JP7190327B2 - MTF measuring device and its program - Google Patents

Description

The present invention relates to an MTF measuring apparatus and program for measuring an MTF (Modulation Transfer Function) indicating spatial frequency characteristics of an imaging system.

The greatest feature of high-definition television is high spatial resolution, and the resolution characteristics of cameras are important. The resolution characteristic of a camera can be expressed in MTF.
Currently, as a method for measuring MTF, the size of the measurement chart captured by the camera is relatively small, and the Slanted-edge method (hereinafter referred to as edge method) is generally used (see Patent Documents 1 and 2 and Non-Patent Documents 1 to 4).

Here, MTF measurement by the conventional edge method will be described with reference to FIG.
First, in the edge method, as shown in FIG. 12A, an ROI (Region Of Interest) to be measured for MTF is selected in an image (edge image I) obtained by imaging a chart including edges. .
Next, in the edge method, as shown in FIG. 12(b), an edge e is detected from the ROI and its inclination θe is obtained. Then, the edge method projects each pixel of the ROI along the edge e onto the projection axis (x-axis) along which equally spaced bins are arranged. At this time, the bin width is set to a sub-pixel width such as 1/4 or 1/8 of the width of one pixel of the ROI.

Then, the edge method averages the values of the pixels projected onto the bins of the projection axis in each bin to obtain an oversampled edge spread function (ESF) as shown in FIG. 12(c). Ask for
Further, the edge method obtains a line spread function (LSF) as shown in FIG. 12(d) by differentiating the edge spread function.
Finally, the edge method obtains the MTF as shown in FIG. 12(e) by Fourier-transforming the edge spread function, taking the absolute value, and normalizing it with a direct current (spatial frequency "0").

JP 2010-237177 A JP 2018-13416 A

ISO 12233:2017, “Photography - Electronic still picture imaging - Resolution and spatial frequency responses” K. Masaoka, K. Arai, and Y. Takiguchi, “Real-time measurement of ultra-high definition camera modulation transfer function,” SMPTE Motion Imaging Journal, 127(10), 2018, doi: 10.5594/JMI.2018.2868435. in press) K. Masaoka et. al., Modified slanted-edge method and multidirectional modulation transfer function estimation,” Optics Express 22(5):6040-6046, March 2014 "Resolution Characteristic Measurement Method for Television Camera Systems Technical Data (ARIB TR-B41)", Version 1.0, P3-7, Association of Radio Industries and Businesses, Established September 29, 2016

In the conventional edge method, there is a problem that calculation cost is required to detect the edge e in the ROI shown in FIG. 12(b). Furthermore, the conventional edge method has a problem that the calculation accuracy of the MTF drops if the edge is not a straight line.
SUMMARY OF THE INVENTION The present invention has been made in view of such problems, and provides an MTF measuring apparatus and a program thereof that can measure the MTF even from a non-straight edge at a lower calculation cost than the conventional edge method. The challenge is to

In order to solve the above problems, an MTF measuring apparatus according to the present invention is an MTF measuring apparatus for measuring an MTF indicating spatial frequency characteristics of an imaging system, comprising ROI setting means, line MTF calculating means, and line MTF averaging. and an aliasing removal means.

In such a configuration, the MTF measuring device uses the ROI setting means to set an ROI, which is a rectangular area including edges, in an edge image including inclined edges captured by the imaging system. This rectangular area is the MTF measurement range.
Then, the MTF measuring device calculates the line MTF, which is the MTF for each line, by the line MTF calculation means by the discrete Fourier transform for each line including the edge in the ROI.
Then, the MTF measuring device calculates an average MTF by averaging the line MTF by the number of lines of the ROI for each predetermined spatial frequency by the line MTF averaging means. This allows the MTF measuring device to measure the MTF of the ROI without detecting the slope of the edge and even if the edge is not straight.

Furthermore, the MTF measurement device performs correction to remove aliasing components from the average MTF by the aliasing removal means. This correction is, for example, a monotonically decreasing function in which the MTF value at spatial frequency ξ=0.5 is the value obtained by dividing the average MTF value by 4/π, and the MTF value is 0 at spatial frequency ξ=1. can be performed using a function approximated by

The MTF measurement apparatus can be operated with an MTF measurement program for causing the computer to function as the ROI setting means, line MTF calculation means, line MTF averaging means, and aliasing removal means.

