JPH01123214A - Image pickup optical system - Google Patents

Abstract

PURPOSE:To obtain an image pickup optical system which is capable of most effectively removing false signals in various photographing states by specifying the disposition of an optical low-pass filter and variable power lenses. CONSTITUTION:The lenses or lens groups which can vary the imaging magnification in the sate of fixing the positions of an object and imaging plane are provided in the optical system and the optical low-pass filter 18 consisting of a double refractive material is disposed on the side nearer the object than at least one among these lenses and lens groups. The magnification of the lenses or lens groups existing nearer the image side than the optical low-pass filter is thereby changed, by which the imaging magnification on the prescribed imaging plane is changed and the good relation between the space frequency spectra of the object image and the frequency characteristics of the optical low-pass filter is maintained at any imaging magnifications. The generation of the false signals is thus effectively prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an imaging optical system used in television cameras, electronic still cameras, electronic endoscopes, and the like.

[Prior art]

In optical devices such as television cameras, electronic still cameras, and electronic endoscopes that use solid-state image sensors or image pickup tubes to obtain color images, the pixel arrangement of the solid-state image sensor, the solid-state image sensor, Interference between the sampling frequency determined by the pitch of the color encoding filter installed in front of the WK picture tube and the spatial frequency components of the object image formed on these light-receiving surfaces causes false signals called moiré, aliasing, etc. occurs, and is a major cause of image quality deterioration. In order to remove such false signals, an optical low-pass filter made of a birefringent plate such as quartz has been provided in the imaging optical system that forms an object image on the light-receiving surface of the image sensor ( For example, see Japanese Patent Publication No. 51-14033).

By the way, a zoom lens is often used as the above-mentioned imaging optical system, but since conventional optical low-pass filters are installed between the imaging element and the lens, When capturing an image of an object having a frequency spectrum, there is a problem in that at a magnification other than a specific magnification, removal of false signals becomes insufficient or becomes impossible at all, resulting in significant deterioration of image quality. Such a phenomenon occurs conspicuously when a television camera is attached to the eyepiece of a fiberscope to take images, so this example will be explained in detail.

Figure 15 schematically depicts a state in which a television camera is attached to the eyepiece of a fiberscope. Lens 4. Optical low-pass filter consisting of a birefringent plate5. A television camera 7 with a built-in CCD image sensor 6 is attached, and an object image formed on the exit end face of the image guide fiber bundle 1 is captured through an eyepiece 2. An image is taken by re-forming the image from the photographing lens 44 through the optical low-pass filter 5 onto the light receiving surface of the CCD image sensor 6. As is well known, the image guide fiber bundle consists of a large number of optical The fibers are bundled in a so-called hexagonal dense bundle, and when the exit end face is enlarged, only the regularly arranged core portion 8 of each fiber shines brightly, as shown in FIG. Therefore, the image formed on the exit end surface can be considered to be the light spot array of this core section 8 modulated by the brightness distribution of the object, and the spatial frequency spectrum of this object image has a fundamental frequency determined by the arrangement of the core section. It has a large spectral component.

A false signal is generated due to interference between this fundamental frequency and the sampling frequency of the CCD 11 image element 6. However, if the photographic lens 4 is a zoom lens, the above fundamental frequency changes with zooming, so a false signal is generated. removal becomes insufficient.

FIGS. 17 and 18 explain this situation.

In FIG. 17, the photographing lens is the first imaging lens 9. Variator lens for changing photographic magnification 10. Compensator lens 11 for compensating for movement of image position due to zooming. It consists of four lens groups including a second imaging lens 12, a variator lens 10 and a compensator lens 1.
1 along the optical axis, the imaging magnification βL
From OW (Figure 15 (A)), the imaging magnification βHtag
Any magnification up to (FIG. 15(B)) can be taken. When this photographic lens captures an image of the light-dark pattern 13 formed by the core portion 8 of the image guide fiber bundle having a repetition pitch P, the image will appear on the light-receiving surface with the smallest repetition pitch P×βLOW (Fig. 15 (
A)) to the largest P×β)IIG (Fig. 15 (B)
)), and the fundamental frequency mentioned above is also 1/P×βLOW to 1/P×βN11i
It will change between NO.

