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The True Story of Imagesetters
Halftone dots generated by imagesetters vary in quality according to the resolution
of the system. For example, the lower the resolution, the more ragged the dot edge. This causes greater dot change and unpredictability when making film contacts, dupes, plates, and on press.
When examining films under moderate magnification (10x or similar) the results may
look good. However, under higher magnification (50x or 100x) deficiencies in the dot structure and emulsion artifacts become evident.
To produce the best results with an imagesetter, the following procedures must be
followed.
- Use the recommended chemistry for the film. Mix carefully to avoid contaminating the developer with even a drop of fixer. Keep the processor clean.
- When establishing your starting point, follow the film manufacturer's recommendation for processing time, temperature, and replenishment.
- Adjust the laser energy and processing to produce the recommended image density, measured with an accurate transmission densitometer. This will maximize
the film's resolution.
- Check the minimum density against the film manufacturer's recommendation. High Dmin can be caused by light fog, chemical contamination, or improper
processing.
If the minimum density is too high the dots may be ragged with soft edges, and
subject to greater changes than hard edge dots. The Dmin may be within tolerances on a clear area of the film, but may actually be much higher between the dots (due to high density specs or artifacts caused by laser
scattering within the emulsion). It can also be caused by a low density veiling between the dots. These two different effects can cause great confusion when measuring and examining halftones.
Tint patches on films processed under different conditions may produce identical
densitometer readings, yet appear markedly different under visual examination.
These discrepancies are due in large part to the integrating nature of the
densitometer and the analytical nature of the human eye. The eye will ignore scattered specs between the dots, whereas the densitometer will average them into the measurement. At 10x, the eye cannot detect a low
density image at the perimeter of the dot, however examination with 50x or 100x microscope will reveal this image defect.
The 50% tint with hard edges will accurately measure 50%, while the 50% tint with
soft edges, high minimum densities, and artifacts between the dots, can measure 40% in negative mode, or 60% in positive mode.
Hard dot halftones normally gain 4% to 6% on negative plates, while soft edge dots
can gain 8% to 16% or more. Negative plates cause an increase in dot size, while positive plates cause a change in the opposite direction, decreasing in dot size.
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...the Rest of the Story
Analyzing the Plate
When the plate is exposed and developed it should be measured with a precise plate
densitometer.
The Betalog Platemaster is specifically designed for this purpose. It accurately
measures all offset plates, regardless of graining, image color, or contrast.
At first the measurements may be difficult to believe because we were
schooled by the plate manufacturers to believe that plates exhibit little or no dot gain. However, when the plate image is examined with a 100x measuring microspcope and compared to the image on the film, it
becomes obvious that the dot has undergone a substantial size change. The Beta Color Viewer is ideal for this purpose, combining illumination, high magnification, and contrast enhancing filters. Process color images on
proofs and press sheets can also be measured with equal ease and accuracy, tracking dot size changes through the entire process.
It is important to use a UGRA, GATF, or RIT plate scale to determine optimum exposure
and resolution. Follow the plate manufacturer's guidelines to achieve best results and run length. Cleanliness, drawdown, and proper exposure integration must be maintained to achieve consistent and
repeatable results. The static eliminating BETA Brush will reduce vacuum draw-down times and keep the image dust free.
The advent of Stochastic Printing has further
aggravated the problems of dot size (pixel) change. The need for accurate densitometers to measure film, plates, proofs, and press sheets is greater than ever. Moreover, the use of microscopes for visual
evaluation of stochastic pixels is an absolute must.
Analyzing Proofs and Press Sheets
Color proofing systems attempt to predict,
within reason, the appearance of the printed sheet. There are two basic types, those with dots, e.g. Cromalin, Approval, Matchprint, Intelliproof, and those without dots, e.g. Iris
and Rainbow. Both systems can emulate dot gain on press, a necessary requirement to achieve high print quality and gray balance. Additionally, both systems mimic ink trapping
efficiency and color sequence effects when measured with special Betalog Color
Reflection Densitometers.
Press measurements with a calibrated densitometer are an absolute necessity in
today's demanding market. However, the densitometer alone cannot reveal all the problems that may exist. For example, mechanical problems involving slurring, doubling, ink and water balance, and register
cannot be accurately measured and corrected with a densitometer alone. Examining press sheets with a 50x or 100x microscope will reveal these defects. The Beta Color Viewer is ideally suited for this
task. It will reveal changes in any color, including the nearly invisible yellow. Make-ready is faster with less waste and the jobs runs more smoothly.
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