ADVANTAGE OF THE INVENTION This invention has the outstanding effect shown below.
According to the present invention, unlike the conventional edge method, the slope of the edge is not detected, so the calculation cost can be suppressed. This allows the present invention to be implemented even with a small-scale FPGA (Field-Programmable Gate Array).
Also, according to the present invention, the MTF can be measured even if the edge is not straight. This makes it possible for the present invention to measure the MTF simply by capturing an image of a portion having an edge without using a dedicated chart.

1 is a block configuration diagram showing the configuration of an MTF measuring device according to an embodiment of the present invention; FIG. FIG. 4 is a diagram for explaining ROIs, in which (a) is a vertical ROI (vertical edge image) and (b) is a horizontal ROI (horizontal edge image). FIG. 4 is an explanatory diagram for explaining the MTF for each line calculated by line MTF calculation means; FIG. 4 is a graphical representation of the original MTF and the aliased MTF; FIG. 4 is a graph diagram for explaining the original MTF, the MTF for each line including aliasing, and the average thereof; FIG. 4 is a graph diagram for explaining the original MTF and the average MTF including aliasing; FIG. 4 is an explanatory diagram for explaining a composite vector of original MTF and aliasing MTF; FIG. 3 is a graph showing the relationship between r(ξ) and <|F _R (ξ)|> _CORR /|F(ξ)|; FIG. 4 is a graph showing measurement results of MTF measured by the MTF measuring device according to the embodiment of the present invention; 4 is a flow chart showing the operation of the MTF measuring device according to the embodiment of the present invention; FIG. 10 is an explanatory diagram for explaining an example in which the MTF is approximated by a monotonically decreasing function as a modified example of the MTF measuring device according to the embodiment of the present invention; FIG. 10 is an explanatory diagram (a) to (e) for explaining the measurement procedure of the conventional edge method;

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[Configuration of MTF measuring device]
First, referring to FIG. 1, the configuration of an MTF measurement device 1 according to an embodiment of the present invention will be described.

The MTF measuring device 1 measures MTF representing the spatial frequency characteristics of the imaging system 2 . Here, the MTF measuring device 1 connects the imaging system 2 and the display device 3 .
The imaging system 2 is an object to be measured by MTF, and is a video camera, a still camera, a camera including a lens to be measured by MTF, or the like.
The imaging system 2 outputs an edge image (moving image or still image) obtained by imaging a chart CH including edges (borders with different contrasts) to the MTF measuring device 1 .

The chart CH is a test chart including an edge pattern in which the inclination of the edge is several degrees from the vertical direction or horizontal direction with respect to the imaging device (not shown) of the imaging system 2 .
This chart CH is, for example, a test chart for MTF measurement used in the edge method. In the present invention, the edges of the chart CH do not necessarily have to be straight lines, and may have distortion such as curved lines. For example, it may be a signboard or the like on which characters having edges are drawn.
Here, when measuring the frequency characteristics in the horizontal direction, the image pickup system 2 picks up the chart CH with the edge slightly inclined (about several degrees) in the oblique vertical direction, as shown in FIG. 2(a). , to obtain an edge image I. When measuring the frequency characteristics in the vertical direction, the imaging system 2 captures the chart CH with the edges in the oblique horizontal direction to obtain an edge image I, as shown in FIG. 2B.

The display device 3 provides a user interface for operating the MTF measurement device 1, and displays edge images captured by the imaging system 2, graphs of measurement results, and the like. For example, the display device 3 is a liquid crystal display, an organic EL display, or the like.
Note that the display device 3 may be separately provided with a display device for displaying the edge image captured by the imaging system 2 and a display device for displaying the measurement result.

The configuration of the MTF measurement apparatus 1 for measuring the MTF of the imaging system 2 using edge images captured by the imaging system 2 will be described in detail below.
As shown in FIG. 1, the MTF measurement device 1 includes edge image storage means 10, ROI setting means 11, line MTF calculation means 12, line MTF averaging means 13, aliasing removal means 14, and MTF output means. 15 and.

The edge image storage means 10 stores an image (edge image) of the chart CH captured by the imaging system 2 . If the output of the imaging system 2 is a moving image, the edge image storage means 10 sequentially receives edge images as frame images from the imaging system 2 in time series via video input means (not shown). This edge image storage means 10 is, for example, a general storage device such as a hard disk or memory.
The edge image stored in the edge image storage means 10 is output to the display device 3 via video output means (not shown).

