JP2004134861A - Resolution evaluation method, resolution evaluation program, and optical apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for resolution evaluation of an optical apparatus such as an electronic camera and a scanner, which is hardly vulnerable to the mechanical vibration of the optical apparatus. <P>SOLUTION: The resolution evaluation method includes the steps of: selecting the edges of a pattern drawn on an original; obtaining the distribution of luminance values near the edges of the pattern by a plurality of lines; obtaining the edge coordinates of each of a plurality of the lines; obtaining the inclination of the edges at the edge coordinates of at least one line among the plurality of lines; selecting measured data from the distribution of the linearly measured luminance values at each prescribed interval, obtaining the difference between the measured data adjacent to each other and obtaining discrete difference data; applying Fourier transform to the discrete difference data to obtain a plurality of modulation transfer functions; and evaluating the resolution on the basis of the plurality of modulation transfer functions. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for evaluating the resolution of an optical device such as an electronic camera or a scanner.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, as a first resolution evaluation method of an optical device such as an electronic camera or a scanner, there is known a method of photographing a striped document in which a low-brightness portion and a high-brightness portion are mutually arranged (for example, see Patent Reference 1). More specifically, resolution evaluation is performed by photographing a plurality of originals having different stripe widths and analyzing the data. In this resolution, as the width of the stripe becomes narrower, it becomes more difficult to distinguish between a low luminance portion and a high luminance portion, so that the resolution decreases.
[0003]
As a second resolution evaluation method, there is known a method in which a document having a boundary portion (hereinafter, referred to as an edge) between a low-luminance portion and a high-luminance portion is prepared in advance, and this document is photographed (for example, Non-Patent Document 1). Reference 1).
[Patent Document 1]
JP-A-1999-313186 [Non-Patent Document 1]
"Photography-Electronic still picture cameras-resolution measurements", Draft International Standards, USA, International Organization for International Standards.
[0004]
[Problems to be solved by the invention]
However, in the first resolution evaluation method described above, a plurality of originals have to be photographed at once or a plurality of times. When photographing a plurality of documents at once, it is difficult to compare partial resolutions within one image, and when photographing a plurality of documents, the arrangement of the documents must be changed many times, which is troublesome. There was a problem.
[0005]
Such a problem does not occur in the second resolution evaluation method, and the second resolution evaluation method is more general than the first resolution evaluation method. However, the second resolution evaluation method has a problem that the measurement data becomes inaccurate due to mechanical vibration of the optical device.
The present invention has been made to solve such a conventional problem, and has as its object to provide a resolution evaluation method which is hardly affected by mechanical vibration of an optical device.
[0006]
[Means for Solving the Problems]
2. The resolution evaluation method according to claim 1, wherein the step of selecting an edge of the pattern drawn on the document, the step of obtaining a plurality of luminance distributions in the vicinity of the edge of the pattern, and the step of: Obtaining coordinates, obtaining an inclination of an edge at the edge coordinates in at least one of the plurality of lines, and determining, for each of the plurality of lines, a predetermined interval from a distribution of luminance measured in a line shape. Selecting measurement data for each, obtaining a difference between adjacent measurement data, obtaining discrete difference data, obtaining a plurality of modulation transfer functions by Fourier-transforming the discrete difference data; and Evaluating the resolution based on the modulation transfer function.
[0007]
3. The resolution evaluation method according to claim 2, wherein the step of selecting an edge of the pattern drawn on the document, the step of obtaining a plurality of lines of the luminance distribution near the edge of the pattern, Obtaining coordinates, obtaining an inclination of an edge at the edge coordinates in at least one of the plurality of lines, and determining, for each of the plurality of lines, a predetermined interval from a distribution of luminance measured in a line shape. Selecting measurement data for each, determining the difference between adjacent measurement data, obtaining discrete difference data, for each of the plurality of lines, fitting an approximate curve to the discrete difference data, Fourier transforming the plurality of approximate curves to obtain a modulation transfer function; and Characterized by a step of evaluating can resolution.
[0008]
According to a third aspect of the present invention, in the resolution evaluation method according to the first or second aspect, in order to calculate the edge coordinates, an average value and a high luminance of a low luminance portion near an edge of the subject are calculated. Find the average value of the portion, find the edge threshold from the average value of the low luminance portion and the average value of the high luminance portion, inspect the measurement data from one of the lines and determine the position where the luminance crosses the edge threshold. It is characterized by detecting.
