JP2012088601A - Method for manufacturing optical element and optical element manufactured by the method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing an optical element capable of accurately manufacturing an optical element with injection molding.SOLUTION: The method for manufacturing the optical element which manufactures an optical element that is used for an optical scanning device and has refractive power by molding the optical element with a mold includes: an injection molding step; a step of measuring optical performance; a step of evaluating optical performance; a step of measuring a shape; a step of modeling a shape error; a step of simulating the shape; a step of checking consistency; a step of correcting a shape error function; a step of calculating a processing value of the mold; a step of processing the correction; and a step of remolding.

Description

  The present invention relates to a method for manufacturing an optical element, and particularly suitable for manufacturing an optical element in an optical scanning apparatus mounted on an image forming apparatus such as a laser beam printer having an electrophotographic process, a digital copying machine, or a multifunction printer. It is.

  Conventionally, in an optical scanning device such as a laser beam printer (LBP), a light beam modulated and emitted from a light source means according to an image signal is periodically deflected by, for example, an optical deflector formed of a rotating polygon mirror (polygon mirror). I am letting. Then, the light beam deflected by the optical deflector is focused in a spot shape on the surface of the photosensitive recording medium (photosensitive drum) by the imaging optical element having the fθ characteristic, and image recording is performed by optically scanning the surface of the photosensitive drum. Is going.

  In recent years, since an optical element constituting an imaging optical element is easy to manufacture, it has been made from resin. Resin is a material that can be easily molded, and is easy to manufacture for glass that requires grinding and polishing. On the other hand, a high production system is required for optical elements in order to form high-quality images. In particular, it is required that the surface shape of the optical surface of the optical element during molding can be accurately controlled. For this reason, the following points have become important for optical elements used in optical scanning devices.

(1) Accurate function approximation of actual optical surface of injection molded optical element (2) Performed when actual measurement result of optical performance such as focus position and simulation result of optical performance expected from optical surface shape do not match The simplicity of optical surface shape correction (1) is as follows. The surface shape of the measured optical surface of the injection-molded optical element is usually obtained as point sequence data. On the other hand, in order to simulate the optical performance expected from the optical surface shape or to obtain the corrected shape of the mold, it is necessary to express the optical surface by some function.

  At this time, if there is a difference between the above point sequence data and the optical surface expressed as a function, a correct result cannot be obtained. Therefore, in order to correct the surface shape of the mirror piece with high accuracy, it is important to express the surface shape of the actual optical surface more accurately as a function.

  Regarding (2), it is as follows. The actual measurement result of the optical performance such as the focus position and the simulation result of the optical performance expected from the surface shape may not match due to factors other than the measurement error and the optical surface shape. For example, in the case of a plastic lens, a difference in refractive index is likely to occur between the inside and the periphery of the lens, and the focus position may shift even if the shape is as designed due to the influence of power generated by the difference in refractive index. If the simulation result based on the measurement shape does not match the actual measurement result, the cause as described above is considered. For this reason, it is important to match the two by correcting the function representing the measured surface shape.

  However, calculating the function from the beginning is time consuming and error prone. Therefore, it is easy to modify the function so that each term of the function has a correlation with the specific optical performance, for example, it is only necessary to modify the specific term part to match the specific optical performance. Sex is important.

  When an optical element is manufactured by injection molding using a mold, the optical element taken out from the mold contracts immediately, so that the shape of the mold does not match the actual shape of the optical element. Conventionally, there has been known a method of manufacturing an optical element having a predetermined shape by correcting the shape of the mold for the error at this time (Patent Documents 1 and 2).

JP 2002-254490 A JP 2003-094441 A

  Conventionally, when manufacturing an optical element using a mold, it was almost always designed with a circular arc for the cross section of the strand, and when the surface shape was modeled, the cross section of the strand was fitted with a circular arc. . The actual surface shape is often different from the circular arc, and depending on the degree of fitting error, the optical performance after correction, for example, the spot shape on the image surface and the irradiation position of the light beam are in the desired state. There were times when it was off.

  In addition, in response to the trend toward higher definition of image quality, the cross section of the child wire is also designed with a non-arc shape. For such optical elements, modeling of the shape by arc fitting is insufficient, but even if the aspheric coefficient is increased in the dark cloud, it becomes easy to fall into the local minimum at the time of fitting, and some fitting method can be used. Need to be established. Furthermore, if there is a modeling method that can easily correct the surface shape when there is a discrepancy between the optical performance expected from the shape error and the actual measurement value, a complicated calculation is required to determine the correction amount. It becomes necessary and makes an error.

  Therefore, in order to manufacture a high-precision optical element, it is important to perform a more accurate function approximation of the actual optical surface of the molded optical element. Further, it is important to appropriately correct the surface shape of the mold, which is performed when the actual measurement result of the optical performance such as the focus position and the simulation result of the optical performance predicted from the surface shape are inconsistent.

  An object of this invention is to provide the manufacturing method of the optical element which can manufacture an optical element with high precision by injection molding, and the optical element manufactured using the method.

