JP5249813B2 - MTF measurement chart, MTF measurement apparatus, and MTF measurement program - Google Patents

Description

  The present invention relates to an MTF measurement chart, an MTF measurement apparatus, and an MTF measurement program for measuring an MTF that represents a spatial frequency characteristic of an imaging system.

Conventionally, in order to evaluate the quality of an imaging system (imaging lens), a technique using an MTF (Modulation Transfer Function) representing a spatial frequency characteristic as an index is known. This MTF expresses how faithfully the contrast of a subject to be imaged can be reproduced as a spatial frequency characteristic.
As a method for measuring the MTF, a method using an edge image having an inclination described in the MTF measurement chart (Slanted-edge method; see Non-Patent Document 1 and Patent Document 1), a sine wave is arranged on the circumference. A method (Modulated Siemens Star method; see Non-Patent Document 2) using the measured MTF measurement chart (Modulated Siemens Star chart) is known.

  In this Slanted-edge method, after obtaining a line spread function (LSF) from an image (ROI: Region Of Interest) of a measurement target portion whose boundary is a black and white binary having a straight line in an MTF measurement chart, MTF is obtained by performing Fourier transform. For example, the Slanted-edge method uses a ROI image in which an edge having a slope shown in (a-1) in FIG. The characteristic is obtained, and the horizontal frequency characteristic is obtained as the MTF by using the ROI image in which the edge having the inclination shown in (a-2) of FIG.

  In the Modulated Siemens Star method, for example, as shown in the MTF measurement chart of FIG. 14B, sine waves of different spatial frequencies are arranged on the circumference, and the sine of the spatial frequency corresponding to the radius. Finding the degree of wave modulation. As described above, the Modulated Siemens Star method obtains the MTF by a sine wave on the circumference, and therefore can measure the MTF in any direction other than the horizontal direction and the vertical direction.

Japanese Patent No. 4092853

ISO 12233: 2000, “Photography-Electronic Still-picture Cameras-Resolution Measurements” C. Loebich, D. Wueller, B. Klingen, A. Jaeger, “Digital camera resolution measurement using sinusoidal Siemens stars”, Proc. SPIE, 6502, 65020N (2007)

Although the conventional Slanted-edge method can obtain the MTF in one direction from the edge image, there is a problem that the MTF in an arbitrary direction cannot be measured at a time.
On the other hand, the conventional Modulated Siemens Star method can obtain the MTF in an arbitrary direction such as an oblique direction in addition to the horizontal direction and the vertical direction. However, since the Modulated Siemens Star method uses a complex layout and a shaded chart, it is difficult to create a chart, and it is easily affected by the gradation accuracy of the chart itself, compared with the ROI of the Slanted-edge method. In addition, since the area required for the analysis is large, there is a problem that the MTF measurement is easily affected by the nonuniformity of illumination.

  In addition, the Modulated Siemens Star method has a problem that the MTF measurement is easily affected by the chart size and illumination non-uniformity because the frequency differs between the image center and the periphery of the MTF measurement chart to be used.

  The present invention provides an MTF measurement chart that can reduce the influence on MTF measurement due to imaging conditions such as illumination and chart size, and that can measure MTF in the horizontal direction, vertical direction, and oblique direction at the same time, MTF An object is to provide a measuring apparatus and an MTF measuring program.

The present invention was devised to achieve the above object. First, the MTF measurement chart according to claim 1 is used in an MTF measurement apparatus that measures MTFs representing spatial frequency characteristics of an imaging system. An MTF measurement chart having a predetermined position on the chart surface as a central position, a boundary extending in a horizontal direction and a vertical direction orthogonal to the central position, and an oblique direction with a central angle of 45 ° with respect to the boundary An arbitrary radiation boundary having a radiation area that divides the chart surface by an extended boundary, and a color having a different contrast for each of the radiation areas adjacent to each other at the boundary, and the central angle is present at an interval of 45 °. The chart surface is divided into a first area and a second area with a straight line having a predetermined angle at the center position as a boundary with respect to the reference, It is characterized in that colors are arranged so that a difference in contrast of the radiation region is different between the first region and the second region .

According to this configuration, the MTF measurement chart has an edge formed at the boundary of the region due to the contrast difference. Further, since the edges are formed at intervals of at least a central angle of 45 °, an image in which the edges are formed in the horizontal direction, the vertical direction, and the oblique direction is captured in the imaging system to be measured. . For this reason, it is possible to measure the MTF in the horizontal direction, the vertical direction, and the oblique direction at a time by using the chart image obtained by imaging the MTF measurement chart. Furthermore, an edge is formed by two boundaries having different contrast differences in the horizontal direction, two boundaries having different contrast differences in the vertical direction, and two boundaries having different contrast differences in the oblique direction. For this reason, by using the chart image obtained by imaging the MTF measurement chart, it is possible to measure MTFs in the horizontal direction, the vertical direction, and the oblique direction with different contrast differences at a time.

The MTF measuring apparatus according to claim 2 or 3 is an MTF measuring apparatus for measuring an MTF representing a spatial frequency characteristic of an imaging system, and comprises a chart image storage means, a chart center obtaining means, and a reference image position obtaining. And a relative position determination unit, a direction-specific image position specifying unit, an image extracting unit, and an MTF calculating unit.

In such a configuration, the MTF measuring apparatus according to claim 2 has the chart image storage means with a predetermined position on the chart surface of the chart for MTF measurement as a central position and a boundary extending in the horizontal direction and the vertical direction orthogonal to the central position. And a radiation region in which the chart surface is divided by a boundary extending in an oblique direction with a central angle of 45 ° with respect to the boundary, and colors having different contrasts are arranged for each radiation region adjacent to the boundary. Regarding the MTF measurement chart, of the boundaries existing at intervals of 45 ° in the central angle of the MTF measurement chart , arbitrary orthogonal boundaries are defined as a horizontal boundary and a vertical boundary, and the horizontal boundary and the vertical boundary are covered. A chart image, which is a captured image of a chart for MTF measurement that is imaged by a predetermined angle rotated by an imaging system to be measured, is stored. The predetermined angle is an angle that is predetermined as an angle at which various phases can be measured by the Slanted-edge method, and is about 1 ° to 5 °, for example.