[Problem that the invention seeks to solve]

An optical low-pass filter prevents interference between the spatial frequency components of the object image and the sampling frequency of the image sensor by reducing the resolution of the object at frequencies higher than a specific spatial frequency. When the frequency characteristics of an optical low-pass filter are shown with frequency as the horizontal axis, the fundamental frequency is 1/P×β during low magnification imaging, as shown by the solid line in FIG. If an optical low-pass filter is configured so that the MTF becomes zero at has a large value, so the resolution is not sufficiently reduced and false signals cannot be removed.On the other hand, the frequency is 1/P×β□ so that false signals are removed at high magnification. , if the MTF is set to zero at low magnification, then 1/P×β. Although it is desirable for the MTF to have a large value at frequencies below 8,
Since the MTF becomes zero at a frequency of /P×β,IIG, the resolution is lowered more than necessary, resulting in a loss of image quality.

In this way, in an imaging optical system configured with an optical low-pass filter placed between the zoom lens and the image plane, it is difficult to remove false signals, especially when imaging an object that has a large spectral component at a specific spatial frequency. There are various problems associated with this.

Separately, when multiple fiberscopes are selectively attached to a single television camera and an object image transmitted by the bundle of image guide fibers is captured, an image guide fiber is attached to each fiberscope. The spatial frequency spectrum of the object image may vary due to reasons such as the thickness of each fiber in the bundle being different or the magnification of the eyepiece being different for each fiberscope. In such a case, regardless of whether the possessing lens is a zoom lens or not, an optical low-pass filter placed between the lens and the image sensor can remove false signals for one fiberscope, but can remove false signals for other fiberscopes. It is completely useless against other fiberscopes and can produce significant false signals.

The present invention has been made in view of these problems, and it is an object of the present invention to provide an imaging optical system that can always obtain a good false signal removal effect in various situations.

[Means and effects for solving the problems] The present invention has the following features:
An imaging optical system for forming an object image on a predetermined imaging plane includes at least one lens or lens group whose imaging magnification can be changed while the positions of the object and the imaging plane are fixed. The present invention is characterized in that an optical low-pass filter made of a birefringent material is disposed closer to the object than at least one of these lenses or lens groups.

According to such a configuration, the imaging magnification on a predetermined imaging plane changes by changing the magnification of the lens or lens group located on the image side of the optical low-pass filter, and as a result, the spatial frequency spectrum of the object image changes. Even if the frequency characteristics of the optical low-pass filter change, the frequency characteristics on the imaging plane of the optical low-pass filter will also change, so even at a different imaging magnification, the difference between the spatial frequency spectrum of the object image and the frequency characteristics of the optical low-pass filter A good relationship can be maintained and the generation of false signals can be effectively prevented.

In addition, a lens or lens group with variable imaging magnification is provided on the object side of the optical low-pass filter, and its imaging magnification is the same as that of a lens or lens group with variable imaging magnification provided on the image side of the optical low-pass filter. If the relationship between the changes in magnification of the front and rear lenses or lens groups of the optical low-pass swivel filter is set so that it changes in inverse proportion to the change in at least one imaging magnification, these changes will be effective for imaging an object. While the changes in magnification cancel each other out and have no effect, on the other hand, the effect of the optical low-pass filter is not canceled out because only the change in magnification on the image side has an effect. As a result, only the frequency characteristics of the optical low-pass filter can be changed without changing the spatial frequency spectrum of the object image, so that false signals can be effectively removed when imaging various objects.

[Example] Hereinafter, an example will be described based on the drawings, but members having the same functions as those of the conventional example are given the same numbers, and detailed explanation thereof will be omitted.

