What is the difference between conventional digital and post ripped color proofs?

Answered by Stephen Beals, Digital Pre Press Manager and Writer

Can you tell me what is the difference between conventional digital color proofs and post ripped color proofs? As far as I understand, the same RIP is used for all the output devices, which ensures consistency of image color whether on screen, pre-printed or the printed item itself. Does post ripped mean that the Postscript file has already been ripped/processed, is stored on the server and will be used to both produce the proofs I see and subsequently the output of the plates the printer will use?


The heart of the question is: what exactly is Raster Image Processing (RIP)? The truth is that the definition of RIP is about to change pretty significantly with the advent of Adobe's new PDF Print Engine. There are currently two different types of RIP processes: ROOM (RIP Once Output Many) and NORM (Normalize Once RIP Many).

Different printing devices require different Raster sizes to output properly. Commercial printing rasterizes continuous tone objects (raster files like photographs at generally about 300 dots per inch and uses a screening algorithm to make dots ranging from 65 lines per inch for some newspapers to 600 lines per inch for very high end sheetfed presses). With digital screening there are many options for doing this depending on the inks, press and paper being used. Vector images like text are output at between 1200 and perhaps 2540 line per inch.

Laser printers rasterize everything: type and images, at generally 600dpi. Ink jets rasterize everything at perhaps 720 or 1440 dots per inch.

It is, of course, quite possible to print your file to an ink jet printer for a "proof". That's also a RIP. Your ink jet printer has to RIP the file and break it into tiny dots for printing. But the RIP your ink jet printer or laser printer uses is almost certainly not the same as what your printer will use. Not only is the color very likely to shift, but objects and text may not print the same way. It is particularly problematic when things like transparency are thrown into the mix.

In almost all commercial print applications, the printer makes the proof from the file after it has been RIPped. Using a printer that is calibrated according to the printer's press output will give a very good representation of what will be printed. But in most cases you will not see the exact dot structure you will see on press, and spot colors will be represented by CMYK equivalents. Proofing with the same raster file as you will print with is called a ROOM workflow. Even though the file must be massaged a bit to create the proof, this does not affect the integrity of the file; the original remains intact and sits on a server waiting for your OK to output. So there is a small caveat: yes, the same file is used to output both the proof and the plate, but the proof is generally not the same resolution as the plate, and in the case of spot colors, the color will almost always be simulated.

More and more you will see NORM workflows. This system allows printers to keep all of the vector files intact without converting them to dots. NORM workflows use PDF files that have been verified, have all fonts embedded and have passed all the system checks required to know they are essentially "bulletproof". (Even ROOM files are not absolutely immune from problems). The beauty of NORM is that you can output the exact same file to any device. Want to send the file to a 600dpi device AND a commercial print shop AND the web? No problem. The same file can be rendered on all of those devices. A proof from a NORM workflow can be just as accurate as a proof from a ROOM workflow. In either case the printer needs to know the output intent to give you a proof that matches the device you will be printing on. But with a NORM workflow the intended device can be changed without building a new file.

Color accuracy is really a different question. Both NORM and ROOM workflows can create accurate and inaccurate color proofs. That depends on your printer's implementation of a good color management system.

Stephen Beals is a digital pre-press manager and has been writing for major print publications for many years. He is the author of A Practical Primer for Painless Print Production. He can be reached at stephenbeals@mac.com.

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