The ROI setting means 11 sets an ROI (region of interest), which is a target area for MTF measurement including inclined edges, in the edge image captured by the imaging system 2 . For example, the ROI setting means 11 designates a rectangular (rectangular) region in the edge image displayed by the display device 3 with a pointing device (not shown) operated by the measurer, thereby setting the ROI position. , size, etc.

Here, when measuring the frequency characteristics in the horizontal direction, the ROI setting means 11, as shown in FIG. The ROI is set by being specified. In this case, the ROI will identify a vertical edge image that varies in brightness in the horizontal direction.
When measuring the frequency characteristics in the vertical direction, the ROI setting means 11 designates a horizontally long rectangular region from the edge image I obtained by capturing the chart CH, as shown in FIG. 2B. ROI is set by In this case, the ROI will identify a horizontal edge image that varies in brightness in the vertical direction.
Note that the direction of the edge (vertical, horizontal) may be specified by the measurer. In that case, the ROI setting means 11 may specify a horizontally long rectangular area in FIG. 2(a) or a vertically long rectangular area in FIG. 2(b).
Also, if it is desired to set each of the top, bottom, left, and right positions as the ROI instead of the center on the screen, the direction of the imaging system 2 or the position of the chart CH should be changed so that the edges come to the positions. Just do it.

Also, the ROI setting by the ROI setting means 11 can be set once if there is no need to change the ROI in the captured images that are sequentially input.
The ROI setting means 11 outputs information indicating the position, size, etc. of the set ROI to the line MTF calculating means 12 .

The line MTF calculation means 12 calculates the MTF (hereinafter referred to as line MTF) for each line in the image (ROI image) specified by the ROI of the edge image set by the ROI setting means 11 . When the ROI image is the horizontal edge image shown in FIG. 2(b), the line MTF calculator 12 rotates the horizontal edge image to obtain the vertical edge image shown in FIG. 2(a). A line is image data corresponding to 1 pixel in the direction of the opposite side (short side) of the ROI that intersects the edge×the number of pixels on the side.
As shown in FIG. 3, the line MTF calculation means 12 calculates, for each line of the image specified by the ROI (L ₁ , L ₂ , L ₃ , . . . , LR; _R is the number of lines in the ROI) The line MTF is calculated by performing a discrete Fourier transform (one-dimensional discrete Fourier transform) on a value obtained by differentiating the density change of the image data of .

Specifically, the line MTF calculation means 12 uses ξ as a spatial frequency (cycles/pixel) that is a light-dark cycle per pixel, N as the number of pixels in one line, and n as a pixel position index (0 or more and less than N). , x(n) is the pixel value at the pixel position n, the line MTF (|F _R (ξ)|) is calculated for each ξ by the following equation (1).
Here, every ξ means every predetermined spatial frequency. This spatial frequency is determined by the number of pixels N in one line of the ROI, and ξ=[0, 1/N, . . . , (N−1)/N].

The line MTF calculation means 12 outputs the calculated line MTFs (|F ₁ (ξ)| to |F _R (ξ)|) for the number of lines of the ROI to the line MTF averaging means 13 .
Before calculating the line MTF (|F ₁ (ξ)| to |F _R (ξ)|) for the number of lines of the ROI, the line MTF calculation means 12 averages the ROI image up to the previous frame. , to calculate the line MTF. This can remove noise in the ROI.

The line MTF averaging means 13 averages the line MTFs for the number of lines of the ROI calculated by the line MTF calculating means 12 .
That is, the line MTF averaging means 13 averages the line MTFs (|F _R (ξ)|) for the number of lines (R) calculated by Equation (1) for each ξ to calculate the average MTF. Hereinafter, the average MTF averaged for each ξ is expressed as <|F _R (ξ)|>.

Here, an ideal ensemble average when the size of the ROI is assumed to be infinite will be described. The ideal ensemble average is hereinafter denoted as <|F _S (ξ)|>.
Assuming that the edge phases of the ROI are uniformly distributed with respect to the sampling positions, <|F _S (ξ)|> can be expressed by the following equation (2).

