The ISOCONTOURS* diagram serves to visualize, analyze and evaluate analog/digital transfer processes in printing and prepress. With every transfer process along the entire manufacturing chain (film, analog platemaking, CtP, proof, printing process, digital printing, etc.), the original halftone values or data changes. The ISOCONTOURS* diagram shows whether these deviations are standard-compliant or whether anomalies have occurred in the transfer process.
The ISOCONTOURS* diagram is shown as an X/Y diagram. All tonal value gradations between 0% and 100% are portrayed in the horizontal X-axis, and tonal value or halftone dot changes are shown in % in the vertical Y-axis, and this information is used to determine a characteristic curve. A comparison between the measured characteristic curve, the standard characteristic curve, and the theoretically calculated ISOCONTOURS* provides important information on transfer behavior.
The ISOCONTOURS* diagram is an important component of the System Brunner PRINT EXPERT* 2000 system solutions PLATE-CHECKER™, PRINTCURVE-CHECKER™, and PROOF-CHECKER™. Users receive immediate assurance that their manufacturing process is standard-compliant, otherwise they can take selective corrective measures.
Under the brand name “ISOCONTOURS* diagram”, System Brunner lays claim to the following intellectual properties:
Background information and development of the ISOCONTOURS* diagram (Felix Brunner)
At the end of the sixties when printing ink manufacturers defined the Euroscale, halftone letterpress printing was still being very widely used. The forms for picture reproduction were called blocks, made up of partially etched metal plates mostly made of zinc, which were mounted on lead bases to bring them to the same height as the lead type.
Quality control by the professionals in those days mainly consisted of looking through a magnifying glass thus making the halftone dots in the form and in the print visible.
A characteristic of letterpress printing compared to offset was a noticeable accumulation of ink at the edges of the halftone dots and characters. This is caused by the mechanics of letterpress printing which is a relief process. Since offset printing is a planographic process, it does not have this characteristic.
In both letterpress and offset printing, printers were above all concerned with fully reproducing all screen gradations, paying special attention to the so-called positive and negative highlights at the beginning and end of the halftone scale. Their efforts were concentrated on preventing the smallest negative screen dot from "filling-in" in order to preserve the shadow details.
As Felix Brunner started talking about "dot gain" at the end of the sixties because he believed this to be the main cause of deviations in black/white and color illustration printing, print experts were initially offended, maintaining that it was a matter of professional pride that their efforts were precisely directed at transferring all screen dots to paper with no dot gain or any other deformation.
It needed a good deal of clarification work at the beginning of the seventies to make printers aware that dot gain was an issue that could not be ignored.
Important help in this regard came from research conducted in the United States which became known in Europe linked with the names Yule, Nielsen, and Murray-Davies.
The Yule-Nielsen research appears to be particularly valuable – this explains the "light gathering" phenomenon which is called optical dot gain. Light gathering cannot be seen with a magnifying glass or a microscope but it is nevertheless very important in viewing printed illustrations.
When Felix Brunner asked print experts about the area where dot gain is most pronounced in his seminars at the beginning of the seventies, the – wrong – answer was always "between 90% and the closure dot, the last open printing dots before the solid tone area".
It is indeed not easy to understand why dot gain is most pronounced somewhere else – in the mid-tone area to be precise– and just when the previously isolated screen dots make contact. Dot gain reaches its maximum at this percent stage and then decreases again.
Consequently if one depicts dot gain as a curve, which Felix Brunner was the first to do in the early seventies, it is hill-shaped: the original values of the screen gradations from 0% to 100% on the horizontal x-axis and the values of the changes on the exaggerated y-axis. The exaggeration serves to better illustrate the hill form.
From the early seventies this type of diagram has systematically spread through the industry under the copyrighted designation “ISOCONTOURS* diagram”. It is considered to be one of System Brunner’s core elements and any breach of copyright will have legal repercussions.
The apex of the hill-shaped dot gain curves depend a great deal on the shape of the screen dot: square screen dots make contact at 50%, elliptical ones make contact in two phases – firstly at the ends of the longer axis and then at the ends of the shorter axis. Circular screen dots only make contact at Pi/4, i.e. at the 78.5% stage, an amazing fact that so far no expert has been able to explain right away from experience. This is a question that is asked time and again at System Brunner training courses.
Felix Brunner used the so-called border zone model for better mathematical recording of screen dot changes and assumed with the working hypothesis that changes to screen dots generally take place in border zones of the same width.
Graphic research institutes were rather derogatory about this model, especially the IGT institute in Holland, but Felix Brunner was able to prove to IGT that the border zone model conformed very closely to technical reality.
The model was also very helpful in the development of the micro-measurement elements that were patented by System Brunner as early as 1973 and have been a great success.
In Corippo/Switzerland in the summer of 1975, Felix Brunner calculated numerous border zone area coverage values. One must not forget that back then the logarithmic pocket calculators from Texas Instruments and HP had not yet been invented and so the calculations had to be made using the classic engineer’s slide rule.
The purpose of the calculations was to construct curves that described border zone changes with equal border zone widths. These curves were named ISOCONTOURS, iso for equal and contours for border zones.
ISOCONTOURS* were calculated for different screen dot shapes – square, round, rhombohedral, and also combinations.
The essence of the concept is to compare the theoretically calculated curves with real characteristic curves ascertained by measurement in order to draw process-technical conclusions. Meanwhile this method has proven itself thousands of times over.
With analog offset platemaking the halftone dots also change in conformity with the border zones but instead of spreading like they do in printing they become smaller due to light source undercutting. Print professionals refer to this as "sharpening".
In platemaking negative values of halftone dot area coverage changes occur as a rule. In order to portray these changes as platemaking characteristic curves in an ISOCONTOURS* diagram, they are correspondingly extended downwards.
The advantage of this portrayal is that print characteristic curves and platemaking characteristic curves are drawn symmetrically to the zero axis, which permits the sum of two important worksteps to be established and analyzed.
The ISOCONTOURS* diagram was also soon used for portraying other transfer processes as well, such as the proof characteristic curve with the DUPONT CROMALIN* analog proofing process, and later also for the CROMALIN* digital proofing process. It hardly matters that the ink droplets of the Inkjet technology appear in the portrayal as virtual screen dots.
After offset printing, ISOCONTOURS* diagrams were soon to be used for newspaper printing, gravure, and flexographic printing as well.
Data acquisition was accelerated dramatically by the Zebrastrip* developed by System Brunner and by the use of scanning measuring devices.
The ISOCONTOURS* method greatly improved the understanding and mastery of all printing and proofing processes.
