We consider the methods of elemental recording of raster binary images on photographic or form material for systems of printing reproduction. In the process of tuning and comparing such processes, it is necessary to evaluate their sensitometric and structural-metric properties, which determine the quality of reproduction of the minimum image elements - raster points and strokes on the material carrier. The use of traditional sensitometry and structural analysis, which are used to calculate photographic processes, does not allow such a full assessment due to the difference between format and elemental recording methods, semitone and binary images, photographic and form materials. The peculiarities of the methods of reproduction of an image elements with elemental recording revealed to be expedient to be used at technological adjustment of printing reproduction system processes.
The basic working properties of the image elements on the material carrier are determined by the gradation parameter. As a gradation parameter, optical density [1, 10] is often used to characterize the property of photographic forms to pass or delay radiation in the molding process, or the relative thickness of the printing layer. The register layer on the printing elements, which characterizes the property of printing forms to transfer ink in printing process [2]. For binary images, the gradation parameter has two levels - the upper lu and the lower ll, hich, depending on the polarity of the process, correspond to the active elements, purposefully formed radiation, and passive formed in the absence of radiation. If the active elements correspond to the lower level, then the process is positive (Fig. 1a), if to the upper one then the process is negative (Fig. 1b). The sensitometric curve (Fig. 1) shows the threshold exposure Hthr to which the basic working property of the passive image elements is provided, and the minimum exposure Hmin from which the basic working property of the active elements is provided. If Hmin Hthr we get a threshold or step change in the gradation parameter.
l Fig. 1. Sensitometric curves of the elemental recording process: a - positive process; b - negative process
Sensitometric process curves will be used to identify the features of methods of reproducing image elements with elemental recording. 2
The basis of the developed system of structurometry of the elemental recording process is based on the normalized functions of distribution of effective energy density: for the pixel - the function of pixel reproduction (FPR) and for the edges of the energy plate - the boundary functions in the direction of personnel (BFP) and line scan (BFL) [3, 11].
Reproduction functions are functions of a discrete argument. We use the following notations: P(y, x) – for FPR and Ek y – for BFP. The x and y arguments related to the line and frame scans have a definition area [4]: 1 b : x, y b, b d, ,0, ,b, where d – sampling step N, b N. N
Kernel Energy Density - The fraction of energy density in the center of the image
element from the maximum level of the energy plate obtained while recording pixels
in all positions, and pixel kernel functions - the dependence of the kernel energy
density on the number of pixels forming active and passive elements, hka k and hkp k .
For the purpose of accuracy of estimation, it is advisable to use a balanced raster
structure [5], for which on the matrix of balance points with recorded pixels, active
raster points are formed, and unwritten ones form passive points, with points in light
and dark regions being formed by identical and round shape pixels. For the balance
structure, formulas for calculating the energy density of the kernel for k-pixel image
elements can be written using FPR:
hka 1 P 0, 0, hka 2 2P 0;0,5 etc.
(
Depending on the FPR, a constant unit level or periodic oscillations of the energy density level in the direction and with the step of the frame scan can be formed on the energy plate (Fig. 2). In this case, a maximum is formed in integer coordinates hdmax 1 (rationing to the maximum), and in half offset coordinates, – minium hпmлin (Fig. 2, b). The average level of energy density on the plate is equal to that specified Rn
2 1 hdmin in the exposure system Hw , then, in the transition from relative values to absolute – is the conversion factor to the average level. In the presence of oscillations it is necessary to distinguish two values for the threshold and minimum exposure - local and average. The first is related to the action of energy density in the maximum or minimum, the second - with the average value of the energy density on the plate and the exposure specified in the system.
For offset printing forms, the main working property of the whitespace elements is provided in the complete absence of the printing layer ll 0 , and printing elements at full thickness (lu 1) [6]. Then the main working property of passive elements with exposure increasing begins to break at the maximums in which the energy density operates at Htlhorc , with system exposure Hthr :
lows in which the energy density operates at H mloicn , at exposure Hmin :
Exposure values Hthr and Hmin can be determined experimentally, but values
Htlhorc and H mloicn can be calculated by the formulas (
Starting with Hmin the basic working properties of the long-lasting elements of the image will be provided, and this exposure can be considered to be minimally sufficient to reproduce them.
Image elements in the center of which the basic working property is provided are considered reproducible and form a pixel reproduction range of kа k p , where k а – is the number of recorded pixels that form the minimally reproduced active element of an image, k p – is the number of unwritten pixels that form the minimally reproduced passive image element. The pixel reproduction range is related to the reproduction range of tone values [7], which is one of the basic parameters that determine the visual properties of printed product. The pixel width equals p kа k p 1 , where p – is the number of pixels in a raster cell, and increases with decrease of kа k p .
We will investigate the factors that affect this amount and width of the range. In the
center of the k-pixel active image element the energy density acts like this:
hkа k RnHw , which for its reproduction should be not less then H mloicn , then,
considering (
H mloicn d
hminHmin RnHw Hw
In the center of k-pixel of the passive element, the image gives energy density
hkp k RnHw , which should be smaller then Htlhorc to recreate it, and considering (
Analysis (
Let's consider the graphs of pixel functions for active hkа k and passive elements hkp k , which are piecewise linear, in one coordinate system (Fig. 3). The line hmin at the intersection with the curve hkа k will give us the point whose abscissa, when rounded, is greater than the number of pixels k а , that form the minimally reproduced active element. Similarly, the line hthr at the intersection with the curve hkp k gives us the point whose abscissa, when rounded to a greater number of pixels k p , that form the minimally reproduced passive element.