ξ indicates the spatial frequency (cycles/pixel).
step(x−s) denotes a step function representing an ideal edge density change whose value transitions from 0 to 1 at position s.
f(x) denotes the line spread function (LSF) of the camera system (imaging system 2 here) in which sampling is represented by a comb function comb(x).
δ(x) is the delta function and {δ(x)−δ(x−1)} denotes a difference filter that approximates the one-dimensional derivative of the edge spread function (ESF).
F(ξ) indicates an optical transfer function (OTF) calculated by Fourier transform of f(x).
sinc(ξ) is a sinc function.

As shown in this equation (2), <|F _S (ξ)|> contains the damping component sinc(ξ), so it is necessary to compensate for the damping component.
The line MTF averaging means 13 regards the average MTF <|F _R (ξ)|> calculated from the finite size ROI as the ideal average <|F _S (ξ)|>, and formula (3) Attenuation compensation is performed as shown.
That is, the line MTF averaging means 13 multiplies <|F _R (ξ)|> by the inverse function of sinc(ξ) (1/sinc(ξ)) as shown in the following equation (3). to compensate for the attenuation component. Here, the average after compensation is expressed as <|F _R (ξ)|> _CORR .

The line MTF averaging means 13 outputs the compensated average <|F _R (ξ)|> _CORR to the aliasing elimination means 14 as the average MTF.

The aliasing removing means 14 corrects the average MTF calculated by the line MTF averaging means 13 by removing aliasing components.
<|F _R (ξ)|> _CORR is obtained based on the sampling position (fixed phase) of the imaging device (not shown) of the imaging system 2, and is obtained by various edge phases like the conventional edge method. Not the requested MTF. Therefore, <|F _R (ξ)|> _CORR suffers from aliasing and contains errors.
Therefore, the aliasing removal means 14 removes aliasing components from the average MTF <|F _R (ξ)|> _CORR calculated by the line MTF averaging means 13 .

Here, the relationship between the average MTF <|F _R (ξ)|> _CORR including the aliasing component and the original MTF will be described with reference to FIGS. 4 to 8 and formulas.
As shown in FIG. 4, the original MTF (|F(ξ)|) (MTF _true indicated by the solid line in FIG. 4) obtained by Fourier transforming f(x), which is a line spread function (LSF), is Comb(x) shown in equation (2) is convoluted and overlaps with the aliasing MTF (MTF _aliasing indicated by the dashed line in FIG. 4) with a phase difference of e ^−2πsξ e ^−jπξ .

More specifically, the MTF obtained from 100 horizontal pixels and 200 vertical pixels as the vertical edge image (ROI) shown in FIG. 2A will be described with reference to FIG.
In FIG. 5, MTF _true indicates the MTF (original MTF) obtained by the conventional edge method, and MTF _line indicates the multiple line MTFs calculated by the line MTF calculation means 12 .
In this case, the MTF obtained by simply averaging a plurality of line MTFs (MTF _line ) is obtained as MTF _{average+aliasing} .
That is, even if the _{MTF average + aliasing} calculated by the line MTF calculating means 12 is equal to or lower than the Nyquist frequency _ξN , as shown in FIG. Estimated.

However, the MTF _{average+aliasing} cannot simply be the original MTF plus the aliasing MTF due to the phase difference between the original MTF and the aliasing MTF.
Therefore, the ratio between the original MTF and the aliasing MTF is specified under a predetermined constraint.
First, as a constraint, |F(ξ)|, which is the original MTF, is set to "0" when ξ≧1 and ξ≦−1. Here, if |F(ξ)| is obtained in the range of 0≤ξ≤1, the aliasing MTF is |F(1-ξ)|.

In this case, as shown in FIG. 7, ξ is equal to <|F _R (ξ)|> _CORR calculated by the line MTF averaging means 13 . Since this average is the average for one rotation, it does not depend on the phase difference (-jπξ (see equation (2))) between F(ξ) and F(1−ξ).
Here, the ratio r(ξ) between |F(ξ)| and |F(1−ξ)| is defined by the following equation (4).

When the phase difference between F(ξ) and F(1−ξ) is 2θ, the total length of the composite vector v is (1+2r(ξ) cos(2θ)+r(ξ) ² ) ^1/2 , An average MTF<|F _R (ξ)|> _CORR , which is the average length of the composite vector v, can be obtained. This relationship is shown in the following equation (5).

Here, E( ) is the complete elliptic integral of the second kind shown in the following equation (6).

