The best reproduction of tone (level) (2)

Second, the best tone reproduction of black and white images


1. Material and printing parameters are initial conditions. The best gradation reproduction is the most important process for obtaining high quality black and white or color printed images. Most of the bad prints are caused by issues with tone reproduction.

The variables affecting the quality of the reproduced copy of the printed image are mainly the quality of the original image, the ink characteristics, the density of the solid ink, the number of screens, the characteristics of the paper, the shape of the dots, and the print characteristics parameters (such as dot gain, dot distortion, overprint, and stencil). Wait.


Table 3-1 Density range of originals and prints



Black and white prints color prints
High light 0.06 0.06 0.20

Dark tone 2.00 2.00 2.90

Density range 1.94 1.94 2.70


Table 3-2 Reproducible Density Ranges for Different Types of Paper



Coated paper Non-coated paper Newsprint
White paper 0.06 0.10 0.20

Only black ink 1.60 1.40 1.20

Tricolor Printing 1.50 1.30 1.20

Four-color printing 2.00 1.80 1.40

Black and white copy maximum density range 1.54 1.30 1.00

Four-color replication maximum density range 1.94 1.70 1.20

The most obvious requirement for perfect image copying is that the copied image is visually compatible with the original, but in fact it is impossible in most cases. Table 3-1 and Table 3-2 list typical manuscript density and print density. Obviously, in most cases the density of the original document is greater than the density range of the printed image, and as the quality of the paper decreases, the range of the reproducible density decreases, as does the saturated color. Materials and printing parameters are the initial conditions that restrict the quality of image reproduction. Without the actual printing conditions and material conditions, the tone reproduction does not have high quality.

2. Determine print characteristics by printing experiments and measurements. Since the printed image cannot usually be matched with the density range of the original, a solution to this problem is to compress the gradation range of the original to match the density range that the paper and the ink can reproduce.

The so-called gradation compression is based on the compression and reduction of the original tone, print copy, but the appearance of the printed image looks normal. Whether printed on coated paper or printed on newsprint, gradation compression is an important step in determining copy quality.

The job of optimizing tone reproduction should start with analyzing the printing conditions. The relationship between the dot size on the platemaking film and the corresponding grey tone on the printed sheet produced by it must be sought. This requires the measurement of the standard printed sample and the film from which the sample was produced. When analyzing printed dots on paper, dot density values ​​are measured instead of dot size values.

In order to compare the dot size of each tone area on the film with the corresponding density on the print sheet, it is more convenient to use the gray scale. However, the number of mesh lines on the ladder must be the same as the number of mesh lines actually printed. There should be at least 10 steps on the network. Specialized test strips can also be used, such as the RIT symmetry scale (see Figure 3-3) to analyze printing characteristics. This ladder consists of 25%, 50%, 75% dot and field blocks, 50% horizontal and vertical lines, and 50% dot blocks are divided into two types of dots: diamond and square. The RIT test strip also includes the RIT highlight/darkness scale. Symmetrical scales contain fewer tonal values, but they can be used to study the five most important tone areas of a printed image.

The dot coverage for each rung on the film can be measured either with a dot coverage meter, or with a standard transmission densitometer, with the densitometer zeroed on the substrate, and with care taken on the positive and negative measurements (if used The RIT scale does not need to measure the dot area because the data marked on these blocks is accurate.

Measure the density of each ladder on the printed ladder on the printed sheet: For ease of analysis, the reflective densitometer should be zeroed and calibrated first, and then zeroed on the measured paper, taking care not to change the adjustment of the high scale end of the densitometer . When measuring on a printed sheet, although the measured value already includes the dot gain, but do not worry, what is done here is the integral density measurement, which determines the average density value formed by dots and spaces together. It is not necessary to use the method described here to decide Print network coverage value.

