PATTERN AREA RATIO MEASURING APPARATUS

Abstract: A pattern area ratio measuring apparatus includes a diffusing unit. The diffusing unit is arranged before light sources and irregularly reflects radiation light to a plate to be measured.

Description: Background of the Invention The present invention relates to a pattern area ratio measuring apparatus for measuring an area ratio of a pattern drawn on a plate or lithographic printing plate to be measured, and in particular to an apparatus of this type comprising a plurality of parallel light sources and radiating light from these light sources onto a plate to be measured, thereby measuring an area ratio of a pattern drawn on the plate to be measured on the basis of light reflected by the plate to be measured, this apparatus further comprising diffusing means arranged before said light sources, for irregularly reflecting radiation light to said plate to be measured. Figs. 9A and 9B show a generally known structure of a pattern area ratio measuring apparatus without any diffusing means. Referring to Figs. 9A and 9B, reference numeral 1 denotes a plate to be measured; 2-1 to 2-n; light sources (halogen lamps) arranged parallel to each other in the widthwise direction of the plate 1 to be measured with intervals of several cm therebetween; and 3, a CCD camera. In the pattern area ratio measuring apparatus having the above arrangement, light from the halogen lamps 2-1 to 2-n is radiated onto the plate 1 to be measured, and light reflected by the plate 1 to be measured is incident on the CCD camera 3. An area ratio of a pattern drawn on the plate 1 to be measured is measured on the basis of an output voltage from the CCD camera 3 generated in correspondence with the input reflected light. The above conventional pattern area ratio measuring apparatus can measure a pattern area ratio without any trouble provided that a presensitized plate (to be referred to as a PS plate hereinafter) is used as the plate 1 to be measured. If, however, a waterless plate is used as the plate 1 to be measured, the pattern area ratio cannot be correctly measured by this apparatus. That is, since the surface of a PS plate is grained, light is irregularly reflected and uniformized by the surface, and this uniformized light is incident as reflected light on the CCD camera 3. Therefore, a pattern area ratio of such a PS plate can be correctly measured. However, since the surface of a waterless plate is mirror-finished, its reflectivity is much higher than that of the PS plate. For this reason, the intensity of light reflected by a portion around an intersection with respect to each of optical axes L₁ to L n of the halogen lamps 2-1 to 2-n is increased, and the output voltage value from the CCD camera 3 corresponding to the portion exceeds an upper voltage limit V UP , as shown in Fig. 10. As a result, a pattern area ratio different from the actual pattern area ratio is measured. In order to be able to minimize measuring errors caused by possible waviness of surface areas to be opto-electronically identified it is known from the disclosure of US-A-44 06 545 to use diffusing means downstream of the plate to be measured, to which end a diffusor-illuminator screen for generating diffused light is described. If, however, the positions of the light sources are to be changed, the thus resulting differences in reflection cannot be coped with. Summary of the Invention The present invention has been made in consideration of the above described drawbacks and has as its object to provide a pattern area ratio measuring apparatus which can measure a pattern area ratio of a plate having a mirror-finished surface such as a waterless plate with substantially the same precision as that obtained by using a PS plate. According to the present invention, this object is achieved by providing a pattern area ratio measuring apparatus of the type described in the preamble of claim 1, in which the diffusing means are adapted to switch the light sources between first and second positions and to effect the irregular reflection of the radiation light to the plate to be measured when the light sources are switched to the second switching position. Thus, different types of applications become possible with the same apparatus simultaneously safeguarding accurate and highly precise measuring results for PS plates as well as for plates having a mirror-finished surface such as waterless plates by switching the light sources to a predetermined switching position, so that light from the light sources is irregularly reflected through the diffusing means and radiated on the plate to be measured. According to a preferred embodiment of the present invention, there is provided an apparatus which includes, for obtaining a voltage value as a measurement basis corresponding to light reflected by the plate to be measured, first voltage adjusting means for adjusting a voltage value corresponding to light reflected by a black level of the plate to be measured to a predetermined value Va, and second voltage adjusting means for adjusting a