JPH0670583B2 - 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 the 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 up. Is known to occur. 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. 10 (a), when the first slit 2 crosses the elliptical beam spot BS, the photodetector 4 outputs a detection signal as indicated by S _x in the figure. This detection signal
The time required for the first slit 2 to cross the beam spot BS is obtained by counting the clock pulses CK over a period (t _{0 to} t ₁ ) in which S _x is equal to or higher than a predetermined level. 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. 10 (b).

As the second method, there is a method described in JP-A-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 obtaining 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. 10 (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. 10 (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. 11 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. 11 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 the scanning line may be broken as shown in Fig. 12 (b).

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 the beam spot on the scanning surface in the scanning optical device, and in the case where the relative moving speed of the light beam and the slit is not known in advance. And at least 3 having different inclination angles with respect to the relative movement directions of the light beam and the slit.
The amount of light passing through each slit is detected using one slit and one slit having the same inclination angle as one of the three slits, and based on the detected light amount relating to the two slits having the same inclination angle, , The relative moving speed of the light beam and the slit is obtained, and the time required for the light beam to cross each slit is obtained based on the detected light amounts of the three slits having different inclination angles. The relative moving distance required for the light beam to cross each of the three slits is calculated based on each of the times, 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 the beam spot on the scanning surface in the scanning optical device, and the relative moving speeds of the light beam and the slit are known in advance. In this case, three slits each having a different inclination angle with respect to the relative movement direction of the light beam and the slit are used to detect the light amount passing through each slit, and the light beam is detected based on each detected light amount. The time required to traverse each slit is determined, and the relative movement required for the light beam to traverse each of the three slits based on the relative moving speed of the light beam and the slit and each time. The distance is calculated, and the shape of the beam spot is obtained based on the three moving distances.

<Operation> According to the invention of each of the above claims, the time required for the light beam to cross three slits each having a different inclination angle is obtained, and the relative moving speed of the light beam and the slit and the respective times are calculated. Based on this, the relative movement distance required for the light beam to cross the three slits 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 deflecting means for deflecting the light beam B emitted from the laser light source 10, 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 a reflection mirror 13 for reflecting the light beam emitted from the fθ lens 12 toward the scanning surface 14 and the like. Scanning surface 14
Corresponds to the surface of the photosensitive material placed on a flat table (not shown). The rotation of the polygon mirror 11 moves the beam spot of the light beam on the scanning surface 14 in the main scanning direction, and the movement of the flat table causes the beam spot to cross the scanning surface 14 at right angles 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 slit 20 is arranged at an appropriate position on the same surface as the scanning surface 14. The M-shaped slit 20 is composed of four slits 21 to 24, as shown in FIG. Of these, the three slits 21 to 23 related to the shape detection of the beam spot BS are, for example, 90 °, 150 with respect to the traveling direction (main scanning direction) of the beam spot BS.
There are different inclination angles of 30 ° and 30 °. Further, the slits 21 and 24 for detecting the moving speed of the beam spot BS
Have the same inclination angle, and the separation distance d is set to, for example, 1 mm. The width of each slit is preferably 20% or less of the average diameter of the beam spot BS. Practically, there is no problem even if it is about 50%, but if it exceeds 60%, the error becomes large.

Below the M-shaped slit 20, a photodetector such as a photodiode 25 for detecting the light amount of the light beam that has passed through the slits 21 to 24 is provided. The photodiode 25 is
It may be a single one having substantially the same size as the M-shaped slit 20, or may be composed of four photodiodes facing each slit 21-24.

When the elliptical beam spot BS as shown in FIG. 2 crosses the M-shaped slit 20, the photodiode 25
Outputs a detection signal as shown in FIG. In FIG. 3, S1 indicates the light quantity of the light beam that has passed through the first slit 21, S2 the second slit 22, S3 the third slit 23, and S4 the fourth slit 24, respectively.

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 the light amount data stored in the memory 28, and performs the following arithmetic processing to calculate the major axis of the beam spot BS, the minor axis, and the angle of the beam major axis with respect to the beam traveling direction, respectively. The result is output to the display device 30 or the like.

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

As shown in FIG. 3, the peak values P ₉₀ , P ₁₅₀ , and P ₃₀ of S1, S2, and S3 are detected from the light amount data read from the memory 28, and a constant coefficient C is determined for these peak values. The light intensity levels C, P ₉₀ , C, P ₁₅₀ , and C, P ₃₀ multiplied by are set respectively. 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 = 2.71828 ...) is set. The time of each data S1, S2, S3 that exceeds such light intensity levels C / P ₉₀ , C / P ₁₅₀ , C / P ₃₀
Find t ₉₀ , t ₁₅₀ , t ₃₀ . Note that t ₉₀ may be the average value of the data S1 and S4.

T ₉₀ , t ₁₅₀ , t ₃₀ obtained in this way is the beam spot
This corresponds to the time required for the BS to cross the first slit 21, the second slit 22, and the third slit 23. Next, the time t _P between the peaks of the first light amount data S1 and the fourth light amount data S4 is obtained.

