JPH06288819A - Method for measuring quantity of face inclination of scanning optical system - Google Patents

Abstract

PURPOSE:To provide a method for measuring the quantity of face inclination of a scanning optical system in which the position of the spot of a laser beam in the sub-scanning direction at the time of optically scanning a face to be scanned is detected in only one scanning for one reflection face and rotation fluctuation of a rotating polygon mirror does not influence thereon. CONSTITUTION:A beam which is reflected from a rotating polygon mirror and made to scan the surface to be scanned is received by a photodetector means 2 for every scanning. A plurality of pieces of data taken by the photodetector means at every scanning is stored in real time in a memory 7. The position of the spot of the beam in the sub-scanning direction perpendicular to the main scanning direction is detected by each scanning by utilizing the data stored in the memory to measure the quantity of the face inclination of the rotating polygon mirror.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring the amount of surface tilt of a scanning optical system, and more particularly to each reflecting surface (deflecting surface) of a rotary polygon mirror (optical deflector) consisting of a plurality of reflecting surfaces constituting the scanning optical system. ), It is possible to measure (evaluate) with high accuracy the variation (interval unevenness) in the scanning position of the laser beam (light beam) in the sub-scanning direction when optically scanning the surface to be scanned due to manufacturing errors, etc. The present invention also relates to a method of measuring the amount of surface tilt of a scanning optical system used in a device such as a laser beam printer (LBP).

[0002]

2. Description of the Related Art In recent years, an image recording apparatus such as a laser beam printer (LBP) has been widely used which records an image by using a laser beam to optically scan the surface of a photosensitive drum, which is a recording medium, with the laser beam. It is used.

FIG. 7 is a schematic view of a main part of a scanning optical system used in this type of image recording apparatus. In the figure, a laser beam emitted from a light source means 71 composed of a semiconductor laser or the like is made into a parallel laser beam by a collimator lens 72, and after being condensed by a cylindrical lens 73 having a predetermined refracting power in the sub-scanning direction. The light is incident on the reflecting surface (deflecting surface) 74a of the optical deflector 74 formed of a rotating polygon mirror (polygon mirror).

The laser beam reflected and deflected by the reflecting surface 74a of the rotary polygon mirror 74 is passed through the imaging lens (f.theta. Lens) 75 having the f-.theta. Characteristic to the surface to be scanned, for example, the surface of the photosensitive drum 76. It is guiding light. By rotating the rotary polygon mirror 74 in the direction of arrow A at a constant speed, the surface to be scanned is optically scanned in the direction of arrow B at a constant speed.

At this time, if the plurality of reflecting surfaces of the rotary polygon mirror 74 are not at the same angle with respect to the rotation axis O but the inclinations of the reflecting surfaces vary, the so-called surface tilt will occur and the sub-surface on the surface to be scanned will appear. There is a problem in that the laser beam in the scanning direction becomes uneven in spacing and an accurate output image cannot be obtained.

Conventionally, as a method of measuring the amount of surface tilt at this time, for example, a one-dimensional line sensor (CCD) in which a plurality of pixels (detection bits) are arranged as light receiving means at an image forming position instead of the photosensitive drum is used. The laser beam reflected by each reflecting surface of the rotary polygon mirror is made incident on the line sensor surface by arranging the laser beam at one or several places in a direction (sub-scanning direction) orthogonal to the scanning direction (main scanning direction) of the laser beam. There is.

Then, the scanning position of the laser beam incident on the line sensor is detected to measure the unevenness (variation) of the scanning line spacing in the sub-scanning direction due to the influence of the surface tilt of each reflecting surface.

As a method of measuring the amount of tilt (unevenness of spacing) on the other surface, for example, a slit having a V-shaped opening is arranged at the image forming position and a pin photo is formed behind the slit, as in the above-described case. A photodetector such as a diode is provided, and the time difference when the laser beam reflected and deflected by each reflecting surface of the rotating polygon mirror scans (passes) the two openings of this slit is detected by the photodetector. , The surface tilt amount of the rotating polygon mirror is measured by the time conversion.

[0009]

However, in the conventional measuring method for measuring the amount of surface tilt using the one-dimensional line sensor (CCD) as the light receiving means for measurement, the amount of surface tilt per one reflecting surface is measured. For each position detection, a large amount of 200 to 1000 pieces of data was read from the line sensor, and the scanning position of the laser beam in the sub-scanning direction was calculated and detected using the data.

Therefore, it takes 100 msec or more to measure the amount of surface tilt per reflecting surface.

