JPS6341487B2 - - Google Patents

Description

[Detailed Description of the Invention] <Industrial Application Field> This invention relates to plate-shaped offset printing in which an image is formed on the surface and the reflectance is different between the image area and the other non-image area. The present invention relates to a device for measuring the area ratio of an image on a plate.

<Prior art> As seen in JP-A-56-24508, JP-A-49-67714, and JP-A-51-25056, the area ratio of the image area of an offset printing plate is determined by optical means. The ink supply amount is set before printing starts by adjusting the opening of the ink key of the printing press to which the offset printing plate is attached based on this measured value, and this allows calibration to be performed immediately after printing starts. It has been proposed to obtain printed matter equivalent to that of printed matter, to reduce paper waste, and to simplify the printing process, such as by adjusting ink keys.

However, according to the above-mentioned conventional example, the optical means for measuring the area ratio of the image area of the offset printing plate is realized by a normal light source such as a fluorescent lamp or an incandescent bulb and a large number of light receiving elements such as photodiodes. Therefore, it is difficult to equalize the sensitivity and other characteristics of individual light-receiving elements and maintain stability over a long period of time, making it difficult to obtain accurate measurement values. Since the system transports and scans the entire surface, it takes a long time to measure and it is difficult to increase the speed.

Japanese Patent Application No. 56-202421 (Japanese Unexamined Patent Publication No. 58-105007) has been proposed to solve the drawbacks of the conventional example.
As disclosed in the specifications and drawings of , an offset printing plate is scanned with a laser beam using a means such as a rotating mirror, and this reflected light is received by a light receiving element such as a photomultiplier to determine the area ratio of the image area. A system is being developed to measure the

According to this, the light source is one laser light source and only one light receiving element is required, so the concerns about variations in characteristics between individual elements seen in the above conventional example are eliminated, and more accurate measurements are possible. Furthermore, since it is an optical scanning method in which the offset printing plate is scanned by changing the irradiation direction of the laser beam, short-time measurement is possible.

Normally, an offset printing plate has a structure in which a photosensitive layer is coated on the surface of an aluminum substrate, and the color of this photosensitive layer is cyan or green, so the printed area where the photosensitive layer remains appears cyan or green. Since this photosensitive layer has been removed in non-image areas, the color has the color of a grained aluminum base. Therefore, as a laser light source
When a He--Ne laser (wavelength: 6328 Å) or the like is used, most of the laser beam is absorbed in the image area, while it is diffusely reflected in the non-image area.

Offset printing plates have color gradations based on the size of the halftone dots, so in highlight areas, the area of halftone dots corresponding to the image area is small, so there is a large amount of diffuse reflection, while in shadow areas, the amount of diffused reflection increases. Since the halftone dot area corresponding to the line portion is large, the amount of diffused reflection is reduced.

Therefore, the condition of the surface of the printing plate can be determined by the amount of this diffusely reflected light (scattered light), and the image on the printing plate is determined by receiving the diffusely reflected light by a photoelectric element, photoelectrically converting it, and calculating the strength and weakness of the electrical signal. The area ratio of the line part can be measured.

<Problems to be Solved by the Invention> However, if the optical system as described above reflects the laser beam directly onto the surface of the printing plate and receives the reflected light on the light receiving element, in the scanning process, not only the scattered light but also the direct reflected light is generated. There are regions where light (regularly reflected light) enters the light receiving element and regions where it does not enter the light receiving element, and since the directly reflected light is optically larger than the diffusely reflected light, the image state in the above two regions is the same. However, there is a difference in the amount of light received by the light-receiving element, which causes measurement accuracy to deteriorate.

<Means for Solving the Problems> The present invention has been made to solve the above problems, and it has been made to improve the reflectance of light of a specific wavelength in the image areas and non-image areas of an image formed on the surface of an offset printing plate. This is an image area ratio measurement device that measures the area ratio of the image area using the difference between means for scanning the entire surface of the offset printing plate with a laser beam, a first polarizing means provided during the irradiation process of the laser beam from the laser light source onto the surface of the offset printing plate, and a means for scanning the reflected light from the surface of the offset printing plate. It is characterized by comprising a light receiving element that receives light and converts it into an electrical signal, and a second polarizing means that is provided immediately before the light receiving element and has a polarization angle orthogonal to the first polarizing means. .