  An optical element manufacturing method of the present invention is an optical element manufacturing method in which an optical element having refractive power used in an optical scanning device is molded using a mold, and the optical element is injection molded using a mold. Injection molding process, optical performance measurement process to measure the optical performance of injection molded optical elements, whether the optical performance of injection molded optical elements is within the allowable value, and if it is within the allowable value, gold Finish the mold correction of the specular piece of the mold and proceed to the next step if it is outside the allowable value, for the injection molded optical elements that are not within the allowable value, the optical surface at multiple locations in the sub-scanning direction The shape measurement process for measuring the shape of the optical element, the two-dimensional shape error function of the optical element is approximated by a polynomial for each cross section of the optical element based on the measurement results in the shape measurement process, and the coefficients of the same order of the polynomial In the main scanning direction The shape error modeling process obtained by approximation as a number, the shape simulation process that simulates the optical performance of the optical element using the shape error function obtained in the shape error modeling process, and the optical performance and optical performance of the shape simulation process A shape error function that compares the measured values in the measurement process and checks the consistency of the measurement results. If there is no consistency in the consistency check process, the shape error function is corrected and returned to the shape simulation process. If there is consistency in the correction process, the consistency check process, the mold machining value calculation process, the mold machining value calculation process, which calculates the shape of the mirror surface piece of the mold using the correction value that functions as the shape error in the shape simulation process Correction process that corrects the shape of the specular piece using the correction value obtained in step 5, and optical using the mold that has undergone the correction process. Reshaping process of reshaping the child back to the step of measuring the optical performance for measuring the optical performance of the optical element obtained by reshaping process is characterized by comprising the steps of subsequently repeating the series of steps.

  ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the optical element which can manufacture an optical element with high precision by injection molding is obtained.

FIG. 5 is a flowchart of a method for manufacturing the optical element of Example 1 according to the present invention. Detailed flow chart of manufacturing method of optical element of embodiment 1 in the present invention The figure which shows the shape measurement location of the optical element in this invention Another figure which shows the shape measurement location of the optical element in this invention FIG. 1 is a main scanning sectional view of Example 1 of an optical scanning device using an optical element created by the optical element manufacturing method of the present invention. Main scanning sectional view of Example 2 of the optical scanning device using the optical element produced by the optical element manufacturing method of the present invention Illustration of shape error of imaging means Illustration of approximation error when optical elements are approximated step by step Illustration of approximation error when optical elements are approximated collectively

  In the present invention, the optical surface of the optical element formed by molding can be corrected more accurately and the surface shape can be easily corrected later in view of the measurement result of the optical performance. In the present invention, the optical surface of the molded optical element is first fitted in the sub-scanning cross-sectional direction, then fitted in the main-scanning cross-sectional direction, and finally a two-dimensional shape error function is obtained to calculate a corrected shape.

  The following steps are used in the method of manufacturing an optical element in which an optical element having a refractive power used in the optical scanning apparatus of the present invention is molded using a mold. It has an injection molding process (step 1) in which an optical element is injection molded using a mold. An optical performance measuring step (step 2) for measuring the optical performance of the injection-molded optical element; It is evaluated whether or not the optical performance of the injection-molded optical element is within an allowable value. If the value is within the allowable value at this time, the mold correction of the mirror surface piece of the mold is finished because there is no problem in the injection molding by the mold. If it is outside the allowable value, an optical performance evaluation process (step 3) that proceeds to the next process is included.

  As a result of the evaluation in the optical performance evaluation process, the injection molded optical element that does not fall within the allowable value has a shape measurement process (step 4) for measuring the shape of the optical surface. At this time, measurement is performed at a plurality of locations in the sub-scanning direction of the optical element. It has a shape error modeling (calculation) step (step 5) for obtaining a two-dimensional shape error function of the optical element from the measurement result in the shape measurement step. Here, the shape error is approximated by a polynomial for each of the plurality of sub-line cross-sections based on the results measured in the sub-scanning direction at a plurality of locations.

Next, coefficients of the same order of the polynomial are approximated as a function in the main scanning direction. The final polynomial is obtained when the coordinate in the main scanning direction is Y, the coordinate in the sub-scanning direction is Z, the shape error of the optical surface is X (Y, Z), and the coefficient is A (Y) or less X (Y, Z)
= A (Y) + B (Y) · Z + C (Y) · Z ² + D (Y) · Z ³ + E (Y) · Z ⁴ +
It is.

A shape simulation step (step 6) for simulating the optical performance of the optical element using the shape error function obtained in the shape error modeling step. It has the consistency check process (step 7) which compares the optical performance in a shape simulation process, and the measured value in an optical performance measurement process (step 2), and checks the consistency of a measurement result. When there is no consistency in the consistency check step (step 7), the shape error function is measured and the shape error function is corrected (step 8) to be returned to the shape simulation step (step 6).

  If there is consistency in the consistency check process (step 7), a mold machining value calculation process (step 9) that calculates the shape of the mirror surface piece of the mold using the correction value that functions as a shape error in the shape simulation process (step 6). ). The correction value is calculated by including the shrinkage rate when the optical element is injection molded. A correction processing step (step 10) for correcting the shape of the mirror piece using the correction value obtained in the mold processing value calculation step (step 9); It has a re-molding process (step 11) which re-molds an optical element using the metallic mold which performed a correction processing process (step 10). In order to measure the optical performance of the optical element obtained in the reshaping process (step 11), the process returns to the optical performance measurement process (step 2), and then a series of processes are repeated.

[Example 1]
1 and 2 are flowcharts of Embodiment 1 in the method for manufacturing an imaging optical element of the present invention. FIG. 2 is a flowchart showing in detail the steps involved in calculating the shape error. Each process of the manufacturing method of the optical element of a present Example is as follows. As an example, the optical element of the present embodiment is intended for an incident optical system used in an optical scanning device and an imaging optical element having fθ characteristics.

