JP2010210507A - Measuring device and measuring method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To simply measure an MTF frequency characteristic of an optic system to be inspected. <P>SOLUTION: The measuring device includes a projection means (10) for forming an image of a striped chart on the optic system to be inspected by projecting an exiting light from a striped chart (16) in which high luminance areas and low luminance areas are alternately arranged at a constant pitch to the optic system to be inspected (100), a detection means (20) for detecting the intensity distribution, at least to the pitch direction of the image formed by the optic system to be inspected, a memory means (37) for storing the luminance distribution, at least to the pitch direction of the striped chart, and a calculation means (33) for calculating the MTF frequency characteristic of the optic system to be inspected based on the intensity distribution detected by the detection means and the luminance distribution stored in the memory means. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a measuring apparatus and a measuring method for measuring MTF frequency characteristics (spatial frequency characteristics of MTF) of a test optical system such as a photographing lens of a camera.

  Patent Document 1 discloses a measurement method for accurately measuring a just focus position of a test optical system such as a photographing lens of a camera. In this measurement method, an image sensor is arranged in the vicinity of the design focal plane of the test optical system, and the chart formed by the test optical system is changed while changing the distance (measurement distance) from the test optical system to the image sensor. Images are repeatedly taken to obtain a plurality of images.

  The chart normally used in this measurement is a sequence in which a plurality of types of stripe patterns having different pitches are arranged in a predetermined positional relationship. In this case, since a plurality of types of stripe images appear in one image, the MTF frequency characteristics of the test optical system can be calculated based on the maximum luminance and the minimum luminance of the stripe images. If this calculation is performed for all images, the MTF frequency characteristic of the test optical system at each measurement distance can be obtained, so that the just focus position can be obtained accurately.

Japanese Patent Publication No. 2007-94331

  However, in the conventional measurement method, the relationship between each of the plurality of types of stripe patterns and the optical axis of the test optical system needs to be a known relationship (or a predetermined relationship), so the chart and the test optical It was difficult to align with the system.

  Accordingly, an object of the present invention is to provide a measuring apparatus and a measuring method that can easily measure the MTF frequency characteristic of a test optical system.

  One aspect of the measuring apparatus exemplifying the present invention is to project emission light from a stripe chart in which high-luminance regions and low-luminance regions are alternately arranged at a constant pitch onto a test optical system, and stripes on the test optical system Projection means for forming a stripe image, which is an image of the chart, detection means for detecting an intensity distribution in at least the pitch direction of the stripe image formed by the optical system to be tested, and brightness distribution in at least the pitch direction of the stripe chart are stored in advance. The storage means and a calculation means for calculating the MTF frequency characteristics of the optical system to be measured based on the intensity distribution of the stripe image detected by the detection means and the luminance distribution of the stripe chart stored by the storage means.

  One aspect of the measurement method illustrating the present invention is to project light emitted from a stripe chart in which high-luminance regions and low-luminance regions are alternately arranged at a constant pitch onto a test optical system, and to stripe the test optical system. Projection procedure for forming a stripe image, which is an image of the chart, detection procedure for detecting an intensity distribution in at least the pitch direction of the stripe image formed by the optical system to be tested, and storage for storing a luminance distribution in at least the pitch direction of the stripe chart And a calculation procedure for calculating the MTF frequency characteristics of the optical system to be detected based on the intensity distribution of the stripe image detected in the detection procedure and the luminance distribution of the stripe chart stored in the storage procedure.

  According to the present invention, a measuring apparatus and a measuring method capable of easily measuring the MTF frequency characteristic of a test optical system are realized.

Configuration diagram of the measuring device The figure explaining chart 16 Operation flowchart of the CPU 33 during measurement The figure explaining stripe image I ' Schematic diagram showing the MTF frequency characteristics for each measurement distance Example of MTF distance characteristics for each spatial frequency Example of measurement results

[Embodiment]
Hereinafter, a measuring apparatus according to an embodiment of the present invention will be described.

  FIG. 1 is a configuration diagram of a measuring apparatus. As shown in FIG. 1, the measurement apparatus 1 includes a projection optical system 10, a test lens 100, a measurement apparatus main body 20, and a calculation / control apparatus 30. The test lens 100 is an imaging optical system, for example, a camera taking lens.

  In the projection optical system 10, a light source 11, a reflecting mirror 12, a diffusion plate 13, a condenser lens 14, a filter 15, a chart 16, and a collimator lens 17 are disposed. The filter 15 is a filter for preventing light having an unnecessary wavelength from entering the lens to be examined 100.