Since the distance d between the first slit 21 and the fourth slit 24 is known, the moving speed of the beam spot BS is calculated from d / t _P. By multiplying this moving speed by t ₉₀ , t ₁₅₀ , t ₃₀ , respectively, as shown in FIG. 4, the moving distance 2T required for the beam spot BS to cross each slit 21, 22, 23 respectively. it is possible to know the _90, 2T _150, 2T _30.
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 as shown in FIG. 4 can be obtained. Angle θ (0 ° ≦ θ
<90 °) can be calculated. When the beam spot BS is inclined as shown in FIG. 11, in the formulas, 2a is the minor axis, 2b is the major axis, and θ is the minor axis.

However, when T ₃₀ = T ₁₅₀ , θ = 0, 2a = 2T ₉₀ , 2b = 2 {(T ² ₃₀ -T ² ₉₀ ) / 3} ^{1/2 As} shown in FIG. Parallel hexagons
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 above-described embodiment, the M-shaped slit 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 M-shaped slits 21 to 24 are not limited to those in the above embodiment, and can be set arbitrarily.

Further, the slits 21 to 24 do not necessarily have to be arranged in an M shape, and can be arranged in an arbitrary array as shown in FIGS. 5 (a) and 5 (b), for example.

Second Embodiment In the first embodiment, the light beam B is scanned and the M-shaped slit is formed.
The light beam B and the M-shaped slit 20 were moved relative to each other by stopping the light beam 20, but the light beam B was stopped and the M-shaped slit 20 was moved to change the beam spot.
It is also possible to detect the shape of the BS.

For example, as shown in FIG. 6, a disk 31 having an M-shaped slit 20 formed therein is arranged perpendicularly to the light beam B, and this disk 31 is rotationally driven by a motor 32. A photodiode 25 is arranged on the rear side of the disk 31 on the optical path of the light beam B, and the light quantity of the light beam passing through each slit is detected by this photodiode 25, so that a beam spot is obtained as in the first embodiment. The shape of can be detected.

Alternatively, as shown in FIG. 7, a drum 33 having an M-shaped slit 20 formed therein is rotationally driven, and a photodiode 25 provided inside the drum 33 detects the light amount of the light beam passing through each slit. Thus, it is possible to detect the shape of the beam spot.

When the M-shaped slit is used, the relative speed between the light beam B and the slit is detected by the two slits 21 and 24 having an inclination angle of 90 °. Therefore, due to the uneven rotation speed of the slit driving motor, The measurement accuracy does not decrease.

Third Embodiment In the first and second embodiments, when the relative moving speeds of the light beam B and the M-shaped slit 20 are not known in advance, the four slits 21 to 24 are used and the two slits 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 slit is known in advance, as shown in FIGS. 8 (a) and 8 (b), the inclination angles differ with respect to the relative moving directions of the beam spots. The shape of the beam spot can be detected by the three slits 21 to 23.

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

[Brief description of drawings]

1 to 4 relate to a first embodiment of the present invention.
FIG. 4 is a block diagram showing its schematic configuration, FIG. 2 is an explanatory diagram of an M-shaped slit, FIG. 3 is a waveform diagram of the amount of light detected through the M-shaped slit, and FIG. 4 is a beam spot shape detection. Explanatory drawing, FIG. 5 is explanatory drawing of the modification of a slit. FIG. 6 is an explanatory diagram of the second embodiment, and FIG. 7 is an explanatory diagram of its modification. FIG. 8 is an explanatory diagram of the slit according to the third embodiment. FIG. 9 is a perspective view showing the configuration of the main part of the conventional method, FIG. 10 is an explanatory view of the operation of the conventional method, and FIGS. 11 and 12 are explanatory views of problems of the conventional method. 20 …… M-shaped slit, 21 …… first slit 22 …… second slit, 23 …… third slit 24 …… fourth slit, 25 …… photodiode BS …… beam spot

Claims (2)

[Claims] 1. A method for detecting the shape of a beam spot on a scanning surface in a scanning optical device, wherein the relative speed of movement of a light beam and a slit is not known in advance. At least 3 having different inclination angles with respect to relative movement directions
The amount of light passing through each slit is detected using one slit and one slit having the same inclination angle as one of the three slits, and based on the detected light amount relating to the two slits having the same inclination angle, , The relative moving speed of the light beam and the slit is obtained, and the time required for the light beam to cross each slit is obtained based on the detected light amounts of the three slits having different inclination angles. A beam which is characterized in that a relative moving distance required for the light beam to cross each of the three slits is calculated based on each of the times and the shape of the beam spot is obtained based on the three moving distances. Spot shape detection method. 2. A method for detecting the shape of a beam spot on a scanning surface in a scanning optical device, wherein when the relative moving speed of the light beam and the slit is known in advance, the light beam and the slit It is necessary to detect the amount of light passing through each slit by using three slits each having a different inclination angle with respect to the relative moving direction of the light beam, and for the light beam to cross each slit based on each detected light amount. The time is obtained, and the relative moving distance required for the light beam to cross each of the three slits is calculated based on the relative moving speed of the light beam and the slit and each time, A method for detecting the shape of a beam spot, which is characterized in that the shape of the beam spot is obtained based on the moving distance.