JPH0365919A - scanning optical device - Google Patents

Abstract

PURPOSE:To exactly detect the diameter of a scanning luminous flux without using an ultra-high speed processing circuit and intricate CCD processing circuit by detecting the spot diameter of the scanning luminous flux in plural detecting parts having the scanning direction widths of plural values. CONSTITUTION:A stripe filter 11 formed with plural slits 11a extending perpendicularly in a main scanning direction and having different widths is disposed on a surface to which the scanning luminous flux is made incident. The scanning luminous flux past the filter is modulated to a bright and dark signal and is condensed and imaged on the surface of a photodetector 13 by the effect of a condenser lens 12. The spot diameter of the scanning luminous flux is detected by using the photodetecting means having plural kinds of the scanning direction widths of the plural detecting parts. The optimum focus adjustment in correspondence to various spot diameters is executed in this way without requiring the ultra-high speed and intricate processing circuits.

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a scanning optical device that scans a light beam from a light source onto an irradiated object such as a photosensitive drum to form an image on the irradiated object. The present invention relates to a scanning optical device including spot diameter detection means.

[Conventional technology]

In a conventional laser beam printer, the scanning light beam on the photosensitive drum has a diameter of about 90 am in the main scanning direction, IlI! The diameter in the scanning direction is about 1100a. Therefore, the depth of focus of the collimator lens at this time is as deep as ±5 to 6 μm, and even if the collimator lens moves due to expansion or contraction of the collimator lens barrel caused by environmental changes, it will not be affected. This is a negligible depth of focus value. However, in recent years, laser beam printers that can record high-definition images with a dot density of about 600 dpi to 800 dpi have been developed. The diameter of the scanning light beam of the scanning optical system used here in the main scanning direction is about 40 μm, the diameter in the sub-scanning direction is about 60 μm, and the depth of focus of the collimator lens is less than ±1 μm. Therefore, it has become almost impossible to suppress the amount of deviation of the collimator lens due to environmental changes to within the depth of focus.Therefore, we measured the scanning beam diameter at a position corresponding to the photosensitive drum.
Based on this measurement II! A method has been proposed to prevent focal shift due to environmental changes by moving a collimator lens to a certain position. For example, there is a method in which the diameter of the scanning light beam in the main scanning direction is detected by a beam detector for obtaining a horizontal synchronizing signal that controls the image writing timing of a laser beam printer. In this case, the beam detector is generally a combination of a P■N photodiode and a knife, and an example has already been proposed in which the diameter of the scanning beam in the main scanning direction can be determined from the waveform when the scanning beam passes through the knife. (Excerpt from Japanese Patent Publication No. 6'0-9243.) The method on the other hand determines the main scanning direction from the slope of the rising waveform of the output signal from the beam detector, but the response time of the sensor and detection circuit is slow. This has the drawback that the rise waveform mentioned above is distorted, making accurate measurement impossible.As another method to overcome these drawbacks, a detector using a CCD element has also been put into practical use, and a method using a CCD element. There are two types of methods: one is a method using a CCD linear sensor, and the other is a method using a CCD area sensor. [Problems to be solved by the invention] The method using a CCD linear sensor is mainly This method directly reads the spot diameter by arranging a CCD linear sensor perpendicular to the scanning direction.The disadvantages of this method are that the CCD linear sensor has low responsiveness and cannot measure the accurate spot diameter of the moving scanning light beam. In some cases, the main scanning diameter cannot be measured using only the sub-scanning diameter.In this case, measurement should be performed either by temporarily stopping scanning on the CCD linear sensor or by emitting laser light in pulses on the CCD linear sensor. However, in both cases, not only is alignment difficult, but even slight fluctuations cause the laser spot to shift from the top of the CCD linear sensor, making it difficult to put it to practical use.On the other hand, measuring the diameter in the main scanning direction As the linear sensor needs to be placed on the main scanning line, positioning is difficult as described above.One way to improve this shortcoming is to use a CCD area sensor.This method uses a CCD area sensor. The light flux passing through the sensor is fixed at a fixed position within the CCD area sensor.