  Furthermore, the MTF measurement device acquires the center position of the radial boundary in the chart image by the chart center acquisition means. The center position may be obtained by displaying a chart image on a display device and instructed by an operator, or by detecting a plurality of straight lines from the boundary by image processing and obtaining intersections thereof. It is good also as acquiring.

Then, the MTF measuring apparatus designates a rectangular region straddling between radiation regions having different horizontal or vertical contrasts in the chart image by the reference image position acquisition unit, so that the reference image that is the reference measurement target image is displayed. Get position and size. Since this reference image is a rectangular region straddling between the radiation regions, the reference image includes an edge that becomes a boundary of the region due to a contrast difference. The rectangular area can be designated by displaying a chart image on a display device and instructing by an operator, like the chart center obtaining means.
Then, the MTF measuring device calculates the relative position with respect to the center position of the reference image based on the position of the reference image acquired by the reference image position acquisition means and the center position acquired by the chart center acquisition means by the relative position determination means. judge.

  Then, the MTF measuring apparatus uses the direction-specific image position specifying unit to set the horizontal direction around the center position based on the relative position determined by the relative position determining unit and the size of the reference image acquired by the reference image position acquiring unit. The position and size of the measurement target image in the vertical direction and the oblique direction are specified. Thus, by specifying the position and size of one reference image, it is possible to specify the measurement target images in the horizontal direction, the vertical direction, and the oblique direction. In the MTF measurement chart, the boundaries of the radiation areas are formed at intervals of a central angle of 45 °, and the reference image is an image including an edge straddling the radiation areas, so the horizontal direction, the vertical direction, and the diagonal direction The measurement target image includes an edge straddling the radiation regions.

Thereafter, the MTF measuring apparatus extracts and reads out the measurement target image specified by the image position specifying unit for each direction from the chart image storage unit by the image extracting unit.
Then, the MTF measuring device calculates the MTF for the horizontal frequency component or the vertical frequency component corresponding to each direction in the measurement target image in each direction extracted by the image extracting unit by the MTF calculating unit. As a result, the MTF is measured for the measurement target images in the horizontal direction, the vertical direction, and the oblique direction.

Furthermore, the MTF measuring device according to claim 4 is the same as the MTF measuring device according to claim 2 or 3 , further comprising reference image position correcting means.

  In such a configuration, the MTF measuring apparatus detects the edge where the difference in contrast of the reference image acquired by the reference image position acquisition unit is generated by the reference image position correction unit so that the center of the reference image is on the edge. Correct the position of the reference image. At this time, based on the relative position determined by the relative position determination unit, the reference image position correction unit sets the position of the reference image in the horizontal direction when the relative position of the reference image is present in the vertical direction with respect to the center position. Move to. On the other hand, when the relative position of the reference image exists in the left-right direction with respect to the center position, the position of the reference image is moved in the vertical direction. As a result, regions having different contrasts are uniformly arranged in the reference image.

Further, the MTF measuring device according to claim 5 is the MTF measuring device according to any one of claims 2 to 4, wherein the direction-specific image position specifying means respectively correspond to four oblique directions. As the measurement target image, a parallelogram image in which the vertical height of the measurement target image in the horizontal direction is the base length and the horizontal width of the measurement target image in the horizontal direction is the height, and the horizontal width of the measurement target image in the vertical direction Is a parallelogram image with the length of the base and the vertical height of the image to be measured in the vertical direction, the angle on the acute angle between the base of each parallelogram and the side corresponding to the diagonal direction It was set as the structure which identifies two images from which becomes 45 degrees.

  In such a configuration, the MTF measuring apparatus uses the image position specifying unit for each direction to measure the length and height of the base of the parallelogram as a single measurement object image in the oblique direction, and the measurement object image in the vertical direction and the horizontal direction, respectively. Two measurement target images having the same width and height as each of the measurement target images are identified. Of the two measurement target images in the oblique direction specified as described above, the measurement target image whose bottom is a horizontal line segment is an image in which an edge can always be detected in the horizontal direction. In addition, a measurement target image whose bottom is a vertical line segment is an image in which an edge can always be detected in the vertical direction.

Furthermore, the MTF measuring apparatus according to claim 6 is the MTF measuring apparatus according to claim 5 , wherein the image extraction means includes oblique direction image shaping extraction means.

  In such a configuration, the MTF measuring apparatus shapes and extracts a parallelogram image, which is an image to be measured in an oblique direction, into a rectangular image having a base and a height as one side by an oblique direction image shaping and extracting unit. As a result, all the measurement target images have the same shape and the same area.

The MTF measurement program according to claim 7 , wherein a center position is a predetermined position on a chart surface of the MTF measurement chart, a boundary extending in a horizontal direction and a vertical direction orthogonal to the center position, and a center with respect to the boundary An MTF measurement chart having a radiation area in which the chart surface is divided by a boundary extending in an oblique direction with an angle of 45 °, and a color having a different contrast for each radiation area adjacent to the boundary. Of the boundaries where the central angle of the measurement chart exists at intervals of 45 °, arbitrary orthogonal boundaries are defined as a horizontal boundary and a vertical boundary, and the horizontal boundary and the vertical boundary are set at a predetermined angle by the imaging system to be measured. From the chart image, which is a picked-up image of the chart for MTF measurement, picked up by rotation and stored in the chart image storage means, the image pickup system Configuration for causing a computer to function as a chart center acquisition unit, a reference image position acquisition unit, a relative position determination unit, a direction-specific image position specification unit, an image extraction unit, and an MTF calculation unit in order to measure the MTF representing the spatial frequency characteristics It was.

  In such a configuration, the MTF measurement program acquires the center position of the radial boundary in the chart image by the chart center acquisition unit. Further, the MTF measurement program specifies a rectangular region straddling between the radiation regions having different horizontal or vertical contrasts in the chart image by the reference image position acquisition unit, so that the reference image that is the reference measurement target image is displayed. Get position and size.

Then, the MTF measurement program calculates the relative position with respect to the center position of the reference image based on the position of the reference image acquired by the reference image position acquisition means and the center position acquired by the chart center acquisition means by the relative position determination means. judge.
Then, the MTF measurement program uses the image position specifying unit for each direction to determine the horizontal direction around the center position based on the relative position determined by the relative position determination unit and the size of the reference image acquired by the reference image position acquisition unit. The position and size of the measurement target image in the vertical direction and the oblique direction are specified.