(Step 1) Initial molding process (injection molding process)
First, a mirror surface piece is processed so as to have a design shape or a design shape, and a die is created. When the mold release amount and shrinkage rate at the time of injection molding can be predicted, the machining shape of the mirror piece may be determined so as to cancel the influence. Next, an optical element is injection-molded by using this processed mold. At this time, the molding conditions are thoroughly examined and stored so that the optical element in the shape of the molded product can be stably injection-molded with the same shape.

(Step 2) Optical Performance Measuring Step The optical element that has been injection-molded is incorporated into an optical measuring device, and the optical performance is measured. Specific measurement items include, for example, spot diameters in the main scanning direction and sub-scanning direction, field curvature, partial magnification, overall magnification, scanning line curvature, scanning line pitch interval, and the degree of spot shape distortion. To do. It is not always necessary to evaluate all of these measurement items, and necessary items are measured according to the purpose. The measurement is desirably performed at a plurality of locations on the image plane (on the surface to be scanned) at intervals of about 10 mm with respect to the main scanning direction (hereinafter referred to as Y direction). It is desirable to measure a measurement range that is equal to or wider than the range used in the optical scanning device.

  For example, the measurement regarding the field curvature of the optical element is performed as follows. An image acquisition device such as a CCD camera is arranged at a position corresponding to the image plane of the measurement device, and the spot diameters of the main scanning section and the sub-scanning section are measured while moving the image acquisition apparatus in parallel with the light beam traveling direction. A defocus area where the separately determined spot diameter is equal to or less than the standard is obtained, and the center is evaluated as the image plane position. The standard of the spot diameter is preferably about 1.2 times the minimum spot diameter in the design value.

  Further, the measurement regarding the partial magnification and the overall magnification of the optical element is performed as follows. An optical element corresponding to a deflecting means (polygon mirror) is rotated at equal intervals, and a light beam passing position on the image plane is measured. Separately, a design light passage position obtained from the fθ coefficient is obtained, and the partial magnification is evaluated from the ratio of the distance between the actual light passage positions, and the overall magnification is evaluated from the ratio of the distance between the two most peripheral points.

  Further, the measurement regarding the distortion of the spot shape is performed as follows. The spot diameter is measured with a section obtained by rotating the main scanning section and the sub-scanning section about 45 degrees around the optical axis of the optical element, not the spot diameters of the main scanning section and the sub-scanning section used for measuring the field curvature. The measured spot diameter difference of each cross section is defined as the spot diameter distortion and measured. The distortion of the spot diameter may appear by defocusing even if it does not occur in a state where it is not defocused. Therefore, it is more preferable to calculate the spot diameter distortion for each defocus position and evaluate the spot diameter distortion with respect to the defocus. Each measurement may be performed independently for each item or simultaneously.

(Step 3) Optical Performance Evaluation Step Optical performance that can be allowed for a single optical element is determined in advance for each item. Then, it is determined whether all items are within the allowable range. As a result, when all items are within the allowable range, the mold correction is completed. If even one item is out of the allowable range, the steps after the shape measuring step in step 4 to be described later are sequentially performed.

(Step 4) Shape Measurement Step The shape of the optical surface of the molded optical element is measured. It is desirable that the optical element used for the shape measurement is the same as the optical element used in the optical performance measurement step in Step 2. However, there are cases where the shape cannot be measured with the same optical element for the convenience of measurement, for example, work is performed in parallel with the optical performance measurement step of Step 2. At this time, the shape may be measured for the optical element molded under the same conditions and the optical element used in the optical performance measurement step of Step 2 and the optical element whose molding timing is substantially the same.

  Since the optical element formed by molding is often a resin, the surface of the optical surface is easily damaged. Therefore, it is desirable that the shape measuring machine is a non-contact type. However, a contact-type shape measuring machine may be used on the premise that the optical performance is not evaluated after the shape measurement. The measurement direction is preferably measured at a plurality of locations in the sub-scanning direction along the sub-scanning section as shown in FIG. In contrast to the thin line indicating the outer shape of the optical element in FIG. Although the central part of the optical element is also measured, it is omitted here. The reason for measuring along the sub-scanning section is to first perform the fitting calculation on the sub-scanning section at the time of shape fitting described later.

  In general, habit is more likely to occur in the sub-scanning direction than in the main-scanning direction, and depending on the height of the optical element, the optical performance is likely to be greatly affected. On the other hand, the main scanning direction hardly changes locally, and even if it occurs, it is often close to the end of the optical element. Therefore, the measurement accuracy and fitting accuracy in the sub-scanning direction with respect to the main scanning direction are further required. In the present embodiment, a plurality of locations are measured along the sub-scanning section, and fitting is first performed in the sub-scanning direction. In the case of the method of measuring a plurality of locations along the main scanning direction, there is a concern that the accuracy of fitting of the surface particularly in the sub-scanning direction is deteriorated because the point sequence data in the sub-scanning direction is generally reduced.

  In this embodiment, approximation in the sub-scanning direction is performed, and thereafter, stepwise approximation is performed such that each order coefficient is approximated in the main scanning direction. On the other hand, there is a method of performing approximation in a lump, but in this case, fitting calculation is performed on a large amount of data using many variables. For this reason, it takes a lot of time until the calculation is completed, and it is easy to catch the local minimum. Therefore, the optimal solution is not always reached.

