Photographic quality monitoring in photocopying system
2018-06-25 13:00:21
With the development of prepress technology, the film output (including self-discharge and self-export, external output and remote receiving and output, etc.) undertaken by each newspaper company and printing factory photocopying center has become increasingly heavy. However, due to different standards and requirements, the quality of the film output by each unit is also uneven, which directly affects the quality of printing. For example, some films have too low transmission density (some are less than 2.0). The images are gray and not black; some are yellowish and poor quality; some dark shadows are blurred, words are stylized, and they lose their handsomeness... if used directly in printing. , the printed image is blurred, the lack of writing style, can not reach the basic requirements of quality, all these are not suitable for printing or film does not meet the requirements of the printing process. It can be seen that to ensure the printing quality, the output quality of the film must be monitored so that the film quality can meet the printing requirements.
Film quality monitoring includes both optical quality monitoring of the film and process quality monitoring.
The so-called optical quality refers to the film's transmission density, fog density, dot blackness, dot halo, etc., which directly determine whether it is suitable for printing. Process quality is some of the higher-level characteristics of the film, such as the number of lines of the mesh, the accuracy of the percentage of outlets, and the quality of outlets. The above indicators are closely related to the printing process and printability, and also determine whether the ideal reproduction effect can be obtained. High-quality films should be films that are both optical quality and process quality.
First, the optical quality of the film
The optical quality indicators of the film include optical density in the field, density of fog, blackness of dots, and virtual halo of dots. In addition, the appearance quality of the film should be as clean and tidy as possible, without scratches and dirt.
The so-called optical density refers to the degree of light blocking of the material, that is, the degree of blackening of the image formed after the photosensitive material is exposed and washed, and is usually represented by the logarithm of the reciprocal of the light transmittance.
1. The meaning of density in the field, density of fog, density of dots, and halo of virtual dots
(1) Field density
The offset printing process requires that the dots (including the ground) on the film be completely transferred to the printing plate, so that the covered portions (ie, the graphic points) on the film cannot be exposed to light by the photosensitive layer of the printing plate, so dots (including the field) Need to have enough light blocking ability.
Field density is measured as the average density in the field.
(2) Fog density
Fog density can be seen as the density of exposure equal to zero. That is, the transparent portion (blank portion) on the film must have a sufficient amount of light to pass through, so that the photosensitive layer of the printing plate is sufficiently light-sensitive and dissolved during development, so the film must have sufficient light-transmitting capability.
Fog density is measured as the average density of blank areas of the film.
(3) The ideal halftones for the darkness of the network and the virtual halo of the network should be absolutely clear, and the blackness within the plane covered by the dots is equal. In fact, due to the existence of light scattering and refraction phenomena, the dots formed by different films have varying degrees of presence of virtual halo edges (ie, the blackness at the edge of the dots is less than the blackness at the center portion of the dots) due to the phenomenon on the film. The presence of virtual halo edges in the outlets results in the differences in the size of the graphic versions due to the fact that the light blocking ability of the edge areas of the screens is weaker than the central area of ​​the screens, and the size of the PS version of the Internet point and the film dot are changed. In order to reduce this change, the virtual halo edge should be made as small as possible, ie, the edge of the halftone dot should have sufficient sharpness.
The blackness of the dot is represented by the concept of dot density at the center of the dot; the virtual halo of the dot is expressed by the density of the edge of the dot.
2. Optical quality inspection control standards
At present, the optical quality of film is mainly determined by measuring the optical density of each index. When measuring, the density of the field and the density of the fog are measured by an ordinary transmission densitometer. The density of the center of the network is measured by a micrometer, and the dot is observed. For edge definition, dot test strips and a magnifying glass are used.
Since the film's on-site density, fog density, dot blackness and dot halation are mainly determined by the film base used for the film and the photographic performance of the photo-sensitive film, the photo-discharge center cannot be controlled. In order to ensure the film's on-site density and fog, the photocopy center can repeatedly test according to the photosensitivity of the photo-composing machine when selecting the film supply, and select the suitable film.
On January 1, 2000, China formulated the equivalent international standards for CY/T30 (offset printing plate production). This standard unifies the basic conditions, operating specifications, and quality requirements for printing, and stipulates the quality requirements for the original color separation film. It is highly operational and has a strong guiding role in practice. According to the standard requirements, the film quality should be as follows:
(1) The density of the center of the halftone film film dividing dots should be greater than the density of the blank film (the film density and the fog) 2.5 or more;
2 The transmission density of the blank film (base density plus fog density) should not exceed 0.15;
3 The density of the central part of the blank dots (that is, the blank part between the dots) can not be greater than the density of the large blank part + 0.1;
The dot density of the 4 dot network can not be greater than 1/40 of the width of the cable, and the dots cannot have obvious cracks.
