JPH02277016A - Image pickup device - Google Patents

Abstract

PURPOSE:To disappear a fiber arranging pattern at the time of observation by a fiberscope connected with a TV camera by arranging an optical element which limits the spatial frequency response of an image pickup optical system in an optical path from the emitting end surface of an image guiding fiber bundle to an image pickup element to about 0.5 or below. CONSTITUTION:A fiberscope guides an image from an objective lens 3 to the solid-state image pickup element 10 of a color camera 2 via the image guide fiber bundle 4 to observe the image on the TV monitor 12. Where the sampling frequency in the vertical direction of the image pickup element 10, Ny; the sampling frequency in its horizontal direction Nx; the basic spatial frequencies in the vertical and horizontal directions of a fiber arranging pattern on the emitting end surface of the fiber bundle, Ux and Uy respectively, this fiberscope satisfies Uy<Ny/2 and/or Ux<Nx/2. An optical low-pass filter 8 which limits the spatial frequency response of an image forming optical system to about 0.5 or below when Uy and/or Ux are given is arranged in the optical path from the emitting end surface of the fiber bundle to the image pickup element 10.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an imaging device for imaging an image appearing on an end face of an image guide fiber bundle of a fiberscope.

[Conventional technology]

Recently, it has become popular to attach a TV left camera to the eyepiece of a fiberscope or the like to observe the inside of a body cavity on a TV monitor.

However, the end face of an image guide fiber bundle used in a fiberscope has a regular bright and dark pattern (network structure) determined by the arrangement of the fiber cores. On the other hand, if a TV left camera or camera using an image pickup tube has a color stripe filter in front of its light-receiving surface, or if a solid-state image sensor has a color mosaic filter in front of it, The color elements of the filter have a regular arrangement, and even in the absence of a mosaic filter, the picture elements of the solid-state image sensor have a regular arrangement. Therefore, there is a problem in that both regular structures interfere with each other, causing moiré in the TV image.

In recent years, the number of pixels in the horizontal scanning direction of solid-state image sensors has increased, and the occurrence of moiré has tended to decrease. That is, the value of the spatial frequency spectrum of an object generally decreases as the frequency increases, so as the number of pixels increases and the sampling frequency increases, moire decreases.

[Problem to be solved by the invention]

However, a fiberscope inserted into a blood vessel or the like is very thin, and therefore its image guide fiber bundle is also thin. The magnification of the optical system that forms the image on the image sensor increases, and as a result, each fiber in the image on the image sensor becomes very thick (the spatial frequency spectrum becomes low frequency).
Although moire is less likely to appear, a problem has arisen in that each fiber image can be seen clearly, which obstructs observation.

SUMMARY OF THE INVENTION In view of the above-mentioned problems, an object of the present invention is to provide an imaging device in which an obstructive fiber arrangement pattern becomes invisible when a TV camera is connected to a thin fiberscope to observe an image.

[Means and effects for solving the problems] An imaging device according to the present invention includes an image guide fiber bundle, an imaging optical system that forms an image of an exit end face of the image guide fiber bundle, and an image forming system that receives an image of the exit end face of the image guide fiber bundle. Equipped with an image sensor,
The vertical sampling frequency of the image sensor is Ny, and when the image sensor also performs sampling in the horizontal direction, the sampling frequency is Ny, and the horizontal and vertical directions of the fiber arrangement pattern in the image of the exit end surface are The fundamental spatial frequency of each is Uo.

When Uy, Uy<Ny/2 and/or Uy<N8/
In an imaging device that satisfies condition 2, the spatial frequency response of the imaging optical system is expressed as Uy and/or in the optical path from the exit end surface of the image guide fiber bundle to the imaging element.
Alternatively, by arranging an optical element with a value of about 0.5 or less in U8, the fiber arrangement pattern in the image of the exit end face of the image guide fiber bundle can be seen even when moire does not occur.

[Example] Hereinafter, the present invention will be described in detail based on the illustrated example.

FIG. 1 shows a first embodiment, in which a color camera 2 using a mosaic filter is attached to a fiberscope 1 for blood vessels. In the fiberscope 1, 3 is an objective lens, and 4 is an image guide fiber bundle. In the color camera 2, 5 is an imaging lens, 6 is an optical low-pass filter, 7 is an infrared light cut filter or a YAG laser cut filter, 8 is an optical low-pass filter, and 9 is a mosaic filter 10, which is a solid-state image sensor. 11 is a camera control unit connected to the solid-state image pickup device IO, and 12 is a TV monitor.

Here, we will discuss under what circumstances moiré will not occur and the fiber arrangement pattern in the exit end face image of the image guide fiber bundle 4 will interfere with observation.

First, let's consider a case where moiré does not occur.

The horizontal sampling frequency of the solid-state image sensor lO (horizontal repetition frequency of the mosaic filter) is Ny
The vertical sampling frequency (the vertical repetition frequency of the mosaic filter 9 or twice it; which value varies depending on the format of the solid-state image sensor IO and the mosaic filter 9) is set to Ny. If the fundamental spatial frequencies in the horizontal direction and vertical direction of the fiber arrangement pattern in the image on the solid-state image sensor lO of the exit end surface of the image guide fiber bundle 4 are respectively Ux and Uy, then N.

[J,<□...If (1), moire does not occur because the frequency of the fiber arrangement pattern is within the Nyquist limit.

