JPS58213226A - Mtf measuring apparatus - Google Patents

Abstract

PURPOSE:To miniaturize an apparatus and to reduce the cost of the apparatus, by using a self-scanning type solid image pickup element satisfying each prescribed condition of a space of a photoelement and the effective photodetecting face. CONSTITUTION:Plural photoelements are arranged in a straight line on a self- scanning type solid image pickup element 21 as a photodetecting element. A space DELTAl of this photoelement is established so as to satisfy the expression-1 by a cut-off frequency UC, a measured maximum frequency UM and a projection minimum scale factor mmin. Further, the effective photodetecting face of the element 21 is the maximum object height at least and is the minimum object height from an image point of the maximum projection scale factor and then, the section to the image point of the minimum projection scale factor is included. By such formation, the calculation processing of Fourier transformation is simplified and also, the number of the element 21 is decreased. Accordingly, the apparatus is miniaturized and its cost is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an MTF measuring device using a self-scanning solid-state image sensor.

For devices that measure MTF of photographic lenses, etc., 20~
It is necessary to measure the object by enlarging it and projecting it at a finite distance of 50 times. This generally means that the imaging performance of photographic lenses, etc. is a long way #I.
It is superior to K and decreases as the distance gets closer, so
This is because imaging performance at a short distance of about 1 m to 2 m is most important.

There are various methods for measuring MTF, but when measuring the MTF of a test lens with a high projection magnification such as a photographic or true lens, an extremely thin slit of several μm to 20 μm is generally used, and the magnification by the test lens is The light intensity distribution in the width direction of the image is received by a light receiver, and the light receiver is scanned to generate a time series signal, which is then discretely Fourier transformed to obtain MTF t.
A method of determining z is used.

The receiver used here may have a single receiving surface, but in such a case mechanical scanning is required for each measurement point, which slows down the measurement speed. Moreover, there are disadvantages in that the operability is poor and the mechanism is complicated. In order to eliminate this drawback, self-scanning solid-state imaging devices such as charge-coupled devices, photodiode arrays, and bucket-loaded devices used in scanners such as facsimile machines and optical character reading devices have been used.

These solid-state imaging devices have photo elements that are extremely small in size due to the requirement for high resolution; for example, in currently commercially available charge-coupled devices, the photo element spacing is about 13 μm.

Conventional MTF measurement equipment uses solid-state imaging devices with extremely small photo elements to measure MTF Y, so a solid-state imaging device is used for each measurement point, making the equipment larger and more complex. This was a factor in increasing costs. Furthermore, if the projection magnification or measurement point is different, the solid-state image pickup device must be scanned individually, which makes the device complicated, takes a long time to set up, and has poor operability.

SUMMARY OF THE INVENTION An object of the present invention is to provide an MTF measurement device that improves the above-mentioned drawbacks, reduces device size and cost, and improves operability.

The present invention will be described below by way of examples with reference to the drawings.

Recently, in facsimiles and the like, so-called 1-magnification scanners have begun to be put into practical use, in which the photoelement spacing of a solid-state image sensor is set to about 100 μm, and miniaturization is achieved by combining this with an optical system such as a lens array.

Embodiments of the present invention are used in such a full-size scanner,
This uses a solid-state image sensor with large photoelement spacing.

By the way, to briefly describe the characteristics of the discrete Fourier transform, as shown in FIG. 1(a), when the slit image is a continuous signal, the MTF is also represented in a corresponding form as a continuous signal. On the other hand, if the slit image is sampled at an interval of Δl as shown in Figure 1(bl), the MTF will be converted repeatedly at a period of 1/Δl, and as shown in Figure 1(e), the sampling interval Δl will be If it is large (in the case of undersampling), the high frequency part of the MTF is superimposed and expressed, and the MTF error (this is called an aliasing error) becomes large in this frequency domain.On the contrary, as shown in Figure 1 (C
), when the sampling interval is made small, no MTF error occurs, but the Fourier transform calculation process becomes enormous, which not only slows down the calculation speed but also requires a large memory capacity.