  FIG. 7 is an explanatory diagram showing an example of the shape error on the incident surface side of the imaging optical element (optical element) 6b constituting the imaging optical system used in the “optical scanning device 1” described later. In FIG. 7, the horizontal direction indicates the longitudinal direction (main scanning direction) of the optical element, and the vertical direction indicates the short direction (sub-scanning direction) of the optical element, and the shape error is indicated on the contour lines. The unit is mm in the vertical and horizontal directions, and the shape error is μm. The shape error in this example was 36.9 μm in width. FIG. 8 shows an example of the fitting error in the case where the shape error is approximated in the sub-scanning direction as described above, and thereafter, the coefficients of the respective orders are approximated in the main scanning direction. Show. The unit is the same as in FIG. The approximate error at this time was 2.9 μm in width.

  On the other hand, FIG. 9 shows the fitting error when fitting all at once. The approximate error at this time was 15.7 μm in width. Comparing FIG. 8 and FIG. 9, it can be seen that the error is larger when the fitting is performed collectively. Therefore, it can be seen that it is preferable to approximate in stages as in this embodiment.

(Step 5) Shape error modeling process (calculation process)
A shape error is calculated by subtracting the design shape from the shape data of the optical element measured in the shape measurement step in step 4. Next, the shape error is approximated by a polynomial. At this time, the order of the shape error function is preferably the same as or larger than the order used in the design. In general, the shape of an actual optical element is often a complicated shape compared to the surface shape of a design value due to the influence of habit and the like. In particular, there may be abrupt changes at the end of the optical surface, and in order to perform polynomial approximation including such a shape error, it is necessary to express it with a higher-order function than the design value.

  The surface shape at the time of design is generally expressed as an optical surface with an aspherical coefficient of the necessary and sufficient order, and is necessary when the polynomial that expresses the error shape has an order less than the design value. It can be easily predicted that the accuracy is not obtained. When the fitting accuracy is poor, the fitting may be performed by limiting the light beam passing range in consideration of tolerances such as assembly accuracy. FIG. 4 shows a light beam passing range on the optical surface of the optical element in consideration of tolerances such as assembly accuracy, and a region surrounded by a dotted line in the figure corresponds.

  By limiting the shape fitting range from the entire measurement region to only the really necessary region surrounded by the dotted line in FIG. Improvement of fitting accuracy can be expected. In this embodiment, the sub-scanning cross-sectional direction is designed using aspherical coefficients up to the fourth order, and thus approximated by a quartic function as shown below.

X = Ay + By · Z + Cy · Z ² + Dy · Z ³ + Ey · Z ⁴
However, X: Shape error Z: Coordinate in the sub-scanning cross-sectional direction Subscript y: Coordinate in the main-scanning cross-sectional direction on the optical surface The above equation was obtained for each measurement position y. In addition, when the sub-scan section is designed as an arc, it may be approximated using a polynomial as described above, or may be fitted with a curvature or a radius of curvature. However, if the optical surface is bad and cannot be fitted by curvature, fitting accuracy can be improved by approximating with a polynomial. For this reason, it is desirable to approximate the shape error with a polynomial in the sub-scanning direction (child line cross section). Note that the order may be increased from the fourth order in order to increase the fitting accuracy.

The above shape error approximation is performed over the entire scanning region. As a result, the aforementioned shape error function is obtained for each measurement position. Subsequently, a coefficient relating to the same order of the shape error function is taken out and approximated by a polynomial related to the main scanning section direction. For example, for primary coefficients B ₅₀ , B ₄₉ , B ₄₈ , ..., B _-50 and Z secondary coefficients C ₅₀ , C ₄₉ , C ₄₈ , ..., C _-50
B (Y) = P ₁ + Q ₁・ Y ² + R ₁・ R ⁴ + S ₁・ R ⁶ …
C (Y) = P ₂ + Q ₂・ Y ² + R ₂・ Y ⁴ + S ₂・ Y ⁶ …
And each coefficient of Z is expressed as a function of Y. By performing this approximation for all orders of Z, the shape error on the optical surface can be expressed by the following one expression.

X (Y, Z) = A (Y) + B (Y) ・ Z + C (Y) ・ Z ² + D (Y) ・ Z ³ + E (Y) ・ Z ⁴
In the present embodiment, the polynomial is approximated by the above polynomial in each region with the optical axis as a boundary in the main scanning direction (Y direction). The order is an even function up to the 16th order according to the design value. At that time, the same coefficient is used in each region for the function of the second order or less of Y. The function used for the fitting does not necessarily need to be an even function. For example, the shape error may be expressed by one function using both even and odd terms without dividing the region.

  Employing such an expression yields the following advantages. One expresses the shape error with a function, so it is easy to simulate changes in optical performance due to the shape error. As another one, A (Y) means the bus shape error of the optical element, B (Y) means the inclination error of the sub-scanning section, and C (Y) means the curvature error of the sub-scanning section. For this reason, when the simulation result and the result of the optical performance measurement process in step 2 do not match, it is possible to easily determine which coefficient function should be corrected. For example, when the scanning line curvatures do not match, the scanning line curvature is greatly affected by the inclination of the optical surface, and thus it is understood that B (Y) may be modified.

  The calculation of the shape error needs to be performed on all the optical surfaces of the optical elements evaluated in the optical performance measurement process in Step 2. For example, when evaluating optical performance, if two lenses are evaluated as a set, it is necessary to calculate a function of shape error for four surfaces. This is because the result of the shape simulation cannot be compared with the measurement value of the optical performance measurement process in Step 2 unless it is performed for all optical surfaces.

(Step 6) Shape Simulation Step Based on the two-dimensional shape error function X (Y, Z) obtained in the shape error calculation step in Step 5, the optical performance with the shape error is simulated by an optical simulator. At that time, the optical performance is simulated using the error calculation function of all the optical surfaces calculated as described above.