[0009]
A resolution evaluation method according to a fourth aspect is the resolution evaluation method according to any one of the first to third aspects, wherein the evaluation of the resolution is performed based on an average value of the plurality of modulation transfer functions. It is characterized by the following.
A resolution evaluation method according to claim 5, wherein in the resolution evaluation method according to any one of claims 1 to 3, the evaluation of the resolution is performed by calculating the maximum of the plurality of modulation transfer functions for each frequency to be evaluated. It is characterized in that it is performed based on the value.
[0010]
A resolution evaluation method according to claim 6, wherein in the resolution evaluation method according to any one of claims 1 to 3, the evaluation of the resolution is performed by minimizing the plurality of modulation transfer functions for each frequency to be evaluated. It is characterized in that it is performed based on the value.
The resolution evaluation program according to claim 7, further comprising: a step of selecting an edge of a pattern drawn on the document; a step of obtaining a plurality of lines of luminance distribution in the vicinity of the edge of the pattern; A step of obtaining coordinates, a step of obtaining an inclination of an edge at the edge coordinates in at least one of the plurality of lines, and a predetermined interval from a distribution of luminance measured in a line shape for each of the plurality of lines. Selecting measurement data for each, obtaining a difference between the adjacent measurement data, obtaining discrete difference data, a procedure for obtaining a plurality of modulation transfer functions by Fourier-transforming the discrete difference data, And a procedure for evaluating the resolution based on the modulation transfer function.
[0011]
9. The resolution evaluation program according to claim 8, further comprising: a step of selecting an edge of a pattern drawn on the document; a step of obtaining a plurality of lines of a luminance distribution in the vicinity of the edge of the pattern; A step of obtaining coordinates, a step of obtaining an inclination of an edge at the edge coordinates in at least one of the plurality of lines, and a predetermined interval from the distribution of the luminance measured in the form of a line for each of the plurality of lines. Selecting measurement data for each, determining the difference between adjacent measurement data, obtaining discrete difference data, and for each of the plurality of lines, fitting an approximate curve to the discrete difference data, Fourier transforming the plurality of approximate curves to obtain a modulation transfer function, and evaluating the resolution based on the plurality of modulation transfer functions To perform the procedure on your computer.
[0012]
The resolution evaluation program according to claim 9 is the resolution evaluation program according to claim 7 or 8, wherein the average value and the high luminance of a low luminance portion near the edge of the subject are calculated to calculate the edge coordinates. Find the average value of the portion, find the edge threshold from the average value of the low luminance portion and the average value of the high luminance portion, inspect the measurement data from one of the lines and determine the position where the luminance crosses the edge threshold. Causes the computer to execute the detection processing.
[0013]
A resolution evaluation program according to a tenth aspect is the resolution evaluation program according to any one of the seventh to ninth aspects, wherein the evaluation of the resolution is performed based on an average value of the plurality of modulation transfer functions. Causes the computer to execute the processing.
A resolution evaluation program according to claim 11, wherein in the resolution evaluation program according to any one of claims 7 to 9, the resolution evaluation is performed by determining a maximum of the plurality of modulation transfer functions for each frequency to be evaluated. Causes the computer to execute processing based on the value.
[0014]
According to a twelfth aspect of the present invention, in the resolution evaluation program according to any one of the seventh to ninth aspects, the resolution evaluation is performed by determining a minimum of the plurality of modulation transfer functions for each frequency to be evaluated. Causes the computer to execute processing based on the value.
According to a thirteenth aspect of the present invention, an optical apparatus is capable of executing the resolution evaluation program according to any one of the seventh to twelfth aspects.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail with reference to the drawings.
FIG. 1 shows the flow of an embodiment of the resolution evaluation method of the present invention. FIG. 2 shows a data collection method for the resolution evaluation of FIG.
As shown in FIG. 2, in order to collect data, a measurement document 3 on which a pattern is drawn is arranged in front of a blackout curtain 1. Two lamps 5 are arranged diagonally forward of the document 3. Further, an electronic camera 7 as an optical device is arranged in front of the document 3. A light-shielding plate 9 is provided on the electronic camera 7 side of the lamp 5 so that light from the lamp 5 does not directly go to the electronic camera 7.
[0016]
In the resolution evaluation method of the present invention, as shown in FIG. 1, first, in step S1, after preparing for photographing, the electronic camera 7 photographs the original 3 composed of a uniform high-luminance portion and a low-luminance portion. Do. Here, the document 3 only needs to include an edge.
