by clicking the arrows at the side of the page, or by using the toolbar.
by clicking anywhere on the page.
by dragging the page around when zoomed in.
by clicking anywhere on the page when zoomed in.
web sites or send emails by clicking on hyperlinks.
Email this page to a friend
Search this issue
Index - jump to page or section
Archive - view past issues
FLEXO Magazine : June 2014
Status T density can now be tracked using sharpened tristimulus values, either as linear functions or as logarithmic functions. Simi- larly, other print properties of interest that are currently derived from density could be de- rived from the sharpened color values. While it is possible to derive such transformations and convert them via a logarithm transform, these functions will have the same limitations and problems as current ISO status density. Thus, one must ask: Is it reasonable to try to simulate ISO status density with color values? It has been shown that it is possible to use colorimetry to duplicate the successes of densitometry, as well as the requirements for a modern process control system based on visual reproduction. But this approach may not be necessary to establish reasonable process control aims. It is possible to set controls based on colorimetry and not on densitometry. From the plots of density and color differ- ence, it is observed that over the range of typical offset press operating characteristics (film weight of 0.7 -μm. to 1.3 -μm.) the density changes by about 25 to 30 points and the color difference by from 6 to 8 CIE Lab units. A typical press control might be ±10 to ±15 points in density and that corresponds to about 0.1 -μm. film thickness control. That level of control represents a color difference of2to3CIELabunitsorabout1to1.5 CIEDE2000 units. That is a tight commercial tolerance and would provide a visually indistinguishable reproduction, impression to impression. In actual practice, critical print buyers will generally accept up to 2 CIEDE2000 units of difference and begin rejecting prints at about 2.5 units, while complaining that their design intent is not quite right at color differences between 2 and 2.5 units. While density does not measure color, we know that the appear- ance is dependent on the density. If density changes by a few points, then the color changes by a few difference units. So color is a more critical control parameter than density. This means that tolerances on color have to be made carefully, with an under- standing that differences near zero are a long way away from reality and are not pragmatic. This is one of the reasons that process control standards like ISO 12647-2 have fairly broad tolerances for the deviation and variation tolerances. CONCLUSION From the data shown in this article, it can be seen that colorimetric values can be utilized to set process aims and to follow those aims. Image 6: Correlation of ISO Status Density and Sharpened Colorimetric Density JUNE 2014 | FLEXO 43