GB2185654A

GB2185654A - Shading correction of image signal

Info

Publication number: GB2185654A
Application number: GB08703957A
Authority: GB
Inventors: Kunio Tomohisa; Kiyoshi Maeda; Masamichi Cho
Original assignee: Dainippon Screen Manufacturing Co Ltd
Current assignee: Dainippon Screen Manufacturing Co Ltd
Priority date: 1983-12-06
Filing date: 1987-02-20
Publication date: 1987-07-22
Also published as: GB8703957D0; GB2185654B

Abstract

The scanned laser beam reflected from an original is detected by an array of photosensors 18 which provide output signals to an image processing circuit. Correction for a shading effect is made by adjusting the gain of amplifiers 21a-21n or 23a-23d, each of which is arranged to amplify the output signal of one, Fig. 10, or a set, Fig. 11, of the photosensors of the array. The array may be divided into two sub-arrays of alternate photosensors, the signals from them being multiplexed. <IMAGE>

Description

SPECIFICATION A Method and System for Compensating for a Shading Phenomenon Field of the Invention This invention relates to a compensation for a shading phenomenon which is caused in reproducing images by using a laser beam scanner fatally having a non-linear characteristic of its scanning speed on an original.

Background of the Invention An optical system of a laser beam scannerfor scanning an original is as shown in Fig. 1. In Fig. 1, a beam emitted from a laser beam producer 11a diverges into a grating beam BG and a scanning beam Bp by means of an optical instrument such as a half mirror. The scanning beam Bp is expanded by a beam expander 12p to have a fixed diameter, and is brought to a galvano mirror 13 to be polarized.

After passing through an object lens 14, the scanning beam Bp is reflected at a mirror 15 to scan an original A in the direction indicated by an arrow in Fig. 1. Then each element of a photo-sensor array 16 receives a reflected beam from the original Ato convert it into a corresponding voltage signal.

In the meantime, said grating beam BG is expanded to have a fixed diameter by a beam expander 12, and is brought to the galvano mirror 13to be polarized. After passing through the object lens 14, the grating beam BG is brought to an optical grating 17. Then the grating beam BG is picked up by each element of a photo-sensor array 18 to produce a grating signal f,,.

In thus-constructed optical system, the galvano mirror 13 is driven by a signal V (Voltage) expressed by an equation: V=VO sin tot (1) wherein V0 is the maximum amplitude of the signal V, while co is an angular velocity. Therefore, the swing angle 6 (radian) of the galvano mirror 13 is expressed by an equation: 6=60 sin tot (2) wherein 80 is the maximum swing angle of the galvano mirror.

Fig. 2 shows the detail of the action of the scanning beam Bp which is scanning an original A being polarized by the galvano mirror 13, from which the mirror 1-5 is omitted for simplification.

In Fig. 2, the scanning length L (mm) of the scanning beam Bp can be expressed by an equation: L=2 . y=2 . f . tan 2# =2 . f . tan (2 . #0 . sin #t) (3) wherein f (mm) is the focal distance of the object lens 14.

On the other hand, the scanning speed V of the scanning beam Bp can be expressed by an equation:

wherein K1 is Kt=4- f f 6o By the way, assuming that the projection area (diameter) of the scanning beam Bp on the original A is constant regardless of the swing angle 6 of the galvano mirror 13, the time T that the spot of the scanning beam Bp on the original A moves per a unit length I can be expressed by an equation:

therefore, the integrated beam quantity E (per a unit time and a unit length) can be expressed by an equation:

wherein P is the intensity (W) of the scanning beam Bp and K2 is K2= I .

k1 Fig. 3 shows a graph of the variation of the integrated beam quantity E of one scanning line obtained by specifying the parameters of the right member of the equation (6). When an image signal is obtained by using the scanning beam Bp having such a characteristic, the voltage of the signal corresponding to the central portion of the scanning line becomes lower and that of the image signal corresponding to the edge portion thereof becomes higher. Therefore, a compensation process for such a shading phenomenon is carried out in conventional laser beam scanners. To resolve the above-mentioned problem, Japanese Patent laid Open No. 58-27466 discloses the following method.