(Step 7) Consistency Check Process of Shape Simulation and Measurement Result The optical performance taking into account the shape error obtained in the shape simulation process of Step 6 is compared with the measured value of the optical performance measurement process of Step 2, and each evaluation item Make sure that the difference is at least acceptable. Note that it is expected that the difference between the optical performance taking into account the shape error obtained in the shape simulation process in Step 6 and the measurement value in the optical performance measurement process in Step 2 will remain after the mold correction. For this reason, when the difference is large, it is expected that the mold correction is necessary again. Therefore, it is necessary to start again from the fitting of the shape error or to correct the shape error function.

(Step 8) Correction process of shape error function As described in the calculation process of shape error in step 5, the shape error function is as follows.
X (Y, Z) = A (Y) + B (Y) ・ Z + C (Y) ・ Z ² + D (Y) ・ Z ³ + E (Y) ・ Z ⁴
A (Y) means the bus shape error of the optical element, B (Y) means the tilt error of the sub-scan section, and C (Y) means the curvature error of the sub-scan section. For this reason, when the simulation result and the result of the optical performance measurement process in step 2 do not match, it is possible to easily determine which coefficient function should be modified. For example, when the scanning line curvature does not match, the scanning line curvature is greatly influenced by the inclination of the optical surface, and thus it is understood that B (Y) may be corrected. Hereinafter, a specific correction method will be described taking scanning line curvature as an example.

  First, the sensitivity that relates the irradiation position in the sub-scanning direction and the tilt of the sub-scanning section is calculated. Next, the surface to be corrected is determined. In particular, when there is a highly sensitive surface among the optical surfaces to be corrected, it is desirable to correct the error function of that surface. This is because, compared to the case where the coefficients of other surfaces are modified, it is possible to modify the shape without greatly changing the shape and to suppress the influence on other optical performances as much as possible. Can be mentioned. The other is because of the high sensitivity, the effect of the fitting error is likely to appear, and there is a high possibility that the fitting error of the highly sensitive surface is causing the mismatch.

From the obtained sensitivity of the irradiation position in the sub-scanning direction and the amount of irradiation position to be corrected, an inclination amount to be corrected for B (Y) of the shape error function is obtained for each Y. The corrected inclination function dB (Y) is obtained by fitting the amount of inclination to be corrected using the same method as that for determining B (Y). Therefore,
B (Y) → B (Y) + dB (Y)
Incorporation into the miscalculation function X (Y, Z) makes it possible to correct the inclination. If the shape error function is modified as follows using the same method as the scanning line curve, the field curvature in the main scanning direction, the partial magnification is A (Y), and the sub-scanning field curvature is C (Y). Good.

X (Y, Z) = A (Y) + B (Y) ・ Z + C (Y) ・ Z ² + D (Y) ・ Z ³ + E (Y) ・ Z ⁴
→ X (Y, Z) = {A (Y) + d A (Y)} + {B (Y) + dB (Y)} ・ Z + {C (Y) + dC (Y)} ・ Z ² + {D (Y) + dD (Y)} ・ Z ³ + {E (Y) + dE (Y)} ・ Z ⁴

The surface to be corrected is desirably selected for each coefficient in consideration of sensitivity. This is because a highly sensitive surface varies depending on the evaluation item. Further, it is desirable to select any one surface to be corrected. This is because the calculation is not complicated. However, when the amount of correction is large and the surface shape is greatly changed, the correction may be shared by a plurality of surfaces. After this, the shape simulation in step 7 and the consistency check process of the measurement result are performed again, and it is confirmed that the difference between the measured value and the simulation value using the shape error function is at least acceptable. To do. If it is not less than the allowable value, the shape error function may be corrected by obtaining the correction function of the correction function again by using the method of the shape error function correction step in step 8.

(Step 9) Mold Machining Value Calculation Step By performing the steps so far, a shape error function that substantially reproduces the measurement value obtained in the optical performance measurement step in Step 2 can be obtained. The corrected shape for the optical element is a shape obtained by inverting the sign of the shape error. However, when the optical element is taken out from the mold, the shape of the mold and the shape of the optical element are strictly different due to the stress inside the optical element and the influence of the temperature difference between the mold and the outside air. For this reason, even if the shape of the mold is corrected according to the corrected shape, the shape of the optical element may not be corrected as desired. There is no problem if the shape shift at this time hardly affects the optical performance. However, when the correction amount is large or when an optical surface with high sensitivity is corrected, the optical performance is likely to be affected. Therefore, it is necessary to obtain (calculate) the die machining value in consideration of the above effects.

  A specific method is to compare the shape of the current mold with the shape of the optical element to obtain the shrinkage rate, and when it is shrunk at this shrinkage rate, the correct shape of the mold is such that the optical element becomes the corrected shape. Can be obtained by calculating back.

  As a method for obtaining the shrinkage rate, there are the following methods. The shape of the mold is deformed at an arbitrary shrinkage rate on a computer, and the shape obtained at this time is compared with the optical surface shape of the actually molded optical element. This comparison is performed while changing the shrinkage rate, and the shrinkage rate that minimizes the difference is searched. The shrinkage rate when this difference is the smallest is taken as the shrinkage rate for this optical surface. As another method, a plurality of marks are provided on the surface of the mold corresponding to the optical surface of the optical element. The shrinkage rate may be obtained by comparing the interval between the marks provided on the mold and the interval between the marks transferred to the optical element side. Alternatively, the contraction rate may be obtained by comparing the length of the part on the mold side to which the optical surface of the optical element is transferred and the width of both ends on the optical element side corresponding to the edge portion of the part.

