JP2505165B2 - Resolution measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to the resolution of an image pickup apparatus, and in particular to a comprehensive MTF (Modulation Tran) including a lens, an image pickup element, and a signal processing circuit.
sfer Function) A device for measuring characteristics.

[Summary of the Disclosure] The present invention provides a false signal in a resolution measuring device of an image pickup device, which can more easily and accurately measure the resolution and can generate a false signal due to aliasing such as a solid-state image pickup device. Measure resolution without being affected,
Furthermore, the characteristics of the frequency band that is the false signal component can be measured.

[Prior Art] Conventionally, various devices for obtaining the MTF of a signal processing circuit are known, but a rectangular wave chart based on a black and white pattern drawn on a plate or a paper panel is used for measuring the resolution of an image pickup device. It is common to measure the used AR (amplitude response). In addition, MTF is obtained by approximate calculation from the above AR.

[Problems to be Solved by the Invention] In the conventionally known AR calculation method, the vibration of a rectangular wave is obtained by an oscilloscope or the like using a panel chart made of a plate or paper on which a rectangular wave luminance change pattern is drawn. Therefore, personal differences among measuring persons, such as reading errors and deviations of the presentation position of the panel chart, also cause the errors. Further, although there is no particular problem with the image pickup tube, particularly in the case of a solid-state image pickup device or the like, it is difficult to measure the waveform amplitude because of a false signal generated from the harmonic component of the rectangular wave pattern.

Therefore, an object of the present invention is to provide a measuring device configured to measure the resolution of a solid-state imaging device (resolution including an optical system and a signal processing circuit) without being affected by aliasing distortion due to spatial sampling. It is in.

Another object of the present invention is to eliminate the measurement error due to the false signal described above by using a television monitor that electronically generates a free waveform other than a rectangular wave as a measurement chart, and to provide an apparatus suitable for automation. In addition to the above, the present invention is intended to provide a high-accuracy and high-speed resolution measuring device which is free from error by a measurer.

Another object of the present invention is to provide a resolution measuring device that can measure a frequency band that is a false signal component that could not be conventionally measured, and that can accurately measure MTF.

[Means for Solving Problems] In order to achieve such an object, in the present invention, a single sine wave pattern displayed as a luminance change pattern on a television monitor, or n-1 times the sine wave pattern is used. An image to be measured is used to image a pattern obtained by combining sine waves of different frequencies between n + 1 times (n is a natural number) or a pattern obtained by modulating a single sine wave with a function of a low frequency. The resolution of the AR of the obtained image pickup output signal is analyzed by using the Fourier transform means.

[Examples] Examples of the present invention will be described with reference to the drawings.

FIG. 1 is an overall configuration diagram of an embodiment to which the present invention is applied. Reference numeral 1 shown in the figure is an imaging device (device under test) such as an imaging tube camera or a solid-state camera. Reference numeral 2 is an apparatus main body for performing measurement analysis and control according to the present invention. A display unit 3 according to the present invention uses a television monitor.

In this embodiment, the sine wave function displayed as the brightness change pattern on the television monitor 3 is imaged by the imaging device 1. The video signal output from the image pickup apparatus 1 is input to the resolution measuring apparatus 2 according to this embodiment. That is, the front-end circuit 4 performs predetermined band limitation and amplification, and the A / D
The converter 5 converts the signal into a digital signal, the signal processing circuit 6 obtains AR from spectrum analysis using Fourier transform, and the MTF characteristic detection circuit 7 calculates the resolution. At this time, the sine wave function displayed on the television monitor 3 is generated by the sine wave generator 8, and an appropriate gamma correction is performed by the gamma correction circuit 9 so as not to cause luminance distortion when displayed on the television monitor 3.

With the above-described configuration, the television monitor 3 is configured to appropriately reduce the size of the display image so that the frequency characteristic (MTF) of the display device does not affect the measurement.
And the distance between the image pickup device 1 are set appropriately. For the sake of simplicity, the size ratio of the displayed image and the captured image is 2: 1.
The case where the settings are made so that

Under the above settings, the relationship between the frequency of the signal generated from the sine wave generator 8 and the spectrum frequency converted by the signal processing circuit 6 is exactly 1: 2. Therefore, it becomes easy to detect the spectrum component included in the video signal imaged at an arbitrary frequency, and it becomes possible to measure a finer frequency interval as compared with the method using the conventional chart. Moreover, since the sine wave is used, the MTF can be directly calculated.

In this embodiment, by converting the signal into a digital signal and performing a discrete Fourier transform, it is possible to accurately obtain a spectral component of a target frequency in a short time by using a method such as a fast Fourier transform (FFT).