According to the result of the measurement, draw the curve shown in Figure 3-4, and call it the print characteristic curve. The abscissa indicates the dot coverage on the film, and the ordinate indicates the corresponding printing density. A smooth curve is used to connect the corresponding points instead of a polyline. When the paper type is different, the curves are different. This analysis method is applicable to any printing method. For photo gravure and positive PS plates, the abscissa is replaced with positive data.

3. Determine the best tone (level) curve based on the original features. After collecting the above print data, it is possible to determine the best possible tone reproduction within the limits of the paper brightness and the maximum printing solid density. For this purpose, a curve is drawn to describe the relationship between the imaginary normal manuscript density and the print density per step of the gray ladder. The curve should have the shape shown in FIGS. 3-5, with the horizontal axis representing the density of the original and the ordinate axis representing the density of the printed copy.

If a normal manuscript is perfectly reproduced, then each tone on the scale can be accurately reproduced. In this case, a 45° straight line can be used to express the ideal contrast. The value of the contrast γ can be calculated by the following formula:


γ=y/x


Where: x represents the horizontal axis coordinate and y represents the coordinate value of the vertical axis corresponding to x. When the angle between the replication curve and the horizontal axis is less than 45°, the print replica loses contrast. Occasionally, the replica has a high contrast in the highlight and the contrast in the dark tone is low.

The limiting factor is the printing conditions: The optimal copy cannot exceed the maximum print density.

The quality of gradation reproduction depends on the gradation compression and correctness of the original. The contrast of the best gradation reproduction is generally lower than that of the original, and therefore always a straight line of less than 45°.

When drawing the gradation reproduction curve, first determine the correct dark tone density of the original and the highest reproducible printing density, and let the perpendicular and horizontal lines intersect at a point from these two points, which of course lies below the theoretical optimal reproduction curve.

The highlights of the printed copy are visually more important than the darker notes, and slight deviations in the highlights can be detected by the eye. If a line is drawn from the zero density to the highest density of the copy, the contrast of the bright tone becomes too low. This is not to say that the dark tone is not important, and the dark tone zone cannot be leveled so that all details are lost. Clearly, a compromise has to be made.

To construct a basic replication curve, select a point between the ideal and the actual achievable dark tone, and then use a straight line to join this point with the zero point (Figure 3-5). From the density density of about 0.80 to the actual achievable dark tone point, a smooth curve is combined to form the best tone reproduction curve. This curve becomes flatter in the dark tone area.

Because this method of optimizing tone reproduction is based on actually printed data, tone reproduction can be standardized.


When copying the best tone, there are more compressions in the shadow zone than in the bright tone zone. Under ideal printing conditions, even if there is a slight deviation in the dark tone on the film, it may not be visible on the printed copy image. Because of this variation there is only a small deviation in the printed image. However, in order to obtain the highest possible contrast in the highlight area, gradation compression is relatively small.

Next, the dot coverage for the halftone dots is determined so that the best reproduction of the tone can be achieved under the given printing conditions. To this end, Figure 3-4 and Figure 3-5 can be combined to form the conversion shown in Figure 3-6. The lower left quadrant in the figure is the desired dot coverage curve, which expresses the relationship between the dot coverage of the copied film and the density of the original. In order to obtain this relationship curve, a 45° conversion money can be drawn in the lower right quadrant so that the density of the manuscript on the abscissa axis can be converted to one below the vertical axis.


In order to obtain the dot coverage curve shown in the lower left quadrant, some control points can be selected on the print characteristic curve of the upper left quadrant, such as dot coverage of 5%, 25%, 50%, 75% and 95%. 5 control points , from the five points down the vertical line to the lower left quadrant. Still use this 5 points as the starting point to make the horizontal line to the right intersect with the best copy curve, and then intersect the conversion money with the perpendicular line from the intersection point to the right lower quadrant. From the intersection point to the left, make a straight line in the lower left quadrant and the other direction The vertical lines drawn from each other intersect to obtain five points. These five points of intersection are combined with smooth curves to obtain the best dot reproduction curve of the film.


Source: Bison

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