On the other hand, one scanning time by the scanning optical system is as short as about 3 msec, so that the scanning position of the laser beam cannot be continuously detected for each scanning. Therefore, the line sensor stores data for several tens of scans per reflecting surface in the memory, and then reads the data to detect the scanning position of the laser beam in the sub-scanning direction.

Further, the pixel width of each detection bit (pixel) of the line sensor (CCD) used as the light receiving means is generally set to be narrow, for example, about 7 μm × 7 μm due to manufacturing conditions. Therefore, for example, 300m / sec
In the scanning optical system that optically scans the surface to be scanned at the above scanning speed, the accumulation time of the light amount of the laser beam read by the detection bit is only 2.3 × 10 ⁻⁸ sec. There is a problem that the scanning position of the laser beam cannot be detected by the light amount.

Therefore, in the same manner as described above, the line sensor side must store data for 10 to several tens of scans per reflecting surface in the memory, and then read the data to detect the scanning position of the laser beam in the sub-scanning direction. I had to do it.

As described above, in the above-described conventional method for measuring the amount of surface tilt (unevenness of spacing), the amount of surface tilt must be measured using data for at least 10 to several tens of scans.
Therefore, strictly speaking, it does not capture the behavior for each scanning, and it is difficult to use it for measurement (evaluation) of a high-resolution image recording device having a smaller allowable amount of the amount of tilt error, for example. There was a problem.

Further, in the method of measuring the amount of surface tilt using the slit having the V-shaped opening, it is possible to detect the scanning position of the laser beam by one scanning per one reflecting surface, but it is possible to detect the scanning position from the time difference. Because of conversion, not only the amount of face tilt, but also the influence of rotation unevenness (jitter component) of the motor etc. for driving the rotary polygon mirror is added, so the true amount of face tilt is measured (evaluation).
It wasn't.

Therefore, similar to the above-mentioned method of measuring the amount of face tilt,
There is a problem in terms of accuracy in the measurement (evaluation) of a high-resolution image recording device having a smaller allowable amount of surface tilt.

A first object of the present invention relates to the amount of surface tilt for each scan captured by the light receiving means when measuring (evaluating) the amount of surface tilt of an optical deflector such as a rotary polygon mirror having a plurality of reflecting surfaces. Scanning the laser beam by storing a plurality of data in a memory in real time and detecting the scanning position of the laser beam in the direction (sub-scanning direction) orthogonal to the scanning direction using the data read from the memory. To provide a method for measuring the amount of surface tilt of a scanning optical system that can detect the position in real time with only one scan per reflecting surface and can be measured with high accuracy without being affected by uneven rotation of a motor or the like. is there.

A second object of the present invention is to receive light in which a plurality of detection bits (pixels) having a light receiving surface having a shape longer in the scanning direction than the direction (sub-scanning direction) orthogonal to the scanning direction are arranged in the sub-scanning direction. By sufficiently securing the accumulation time of the light quantity per one scanning by using the means, the scanning position of the laser beam in the sub-scanning direction can be detected by only one scanning per one reflecting surface, and the uneven rotation of the motor etc. Another object of the present invention is to provide a method for measuring the amount of surface tilt of a scanning optical system, which can perform highly accurate measurement without being affected.

[0019]

The method of measuring the amount of surface tilt of a scanning optical system according to the present invention comprises: (1-a) a beam from the rotary polygon mirror when the rotary polygon mirror is rotated onto a surface to be scanned. A beam for guiding and optically scanning is received by a light receiving unit for each scanning, and a plurality of data for each scanning captured by the light receiving unit is stored in a memory in real time, and the data from the memory is used. The scanning position of the beam in the sub-scanning direction orthogonal to the main scanning direction is detected for each scanning, and the amount of surface tilt of the rotary polygon mirror is measured.

(1-b) When the rotating polygon mirror is rotated, the beam from the rotating polygon mirror is guided onto the surface to be scanned and optically scanned, and the beam is scanned in the sub-scanning direction orthogonal to the main scanning direction. A detection bit having a detection surface having a shape longer in the main scanning direction is guided to a light receiving means arranged in the sub scanning direction, the beam for each scanning is received by the light receiving means, and a signal from the light receiving means is received. Is used to detect the scanning position of the beam in the sub-scanning direction, and the amount of surface tilt of the rotary polygon mirror is measured.