That is, by previously inputting proper shading compensation coefficients for specific points of one scanning line to a memory, image signal from a photo-sensor is compensated for the shading phenomenon. Inconveniently, this method naturally requires a certain amount of memory capacity, furthermore, since the coefficients for the points other than the specific points are computed through an interpolation process in the method, any system for embodying the method must be provided with a computation device and a soft ware for the above computation process. What is more inconvenient of such a computer is its inability of processing image data in real time.

Japanese Patent Publication No. 58-19187 discloses another method as follows. That is, the inverse values of shading compensation coefficients are input to a memory beforehand instead of the coefficients themselves, and an image signal is compensated for the shading phenomenon by means of a multiplication between the coefficient and the image signal. Practically, the inverse coefficients are reduced by a certain value in being input to the memory to allow the memory capacity to be smaller, however, the method has the same drawbackthatthe previous method has.

Japanese Patent laid Open No. 57-119565 discloses yet another method as follows. That is, as well as the above-mentioned two methods, the compensation process is carried out by using coefficients stored in a memory beforehand, in addition, this method adopts a trouble-some way of determining the coefficients by previously scanning a reference (white) original.

Summary of the Invention This invention is proposed to resolve the above conventional drawbacks.

An object of this invention is to provide a method and system for compensating for a shading phenomenon using no expensive memory for storing shading compensation coefficients. Another object of this invention is to provide the above method being free from a soft ware for computing shading compensation coefficients.

To achieve the above objects, this invention carries out the following processes on an input and/or an output signal(s).

In the input side, image signal, i.e., a voltage signal being in inverse proportion to the scanning speed of a scanning beam polarized by a beam polarizing means is multiplied by a compensation signal (mentioned afterwards), i.e., a voltage signal being proportional to the scanning speed of the input scanning beam to obtain a compensated image signal. The above-mentioned compensation signal can be obtained by adjusting the offset value and the amplitude of the output signal of a low-pass filter of a PLL circuit when a signal obtained from an optical grating is multiplied by the PLL circuit.

In the output side, image signal, i.e., a voltage signal is multiplied by a compensation signal, i.e., a voltage signal being in proportion to the scanning speed of a recording beam polarized by a polarizing means to obtain a recording signal, and then the recording signal is used for driving the recording beam in exposing a photosensitive material. This recording compensation signal can be obtained in the same manner as mentioned on the input side.

By using above-mentioned method and system, a real-time shading compensation process can be pqrformed by using a circuit being simpler than that used in any other conventional systems.

The above and other objects and features of this invention can be appreciated more fully from the following detailed description when read with reference to the accompanying drawings.

Brief Description of the Drawings Fig. 1 shows an optical system of the input side of an image reproducing system.

Fig. 2 shows the main part of the optical system shown in Fig. 1.

Fig. 3 shows a graph of the variation of the integrated beam quantity E of one scanning line.

Fig. 4 shows a PLL circuit.

Fig. 5 shows the wave forms of the voltages of several points of the PLL shown in Fig. 4.

Fig. 6 shows the input scanning part of an image reproducing system to which the method of this invention is applied.

Fig. 7 shows the wave forms of the voltages of several points of the system shown in Fig. 6.

Fig. 8 shows the recording part of an image reproducing system to which the method of this invention is applied.

Fig. 9 shows the wave forms of the voltages of several points of the system shown Fig. 8.

Fig. 10 shows a circuit for compensating for a shading phenomenon simply and the characteristic thereof.

Fig. 11 shows another circuit for compensating for a shading phenomenon simply.

Fig. 12 shows an output signal of a photo-sensor array containing a distortion.

Fig. 13 shows another circuit for compensating for a shading phenomenon and the section shape of the photo-sensor thereof.

Fig. 14 shows several signals in correspondence with photo-sensors.

Fig. 15 shows a relation between two photosensors.

Preferred Embodiment of this Invention Fig. 4 shows a conventional PLL (Phase Lock Loop) circuit 3 for obtaining a synchronization signal requisite for controlling an image reproducing system. The PLL circuit 3 is usually composed of a phase comparator 31, a low-pass filter 32, a voltagecontrolled oscillator 33 and a frequency divider 34.