Next, in step S2, an area as indicated by a dotted line in FIG. 3 is designated from the captured image so as to include an edge. This designation is performed, for example, by connecting a notebook computer to the electronic camera 7 and using dedicated software.
[0017]
Next, in step S3, a high-luminance portion is measured and an average value is calculated.
Calculating the average value means calculating the average value of the luminance when the luminance is divided by, for example, 8 bits (256 gradations) in the gray scale.
Next, in step S4, a low luminance portion is measured, and an average value is calculated similarly to the high luminance portion.
[0018]
Next, in step S5, an edge threshold value assumed to be a boundary between the high luminance portion and the low luminance portion is obtained from the average value of the low luminance portion and the average value of the high luminance portion.
Specifically, it is calculated by further obtaining the average of the average value of the high luminance portion and the average value of the low luminance portion.
Next, in step S6, line-shaped luminance data (hereinafter, referred to as line data) near the edge as shown in FIG. 4 is extracted from the area for resolution calculation indicated by the dotted line in FIG. 3 and recorded in the memory. . In this embodiment, line data of 30 lines L1 to L30 is recorded.
[0019]
Next, in step S7, line data is searched from one of the lines at predetermined intervals to detect a position where the luminance crosses the edge threshold. Then, edge coordinates are obtained by linear interpolation from the coordinates of the positions before and after the luminance crosses the edge threshold value and the luminance at the coordinates.
Next, in step S8, it is determined whether or not edge coordinates have been obtained for all of the 30 lines L1 to L30. If not, the process returns to step S7. On the other hand, if the calculation has been completed, the process proceeds to step S9.
[0020]
Next, in step S9, a cycle is calculated. The period refers to the number of lines until the edge coordinates become (E1 + 1) when the edge coordinates of the line L1 are E1, and the number of lines obtained in this manner corresponds to the slope. Note that such calculation cannot be performed if the edge is not inclined at all, but since it is not considered that the edge is not actually inclined at all, the period is always calculated and the calculation result may be obtained. it can. Hereinafter, an example in which the number of lines obtained as a cycle is 25 will be described.
[0021]
Next, in step S10, data is selected at predetermined intervals from the line data as shown in FIG. 5, a difference between adjacent measurement data is obtained, and discrete difference data is calculated. Note that the vertical axis of the graph in FIG. 5 indicates the gray scale value when the gray scale is divided into 255 levels, and the horizontal axis indicates the position of the line data on the line.
[0022]
Next, in step S11, an approximation curve as shown in FIG. 6 is fitted to the discrete difference data obtained in step S10 by a least squares method (LSM: Last Squares Method). The vertical axis of the graph of FIG. 6 indicates the gradation difference between adjacent pixels, and the horizontal axis indicates the position of the line data on the line.
Next, in step S12, an approximate curve for the number of lines corresponding to the inclination calculated in step S9 is adopted as a measurement range.
[0023]
Next, in step S13, the approximate curve obtained by the least square method is subjected to Fourier transform. The Fourier transform is performed by a discrete Fourier transform (DFT).
Next, in step S14, the calculation result obtained in step S13 is recorded in the memory.
[0024]
Next, in step S15, it is determined whether or not the number of Fourier-transformed lines has reached a predetermined number (the cycle obtained in step S9) of 25 lines. If the number is less than 25, the process returns to step S10, and the same processing is performed on the next line. On the other hand, if the number of lines has reached 25, the process proceeds to step S16.
In step S16, an average value of the 25 lines after the Fourier transform is obtained based on the data on the inclination of the edge obtained in step S9 and the result of the Fourier transform performed in step S13. Then, the resolution is estimated from the obtained line shape of the average value of the 25 lines, and the resolution is evaluated. FIG. 8 shows an example of calculating the average value. The vertical axis of the graph in FIG. 8 indicates the response after DFT, and the horizontal axis indicates the frequency. Further, in the graph, data indicated by a dotted line indicates lines L1 to L4 among the 25 lines for simplicity of explanation, and a solid line indicates an average value thereof.
[0025]
Note that a program for performing the above-described resolution evaluation is recorded on a recording medium and provided in the electronic camera 7.
In the resolution evaluation method according to this embodiment, 30 lines of luminance distribution near the edge of the pattern drawn on the document 3 are obtained, Fourier transform is performed for each line, and a modulation transfer function (MTF: MTF). ), And finally calculate the average value of the MTF for each line. Therefore, the accuracy and precision can be increased by performing a Fourier transform on a larger number of lines in a narrower range. As a result, the electronic camera 7 can be less affected by mechanical vibration.
