JPH03223630A - Method for detecting shape of beam spot - Google Patents

Abstract

PURPOSE:To perform simple and accurate detection by computing the relative moving distance required for crossing a knife edge whose angle is different from those of two knife edges having the same slant angle. CONSTITUTION:The beam spot of a light beam B from a laser light source 10 is moved on a scanning surface 14 in the main scanning direction (a) by the rotation of a polygon mirror 11. The beam spot is relatively moved in the secondary scanning direction (b) which is perpendicular to the main scanning direction by the movement of a flat table. A mask 20 having the knife edges which are arranged in an M pattern is arranged on the appropriate position on the same surface as the surface 14. When the elliptical beam spot crosses the mask 20, a photodiode 25 outputs the detected light-amount signal. The signal is stored in a memory 28 through an amplifier 26 and an A/D converter 27. A CPU 29 operates the light-amount data in the memory 28 and computes the major axis and the minor axis of the beam spot and the angle of the longitudinal axis of the beam with respect to the advancing direction of the beam. The results are outputted into a display device 30. Thus, the detection can be performed simply and accurately.

Description

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

<Prior Art> In this type of scanning optical device, if the shape of the beam bond on the scanning surface deviates from the target value due to an error in the optical system, for example, the edges of the recorded pattern may become blurred or the scanning line may be distorted. It is known that a gap may occur between scanning lines. Therefore, the following methods have been proposed and implemented in the past in order to detect the shape of a beam spot on a scanning surface.

The first method will be explained with reference to FIG.
Two slits 2 and 3 are formed with an angle of 5". A light beam is irradiated from the outside of the drum 1 perpendicularly to the peripheral wall. A beam spot B is formed on the peripheral surface of the drum 1.
A photodetector 4 is provided inside the drum 1 so as to face S.

As shown in FIG. 8(a), when the first slit 2 crosses the elliptical beam bond BS, the photodetector 4 outputs a detection signal as indicated by SX in the same figure. The period during which this detection signal sx exceeds a predetermined level (t6
-Ll), the time required for the first slit 2 to cross the beam spot BS is determined by counting the clock pulses CK. If the count value is N, then the length of the beam bond BS in the X direction (that is, the direction perpendicular to the first slit 2) is NX×sin
given by 45°. Similarly, the length of the beam spot BS in the Y direction (that is, the direction perpendicular to the second slit 3) is Ny
45”.

As a second method, there is a method as described in Japanese Patent Laid-Open No. 13514/1983. This method detects the shape of the beam bond BS by providing a slit whose optical distance from the light source is equivalent to the scanning plane, scanning the light beam across the slit, and detecting the light that has passed through the slit. are doing.

Specifically, the angle tilted with respect to the scanning direction of the light beam is
and a second slit perpendicular to the scanning direction, and based on measuring the time required for the beam spot to cross each slit, determine the beam spot diameter in the direction perpendicular to each slit. I'm looking for.

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

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

The first and second methods of determining the beam spot diameter using two slits basically have the same slope as the first and second slits and are circumscribed to the beam spot BS, as shown in FIG. A parallelogram ABCD is determined, and the shape of the beam spot BS is determined based on the distances #X and Y between opposing sides. For example, sides BC and AD are parallel lines determined by the first slit 2 in FIG. 8(a), and sides AB and DC are parallel lines determined by the second slit 3 in FIG. 8(b). be.

By the way, the beam spot on the scanning plane is usually elliptical, and the elliptical shape inscribed in the parallelogram ABCD is not uniquely determined but exists in an infinite number. For example, Beamsbond BS' as shown by the chain line in FIG.
It is inscribed in parallelogram ABCD. In other words, the first
According to the second method, beam spots BS and BS' having different shapes are mistakenly detected as having the same shape, and it is impossible to determine whether the beam spot has an elliptical shape.

Since beam spots Bs and BS' in FIG. 9 have different beam widths in the sub-scanning direction, for example, beam spot B
Even if scanning and recording is performed normally at beam spot S as shown in Figure 10(a), beam spot BS' is scanned and recorded normally as shown in Figure 10(b).
There is also a risk that scanning line wrinkles may occur as shown in FIG.

In addition, the first method using a slit. In the second method, the timing of the rise and fall of the light intensity detection signal is affected by the width of the slit, so in order to accurately detect the shape of the beam spot, it is necessary to perform correction according to the width of the slit. be. Also, if the slit width is large, noise components included in the light intensity detection signal will increase due to the influence of ambient light, and conversely, if the slit width is too small, the amount of light transmitted through the slit will be extremely small, and the level of the light intensity detection signal will decrease. In either case, the measurement accuracy decreases, which is disadvantageous because the light intensity detector becomes susceptible to the influence of internal noise.