Here, the first polarizing means is a polarizing beam splitter or a polarizing filter, which reflects the reflected laser beam onto the surface of the printing plate as either a p-polarized component or an s-polarized component perpendicular thereto. The second polarizing means is a polarizing filter or a polarizing beam splitter having a polarization angle perpendicular to that of the first polarizing means, and specifically, the laser beam reflected by the first polarizing means is divided into p-polarized components. If the laser beam is to be a laser beam made up of If the laser beam is made up of only the s-polarized light component, the second polarization means will transmit only the p-polarized light component orthogonal to the s-polarized light component.

<Operation> According to the above configuration, the laser beam reflected on the printing plate surface by the first polarization means is p
It can be made up of only a polarized light component or only an s-polarized light component. If the first polarization means is selected so that the laser beam reflected on the surface of the printing plate is only p-polarized, the light reflected from the printing plate will be divided into directly reflected light and scattered light, and the directly reflected light will be p-polarized. The scattered light includes only a p-polarized light component and an s-polarized light component. Since the second polarization means passes only the s-polarized light component that is orthogonal to the p-polarized light component, the p-polarized light component is blocked from both the direct irradiation light and the scattered light, and only the s-polarized light component of the scattered light is transmitted to the light receiving element. It will be reflected. Also,
Even if the first polarization means makes the laser beam only the s-polarized component and the second polarization means makes it possible to reflect only the p-polarized component to the light receiving element, the same principle applies to the reflected light. Of the scattered light p
Only the polarized light component can be reflected to the light receiving element. Therefore, it is possible to uniformly block direct reflected light over the entire surface of the printing plate, and only scattered light is reflected to the light receiving element, which is a problem that has hindered accurate measurement of area ratio. can be removed.

<Example> The present invention will be described in detail below based on an example shown in the drawings.

FIG. 1 is an external view of an apparatus for measuring the area ratio of the image area of an offset printing plate according to the present invention, and FIG. 2 is an explanatory view of the optical system.

As shown in FIG. 1, an image area ratio measuring device 1 for an offset printing plate (hereinafter simply referred to as a printing plate) is used for printing on which the upper surface is the printing plate mounting surface 2 and the area ratio of the image area is to be measured. A printing plate P can be placed thereon, and a control device for controlling the operation of this device, a calculation device for processing measured data, etc. are arranged on the side of the printing plate device surface 2.

The tip of the optical system installation section 3 is configured to be located above the center of the printing plate mounting surface 2, and therein is a laser light source 11 and a photomultiplier, which is a light receiving element, which will be explained in detail later. An optical system such as a (photomultiplier tube) 12 is installed, and its tip irradiates a laser beam and receives reflected light from the printing plate P.

The operation panel 4 has a group of keys such as a control key, a numeric keypad, and a set key, a display element such as a phototube, and an output device that outputs the area ratio data that is the measurement result to a magnetic card or the like. By inputting data such as the machine's ink key pitch and plate size through this key group, the measured data is processed and output from the output device as data readable by the printing machine.

In such a device, the optical system that irradiates the printing plate P with a laser beam, receives this reflected light by a photoelectric element such as a photomultiplier, and converts the strength of the reflected light into the strength of an electrical signal is shown in Figure 2. The explanation will be based on.

A laser beam emitted from a laser light source 11 is adjusted to a certain beam diameter by a collimator lens 13 and reflected by a mirror 14 whose rotation is controlled by a galvanometer 19, and is reflected onto a printing plate P placed on a printing plate mounting surface 2. irradiated.

To explain the configuration in which the entire surface of the printing plate P is scanned by this laser beam and the print area ratio of the printing plate P is measured, first, the laser beam is positioned at the origin SP, which is set in advance at one corner of the printing plate P. The galvanometer 19 and motor 22 are driven so that the printing plate P is rotated by the galvanometer 19 starting from this origin SP.
scan in the horizontal direction. From origin SP to printing version P
After scanning to the horizontal end, the galvanometer 19 is rotated in the opposite direction to return the laser beam to the end of the printing plate P on the origin SP side, and the motor 22 rotates the rotary plate 21 to which the galvanometer 19 is attached. From this state, the galvanometer 19 is driven to scan in the horizontal direction in the same way as before, and by repeating such horizontal scanning and vertical scanning, the printing plate is The entire surface of P can be scanned. Note that the oscillation of the laser beam is controlled by a laser drive control device 23 connected to the laser light source 11, and the operation of the galvanometer 19 and motor 22 related to laser beam scanning is controlled by an optical system drive control device 24.

