JPH076836B2 - Beam spot shape detection method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention relates to an image of a document on a scanning surface by optically scanning a light beam, such as a plate making scanner, a laser printer, a laser plotter. Read information,
It also relates to a scanning optical device for recording a desired pattern on a photosensitive material on a scanning surface, and more particularly to a method for detecting the shape of a beam spot on the scanning surface.

<Prior Art> In this type of scanning optical device, when the shape of the beam spot on the scanning surface deviates from a target value due to, for example, an error in the optical system, the edge of the recorded pattern is blurred or the scanning line is not lined. Is known to occur (a gap occurs between scanning lines). Therefore, conventionally, the following method has been proposed and implemented in order to detect the shape of the beam spot on the scanning surface.

The first method will be described with reference to FIG. On the peripheral wall of the drum 1 that rotates at a constant speed, positive and negative 45
Two inclined slits 2 and 3 are formed. The light beam is emitted from the outside of the drum 1 perpendicularly to the peripheral wall. A photodetector 4 is provided inside the drum 1 so as to face the beam spot BS formed on the peripheral surface of the drum 1.

As shown in FIG. 8 (a), when the first slit 2 crosses the elliptical beam spot BS, the photodetector 4 outputs a detection signal as shown by S _X in FIG. This detection signal
By counting the clock pulse CK over a period (t _{0 to} t ₁ ) at which S _X is equal to or higher than a predetermined level,
The time required for the first slit 2 to cross the beam spot BS is obtained. When the count value and N _X, X direction of the beam spot BS (i.e., a direction perpendicular to the first slit 2) the length of is given by N _X × sin45 °. Similarly, the length of the beam spot BS in the Y direction (that is, the direction perpendicular to the second slit 3) is given by N _Y × sin 45 ° as shown in FIG. 8 (b).

The second method is the method described in Japanese Patent Laid-Open No. 64-13514. This method provides a slit whose optical distance from the light source is equivalent to the scanning surface, scans the light beam across the slit, and detects the light passing through the slit to detect the shape of the beam spot BS. is doing. Specifically, the first slit inclined with respect to the scanning direction of the light beam and the second slit perpendicular to the scanning direction are used to measure the time required for the beam spot to cross each slit. Based on this, the beam spot diameter in the direction perpendicular to each slit is obtained.

The third method is to irradiate a two-dimensional image pickup device such as a CCD with a beam spot in an enlarged manner, and directly measure the beam spot diameter from the image.

<Problems to be Solved by the Invention> However, the above-described conventional example has the following problems.

The first and second methods of determining the beam spot diameter using two slits, in short, have the same inclination as the first and second slits and are circumscribed to the beam spot BS, as shown in FIG. The parallelogram ABCD is determined, and the shape of the beam spot BS is obtained based on the distances X and Y between the opposing sides. The sides BC and AD are, for example, FIG. 8 (a).
Is a parallel line determined by the first slit 2 of
B and DC are parallel lines determined by the second slit 3 in FIG. 8 (b).

By the way, the beam spot on the scanning surface is usually an elliptical shape, and the elliptical shape inscribed in the parallelogram ABCD is not uniquely determined and there are innumerable numbers. For example, a beam spot BS 'shown by a chain line in FIG. 9 is also inscribed in the parallelogram ABCD. In other words, according to the first and second methods, the beam spots BS and BS ′ having different shapes are erroneously detected as having the same shape, and the elliptical shape of the beam spot cannot be discriminated. The beam spots BS and BS 'in FIG. 9 have different beam widths in the sub-scanning direction. Therefore, for example, the beam spot BS can be scanned and recorded normally as shown in FIG.
In BS ', there is a possibility that scanning lines may be broken as shown in Fig. 10 (b).

In addition, in the first and second methods using the slit, the rising and falling timings of the light amount detection signal are affected by the width of the slit, so if the shape of the beam spot is accurately detected, It is necessary to make a correction according to the width. Also, if the slit width is large, the noise component included in the light amount detection signal increases due to the influence of ambient light,
On the other hand, if the slit width is made too small, the amount of light transmitted through the slit will become extremely small, and the level of the light amount detection signal will drop, making it more susceptible to the internal noise of the light amount detector. This causes the inconvenience of lowering.

On the other hand, according to the third method, although the elliptical shape can be discriminated, the problem that the adjustment of the optical system for expanding and irradiating the beam spot is complicated and the apparatus becomes large, blooming, and crossing are required. It is also necessary to consider the effects of talk.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a beam spot shape detection method capable of accurately and easily detecting the shape of a beam spot.

<Means for Solving the Problem> The present invention has the following configuration in order to achieve such an object.