(1-c) The light flux from the light source means is guided to the rotary polygon mirror, and the reflected light flux from the rotary polygon mirror is guided to the surface to be scanned for optical scanning. A line sensor is provided at a corresponding position so that a plurality of elements are arranged in a sub-scanning direction orthogonal to the main scanning direction, and a plurality of data for each scan from the line sensor is stored in a memory in real time. It is characterized in that the scanning position in the sub-scanning direction is obtained for each scan using the data stored in the memory, and the plane tilt of the rotary polygon mirror is measured.

(1-d) When the light flux from the light source means is guided to the rotary polygonal mirror and the reflected light flux from the rotary polygonal mirror is guided to the surface to be scanned for optical scanning, it is orthogonal to the main scanning direction. A plurality of detection bits having a light receiving surface having a shape longer in the main scanning direction than in the sub scanning direction, and a line sensor arranged in the sub scanning direction is provided at a position corresponding to the surface to be scanned, and an output from the line sensor It is characterized in that the plane tilt of the rotary polygon mirror for each scanning is measured using a signal.

[0023]

EXAMPLE FIG. 1 is a block diagram of the main part of a measuring machine for measuring the amount of surface tilt of Embodiment 1 of the present invention, and FIG. 2 is a flow chart showing the method of measuring the amount of surface tilt in this embodiment.

Next, an outline of the measuring machine shown in FIG. 1 will be described. In the figure, reference numeral 1 denotes an objective lens, which enlarges a laser beam (light beam diameter) reflected and deflected by a rotary polygon mirror of a scanning optical system and guides it to a MOS linear image sensor 2 as a light receiving means for measurement. MOS linear image sensor (hereinafter also simply referred to as "MOS sensor")
Reference numeral 2 receives the laser beam reflected and deflected by each reflecting surface of the rotary polygon mirror magnified by the objective lens 1 and detects the scanning position of the laser beam in the sub-scanning direction.

This MOS type sensor 2 has a high ultraviolet sensitivity, a low dark current, and a large saturated charge amount, so that it is about 44,000.
Since it has a wide dynamic range and the sensitivity unevenness is as small as 1% or less, it has a feature that sufficient measurement sensitivity can be obtained even with an exposure amount of only one scanning.

A multi-channel detector (sensor drive circuit) 3 drives the MOS sensor 2 based on the pulse signal from the pulse generator 4. 11
Is a pulse signal supply unit, which supplies a pulse signal used to drive the MOS sensor 2
And a DC power source 5. An A / D converter 6 converts an analog signal from the MOS sensor 2 into a digital signal.

Reference numeral 7 denotes a memory (storage device), which is an A / D
The digital signal (output value) converted by the converter 6 is also temporarily stored. A controller (control unit) 8 receives a beam trigger signal from the scanning optical system and controls the timing of driving the MOS type sensor and A / D conversion.

Reference numeral 9 denotes a host computer, which reads an instruction to the controller 8 and data (sensor output value) stored from the memory 7 to calculate the amount of tilt of the rotary polygon mirror, and displays the calculation result (data). It is displayed on the section (display) 10.

In this embodiment, a plurality of data for each scan captured by the MOS type sensor 2 is stored in the memory 7 in real time, and the laser beam is transmitted by the host computer 9 using the data from the memory 7. The variation in the scanning position in the sub-scanning direction (interval unevenness), that is, the so-called surface tilt amount is measured.

Next, the details of the surface tilt amount measuring method of this embodiment will be described with reference to the flow chart shown in FIG.

First, the dark level is set so that unnecessary light does not enter the MOS type sensor 2 of the measuring instrument shown in FIG. Next, the scanning optical system to be measured is set at a predetermined position of the measuring machine, and a laser beam is emitted from a light source made of, for example, a semiconductor laser to perform optical scanning. Then, for example, a rotating polygon mirror (polygon mirror) to be measured by the measuring machine.
The number of reflection surfaces (polygon surfaces), the number of data to be measured, and conditions such as the MOS type sensor drive timing are input and set.

In this measuring machine, a controller 8 receives a scanning timing signal of a laser beam emitted from an object to be measured (scanning optical system), the trigger signal is used to drive the MOS type sensor 2 after a certain time, and then the MOS type sensor 2 is driven. A signal (data) relating to the surface tilt accumulated in each bit of the sensor 2 is sequentially read, and this signal is used as calculation data of the scanning position of the laser beam for one scanning.

Therefore, after the laser beam scans (passes) over the surface of the MOS type sensor 2, the MOS type sensor 2 is scanned.
The sensor drive timing is set so as to sequentially read the signals from.