A portion of the laser beam irradiated onto the printing plate P through such scanning is absorbed by the surface of the printing plate, and a portion is diffusely reflected and received by the photomultiplier 12 located above the printing plate P, where it is photoelectrically transmitted. converted. As mentioned above, the color of the photosensitive layer of a printing plate (positive type PS plate) is usually cyan or green, so the image area where this photosensitive layer remains is cyan or green, and is not a non-image area. Since this photosensitive layer has been removed in the lined areas, they take on the color of a grained aluminum base.
Therefore, when a He--Ne laser (wavelength: 6328 Å) is used as a laser light source, most of the laser beam is absorbed in the image area, while being diffusely reflected in the non-image area.

Offset printing plates have color gradations based on the size of the halftone dots, so in highlight areas, the area of halftone dots corresponding to the image area is small, so there is a large amount of diffuse reflection, while in shadow areas, the amount of diffused reflection increases. Since the halftone dot area corresponding to the line portion is large, the amount of diffused reflection is reduced.

Therefore, the condition of the surface of the printing plate can be determined based on the amount of this diffusely reflected light.The diffusely reflected light is received by the photo multiplier 12 and photoelectrically converted, and the strength and weakness of the electric signal is processed and processed to determine the image area of the printing plate. The area ratio of can be measured.

According to the present invention, during the course of the laser beam before being irradiated onto the printing plate P, as shown in FIG. or a polarizing beam splitter) 18 is provided.

That is, as shown in FIG. 5, the laser beam is directly reflected onto the printing plate P without using a polarizing beam splitter or polarizing filter as described above.
If the optical system is such that the reflected light is received by the photo multiplier 12, the light reflected to the photo multiplier 12 in area A on the printing plate P is scattered light, and in area B it is directly reflected light. A mixture of both types of scattered light is received, and the amount of directly reflected light is larger than that of scattered light. Therefore, the image on the surface of printing plate P is the same in area A and area B. Also, the amount of light received in area B is larger than in area A, which causes errors in measurement data.

The polarizing beam splitter 16 and polarizing filter 18 as described above are provided for the purpose of preventing such defects.

Explaining according to the embodiment, a polarizing beam splitter 16 is provided before the laser beam emitted from the laser light source 11 is reflected on the printing plate P. The polarizing beam splitter 16 converts the laser beam into light consisting of either only the p-polarized component or only the s-polarized component, so that the laser beam consisting only of the p-polarized component or only the s-polarized component is printed. The plate P will be irradiated. In general, a polarizing beam splitter has the characteristic of passing the p-polarized light component of a laser beam and reflecting the s-polarized light component, so it is possible to cut either the transmitted light or the reflected light from the polarized beam splitter and cut the other. The light should be incident on the printing plate.

Although the installation position of such a polarizing beam splitter is between the laser light source 11 and the collimator lens 13 in FIG. It may be installed in between.

In addition, the same purpose can be achieved by using a polarizing filter instead of a polarizing beam splitter, and the point is that before irradiating the printing plate with a laser beam, the laser beam consists of only a p-polarized component or only an s-polarized component. Any optical device having such a function can be used in place of the polarizing beam splitter.

By adopting the above configuration, the printing plate P
The laser beam irradiated can be made to consist only of p-polarized light component or s-polarized light component, but in the following explanation, for simplicity, the laser beam will be made to consist of only p-polarized light component. I will explain about it.

As mentioned above, the reflected light from the printing plate P that has been irradiated with a laser beam consisting only of p-polarized light components can be divided into direct reflected light and scattered light. Directly reflected light consists only of p-polarized light components, and scattered light The light contains a p-polarized component and an s-polarized component.

If such reflected light is received as is, both the directly reflected light and the scattered light will be received, so the above-mentioned measurement error will still occur. Therefore, a polarizing filter 18 is provided immediately before the photomultiplier 12 to cut out the p-polarized light component and pass only the s-polarized light component. With this configuration, all directly reflected light consisting only of the p-polarized light component is blocked, and the p-polarized light component of the scattered light is blocked, and as a result, only the s-polarized light component of the scattered light enters the photo multiplier 12. will be done.

Thereby, the reflected light that is received by the photomultiplier 12 at any position on the printing plate P is only the s-polarized component of the scattered light, and it is possible to prevent the above-mentioned measurement error from occurring.

Note that when the printing plate P is irradiated with a laser beam consisting only of the s-polarized light component, the directly reflected light is only the s-polarized light component, and the scattered light contains both the s-polarized light component and the p-polarized light component. If a polarization filter is provided immediately before the multiplier 12 to block the s-polarized light component, the directly reflected light can be blocked, and only the p-polarized light component of the scattered light can be allowed to enter the photomultiplier 12. Therefore, the same effect as when the printing plate is irradiated with a laser beam consisting only of p-polarized light components can be obtained.