  The light emitted from the light source 11 is reflected by the reflecting mirror 12 and then made uniform by the diffusion plate 13, and the chart 16 is illuminated almost uniformly from the front through the condenser lens 14 and the filter 15. Since the chart 16 is arranged at the focal position of the collimator lens 17, the light that has passed through the chart 16 is incident on the test lens 100 as a parallel light beam L <b> 1 by the collimator lens 17.

  The measurement apparatus main body 20 includes an area sensor 22 such as a CCD that receives the condensed light beam L2 emitted from the test lens 100, and a distance in the optical axis direction from the test lens 100 to the imaging surface of the area sensor 22 (measurement distance). And a moving stage 24 for changing L).

  An image of the chart 16 by the test lens 100 is formed on the area sensor 22, but when the imaging surface of the area sensor 22 is at the just focus position of the test lens 100, the contrast of the image of the chart 16 is When the image pickup surface of the area sensor 22 is shifted from the just focus position of the lens 100, the contrast of the image on the chart 16 is lowered.

  The arithmetic / control device 30 includes an A / D converter 31 that performs A / D conversion on an image signal output from the area sensor 22, a CPU 33 that performs processing on the image signal output from the A / D converter 31, and And a memory 37 for storing information (such as an operation program of the CPU 33) necessary for the processing. The CPU 33 has a function as a control unit that controls the measurement apparatus main body 20 and a function as an arithmetic unit that processes an image signal acquired by the measurement apparatus main body 20.

  FIG. 2A is a view of the chart 16 as viewed from the front (from the optical axis direction of the test lens 100). As shown in FIG. 2A, the chart 16 is a partial light shielding mask in which a transmission region 16b that transmits incident light and a light shielding region 16d that blocks incident light are arranged, and the central portions of the chart 16 are equal to each other. A stripe pattern P in which line-shaped transmission regions 16b and line-shaped light shielding regions 16d having a width are alternately and repeatedly arranged.

  In the chart 16 of this embodiment, it is not necessary to form a plurality of types of stripe patterns P having different pitches, and it is sufficient that at least one type of stripe pattern P is formed. Hereinafter, the pitch direction of the stripe pattern P is defined as the x direction, and the line direction of the stripe pattern P is defined as the y direction.

  Further, it is assumed that the optical axis of the above-described lens 100 is located near the center of the stripe pattern P. However, in the present embodiment, since other types of stripe patterns are not used, the optical axis position of the test lens 100 in the stripe pattern P need not be known. Therefore, in the present embodiment, alignment between the chart 16 and the test lens 100 is easy.

  In addition, the visual field F of the above-described test lens 100 captures a part (at least one pitch) of the stripe pattern P.

  When the light source 11 is turned on, the entire chart 16 is illuminated almost uniformly. In this case, the luminance distribution I (in the pitch direction (x direction) of the stripe pattern P viewed from the lens 100 to be tested is used. x) has a rectangular wave shape as shown in FIG. It is assumed that information on the luminance distribution I (x) is input in advance from the user to the arithmetic / control device 30 and stored in the memory 37 described above.

  The information of the luminance distribution I (x) stored in the memory 37 does not need to indicate the absolute luminance at each position of the stripe pattern P, but indicates the relative luminance at each position of the stripe pattern P. It is enough.

  Further, the information of the luminance distribution I (x) stored in the memory 37 does not have to be the entire information of the stripe pattern P, and includes information on at least a portion captured by the field of view F of the lens 100 to be examined. It is enough.

  FIG. 3 is an operation flowchart of the CPU 33 during measurement. This operation flowchart is executed according to the operation program described above. Hereafter, each step of FIG. 3 is demonstrated in order.

  Step S11: The CPU 33 drives the moving stage 24 as necessary, and sets the measurement distance L to an initial value.

  Step S12: The CPU 33 drives the area sensor 22 and captures an image signal for one frame from the area sensor 22. This image signal shows an image captured by the field of view F of the test lens 100. In the image, as shown in FIG. 4A, the field of view F of the test lens 100 is included in the stripe pattern P. An image of the captured portion (hereinafter referred to as “striped image I ′”) appears. Note that the contrast of the stripe image I 'increases as the current measurement distance L is closer to the just focus position.