[0026]
Further, in the resolution evaluation method of this embodiment, 30 lines of luminance distribution near the edge of the pattern drawn on the document 3 are obtained, and the inclination of the edge at the edge coordinate E1 is calculated. The adverse effects on the electronic camera 7 side, including the focusing accuracy and the degree of camera shake, can be quantitatively grasped. Therefore, when the same scene is shot under different shooting conditions, whether the image is relatively focused or relatively free from camera shake can be determined by checking the resolution. Can be determined. In addition, in a display having a relatively low resolution, it is particularly effective in this case because it is desired for the observer to observe the image with a little better resolution.
[0027]
In the resolution evaluation program of this embodiment, 30 lines of luminance distribution near the edge of the pattern drawn on the document 3 are obtained, Fourier transform is performed for each line, an MTF is obtained, and finally the average value of the MTF for each line is obtained. Is calculated, the accuracy and precision can be increased by executing this program so as to perform a Fourier transform on a larger number of lines in a narrower range.
[0028]
In the above-described embodiment, an example in which DFT is used as Fourier transform has been described, but fast Fourier transform (FFT: Fast Fourier Transformation, hereinafter referred to as FFT) may be used.
Also, in the above-described embodiment, an example has been described in which the evaluation of the resolution is performed by calculating the average value of a plurality of MTFs. However, the one having the maximum value among the plurality of MTFs is used for each frequency to be evaluated. You may do it. Alternatively, one having a minimum value among a plurality of MTFs for each frequency to be evaluated may be used. In short, any mathematical processing may be performed on a plurality of MTFs as long as the mathematical processing associated with each other can be performed and an accurate MTF can be calculated.
[0029]
Further, in the above-described embodiment, the electronic camera 7 has been described as the optical device. However, any optical device that can photograph the document 3 for evaluating the resolution may be used. For example, a scanner or a microscope may be used.
[0030]
【The invention's effect】
As described above, in the resolution evaluation method of the present invention, a plurality of luminance distributions in the vicinity of the edge of a pattern drawn on a document are obtained, a Fourier transform is performed for each of the plurality of lines, an MTF is obtained, and finally a line is obtained. Since the average value of the MTF for each is calculated, the accuracy and precision can be increased by performing a Fourier transform on a larger number of lines in a narrower range. As a result, the optical device can be made less susceptible to mechanical vibration.
[0031]
In the resolution evaluation program according to the present invention, a plurality of luminance distributions in the vicinity of edges of a pattern drawn on a document are obtained, a Fourier transform is performed for each of the plurality of lines, an MTF is obtained, and finally, an MTF is obtained for each line. Since the average value of the MTF is configured to be calculated, the accuracy and precision can be increased by executing this program so as to perform a Fourier transform on a larger number of lines in a narrower range.
[0032]
Further, in the optical apparatus of the present invention, a plurality of luminance distributions in the vicinity of the edge of the pattern drawn on the document are obtained, a Fourier transform is performed for each of the plurality of lines, an MTF is obtained, and finally, an MTF for each line is obtained. Since the program for calculating the average value of can be executed, the accuracy and precision can be increased by performing a Fourier transform on a larger number of lines in a narrower range. As a result, the optical device can be made less susceptible to mechanical vibration.
[Brief description of the drawings]
FIG. 1 is a flowchart showing one embodiment of a resolution evaluation method of the present invention.
FIG. 2 is an explanatory diagram showing a photographing method when photographing a document to be evaluated for resolution.
FIG. 3 is an explanatory diagram showing a portion to be evaluated for resolution.
FIG. 4 is an explanatory diagram showing a line of an edge portion to be evaluated for resolution.
FIG. 5 is an explanatory diagram showing a gradation change of line data.
FIG. 6 is an explanatory diagram when fitting is performed on a gradation difference at predetermined intervals of line data.
FIG. 7 is an explanatory diagram showing a range for performing Fourier transform.
FIG. 8 is an explanatory diagram showing a Fourier transform result.
