Kodak provides various raster file targets to aid in exposure setting. These high-resolution 1-bit TIFFs can be used without having to configure and run test files through the Prinergy or Prinergy Evo workflow system.
For more information and a list of Kodak raster file targets, see Kodak raster file targets.
To determine minimum dot value, test targets are exposed and plates produced for each different media and line frequency used at a customer site. Generally, for the same line frequency, as media thickness decreases, the minimum dot also decreases. For example, 100 line on 0.120 media may have a 4% minimum dot value, but 100 line on 0.045 media may have a 1% minimum dot value.
For the same media, as line frequency increases, minimum dot value generally increases. For example, 0.045 at 100 line may have a minimum dot value of 1%, but 0.045 at 150 line may have a minimum dot value of 5%. Some trade shops produce the same frequency and plate thicknesses with different vendor media. Even though the line screen is the same, separate minimum dot value determinations for each media are required.
Minimum dot value can usually be determined by placing the processed plate exposed side down on a light table, and inspecting the tips of the dots with a microscope. How well the dots contact the glass and transmit light, and how consistent the resulting dot shapes are, is a good indication of minimum dot value. In the image below, printing dots with shoulders not well-formed are shown on the left, and dots with well-formed shoulders (a good minimum dot) are shown on the right.
Figure 1. Dots viewed on light table with microscope. Printing dots, with shoulders not well-formed (left), and dots with well-formed shoulders (right). Dot form on right is likely a good minimum dot.
Dots may look good through a microscope, but they must also be measured with a micrometer to ensure that they rise to full relief height. Dots that are below full relief height may be subject to ink buildup and poor ink transfer, and are known as scum dots.
In actual practice, it can be difficult to use a micrometer on small highlight dots, because the dots may compress slightly and produce a false low reading. It is a good idea to cut out several rows of dots, with a full-height reference such as a keyline included, in order to examine the cross-section to determine if the dots are rising to full relief height.
Figure 2. Magnified dots
Dots must be examined under magnification. It can appear that a particular percentage is holding on plate, when in reality dots are not well formed and will not print.
A media representative can make a well-informed determination about minimum dot value, but ultimately the results must be confirmed by a press run. Dots that may look well-formed may still not transfer ink efficiently. Use a reflection densitometer on print samples, with attention to print values that increase instead of decreasing. For example, a nominal 5% patch may reproduce at 10% on press, but the 4% patch may reproduce at 13%. This indicates that the 4% dot is not well-formed, and that it is transferring ink with too much gain—it should normally print lighter than the 5% patch. These types of changes can not normally be detected with the human eye, and you should rely on densitometers to spot these kinds of unwanted changes. In the image below, a densitometer was used to measure print results and the results were printed to the right. The 4% dot may have looked good under the microscope, but a print test shows that 4% prints with excessive gain and is printing darker than 5%. In this case, 5% should be used as the minimum dot value.
Figure 3. Printed step wedge used to confirm minimum exposure value
Three factors contribute to the ability to hold a small dot on the plate:
- The size of the opening in the mask
- The relative spacing of a dot to its nearest neighbor object
- The relief depth of the plate
Placing dots closer together allows smaller dot sizes to be held on plate, which means that higher frequencies may allow smaller minimum dots.
When minimum dot value has been determined for each media and frequency,byou can move on to creating bump curves and fingerprinting to determine tone reproduction curves.