Furthermore, in place of the polarizing filter 18, it is also possible to use a device capable of achieving the same purpose, such as a polarizing beam splitter, as a means for blocking the p-polarized light component or the s-polarized light component.

Furthermore, it is also possible to add the following configuration to the image area ratio measuring device according to the present invention.

In the optical system as described above, the laser beam is refracted by the rotating mirror 14 and the printing plate P
Since the laser beam is reflected and scanned, the angle of the mirror relative to the laser beam changes over time. The reflection characteristics of the laser beam in this situation are as shown in Figure 3. If the horizontal axis is the wavelength and the vertical axis is the reflectance, then the angle of reflection of the laser beam on the mirror 14 is 45 If the temperature is ゜,
Its characteristics have a peak at the He-Ne laser wavelength of 6328 Å as shown in curve A, but when the incident angle changes to 60°, its reflection characteristics have a peak at the He-Ne laser wavelength as shown in curve B.
A phenomenon occurs in which the wavelength of the Ne laser shifts from 6328 Å to the shorter wavelength side, and at the wavelength of 6328 Å, its reflectance decreases from Y to Y', so that the laser beam passes through the rotating mirror 14 to the printing plate P. The amount of reflected light increases or decreases as the angle of the mirror 14 changes, making uniform irradiation impossible. As will be described later, just before the photo multiplier 12, external light is blocked and only the wavelength of the laser beam (6328 Å for the He-Ne laser) is used so that only the reflected light from the printing plate of the laser beam, which is the signal light, can be received. An interference filter 17 is provided to allow the light to pass through. Therefore, if uniform irradiation at a specific wavelength is insufficient, good measurement results cannot be obtained.

This problem can be solved by using a mirror 14 with characteristics such that its reflectance is constant in a certain wavelength range X ₁ to X ₂ including the He-Ne laser wavelength of 6328 Å, as shown in Figure 4. It can be solved if

That is, when the maximum swing angle of the mirror 14 necessary for scanning the printing plate P in the horizontal direction or the vertical direction is 30 degrees, the reflection angle of the laser beam on the mirror 14 is
If X ₁ and X ₂ are set so that the reflectance at the wavelength of 6328 Å remains constant even if the peak wavelength shifts by 45° ± 15°, the printing plate P can be adjusted regardless of the rotation of the mirror 14.
It becomes possible to irradiate the target area with a uniform laser beam with a wavelength of 6328 Å.

In addition, in this example, we explained that uniform irradiation at the wavelength of 6328 Å is possible because a He-Ne laser is used as the laser light source, but if other lasers are used, uniform irradiation at the wavelength of that laser is possible. It would be better to make it possible.

To give a specific example of a mirror with such characteristics, two types of dielectrics with different refractive indexes (for example,
Alternating multilayer deposition of SiO ₂ and TiO ₂ (usually 19 to 23
layer). Here n is an integer. By using such a mirror, it is possible to have the characteristics described above, and because the light is reflected while interfering, it can achieve a reflectance of over 99%, compared to the reflectance of a general mirror, which is around 80%. It is possible,
The amount of light reflected onto the printing plate P can be increased.

In addition, a 1/4 wavelength plate 1 is installed immediately after the laser light source 11.
5 to circularly polarize the light and reflect it to a first polarizing means, which will be described later.

However, in the case of the device of the present invention, it is possible to obtain practically acceptable accuracy even without particularly providing this quarter-wave plate.

Furthermore, even if only the scattered light from the printing plate could be transmitted as described above, the photomultiplier 12 would receive this signal along with external light such as reflected light from indoor lighting, which would interfere with measurement. will occur.

In order to avoid the influence of such external light, an interference filter 17 having characteristics as shown in FIG. 6 is used.
is provided immediately before the photo multiplier 12 together with a polarizing filter. As is clear from FIG. 6, the interference filter 17 transmits only light in a specific wavelength range, and in this embodiment,
Because it uses a He-Ne laser, its wavelength is 6328
It has a transmittance peak at Å, and its half-width is approximately
If an interference filter such as 50 Å is used, only light in the vicinity of the laser wavelength is transmitted, so that the photomultiplier 12 receives almost only the laser reflected light, making it possible to perform highly accurate measurements.

Furthermore, in this embodiment, the laser beam is swung laterally by a mirror 14 rotated by a galvanometer 19, and is scanned vertically by a rotary plate 21 driven by a motor 22 to scan the entire surface of the printing plate P. As a result, the following phenomena occur.