This causes an increase in costs. In other words, when performing discrete Fourier transform, it is necessary to determine the sampling interval to the minimum necessary, and by sampling the slit image at this interval, the cutoff frequency Uc of the test lens is MT with no aliasing error up to
F can be measured. This sampling is called Nyquist sampling, and its sampling interval Δl(
1/At is called the Nyquist frequency) is Δl =--2U, where m is the projection magnification of the lens to be tested.

is given by However, for photographic lenses, the cutoff frequency U0 of the tested lens is 200 c/sw to 300 c/sw.
m, and from the resolution performance of photographic film and the resolution performance of humans, it is sufficient that the spatial frequency required of the MTF measuring device is at most 1oo c/MM. Therefore, even if it is substantially undersampled, if the maximum spatial frequency required for the MTF measuring device is U, then UM
It is M that there is no aliasing error from o to U.
This is a condition for the TF measuring device.

Figure 2 shows this relationship and shows the sampling interval (
The photoelement spacing of the solid-state image sensor) Δl is 2Uo2kUM UM However, it is sufficient if both real number expressions are satisfied. If the projection magnification changes, the minimum magnification is mm. Then, Δl is 20o2 k
UM2U.

It is sufficient if both equations are satisfied.

Figure 6 shows an example of the configuration of a self-scanning solid-state image sensor, Δl
is determined by the formula fl> (2+ or (31). /XN.

4 and 5 show the optical system in each embodiment of the present invention, and FIG. 6 shows the chart thereof, the relationship between the lens to be tested and the film surface. A slit 156 is formed in the chart 14 to correspond to a predetermined object height and azimuth angle, and the lamp 16 illuminates the slit 156 through the optical fiber 1. Slit 1 facing the tanjiennyar direction
Images 5 to 15 are projected perpendicularly onto a self-scanning solid-state image sensor installed parallel to the radial direction via the inspection lens 18 and the imaging mirror 19.20. This solid-state image sensor has Δl (31(
41”, and in the example shown in Figure 4, 1 piece 2
1, but in the example of FIG. 5, there are two 214 and 212. The mirrors 19 and 20 are vertically movable, and their positions are adjusted in the vertical direction according to the projection magnification.

The lens to be tested 18 is easily installed on the mount 22 and can be rotated arbitrarily by the rotation ring 23, making it possible to measure the MTF in the concentric direction of the measurement point corresponding to the film/surface 24. There is. Solid-state image sensor 21
Alternatively, 214 and 212 are fixed to a mounting board 25, and this mounting board 25 can be scanned by a motor or the like at a constant speed in the direction in which the solid-state image sensor 21 or the photo elements are arranged at 211° and 21□. The solid-state image sensor 21 or 214, 212 is driven by a drive circuit (not shown), and its output signal is processed by a signal processing circuit to obtain the MTF.

In the MTF measuring device configured in this way, when measuring MTF y,6 in the radial direction, the slits 151 to 153 in the tangential direction are projected onto the solid-state image sensor 21 or 211, 212 perpendicularly to the direction in which the photo elements are arranged. When the scanning of the mounting board 25 is stopped, the light intensity distribution in the width direction (radial direction) of the slit image is measured by the solid-state image sensor 21 or 211, 212, and stored in the -H memory by the signal processing circuit. Converted MTF
is required. On the other hand, when measuring the MTF in the Kunjential direction, the slits 154 to 156 in the radial direction
is projected parallel to the array direction of the solid-state image sensor 21 or 211°212vc photo element, and the light intensity distribution in the width direction (tangential direction) is scanned at a constant speed by a motor or the like of the mounting board 25. ,21
2, is stored in a -H memory by a signal processing circuit, and then subjected to Fourier transformation to obtain the MTF.

FIG. 7 shows the minimum object height Pnl of the above MTF measuring device. .

Maximum object height PIIlax and minimum projection magnification ml1l.