On the other hand, according to the third method, it is possible to distinguish elliptical shapes, but the adjustment of the optical system for enlarging the beam spot is complicated, and there are problems such as major damage to the device, blooming, It is also necessary to consider effects such as crosstalk.

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

Means for Solving the Problems> In order to achieve the above objects, the present invention has the following configuration.

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 method comprises: detecting the shape of a beam spot on a scanning surface in a scanning optical device; at least three knives each having a different angle of inclination with respect to the direction of relative movement of the light beam and the knife when the relative speed of movement is not known in advance; one knife having an angle, detecting the light intensity of the light beam passing through each knife, performing differential processing on the detected light intensity, and obtaining a differential value of the detected light intensity with respect to two knives having the same inclination angle. Based on this, calculate the relative moving speed of the light beam and the knife, and also calculate the time required for the light beam to cross each knife based on the differential value of the amount of detected light for the three knives with different inclination angles. , Calculating the relative travel distance required for the light beam to cross each of the three knives based on the travel speed and each time, and determining the shape of the beam spot based on the three travel distances. It is.

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: When the relative speed of movement is known in advance, three knives having different inclination angles with respect to the direction of relative movement between the light beam and the knife are used to calculate the light beam passing through each knife. , detecting the amount of light of , differentially processing the detected amount of light, determining the time required for the light beam to cross each knife edge based on the differential value of each detected light amount, and determining the relative amount of the light beam and the knife edge. The relative moving distance required for the light beam to cross each of the three knife edges is calculated based on the moving speed and each of the above-mentioned times, and the shape of the beam spot is determined based on the three moving distances. be.

According to the invention of each of the above claims, the time required for the light beam to cross three knife blades having different inclination angles is determined, and the relative moving speed of the light beam and the knife blade is calculated based on the respective times. Based on this, the relative travel distance required for the light beam to traverse the three knives can be calculated.

Once such a moving distance is determined, a hexagon that circumscribes the beam spot and whose opposing sides are parallel is determined. Since there is only one ellipse inscribed in this hexagon, the shape of the beam spot, specifically, the major axis, minor axis, and beam of the beam long axis of the elliptical beam spot, is determined based on the above three moving distances. The angle with respect to the direction of movement is uniquely determined.

<Example> Hereinafter, an example of the present invention will be described in detail based on the drawings.

FIG. 1 is a schematic block diagram showing an example in which the method of the present invention is applied to detecting the shape of a beam spot of a plane scanning optical device. However, the present invention is not limited to a plane scanning optical device, but can also be applied to a rotational scanning optical device.
It can be used regardless of the recording system or reading system.

The plane scanning optical device shown in FIG.
♂, this laser light source] A polygon mirror 11 as a means for deflecting the light beam B irradiated from O, and an fθ lens 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. 12, and the light beam emitted from this rθ lens 12 is directed to the scanning surface 14.
It is equipped with a reflective mirror 13 that reflects the light toward the target. The scanning surface 14 corresponds to the surface of a photosensitive material placed on a flat table (not shown). By the rotation of the polygon mirror II, the beam spot of the light beam moves on the scanning surface 14 in the main scanning direction, and by the movement of the flat table, the beam spot moves on the scanning surface 14 perpendicular to the main scanning direction. relative movement in the sub-scanning direction.

In order to detect the shape of the beam spot on the scanning surface 14 in such an optical device, an M-shaped knife (i.e., a linear light-shielding/transmitting knife) is placed at an appropriate position on the same plane as the scanning surface 14. A mask 20 with a light boundary) is placed. As shown in FIG. 2(a), the mask 20 has two triangular light-transmitting regions G+,
Knives 21 and 22, which are provided with a light-transmitting region Q, , Gx, and have vertical edges and inclined edges arranged in an M-shape.
．． 23.24 is formed. Among these, the three knives 21 to 23 involved in detecting the shape of the beam spot BS have different inclination angles of, for example, 90°, 150a, and 30@ with respect to the moving direction (main scanning direction) of the beam pants 1-BS. There is. Furthermore, the knives 2I and 24, which are used to detect the moving speed of the beam spot BS, have the same inclination angle, and the distance d between them is set to, for example, 1 mm.