  Step S13: The CPU 33 determines whether or not the number of changes of the measurement distance L has reached a predetermined value. If it has not reached the predetermined value, the process proceeds to step S14, and if it has reached the predetermined value. The process proceeds to step S15. Here, it is assumed that the stripe image I ′ is acquired at each of the nine measurement distances L, and it is assumed that the predetermined value that is the determination criterion of this step is set to “8”. Thereby, in this embodiment, steps S12 and S13 are repeated nine times, and as a result, nine stripe images I 'are acquired.

  Step S14: The CPU 33 drives the moving stage 24 to shift the measurement distance L by one step (after setting L = L + Δ), and then returns to step S12. Note that the change amount Δ for one step of the measurement distance L is set to 25 μm, for example. Therefore, the measurement distance L differs by 25 μm between the nine stripe images I ′ acquired in the present embodiment.

  Step S15: The CPU 33 individually performs MTF calculation on the nine stripe images I ′ acquired in the loop of steps S11 to S14. The MTF calculation procedure includes the following procedures (a), (b), and (c).

  (A) The brightness distribution I ′ (x) in the pitch direction of the stripe image I ′ is calculated as shown in FIG. 4B by averaging the stripe image I ′ in the line direction (y direction).

  (B) Based on the luminance distribution I ′ (x) calculated in the procedure (a) and the luminance distribution I (x) stored in the memory 37, the MTF frequency characteristic of the test lens 100 is calculated.

  Here, when the MTF frequency characteristic is “MTF (ω)”, MTF (ω) is expressed by the following equation (1).

但し、上式（１）におけるＦ｛Ａ（ｘ）｝は、分布Ａ（ｘ）のフーリエ変換であり、ωは空間周波数である。

However, F {A (x)} in the above equation (1) is a Fourier transform of the distribution A (x), and ω is a spatial frequency.

  In this procedure (b), MTF (ω) is calculated based on the equation (1). Since the luminance distribution I ′ (x) acquired in the procedure (a) is actually measured data and is discrete data, the MTF (ω) calculated in the procedure (b) is also discrete data.

  (C) The MTF frequency characteristic MTF (ω) calculated in the procedure (b) is normalized so that the MTF value at ω = 0 is 1. Hereinafter, the normalized MTF frequency characteristic MTF (ω) is simply referred to as “MTF frequency characteristic MTF (ω)” (procedure (c)).

In this step (step S15), the above MTF calculation is performed for each of the nine stripe images I ′. Therefore, as schematically shown in FIG. 5, nine MTF frequency characteristics MTF _L1 (ω), MTF _L2 (ω),..., MTF _L9 (ω) is acquired. These MTF frequency characteristics _{MTF L1 (ω), MTF L2} (ω), ..., Between the _{MTF L9} (omega), the measurement distance L are different each 25 [mu] m.

Step S16: Based on the MTF frequency characteristics for each measurement distance, that is, the MTF frequency characteristics MTF _L1 (ω), MTF _L2 (ω),..., MTF _L9 (ω), the CPU 33 determines the spatial frequency as shown in FIG. MTF distance characteristics (MTF distance characteristics), that is, MTF distance characteristics MTF _ω1 (L), MTF _ω2 (L),..., MTF _ω8 (L) are calculated (Note that the MTF distance characteristics are sometimes referred to as MTFD). .) These MTF distance characteristics MTF _ω1 (L), MTF _ω2 (L),..., MTF _ω8 (L) have different spatial frequencies ω.

Here, the spatial frequency ω ₁ of the MTF distance characteristic MTF _ω1 (L) is 20.089286 [LP / mm], and the spatial frequency ω ₂ of the MTF distance characteristic MTF _ω2 (L) is 24.553571 [LP / mm]. The spatial frequency ω ₃ of the MTF distance characteristic MTF _ω3 (L) is set to 29.017857 [LP / mm], the spatial frequency ω ₄ of the MTF distance characteristic MTF _ω4 (L) is set to 33.4482143 [LP / mm], and the MTF The spatial frequency ω ₅ of the distance characteristic MTF _ω5 (L) is 37.946429 [LP / mm], the spatial frequency ω ₆ of the MTF distance characteristic MTF _ω5 (L) is 42.410714 [LP / mm], and the MTF distance characteristic. MTF _Omega7 spatial frequency omega ₇ (L) are set to 46.875 [LP / mm], the spatial frequency omega ₈ of MTF distance characteristics MTF _Omega8 (L) 51. 39,286 was [LP / mm]. In FIG. 6, each of the MTF distance characteristics MTF _ω1 (L), MTF _ω2 (L),..., MTF _ω8 (L) is drawn as a curve, but is actually calculated as discrete data.