FIG. 8 shows the relationship between r(ξ) and <|F _R (ξ)|> _CORR /|F(ξ)|.
Thus, the average MTF 〈|F _R (ξ)|〉 _CORR calculated by the line MTF averaging means 13 and the original MTF |F(ξ)| It changes according to the ratio r(ξ) to MTF|F(1−ξ)|.
In the case of ξ=0.5, r(ξ)=1 from the above equation (4), and | _F (0.5)| It can be obtained by dividing _CORR by 4/π (=1.2732).
That is, at ξ=0.5, the original MTF value can be fixedly obtained as a value obtained by dividing <|F _R (0.5)|> _CORR by 4/π. However, when ξ≠0.5, the original MTF value cannot be obtained because r(ξ) in Equation (4) is undetermined.
Therefore, the aliasing removing means 14 corrects the average MTF <|F _R (ξ)|> _CORR so as to approximate a monotonically decreasing function passing through a predetermined specific point, and obtains the MTF characteristic.

Here, for simplicity of calculation, the average MTF 〈|F _R (ξ)|〉 _CORR approximates the original MTF characteristics by p-th power of the sinc function, which is the Fourier transform of the rectangular function, and the line MTF averaging The average MTF calculated by means 13 is corrected. A sinc function is a function passing through a specific point where sinc(0)=1 and sinc(1)=0.
Here, |F(ξ)|, which is the original MTF characteristic, is assumed to be sinc ^p (ξ) (0≦ξ≦1). p of sinc ^p (ξ) can be obtained by the following equation (7), which is a modified equation (5), with ξ=0.5 (r(0.5)=1). If ξ=0.5 does not exist, <|F _R (0.5)|> _CORR is obtained by interpolation from values <|F _R (ξ)|> _CORR corresponding to adjacent spatial frequencies. Just ask.

The aliasing removal means 14 uses p calculated by the equation (7) to calculate the ratio r(ξ) between |F(ξ)| and |F(1−ξ)| Calculate for each ξ. Here, every ξ is every spatial frequency predetermined by the number of pixels of one line of the ROI.

The aliasing removing means 14 calculates the average MTF < |F _R (ξ)|> _CORR is corrected to obtain |F(ξ)|.

Thereby, the aliasing removing means 14 can remove the aliasing component of the average MTF<|F _R (ξ)|> _CORR in the range of 0≦ξ≦0.5 to estimate the original MTF.
The aliasing removing means 14 outputs |F(ξ)| obtained by correcting <|F _R (ξ)|> _CORR to the MTF outputting means 15 .

The MTF output means 15 outputs the MTF, which is the measurement result, to the outside (for example, the display device 3).
This MTF output means 15 may output the MTF value for each spatial frequency, or may output it as a graph.
As a result, the MTF measurement apparatus 1 can remove aliasing from MTF _{average+aliasing} , which includes aliasing in the average MTF for each line, and measure the original MTF _true , as shown in FIG.

By configuring the MTF measuring apparatus 1 as described above, the MTF measuring apparatus 1 can measure the MTF without obtaining the slope of the edge, and the calculation cost can be reduced as compared with the conventional edge method. .
In addition, the MTF measuring apparatus 1 calculates and averages the MTF (line MTF) for each line of the ROI, so the edges do not necessarily have to be straight lines.
Note that the MTF measurement apparatus 1 can be operated by a program (MTF measurement program) for causing a computer to function as each means described above.

[Operation of MTF measuring device]
Next, the operation of the MTF measurement apparatus 1 according to the embodiment of the present invention will be described with reference to FIG. 10 (see also FIG. 1 for the configuration).
Here, the MTF measurement device 1 stores an edge image obtained by imaging the chart CH, which is input from the imaging system 2, in the edge image storage means 10, and displays the edge image on the display device 3. FIG. The operation of the MTF measurement device 1 to measure the MTF from the edge image will be described in detail below.

In step S1, the ROI setting means 11 sets the position, size, etc. of the ROI. For example, the ROI setting means 11 sets the ROI by designating a rectangular region in the edge image displayed by the display device 3 with a pointing device (not shown) operated by the measurer.

