JPH10221509A - Image-forming optical system and photographic measuring method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image-forming optical system and a photographic measuring method with which a position of the gravitational center of an object image can be precisely measured when photographic measurement can be carried out by using an electronic camera or the like. SOLUTION: An image-forming optical system 1 is composed of image pick-up lenses 5a to 5c, a diaphragm 7, an ND filter 9, pick-up lenses 5d to 5f, a relay lens system 11 and a CCD (image pick up element) 13 which are arranged in the mentioned order in a direction from the object side to the CCD side. The diaphragm 7 is provided between the group of the lenses 5a to 5c and the group of the lenses 5d to 5f. The ND filter 9 having transmissivity with a radial Gaussian distribution is located at the position of the diaphragm 7. The provision of such a ND filter 9 can prevent generation of diffraction rings which are likely to occur in a normal lens system, and accordingly, it is possible to prevent occurrence of measuring deviation of a spot position caused by the diffraction rings.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming optical system for an electronic camera having a CCD image pickup device. The present invention also relates to a method for performing photo measurement (including surveying) using such a camera.

[0002]

2. Description of the Related Art In recent years, photographic measurement has been actively performed in the fields of industrial measurement and surveying.
Photographic measurement can be performed even in a situation where it is difficult for a measurer to approach a required measurement point, such as in the case of shape measurement of a large structure such as a building or a bridge. Furthermore, unlike a normal surveying instrument, there are various advantages such as the ability to measure a time-varying shape over the entire structure in a short time.
Also in the field of surveying, photogrammetry is being used as a simple method instead of flat plate surveying, because it can measure many measurement points simultaneously.

In the conventional analog-type photo measurement, a stereo photograph is taken by a silver halide camera using a photographic dry plate or the like, and the photograph is taken, for example, by a device called a plotter. I was working on it.
However, recently, an analysis plotter using a computer has appeared, and the plotting work can be performed almost automatically.

In photograph measurement, an electronic camera is often used at the time of photographing. This is because when a photograph is taken with an electronic camera, the image can be directly taken into a computer without passing through a scanner or the like, and can be applied to an analytical plotting device. Another advantage is that, unlike a photographic plate or a photographic film, the image is digitized (digitalized), so that data is not degraded even during long-term storage.

[0005] An electronic camera detects a real image of an object formed by an imaging lens, usually by a photodetector array using a CCD. The data obtained here is subjected to image processing or the like in consideration of the positional relationship on the CCD, or is displayed on a display device in the same order as the array of detectors that have taken an image. In photo measurement, the distance between any two points in an image is measured on a computer in consideration of the positional relationship on the CCD, and the distance is converted to the length on the object, and the size is measured. Measurement.

In actual photograph measurement, the following work is usually performed. A plurality of reflectors called targets are attached to measurement points of the object to be measured, and the reflection of light from the targets is photographed. Alternatively, a measurement point on the measurement target is illuminated in a point shape with a laser or the like toward the measurement target and photographed. In this way, the measurement points on the measurement target are recorded as images.

Next, the distance between the targets on the image and the distance from the position where the measurement is performed, that is, the camera position to the target, are calculated, and the shape of the measurement object is measured or measured based on the calculated distance. Measure the distance to the target.
Therefore, when taking a photograph of the object, it is necessary to take a photograph with sufficiently high resolution. For this purpose, it can be said that the fineness of the CCD (element density) is the most important.

[0008] In other words, silver halide photography is basically an analog record, and it is possible to make very detailed measurements if the image is emphasized. You can only get as. As a result, for example, data captured by an electronic camera using a CCD of 3,000 × 2,000 pixels basically has only 3,000 × 2,000 data. This means that the measurement accuracy is only about 1/2000 of the imaging span.

Therefore, in order to achieve higher accuracy,
Statistical processing of photographs of points on the CCD where the target is actually photographed from multiple points is used to statistically process the target to determine the position of the target with an accuracy of less than the pixels of the CCD, or the center of gravity of the image of the target spread around. By finding C
For example, measurement is performed with an accuracy equal to or less than the CD pixel.