When the aforementioned grating signal fin (shown in Fig. 5(a)) is input to the PLL circuit 3, ordinarily a synchronization signal fout as shown in Fig. 5(c) can be obtained by multiplying the frequency of the grating signal fin. Meanwhile, the output voltage VLPF of the low-pass filter 32 employed in the PLL circuit 3 corresponds to the scanning speed of the scanning beam Bp as shown in Fig. 5(b), and can be expressed by an equation: VLpFOOK4 V (7) wherein K4 is a constant.

Therefore, when a signal\l,,, obtained from the output signal VLPF of the low-pass filter 32 by adjusting the offset value and the amplitude thereof is multiplied by said integrated beam quantity E of the equation (6), a constant value free from the scanning speed V of the scanning beam can be obtained as expressed by the following equation (8).

VLpFXE P I K4 (constant) (8) Fig. 6 shows the input scanning part of an image reproducing system to which the method of this invention is applied, while Fig. 7 shows the wave forms of the voltages of several points of the system shown in Fig. 6.

At first, an image signal Vp obtained by scanning a reference original such as a white original is input from a photo-sensor array 16 via an amplifier 21 to one terminal of a multiplier 22a as a reference signal VPU (shown in Fig. 7(d)). On the other hand, the output signal VLPF of the low-pass filter 32 as shown in Fig. 7(b) is adjusted of the offset value and amplitude thereof by a signal processor 23a to be said signal VLpF, and then input to the other terminal of the multiplier 22a. The multiplier 22a multiplies the value of the reference signal Vpu by that of the signal V,pFto obtain an image signal Vdata being compensated for the shading phenomenon.The image signal Data then undergoes A/D conversion in the AID converter 24 to be input to an image processor 25 which carries out several works such as a color correction work on the image signal Vdata.

Fig. 8 shows the recording part of an image reproducing system to which the method of this invention is applied, while Fig. 9 shows the wave forms of the voltages of several points of the system shown in Fig. 8.

The optical system of Fig. 8 comprises almost the same unit that the input system shown in Fig. 1 has except the fact that a recording beam BR is modulated by an audio-optic modulator (AOM) 6 being driven by an AOM driving circuit 5, Precisely, a beam emitted from a beam producer 11b diverges into a recording beam BR and a grating beam BG.

The recording beam BR is brought to a photosensitive material E mounted on a recording drum D via a galvano mirror 13, an object lens 14 and a mirror 15b An optical system for obtaining a grating signal for the output side and a PLL circuit 3 are as same as the corresponding devices of the input side, and one set of the devices can also be common for both sides. After passing through an optical grating 17, a grating beam BG is converted into a corresponding voltage signal by the elements of a photo-sensor array 18 and is input to the PLL circuit 3. Finally, the signal VLPF as shown in Fig. 9(c) is output from a signal processor 23b to an analog multiplier 22b in the same manner described on the input side.

Meanwhile output signal of a voltage-controlled oscillator of the PLL circuit 3 is input to an image processor4to undergo a gradation correction process as well as to be converted to a corresponding halftone dot signal Vs as shown in Fig. 9(d) and is input to the analog multiplier 22b.

The halftone dot signal V9 is multiplied by said signal VLPF in the analog multiplier 22b to be input to the AOM driving circuit 5 as an output signal V,, as shown in Fig. 9(e). The AOM driving circuit 5 outputs a signal VAOM as shown in Fig. 9(f) to the AOM 6. The AOM 6 modulates the recording beam BR obtained from a beam emitted from the beam producer 11,.

Consequently, by polarizing the modulated recording beam BR by the galvano mirror 13 being driven by the aforementioned signal V0 (=sin ot) in exposing a photosensitive material, a reproduction image as shown in Fig. 9(h) can be recorded.

The above-mentioned method and system can also be applied to both the input and the output side simultaneously. Besides, it can of course be applied to an independent input scanning system orto an independent recording system.

As mentioned above, materialization of this invention provides a simple and economical image reproducing system being free from a shading phenomenon without using a memory for storing compensation coefficients and a computation device because this invention adopts a simple real-time shading compensation method in which input image signal or the output recording signal is multiplied by a shading compensation signal corresponding to the variation of the scanning speed of the input scanning or the recording beam.

Figs. 10,11,12,13, l4and 15showanother method for compensating for a shading phenomenon.