A photodetector such as a photodiode 25 is provided under the mask 20 to detect the amount of light beam passing through each transparent region C, , C,. The photodiode 25 is located in the light-transmitting region G, , (1;) of the mask 20.

It may be of a single shape slightly larger than the above, or it may be composed of four segment-shaped photodiodes arranged opposite each knife 21 to 24.

When this mask 20 is crossed by an elliptical beam bond BS as shown in FIG. 2(a), the photodiode 25 outputs a light amount detection signal as shown in FIG. 2(a).

The detection signal of the photodiode 25 is amplified by an amplifier 26, then converted to a digital signal by an A/D converter 27, and stored in a memory 28. The CPU 29 is the memory 2
By reading the light amount data stored in 8 and performing differentiation processing, a light amount differential value as shown in FIG. 2(C) is calculated. By performing arithmetic processing on this light amount differential value, the major axis and minor axis of the beam spot BS, and the angle of the beam major axis with respect to the beam traveling direction are respectively 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 Figure 2 (C), from the light amount differential value data,
Husband 31. corresponding to Naifetsuji 21.22.23. S2,
Each peak value P of S3. + P 180 + P
1° and a constant coefficient C for these peak values.
The amount of light multiplied by Leher C・[),. , C-P+s. ,c-
p.

(However, for the set of light quantity level c-p, ., C-P3o, and C
・PIS. (with a different sign) for each.

Here, the coefficient C is a coefficient for determining the beam spot diameter of a light beam whose light amount has a normal distribution; for example, I
/ e ′ (e = 2.71828...). Such a light quantity Le Le C-P. , C.1),,
, , C-Tゝ, each peak exceeding 0 51.52S
3 time t,. ,t,,. , ti. seek. In addition, L. The average value of the peaks S1 and S4 may be used.

In this way, you can ask for it. r LlsO+ t
,. The beam spot BS is the first knife 21
．． Second Naifetsuji 22. This corresponds to the time required to cross the third knife ledge 23. Next, the time interval between the first beak S1 and the fourth beak S4 is determined.

Since the distance d between the first knife 21 and the fourth knife 24 is known, the moving speed of the beam spot BS is calculated from a/lP. This moving speed and the above,
. +LISO+L3°, the travel path #2T required for the beam bond BS to traverse each knife edge 21, 22, 23, respectively, as shown in FIG. , 2T+s. .

2T. can be known. Here, Tea, T
ea.

T. is given by the following equation.

2　　　　　t.

2　　　　　t.

■～■T, which was asked for in 2. , TI5゜, T,. of,
By substituting the following equations (■ to ■), the major axis 2a, minor axis 2b of the elliptical beam spot BS as shown in Fig. 3, and the angle θ (0°≦θ <90°) can be calculated. In addition, in the case where the beam probe LBS is tilted in the opposite direction as shown in Fig. 9, in formulas 0 to 0, 2a is the minor axis,
2b is the major axis, and θ is the angle of the minor axis.

2 a = [(1/3)(Tx+so+T'to
+47”qo) +(]/l-71)×(↑”+5
e-T"zo) ((1+tan"2θ) /la
n'2θl I/! l l/ffi...
・■ 2 b-[(1/3) (T'+so+T"3a+JT
”vo) (1/Jko)X (T!1so-T”s
o) ((1+jan"2θ) /lan"2θ
l l/2 ] 1/!・・・・・・■ θ−(1,/2) tan−' [1 piece (T”+5o−
T"so)/(8T'vo-T'1so-T"so)
１・・・・・・■ However, T. -Tea. When θ−0, 2a=2Tqo, 2 b = 2 [(T”5
o-T”qo)/3 As shown in Figure 3, there is one ellipse inscribed in the hexagon ABCDEF whose opposing sides are parallel, so the shape of the beam bond BS can be correctly determined using the above formulas can be measured.

In the optical system of the scanning optical device as shown in FIG. 1, the beam spot shape may vary 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.

Incidentally, the inclination angles of the knives 21 to 24 are not limited to those in the above embodiments, and can be set arbitrarily. Furthermore, the knives 21 to 24 do not necessarily have to be arranged in an M-shape (for example, they can be arranged in any arbitrary arrangement as shown in FIGS. 4(a) to 4(C)). a) and each light shielding area G, , Gt shown in FIG.
A mask may be used in which the area is a light-shielding area and the surrounding area is a light-transmitting area.