The above theoretical requirements, but because the general printing companies do not have micro-density meter and other test equipment, in order to facilitate the evaluation of the density of network centers, the standard made a note:
In practical applications, the density of the center of the dot can be measured using the on-site density. If the density of the large-area solid is greater than the density of the transparent film, the density of the center of the dot is acceptable. That is, as long as the density is measured by an ordinary transmission densitometer, it is more than 3.5 or more than the base density. If there is no transmission densitometer, an experience method is given in the appendix of the standard: observation with a thin line icon and a 60-100 magnification magnifier. If there is no measurement bar, a method is also given, which is to observe with two color plates and a high magnification magnifying glass. These empirical methods are useful in practical work.
The dot density of the dot (control film imaginary haze) refers to the average distance between the contour lines corresponding to 10% and 90% of the dot center density, and the line width refers to the reciprocal of the dot line number. For example, for a 100-line/inch cable, where the cable width is about 0.254mm (25.4mm divided by 100), the edge width of the dot should not be greater than 40% of 0.254mm, ie, about 0.006mm (6 microns). In practical applications, it is often estimated by comparing the width of the thin line on the control strip.
The optical quality of the film depends on the output quality of the dispensing system itself, the properties of the film material, and the processing conditions. In practical work, we should use various means as far as possible to improve these factors and maximize the requirements of industry standards. This will be conducive to the stability and improvement of quality, but also make the export market "have laws to follow," and eventually move closer to internationalization.
Second, film quality control
The process quality of the film includes the number of screen lines, dot shapes, screen angles, dot percentage accuracy, and accuracy of four-color overprinting selected for the output. These characteristics of the film are closely related to the printing process and printability, that is, corresponding to different printing materials and copying emphasis, relatively different parameters are selected for output. If the optical quality of a film is good, improper selection of output parameters can also result in poor printing quality. Obviously, the process quality of the film is not appropriate, and it cannot be considered as a high-quality film.
1. The number of mesh lines
The number of mesh lines: the number of one-way parallel lines within a unit area. The higher the number of lines, the smaller the dots and the finer the prints. However, it is not that the higher the number of lines is, the better, but it should be selected according to the printing and material process requirements. The theoretically calculated results show that the fine screen is much more sensitive to changes in printing than the coarse screen. When the quality of the paper cannot reach the corresponding standard, the accuracy of the machine is not high, the quality of the ink is poor (grain is coarse, etc.), and the pressure is large and the dot enlargement rate is high at the time of printing. Color, etc., so the number of lines in the screen should match the actual print. For example, newspapers printed on the screen should be 100 lines/inch or so. If the printability conditions are good, if the quality of the paper is high and the process error can be effectively controlled, it can be higher. The number of screens printed by color newspapers in Hong Kong and Taiwan is higher than that of the mainland. The number of screens available is 133 lines per inch.
It is worth noting that, at a certain resolution of the imagesetter, the number of meshes is inversely proportional to the level of the image. The number of network lines is high, and the number of levels is relatively small, but vice versa. Newspaper printing should generally ensure that each color version has at least 100 or more levels. In this case, if the number of meshes is 100 lines/inch, the resolution of the imagesetter should be at least 1000 dpi or more. In order for both to be taken into account, the resolution of the imagesetter should be sufficiently high.
2. Dot shape
Dot shape, ie the geometry of a single dot. The effect of dot shape on the copying effect is not as obvious as the number of meshes, but the shape of different dot shapes is different due to the change in pressure (percentage of dots). Based on this, for the key adjustment range to be replicated, the shape of the dot which has the skipping phenomenon in this range should be avoided as much as possible. The hops occurring in different shapes of dots are usually:
Square dots are at 50%, diamond (elliptical) dots are at 35% and 65%, and circular dots are at 78.5%. Therefore, when dealing with different images, the characteristics of the dot should be taken into account. When selecting the time, it should be selected, high-profile, and avoid the jumping zone of the dot.
3. Web corner
The net angle refers to the angle between the vertical line and the horizontal line at the center of the dot. It reflects the orientation of the dots. When printed in monochrome, the human eye has the best visual effect on a 45 degree screen angle. In the case of multi-color overprinting, due to the inaccuracy of the overprinting of the dots, there may be a phenomenon of "rhizome". Selecting a suitable combination of mesh angles is one of the methods to reduce the occurrence of moiré. Under normal circumstances, four-color printing will be the main color at 45 degrees, the screen angle is:
Based on characters, CMYK takes 15, 45, 90, and 75 in order;
Based on landscapes, CMYK takes 45, 15, 90, and 75 in order.