voltage value corresponding to light reflected by a white level of the plate to be measured to a predetermined value Vb. According to this aspect of the present invention, light from the light sources is irregularly reflected through the diffusing means and radiated onto the plate to be measured, and a voltage value corresponding to light reflected by the plate to be measured is adjusted to change between the predetermined values Va and Vb. Brief Description of the Drawings Figs. 1A and 1B are side and front views, respectively, showing an embodiment of a pattern area ratio measuring apparatus; Fig. 2 is a graph showing an output voltage value from a CCD camera of the pattern area ratio measuring apparatus shown in Figs. 1A and 1B obtained when a waterless plate is used as a plate to be measured; Figs. 3A and 3B are side and front views, respectively, showing pattern area ratio measuring apparatus according to the present invention; Fig. 4 is a side view showing a switching angular position of a cover member formed integrally with halogen lamps of the pattern area ratio measuring apparatus shown in Figs. 3A and 3B; Fig. 5 is a block diagram showing a processor for receiving an output voltage from a CCD camera of preferred embodiment of the pattern area ratio measuring apparatus; Fig. 6 is a plan view showing a black level adhered on the peripheral surface of a waterless plate; Fig. 7 is a flow chart for explaining the operation of the pattern area ratio measuring apparatus shown in Fig. 5; Figs. 8A and 8B are graphs showing output voltage values of CCD cameras upon pattern area ratio measurement for a PS plate and a waterless plate according to the present invention and a conventional apparatus; Figs. 9A and 9B are side and front views, respectively, showing a structure of a conventional pattern area ratio measuring apparatus; and Fig. 10 is a graph showing an output voltage value from a CCD camera of the conventional pattern area ratio measuring apparatus shown in Figs. 9A and 9B obtained when a waterless plate is used as a plate to be measured. Detailed Description of the Preferred Embodiments A pattern area ratio measuring apparatus according to the present invention will be described in detail below with reference to the accompanying drawings. Figs. 1A and 1B show an embodiment of a pattern area ratio measuring apparatus of a hither to known concept. In Figs. 1A and 1B, the same reference numerals as in Figs. 9A and 9B denote the same parts and a detailed description thereof will be omitted. In this embodiment, a diffusing plate 4 is arranged before halogen lamps 2-1 to 2-n. This diffusing plate 4 consists of a semitransparent plate. In the pattern area ratio measuring apparatus having the above arrangement, light from the halogen lamps 2-1 to 2-n is radiated onto a plate 1 to be measured through the diffusing plate 4, and light reflected by the plate 1 to be measured is incident on a CCD camera 3. That is, the light from the halogen lamps 2-1 to 2-n is irregularly reflected and uniformized through the diffusing plate 4 and radiated onto the plate 1 to be measured. If a waterless plate is used as the plate 1 to be measured, light irregularly reflected and uniformized through the diffusing plate 4 is radiated onto the surface of the waterless plate. Therefore, the conventional problem of an increase in intensity of light reflected by a portion around an intersection with respect to each of optical axes L-1 to L-n of the halogen lamps 2-1 to 2-n can be solved. Therefore, as shown in Fig. 2, since the output voltage value from the CCD camera 3 changes in a region below its upper voltage limit V UP , a pattern area ratio can be correctly measured on the basis of the output voltage value from the CCD camera 3. Figs. 3A and 3B show an embodiment of the pattern area ratio measuring apparatus corresponding to the main aspect of the present invention. To this end, a diffusing portion 5-a consisting of a semitransparent member is provided to a cover member 5 of halogen lamps 2-1 to 2-n. By rotating the cover member 5 formed integrally with the halogen lamps 2-1 to 2-n clockwise through 90°, its angular position can be switched from that shown in Fig. 3A to that shown in Fig. 4. In the pattern area ratio measuring apparatus having the above arrangement, if a PS plate is used as a plate 1 to be measured, the angular position of the cover member 5 formed integrally with the halogen lamps 2-1 to 2-n is normally set as shown in Fig. 3A. That is, when the cover member 5 is located in the normal angular position, light from the halogen lamps 2-1 to 2-n is directly radiated onto the plate (PS plate) 1 to be measured, and light reflected by the plate 1 to be measured is incident on a CCD camera 3. When a waterless plate is used as the plate 1 to be measured, the cover member 5 formed integrally with the halogen lamps 2-1 to 2-n is rotated clockwise through 90°C. That is, the cover member 5 is switched to the angular position shown in Fig. 4. When the cover member 5 is switched to the angular position shown in Fig. 4, light