Chapter 3 ― Copydot Scanning & Big File Management

When you make the transition to CTP you may find that several of your customers who supply film are not immediately capable or willing to provide digital files. It isn’t practical, nor is it financially viable to maintain two production workflows: film and CTP. As a result, most CTP users have implemented copydot scanning. Copydot scanners are high-resolution film scanners that may have 10,000 dpi or higher resolution. The high resolution is necessary capture the dot structure imaged on the film.  

Copydot Scanning. Some CTP users feel that copydot scanning is as good as contacting film, which has been a normal part of daily production for many years. Copydot scanned films are captured in a raster image, one for each plate, and are treated throughout the rest of the CTP process as an image. A single halftone dot may be constructed of many “samples” captured in the scanning process. Most CTP imagesetters construct halftone dots from multiple “spots” imaged to the plate. Hence, the copydot scanning workflow allows dot-for-dot reproduction of the film image to the plate.

Quality Control and Copydot Scanning. Quality control is important to the copydot scanning process and some of the issues include:

·        Mistakenly transposing films during scanning.

·        Keeping the work environment free of dirt.

·        Handling films to prevent damages and dust.

·        The coating on the diffuser sheet needs to be free of adhesives or dirt.

·        Making sure that film mounting and fit in the scanner is correct.

 “Dirty film is going to reproduce as dirty film,” says Kevin Klein of Perry Judd’s. Pinholes and abrasion wear will be reproduce through copydot scanning. Pinhole paints (opaque) don’t work, because the light in the CTP device is so intense, it “shots right through.” It is recommended that red tape be used to cover pin holes and image defects. Dust that falls on the black portion of a negative film will reflect light and will show up as pinhole on the plate. For that reason, some CTP users feel that positive films are preferred for film scanning. CTP systems vendors, such as CreoScitex also provide software that is used to clean-up image defects directly in the file created by the copydot scanner.

The focus of the copydot scanner is also critical. If you’re set up for 6-mm thickness film, but scan 4-mm film, you’ll lose your highlights. Whatever you have on film, in terms of line screen and resolution, is what you’ll print with. Keep in mind that if the films are not appropriate for your press, the scans will not print well either.

If you mount films in the scanner incorrectly the image files will have errors. All portions of the film must be in contact with the mounting surface. It is also advisable that you scan in the orientation that you’ll print the image in. If the RIP needs to rotate an image in the RIP process, it will take much longer than if you scanned to proper orientation. There are tools available that will seek out crop marks or corner in the images to correct orientation automatically during the scan and users have found that these software applications are effective about 90% of the time.

Ramifications of Really Big Files. Copydot scanning produces a very large file that can be problematic for both proofing and customer layout creation. A single color image (imposed flat imaged to a plate) could take as much as 66 MB of storage and a color plate could use up to a gigabyte of storage. On average, it is recommended that you factor in 25 MB per four-color page when estimating your network and storage needs.  CCITT Group III and IV compression can be applied to copydot scan files to help reduce storage requirements. Group III compressed files can be edited in Adobe’s PhotoShop. When planning your network, server, and storage capacity, survey your current operations over the course of a couple of months and consider:

1.      The number of pages in production on any given day to include the daily average and the maximum number in a given day.

2.      The number of plates currently produced per day.

3.      The average length of life for a job defined as the time from receipt of customer files to removal of films, plates, or files to archival storage.

After factoring in the aforementioned average page and plate file sizes, don’t be surprised if terms such as Terabytes [plural intended], Gigabit Ethernet, and Dense Wavelength Division Multiplexing start creeping into conversations at the plant!

CTP users must down-sample copydot scan files in order to create proofs. Images are not re-scanned to create proof files, but there is additional processing time required. CreoScitex, Kodak Polychrome Graphics, Shira, and others have software that allows users to proof combinations of pre-separated and separated images.  Some CTP users are comfortable providing soft proofing (monitor display) to press operators; however, preview files don’t show subtle details such as image trapping. Down sampling of copydot scan files (or up sampling any raster file) often produces image and pattern-interference artifacts. Another problem is generating low-resolution file formats for FPO images that clients can place in the layout file.

Selecting a Scanning Resolution. Some users have expressed frustration with copydot scanning, particularly where multiple platesetters must be supported. It’s desirable to scan the copydot file at the resolution of the intended output device or at a multiple of the output device. For instance, if your output device operates at 2400 dpi then it is desirable to scan at 2400, 4800, or 9600 dpi. However, if your output device operates at 1905 dpi, then 1905, 3810, or 7620 dpi is desirable. If you are supplying files to multiple plants you may find it necessary to scan each set of client films twice for the two different output devices.

Sampling up files is not recommended. Sampling down files irrationally (where a multiple of the output resolution is not the source or scan resolution) may produce image artifacts, such as a slight moiré in shadow areas or problems in tonal graduations. These image artifacts may be acceptable to you and your customers depending on the severity of the artifacts and the sensitivity of the subject imagery. Using a single scanning resolution to support multiple output resolutions is possible, but should be tested before applying it to live jobs. There are two cost factors to scanning at multiple resolutions: the cost of the scanning and the cost of the added logistics of file tracking.

From a quality control and management perspective, establishing a consistent setting for each scanner produces better quality overall, as opposed to adjusting the scanner for each set of film provided by customers. If a scanner is not set-up correctly, as much as 2% of image data can be lost from both the highlights and shadows. Adjusting the scanner’s threshold is necessary to capture the upper and lower tint values. Some CTP Users report found that adding a little “noise” to the images could help reduce or eliminate artifacts. However, “de-screening” methods available on the market are not recommended, as they tend to produce significant artifacts and fuzzy type.