1 to 5 show a first embodiment, and are schematic representations of a state in which a television camera with a built-in CCD TI image element 6 is attached to the eyepiece of a fiberscope 3. . In this state, as shown in Figure 2, the arrangement direction of each fiber of the image guide fiber bundle and the CCD
It is configured so that the horizontal scanning direction of the ll image element 6 substantially coincides with the horizontal scanning direction. The output signal of the CCD image sensor 6 is supplied to the television monitor 17 via the camera control unit 19, so that the object image formed on the exit end face of the image guide fiber bundle 1 can be observed.

In this embodiment, the optical low-pass filter 18 is arranged between the first imaging lens 9 and the variator lens 10 of the zoom lens 5. FIG. 3 shows the specific structure of the optical low-pass filter for one day, in which crystal plates 20, 21, and 22 are stacked with A wavelength plates 23, 24 interposed therebetween in order from the light incident side. Figure 4 shows the separation direction when each crystal plate separates an incident ray into an ordinary ray and an extraordinary ray due to its birefringence. 20, 21, and 22 have separation directions of -60", 30', and 30 degrees, respectively.

When a single ray of light enters the optical low-pass filter 18, it is first separated by the first crystal plate 20 into an ordinary ray H2 and an extraordinary ray L2, as shown in FIG. After all of these are converted into circularly polarized light by the A wavelength plate 23,
The ordinary rays and extraordinary rays, that is, L3. It is separated into Lx and L4. These are further converted into circularly polarized light by the A wavelength plate 24, and again converted into ordinary and extraordinary rays by the third crystal plate 22, that is, Ll and L.
It is separated into s, Ls and L'l, Lx-tori, and L, tori. In this way, the incident light beam is separated into eight beams, so that eight object images whose positions are shifted from each other are formed on the CCD image sensor 6. -When two images separated by a distance d are formed by a crystal plate,
The spatial frequency characteristic of the crystal plate in a direction forming an angle θ with the separation direction is expressed by an MTF as shown in FIG. 6 having a zero point at the frequency 1/acos θ. Since it includes two crystal plates, the overall spatial frequency characteristic is expressed as the product of the MTF of each crystal plate.

Here, in this embodiment, the optical low-pass filter 18 is located closer to the object than the variator lens 10, so when the variator lens 10 is moved to change the imaging magnification, the distance between the two separated images on the image plane also changes. Although the zero point of the MTF moves, since both the spatial frequency spectrum of the object image and the zero point of the optical low-pass filter 18 change in inverse proportion to the imaging magnification, it can be said that the manner in which they change is exactly the same. Therefore, if the relative relationship between the spatial frequency spectrum of the object image and the MTF of the optical low-pass filter 18 is determined so that false signals can be sufficiently removed at a certain imaging magnification, the relationship between the two will always be maintained at any imaging magnification. It becomes constant, and the effect of removing false signals can be maintained as it is.

The specific configuration of the optical low-pass filter may be determined as appropriate depending on the application, but the filters shown in FIGS. 7 to 9 are also used for television cameras that capture images of fiberscopes. As shown in FIG. 7, this optical low-pass filter consists of three crystal plates 25.
26°27 are directly superimposed. The separation direction of each crystal plate is −60° with respect to the horizontal scanning direction H of the CCD image sensor 6, as shown in FIG.

-15° and 30°, and by this combination, one incident light beam is separated into eight light beams arranged as shown in FIG. Since the function of each crystal plate is similar to that of the optical low-pass filter described with reference to FIGS. 3 to 6, the details thereof will be omitted.

In this embodiment, the compensator lens 11 is placed closer to the image side than the optical low-pass filter 18, but this function may be provided to the first imaging lens 9.
Focusing may be performed by moving the first imaging lens 9 back and forth.If the lens closer to the object than the optical low-pass filter 18 is moved in this way, the change in magnification will only affect the object image. , the relationship between the spatial frequency spectrum of the object image on the CCD image sensor 6 and the MTF of the optical low-pass filter changes slightly, but the magnification change due to movement for focusing and image position correction is very small, so there is no false signal. It can be assumed that the removal effect remains virtually unchanged, and since the distance between the two images separated by the crystal plate is proportional to their thickness, imaging using a lens on the exit side of the optical low-pass filter is recommended. In an optical system, it is preferable that the imaging magnification of the lens is higher, since the thickness of the crystal plate can be made thinner to obtain the same separation distance.