In step S2, the line MTF calculation means 12 reads an image (ROI image) specified by the ROI set in step S1 from the edge images stored in the edge image storage means 10. FIG.
In step S3, the line MTF calculation unit 12 averages the ROI image read out in step S2 with the ROI images up to the previous frame.
In step S4, the line MTF calculation means 12 performs the discrete Fourier transform of the above equation (1) on the value obtained by differentiating the density change of the image data for one line for each line of the ROI image. Calculate the MTF.

In step S5, it is determined whether or not line MTFs have been calculated for all lines of the ROI image.
Here, if the line MTF has not yet been calculated for all the lines of the ROI image (No in step S5), the MTF measurement device 1 returns to step S4, and the line MTF calculation means 12 calculates the next line. A line MTF is calculated as a target.
On the other hand, when line MTFs have been calculated for all lines of the ROI image (Yes in step S5), the MTF measuring device 1 proceeds to step S6.

In step S6, the line MTF averaging means 13 averages the line MTFs corresponding to the number of lines of the ROI for each spatial frequency ξ (0≦ξ≦0.5) to obtain an average MTF <|F _R (ξ)|> Calculate
In step S7, the line MTF averaging means 13 calculates <|F _R (ξ)|> _CORR in which the attenuation component of the average MTF <|F _R (ξ)|> is compensated by the above equation (3).

In step S8, the anti-aliasing means 14 approximates <|F _R (ξ)|> _CORR as a function of correcting the average MTF after compensation <|F _R (ξ)|> _CORR through a particular point. A parameter is determined that specifies the shape of a predetermined monotonically decreasing function. Here, the aliasing removal means 14 uses the above equation (7) to obtain the MTF value "0" at ξ=1, and the MTF value obtained by the line MTF averaging means 13 at ξ=0.5 <|F Determine the parameter (here, p) of the function (here, sinc ^p (ξ)) passing through each point such that _R (0.5)|> _CORR /(4/π)). If ξ=0.5 does not exist, the aliasing removing means 14 uses <|F _R (0.5)|> _CORR obtained by interpolation.

In step S9, the aliasing removal means 14 uses a function specified by the parameter p to determine the ratio r(ξ) between unknown |F(ξ)| and |F(1−ξ)| It is calculated for each spatial frequency .xi.(0.ltoreq..xi..ltoreq.0.5) by the above equation (8).
In step _S10 , the aliasing removing means 14 calculates the spatial frequency _ξ |F(ξ)|, which is the MTF value for each
In step S<b>11 , the MTF output unit 15 graphs the MTF value for each spatial frequency ξ calculated in step S<b>10 and outputs the graph to the display device 3 .

Here, when the end of the measurement is instructed from the outside (Yes in step S12), the MTF measurement device 1 ends the operation.
On the other hand, when continuing the measurement operation (No in step S12), the MTF measuring apparatus 1 returns to step S2. In addition, when resetting ROI, it returns to step S1 (not shown).

By the above operation, the MTF measuring apparatus 1 can measure the MTF of the entire ROI from the MTF of each line of the ROI without obtaining the slope of the edge.
Although the embodiments of the present invention have been described above, the present invention is not limited to these embodiments.

(Modification 1)
Here, the MTF measurement device 1 successively calculates the ratio r(ξ) of the above equation (8) in the aliasing removing means 14 .
However, the calculation of this ratio r(ξ) may be omitted when the ROI image is averaged for a predetermined time or number of times after setting the ROI.
In this case, the MTF measurement device 1 performs averaging of the ROI images for a predetermined time or a predetermined number of times after setting the ROI by the aliasing removing means 14, and then stores the space in a storage unit (not shown). Maintain the frequency ξ and the ratio r(ξ) as a one-dimensional lookup table. Then, in step S8, the aliasing removing means 14 reads the ratio r(ξ) from the one-dimensional lookup table for each spatial frequency ξ. Thereby, the MTF measuring device 1 can reduce the amount of calculation.

(Modification 2)
Here, the aliasing removing means 14 uses a sinc function as a function for approximating the MTF obtained by removing aliasing components from the MTF of the ROI calculated by the line MTF averaging means 13 .
However, this function is not limited to the sinc function.
<|F _R (ξ)|> _CORR calculated by the line MTF averaging means 13 is less affected by aliasing as the spatial frequency ξ is closer to "0".