Although only pulses are lit, the CCD area sensor has a coarse element density of several tens of microns, which requires magnification and projection of the spot, making it complicated. In addition, in order to accurately measure the spot diameter in the main scanning direction, the pulse lighting time must be kept very short, otherwise the spot will fall in the scanning direction and accurate spot diameter measurement will not be possible. If it was shortened, the amount of light to the CCD area sensor was not sufficient, and measurement could become impossible due to insufficient sensitivity. Therefore, an object of the present invention is to provide a scanning optical device equipped with a detection means that can accurately detect the diameter of a scanning light beam without using an ultra-high-speed processing circuit or a complicated COD processing circuit, in order to solve the above-mentioned drawbacks. There is a particular thing. [Means for Solving the Problems 1] In the present invention for achieving the above object, the scanning light beam is received at a position optically approximately equivalent to the irradiated object, such as a photosensitive drum, on which the light beam from the light source is scanned. The detection means is provided with a plurality of detection portions and non-detection portions arranged alternately along the scanning direction, and the detection portions have a plurality of widths in the scanning direction. The specific form of the detection part and the non-detection part is a filter with multiple slits placed in front of the light receiving element. There are those that are gradually constant and those that change by ffi. [Function 1] Since the scanning light beam is detected by a plurality of detection portions having a plurality of widths in the scanning direction, the detection light can be processed flexibly and an excessive burden is not placed on the design of the processing means. [Embodiment] FIG. 1 is a schematic diagram of an embodiment of the present invention. In the figure, a light beam emitted from a light source l such as a semiconductor laser passes through a convex lens 2 and a convex lens 3 that constitute a collimator system. The light beam passing through this becomes a parallel light beam. Reference numeral 4 denotes a variable aperture, and the aperture diameter that provides the optimum spot diameter is calculated from the image data, and the aperture 4 is operated accordingly. If the aperture diameter is made smaller, the spot diameter formed on the photosensitive drum 20 becomes larger, and vice versa, the spot diameter becomes smaller. The light beam focused by the variable diaphragm 4 is applied to the reflective surface 6a of the polygon mirror 6 by the cylindrical lens 5, with a diameter in the main scanning direction (a direction perpendicular to the optical axis within the scanning plane formed by the scanning light beam over time). remains as it is, and is focused and guided only in the sub-scanning direction (direction perpendicular to the scanning plane). The light flux that has entered the polygon mirror 6 in this manner is deflected and scanned by the rotation of the polygon mirror 6, and is scanned as an image on the photosensitive drum 20 by the action of the spherical lens 7 and the toric lens 8. Reference numeral 9 denotes a beam detector placed at a location with an optical path length approximately equal to that of the photosensitive drum 20, which detects the scanning light flux, and the timing for starting image writing for each scan is determined by the output from this. Reference numeral 10 denotes a spot diameter detector for detecting the diameter of the scanning light beam in the main scanning direction, and the convex lens 2 is moved in the optical axis direction in response to the output signal from this detector so that the optimum spot diameter can always be obtained on the photosensitive drum 20. I'm trying to be able to do it. Such a scanning light flux forms an exposure distribution for one scan of an image on the surface of the photosensitive drum 20, and further, for each scan, the photosensitive drum 20 is rotated by a predetermined amount in the sub-scanning direction to form an exposure distribution on this drum 20 according to the image signal. A latent image is formed having
This latent image is then recorded as a visible image on recording paper using a well-known electrophotographic process. FIG. 2 shows the details of the spot diameter detector 10. On the surface on which the scanning light beam is incident, there is a stripe filter 11 in which a plurality of slits lla extending perpendicularly to the main scanning direction and having different widths are formed. The scanning light beam that has passed through ζ is modulated into a bright/dark signal, and is focused or imaged on the surface of the light receiver 13 by the action of the focusing lens 12. FIG. 3 is a detailed diagram of the stripe filter 11. The stripe filter 11 has 15 slits lla, each of which has a different width. The slit width on the scanning start side (left l1l in the figure) is 100 μm.
, the next slit width is 95 um, the next slit width is 90 pm, and so on, and the width is narrowed by 5 μm. The final slit width is 30 um. Fig. 4 shows how the output signal of the spot diameter detector 10 changes depending on the change in spot diameter.When the main scanning diameter is 60 μm, as shown in Fig. Incidentally, since the slit width is wider than the main scanning diameter, the output from the spot diameter detector 10 is also large, but as the slit width becomes narrower, the output becomes smaller. Since the relationship between the spot diameter of the scanning light beam and the output from the detector 10 changes depending on the response speed and sensitivity of the spot diameter detector 10, as well as the scanning speed and light intensity, these relationships are experimentally determined in advance. As shown in FIG. 4, the output signal from the spot diameter detector 10 is sliced at a specific slice level and the number of signals is counted, thereby making it possible to measure the spot diameter. Change the aperture and diameter! ! When the spot diameter on the photosensitive drum 20 is changed according to the density, the optimal spot diameter at that time can be recognized by counting the number of signals as described above, and based on this, the collimator lens system 2 is adjusted to the optimal focus position. , 3 can be adjusted. This focus 3! An example of the Iu method will be explained. First, when the recording density of the image signal is determined, the aperture diameters of the collimator lens systems 2 and 3 are calculated, and the aperture 4 is operated accordingly. Here, the scanning light beam is run, and the amount of light passing through the first slit lla of the filter 11 (the rightmost one in FIG. 3) is measured. By repeating this operation several times, Find the average value. This average value becomes the reference value, and the light amount is adjusted repeatedly, and then the spot diameter is measured. To measure the spot diameter, the signal detected by the light receiving element 13 when the scanning light beam crosses the filter 11 is amplified and widened by an AC amplifier, and the number of signals of this amplified signal is set to a predetermined slice level (the above-mentioned standard). This is done by counting with a counter set to count based on the value). Since accurate measurements cannot be made if there are variations in light intensity, these operations are repeated several times, and the average value Ct