That is, the invention described in claim (1) is a method for detecting the shape of a beam spot on a scanning surface in a scanning optical device, which comprises a light beam and a knife edge which is a linear light-shielding / light-transmitting boundary. Of the at least three knife edges each having a different inclination angle with respect to the relative moving direction of the light beam and the knife edge, when one of the three knife edges is not known. And one knife edge having the same inclination angle, and detecting the light quantity of the light beam passing through each knife edge, differentiating the detected light quantity, and dividing the detected light quantity into two knife edges having the same tilt angle. Based on the differential value of the detected light amount, the relative moving speed of the light beam and the knife edge is obtained, and the differential value of the detected light amount of the three knife edges having different tilt angles is obtained. The time required for the light beam to cross each knife edge is determined based on the above, and the relative moving distance required for the light beam to cross each of the three knife edges based on the moving speed and each time. Is calculated, and the shape of the beam spot is obtained based on the three moving distances.

On the other hand, the invention according to claim (2) is a method for detecting the shape of a beam spot on a scanning surface in a scanning optical device, which comprises a light beam and a knife edge which is a linear light-shielding / light-transmitting boundary. When the relative moving speed of each of the knife edges is known in advance, three knife edges having different inclination angles with respect to the relative moving direction of the light beam and the knife edge are used to pass each of the knife edges. The light amount of the light beam is detected, the detected light amount is differentiated, based on the differential value of each detected light amount, the time required for the light beam to cross each knife edge is obtained, the light beam and the knife edge And a relative moving speed of the light beam and each of the three times, the relative moving distance required for the light beam to cross each of the three knife edges is calculated. Based, and requests the shape of the beam spot.

<Operation> According to the invention of each of the above claims, the time required for the light beam to cross three knife edges having different inclination angles is obtained, and the relative moving speed between the light beam and the knife edge and each of the above-mentioned times are obtained. Based on, the relative distance traveled by the light beam to cross the three knife edges can be calculated. When such a moving distance is obtained, a hexagon circumscribing the beam spot and having parallel opposing sides is determined.
Since there is one elliptical shape inscribed in this hexagon, the shape of the beam spot, specifically, the major axis of the elliptical beam spot, the minor axis of the elliptical beam spot, and the beam of the major axis of the beam, based on the above three movement distances The angle with respect to the moving direction is uniquely obtained.

<Example> Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

First Embodiment FIG. 1 is a schematic block diagram showing an example in which the method of the present invention is applied to shape detection of a beam spot of a plane scanning optical device. However, the present invention can be applied not only to the plane scanning type optical device but also to the rotary scanning type optical device, and can be applied regardless of the recording system and the reading system.

The plane scanning optical device shown in FIG.
And a polygon mirror 11 as a means for deflecting the light beam B emitted from the laser light source 10, and an fθ lens 12 that converges the incident light beam and emits the incident light beam with a constant angular velocity as a light beam with a constant velocity. And this fθ
A reflection mirror 13 for reflecting the light beam emitted from the lens 12 toward the scanning surface 14 is provided. The scanning surface 14 corresponds to the surface of the photosensitive material placed on a flat table (not shown). Due to the rotation of the polygon mirror 11, the beam spot of the light beam moves on the scanning surface 14 in the main scanning direction,
Further, the beam spot relatively moves on the scanning surface 14 in the sub-scanning direction orthogonal to the main scanning direction by the movement of the flat table.

In order to detect the shape of the beam spot on the scanning surface 14 in such an optical device, a knife edge (that is, a linear light shield A mask 20 having a transparent boundary is arranged. As shown in FIG. 2 (a), the mask 20 has two light-transmitting regions G ₁ and G ₂ in a triangular shape in a light-shielding region H,
Vertical edges and inclined edges of the respective light-transmitting areas G ₁ and G ₂ form knife edges 21, 22, 23 and 24 arranged in an M shape. Of these, the three knife edges 21 to 23 relating to the shape detection of the beam spot BS have different inclination angles of 90 °, 150 °, 30 ° with respect to the moving direction (main scanning direction) of the beam spot BS. There is. Also, beam spot BS
The knife edges 21 and 24 for detecting the moving speed of the have the same inclination angle, and their separation distance d is, for example, 1 m.
It is set to m.

Below the mask 20, there is provided a photodetector such as a photodiode 25 that detects the light amount of the light beam that has passed through the light-transmitting areas G ₁ and G ₂ . Photodiode 25 is mask 20
May have a single shape slightly larger than the light-transmitting areas G ₁ and G ₂ , or may be composed of four segment-shaped photodiodes arranged facing each knife edge 21-24. Good.

When the elliptical beam spot BS as shown in FIG. 2 (a) crosses the mask 20, the photodiode 25
Outputs a light amount detection signal as shown in FIG. 2 (b).