The laser beam from the reflecting surface to be measured scans the surface of the MOS type sensor 2 and the laser beam from the next reflecting surface scans the surface of the MOS type sensor 2 until the laser beam is scanned. Since it is necessary to read out the signals accumulated in all bits of the type sensor 2, for example, the MOS
The driving frequency f of the MOS sensor 2 is set such that 1 / f × the number of bits of the MOS sensor <one scanning cycle of the laser beam, where f is the driving frequency of the mold sensor 2.

Therefore, the driving frequency f of the MOS sensor 2 is set so as to correspond to the specifications of the scanning optical system having the shortest one scanning cycle.

As described above, the laser beam from the reflecting surface of the rotary polygon mirror scans (passes) the MOS type sensor 2 only once, and at a scanning speed of 300 m / sec or more. The exposure amount on 2 is 1n
Only about j / cm ² .

Therefore, it is a condition that the sensor used for this measurement has high accuracy and that the sensitivity unevenness of each bit of the sensor is small.

Therefore, the dynamic range of a one-dimensional line sensor (CCD) conventionally used as a light receiving means for measurement is as narrow as about 100, and the sensitivity unevenness of each bit is as large as 10%. Since the amount is small, the measurement reproducibility is greatly affected. Therefore, it is not suitable as a sensor for detecting the scanning position of the laser beam by only one scanning per one reflecting surface as in the present embodiment.

On the other hand, the MO used in the measuring machine of this embodiment.
As described above, the S-type sensor 2 has a high ultraviolet sensitivity, a low dark current, a large saturated charge amount, and a wide dynamic range of about 44000. Also, the sensitivity unevenness is as small as 1% or less. Sufficient measurement sensitivity can be obtained with only the exposure amount.

Next, after setting the condition of the measuring object to the measuring machine, the data acquisition of the measuring machine is started. After detecting the trigger signal, the MOS sensor 2 is scanned every scanning.
The analog signal from is read out, the signal is converted into a digital signal by the A / D converter 6 and stored in the memory 7 in real time. This processing procedure is repeated for the set number of data.

In the present embodiment, the storage capacity of the memory 7 is 256 Kbytes (2 pixels per pixel of the MOS type sensor) so that a maximum of 1000 pieces of measurement data can be stored.
56 levels of multi-valued data = 8 bits (1 byte), sensor pixel number = 256 pixels, so memory capacity = 1 byte x 2
(56 pixels × 1000 data = 256 Kbytes) is set, but this storage capacity can be changed according to the amount of measured data.

After the measurement data is taken in, each data is read from the memory 7 to calculate the scanning position (irradiation position) of the laser beam on the MOS type sensor 2, that is, the barycentric position of the data is calculated for each scanning. It is calculated by the calculation formula (1) shown below from 9.

That is, the output corresponding to the exposure amount for each bit of the MOS type sensor 2 is Vi, the maximum output value is Vp, the bit position of the sensor is i, the slice level is V ₁ , and the sensor output is the slice level value. When the bit position of each sensor is a and b, the barycentric position Z of the data for each scanning is

[0044]

[Equation 1] Becomes

FIG. 3 is an explanatory diagram showing the relationship between the sensor output from the MOS type sensor 2 and the bit position of the sensor at this time.

Thus, the scanning position of the laser beam of all data is calculated, and then the MAX position or the MIN position (the arrangement direction of the detection bits is the up and down direction) for each sensor output graph and each reflecting surface based on the calculated data. (Since it is in the sub-scanning direction, the one with the highest laser beam scanning position is the MAX position and the one with the lowest laser beam position is the MIN position.) Then, calculation of the average scanning position, etc., and scanning of each reflecting surface The difference between the positions (tilts) = the amount of surface tilt (measurement resolution of the sensor × center-of-gravity position difference (number of bits)) is calculated, and the data of each calculation result is displayed on the monitor 10 to complete the measurement.

If the surface tilt of the rotary polygon mirror is corrected by, for example, a surface tilt correction optical system based on the measurement result at this time, highly accurate optical scanning can be performed.

As described above, in this embodiment, the laser beam which is optically scanned by rotating the rotary polygon mirror as described above is received by the MOS type sensor 2 for each scanning, and is taken in by the MOS type sensor 2. A plurality of data for each scan is stored in the memory 7 in real time. The host computer 9 uses the data from the memory 7 to detect the scanning position of the laser beam in the sub-scanning direction in real time, and the difference between the relative scanning positions of the reflecting surfaces of the rotary polygon mirror (the amount of surface tilt). ) Is measured with high accuracy.

