were of very poor quality. Imaging technology has allowed for better resolution, coupled with advanced color management, to yield color accurate proofs. Today, contone digital proofers are both accurate and fast (faster than a halftone proofer), not to mention that they cost a fraction of what a halftone digital proofer costs.

Color management is critical to a good contone digital proofing system. The technology determines how to best lay down a layer of ink or dye in the right amounts to accurately represent the color on press (which, incidentally, is always made up of halftone screens). Some people have a hard time justifying the comparison between a proof with no dots and a press sheet with dots, but others can look only at the color and make judgment calls based on that alone.

The halftone dot proof provides comfort (when looking at it up close or under a loupe), but when you look at halftone and contone proofs at a normal viewing distance (i.e. 18"), the halftone screen pattern is indistinguishable and the two proofs will appear nearly identical. Since contone proofers do not produce a halftone screen, their resolution requirements are much less (i.e. 600 - 1400 dpi).

Both proofing systems have their purpose, so there will always be a place for each. It is important to determine where in the workflow a contone proof is acceptable and where a halftone dot proof would be better served. The level of printing also has some bearing on this. For example, high-end coffee table books or critical advertising would tend to be done on a halftone dot proofer, while some book printing or packaging might be able to use contone proofing methods. Ultimately, each case is unique and depends on what the customer is willing to accept and sign-off on.
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