Thereafter, the MTF measurement program extracts and reads out the measurement target image specified by the direction-specific image position specifying means from the chart image storing means by the image extracting means.
Then, the MTF measurement program calculates the MTF for the horizontal frequency component or the vertical frequency component corresponding to each direction in the measurement target image extracted by the image extraction unit by the MTF calculation unit. As a result, the MTF is measured for the measurement target images in the horizontal direction, the vertical direction, and the oblique direction.

The present invention has the following excellent effects.
According to the first aspect of the present invention, the MTF measurement chart is distinguished into radiation areas having boundaries at intervals of 45 ° of the central angle, and there is a difference in contrast at the boundaries of the respective radiation areas. By measuring the MTF from an image obtained by imaging the chart, the MTF in each direction can be measured at a time using the edges in the horizontal direction, the vertical direction, and the oblique direction. Furthermore, since the difference in contrast of the radiation region differs between the first region and the second region, in addition to the effect that the edges in the horizontal direction, the vertical direction, and the oblique direction can be measured at a time, the horizontal direction In the vertical direction and the oblique direction, MTFs having different contrast differences can be measured at a time. Moreover, since the chart for MTF measurement has a simple configuration in which the chart surface is radially divided, the chart can be easily created.
Furthermore, since the MTF measurement chart allows the MTF to be measured by the edge generated at the boundary of the radiation region, if a large chart is prepared, the measurement can be performed with a wide range of magnifications.

According to the second, third, and seventh aspects of the present invention, the MTF in each direction is obtained from the chart image obtained by imaging the MTF measurement chart using horizontal, vertical, and diagonal edges in one measurement. (Horizontal frequency component or vertical frequency component) can be measured.

According to the fourth aspect of the present invention, by correcting the position of the reference image so that the center of the reference image is on the edge, the two color regions in the reference image can have the same area. For this reason, similarly, other measurement target images extracted based on the position of the reference image can have the same area for the two colors, and the measurement of the MTF in each measurement target image can be performed under the same conditions. it can.

According to the fifth aspect of the present invention, as the measurement target image in the oblique direction, the measurement target image for measuring the MTF by detecting the edge based on the vertical direction and the MTF by detecting the edge based on the horizontal direction. A measurement target image for measurement can be extracted, and horizontal and vertical MTFs can be measured by one measurement. At this time, the MTF in the diagonal direction can be measured by dividing it into a horizontal frequency component and a vertical frequency component.

According to the sixth aspect of the invention, since the shape and size of the measurement target images in the horizontal direction, the vertical direction, and the diagonal direction can all be made the same, the measurement of the MTF in each measurement target image is performed under the same conditions. MTF can be measured with high accuracy.

It is a block diagram which shows the structure of the MTF measuring apparatus which concerns on embodiment of this invention. It is a pattern figure which shows the chart for MTF measurement which concerns on embodiment of this invention. It is explanatory drawing for demonstrating the pattern structure of the chart for MTF measurement. It is explanatory drawing for demonstrating the relationship between the chart for MTF measurement, and the chart image which imaged the said chart for MTF measurement. It is explanatory drawing for demonstrating the center position on a chart image, and the position of a reference | standard image. It is explanatory drawing for demonstrating position correction of a reference | standard image (reference | standard ROI). It is explanatory drawing for demonstrating the position of the measuring object image of the horizontal direction on a chart image, a vertical direction, and a diagonal direction. It is explanatory drawing for demonstrating the relationship between the measuring object image of a diagonal direction, and the measuring object image of a horizontal direction and a vertical direction. It is explanatory drawing for demonstrating the content of shaping to the rectangular shape of the measuring object image of the diagonal direction. It is a figure which shows the measuring object image of the horizontal direction, the vertical direction, and the diagonal direction extracted from the chart image. It is a flowchart which shows operation | movement of the MTF measuring apparatus which concerns on embodiment of this invention. It is another pattern figure of the chart for MTF measurement concerning the embodiment of the present invention. It is another pattern figure of the chart for MTF measurement concerning the embodiment of the present invention. FIG. 4 is a pattern diagram of a conventional MTF measurement chart, where (a) is a part of a chart used in the Slanted-edge method, and (b) is a Modulated Siemens Star chart.

Embodiments of the present invention will be described below with reference to the drawings.
[Chart for MTF measurement]
First, the MTF measurement chart used in the embodiment of the present invention will be described with reference to FIG. 2 and FIG. Here, the MTF measurement chart shown in FIG. 2 is described with reference numerals in FIG.
This MTF measurement chart CH is a chart used in an MTF measurement apparatus that measures MTF (Modulation Transfer Function) representing the spatial frequency characteristics of the imaging system. That is, the MTF measurement chart CH is imaged through an imaging system (lens, camera, or the like) that is an object to be measured by the MTF, and is used when measuring the MTF in an MTF measurement apparatus described later.

As shown in FIG. 3, the MTF measurement chart CH has a center position O such that a predetermined position on the chart surface is a center position O, and a boundary dividing at least a region is formed at intervals of 45 ° of the center angle. The chart surface is divided into radiation areas B (B ₁ , B ₂ ,..., B ₁₀ ), which are a plurality of areas, at a boundary extending radially from.
Further, in the MTF measurement chart CH, colors having different contrasts are arranged in the radiation region B (B ₁ , B ₂ ,..., B ₁₀ ) for each adjacent radiation region B. Here, for each adjacent radiation region B, white and black, and two gray colors having different shades, which are intermediate colors of white and black, are arranged.

The MTF measurement chart CH is based on an arbitrary one of boundaries existing at intervals of 45 ° in the central angle, and a straight line having a predetermined angle θ at the central position O with respect to the reference as a boundary. surface is divided into a first region a ₁ and the second region a ₂ regions. Here, as one of the boundaries existing at intervals of 45 ° of the central angle, the chart surface is first defined by a straight line having a predetermined angle θ at the central position O with reference to the boundaries of the radiation regions B ₁ and B ₉ . It is divided into regions _{a 1} and the second region _{a 2.}

  The angle θ is an image obtained by imaging the MTF measurement chart CH, and a range in which a rectangular region partially including each adjacent radiation region can be specified on a boundary existing at intervals of 45 ° of the central angle. If it is. For example, it may be about 10 ° to 30 °. The designation of the rectangular area will be described in the description of the MTF measuring apparatus described later.