4. Output percentage accuracy
The dot percentage accuracy refers to the error of the percentage of dots on the film after output and the percentage of dots set before the output. For a particular ink, a CMYK color dot combination determines a particular color. Ideally, the percentage set by the front-end processing is output to the film with accuracy. However, in reality it is inevitable that there are errors and must be strictly controlled. If the dot expansion on the film is very large, it will inevitably cause the screen to become dark, dull, and refreshed after printing. The reason for the error is mainly due to factors such as the quality of the discharge, the performance of the photosensitive material, and the process of washing. Among them, the quality of the discharge and discharge of the imagesetter is the most important factor.
Film quality monitoring includes both optical quality monitoring of the film and process quality monitoring.
The so-called optical quality refers to the film's transmission density, fog density, dot blackness, dot halo, etc., which directly determine whether it is suitable for printing. Process quality is some of the higher-level characteristics of the film, such as the number of lines of the mesh, the accuracy of the percentage of outlets, and the quality of outlets. The above indicators are closely related to the printing process and printability, and also determine whether the ideal reproduction effect can be obtained. High-quality films should be films that are both optical quality and process quality.
First, the optical quality of the film
The optical quality indicators of the film include optical density in the field, density of fog, blackness of dots, and virtual halo of dots. In addition, the appearance quality of the film should be as clean and tidy as possible, without scratches and dirt.
The so-called optical density refers to the degree of light blocking of the material, that is, the degree of blackening of the image formed after the photosensitive material is exposed and washed, and is usually represented by the logarithm of the reciprocal of the light transmittance.
1. The meaning of density in the field, density of fog, density of dots, and halo of virtual dots
(1) Field density
The offset printing process requires that the dots (including the ground) on the film be completely transferred to the printing plate, so that the covered portions (ie, the graphic points) on the film cannot be exposed to light by the photosensitive layer of the printing plate, so dots (including the field) Need to have enough light blocking ability.
Field density is measured as the average density in the field.
(2) Fog density
Fog density can be seen as the density of exposure equal to zero. That is, the transparent portion (blank portion) on the film must have a sufficient amount of light to pass through, so that the photosensitive layer of the printing plate is sufficiently light-sensitive and dissolved during development, so the film must have sufficient light-transmitting capability.
Fog density is measured as the average density of blank areas of the film.
(3) The ideal halftones for the darkness of the network and the virtual halo of the network should be absolutely clear, and the blackness within the plane covered by the dots is equal. In fact, due to the existence of light scattering and refraction phenomena, the dots formed by different films have varying degrees of presence of virtual halo edges (ie, the blackness at the edge of the dots is less than the blackness at the center portion of the dots) due to the phenomenon on the film. The presence of virtual halo edges in the outlets results in the differences in the size of the graphic versions due to the fact that the light blocking ability of the edge areas of the screens is weaker than the central area of ​​the screens, and the size of the PS version of the Internet point and the film dot are changed. In order to reduce this change, the virtual halo edge should be made as small as possible, ie, the edge of the halftone dot should have sufficient sharpness.
The blackness of the dot is represented by the concept of dot density at the center of the dot; the virtual halo of the dot is expressed by the density of the edge of the dot.
2. Optical quality inspection control standards
At present, the optical quality of film is mainly determined by measuring the optical density of each index. When measuring, the density of the field and the density of the fog are measured by an ordinary transmission densitometer. The density of the center of the network is measured by a micrometer, and the dot is observed. For edge definition, dot test strips and a magnifying glass are used.
Since the film's on-site density, fog density, dot blackness and dot halation are mainly determined by the film base used for the film and the photographic performance of the photo-sensitive film, the photo-discharge center cannot be controlled. In order to ensure the film's on-site density and fog, the photocopy center can repeatedly test according to the photosensitivity of the photo-composing machine when selecting the film supply, and select the suitable film.
On January 1, 2000, China formulated the equivalent international standards for CY/T30 (offset printing plate production). This standard unifies the basic conditions, operating specifications, and quality requirements for printing, and stipulates the quality requirements for the original color separation film. It is highly operational and has a strong guiding role in practice. According to the standard requirements, the film quality should be as follows:
(1) The density of the center of the halftone film film dividing dots should be greater than the density of the blank film (the film density and the fog) 2.5 or more;
2 The transmission density of the blank film (base density plus fog density) should not exceed 0.15;
3 The density of the central part of the blank dots (that is, the blank part between the dots) can not be greater than the density of the large blank part + 0.1;
The dot density of the 4 dot network can not be greater than 1/40 of the width of the cable, and the dots cannot have obvious cracks.
The above theoretical requirements, but because the general printing companies do not have micro-density meter and other test equipment, in order to facilitate the evaluation of the density of network centers, the standard made a note:
In practical applications, the density of the center of the dot can be measured using the on-site density. If the density of the large-area solid is greater than the density of the transparent film, the density of the center of the dot is acceptable. That is, as long as the density is measured by an ordinary transmission densitometer, it is more than 3.5 or more than the base density. If there is no transmission densitometer, an experience method is given in the appendix of the standard: observation with a thin line icon and a 60-100 magnification magnifier. If there is no measurement bar, a method is also given, which is to observe with two color plates and a high magnification magnifying glass. These empirical methods are useful in practical work.
