METHODS OF CALCULATION OF TVI

Have you ever wondered what printed paper is made of? which we use in our daily life ,like currency, newspaper , books , magazine , food container etc. this all things are made up of tiny things named dot .During the printing process dot (halftone) is most important variable because every print is made up of many tiny sized dots. The change in the size of dot is called dot gain. Dot gain is defined as increment in half dot area which was printed on substrate as compared to the dot area which was generated on film or plate. There are total two types of dot gain – Physical (mechanical) dot gain and Optical dot gain. In offset printing, most of physical dot gain occurs before the ink touches the paper surface. Reasons behind the physical dot gain is the higher pressure between the plate to blanket and blanket to impression cylinder and also increment in ink film thickens, ink, ink quantity, temperature in inking unit, dampening solution. Optical dot gain originates from light scattering inside the substrate, causing light exchanges between different chromatic areas. Dot shape and paper surface coating are the factors affecting optical dot gain.

To avoid dot gain during the printing process, dot size must be measured. Densitometer is one of the most important and widely used instrument to measure dot gain. There are two types of densitometer, transmission densitometer and reflection densitometer. Working method of the transmission densitometer measure the amount of light that is transmitted through a transparent material such as film base. Reflection densitometer measure the amount of light reflected from a printed image and densitometer use Marry-deivies and Yule-nielson equation to define the dot size.

In printing process, paper characterization and dot shape effects physical and optical dot gain. Coated paper has high physical dot gain because of dot spreads more on the surface of the paper and the uncoated paper absorbs ink more than coated paper. So that uncoated paper has low physical dot gain but the optical dot gain is vice-versa. Total dot gain is higher for uncoated paper as compared to the coated paper so that to avoid dot gain, coated paper is used. Thus, it provides good printability. We are seen that most dot gain in middle tones, middle tone is basically 30% to 70% dot area covered. Dot size and arrangement of dot help to avoid dot gain And also AM and FM 2nd big halftone techniques are avoid to dot gain specially in middle tones.

During the printing process , ink film thickens and ink viscosity level are affecting the dot size. It is Important to the analyzed tone value results it is very difficult to determine which ink viscosity level to application is the most efficient because, different ink viscosity levels have different impacts on tone value. Dot gain is caused by ink spreading around halftone dots basically there are three types of ink viscosity level are used low, normal and high. Ink viscosity is a contributing factor with coated paper, higher viscosity ink can resist the pressure better than lowest viscosity ink. Magenta ink produces the best TVI value for low ink viscosity level. Normal ink viscosity level is best for cyan and yellow. High ink viscosity level generates the best results for black ink.It seems that the densitometer has a slight problem measuring small densities or small dot areas. 0% dot means that no image was here. The Transition Area (TA) approach is shown to give a good discrimination between printing ink and the printing media. Now a days we can measure dot area by densitometer but when densitometer was not found the dot area was measured by the physical measurement methods. Dot area reading on the press sheet shows greater difference when comparing densitometer and physical measurements because the densitometer reading account for change in dot size and also depend on optical phenomena whereas physical measurements account for only the physical size of the dot. And it should be noted that the physical measurements of the dot area indicate accurate tints at 40% to 80%. The densitometer readings and physical measurements were within 1% of each other at 30% to 70% tints, in agreement at 20% and were 4% different at the 90% tints. But densitometer use is easier and user friendly compared to physical method.

If we are talking about working principle of densitometer, densitometer use MD (Marry-deivies ) and YN (Yule-nielson) equation. But which one is better? For this, we use three different tints (i.e., 25%, 50%, 75%) on the printing plate. Then with the help of densitometer we measure all dot size with both equation and compare with OICS (optical image capture system). These results suggest that the Yale-Nielsen reading were better correlated with the OICS readings than the Marry-Davies readings for all three tints. In densitometer YN equation, with an appropriate ‘n’ factor is a better method for measuring dot area on offset printing plate.

Mr. Fenilkumar R. Pethani

TE Printing

PVG COET & GKPIM

REFERENCES

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6. Wedin, M., & Kruse, B. (1995, October). Mechanical and Optical Dot Gain in Halftone Colour Prints. In INTERNATIONAL CONGRESS ON ADVANCES IN NONIMPACT PRINTING TECHNOLOGIES (pp. 500-506). IS&T AND SEPJ

7. Adamcewicz, Justine E., (1994). "A Study on the effects of dot gain, print contrast and tone reproduction as it relates to increased solid ink density on stochastically screened images versus conventionally screened images"