is required. Next, the convex lens 3 is directed toward the polygon mirror 6 (hereinafter, this direction will be referred to as the child direction, and the opposite direction will be referred to as the one direction).
Similarly, the number of signals of the scanning light flux crossing the filter 11 is counted, and the average value C2 is determined. The moving direction of the convex lens 3 is determined by comparing C1 and 02 described above. This will be further explained with reference to the flowchart shown in FIG. When the measurement sequence is started, the signals passing through the slit when scanning at the current position of the convex lens 3 are counted, and the count value Ct of the signal at this time is calculated and stored in the memory. move a predetermined amount in the direction,
The slit passing signals are counted again, and the count value C2 of the signal at this time is calculated and stored in the memory. CI and C2 are compared by a calculation routine, and when C1=C2, the convex lens 3 is moved in the child direction and the slit passing signals are counted again. The count value at this time is stored as 03 in the memory. Next, as in the previous case, C2 and C3 are compared, and when C2=C3, the measurement sequence ends and the next sequence begins. If C2 and C3 are not equal, the convex lens 3 is moved in one direction and the measurement sequence ends. C1 and C2
If they are not equal, Ct and 02 are compared again, and if 02 is greater than C1, the value of C2 is transferred to the memory where the result of C1 is stored. After that, go to the routine to move the convex lens 3 in the child direction and repeat. Also, when C1 is larger than C2, move the convex lens 3 in one direction and change the value of C2 to the memory where the result of 01 is stored. Go to the measurement routine again and repeat. The above is based on the following principle. When the convex lens 3, which is a part of the collimator lens, moves, the light beam emitted from the collimator lens does not become a parallel light beam, but is focused on the slit of the spot diameter detector 10 in an out-of-focus state. As a result, the main scanning beam diameter of the scanning light beam becomes larger than when it is in focus.If the focus is blurred and the beam diameter increases, the output when passing through the narrow slit width portion becomes smaller. Since the beam diameter is the smallest when the beam is in focus, the output signal can be sliced and counted at a predetermined slice level, and when the count reaches the largest number, the beam is in focus. . The variable diaphragm 4 is changed by a mechanism (not shown), and is provided after the light beam is made into a parallel light beam by a mechanism similar to a diaphragm in a camera or the like. When the diameter of the aperture is large, the spot diameter at the focal position of the light beam passing through the imaging lens is small, and when the diameter of the aperture is small, the spot diameter becomes large. The depth of focus changes in proportion to this, being shallower when the spot diameter is small and deeper when the spot diameter is large. This means that the focus state changes sensitively in accordance with the movement of the convex lens 3. In order to overcome this drawback, when the diameter of the aperture is large, the amount of movement of the convex lens 3 is made small, and when the diameter of the aperture is small, the amount of movement of the convex lens 3 is increased. In this way, by changing the amount of movement of the convex lens 3 per step in response to changes in the aperture diameter of the variable aperture 4, highly accurate focus adjustment becomes possible. In the above, the focus was adjusted by moving the convex lens 3 of the convex lenses 2 and 3 that make up the collimator lens system, but it is also possible to move the convex lenses 2 and 3 as a unit. It does not matter if it is a one-piece structure instead of a two-piece structure, but in this case,
The drawback is that the depth of focus of the collimator lens system is shallow, several microns, and the adjustment accuracy is high. For this reason, it is advantageous to configure the collimator lens system with two lenses and to move the front lens whose focal length changes less with respect to the amount of movement. Although the spot diameter detector 10 is provided with a stripe filter 11 in front of the light receiving element 13, it is also possible to configure the sensor itself in a stripe shape. In this case, the condenser lens 12 may be omitted. Further, a stripe filter may be formed on the condenser lens 12 itself. Further, although the beam detector 9 and the spot diameter detector 10 are provided separately, it is also possible to make them a separate body. Furthermore, the beam detector 9 is a spot diameter detector 1.
It is also possible to use the portion of any of the slits lla. [Effects of the Invention] As explained above, according to the configuration of the present invention, the spot diameter of the scanning light beam is detected using a light detection means in which a plurality of detection portions have a plurality of widths in the scanning direction. It is possible to perform optimal focus am corresponding to various spot diameters without requiring a high-speed or complicated processing circuit.