Therefore, as shown in FIG. 11, the function g(ξ) that inputs the spatial frequency ξ and outputs the MTF value is a predetermined spatial frequency that is less affected by aliasing, for example, ξ≦0.3 or less. <|F _R (ξ)|> _CORR calculated by the MTF averaging means 13 is used as it is. Then, point P1 of g(0.3)=<|F _R (0.3)|> _CORR and g(0.5)=<|F _R (0.5)|> _CORR /(4/π ) and a point P3 at g(0.7)=0. Assuming that the influence of aliasing is small at ξ≦0.3, g(ξ)=0 at 0.7≦ξ≦0.
Any monotonically decreasing function such as a quadratic function, a cos function, a spline function, or the like can be used as the function g.
In this case, instead of steps S8 to S10 in FIG. 10, the aliasing removal means 14 may find a function passing through the specific points shown in FIG. 11 and calculate the MTF value for each spatial frequency.

Of course, the anti-aliasing means 14 has g(ξ ), all or part of the average MTF value calculated by the line MTF averaging means 13 is approximated by a monotonically decreasing function passing through points P2 and P3 shown in FIG. good too.

(Modification 3)
Here, the ROI setting means 11 can set the ROI using an edge image whose edges are close to the vertical direction (see FIG. 2(a)) or an edge image whose edges are close to the horizontal direction (see FIG. 2(b)). did.
However, the ROI setting unit 11 may set an ROI including edges in an edge image including edges greatly inclined from the vertical or horizontal direction (for example, the inclination is about 45°).
In that case, the line MTF calculation means 12 obtains the slope of the edge from the image specified by the ROI (ROI image), as in the conventional edge method, and calculates the pixel interval of the edge spread function as shown in the following reference. is scaled according to the inclination angle of the edge to calculate the MTF for each line. As a result, the line MTF calculation means 12 can absorb the difference in width of the edge in the horizontal direction (or vertical direction) due to the inclination and calculate the line MTF under substantially the same conditions.
<References>
JKM Roland,“A Study of Slanted-Edge MTF Stability and Repeatability,” Proc. SPIE, vol. 9396, p. 93960L, Feb. 2015.

REFERENCE SIGNS LIST 1 MTF measurement device 10 edge image storage means 11 ROI setting means 12 line MTF calculation means 13 line MTF averaging means 14 aliasing removal means 15 MTF output means 2 imaging system 3 display device CH chart image

Claims (6)

An MTF measuring device for measuring an MTF indicating spatial frequency characteristics of an imaging system,
ROI setting means for setting an ROI, which is a rectangular area including the edge, in the edge image including the edge having a slope captured by the imaging system;
Line MTF calculation means for calculating a line MTF, which is the MTF for each line, by performing a discrete Fourier transform for each line including the edge in the ROI;
line MTF averaging means for calculating an average MTF by averaging the line MTF by the number of lines of the ROI for each predetermined spatial frequency;
anti-aliasing means for correcting to remove an aliasing component from the average MTF;
An MTF measurement device comprising: The aliasing removal means is configured such that, in the average MTF value for each spatial frequency ξ, the MTF value at the spatial frequency ξ=0.5 is the average MTF value divided by 4/π, and the spatial frequency ξ 2. The MTF measurement apparatus according to claim 1, wherein the average MTF is corrected as a function approximated by a monotonically decreasing function in which the MTF value is 0 at =1. The anti-aliasing means assumes the original ^MTF characteristic |F(ξ)| ξ)|/|F(ξ)|=(ξ/(1−ξ)) Based on ^p , the value of the MTF for each spatial frequency ξ of the average MTF is expressed as ((2/π) (r(ξ) +1) E (4r (ξ) / (r (ξ) + 1) ² )) (E ( ) is a complete elliptic integral of the second kind) for correction. Device. The edge image is an image continuously captured by the imaging system,
After setting the ROI, the aliasing removing means calculates the ratio r(ξ) in the ROI image obtained by adding and averaging a predetermined time or number of times, and holds it as a one-dimensional lookup table for each spatial frequency ξ. , thereafter, referring to the one-dimensional lookup table and using the value of the ratio r(ξ) to correct the MTF value for each spatial frequency ξ of the average MTF. 's MTF measurement device. The line MTF averaging means compensates for the attenuation component of the average MTF by multiplying the average MTF by 1/sinc(ξ) for each spatial frequency ξ. 5. The MTF measuring device according to any one of items 4. An MTF measurement program for causing a computer to function as the MTF measurement device according to any one of claims 1 to 5.

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