The first area A ₁ and the second region A _2, between the respective regions, the radiation area B, the difference in B ... contrast is color differently. Specifically, in the radiation regions B ₁ , B ₂ , and B _{7 to} B ₉ included in the first region A ₁ , the respective colors are alternately arranged in white and black, and are included in the second region A _2. In the radiation regions B _{3 to} B ₆ and B ₁₀ , each color is an intermediate color between white and black and is alternately arranged in two gray colors having different shades.

By configuring the pattern of the MTF measurement chart CH in this way, when measuring the MTF with an image obtained by imaging the chart, a region straddling the radiation regions B ₂ and B ₉ having edges in the vertical direction, and In the region straddling the radiation regions B ₆ and B ₁₀ , the horizontal frequency characteristics can be measured. Also, areas astride the emission region B ₁ and B ₈ having an edge in the horizontal direction, and, in a region extending over the emission region B ₄ and B _5, it is possible to measure the frequency characteristic in the vertical direction . Further, areas astride the emission region B ₁ and B ₂ having an edge in an oblique direction, region astride the emission region B ₃ and B _4, was astride the emission region B ₅ and B ₆ regions, and the radiation In a region straddling the regions B ₇ and B ₈ , it is possible to measure frequency characteristics in an oblique direction.

As described above, the MTF measurement chart CH can measure the MTF in the horizontal direction, the vertical direction, and the oblique direction at a time with simple patterns of black and white and gray. Furthermore, the MTF measurement chart CH has a black and white edge and a gray edge, so that the MTF can be evaluated based on the contrast difference of the imaging system to be measured.
Further, since the MTF measurement chart CH is divided by straight line boundaries and is configured by a simple pattern in which each region is colored in a single color, chart creation is easy.

(Another pattern example of the MTF measurement chart)
Here, another pattern example of the MTF measurement chart will be described with reference to FIGS.
In the MTF measurement chart CH _B shown in FIG. 12, white and black are respectively arranged in the radiation regions B, B,... Adjacent to each other at a boundary extending radially from the center position O at intervals of 45 ° of the central angle. Thus, the chart surface is divided.

The MTF measurement chart CH _B does not have the first area A ₁ and the second area A ₂ as in the MTF measurement chart CH described in FIG. 3, and is simply colored in white and black alternately. Yes. As described above, the MTF measurement chart CH _B is a simple black and white pattern, and can measure the MTF in the horizontal direction, the vertical direction, and the oblique direction at a time. In addition, the MTF measurement chart CH _B has a sufficiently wide dynamic range of the camera to be measured by the MTF, the white part of the chart does not over clip, the black part does not under clip, and the MTF has the same contrast in eight directions. Suitable when you want to measure. Further, since the chart becomes a black and white binary image, the creation is easy. Further, the MTF measurement chart CH _B may be colored in reverse to white and black.

It should be noted that in the MTF measurement chart CH _B , if the dynamic range of the camera does not fit (the white part of the chart overclips or the black part also underclips), or the MTF in 8 directions with the same contrast is used. If you want to measure, white and black colors, as shown in FIG. 13, to suppress the contrast difference, and color using the two colors of different shades in gray color, may constitute a chart for MTF measurement CH _C .

In addition, the MTF measurement chart CH described in FIG. 3 is configured to measure MTFs in eight directions at once, but with the same contrast, the horizontal direction, the vertical direction, and the diagonal direction (diagonal upper right direction, diagonal right lower direction). It is good also as a structure which measures only these 4 directions. That is, in the MTF measuring chart CH 3, one of the first region A ₁ and the second region A ₂ only to the color scheme, the other areas any color (e.g., white color, etc.) may be.
Hereinafter, the configuration and operation of an MTF measurement apparatus that measures MTF will be described using the MTF measurement chart CH as an example.

[Configuration of MTF measuring device]
First, the configuration of an MTF measuring apparatus according to an embodiment of the present invention will be described with reference to FIG. This MTF measuring apparatus 1 measures MTF representing the spatial frequency characteristics of an imaging system. Here, the MTF measurement apparatus 1 includes a chart image storage unit 10, an ROI position specifying unit 11, an image extraction unit 12, and a calculation unit 13. Here, the MTF measuring apparatus 1 is connected to an imaging system (lens, camera, etc.) 2 to be measured and a display device 3 that provides a user interface for operating the MTF measuring apparatus 1. .

  The chart image storage means 10 stores an image (chart image) obtained by imaging the MTF measurement chart CH described in FIGS. 2 and 3 by the imaging system (lens, camera, etc.) 2 to be measured. It is a general storage device such as a hard disk.

As shown in FIG. 4, the chart image stored in the chart image storage means 10 is an arbitrary boundary orthogonal among the boundaries where the central angle of the MTF measurement chart CH exists at intervals of 45 °. Is a horizontal boundary and a vertical boundary, and the horizontal boundary and the vertical boundary are images that are rotated by a predetermined angle by the imaging system 2 to be measured. The rotation angle theta _G is a predetermined angle as the angle capable of measuring a variety of phases in Slanted-edge method, for example, about 1 ° to 5 °. The chart image IMG _CH may be an image obtained by rotating the MTF measurement chart CH by an angle θ _G in advance when attaching to the wall surface during measurement, or rotating the imaging system 2 by an angle θ _G. It is also possible to pick up images. Alternatively, the chart image IMG _CH may be an image obtained by rotating the chart itself by an angle θ _G in advance.

  The ROI position specifying unit 11 specifies the position of a measurement target portion image (ROI: Region Of Interest) for performing MTF measurement from a chart image obtained by imaging the MTF measurement chart CH. Here, the ROI position specifying unit 11 includes a chart center acquiring unit 111, a reference ROI position acquiring unit 112, a relative position determining unit 113, a reference ROI position correcting unit 114, and a direction-specific ROI position specifying unit 115. I have.

The chart center acquisition unit 111 acquires the center position (center coordinate) of the radial boundary in the chart image stored in the chart image storage unit 10. The chart center acquisition unit 111 displays the chart image IMG _CH shown in FIG. 5 on the display device 3. For example, the operator can determine the center of the chart image IMG _CH by a pointing device (input unit) such as the touch pen 31. By specifying, the center position (center coordinate) O of the chart image IMG _CH is acquired. The chart center acquisition unit 111 may extract a plurality of straight lines that are boundaries of the radiation region B (see FIG. 3) by edge detection in the chart image, and obtain the center coordinates from the intersection of the straight lines.
The chart center acquisition unit 111 outputs the acquired center position (center coordinate) to the relative position determination unit 113 and the direction-specific ROI position specification unit 115.