(Step 10) Correction processing step Based on the corrected shape (correction amount) derived in the die machining value calculation step in Step 9, based on the new surface shape (optical function surface) superimposed on the current mirror surface shape Create (correction process) the mirror surface piece of the mold.

(Step 11) Re-molding process Finally, injection molding is performed under the same molding conditions as the initial molding process of Step 1 with the mirror piece corrected in the correction process. As a result, the optical performance and surface shape of the optical element can be accurately corrected. Further, when the remolding results in different mold release amounts before and after the correction, the molding conditions may be re-examined so that the shape of the molded product can be stably molded with the same shape.

  As described above, an optical element having a desired optical performance can be formed by repeatedly performing the steps 2 to 11 until OK is obtained in the optical performance evaluation step of step 3.

[Example 2]
The first embodiment mainly relates to a method for correcting an optical element in which a light beam passes on an optical axis or a bus. Of course, the method described in the first embodiment is also effective for an optical element in which light does not pass on the bus. In this embodiment, the shape error can be corrected more accurately and easily by changing the equation in the shape error function correcting step in Step 8 for the optical element in which the light beam does not pass on the bus. Next, the method will be described.

  The present embodiment is different from the first embodiment in the shape error function expression method in step 8. As will be described in detail below, the shape error function is expressed not on the basis of the bus but on the basis of the light beam passage position. Thereby, the deviation of the light beam passing position due to the correction of the shape error function is reduced, and the correction is performed with higher accuracy. Therefore, the processes from the initial molding process of Step 1 to the shape simulation of Step 7 and the consistency check process of the measurement results among all processes shown in FIG.

(Step 8) Shape Error Function Correction Process At this time, the shape error function is the same as described in the shape error calculation process in Step 5,
X (Y, Z) = A (Y) + B (Y) ・ Z + C (Y) ・ Z ² + D (Y) ・ Z ³ + E (Y) ・ Z ⁴
It is expressed. If the shape error function is corrected while maintaining the above equation, the incident position of the light beam changes when the higher-order coefficient of the first-order term of Z is corrected or when the light beam does not pass through the bus.

Hereinafter, the adverse effects of correcting the scanning line curvature will be described. To correct scanning line curvature as described in the first embodiment
B (Y) → B (Y) + dB (Y)
By changing the inclination of the optical surface, correction is performed. At this time, since the optical surface is tilted around the generatrix, the area other than the generatrix moves to the back or near as viewed from the deflecting means 5 shown in FIG. During scanning, a light beam incident on the optical surface is generally inclined with respect to the surface normal, so that the light beam passing position is also shifted when the position of the optical surface is shifted in the forward direction.

  Therefore, if the scanning line curvature is corrected in this state, other optical performance such as field curvature may be changed due to the influence of the deviation of the light beam passing position. Therefore, when correcting other optical performances, it is necessary to correct the shape error function in consideration of the influence of the scanning line curvature correction. However, if the curvature of field is corrected, the light beam passing position changes, and it is expected that the performance of the scanning line curve will change again, so that a rough calculation is required.

  Therefore, in the present embodiment, a method that is not affected by the deviation of the light passing position accompanying the correction of the shape error function is used. In order to prevent the deviation of the light beam passing position, it is only necessary to correct the coefficient with reference to the light beam passing position when correcting each coefficient. First, in Example 1, the shape error function related to the sub-scanning cross section is expressed as follows.

Xy (Z) = Ay + By ・ Z + Cy ・ Z ² + Dy ・ Z ³ + Ey ・ Z ⁴
On the other hand, in this embodiment, when the light beam passing height is ky, the shape error function related to the sub-scanning cross section is
Xy (Z) = Ay '+ By' ・ (Z-Ky) + C y '・ (Z-Ky) ² + D y' ・ (Z-Ky) ³ + E y '・ (Z-Ky) ⁴
The expression is changed based on the light passage position as follows.

  When correcting the shape error function, correct the above Ay ', By', Cy ', Dy', and Ey 'so that the ray passage position is not shifted for coefficient terms higher than the first order term of Z. Each optical performance can be modified.

When calculating the sensitivity for associating each optical performance with each coefficient, it is necessary to obtain the sensitivity based on the light passing position instead of changing the coefficient based on the bus. For example, when finding the sensitivity that correlates the irradiation position in the sub-scanning direction with the tilt of the sub-scanning section, it is not tilted around the generatrix but is tilted around the light beam passing position and is sensitive based on the irradiation position deviation at that time. Find the degree. The corrected shape error function finally obtained is expressed as follows.
X (Y, Z) = {A (Y) + d A (Y)} + {B (Y) + dB (Y)} ・ (Z-+ {C (Y) + dC (Y)} ・ Z ² + {D (Y) + dD (Y)} ・ Z ³ + {E (Y) + dE (Y)} ・ Z ⁴
(Step 9) Mold machining value calculation step The same as in the first embodiment.
(Step 10) Correction processing step The same as in the first embodiment.
(Step 11) Remolding Step The same as in Example 1.

  As described above, according to each embodiment, an actual optical surface is modeled as accurately as possible so that an actual measurement value of optical performance such as a focus position can be associated with an optical performance predicted from the surface shape. If the actual measurement and the simulation result do not coincide with each other, the surface shape can be corrected as easily as possible so that the desired optical performance and the surface shape can be accurately manufactured. Further, by using an optical element molded by such a manufacturing method, it is possible to provide an optical scanning device having high-precision performance.