  Step S17: The CPU 33 calculates the just focus position of the test lens 100 by the following procedures (c) to (f).

(C) Approximate curves of MTF distance characteristics MTF _ω1 (L), MTF _ω2 (L),..., MTF _ω8 (L), which are discrete data, are calculated. As a method for calculating the approximate curve, any known calculation method such as spline approximation can be applied. Hereinafter, as shown in FIG. 6, MTF distance characteristics MTF _.omega.1 a trendline (L) _{C 1} Distant, MTF distance characteristics MTF _.omega.2 a trendline (L) _{C 2} Distant, MTF distance characteristics MTF _{[omega] 3} (L) the approximate curve of _{C 3} Distant, MTF distance characteristics MTF _.omega.4 a trendline (L) _{C 4} Distant, MTF distance characteristics MTF _.omega.5 a trendline (L) _{C 5} Distant, MTF distance characteristics MTF _.omega.6 (L ) _{Is set} as C ₆ , the approximate curve of MTF distance characteristic MTF _ω7 (L) is set as C ₇ , and the approximate curve of MTF distance characteristic MTF _{ω 8} (L) is set as C ₈ .

(D) Peak distances of the curves C ₁ , C ₂ , C ₃ , C ₄ , C ₅ , C ₆ , C ₇ , C ₈ (measurement distances that give a peak to the curve) L ₁₀ , L ₂₀ , L ₃₀ , L ₄₀ , L ₅₀ , L ₆₀ , L ₇₀ , and L ₈₀ are calculated. If the test lens 100 is an ideal optical system, the peak distances L ₁₀ , L ₂₀ , L ₃₀ , L ₄₀ , L ₅₀ , L ₆₀ , L ₇₀ , and L ₈₀ are equal to each other. When 100 is an actual optical system, the peak distances L ₁₀ , L ₂₀ , L ₃₀ , L ₄₀ , L ₅₀ , L ₆₀ , L ₇₀ , and L ₈₀ are slightly shifted.

(E) The weighted average value L _ave of the peak distances L ₁₀ , L ₂₀ , L ₃₀ , L ₄₀ , L ₅₀ , L ₆₀ , L ₇₀ , and L ₈₀ is calculated by the following equation (2).

なお、式（２）におけるｉ番目の重み値Ｇ_ｉは、ｉ番目の空間周波数ω_ｉに対して予め決められた値である。重み値Ｇ_１、Ｇ_２、Ｇ_３、Ｇ_４、Ｇ_５、Ｇ_６、Ｇ_７、Ｇ_８の全体の配分は、被検レンズ１００の適用先（具体的には、被検レンズ１００の撮影対象に含まれる空間周波数の分布）などに応じて予め決められている。

Note that the i-th weight value G _i in Equation (2) is a value determined in advance for the i-th spatial frequency ω _i . The overall distribution of the weight values G ₁ , G ₂ , G ₃ , G ₄ , G ₅ , G ₆ , G ₇ , G ₈ depends on the application destination of the test lens 100 (specifically, the imaging of the test lens 100 (Distribution of spatial frequency included in the object) and the like.

(F) The average value L _ave calculated in the procedure (e) is displayed on a monitor (not shown) as information indicating the just focus position of the lens 100 to be tested. FIG. 7 shows the horizontal axis of the graph shown in FIG. 6 as a defocus amount. In FIG. 7, the focus amount when the measurement distance L is L _ave is zero, and the defocus amount when the measurement distance L is other than L _ave is (L−L _ave ) (step S17). .

  As described above, since the measuring apparatus 1 of the present embodiment stores the luminance distribution I (x) in the pitch direction of the stripe pattern P in advance, the test lens 100 is used even though only one type of stripe pattern P is used. It became possible to measure the MTF frequency characteristics of the.

  Further, since the measuring apparatus 1 of the present embodiment has only one stripe pattern used for measurement (only the stripe pattern P shown in FIG. 2), the optical axis position of the lens 100 to be measured in the stripe pattern P is known. There is no need to. Therefore, alignment between the chart 16 and the test lens 100 is easier than in the past. Incidentally, in the present embodiment, it is only necessary that at least the chart surface and the optical axis of the test lens 100 are perpendicular to each other.

  Moreover, since the measuring apparatus 1 of this embodiment has only one stripe pattern type used for measurement, the pattern of the chart 16 can be simplified as shown in FIG.