The reference ROI position acquisition means (reference image position acquisition means) 112 designates a rectangular area straddling between radiation areas having different horizontal or vertical contrasts in the chart image, and thereby a reference ROI that is a reference measurement target image. Is to acquire the position and size. The reference ROI position acquisition unit 112 is configured such that the operator designates a rectangular area with a pointing device such as the touch pen 31 in the chart image IMG _CH shown in FIG. 5 displayed on the display device 3. The position and size of the reference ROI that is the measurement target image is acquired. FIG. 5 shows an example in which a region on the vertical boundary adjacent to the radiation region and above the center position is designated as the reference ROI (R), but the region below the center position is shown. An area may be specified. Or you may designate the area | region of the right side or left side from the center position on the horizontal boundary which the radiation | emission area | region adjoins.
The reference ROI position acquisition unit 112 outputs the position and size of the reference ROI to the relative position determination unit 113 and the reference ROI position correction unit 114.

  The relative position determination unit 113 determines a relative position with respect to the center position of the reference ROI based on the position of the reference ROI acquired by the reference ROI position acquisition unit 112 and the center position acquired by the chart center acquisition unit 111. is there. That is, the relative position determination unit 113 determines in which direction (up / down / left / right) the image is present with respect to the center position (center coordinates). The relative position determining unit 113 outputs the determination result to the reference ROI position correcting unit 114 and the direction-specific ROI position specifying unit 115.

  The reference ROI position correcting means (reference image position correcting means) 114 corrects the position of the reference ROI so that the center of the reference ROI is on the boundary of the radiation region. That is, the reference ROI position correcting unit 114 detects an edge where a difference in contrast occurs in the reference ROI acquired by the reference ROI position acquiring unit 112, and the center (diagonal center) of the reference ROI is on the edge. As described above, the position of the reference ROI is corrected by moving the position of the reference ROI horizontally or vertically based on the relative position determined by the relative position determination unit 113.

  For example, the reference ROI (R) is selected in the vicinity of the vertical boundary above the center position O as shown in FIG. 5, and the center C of the reference ROI (R) is set to be vertical as shown in FIG. If it does not exist on the boundary, the reference ROI position correcting means 114 detects the edge in the horizontal direction within the reference ROI (R), and shifts the center C in the horizontal direction on the edge as shown in FIG. The position of the reference ROI is corrected by moving it. Then, the reference ROI position correcting unit 114 outputs the corrected position of the reference ROI to the direction-specific ROI position specifying unit 115.

  Note that the reference ROI position correcting unit 114 corrects the position of the reference ROI by the same method as described in FIG. 6 even when the reference ROI is selected near the vertical boundary below the center position. To do. On the other hand, when the reference ROI is selected near the horizontal boundary on the left side or the right side of the center position, the reference ROI position correcting unit 114 detects an edge in the vertical direction within the selected reference ROI, and the center of the reference ROI is determined. The position of the reference ROI is corrected by shifting in the vertical direction and moving on the edge.

  The direction-specific ROI position specifying means (direction-specific image position specifying means) 115 is based on the relative position determined by the relative position determining means 113 and the position and size of the reference ROI output from the reference ROI position correcting means 114. The position and size of the measurement target image on the edges in the horizontal direction, the vertical direction, and the oblique direction with the position as the center are specified.

Here, as shown in FIG. 7, the direction-specific ROI position specifying means 115 is symmetric with respect to the reference ROI (here, R ₂ ) with respect to the center O as a vertical direction ROI (R ₂ , R ₄ ). The horizontal direction ROI (R ₁ , R ₃ ) and the diagonal direction ROI (R _{5 to} R ₁₂ ) are specified.
In the example of FIG. 7, R ₃ , R ₄ , and R ₁ are specified at positions where the reference ROI (R ₂ ) is rotated 90 °, 180 °, and 270 ° clockwise with respect to the center O. Further, since the size is obtained by rotating the reference ROI, R _{1 to} R ₄ have the same size although there are differences in the horizontal and vertical directions.

A specific method of the oblique direction ROI (R _{5 to} R ₁₂ ) will be described with reference to FIG. 8 (refer to FIG. 1 as appropriate). In FIG. 8, although a description is given of specific techniques example R ₇ and R ₈ is an upper right of the ROI in FIG. 7, the other direction (upper left, obliquely lower right, lower left) also applies to the ROI It is.

As shown in FIG. 8 (a), the vertical height of R ₂ is a reference ROI selected by the vertical direction upper side L1, a horizontal width L2. At this time, as shown in FIG. 8B, the direction-specific ROI position specifying unit 115 uses the horizontal width L2 of the reference ROI (R ₂ in FIG. 8A) as the measurement target image in the vertical direction as the base, An image of a parallelogram having an acute angle of 45 ° between the base and the side corresponding to the oblique direction is specified as ROI ₇ with the vertical height L1 as the height. Further, the center (diagonal center) of R _{2 as} the reference ROI and the center (diagonal center) of R ₇ have an angle of 45 ° with respect to the center O (see FIG. 7).
As a result, the ROI (R ₇ ) that is the measurement target image shown in FIG. 8B is an image in which an edge can be extracted without fail in the horizontal direction.

On the other hand, as shown in FIG. 8C, the direction-specific ROI position specifying means 115 uses the vertical height L2 of the ROI (R ₁ in FIG. 8A) that is the measurement target image in the horizontal direction as the base and horizontal width. A parallelogram image in which the acute angle formed by the base and the side corresponding to the oblique direction is 45 ° is specified as ROI (R ₈ ) with L1 as the height. Further, the center (diagonal center) of R _{2 as} the reference ROI and the center (diagonal center) of R ₈ have an angle of 45 ° with respect to the center O (see FIG. 7).
As a result, the ROI (R ₈ ) that is the measurement target image shown in FIG. 8C is an image in which an edge can be extracted without fail in the vertical direction.

As described above, the direction-specific ROI position specifying unit 115 specifies the region having the same area (number of pixels) because the ROI in the oblique direction is specified based on the size of the reference ROI.
Returning to FIG. 1, the description of the configuration of the MTF measuring apparatus 1 will be continued.

