Measuring Dots - Mechanical vs. Reflective

Measuring printed dots have become commonplace in flexographic printing. The reasons ranging from obtaining information to perform cutbacks on film negatives, to running control targets to maintain the integrity and quality of a job throughout the printing process. Some measuring devises take dot measuring to a whole new level, measuring the circularity of dots, how much the dot is slurring, and even the density of ink as it varies across the face of a dot. But one thing that dot measuring devises or (IASs) image analysis systems do have in common is that they measure dots or images in one of two fashions.

When measuring a screen area whether it is a patch on a tonal scale, a vignette, or a process area, reflectivity is a common method of calculating dot percentage. When a reflective measuring devise reads a sample in reflective mode it transmits light to the substrate. The ink absorbs some of this light while light that is not absorbed reflects back to the measuring devise. The devise then using a mathematical equation calculates the dot size or percent of coverage given the difference between absorbed and reflected light. This is an optical density calculation.

When measuring an area using a mechanical method, the information obtained is a precise measurement of physical dot size. In some cases the measuring devise will not only give the observer a dot percentage, but also a read of dot diameter and perimeter measurement, along with standard deviations within each of theses measurements, given the total number of dots analyzed.

What is a better method?

When it comes down to which method of measuring is better, it's really rather simple. When using a reflective method of measuring you must remember that as viscosity of inks changes, so do the ink densities. Therefore if there is a density change, a reflective method will absorb different levels of light. So what does this all mean? It means when your viscosity raises the dot percentage increases, when viscosity is lower, the dot percentage decreases even when there is no physical change in size to the dot. So when using a reflective method an operators control or run target may seem to have too much or too little impression given the predetermined dot percentage, when in actuality it may be an issue pertaining to dot density.

Using a mechanical method allows the operator to monitor physical dot size, while using a density feature to analyze the density or ink strength. By breaking the two issues down, it allows the operator to be more precise in operating the press and maintaining quality.

For more information on this issue, please contact: info@teamflexo.com

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