In the case of this embodiment, it is preferable in the above sense that F's<Ftow, where the focal length of the entire system at minimum magnification is FLl1M+ and the focal length of the first imaging lens is F.

In addition, the variator lens 10 and CCD jl may be installed as necessary.
Of course, an optical low-pass filter may also be provided between the image element 6 and the image element 6.

Furthermore, even if the optical low-pass filter is placed closer to the object than the first imaging lens, the same effect as in this embodiment can be obtained.

FIG. 10 shows a second embodiment, in which a retrofocus type zoom lens system 28 uses an optical low-pass filter.

29 is a compensator and focusing lens, 30
is a variator lens. An optical low-pass filter 18 utilizing birefringence is placed in front of the variator lens 30. Even in this case, if it is used as the imaging optical system of the television camera 7 of the first embodiment, the same effects as in the first embodiment can be obtained.

11 and 12 show a third embodiment of the present invention. In this embodiment, although the lens system as a whole does not have the function of a zoom lens, it is possible to arbitrarily change the spatial frequency of the zero point of the MTF of the optical low-pass filter with respect to the spatial frequency spectrum of the object image. 1st
In Fig. 1, the photographing lens 31 has a configuration in which two lens systems 32 and 33, which are themselves zoom lenses, are arranged in order from the object side, and a master lens 38 is arranged on the image side. An optical low-pass filter 18 made of a quartz plate is provided.

The zoom lens 32 is the first imaging lens 9. similar to that of the first embodiment. Variator lens 10. Compensator lens 11. It consists of a second imaging lens 12, and the zoom lens 33 is the zoom lens 32 reversed from front to back, and 34, 35, 36, and 37 are the second imaging lenses, respectively.
compensator lens.

This corresponds to a variator lens and a first imaging lens. The object 39 once forms an intermediate image 40 immediately after the optical low-pass filter 18 between the zoom lenses 32 and 33,
Furthermore, a final image 41 is formed by a zoom lens 33 and a master lens. The imaging magnification of the entire system is β, and the imaging magnification of the zoom lenses 32 and 33 is β, 8. β1. Then, β3. =β,! ×β3. The relationship holds true.

Here, variator lens 10.36. If the compensator lens 11°35 is moved along the optical axis, the magnification of the zoom lens 32°33 changes to different values β, 8', β1, . ′, but by creating a predetermined relationship between the movements of each lens, the magnification of the entire system can be kept constant, that is, β,
l-β,! ×β, 3 - β, 2′×β3. ′-can be constant. If this relationship is satisfied, the spatial frequency characteristics of the optical low-pass filter for one day can be changed. Figure 12 (A).

In (B), (a) shows the cores of two fibers adjacent to each other at a distance P when the exit end face of the image guide fiber bundle is placed as an object, and (b) shows the image at the imaging position 41. Since the optical low-pass filter 18 shown in FIG. Let S be the separation distance when an incident ray is separated into an ordinary ray and an extraordinary ray by the birefringence effect in the optical low-pass filter 18. In FIG. 12 (^), the magnification of the zoom lens 32.33 is β, 2. β1. in the case of,
(B) is also β3. ', β1. ′ is shown, but
At the imaging position 41, in both cases the two cores are PX3. The images are formed at a distance of l. However, since the separation distance S between ordinary light and extraordinary light by the optical low-pass filter 18 is affected only by the zoom lens 33, the distance between the two separated images is S×βss in case (A) and S×βss in case (B). S×β,′, which changes with the magnification of the zoom lens 33. As explained earlier, the separation distance at the final image plane between the image of ordinary light and the image of extraordinary light determines the spatial frequency characteristics of the optical low-pass filter, so in the optical system of this embodiment, the spatial frequency spectrum of the object image This means that only the spatial frequency characteristics of the optical low-pass filter can be changed without changing the optical low-pass filter.