Fig. 10 shows a circuit, in which the outputs of the elements 18a, 18b ... 18n of a photo-sensor array 18 are input in parallel to a differential amplifier 22 via gain-variable differential amplifiers 21,, 21a, 21b 21n respectively.

When a curve a shown in Fig. 10(b) is the wave form of the output of each sensor element of the photo-sensor array 18 obtained by scanning a reference original such as a white original, the output signal of a constant value P as shown in Fig.

10(b) can be obtained by preponderantly reducing the gain of the differential amplifiers corresponding tothe sensor elements of the edge portions.

Fig. 11 shows another circuit, in which the outputs of several sets of consecutive plural (three or four in this case) sensor elements of the photo-sensor array 18 are input in parallel to a differential amplifier 24 via gain-variable differential amplifiers 23a to 23d respectively. After being adjusted of the amplitude by the differential amplifiers 23a to 23d, the outputs of each sets of the sensor elements are output from the differential amplifier 24. Although this circuit is a simplified embodiment and is not capable of outputting a signal of a complete constant value 3 as shown in Fig. 10(b), practically it has no problem in compensating for the shading phenomenon.

Thus an output signal of a constant characteristic can be obtained being free from the unevenness contained in the signals from the sensor elements caused by the variation of the scanning speed of the input scanning beam on an original. Nevertheless, the compensated signal obtained by using the circuits of Fig. 10 or 11 still contains a distortion as shown in Fig. 12. That results mainly from two reasons: one of which is the deteriorated sensitivity of the edge portions of each sensor element brought about in a sensor material (wafer) cutting stage; the other is the gaps between the sensor elements.

When a reproduction image is recorded by using a signal containing such a distortion, portions of the reproduction image corresponding to the distortion ingredients carry a scratch.

To resolve the above-mentioned problem, a method in which two photo-sensors 1 8A and 18B are employed as shown in Fig. 13(a) can be adopted.

Precisely, the photo-sensors 1 8A and 1 8B are arranged symmetrically about a scanning line of an original A as shown in Fig. 13(b), while the elements of both photo-sensors are arranged alternately in the main scanning direction not to make gaps between the sensor elements of both sensors synchronize in the sub-scanning direction (shown as an arrow X) as shown in Fig. 14(a) and (c). When a reference original (white original) is scanned by thus arranged photo-sensors 18A and 18B, they output signals 1A and 1B as shown in Fig. 14(b) and (d) respectively. By switching the output signals 1A and 1. with a multiplexer 19 controlled by a changeover clock S (shown in Fig. 14(e)) output from a changeover signal generator 20 (shown in Fig.

13(a)), a final output image signal Vin containing no said kind of distortion ingredients as shown in Fig.

14(f) can be obtained.

In the above method in which the output signal is obtained alternately from the two photo-sensors, each gap between the sensor elements of the photo-sensors 1 8A and 1 8B can be widened to the width corresponding to the length of a linear characteristic portion of the sensor element as shown in Fig. 15, therefore, each of the photosensors 18A and 18B can be composed of sensor elements of fewer number in comparison with the photo-sensors used in the circuit of Fig. 10.

By the way in the circuits of Figs. 13 and 14, one differential amplifier is provided for one sensor element, however, one differential amplifier can also be shared by a plurality of sensor elements.

The above-mentioned three methods are capable of automatically compensating for a shading phenomenon by previously determining the gain of each differential amplifier without using no memory for memorizing compensation coefficients and no computation device for processing image data using the coefficients.

Claims

1. A method of compensating for a shading phenomenon which occurs in an image signal obtained by scanning an original with a beam scanner, the method comprising scanning an array of photosensors arranged to provide output signals to an image processing circuit with the beam reflected from the original, and adjusting the gain of a plurality of amplifiers, each of which is arranged to amplify the output signal of one or more of the photodetectors of the array.

2. A method according to Claim 1 in which the output signals from the amplifiers are amplified by a further amplifier of adjustable gain before being applied to the image processing circuit.

3. A method according to Claim 1 or Claim 2 in which the array of photosensors is subdivided into two sub-arrays containing alternate photosensors, and multiplexing means is provided for obtaining an output signal alternately from each of the subarrays.

4. A method according to any preceding claim in which the output of sets of consecutive photosensors of the array or sub-array are input in parallel to each of the plurality of adjustable-gain amplifiers.