  In addition, since the measurement apparatus 1 of the present embodiment averages the stripe image I ′ in the line direction when obtaining the luminance distribution I (x) in the x direction of the stripe image I ′, the intensity distribution I ′ (x) SN can be improved and measurement accuracy can be improved.

[Others]
In the above-described embodiment, the MTF calculation for each measurement distance is performed collectively after the stripe images I ′ for all measurement distances are acquired. However, the stripe image I ′ for each measurement distance is acquired. It may be done every time.

  In the above-described embodiment, all of the processing for the stripe image I ′ is executed by software. However, part or all of the processing may be executed by hardware.

  Further, in the above-described embodiment, the processing for the stripe image I ′ is executed by the calculation / control device 30 provided inside the measurement apparatus 1, but part or all of the processing is provided outside the measurement apparatus 1. Or a general-purpose computer.

DESCRIPTION OF SYMBOLS 10 ... Projection optical system, 100 ... Test lens, 20 ... Measuring apparatus main body, 30 ... Calculation / control apparatus, 11 ... Light source, 12 ... Reflector, 13 ... Diffuser, 14 ... Condenser lens, 15 ... Filter, 16 ... Chart: 17 ... Collimator lens, 22 ... Area sensor, 24 ... Moving stage, 31 ... A / D converter, 33 ... CPU, 37 ... Memory

Claims (8)

Projecting light emitted from a stripe chart in which high-intensity areas and low-intensity areas are alternately arranged at a constant pitch to a test optical system, and forming a stripe image that is an image of the stripe chart on the test optical system Means,
Detecting means for detecting an intensity distribution in at least the pitch direction of the stripe image formed by the optical system to be detected;
Storage means for storing a luminance distribution in at least the pitch direction of the stripe chart;
Computation means for calculating MTF frequency characteristics of the optical system under test based on the intensity distribution of the stripe image detected by the detection means and the luminance distribution of the stripe chart stored by the storage means measuring device. The measuring apparatus according to claim 1,
The computing means is
Based on the intensity distribution in the pitch direction of the stripe image I ′ (x), the luminance distribution in the pitch direction of the stripe chart I (x), and the following equation (1), the MTF frequency characteristic MTF (ω )

However, F {A (x)} in Equation (1) is a Fourier transform of the distribution A (x), and ω is a spatial frequency. The measuring apparatus according to claim 2,
The detection means includes
An image sensor that detects the intensity distribution of the stripe image in a two-dimensional direction,
The computing means is
In calculating the MTF frequency characteristic MTF (ω), the intensity distribution I ′ (x) in the pitch direction of the stripe image is obtained by averaging the intensity distribution of the stripe image detected by the detection unit over the line direction. A measuring device characterized by calculating. The measuring apparatus according to claim 1,
A control unit that repeatedly performs detection on the detection unit while changing a measurement distance from the test optical system to the detection unit;
The computing means is
Calculating MTF frequency characteristics for each measurement distance of the optical system under test based on each of the intensity distributions of the plurality of stripe images detected by the detection means and the luminance distribution of the stripe chart stored by the storage means. Characteristic measuring device. The measuring apparatus according to claim 4, wherein
The computing means is
A measuring apparatus that calculates a just focus position of the test optical system based on an MTF frequency characteristic for each measurement distance of the test optical system. The measuring apparatus according to claim 5, wherein
The computing means is
By calculating the MTF distance characteristic for each spatial frequency of the test optical system from the MTF frequency characteristic for each measurement distance of the test optical system, and averaging the peak distances of a plurality of curves drawn by these characteristics, A measuring apparatus for calculating a just focus position of the test optical system. The measuring apparatus according to claim 6, wherein
The computing means is
From the MTF frequency measurement for each measurement distance of the test optical system, the MTF distance characteristics for each spatial frequency of the test optical system are calculated, and the weighted average values of the peak distances of a plurality of curves drawn by these characteristics are calculated as described above. A measuring apparatus that calculates as information indicating a just focus position of a test optical system. Projecting light emitted from a stripe chart in which high-intensity areas and low-intensity areas are alternately arranged at a constant pitch to a test optical system, and forming a stripe image that is an image of the stripe chart of the test optical system Procedure and
A detection procedure for detecting an intensity distribution in at least the pitch direction of a stripe image formed by the test optical system;
A storage procedure for storing a luminance distribution in at least the pitch direction of the stripe chart;
A calculation procedure for calculating an MTF frequency characteristic of the optical system under test based on the intensity distribution of the stripe image detected in the detection procedure and the luminance distribution of the stripe chart stored in the storage procedure. Measuring method to do.

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