, is a diagram showing the relationship of the image point on the projection plane with respect to the maximum projection magnification mmax, in which R8 is the optical axis, R4 is the image point at the minimum magnification and minimum image height, and R2 is the image point at the maximum magnification and maximum image height. be. The effective light receiving length l-Δ/XN of the solid-state image sensor is determined by this relationship. That is, in the example of FIG. 4, the effective light receiving length l of the solid-state image sensor 21 is "mmax" Pma
The number of photo elements is determined so as to satisfy x. Therefore, even if the object height changes due to the conversion of the chart 14 or the projection magnification changes, it is possible to obtain the MTF for any image height.

In the example shown in FIG. 5, the solid-state image sensor 212 has an effective light receiving length l.
is 1>m *P-m@P max max mxn m
The number of photo elements is determined to satisfy in,
Therefore, even if the chart is replaced or the magnification is changed, the off-axis slit image is always projected onto the effective light receiving surface of the solid-state image sensor 21□, and the MTF is measured.

When measuring the MTF in the tangential direction, the scanning speed of the mounting board 25 is determined by the storage time of the solid-state image sensor, but the drive pulse frequency of the stepping motor used to scan the mounting board is determined by the storage pulse frequency of the solid-state image sensor. It also has to be short (in terms of cycle, it has to be long).

When measuring the MTF in the tangential direction, it may take a long time to scan the mounting board 25, but the scanning distance required to obtain the light intensity distribution of the slit image is At most 10m/m~
Approximately 20 m/in is sufficient, but it is possible to measure in about the same time as a conventional MTF measuring device using solid-state image sensors, and the number of solid-state image sensors used can also be increased from on-axis to off-axis. Even if they are provided separately, there are only two solid-state imaging devices, and there is no need to move the solid-state imaging device when changing the magnification or converting the chart, which greatly improves operability and substantially shortens the measurement time. Furthermore, since the number of solid-state image sensors is significantly reduced, the drive circuit for the solid-state image sensors becomes simple, and its control becomes extremely easy, resulting in a significant cost reduction.

As described above, according to the present invention, since the photoelement spacing Δl of the solid-state image sensor is set to 2002kUM20M, there is no aliasing error, Δl is relatively large, and the Fourier transform calculation process is easy and costs can be reduced. . Furthermore, the light-receiving surface of the solid-state image sensor includes at least the section from the image point at the maximum object height and maximum projection magnification to the image point at the minimum object height and maximum projection magnification, reducing the number of solid-state image sensors. This makes it possible to reduce the size and cost of the apparatus, and improves operability since there is no need to move the solid-state image sensor when changing the magnification or replacing the chart.

[Brief explanation of the drawing]

1 and 2 are diagrams for explaining the present invention, FIGS. 3(a) and 3(b) are perspective views and partially enlarged views showing an example of a solid-state image sensor, and FIGS. 4 and 5. 6 is a perspective view showing an optical system of each embodiment of the present invention, and FIGS. 6 and 7 are diagrams for explaining the same embodiment. 211211.112...Self-scanning solid-state image sensor fEJ (C)xv-)4tp aIILILfrontFlsaV-
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Claims (1)

[Claims] A slit is enlarged and projected by a test lens, and the light intensity distribution of this slit image is measured by a light receiving element and Fourier transformed.
The MTF measuring device for measuring F has a self-scanning solid-state image sensor in which a plurality of photo elements are linearly arranged as the light receiving element, and the interval Δl between the photo elements is equal to the cutoff frequency Uc of the lens to be tested. and the measured maximum frequency UM. Projection minimum magnification III [l engineering. 2 Uo for
2k UM 2UM is satisfied, and the effective light-receiving surface of the self-scanning solid-state image sensor includes at least an area from an image point at a maximum object height and a maximum projection magnification to a point at a minimum object height and a minimum projection magnification of 0.4. An MTF measuring device, comprising: a scanning means for scanning the self-scanning solid-state image sensor in a direction perpendicular to the photoelement arrangement direction.