Researching the influence of the values hthr and hmin , де hthr , hmin 0,1 and hthr hmin , the sum of kа k p allows us to draw the following conclusions. The value of kа k p depends on the width of the interval h hmin hthr and its position along the y-axis. The smaller the interval h , the smaller the sum of kа k p and the wider the pixel recreation range. At a fixed value of the interval h , the minimum kа k p corresponds approximately to its central position on the segment of the y-axis, i.e. hmin hthr 1 or Hw Hthr hdmin Hmin . In this case, we obtain a symmetric pixel range with equal boundary values kа k p . Some incertitude in the width of the range is introduced by the piecewise-linear nature of the functions, the angular coefficients of the linear sections of which depend on the FPR and the positions of the pixel recording.
The maximum possible pixel reproducing range can be obtained when
hmin hthr 0,5 , which corresponds to the threshold sensitometric curve
Hmin Hthr . The maximum possible range for a given raster structure is
completely determined by the central part of the FPR.
Judging by (
Hw posure in the system. However, as the exposure increases, the interval from the central position down the ordinate is shifted and k а k p increases. As a result of these factors, in this case, the maximum range is asymmetrical with a smaller value of k а .
The piecewise linear nature of the pixel functions and the rounding of the pixels to integers results in the uncertainty of the bandwidth of up to two pixels. Also, the bandwidth is affected by the lower durability of the small printing elements not taken into account.
Let's investigate the effect of the nature of the FPR distribution on the reproduction of small image elements. In the analytical method of calculating FPR as a factor controlling the distribution proposed analytical method of calculating FPR. As a factor controlling the distribution of the energy density of the FPR, we take r0 – the laser beam radius in the area of constriction at the level e2 at constant scattering parameters, selected to approximate the experimental data obtained for offset heat-sensitive plates [8].
As r0 , decreases , the energy density fraction in the central part of the FPR
increases, the values of the pixel function hkа k increase accordingly, and the values hkp k
decrease (table 1), increasing the pixel range to full k а k p 1 . The energy density
values in the center of the 1-pixel active and passive elements are calculated by (
However, as r0 decreases, the amplitude of the oscillation level of the energy plate
increases (columns hdmin and Rn of table 1), which can lead to the inadmissible
partitioning of image elements into parts. The energy density at each point is formed by
pixels that fall into the FPR definition area, and the minimals are formed in the
coordinates of the frame sweep offset by half a step from the line item positions. Then the
smallest value of the energy density is formed in the center of the 2-pixel element
with the frame placement of pixels, for the calculation of which in formula (
R H n w H w1а
ments of FPR: hmаin 2 2L0,5; 0 . Starting with r0 0,8 the energy density at the
center of such an element begins to fall (Table 1). From (
It should be added that in the case of 4 × 4, a pixel consisting of 16 subpixels is used. With decreasing of r0 the exposure of H w1а decreasesfrom which the 1-pixel active element is reproduced, and the exposure of Hw1p , to which the 1-pixel passive element is reproduced (the table shows the ratio of H w to local values). However, with r0 0,8 the minimum sufficient process exposure begins to be determined by the increased exposure of H int integers required to maintain the integrity of the image w elements (values highlighted in the table in bold). The need to increase exposure contradicts the lack of activity that is characteristic of, for example, the technology of elemental recording of offset printing forms. When using a pixel consisting of subpixels of a larger recording extension [9], a constant single level of the energy plate is formed, and the full pixel range is reached without increasing the exposure (row 4 × 4 of the table). However, this approach requires specific hardware solutions. 4
As a result of the researches the following features of reproduction of image elements in the processes of elemental recording were revealed: To obtain a larger pixel recreation range, the Hthr threshold and the minimum Hmin exposure should be as close as possible to each other. The maximum possible range is reached in the case of a threshold sensitometric curve Hmin Hthr . Choosing a working exposure Hw in a system close to the minimum Hmin results in poor reproduction of the active elements. The pixel range with equal lower and upper bounds is obtained at Hw Hthr hdminHmin , but it is not maximal. The maximum pixel range is shifted toward larger exposures, with active elements reproducing better than passive ones. The result may be due to the lower resistance of small print elements to processing. The integrity of the image elements can be controlled in the center of a 2pixel pixel element. Reducing the laser beam radius r0 allows us to obtain the full pixel range k а k p 1 , but this increases the oscillation amplitude of the energy plate and, as a consequence: increases the minimum Hmin , and decreases the threshold Hthr of the exposure; the integrity of the image elements is impaired and the condition of its preservation determines the minimum sufficient exposure in the system. This increases the working exposure in the system, which may be unacceptable with insufficient activity, which is characteristic, for example, for the technology of elemental recording of offset printing forms. The use of a pixel consisting of sub-pixels of greater recording expansion allows to obtain the full pixel range at a constant level of the power plate.
It is expedient to use the revealed features of reproduction of image elements when designing and technological tuning of processes with elemental recording. 5
Methods of elemental recording of bitmap raster images for photographic and form material for printing systems have been considered. The expediency of maximizing the minimum and maximum exposures with each other is shown, but the maximum pixel range is shifted toward larger exposures. To get the full pixel range, it is advisable to use sub-pixels and reduce the laser pro-radius.