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FLEXO Magazine : October 2012
Technologies & Techniques New Era in Digital Imaging next gen HD Flexo enhances Tonal Range & shelf impact By ian Hole Sharp. Smooth. Dense. Defined. Consistent. In one form or another, high definition flexo imaging now repre- sents more than 30 percent of all the digital installa- tions worldwide. In the three-and-one-half years since it was introduced, the technology has undergone changes that have considerably extended the tonal range and, therefore, increased the packaging’s shelf impact. Meanwhile, the process has maintained its simplicity, even reducing the number of manual steps. It has been an interest- ing set of flexo plate technology and dot discoveries. BETTER DOTS Imaging resolutions for CTP flexo have changed dramati- cally. In 1995, a 2100 pixel per inch (ppi) dot was used. That improved to 2540 ppi in 1998 and, with the advent of HD flexo, 4000 ppi in 2008. While most people understand that higher resolutions offer greater detail, the most important facet of 4000 dpi is that it can help image more gray levels. Of course, the more gray levels there are, the less apparent stepping there is in an image. Files from the designer are built from 256 gray levels, which is the amount that a byte of information can define in an image file. So, these 256 gray levels are needed to fully reproduce the design work images. Many of us know that the number of gray levels is calcu- lated with this formula: Number of natural gray levels = (PPI/ LPI)2 Where PPI is CTP Imaging resolution, and LPI is job line screen. Using this equation, a 150-line screen at 2400 ppi creates a RIPped file using 256 gray levels. However, many converters are challenging quality levels, testing 200, and even 225 line screens—or even more. A 2400 ppi imager can only gener- ate 144 gray levels at 200 dpi, and 92 at 250 dpi. Also, flexo typically requires compensation on the tonal range—e .g. a mid-tone compensation. Each compensation step further reduces the number of available gray levels. Of course, there is so-called “Super Cell Screening” that can artificially create more gray levels by mixing dot sizes into each other, but they need space to work well. While tonal transitions over a larger area can be smoother, short transitions as well as high-detail tonal changes inside images cannot be fixed—which is why you often can see the individual gray steps. A 4000 ppi imager still creates 256 gray levels at 250 dpi and 400 gray levels at 200 lpi. There are so many natural gray levels available that, even after all the compensation steps, there will still be more than 256 gray levels remaining. This creates outstanding image visual appearance, with high detail sharpness and outstanding image contrast. At higher resolutions, another thing occurs. At 4000 ppi, dot shapes are also better, because there are more pixels to HIGHER RESOLUTION, GREATER DETAIL • Full HD Flexo combines the excellent print behavior of round top dots in the highlights with the print behavior of flat top dots in the shadows • Smooth solids and full highlights extend printable color space • Experience shows that 4000 ppi imaging creates a minimum dot percentage in print that is about 40 percent lower than what is possible with 2540 ppi and leads to much better defined structures • Micro screens allow ink to stay in place, rather than pooling • High definition technology has matured and evolved to extend the tonal range of print - 1995 = 10 percent highlight dot and 1.25 SID - 2009 = 0 percent highlight and 1.4 SID with inline UV - 2012= Full HD Flexo using Inline UV2, can deliver highlight dots to zero along with 1.7 SID Figure 1: Dots formed on files at the bitmap pixel level for a 175 lpi screen for both 2540 ppi and 4000 ppi imaging resolutions. 60 FLeXO ocToBeR 2012 www.flexography.org 3rd Annual Plate/ Platemaking/Sleeve Selection Guide