  The image extracting unit 12 extracts an ROI image for each direction specified by the ROI position specifying unit 11 from the chart image. Here, the image extracting unit 12 includes a horizontal / vertical ROI extracting unit 121 and an oblique direction ROI shaping / extracting unit 122.

The horizontal / vertical ROI extraction unit (horizontal / vertical image extraction unit) 121 extracts the horizontal and vertical ROI images from the chart image storage unit 10 among the ROIs for each direction specified by the ROI position specifying unit 11. Extracted and read out. The horizontal / vertical ROI extraction unit 121 outputs the extracted horizontal and vertical ROI images to the calculation unit 13. That is, the horizontal / vertical ROI extraction unit 121 converts R ₂ and R ₄ that are vertical ROIs and R ₁ and R ₃ that are horizontal ROIs into the chart image IMG _CH among the ROIs described in FIG. Extract from

The oblique direction ROI shaping / extracting means (oblique direction image shaping / extracting means) 122, among the ROIs for the respective directions specified by the ROI position specifying means 11, outputs a parallelogram image that is an image of the oblique ROI, It is shaped into a rectangular image and extracted. The oblique direction ROI shaping / extracting means 122 outputs the oblique direction ROI that has been shaped and extracted to the computing means 13. That is, the oblique direction ROI shaping / extracting means 122 uses R _{5 to} R ₁₂ which are ROIs in the oblique direction out of the ROIs described in FIG.

Here, with reference to FIG. 9 (refer to FIG. 1 as appropriate), a method in which the oblique ROI shaping extraction unit 122 shapes the oblique ROI will be described. In FIG. 9, an example of shaping the R ₇ is a diagonal direction of the ROI shown in FIG. 7 will be described.
As shown in FIG. 9A, the oblique ROI shaping / extracting means 122 shapes and extracts the oblique ROI shown in (a-1) into the rectangular image shown in (a-2). The parallelogram image shown in (a-1) has the same number of pixels in the horizontal direction. Further, since the parallelogram has an inclination of 45 °, the horizontal line is sequentially shifted by one pixel in the vertical direction.

Therefore, as shown in FIG. 9B, the oblique ROI shaping / extracting means 122 shifts the oblique ROI shown in (b-1) one pixel at a time for each line, so that (b− As shown in 2), an ROI in which pixels are aligned in the vertical direction is assumed.
As a result, the oblique ROI shaping / extracting means 122 can extract the oblique ROI as an image of a rectangular area having the same shape as the image extracted by the horizontal / vertical ROI extracting means 121.
Returning to FIG. 1, the description of the configuration of the MTF measuring apparatus 1 will be continued.

As described above, the image extraction unit 12 can extract the ROI in the horizontal direction, the vertical direction, and the oblique direction from the chart image. That is, as shown in FIG. 10, the image extraction unit 12 stores the measurement target image (ROI) in each direction in the chart image storage unit 10 as rectangular images (R _{1 to} R ₁₂ ) having the same shape. Extract from chart image.

  The calculating means 13 calculates a spatial frequency characteristic (MTF) from the measurement object image (ROI). Here, the calculation means 13 includes an MTF calculation means 131 and an oblique direction MTF averaging means 132.

  The MTF calculation unit 131 calculates the MTF for the measurement target image in each direction extracted by the image extraction unit 12. The MTF calculating unit 131 calculates the MTF from the edge image serving as the measurement target image using a general Slanted-edge method. That is, the MTF calculating unit 131 calculates the line spread function (LSF) for each measurement target image, and then calculates the MTF by performing Fourier transform.

Thereby, MTF calculating means 131 in each measurement target image shown in FIG. 10, the vertical frequency component of the monochrome image for R _1, horizontal frequency components of the monochrome image for R _2, for R ₃ are shades of gray For the vertical frequency component of the image, and R ₄ , the MTF of the horizontal frequency component of the gray gray image can be obtained.

Note that the MTF calculation means 131 outputs the MTF calculated from the horizontal and vertical measurement target images (R _{1 to} R _{4 in} FIG. 10) as the MTF calculation result as it is. On the other hand, the MTF calculation unit 131 outputs the MTF calculated from the measurement target image in the oblique direction (R _{5 to} R _{12 in} FIG. 10) to the oblique direction MTF averaging unit 132.

  The oblique direction MTF averaging means 132 averages the MTF calculated from the same oblique target processing image with respect to the MTF calculated from the oblique measurement target image calculated by the MTF calculating means 131. . Here, it is assumed that the horizontal frequency characteristic in the oblique direction and the vertical frequency characteristic substantially coincide with each other, and MTF averaging is performed on the horizontal frequency component and the vertical frequency component.

That is, the oblique direction MTF averaging means 132 averages the MTFs calculated from R ₅ and R ₆ which are the same oblique direction measurement target images shown in FIG. Similarly, the diagonal MTF averaging means 132 averages the MTF calculated from R ₇ and R ₈ , averages the MTF calculated from R ₉ and R _10, and calculates the MTF calculated from R ₁₁ and R _12. Is averaged. The oblique direction MTF averaging means 132 outputs the averaged MTFs in the four directions as MTF calculation results to the outside.

As described above, the computing unit 13 calculates MTFs in a total of eight directions from the measurement target image extracted from the chart image by the image extracting unit 12, up and down, left and right, and upper right, lower right, upper left, and lower left. Can do.
By configuring the MTF measurement device 1 in this way, the MTF measurement device 1 measures MTF (horizontal frequency component and vertical frequency component) in the horizontal direction and the vertical direction from the chart image obtained by imaging the MTF measurement chart CH. can do. Further, the MTF measuring apparatus 1 can measure the MTF by dividing the oblique direction into a horizontal frequency component and a vertical frequency component.
The MTF measuring apparatus 1 can be realized by a general computer having a CPU and a memory (not shown). At this time, the MTF measurement apparatus 1 operates according to the MTF measurement program that causes the computer to function as each of the above-described means.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to this structure. For example, when the rectangular area is designated, the reference ROI position acquisition unit 112 displays the rectangular frame and the center thereof together with the chart image, and the operator moves the rectangular frame so that the center overlaps the edge. Thus, the position of the reference ROI may be specified. In this case, the configuration may be a simple configuration in which the reference ROI position correcting unit 114 is omitted.