The detection signal of the photodiode 25 is amplified by the amplifier 26, converted into a digital signal by the A / D converter 27, and stored in the memory 28. The CPU 29 reads out the light amount data stored in the memory 28 and performs a differentiation process to calculate a light amount differential value as shown in FIG. 2 (c). By performing arithmetic processing on this light quantity differential value, the major axis, the minor axis of the beam spot BS, and the angle of the major axis of the beam with respect to the beam traveling direction are calculated, and the results are output to the display 30 or the like.

The arithmetic processing performed by the CPU 29 will be described below.

As shown in FIG. 2 (c), from the light quantity differential value data,
Peak values of S1, S2, S3 corresponding to knife edges 21, 22, 23
Light intensity level C ・ P ₉₀ , C ・ P ₁₅₀ , C ・ P detected by detecting P ₉₀ , P ₁₅₀ , P ₃₀ and multiplying these peak values by a constant coefficient C
₃₀ (However, the combination of light intensity levels C / P ₉₀ and C / P ₃₀ and C / P ₁₅₀
And the sign is different). Here, the coefficient C is a coefficient for determining the beam spot diameter of a light beam having a normal distribution of light quantity, and is, for example, 1 / e ² (e = 2.71).
828 ...) is set. Such light level C ・ P ₉₀ , C
・ Time of each peak S1, S2, S3 exceeding P ₁₅₀ , C ・ P ₃₀ t ₉₀ , t
_{Find 150} , t ₃₀ . The average value of peaks S1 and S4 may be used as t ₉₀ .

T ₉₀ , t ₁₅₀ , t ₃₀ obtained in this way is the beam spot
BS has 1st knife edge 21, 2nd knife edge 22, 3rd
Corresponding to the time required to cross the knife edge 23 of the. Next, the time t _P between the first peak S1 and the fourth peak S4 is obtained.

Since the distance d between the first knife edge 21 and the fourth knife edge 24 is known, the moving speed of the beam spot BS is calculated from d / t _P. As shown in FIG. 3, by multiplying this moving speed by t ₉₀ , t ₁₅₀ , and t ₃₀ , respectively,
The travel distances 2T ₉₀ , 2T ₁₅₀ , 2T ₃₀ required for the beam spot BS to cross each knife edge 21, 22, 23 respectively can be known. Here, T ₉₀ , T ₁₅₀ , and T ₃₀ are given by the following equation.

By substituting T ₉₀ , T ₁₅₀ , and T ₃₀ obtained by the equations into the following equations, respectively, the major axis 2a, the minor axis 2b, and the major axis of the elliptical beam spot BS shown in FIG. 3 can be obtained. Angle θ (0 ° ≦ θ
<90 °) can be calculated. When the beam spot BS inclines in the opposite direction as shown in FIG. 9, in the formula, 2a is the minor axis, 2b is the major axis, and θ is the minor axis.

However, when the _{T 30 = T 150, θ =} 0,2a = 2T 90, 2b = 2 {(T 2 30 -T 2 90/3} as shown in ^half Fig. 3, the opposing sides, respectively Parallel hexagon
Since there is one ellipse inscribed in ABCDEF, the shape of the beam spot BS can be correctly measured by the above formulas.

The optical system of the scanning optical device as shown in FIG. 1 may have a different beam spot shape depending on the scanning position.
According to the embodiment described above, the mask 20 and the photodiode 25
Can be installed at various positions in the main scanning direction of the light beam B, and the beam spot shape at each position can be measured.

The inclination angles of the knife edges 21 to 24 are not limited to those in the above embodiment, and can be set arbitrarily. Further, the knife edges 21 to 24 do not necessarily have to be arranged in an M shape, and can be arranged in any arrangement as shown in FIGS. 4 (a) to 4 (c), for example. Further, a mask may be used in which each of the light-transmitting regions G ₁ and G ₂ shown in FIGS. 2A and 4 is used as a light-shielding region and the surrounding region thereof is a light-transmitting region.

Second Embodiment In the first embodiment, the light beam B is scanned and the mask 20 is stopped to move the light beam B and the mask 20 relative to each other. However, the light beam B is stopped and the mask 20 is moved. As a result, it is possible to detect the shape of the beam spot BS.

For example, as shown in FIG. 5, a disc 31 on which the mask 20 is formed is arranged perpendicularly to the light beam B, and the disc 31 is rotationally driven by a motor 32. A disk 31 on the optical path of the light beam B
By disposing the photodiode 25 on the rear side and detecting the light quantity of the light beam passing through each knife edge with this photodiode 25, the shape of the beam spot can be detected as in the first embodiment.

Alternatively, as shown in FIG. 6, the drum 33 on which the mask 20 is formed is rotationally driven, and the photodiode 25 provided inside the drum 33 detects the light amount of the light beam passing through each knife edge. It is also possible to detect the shape of the beam spot.