In addition, according to the present invention,
(B) Fitting work is performed at each step. For this reason, compared with the case where two-dimensional shape fitting is performed at a time, it is difficult to fall into the local minimum, and even when falling into the local minimum, it is easy to find out by comparing the degree of fitting errors of other cross sections. For this reason, the shape fitting can be performed with high accuracy by changing the state of the shape fitting start.

  (B) When fitting in the main scanning section direction, a plurality of fitting functions of the sub-scanning section are fitted for each order. For this reason, compared to the case of fitting all orders at once, it is less likely to fall into the local minimum, and even if it falls into the local minimum, discovery is easy. As a result, the shape fitting can be performed with high accuracy by changing the start state of the shape fitting start.

  (C) In the case of a scanning optical system, since the light beam scans on the optical surface in the main scanning direction, optical evaluation such as field curvature and scanning line curvature is generally performed at a plurality of locations in the main scanning direction. Is called. Therefore, if the optical performance expected from the result of shape fitting does not match the measured value of the optical performance, and it is necessary to perform some correction on the fitted shape, the main scanning is performed for each order in the sub-scanning direction from the beginning. It is expressed as a function. For this reason, it is easy to correct the fitting shape. For example, when correcting the scanning line curvature, only the function related to the primary term in the sub-scanning direction needs to be modified. When correcting the field curvature in the sub-scanning direction, only the function related to the secondary term in the sub-scanning direction is required. You just have to fix it.

[Optical scanning device 1]
FIG. 5 is a sectional view (main scanning sectional view) of the principal part in the main scanning direction of the optical scanning apparatus according to the first embodiment of the present invention. In the following description, the main scanning direction (Y direction) is a direction perpendicular to the rotation axis of the deflecting unit and the optical axis of the imaging optical system (the direction in which the light beam is reflected and deflected (deflected and scanned) by the deflecting unit). The sub-scanning direction (Z direction) is a direction parallel to the rotation axis of the deflecting unit. The main scanning section is a plane including the main scanning direction and the optical axis of the imaging means. The sub-scanning section is a section perpendicular to the main scanning section. In the figure, reference numeral 1 denotes a light source means, which is composed of, for example, a semiconductor laser. The wavelength of light oscillated from the semiconductor laser 1 of this embodiment is 780 nm.

  The collimator lens 21 converts the divergent light beam emitted from the light source means 1 into substantially parallel light. Thereafter, the light beam incident on the cylindrical lens 22 having refractive power only in the sub-scanning direction is formed as a line image on the deflection surface (reflection surface) 5a of the optical deflector 5 with respect to the sub-scanning section. Reference numeral 3 denotes an aperture stop which shapes the beam shape by limiting the passing light flux. Each optical element such as the collimator lens 21, the cylindrical lens 22, and the aperture stop 3 constitutes one element of the incident optical means LA. An optical deflector 5 serves as a deflecting unit that deflects and scans the light beam guided from the incident optical system LA in the main scanning direction. The optical deflector 5 is composed of, for example, a four-sided polygon mirror (rotating polygonal mirror) having an outer diameter of 20 mm, and is rotated at a constant speed in the direction of arrow PA in the figure by a driving means such as a motor.

  Reference numeral 6 denotes an imaging optical system having a condensing function and an fθ characteristic, and includes imaging lenses 6a and 6b made of a plastic material. The image forming means 6 forms a light beam based on the image information reflected and deflected by the deflector 5 in a spot shape on the photosensitive drum surface 8 as the surface to be scanned in the main scanning section. In addition, the tilt correction is performed by providing a conjugate relationship between the deflection surface 5a of the optical deflector 5 and the vicinity thereof and the photosensitive drum 8 on the sub-scan section. Reference numeral 8 denotes a photosensitive drum as a surface to be scanned.

  In this embodiment, the divergent light beam emitted from the semiconductor laser 1 according to the image information is substantially collimated by the collimator lens 21, and the cylindrical lens 22 produces a line image (on the deflection surface 5 a of the optical deflector 5 in the sub-scan section). The image is formed as a longitudinal line image in the main scanning direction. Further, when the light beam passes through the aperture stop 3, the light beam width is limited so that the light beam has a desired spot diameter on the surface to be scanned.

  The light beam reflected and deflected by the deflecting surface 5a of the optical deflector 5 is imaged in a spot shape on the photosensitive drum surface 8 through the imaging lenses 6a and 6b, and the optical deflector 5 is rotated in the direction of the arrow PA. Thus, optical scanning is performed on the photosensitive drum surface 7 in the arrow PB direction (main scanning direction) at a constant speed. Thereby, an image is recorded on the photosensitive drum surface 8 as a recording medium. In the present embodiment, the imaging lenses 6a and 6b are molded by the optical element manufacturing method described in the first or second embodiment, thereby having desired performance as a high-precision optical scanning device.

[Optical scanning device 2]
FIG. 6 is a sectional view (main scanning sectional view) of the main part in the main scanning direction of the optical scanning apparatus according to the second embodiment of the present invention. In the figure, the same suffix numbers are given to those having the same functions as those in the first embodiment.

  In the figure, reference numeral 1 denotes a light source means, which is composed of, for example, a semiconductor laser. In this embodiment, the wavelength is 790 nm. Reference numeral 2 denotes a coupling lens made of a plastic material and having an anamorphic power. The coupling lens 2 is manufactured by the optical element manufacturing method according to any one of the embodiments of the present invention. The coupling lens 2 converts the divergent light beam emitted from the light source means 1 into substantially parallel light with respect to the main scanning section. The sub-scan section is formed as a line image on a deflection surface (reflection surface) 5a of the optical deflector 5 described later. Further, a diffraction grating is provided on the incident surface side of the coupling lens to reduce the movement of the focus due to temperature fluctuation.

