There are many concepts about density in printing. What are these concepts of density and what are their uses in actual printing? This article makes a brief discussion on this.
1. Concentration
Density is a comprehensive concept, divided into reflection density and transmission density. Taking the reflection density as an example, a beam of light is projected on the surface of the object, the projected luminous flux is lo, and the luminous flux reflected on the object surface is lr, then the reflectivity is: R = lr / lo; the concentration is defined as D = lg (1 / R) .
It can be seen that the greater the D, the lower the reflectivity and the darker the object; the instrument for testing the density in printing is the densitometer.
2. Full version density and tone density
Halftone prints express light and dark levels as a percentage of dots. The concentration of the area where the dot percentage is 100%.
That is the full version density, generally expressed by Dv. In the non-100% dot area, there is a portion covered by ink (its density is called dot density Dv) and a portion not covered by ink (expressed as paper density). The tone density is the density of the paper density and the dot density, which is generally expressed by D1. Therefore, the tone density is also called the density of the dot area and is widely used.
Expand the value of the measured network points. The expansion of dots in printing is inevitable. The dot expansion value should be equal to the percentage of the printed dots minus the dot percentage of the printed film.
The Murray-Davis formula is used to measure the dot percentage of the printed matter, that is: α = (1-1O-D1) / (1-1O-DV) where α is the dot percentage of the printed area and D1 is the order of the measured area Adjust the density, DV is the full version density. The measured dot expansion value can provide pre-compensation parameters for the printing plate.
Measure printing contrast. The formula for printing contrast is K = 1-D1 / DV. D1 is the tone density after 75% dot printing on the film. The greater the K value in printing, the more the dark tone is pulled, and the better the printing quality. The ink supply amount when the K value is the largest is the optimal ink amount for printing control.
By controlling the full plate density DV to control the printing ink supply. We know that the amount of ink supply is the best when the K value reaches the maximum value during printing, and obtaining the full plate density DV at this time can be used as a parameter to control the amount of ink during printing. For example, when the K value of the black plate is the largest, the corresponding DV is 1.65, then the density is generally controlled at 1.60 to 1.70 during printing.
3. Color separation density and color density
In color printing, when the density is measured with a color densitometer for a certain color patch, three values ​​D1, Dg, and Db can be obtained. These three values ​​are called the color separation density of this color patch. The D1 value is the measurement result when the color density meter is added with the red filter, and reflects the absorption capacity of this color block to the red light component in the incident light; the corresponding Dg and Db correspond to the green light and blue light, respectively.
It is often necessary to detect certain characteristics of color inks in printing. For example, to detect yellow ink, the color density can be used to measure the three color separations of the yellow block. It is found that the Db value is large, the D1 and Dg values ​​are small, this is because the yellow ink absorbs most of the blue light and reflects most of the red light And green light. We call the larger Db value the color density of the yellow block. Magenta ink and cyan ink.
The color density of yellow, magenta, and cyan is also called the full-color density of these three colors. In the previous applications 1 to 3, the measurement of full-plate density was mentioned. For the yellow version, the data to be measured is its color density value Db; the magenta version and the cyan version are similar.
Measure the performance parameters of the ink. Commonly detected items include color shift, grayscale and color efficiency. For example, the calculation of the color efficiency of yellow ink is: 1-0.5 × (D1 + Dg) / Db. Among them, D1, Dg, Db are the three-color separation density of yellow ink, Db is its color density or full-plate density, and the other two inks are analogous.
4. Decomposition concentration and synthetic concentration
From a mathematical point of view, these two concentrations, that is, the principle of superposition of concentrations (see Figure 1).
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D1 in the figure represents the density of the first layer of ink color, D2, 1 represents the density of the second layer of ink superimposed on the first layer of ink, D1 + 2 represents the combined density of the two ink layers. Then there is D1 + 2 = D1 + D2, 1 where D1, D2, 1 is called the decomposition concentration, and D1 + 2 is called the synthesis concentration.
Measure the overprint rate of ink. The overprint rate of the ink expressed in density is f = D2, 1 / D2. Where D2,1 represents the density of the second layer of ink color superimposed on the first layer of ink, D2 represents the density of the second layer of ink printed on paper alone. To measure D2 alone, 1 is impossible, according to the above concentration superposition formula D1 + 2 = D1 + D2, 1 can be known as D2, 1 = D1 + 2-D1. Therefore, the formula can be changed to f = (D1 + 2-D1) / D2. Measure D1, D2 and D1 + 2 with the density meter to calculate the overprint rate of the second color ink on the first color ink. Of course, a color filter that complements the second color should also be added for the measurement of mutual overprinting of color inks.
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