[Explanation of symbols]
3 Original 7 Electronic camera E1 Edge coordinates L1 to L30 Line

Claims (13)

Selecting an edge of a pattern drawn on the manuscript;
Obtaining a plurality of lines of luminance distribution near the edge of the pattern;
Obtaining edge coordinates for each of the plurality of lines;
Determining the slope of the edge at the edge coordinates in at least one of the plurality of lines;
For each of the plurality of lines, select measurement data at predetermined intervals from the distribution of the luminance measured in the line shape, determine the difference between adjacent measurement data, obtain discrete difference data,
Fourier transforming the discrete difference data to obtain a plurality of modulation transfer functions;
Evaluating a resolution based on the plurality of modulation transfer functions;
A resolution evaluation method comprising: Selecting an edge of a pattern drawn on the manuscript;
Obtaining a plurality of lines of luminance distribution near the edge of the pattern;
Obtaining edge coordinates for each of the plurality of lines;
Determining the slope of the edge at the edge coordinates in at least one of the plurality of lines;
For each of the plurality of lines, select measurement data at predetermined intervals from the distribution of the luminance measured in the line shape, determine the difference between adjacent measurement data, obtain discrete difference data,
Fitting an approximate curve to the discrete difference data for each of the plurality of lines;
Fourier transforming the plurality of approximate curves to obtain a modulation transfer function;
Evaluating a resolution based on the plurality of modulation transfer functions;
A resolution evaluation method comprising: 3. The resolution evaluation method according to claim 1, wherein an average value of a low-luminance portion and an average value of a high-luminance portion in the vicinity of an edge of the subject are calculated to calculate the edge coordinates. A resolution evaluation method, wherein an edge threshold value is obtained from an average value of a portion and an average value of a high-brightness portion, and the measurement data is inspected from one of the lines to detect a position where the luminance crosses the edge threshold value. The resolution evaluation method according to any one of claims 1 to 3,
A resolution evaluation method, wherein the evaluation of the resolution is performed based on an average value of the plurality of modulation transfer functions. The resolution evaluation method according to any one of claims 1 to 3,
A resolution evaluation method, wherein the evaluation of the resolution is performed based on a maximum value of the plurality of modulation transfer functions for each frequency to be evaluated. The resolution evaluation method according to any one of claims 1 to 3,
A resolution evaluation method, wherein the evaluation of the resolution is performed based on a minimum value of the plurality of modulation transfer functions for each frequency to be evaluated. Steps to select the edges of the pattern drawn on the manuscript,
A procedure for obtaining a plurality of lines of the distribution of luminance near the edge of the pattern;
A procedure for obtaining edge coordinates for each of the plurality of lines;
A step of obtaining an inclination of an edge at the edge coordinates in at least one of the plurality of lines;
For each line of the plurality of lines, select measurement data at predetermined intervals from the distribution of the luminance measured in the line shape, determine the difference between adjacent measurement data, a procedure to obtain discrete difference data,
Fourier transforming the discrete difference data to obtain a plurality of modulation transfer functions;
A step of evaluating the resolution based on the plurality of modulation transfer functions;
A resolution evaluation program for causing a computer to execute. Steps to select the edges of the pattern drawn on the manuscript,
A procedure for obtaining a plurality of lines of the distribution of luminance near the edge of the pattern;
A procedure for obtaining edge coordinates for each of the plurality of lines;
Determining the slope of the edge at the edge coordinates in at least one of the plurality of lines;
For each line of the plurality of lines, select measurement data at predetermined intervals from the distribution of the luminance measured in the line shape, determine the difference between adjacent measurement data, a procedure to obtain discrete difference data,
A procedure for fitting an approximate curve to the discrete difference data for each of the plurality of lines,
Fourier transforming the plurality of approximate curves to obtain a modulation transfer function;
A step of evaluating the resolution based on the plurality of modulation transfer functions;
A resolution evaluation program for causing a computer to execute. In the resolution evaluation program according to claim 7 or 8,
In order to calculate the edge coordinates, an average value of a low-luminance portion and an average value of a high-luminance portion in the vicinity of the edge of the subject are obtained, and an edge threshold is calculated from the average value of the low-luminance portion and the average value of the high-luminance portion. A resolution evaluation program for causing a computer to execute a process of detecting the measured data from one of the lines and detecting a position where the luminance crosses an edge threshold. In the resolution evaluation program according to any one of claims 7 to 9,
A resolution evaluation program for causing a computer to execute a process of evaluating the resolution based on an average value of the plurality of modulation transfer functions. In the resolution evaluation program according to any one of claims 7 to 9,
A resolution evaluation program for causing a computer to execute processing for evaluating the resolution based on the maximum value of the plurality of modulation transfer functions for each frequency to be evaluated. In the resolution evaluation program according to any one of claims 7 to 9,
A resolution evaluation program for causing a computer to execute a process of evaluating the resolution based on a minimum value of the plurality of modulation transfer functions for each frequency to be evaluated. An optical apparatus capable of executing the resolution evaluation program according to claim 7.

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