Chapter 2 ― Plate Measurement and Quality Control

There has been a lot of discussion in the trade media about plate measurement. Traditionally dot area had been estimated in prepress by taking density measurements from films using a transmission densitometer. With the absence of film in the CTP workflow, this traditional quality control process is no longer possible. Early CTP users explored direct measurement of imaged plates, but direct measurement of images on highly reflective and textured surfaces has proven to be problematic.

Plate Measurement. There are three components of a plate measurement and control program: 1. Selecting a target, 2. Measurement, and 3. Data reporting. CTP quality control targets include the GATF/Systems of Merritt Digital Plate Control target and the digital version of the UGRA plate control wedge. The GATF target includes RIP statistics that are extracted from the image processor, as well as pixel width, star patches, and tint value patches.

Dick Fisch, Bob Cavin, and Sharon Bartels of Imation imaged the UGRA plate control wedge to a series of plates and measured each plate with a variety of densitometers and spectrophotometers. The most significant factor found to be influencing measurements by Fisch, et.al. was that the grain-to-instrument angle significantly affects variability in measurements. Furthermore:

·        In general, smaller dots produce more variability in measurements

·        Densitometer measurements of smooth surfaced plates are significantly more variable than medium or rough plates.

·        Contrary to zero-forty-five degree densitometry, spectrophotometers exhibit higher variability for rough plates.

To complete this study, Fisch et. al. made 27,000 measurements. They concluded that densitometers produce unacceptable variability in measurements of smooth plates and that integrating sphere spectrophotometers, specular excluded, and direct measurement of percent dot area is preferred.[1] Fisch et.al. found one integrating sphere device, designated “B,” was found to be satisfactory for measuring dot area of smooth, medium, and rough grained plates. However, there are no integrating sphere devices on the market used to measure density.

Several direct measurement devices, known at plainometers, have been introduced to the graphic arts market. (An example is the Ccdot device produced by Centurfax.) These devices capture a greatly magnified image of the image on the plate surface and use linear measurements of the captured images (i.e., microns or millimeters) to determine average dot area for a given target. When selecting a plainometer the device’s aperture size (also the size of the charge coupled device used to capture the image,) must be large enough to sample a significant number of dots in a target area. A device with a small aperture may not be appropriate for newspapers or silk screen printing where line screens of less than 133 LPI are common. Furthermore, a square aperture may interfere with the line screen’s dot pattern.

Most experienced users have tailored their quality control program to focusing on repeatability, not necessarily accuracy. If plate output is repeatable and consistent, then users can control their process. As a result, many CTP users employ visual plate targets that the operator can use to make a quick determination of whether or not a plate meets house standards. Visual control targets can be both precise and accurate.

Laser imaging devices used in CTP imaging work with different combinations of “on and off” patterning. For instance, a four-by-four image pattern consists of four dots in four consecutive spaces, but it is imaged as slightly overlapping lines. The most sensitive imaging pattern is one-by-one, because it has the most surface area. AGFA constructed a Digital Plate Control Wedge with 8 x 8 pattern checker board targets printed adjacent to 1 x 1, 2 x 2, 3 x 3, etc. targets. When the 8x8-pattern target visually matches all of the other targets, you will know that you have found the optimal laser exposure for your CTP imagesetter. This is not to be confused with linearization of the imagesetter; the imagesetter must be linearized for this approach to work.

Exposure levels affect dot gain by raising or lowering the overall dot gain curve at all points along the curve. It is recommended that printers determine a upper and lower limit to dot gain curves, image a visual target representing the upper and lower limits of acceptability and use those samples as a visual guide in daily operations. Users have also used the GATF and UGRA control targets as visual targets, although they were not developed for this purpose.

Baking Plates. Metallurgists hired to test several types of plates, measuring tensile strength and hardness before and after baking found plates that are over baked were much more likely to crack. Users reported that thermal plates cracked easier if baked at over 500 degrees ― aluminum plates starts to warp if the metal’s temperature is raised above 500 degrees and that the material weakens as a result.