[0010]

However, the above-described method has the following problems. As is well known, not only photographic lenses but also lenses have a diffraction phenomenon. This is because the imaging lens has only a finite size, and there is a sharp change in transmitted light intensity at the edge of the lens. As a result, a so-called diffraction fringe always occurs in a captured image. Diffraction fringes are weak ring-shaped bright areas formed around the center spot.

Approximately, assuming that the effective radius of the lens is α, the focal length is f, and the wavelength of light is λ, the Bessel function is

[0012]

(Equation 1)

When photographing with such an optical system, a point on the object to be measured becomes an image having an intensity distribution as shown in FIG. In FIG. 4, the horizontal axis represents the number of the arranged CCDs, and the vertical axis represents the light intensity. This means that the resolution is about 15
μm, that is, a point image in the case of 2.5 CCD cells having a width of 6 μm, which is almost comparable to the resolution when a normal 35 mm camera lens and a relay lens for forming an image on the CCD are used. I do. Note that the shift of the center position of the image from the center of the CCD is set to 1/4 of the CCD width in the left direction. When this light intensity is detected by the CCD, the CCD
That is, light is detected for each cell, and the intensity distribution is as shown in FIG.

In photographic measurement, it is necessary to find a point to be measured on a photograph. To do so, it is necessary to accurately find the vertex from the light intensity distribution of a captured image. On this occasion,
If a fine intensity distribution as shown in FIG.
Although it is easy to accurately determine the position of the vertex, the intensity distribution actually obtained by an imaging device such as a CCD is shown in FIG.
(B), it is difficult to find the position of the vertex below the pixel size of the CCD. In particular, a bulge at the foot of the intensity distribution, which is seen in both FIGS. 4A and 4B, that is, the diffraction ring may shift the center position of the intensity distribution from the actual position.

For example, in the examples of FIGS. 4A and 4B, the signal intensity of the entire spot is a unit of light intensity,
The signal strength of each cell obtained in D is 0.002 from the left.
6, 0.0169, 0.0058, 0.0675, 0.
3026, 0.3902, 0.1749, 0.013
5, 0.0146, 0.0098, 0.0017, and the barycentric position of the light intensity obtained from this is r = 0.26.
8 compared to the true center position r = 0.25.
An error of about 1.8% occurs with respect to the cell width of the CCD.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an image forming optical system and a photograph measuring method capable of accurately measuring the position of the center of gravity of an object image when measuring a photograph using an electronic camera or the like.

[0017]

In order to solve the above problems, an image forming optical system according to the present invention is an image forming optical system for forming an image of an object on an image sensor arranged in a plane. hand;
A peripheral portion is provided with an ND filter having a low transmittance at a position of a stop ring of the imaging optical system. When photograph measurement is performed using such an imaging optical system, diffraction fringes in the peripheral portion of the image can be eliminated by the ND filter.

The photographic measuring method according to the present invention is a photograph including an operation of forming an image of an object on an image sensor arranged in a plane and processing a signal of the image sensor to obtain a position of a center of gravity of the image. A measurement method, wherein an ND filter having a low transmittance at a peripheral portion is inserted at a position of an aperture ring of the imaging optical system;
The ND filter is used to eliminate diffraction fringes in an image peripheral portion.

[0019]

FIG. 1 is a diagram showing an optical arrangement of an image forming optical system according to one embodiment of the present invention. This imaging optical system 1
Are from the left (object side) to the right (C
Toward the CD), a group of three imaging lenses 5a to 5c, an aperture 7, an ND filter 9, and a group of three imaging lenses 5d to 5d to
f, a relay lens system 11 and a CCD (image pickup device) 13 are arranged. The imaging lens 5 forms an image of the object to be measured, and this image is formed by the relay lens 11 into the CCD 13.
Is imaged on the imaging surface of. The reason why the six imaging lenses are provided is to correct various aberrations.