from the halogen lamps 2-1 to 2-n is irregularly reflected and uniformized through the diffusing portion 5a and radiated on the surface of the plate (waterless plate) 1 to be measured, and light reflected by the plate 1 to be measured is incident on the CCD camera 3. As a result, the same effect and result as obained by the apparatus shown in Figs. 1A and 1B can be obtained in an extremely easy manner by the same basic structure. Another embodiment corresponding to a preferred aspect of the present invention will be described below. Fig. 5 shows a processor for receiving an output voltage from a CCD camera 3, comprising a CPU 6 as a control unit using a microprocessor or the like, an I/O port 7, a memory 8, an A/D converter 9, a black level D/A converter 10, a white level D/A converter 11, a subtractor 12, and an amplifier 13. That is, an output voltage (analog value) from the CCD camera 3 is supplied to the A/D converter 9 via the amplifier 13 and the subtractor 12. The supplied voltage is converted into a digital value by the A/D converter 9 and supplied to the CPU 6. The CPU 6 calculates a pattern area ratio of a plate 1 to be measured on the basis of the digital value supplied from the A/D converter 9. On the basis of a digital value supplied upon measurement of a black level of a waterless plate to be described later, the CPU 6 supplies a feedback value (digital value) to the black level D/A converter 10 so that a voltage to be supplied to the A/D converter 9 is adjusted to a predetermined value Va, thereby executing measurement of a pattern area ratio. On the basis of a digital value supplied upon measurement of a white level on a waterless plate to be described later, the CPU 6 supplies a feedback value (digital value) to the white level D/A converter 11 so that the voltage to be supplied to the A/D converter 9 is adjusted to a predetermined value Vb, thereby executing measurement of a pattern area ratio. When a waterless plate is used as plate 1 to be measured, a black level 1-1 is adhered on the peripheral surface of the waterless plate 1 (Fig. 6). Assume that when measurement of a pattern area ratio is started, light reflected by the black level 1-1 is initially incident on the CCD camera 3. The black level 1-1 is defined as a darkest portion of a pattern 1-2 drawn on the waterless plate 1. Therefore, when pattern area ratio measurement is started in the processor shown in Fig. 5, an output voltage corresponding to light reflected by the black level 1-1 is initially supplied to the A/D converter 9 via the amplifier 13 and the subtractor 12. This supplied voltage is converted into a digital value by the A/D converter 9 and supplied to the CPU 6. The CPU 6 calculates the voltage to be supplied to the A/D converter 9 as a black level voltage value on the basis of the input digital value (black level value) (step 501 in Fig. 7). As the waterless plate 1 is moved, light reflected by a blank portion 1-3 between the black level 1-1 and the pattern 1-2 is incident on the CCD camera 3. In this embodiment, this blank portion 10-3 is defined as a white level corresponding to a brightest portion of the pattern 1-2. An output voltage value corresponding to light reflected by the white level 1-3 is supplied to the A/D converter 9 via the amplifier 13 and the subtractor 12. This supplied voltage is converted into a digital value by the A/D converter 9 and supplied to the CPU 6. The CPU 6 calculates the voltage to be supplied to the A/D converter 9 as a white level voltage value on the basis of the input digital value (white level value) (step 502). The CPU 6 supplies the feedback values to the black and white level D/A converters 10 and 11 so that the black and white level voltage values are adjusted to the predetermined values Va and Vb, respectively (step 503), and executes measurement of the pattern area ratio (step 504). Referring to Fig. 8A, assuming that the output voltage value is the voltage value to be supplied to the A/D converter 9, the predetermined values Va and Vb with respect to the black and white level voltage values can be respectively defined as lower and upper limits for changing the voltage to be supplied within a measurement range L. The feedback value supplied to the subtractor 12 via the black level D/A converter 10 is supplied to perform offset adjustment of the voltage value to be supplied to the A/D converter 9. The feedback value supplied to the subtractor 12 via the white level D/A converter 11 is supplied to perform gain adjustment of the voltage value to be supplied to the A/D converter 9. Fig. 8B shows the result obtained by a conventional apparatus not using the present invention. Referring to Fig. 8B, the output voltage value from the CCD camera 3 is increased as a whole to fall outside the measurement range L. In addition, the output voltage value from the CCD camera 3 corresponding to a portion on a plate surface around an intersection with respect to an optical axis of each halogen lamp sharply increases due to intense light reflected by the portion. In this embodiment, however, as