Further, here, in the direction-specific ROI position specifying means 115, the four directions of the horizontal direction, the vertical direction, and the oblique direction (oblique upper right direction, oblique lower right direction), and the same direction and four directions having different contrast differences. However, the ROI is specified only for four directions (horizontal direction, vertical direction, diagonal direction (diagonal upper right direction, diagonal lower right direction)) having the same contrast difference. It is good also as a simple structure to do. For example, the direction-specific ROI position determining means 115 may be configured to identify an ROI only one of the predetermined first region _{A 1} and the second region _{A 2} of the MTF measuring chart CH in FIG.

  In this example, the computing unit 13 averages and outputs the MTFs in the same direction for the MTFs in the oblique direction, but may output the averaged MTFs without averaging. Alternatively, the direction-specific ROI position specifying unit 115 may extract only one of the ROIs in the same oblique direction. In this case, the configuration can be a simple configuration in which the oblique direction MTF averaging means 132 is omitted.

[Operation of MTF measuring device]
Next, the operation of the MTF measuring apparatus according to the embodiment of the present invention will be described with reference to FIG.

First, the MTF measuring apparatus 1 images the MTF measurement chart CH by the imaging system 2, and stores the captured chart image in the chart image storage means 10 (step S1).
Then, the MTF measuring apparatus 1 displays the chart image on the display device 3 by the chart center acquisition unit 111 of the ROI position specifying unit 11 and designates the center by the operator, whereby the center position (center of the chart image) Coordinate) is acquired (step S2).

  Further, the MTF measuring apparatus 1 is configured to perform a reference measurement by designating a rectangular area by the operator in the chart image displayed on the display device 3 by the reference ROI position acquisition unit 112 of the ROI position specifying unit 11. The position and size of the reference ROI that is the target image are acquired (step S3). In addition, you may perform operation | movement of these steps S2 and S3 by changing the order.

  Then, the MTF measuring apparatus 1 determines that the position of the reference ROI is the center of the chart image based on the center position of the chart image acquired in step S2 and the position of the reference ROI acquired in step S3 by the relative position determination unit 113. It is determined whether a position (relative position) of up, down, left, or right is designated with respect to the position (step S4).

  Then, the MTF measuring apparatus 1 corrects the position of the reference ROI by the reference ROI position correcting means 114 so that the center of the reference ROI is on the boundary (edge) of the radiation region (step S5). If the reference ROI selected in the vicinity of the vertical boundary above or below the center position of the chart image acquired in step 2 is selected as the reference ROI acquired in step S3, the reference ROI position correcting unit 114 Within the reference ROI, an edge is detected in the horizontal direction, and the center of the reference ROI is shifted in the horizontal direction and moved onto the edge, thereby correcting the position of the reference ROI. In addition, when the reference ROI is selected near the horizontal boundary on the left or right side of the center position of the chart image, the reference ROI position correcting unit 114 detects an edge in the vertical direction within the reference ROI. The position of the reference ROI is corrected by shifting the center of the reference ROI in the vertical direction and moving the center on the edge.

Then, the MTF measuring apparatus 1 uses the center position acquired in step S2 as the center based on the relative position determined in step S4 by the direction-specific ROI position specifying unit 115 and the size of the reference ROI acquired in step S3. Then, the positions and sizes of ₁₂ measurement target images (R _{1 to} R _{12 in} FIG. 7) in the eight directions of the horizontal direction, the vertical direction, and the oblique direction are specified (step S6).

Thereafter, the MTF measuring apparatus 1 extracts the horizontal and vertical ROI images from the chart image stored in the chart image storage unit 10 and reads them by the horizontal / vertical ROI extraction unit 121 of the image extraction unit 12 ( Step S7).
Further, the MTF measuring apparatus 1 uses the oblique ROI shaping / extracting means 122 of the image extracting means 12 to obtain a parallelogram image, which is an oblique ROI image, from the chart image stored in the chart image storing means 10. A rectangular image is shaped and extracted (step S8). The operations in steps S7 and S8 may be performed in the reverse order.

Then, the MTF measuring apparatus 1 obtains the line spread function (LSF) for the ROI (measurement target image) extracted in steps S7 and S8 by the MTF calculating unit 131 of the calculating unit 13, and then performs the Fourier transform. By doing so, the MTF is calculated (step S9). Further, the MTF measuring apparatus 1 averages the MTFs calculated from the same diagonal target processing image among the MTFs calculated in step S9 by the diagonal MTF averaging unit 132 (step S10).
And the MTF measuring apparatus 1 outputs the MTF of each direction (8 directions) calculated by step S9 and S10 by the calculating means 13 (step S11).

  With the above operation, the MTF measuring apparatus 1 measures MTF (horizontal frequency component and vertical frequency component) in a total of eight directions in the horizontal direction, the vertical direction, and the oblique direction from the chart image obtained by imaging the MTF measurement chart CH at a time. can do.

DESCRIPTION OF SYMBOLS 1 MTF measuring apparatus 10 Chart image memory | storage means 11 ROI position specific | specification means 111 Chart center acquisition means 112 Reference | standard ROI position acquisition means (reference | standard image position acquisition means)
113 Relative position determination means 114 Reference ROI position correction means (reference image position correction means)
115 Direction-specific position specifying means 12 Image extracting means 121 Horizontal / vertical ROI extracting means (horizontal / vertical image extracting means)
122 Oblique direction ROI shaping extraction means (oblique direction image shaping extraction means)
13 calculating means 131 MTF calculating means 132 diagonal direction MTF averaging means 2 imaging system 3 display _CH, CH _B, CH C MTF measuring chart B emission region A ₁ the first region A ₂ second region

Claims (7)