An aperture 7 is provided between the imaging lenses 5a to 5c and 5d to 5f. An ND filter 9 having a Gaussian distribution transmittance in the radial direction is inserted at the position of the stop 7 of the imaging optical system 1. Such ND
Due to the filter 9, the light intensity distribution of the image formed by the imaging optical system has a Gaussian distribution, and diffraction around the center spot peculiar to the Bessel function type light intensity distribution formed by the ordinary optical system. The occurrence of rings disappears,
Measurement deviation of the spot position due to the diffraction ring does not occur. Therefore, by inserting such an ND filter at the stop position of the optical system, it is possible to provide a photographic optical system capable of performing photographic measurement with high accuracy.

FIG. 2 shows a transmittance distribution of the ND filter according to the present embodiment. This ND filter, the transmittance distribution ^{exp {- 2 (r / 3a} ) 2} (a is effective radius of the optical system) are set to be in. Therefore, the density distribution is adjusted so that the amount of transmitted light at the periphery is reduced to 1 / e ⁶ ≒ 1/400. This is to sufficiently suppress diffraction by the ND filter fixed frame, the aperture, and the lens frame. As long as the aperture ring, lens, etc. have a finite size, the diffraction phenomenon due to the edge cannot be strictly avoided, but as described above, ND
If the amount of diffracted light with respect to the total amount of light is made substantially negligible by inserting a filter, it can be considered that no diffraction has occurred. As an example, such an ND filter can be manufactured by changing the thickness of the light absorbing glass between the center and the periphery and bonding the light absorbing glass to a transparent dummy glass for making the overall thickness of the filter uniform.

FIG. 3A shows an image formed by an optical system using such an ND filter. FIG. 3B shows the signal intensity distribution of each CCD cell when this image is received by the CCD element. Here, the effective diameter, the focal length, the light wavelength, the position of the image, and the like of the optical system were the same as those in the conventional example shown in FIG.

In the case of FIG. 3, it can be seen that no diffraction ring occurs and the light intensity distribution draws a relatively monotonous curve. Further, the signal intensity value of each CCD is from the left,
0.0006, 0.0029, 0.0113, 0.03
34, 0.0767, 0.1364, 0.1877,
0.2001, 0.1652, 0.1056, 0.05
23, 0.0201, 0.0060, 0.0013,
0.0002, and the position of the center of gravity of the image obtained from this is r
= 0.254, which is 0. 0 compared to the true value r = 0.25.
The error was kept to 4%.

[0024]

As is apparent from the above description, according to the present invention, the position of the image of the measurement object formed on the image sensor can be measured with high accuracy, and the accuracy in photographic measurement can be improved. Can be contributed to. In addition, machine vision technology based on photo measurement can contribute to the improvement of object recognition accuracy.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an optical arrangement of an imaging optical system according to an embodiment of the present invention.

FIG. 2 shows a transmittance distribution of the ND filter of the present embodiment.

FIG. 3A shows an imaging result by an optical system using an ND filter according to one embodiment of the present invention. (B) shows the signal intensity distribution of each CCD cell when this is received by the CCD element.

FIG. 4A shows an imaging result by a conventional optical system.
(B) shows the signal intensity distribution of each CCD cell when this is received by the CCD element.

[Explanation of symbols]

 Reference Signs List 1 imaging optical system 3 optical axis 5 imaging lens 7 aperture 9 ND filter 11 relay lens 13 CCD

Claims (4)

[Claims] 1. An imaging optical system for forming an image of an object on an image sensor arranged in a plane, wherein a peripheral portion has a low transmittance at a position of an aperture ring of the imaging optical system. An imaging optical system comprising the ND filter according to (1). 2. The method according to claim 1, wherein the image forming optical system is a photograph measurement (including a survey).
The imaging optical system according to claim 1, wherein the imaging optical system is included in an apparatus, wherein the ND filter eliminates diffraction fringes in a peripheral portion of the image. 3. The imaging optical system according to claim 1, wherein the ND filter has a Gaussian transmittance distribution. 4. A photographic measurement method including an operation of forming an image of an object on an imaging element arranged in a plane and processing a signal of the imaging element to obtain a position of a center of gravity of the image; A photographic measurement method comprising: inserting an ND filter having a low transmittance at a peripheral portion at a position of an aperture ring of the imaging optical system; and eliminating diffraction fringes at a peripheral portion of the image by the ND filter.