Such an optical system is effective in the following cases.

There are various types of fiberscopes depending on their uses, and the fibers in the image guide fiber bundle also have various thicknesses. Therefore, P in FIG.
The value of may differ depending on the fiber scope. When capturing object images from multiple fiberscopes with a single television camera, the most effective way to remove false signals is to install an optical low-pass in the television camera optical system for each fiberscope. Although it is necessary to change the characteristics of the filter, this can be achieved by simply moving some lenses along the optical axis according to this embodiment.

In the above example, the imaging magnification of the photographic optical system as a whole is constant, but if it is desired to make it variable, the master lens 38 in FIG. 11 is replaced with a zoom lens 42, as shown in FIG. The object image 41 is captured by this zoom lens 4 again.
2, and a CCD image sensor or the like may be placed at the image position 43.

FIG. 14 shows a fourth embodiment of the present invention. In this example, the photographing lens 44 is arranged in order from the object side to the first imaging lens 9. First variator lens 10° second imaging lens 45.
It consists of a second variator lens 36.degree. and a third imaging lens 46, with an optical low-pass filter 18 arranged between the two variator lenses 10.36. In such a configuration, the magnification of the two variator lenses 10.36 is β
1°.

β8. When β, . ×β5. 1st so that it is always constant. By interlocking the second variator lens 10.36, exactly the same effect as the lens system of the third embodiment shown in FIG. 11 can be obtained. In addition, the second variator lens 36
If the magnification is changed by moving only the lens, the same effect as in the first embodiment shown in FIG. 1 can be obtained. In this case, the minimum value of the combined focal length of the lens on the object side of the optical low-pass filter 18 is FMIN+the shortest focal length of the entire system is F. 1. , then FMIN<F for the same reason as explained in the first embodiment. It is desirable to set the angle to 1°.

In each of the above embodiments, the optical low-pass filter is provided separately from the lens, but one of the plurality of lenses constituting the lens system is made of a birefringent material such as crystal, and the optical low-pass filter is provided in the lens. The function of the low-pass filter may be taken care of. In this case, if the arrangement relationship between the lens and the variator lens is set in the same manner as the arrangement relationship between the optical low-pass filter and the variator lens in each embodiment, the intended purpose can be achieved. can be achieved.

〔Effect of the invention〕

According to the present invention, by appropriately arranging an optical low-pass filter and a variable magnification lens, it is possible to obtain an imaging optical system that can most effectively remove false signals under various photographing conditions.

[Brief explanation of the drawing]

1 and 2 are diagrams showing the structure of a first embodiment of the present invention, and FIGS. 3 and 4 are diagrams showing a specific structure of an example of an optical low-pass filter used in the embodiment. 5
6 and 6 are diagrams for explaining the operation of this optical low-pass filter, and FIGS. 7 to 9 explain the specific structure and operation of other examples of the optical low-pass filter used in the above embodiment. FIG. 10 is a diagram showing the configuration of the second embodiment of the present invention, FIGS. 11 and 12 are diagrams showing the configuration and operation of the third embodiment of the present invention, and FIG.
The figure shows the configuration of a modification of the third embodiment, FIG. 14 shows the configuration of the fourth embodiment of the present invention, and FIGS. 15 to 18 are diagrams for explaining the conventional example. be.

Claims (1)

[Claims] In an imaging optical system for forming an image of an object on a predetermined imaging plane, a lens or a lens capable of changing its imaging magnification while the positions of the object and the predetermined imaging plane are fixed in the optical system; 1. An imaging optical system comprising at least one lens group, and an optical low-pass filter made of a birefringent material arranged closer to the object side than the lens or at least one of the lens groups.