An MTF measurement chart used in an MTF measurement apparatus for measuring an MTF representing a spatial frequency characteristic of an imaging system,
The chart surface is divided into a predetermined position on the chart surface as a central position, and a boundary extending in the horizontal and vertical directions orthogonal to the central position and a boundary extending in an oblique direction with a central angle of 45 ° with respect to the boundary. The radiation area
For each radiation region adjacent at the boundary, color different contrast is provided,
The chart surface is defined as a first area and a second area with an arbitrary one of the boundaries having a central angle of 45 ° as a reference and a straight line having a predetermined angle at the central position with respect to the reference as a boundary. of divided into regions, the between the respective regions of the first region and the second region, M TF measurement chart it characterized in that the difference in contrast of the radiation area is color differently. An MTF measuring apparatus for measuring an MTF representing a spatial frequency characteristic of an imaging system,
With a predetermined position on the chart surface of the MTF measurement chart as a central position, a boundary extending in the horizontal and vertical directions orthogonal to the central position, and a boundary extending in an oblique direction with a central angle of 45 ° with respect to the boundary The MTF measurement chart has a radiation area that divides the chart surface, and a color having a different contrast is arranged for each radiation area adjacent to the boundary, and the center angle of the MTF measurement chart exists at intervals of 45 ° Of the MTF measurement chart, in which arbitrary orthogonal boundaries are defined as horizontal boundaries and vertical boundaries, and the horizontal and vertical boundaries are rotated by a predetermined angle by the imaging system to be measured. Chart image storage means for storing a chart image as an image;
Chart center acquisition means for acquiring a center position of a radial boundary in the chart image;
Reference image position acquisition means for acquiring the position and size of a reference image that is a reference measurement target image by designating a rectangular region that spans between radiation regions having different horizontal or vertical contrasts in the chart image ,
Relative position determination means for determining a relative position of the reference image with respect to the center position based on the position of the reference image acquired by the reference image position acquisition means and the center position acquired by the chart center acquisition means;
Based on the relative position determined by the relative position determination means and the size of the reference image acquired by the reference image position acquisition means, the position of the measurement target image in the horizontal direction, the vertical direction, and the oblique direction with the center position as a center. And direction-specific image position specifying means for specifying the size,
An image extracting means for extracting and reading out the measurement target image specified by the image position specifying means for each direction from the chart image storage means;
MTF calculating means for calculating the MTF for a horizontal frequency component or a vertical frequency component corresponding to each direction in the measurement target image extracted in each direction by the image extracting means;
An MTF measuring apparatus comprising: An MTF measuring apparatus for measuring an MTF representing a spatial frequency characteristic of an imaging system,
Among the boundaries where the central angles of the MTF measurement chart according to claim 1 exist at intervals of 45 °, arbitrary boundaries orthogonal to each other are defined as a horizontal boundary and a vertical boundary, and the horizontal boundary and the vertical boundary are measured objects. Chart image storage means for storing a chart image that is a captured image of the MTF measurement chart imaged by being rotated by a predetermined angle by the imaging system to be
Chart center acquisition means for acquiring a center position of a radial boundary in the chart image;
Reference image position acquisition means for acquiring the position and size of a reference image that is a reference measurement target image by designating a rectangular region that spans between radiation regions having different horizontal or vertical contrasts in the chart image ,
Relative position determination means for determining a relative position of the reference image with respect to the center position based on the position of the reference image acquired by the reference image position acquisition means and the center position acquired by the chart center acquisition means;
Based on the relative position determined by the relative position determination means and the size of the reference image acquired by the reference image position acquisition means, the position of the measurement target image in the horizontal direction, the vertical direction, and the oblique direction with the center position as a center. And direction-specific image position specifying means for specifying the size,
An image extracting means for extracting and reading out the measurement target image specified by the image position specifying means for each direction from the chart image storage means;
MTF calculating means for calculating the MTF for a horizontal frequency component or a vertical frequency component corresponding to each direction in the measurement target image extracted in each direction by the image extracting means;
An MTF measuring apparatus comprising: Based on the relative position determined by the relative position determination unit so as to detect an edge where a difference in contrast of the reference image acquired by the reference image position acquisition unit occurs and the center of the reference image is on the edge. 4. The MTF according to claim 2 , further comprising reference image position correcting means for correcting the position of the reference image by moving the position of the reference image horizontally or vertically. measuring device. The direction-specific image position specifying means determines the vertical height of the measurement target image in the horizontal direction as the base length and the horizontal width of the measurement target image in the horizontal direction as the measurement target images corresponding to the four oblique directions. A parallelogram image having a height, and a parallelogram image having a horizontal width of the measurement target image in the vertical direction as a base length and a vertical height of the measurement target image in the vertical direction as a height. according to claims 2 in which the acute side of the angle between the side corresponding parallelogram base and in the diagonal direction and identifies the two images to be 45 ° in any one of claims 4 MTF measuring device. The image extraction means comprises oblique direction image shaping and extraction means for shaping and extracting the parallelogram image that is the measurement target image in the oblique direction into a rectangular image having a base and a height as one side. The MTF measuring apparatus according to claim 5 . With a predetermined position on the chart surface of the MTF measurement chart as a central position, a boundary extending in the horizontal and vertical directions orthogonal to the central position, and a boundary extending in an oblique direction with a central angle of 45 ° with respect to the boundary The MTF measurement chart has a radiation area that divides the chart surface, and a color having a different contrast is arranged for each radiation area adjacent to the boundary, and the center angle of the MTF measurement chart exists at intervals of 45 ° Among the boundaries to be measured, arbitrary orthogonal boundaries are defined as a horizontal boundary and a vertical boundary, and the horizontal boundary and the vertical boundary are imaged by a predetermined angle rotated by the imaging system to be measured and stored in the chart image storage unit In order to measure the MTF representing the spatial frequency characteristics of the imaging system from the chart image that is the captured image of the MTF measurement chart, Computer
Chart center acquisition means for acquiring a center position of a radial boundary in the chart image;
Reference image position acquisition means for acquiring a position and a size of a reference image that is a measurement target image serving as a reference by designating a rectangular region straddling between radiation regions having different horizontal or vertical contrasts in the chart image,
Relative position determination means for determining a relative position of the reference image with respect to the center position based on the position of the reference image acquired by the reference image position acquisition means and the center position acquired by the chart center acquisition means;
Based on the relative position determined by the relative position determination means and the size of the reference image acquired by the reference image position acquisition means, the position of the measurement target image in the horizontal direction, the vertical direction, and the oblique direction with the center position as a center. And direction-specific image position specifying means for specifying the size,
An image extracting means for extracting and reading out the measurement target image specified by the direction-specific image position specifying means from the chart image storage means;
MTF calculating means for calculating the MTF for the horizontal frequency component or the vertical frequency component corresponding to each direction in the measurement target image in each direction extracted by the image extracting means;
MTF measurement program to function as