That is, the printing plate P to the photo multiplier 12
If the reflection angle of the reflected light changes over time due to the scanning, and if the optical filter group 25 such as the interference filter 17 and polarization filter 18 provided immediately before the photo multiplier 12 is fixed, then this The angle of incidence of the reflected light on the optical filter group 25 will vary.

However, the spectral characteristics of the interference filter 17 change due to changes in the angle of incidence on the interference filter surface.
The transmittance at the main wavelength of the laser, 6328 Å, also changes. For example, when the angle of incidence of the reflected light on the interference filter is about 30° with respect to the normal plane of the interference filter, the spectral characteristics are shown in Figure 7. As shown, the normal spectral characteristics that have a peak at wavelength X (6328 Å) are shifted to the shorter wavelength side and have a peak at wavelength '' to Y.

In this way, the amount of light transmitted through the interference filter fluctuates due to changes in the angle of incidence, so the printing plate P
Photo multiplier 1 calculates the exact amount of reflected light from
2 causes the inconvenience of not being able to measure.

This phenomenon is similar to the polarizing filter 18, which has the disadvantage that the degree of polarization varies depending on the angle of incidence on the polarizing filter.

Therefore, in the present invention, the optical filter group 25 is designed so that it always faces the irradiation position of the printing plate P with the varying laser beam so that the reflected light from the printing plate P always enters the optical filter group 25 at right angles. Rotate.

As shown in FIG. 2, optical filter group 2
5 is attached to a galvanometer 20, and this galvanometer 20 is attached to a rotary plate 21 to which a galvanometer 19 and a photomultiplier 12 are attached.

With this configuration, the mirror 14 and the optical filter group 25 are attached to the same rotary plate 21 for vertical scanning of the laser beam, so that the angle can always be maintained at a constant angle, and for horizontal scanning, the optical filter group 25 can be maintained at a constant angle. If the galvanometer 20 is controlled by the optical system drive control circuit 24 so that the filter group 25 is rotated in synchronization with the mirror 14, the incident angle of the reflected light to the optical filter group 25 can always be maintained at approximately 90 degrees. As a result, while scanning the entire surface of the printing plate P, the reflected light from the printing plate P can always enter the optical filter group 25 at an angle of approximately 90°, and the above-mentioned defects can be solved and the printing plate Uniform measurement can be performed over the entire surface of P.

In this embodiment, the printing plate P is scanned using one mirror 14 and a rotary plate to which the mirror 14 is attached.
Even if scanning is performed using two rotating mirrors without using a rotating plate, the optical filter group can be operated using a rotating plate, galvanometer, etc. that is synchronized with the rotation of these mirrors based on the same idea. Good.

<Effects> As described above, the present invention measures the area ratio of an image by irradiating the surface of an offset printing plate with a laser beam, and the laser beam irradiated onto the surface of the printing plate has a p-polarized component or a p-polarized component. By containing only either the s-polarized light component and blocking the p-polarized light component or the s-polarized light component immediately before the light-receiving element, direct reflected light can be uniformly blocked over the entire surface of the object to be measured. Since only the scattered light can be incident on the light receiving element, the area ratio can be measured with higher accuracy.

[Brief explanation of drawings]

The drawings show one embodiment of the present invention; FIG. 1 is an external view of the device of the present invention, FIG. 2 is an explanatory diagram of the optical system, and FIGS. 3 and 4 are explanations of the reflection characteristics of the mirror. 5 are explanatory diagrams of laser beam irradiation, and FIGS. 6 and 7 are explanatory diagrams of spectral characteristics of the interference filter. 1... Measuring device, 11... Laser light source, 12
...Photo multiplier, 13...Collimator lens, 14...Mirror, 15...1/4 wavelength plate,
16...Polarizing beam splitter, 17...Interference filter, 18...Polarizing filter, 19,2
0... Galvanometer, 21... Rotating plate, 25...
Optical filter group, P...Printing version.

Claims (1)

[Claims] 1. An image area ratio measuring device that measures the area ratio of an image formed on the surface of an offset printing plate by utilizing the difference in reflectance of specific wavelength light between the image area and the imaged area, A laser light source that oscillates a laser beam, a means for continuously changing the irradiation direction of the laser beam to scan the entire surface of the offset printing plate with the laser beam, and a means for scanning the entire surface of the offset printing plate with the laser beam from the laser light source. a first polarizing means provided during the irradiation process; a light receiving element that receives reflected light from the surface of the offset printing plate and converts it into an electrical signal;
An image area ratio measuring device comprising: a second polarizing means provided immediately in front of a light receiving element and having a polarization angle perpendicular to the first polarizing means.