The dot density of the dot (control film imaginary haze) refers to the average distance between the contour lines corresponding to 10% and 90% of the dot center density, and the line width refers to the reciprocal of the dot line number. For example, for a 100-line/inch cable, where the cable width is about 0.254mm (25.4mm divided by 100), the edge width of the dot should not be greater than 40% of 0.254mm, ie, about 0.006mm (6 microns). In practical applications, it is often estimated by comparing the width of the thin line on the control strip.
The optical quality of the film depends on the output quality of the dispensing system itself, the properties of the film material, and the processing conditions. In practical work, we should use various means as far as possible to improve these factors and maximize the requirements of industry standards. This will be conducive to the stability and improvement of quality, but also make the export market "have laws to follow," and eventually move closer to internationalization.
Second, film quality control
The process quality of the film includes the number of screen lines, dot shapes, screen angles, dot percentage accuracy, and accuracy of four-color overprinting selected for the output. These characteristics of the film are closely related to the printing process and printability, that is, corresponding to different printing materials and copying emphasis, relatively different parameters are selected for output. If the optical quality of a film is good, improper selection of output parameters can also result in poor printing quality. Obviously, the process quality of the film is not appropriate, and it cannot be considered as a high-quality film.
1. The number of mesh lines
The number of mesh lines: the number of one-way parallel lines within a unit area. The higher the number of lines, the smaller the dots and the finer the prints. However, it is not that the higher the number of lines is, the better, but it should be selected according to the printing and material process requirements. The theoretically calculated results show that the fine screen is much more sensitive to changes in printing than the coarse screen. When the quality of the paper cannot reach the corresponding standard, the accuracy of the machine is not high, the quality of the ink is poor (grain is coarse, etc.), and the pressure is large and the dot enlargement rate is high at the time of printing. Color, etc., so the number of lines in the screen should match the actual print. For example, newspapers printed on the screen should be 100 lines/inch or so. If the printability conditions are good, if the quality of the paper is high and the process error can be effectively controlled, it can be higher. The number of screens printed by color newspapers in Hong Kong and Taiwan is higher than that of the mainland. The number of screens available is 133 lines per inch.
It is worth noting that, at a certain resolution of the imagesetter, the number of meshes is inversely proportional to the level of the image. The number of network lines is high, and the number of levels is relatively small, but vice versa. Newspaper printing should generally ensure that each color version has at least 100 or more levels. In this case, if the number of meshes is 100 lines/inch, the resolution of the imagesetter should be at least 1000 dpi or more. In order for both to be taken into account, the resolution of the imagesetter should be sufficiently high.
2. Dot shape
Dot shape, ie the geometry of a single dot. The effect of dot shape on the copying effect is not as obvious as the number of meshes, but the shape of different dot shapes is different due to the change in pressure (percentage of dots). Based on this, for the key adjustment range to be replicated, the shape of the dot which has the skipping phenomenon in this range should be avoided as much as possible. The hops occurring in different shapes of dots are usually:
Square dots are at 50%, diamond (elliptical) dots are at 35% and 65%, and circular dots are at 78.5%. Therefore, when dealing with different images, the characteristics of the dot should be taken into account. When selecting the time, it should be selected, high-profile, and avoid the jumping zone of the dot.
3. Web corner
The net angle refers to the angle between the vertical line and the horizontal line at the center of the dot. It reflects the orientation of the dots. When printed in monochrome, the human eye has the best visual effect on a 45 degree screen angle. In the case of multi-color overprinting, due to the inaccuracy of the overprinting of the dots, there may be a phenomenon of "rhizome". Selecting a suitable combination of mesh angles is one of the methods to reduce the occurrence of moiré. Under normal circumstances, four-color printing will be the main color at 45 degrees, the screen angle is:
Based on characters, CMYK takes 15, 45, 90, and 75 in order;
Based on landscapes, CMYK takes 45, 15, 90, and 75 in order.
4. Output percentage accuracy
The dot percentage accuracy refers to the error of the percentage of dots on the film after output and the percentage of dots set before the output. For a particular ink, a CMYK color dot combination determines a particular color. Ideally, the percentage set by the front-end processing is output to the film with accuracy. However, in reality it is inevitable that there are errors and must be strictly controlled. If the dot expansion on the film is very large, it will inevitably cause the screen to become dark, dull, and refreshed after printing. The reason for the error is mainly due to factors such as the quality of the discharge, the performance of the photosensitive material, and the process of washing. Among them, the quality of the discharge and discharge of the imagesetter is the most important factor.
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