[Brief explanation of drawings]

Figure 1 is a schematic configuration diagram of an embodiment of the present invention, Figure 2 is a detailed diagram of a spot diameter detector, Figure 3 is a detailed example of a stripe filter, and Figure 4 is a diagram showing the output signal of the spot diameter detector. The figure shown in FIG. 5 is an adjustment flowchart of the collimator lens system. DESCRIPTION OF SYMBOLS 1... Light source, 2, 3... Convex lens, 4... Variable aperture, 6... Polygon mirror 9... Beam detector, 10... Spot diameter detector, 11... Strive filter 11a... Slit, 12... Condensing lens, 13... Light receiving element (20... e Photosensitive drum

Claims (1)

[Scope of Claims] 1. In a scanning optical device that scans an irradiated object with a light beam emitted from a light source, a light detection means that receives the scanning light beam is provided at a position optically equivalent to the irradiated object, A scanning optical device characterized in that the light detection means is constituted by a plurality of detection portions and non-detection portions arranged alternately, and the detection portions have a plurality of widths in the scanning direction. 2. The scanning optical device according to claim 1, wherein the width of the detection portion in the scanning direction gradually decreases along one direction. 3. The scanning optical device according to claim 1, wherein the light detection means includes a stripe filter provided in front of the light receiving element and in which a plurality of slits having a plurality of widths in the scanning direction are formed.