  Reference numeral 3 denotes an aperture stop which shapes the beam shape by limiting the passing light flux. The coupling lens 2 may be divided into a collimator lens and a cylindrical lens having the same power in the main scanning direction and the sub-scanning direction. Each optical element such as the coupling lens 2 and the aperture stop 3 constitutes an element of the incident optical system LA. An optical deflector 5 serves as a deflecting unit that deflects and scans the light beam guided from the incident optical system LA in the main scanning direction. The optical deflector 5 is composed of, for example, a polygon mirror (rotating polygonal mirror) having an outer diameter of 34 mm and is rotated at a constant speed in the direction of arrow PA in the figure by a driving means such as a motor. The light beam from the incident optical means LA is incident at an angle of 3 ° in the sub-scan section.

  Reference numeral 6 denotes an imaging optical system having a condensing function and an fθ characteristic, and includes imaging lenses 6a and 6b made of a plastic material. The image forming means 6 forms a light beam based on the image information reflected and deflected by the deflector 5 in a spot shape on the photosensitive drum surface 8 as the surface to be scanned in the main scanning section. Then, the tilt correction is performed by providing a conjugate relationship between the deflection surface 5a of the optical deflector 5 and the vicinity thereof and the photosensitive drum 8 on the sub-scan section. A dust-proof glass 7 prevents dust, toner, and the like from entering the optical scanning device. Reference numeral 8 denotes a photosensitive drum as a surface to be scanned.

  In this embodiment, the divergent light beam emitted from the semiconductor laser 1 in accordance with image information is converted into substantially parallel light in the main scanning section by the coupling lens 2 and is lined to the deflecting surface 5a of the optical deflector 5 in the sub-scanning section. An image is formed as an image (line image elongated in the main scanning direction). When the light beam passes through the aperture stop 3, the light beam width is limited so that the light beam has a desired spot diameter on the surface to be scanned. The light beam reflected and deflected by the deflecting surface 5a of the optical deflector 5 is imaged in a spot shape on the photosensitive drum surface 8 through the imaging lenses 6a and 6b, and the optical deflector 5 is rotated in the direction of the arrow PA. Thus, optical scanning is performed on the photosensitive drum surface 7 in the arrow PB direction (main scanning direction) at a constant speed.

  Thereby, an image is recorded on the photosensitive drum surface 8 as a recording medium. Since the light beam is incident on the deflecting surface 5a at an angle of 3 ° in the sub-scanning direction, when the light beam passes through the imaging lenses 6a and 6b, the light beam passes above the generatrix except for the axis. In this embodiment, the imaging lenses 6a and 6b are molded by the optical element manufacturing method described in the first or second embodiment, so that it is possible to have desired performance as a high-precision optical scanning device.

DESCRIPTION OF SYMBOLS 1 Light source means 2 Coupling 3 Aperture 5 Optical deflector 6 Imaging optical system

Claims (5)

  In an optical element manufacturing method for manufacturing an optical element having a refractive power used in an optical scanning device by molding using a mold, an injection molding process for injection molding the optical element using a mold, an optical element molded by injection molding The optical performance measurement process for measuring the optical performance of the evaluation, whether or not the optical performance of the injection-molded optical element is within the allowable value, and if it is within the allowable value, the mold correction of the specular piece of the mold is terminated, An optical performance evaluation process that proceeds to the next step if it is outside the allowable value, and a shape measuring step that measures the shape of the optical surface at multiple locations in the sub-scanning direction for injection molded optical elements that are not within the allowable value, the shape From the measurement results in the measurement process, the two-dimensional shape error function of the optical element is approximated by a polynomial for each of the cross sections of the plurality of child lines, and the coefficients of the same order of the polynomial are approximated as a function in the main scanning direction. Shape error Comparison of measured values in the optical performance and optical performance measurement processes in the shape simulation process and shape simulation process that simulates the optical performance of the optical element using the shape error function obtained in the modeling process and the shape error modeling process If there is no consistency in the consistency check process or consistency check process for checking the consistency of measurement results, the shape error function is corrected and returned to the shape simulation process. If there is consistency, use the correction value obtained in the mold machining value calculation step to calculate the shape of the mirror surface piece of the mold using the correction value that is the shape error function in the shape simulation step, and the correction value obtained in the mold machining value calculation step. A correction process for correcting the shape of the mirror piece, a re-molding process for re-molding the optical element using the mold subjected to the correction process, Method of manufacturing an optical element, characterized in that the return to the step of measuring the optical performance for measuring the optical performance of the optical element obtained in the form step comprises the steps of thereafter repeating the series of steps.   2. The method of manufacturing an optical element according to claim 1, wherein, in the step of calculating the die machining value, the correction value is calculated by including a shrinkage rate when the optical element is injection-molded. In the shape error modeling step, the polynomial in the main scanning direction is Y, the sub-scanning direction coordinate is Z, the optical surface shape error is X (Y, Z), and A (Y) or less is a coefficient.
X (Y, Z)
= A (Y) + B (Y) .Z + C (Y) .Z ² + D (Y) .Z ³ + E (Y) .Z ⁴ +. . .
The method of manufacturing an optical element according to claim 1, wherein   An optical element manufactured by the method for manufacturing an optical element according to claim 1.   An optical scanning device using the optical element according to claim 4.