Next, a second embodiment of the present invention will be described with reference to FIGS.
Will be explained. FIG. 4 is a configuration diagram of a main part of a measuring machine for measuring the amount of surface tilt according to the second embodiment of the present invention, and FIG. 5 is a flowchart of a measuring method for measuring the amount of surface tilt in the present embodiment. 4, the same elements as those shown in FIG. 1 are designated by the same reference numerals.

This embodiment is different from the above-mentioned first embodiment in that the measuring light receiving means is longer in the main scanning direction than in the direction (sub scanning direction) orthogonal to the scanning direction (main scanning direction). A MOS type linear image sensor (M having a plurality of detection bits (pixels) having a light receiving surface arranged in the sub-scanning direction.
The OS type sensor 12 is used to extend the accumulation time of the light amount in one scanning direction.

The other structures and the method of measuring the amount of surface tilt are basically the same as in the first embodiment. Here, the description will be focused on the points that are partially overlapped but different.

In a conventional line sensor (CCD) used as a light receiving means, a plurality of detection bits (pixels) each having a pixel width of 7 μm × 7 μm are arranged in the sub-scanning direction as shown in FIG. 6A. A laser beam scans the surface of the line sensor 22 at a scanning speed of 300 m / sec. At this time, the exposure time on the surface of the line sensor 22 is only about 2.3 × 10 ⁻⁸ sec. Therefore, in order to measure the scanning position of the laser beam for each scanning as in this embodiment, the exposure amount is It is insufficient and difficult to use.

In the scanning optical system of the high resolution image recording apparatus, the laser beam diameter is small and the light amount itself is small. Similarly, it is difficult to use it as a light receiving means.

On the other hand, the light receiving means 12 used in this embodiment
As shown in FIG. 6 (B), the MOS sensor 12 has a plurality of detection bits arranged in the sub-scanning direction and having a light receiving surface of 25 μm in the sub-scanning direction (plane tilt direction) and a length of 2.5 mm in the main scanning direction. In order to further increase the resolution in the plane tilt direction, the objective lens 1 is used to expand the laser beam diameter, and the resolution is 5 μm.

The length of the detection bit in the main scanning direction is 50.
The length conversion is 0 μm, which is about 71 times longer than the length (7 μm) of the line sensor (CCD) 22 in the main scanning direction, and the exposure time is also proportional to 1.7 × 1.
It is as long as 0 ^-6 sec.

This is longer than the accumulation time for several tens of scans which is conventionally required when the line sensor 22 is used. As a result, in this embodiment, the scanning position of the laser beam in the sub-scanning direction can be detected with only one scanning.

In this embodiment, this MOS type sensor 12 is used as a light receiving means for measurement, and the first embodiment described above is used.
Similarly, the variation of the scanning position of the laser beam in the sub-scanning direction (unevenness of interval), that is, the amount of surface inclination is measured based on the surface inclination measuring method shown in FIG.

Since this measuring method is basically the same as that of the above-described first embodiment, it will be briefly described though it is duplicated here.

First, the scanning optical system of the measuring object is set at a predetermined position of the measuring machine, the conditions of the measuring object are set in the measuring machine, then the data acquisition of the measuring machine is started, and the MOS type is set for each scanning. The signal from the sensor 12 is converted into a digital signal and stored in the memory 7. This processing procedure is repeated for the number of measurement data.

After the data is taken in, each data is read from the memory 7 to calculate the scanning position (irradiation position) of the laser beam, that is, the position of the center of gravity of the data is calculated by the host computer 9 for each scanning. It is calculated based on.

Thus, the scanning position of the laser beam for all data is calculated, and then the sensor output graph for each scanning and the MAX position, MIN position, average scanning position, etc. for each reflection surface are calculated based on the calculated data. The difference between the scanning positions (tilts) of the reflecting surface = the amount of surface tilt is calculated, and then the data of each calculation result is displayed on the monitor 10 to complete the measurement.

As in the case of the first embodiment, based on the measurement result at this time, if the surface tilt of the rotary polygon mirror is corrected by the surface tilt correction optical system, high-precision optical scanning is performed. be able to.