shown in Fig. 8A, light is irregularly reflected and uniformized through the diffusing plate 4 and radiated onto the surface of the plate 1. Therefore, an increase in intensity of light reflected by the portion around an intersection with respect to an optical axis of each halogen lamp can be prevented to eliminate a sharp increase in the voltage value to be supplied to the A/D converter 9. In addition, since the feedback values are supplied to the subtractor 12 via the black and white level D/A converters 10 and 11, an offset and a gain of the voltage value to be supplied to the A/D converter 9 can be adjusted to allow this supplied voltage value to change within the measurement range L. Therefore, measurement of a pattern area ratio can be correctly performed on the basis of the voltage value to be supplied to the A/D converter 9. That is, by properly selecting the diffusing plate 4 and properly determining the predetermined values Va and Vb for the black and white level voltage values, respectively, substantially the same measurement precision as that obtained by a PS plate can be obtained by a waterless plate. The techniques according to the embodiments of the present invention can be applied to precisely measure a pattern area ratio of a plate having lower reflectivity than that of a PS plate by partially changing circuit constants or programs without adjusting hardware. In each of the above embodiments, a waterless plate is used as the plate 1 to be measured having high reflectivity. However, the plate 1 to be measured is not limited to a waterless plate. That is, the present invention can be used to measure pattern area ratios of various types of plates having mirror-finished surfaces such as a waterless plate, resulting in a very wide application. As has been described above, the diffusing means for irregularly reflecting radiation light to a plate to be measured is arranged before light sources. Therefore, light from the light sources is irregularly reflected and radiated onto the plate to be measured. According to the present invention, the pattern area ratio measuring apparatus comprises the diffusing means for irregularly reflecting radiation light to a plate to be measured in its predetermined switching position. Therefore, by switching the diffusing means to the predetermined switching position, light from light sources is irregularly reflected and radiated onto the plate to be measured. Therefore, by properly selecting the diffusing means, a pattern area ratio of a plate having a mirror-finished surface such as a waterless plate can be measured with substantially the same precision as that obtained by using a PS plate. Furthermore, the pattern area ratio measuring apparatus comprises first voltage adjusting means for adjusting a voltage value corresponding to light reflected by a black level of a plate to be measured to a predetermined value Va, and second voltage adjusting means for adjusting a voltage value corresponding to light reflected by a white level of the plate to be measured to a predetermined value vb. Therefore, light from light sources is irregularly reflected by a diffusing means and radiated onto the plate to be measured, and the voltage value corresponding to light reflected by the plate to be measured can be changed between the predetermined values Va and Vb. Therefore, by properly selecting the diffusing means and properly setting the predetermined values Va and Vb, a pattern area ratio of a plate having a mirror-finished surface such as a waterless plate can be measured with substantially the same precision as that obtained by using a PS plate.

Claims: A pattern area ratio measuring apparatus which includes a plurality of parallel light sources and radiates light from said light sources onto a plate to be measured, thereby measuring an area ratio of a pattern drawn on said plate to be measured on the basis of light reflected by said plate to be measured, the apparatus comprising diffusing means, arranged before said light sources, for irregularly reflecting radiation light to said plate to be measured, characterised in that said diffusing means are adapted to switch said light sources between first and second positions and to effect the irregular reflection of the radiation light to said plate to be measured when said light sources are switched to the second switching position. A pattern area ratio measuring apparatus according to claim 1, characterized in that it comprises, for obtaining a voltage value as a measurement basis corresponding to light reflected by said plate to be measured: first voltage adjusting means for adjusting a voltage value corresponding to light reflected by a black level of said plate to be measured to a predetermined value Va; and second voltage adjusting means for adjusting a voltage value corresponding to light reflected by a white level of said plate to be measured to a predetermined value Vb.

Inventors: SHIZUYA, FUMIO
Applicants: KOMORI CORPORATION
Application year: 1990
Main classification: G01B11
IPC codes: G01B11, G01N21, G01J1, B41F31