In the first embodiment, the light beam B and the mask 20 are moved relative to each other by scanning the light beam B and keeping the mask 20 stationary. By doing so, it is also possible to detect the shape of the beam spot BS.

For example, as shown in FIG. 5, a disk 31 on which the mask 20 is formed is placed perpendicular to the light beam B, and the disk 31 is rotated by a motor 32. A photodiode 25 is placed behind the disk 31 on the optical path of the light beam B, and the photodiode 25 detects the light intensity of the light beam that has passed through each knife, thereby determining the beam spot as in the first embodiment. The shape of can be detected.

Alternatively, as shown in FIG. 6, by rotating a drum 33 on which a mask 20 is formed and detecting the amount of light beam passing through each knife with a photodiode 25 provided inside the drum 33. , it is also possible to detect the shape of the beam spot.

In addition, when using knife blades arranged in an M-shape,
Since the relative speed between the light beam B and the knife is detected by the two knives 21 and 24 having an inclination angle of 90 degrees, measurement accuracy will not be degraded due to uneven rotational speed of the slint driving motor.

(11) In the first and second embodiments, when the relative movement speed between the light beam B and the mask 20 is not known in advance, four knife blades 21 to 24 are used, and two knife blades 21 and 24 with a 90° inclination are used. The relative movement speed was calculated from the amount of light detected through the . However, if the relative moving speed of the light beam and the knife blade is known in advance, there is no need to detect the beam shape using the four knives described above, and the beam spot is tilted with respect to the relative moving direction of the beam spot. Three knives with different angles 21-23
It is possible to detect the shape of the beam spot.

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

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

Furthermore, according to the invention described in claims (+) and (2), the amount of light is detected through the knife, and the shape of the beam spot is determined based on the differential value of the amount of light.
There is no need for processing to correct the width of the slit as in the conventional example. Furthermore, since the change in the amount of light detected before and after the knife is larger than the change in the amount of light detected through the slit, highly reliable shape detection that is less susceptible to noise can be performed.

[Brief explanation of drawings]

Figures 1 to 3 relate to the first embodiment of the present invention.
The figure is a block diagram showing its schematic configuration, Figure 2 (a) is an explanatory diagram of the naifetsuji arranged in an M-shape, Figure 2 (Mountain)
is a waveform diagram of the light quantity detection signal detected through the knife, Figure 2 (C) is a differential waveform diagram of the light quantity detection signal, Figure 3 is an explanatory diagram of beam spot shape detection, Figure 4 (a),
Φ), (C) is an explanatory diagram of a modification of the knife. FIG. 5 is an explanatory diagram of the second embodiment, and FIG. 6 is an explanatory diagram of a modification thereof. FIG. 7 is a perspective view showing the main structure of the conventional method, FIG. 8 is an explanatory diagram of the operation of the conventional method, and FIGS. 9 and 10 are explanatory diagrams of problems in the conventional method. 20... Mask 21, 22.23.24... Naifetsuji 25...
Photodiode BS...beam spot

Claims (2)

[Claims] (1) A method for detecting the shape of a beam spot on the scanning surface of a scanning optical device, in which the relative moving speed between the light beam and the knife edge, which is a linear light-shielding/light-transmitting boundary, is not known in advance. at least three knife edges each having a different angle of inclination with respect to the direction of relative movement of the light beam and the knife edge, and one knife edge having the same angle of inclination as one of said three knife edges. Detect the amount of light of the light beam that has passed through each of the knife edges using Determine the relative moving speed of the beam and the knife edge, and also determine the time required for the light beam to cross each knife edge based on the differential value of the detected light amount for three knife edges with different inclination angles, Calculating relative travel distances required for the light beam to cross each of the three knife edges based on the travel speed and each time, and determining the shape of the beam spot based on the three travel distances. A beam spot shape detection method characterized by: (2) A method for detecting the shape of a beam spot on a scanning surface in a scanning optical device, in which the relative moving speed between the light beam and a knife edge, which is a linear boundary between light blocking and light transmission, is known in advance. detect the amount of the light beam that has passed through each of the knife edges, using three knife edges each having a different inclination angle with respect to the direction of relative movement between the light beam and the knife edge; The detected light amount is differentiated, the time required for the light beam to cross each knife edge is determined based on the differential value of each detected light amount, and the relative moving speed of the light beam and the knife edge is calculated. A beam spot characterized in that the relative movement distance required for the light beam to cross each of the three knife edges is calculated based on the time, and the shape of the beam spot is determined based on the three movement distances. shape detection method.