[0063]

According to the present invention, (2-a) when measuring (evaluating) the variation in the scanning position of the laser beam in the sub-scanning direction when optically scanning the surface to be scanned, the so-called surface tilt amount, By performing the measurement based on the measurement method described above, the scanning position of the laser beam in the sub-scanning direction can be measured in real time with only one scanning per reflecting surface, and the unevenness of rotation of the motor or the like for rotating the rotary polygon mirror can be measured. The behavior analysis for each reflecting surface can be performed without being affected, and thus the tolerance of the amount of surface tilt is small, and the trouble of the scanning optical system that can be applied to the measurement of a higher resolution image recording device It is possible to achieve a method for measuring the amount of leakage.

(2-b) When measuring (evaluating) the variation in the scanning position of the laser beam in the sub-scanning direction when optically scanning the surface to be scanned, the so-called plane tilt amount, as described above, the light receiving means serves as the sub-scanning direction. By using a sensor (for example, a MOS type linear image sensor) in which a plurality of detection bits (pixels) having a light receiving surface longer in the main scanning direction are arranged in the sub scanning direction, only one scanning is performed per reflecting surface. The scanning position of the laser beam in the sub-scanning direction can be measured with, and the behavior of each reflecting surface can be analyzed without being affected by the uneven rotation of the motor that rotates the rotating polygon mirror. It is possible to achieve a method for measuring the amount of surface tilt of a scanning optical system, which has a small allowable value of the amount of surface tilt and can be used for measurement of an image recording apparatus with higher resolution.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a main part of a surface tilt amount measuring machine according to a first embodiment of the present invention.

FIG. 2 is a flowchart showing a measuring method according to the first embodiment of the present invention.

FIG. 3 is an explanatory diagram showing a relationship between a sensor output and a sensor bit position according to the first embodiment of the present invention.

FIG. 4 is a configuration diagram of a main part of a surface tilt amount measuring machine according to a second embodiment of the present invention.

FIG. 5 is a flowchart showing a measuring method according to a second embodiment of the present invention.

FIG. 6 is an explanatory diagram comparing a line sensor (CCD) and a MOS linear image sensor.

FIG. 7 is a schematic view of a main part of a conventional scanning optical system.

[Explanation of symbols]

 1 Objective Lens 2, 12 MOS Linear Image Sensor 3 Multi-Channel Detector 4 Pulse Generator 5 DC Power Supply 6 A / D Converter 7 Memory 8 Controller 9 Host Computer 10 Display

Claims (4)

[Claims] 1. A beam from the rotating polygon mirror when the rotating polygon mirror is rotated is guided onto a surface to be scanned and optically scanned by the light receiving means, and the beam is received by each light receiving means. A plurality of captured data for each scanning is stored in a memory in real time, and the scanning position of the beam in the sub-scanning direction orthogonal to the main scanning direction is detected for each scanning by using the data from the memory. A method for measuring a plane tilt amount of a scanning optical system, which comprises measuring a plane tilt amount of the rotary polygon mirror. 2. The beam from the rotating polygonal mirror when the rotating polygonal mirror is rotated is guided to the surface to be scanned and optically scanned by the beam rather than in the sub-scanning direction orthogonal to the main scanning direction. A detection bit having a long detection surface in the main scanning direction is guided to a light receiving means in which a plurality of detection bits are arranged in the sub scanning direction, the light receiving means receives a beam for each scanning, and a signal from the light receiving means is used. A method of measuring the amount of surface tilt of the rotary polygon mirror by detecting the scanning position of the beam in the sub-scanning direction to measure the amount of surface tilt of the rotary polygon mirror. 3. A position corresponding to the surface to be scanned when the light beam from the light source means is guided to the rotary polygonal mirror and the reflected light beam from the rotary polygonal mirror is guided to the surface to be scanned for optical scanning. A line sensor is provided so that a plurality of elements are arranged in a sub-scanning direction orthogonal to the main scanning direction, and a plurality of data for each scanning from the line sensor is stored in a memory in real time and stored in the memory. A method for measuring the amount of surface inclination of a scanning optical system, characterized in that the scanning position in the sub-scanning direction is obtained for each scan using the data obtained and the surface inclination of the rotary polygon mirror is measured. 4. A sub-scan orthogonal to a main scanning direction when a light beam from a light source means is guided to a rotary polygon mirror and a reflected light beam from the rotary polygon mirror is guided to a surface to be scanned for optical scanning. A plurality of detection bits each having a light receiving surface having a shape longer in the main scanning direction than the scanning direction, and a line sensor arranged in the sub scanning direction is provided at a position corresponding to the surface to be scanned, and an output signal from the line sensor is used. A method for measuring the amount of surface inclination of the scanning optical system, wherein the amount of surface inclination of the rotary polygon mirror is measured for each scan.