When performing discrete Fourier transform such as FFT, the width of the time window used for calculation cannot be made larger than one horizontal period (1H), especially for video signals, so n times the frequency with 1H as the period ( Only n frequency components (n is a natural number) are calculated. If the imaged video signal component does not have this frequency exactly, that is, if the above-mentioned reduction ratio is slightly different from the set value,
The window function of the time window causes some error.

In order to reduce these errors, conventionally known as a method of Fourier transform is a method of multiplying by a Hamming window function and then performing Fourier transform. However, even if such an operation is performed, a slight error occurs in the spectral component of the target frequency when the frequency is exactly n times and when the frequency is slightly different from the n times frequency.

Next, an example of further reducing this error will be described. For example, by displaying a waveform in which a plurality of frequency components between the above n-1 times and n + 1 times the frequencies are displayed,
The above-mentioned error can be reduced. That is, since any one of the plurality of frequency components matches the n-fold frequency, the errors are averaged and the accuracy as a whole can be improved. Further, for the same reason, amplitude modulation of the sine wave by an appropriate function makes it possible to appropriately disperse the spectrum and thus improve the accuracy.

Considering the limit example in which the number of frequency components is increased, This is equivalent to using the function of the above formula (1). Where θ = ωt.

Equation (1) above is Is similar to the case where a function of sin θ / θ is used as the above-described function of amplitude modulation. Therefore, by using this function, the amplitude characteristic of the dispersed spectrum can be made almost flat, and higher accuracy can be obtained.

The method described above is effective for the frequency shift due to the graphic distortion of the television monitor and the graphic distortion of the lens, and it becomes possible to provide a highly accurate resolution measuring device.

FIG. 2 shows an embodiment in which a downsized main measuring device is connected to the front surface of the image pickup device. That is, since the resolution measuring device according to the present invention can be easily miniaturized, the angle of view and the position can be easily adjusted by mounting the television monitor 3 on the front surface of the image pickup device 1 via the lens 12 as shown in the figure. Becomes

[Advantages of the Invention] In the prior art, the high-frequency component of the resolution chart appears as aliasing distortion due to the solid-state image sensor with spatial sampling, which hinders the measurement of the true resolution. Since the monitor is used to capture and measure the resolution pattern of the sine wave, the interference due to the aliasing distortion can be removed and the measurement accuracy can be improved.

Further, it is possible to measure a frequency band that is a false signal component due to aliasing distortion, which could not be measured conventionally.

Furthermore, even a composite wave of a plurality of sinusoidal waves having different frequencies can be easily generated electronically, and by using this as a resolution pattern, the resolution can be improved even when the scale ratio during imaging changes. It is possible to measure with high accuracy.

Since the present invention uses a television monitor without using a panel chart made of a board or paper, there is no measurement error due to individual differences of measurers, miniaturization and automation are possible, and a simple and highly accurate resolution measuring device is provided. Can be realized.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a resolution measuring apparatus to which the present invention is applied, and FIG. 2 is a schematic configuration diagram showing a state in which the resolution measuring apparatus according to the present invention is miniaturized and attached to the front surface of an image pickup apparatus. . 1 ... Imaging device, 3 ... Television monitor, 4 ... Front-end circuit, 5 ... A / D converter, 6 ... Signal processing circuit,
7 ... MTF characteristic detection circuit, 8 ... Sine wave generator, 9 ...
Gamma correction circuit, 10 ... Measurement result plotting circuit.

Claims (5)

(57) [Claims] 1. A resolution measuring device for measuring the resolution of the image pickup device under measurement by analyzing the AR (amplitude response) of the obtained image pickup output signal by imaging the resolution pattern of the display device by the image pickup device under measurement. In, a display device including a sine wave generator that generates a sine wave of one or more frequencies and a television monitor that displays the output of the sine wave generator as a brightness change pattern; and Fourier transform the imaging output signal. And a Fourier transform means for measuring the resolution by detecting the signal level of a component corresponding to the frequency of the sine wave forming the resolution pattern. 2. The resolution measuring apparatus according to claim 1, wherein discrete Fourier transforming means is used as the Fourier transforming means. 3. The sine wave generator is ωt / sin ωt sin n
The resolution measuring apparatus according to claim 1, wherein the resolution measuring apparatus generates a waveform signal of ωt. 4. When the resolution pattern of the display device is imaged by the image pickup device under test, the interval between the scanning lines of the television monitor projected on the image pickup surface of the image pickup device under measurement is determined by the distance of the image pickup device under measurement. The resolution measuring apparatus according to claim 1, wherein the resolution measuring apparatus captures an image through a projecting unit that projects by reducing the distance to be equal to or smaller than the interval between the scanning lines. 5. The display device has a γ correction means for correcting and displaying a γ correction having a characteristic opposite to the γ characteristic of the television monitor in a display pattern. The resolution measuring device according to the item.