In addition, when the knife edge arranged in M shape is used,
Since the relative speed between the light beam B and the knife edge is detected by the two knife edges 21 and 24 having the inclination angle of 90 °, the measurement accuracy does not decrease due to the uneven rotation speed of the slit driving motor. .

Third Embodiment In the first and second embodiments, when the relative moving speeds of the light beam B and the mask 20 are not known in advance, the four knife edges 21 to 24 are used and the two knife edges 21 inclined at 90 ° are used. , 2
The relative moving speed was calculated from the amount of light detected through 4. However, when the relative moving speed of the light beam and the knife edge is known in advance, it is not necessary to detect the beam shape using the four knife edges as described above, and the relative movement direction of the beam spot is not detected. The shape of the beam spot can be detected by the three knife edges 21 to 23 having different inclination angles.

<Effects of the Invention> As is clear from the above description, according to the invention described in claim (1), when the relative moving speed of the light beam and the knife edge is not known in advance, on the scanning surface. The shape of the beam spot can be detected relatively easily and accurately.

According to the invention described in claim (2), when the relative velocity between the light beam and the knife edge is known in advance,
The beam spot shape on the scanning plane can be detected relatively easily and accurately.

Further, according to the inventions of claims (1) and (2), the light quantity is detected via the knife edge, and the shape of the beam spot is obtained based on the differential value of the light quantity.
The process of correcting the width of the slit unlike the conventional example is unnecessary. Further, since the change in the amount of light detected before and after the knife edge is larger than the change in the amount of light detected through the slit, it is possible to perform highly reliable shape detection that is not easily affected by noise.

[Brief description of drawings]

1 to 3 relate to a first embodiment of the present invention.
The figure is a block diagram showing its schematic configuration, FIG. 2 (a) is an explanatory view of knife edges arranged in an M shape, and FIG. 2 (b) is a waveform of a light amount detection signal detected through the knife edges. FIG. 2 (c) is a differential waveform diagram of the light amount detection signal,
FIG. 3 is an explanatory diagram of the shape detection of the beam spot, and FIGS. 4 (a), (b) and (c) are explanatory diagrams of a modified example of the knife edge. FIG. 5 is an explanatory view of the second embodiment, and FIG. 6 is an explanatory view of its modification. FIG. 7 is a perspective view showing a main structure of a conventional method, FIG. 8 is an explanatory view of the operation of the conventional method, and FIGS. 9 and 10 are explanatory views of problems of the conventional method. 20… Mask 21,22,23,24… Knife edge 25… Photodiode BS… Beam spot

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kunio Tomohisa 1 Nihon Screen Manufacturing Co., Ltd. 1 No. 1 Tenjin Kitamachi 4-chome, Horikawa-dori Teranouchi, Kamigyo-ku, Kyoto-shi, Kyoto (56) References 3-160329 (JP, A) JP-A-3-120426 (JP, A) JP-A 1-314926 (JP, A)

Claims (2)

[Claims] 1. A method for detecting a shape of a beam spot on a scanning surface in a scanning optical device, wherein a relative moving speed of a light beam and a knife edge which is a linear light-shielding / light-transmitting boundary is previously determined. If not known, at least three knife edges, each having a different tilt angle with respect to the relative movement direction of the light beam and the knife edge, and one having the same tilt angle as one of the three knife edges. The knife edge is used to detect the light quantity of the light beam that has passed through each of the knife edges, and the detected light quantity is subjected to differential processing, and based on the differential value of the detected light quantity relating to two knife edges with the same inclination angle. , The relative moving speed of the light beam and the knife edge is obtained, and the light boom is adjusted based on the differential value of the detected light amount for the three knife edges with different inclination angles. The time required to cross each knife edge is obtained, and based on the moving speed and each time, the relative moving distance required for the light beam to cross each of the three knife edges is calculated, A method of detecting a shape of a beam spot, wherein the shape of the beam spot is obtained based on the three moving distances. 2. A method for detecting the shape of a beam spot on a scanning surface in a scanning optical device, wherein a relative moving speed of a light beam and a knife edge which is a linear light-shielding / light-transmitting boundary is previously determined. If known, three knife edges, each having a different tilt angle with respect to the relative direction of movement of the light beam and the knife edge, are used to detect the amount of light beam passing through each said knife edge. , Differentially processing the detected light quantity, based on the differential value of each detected light quantity, the time required for the light beam to cross each knife edge is obtained, and the relative moving speed of the light beam and the knife edge, The relative movement distance required for the light beam to cross each of the three knife edges is calculated based on each of the times, and the beam spot is calculated based on the three movement distances. Shape detection method of the beam spot, wherein the determination of the shape.