Next, let us consider a case where the fiber arrangement pattern described above interferes with observation. The repetition of fibers in the image is V (c
pd), the spatial frequency of the image of the finest object that can be resolved is V/2 (cpd). The observer sees both the fiber arrangement pattern and the brightness pattern of the object image at the same time, so the fiber arrangement pattern is good (if visible, it will interfere with object observation.The CPD is as shown in Figure 2). It means how many cycles, that is, how many lines there are per 1' of visual angle.

Here, according to Fig. 4 (the main part is shown in Fig. 3) in the right column of page 690 of the literature "Applied Physics" Vol. 49, No. 7 (1980), the MTF at the time of naked eye observation is It has been shown that the spatial frequency response (spatial frequency response) reaches a maximum around 2.5 cpd and decreases around that point. Furthermore, Fig.
, 4 displays a brightness pattern of a single spatial frequency with an average luminance of 20 nits (=cd/rr?) on the entire screen of a TV monitor, and the horizontal axis is the MTF when a subject with normal vision observes this with the naked eye. The spatial frequency (unit: cpd) is plotted along the vertical axis with the magnitude (unit: dB).

When actually looking at an object, it is necessary to consider the magnitude of each spatial frequency component of the object in addition to the MTF.

For example, when V=5 (cpd), V/2 = 2.5 (
cpd), and according to Figure 3, this seems to be sufficient because the MTF at the resolution limit of the object is larger than the MTF at the frequency of the fiber arrangement pattern. However, in real objects, the spectrum of high frequency components Since the size of the spectrum is smaller than the spectrum of low frequency components, the actual appearance is often weak, and it is lost to the fiber arrangement pattern. Also, according to FIG. 3, although the MTF of V=5 is small, V=O,
Same as the MTF of S (because it is leprosy, the fiber arrangement pattern is as noticeable as the coarse pattern in the object. Therefore, ■
It is necessary to make V=20 (cpd
), it is estimated from Fig. 3 that it will be less than -30 dB, which is quite small. On the other hand, V/2=10(cpd
) is also quite small at about -20 dB, but the MTF becomes largest at frequencies slightly lower than this (often found in objects). Therefore, the fiber arrangement pattern becomes less noticeable with respect to the pattern in the object, making it easier to see.

Here, as shown in FIG. 1 (if the length of the vertical side of the TV monitor 12 is H, then as a standard observation condition, the distance S from the TV monitor 12 to the observer is 5=58... - It is said to be about (3). Note that H is often about 15 to 25 cm.

Therefore, as shown in FIG. 4, if the fiber spacing of the exit end face image of the image guide fiber bundle 4 on the screen of the TV monitor 12 is φ, then ≦20 cpd (4) When φ, the fiber The array pattern obstructs observation.

In reality, there are cases where the distance to the TV monitor 12 is about 3H, so when ≦20cpd (5)φ tan −' ()H, the fiber arrangement pattern becomes a hindrance to observation.

Note that when the fiber diameters are random, such as in a bundle of image guide fibers made of quartz, the coarsest fiber spacing may be considered as φ.

Therefore, when formula (5) is satisfied, the fundamental spatial frequency U of the fiber image in the optical low-pass filter 8.

If the MTF of Uy is lowered, an image that is easy to use for practical use can be obtained. Specifically, it is shown on page 691 of the above-mentioned document "Applied Physics" Vol. 49, No. 7 (1980) that if the MTF is set to 0.5 or less, a considerable improvement can be achieved. The above-mentioned literature examines the ease with which the pattern is seen with the naked eye when a light-dark pattern with a high spatial frequency is superimposed on a light-dark pattern with a low spatial frequency. Page 691, right column 11~
According to line 20, the masking pattern (high frequency side)
It is said that when the contrast of 0.5 or less, the influence of the low frequency pattern on the contrast discrimination threshold value decreases rapidly. The fiberscope image consists of the object's light and dark pattern (including color) overlapping the mesh pattern (which has a higher frequency than the object image) due to the fiber arrangement.
Considering this mesh pattern as a masking pattern, if its contrast, or MTF, becomes 0.5 or less, it is thought that the low frequency pattern, that is, the degree of interference when viewing an object image, will be rapidly reduced.

Therefore, it is desirable that the optical low-pass filter 8 has an MTF of 0.5 or less at the spatial frequencies Uy and Uy.

5 and 6 show the structure and cross section of an example of the optical low-pass filter 8, respectively, and the crystal plates 13 and 14
A wavelength plate 15 is sandwiched between them. In FIG. 5, arrows indicate the direction of image separation. Further, FIG. 7 shows the MTF of the optical low-pass filter 8 on a two-dimensional frequency plane, and the dotted line in the figure is the line where the MTF becomes 0, that is, the trap line.

By the way, according to Fig. 4, U8 = - φ (6) □φ However, since the MTF in the UyIUF direction decreases with a cosine function, the MTF at Uy and Uy is set to 0.5. To do the following, the spatial frequency of the tiger-tube line in Figure 7 is Uy. +U
If it is IO, it must be .

The trap line may be selected as shown in FIG. 8 instead of inverted as shown in FIG. This is obtained by rotating the optical low-pass filter 8 shown in FIGS. 5 and 6 at an angle θ around the optical axis, and equations (8) and (9) can be applied in almost the same way.

The optical low-pass filter 8 may be constructed using a liquid crystal polymer plate 9, a phase grating 9, a diffraction grating 9, a lentifural lens, a polygonal lens, a soft focus lens, etc. instead of the quartz plate. Further, the A wavelength plate may also be constructed from a liquid crystal polymer plate. Liquid crystal polymer plates are convenient because they are lightweight and can be shaped freely.

FIG. 9 shows a second embodiment, in which a three-plate color camera 2 is attached to a fiberscope 1, and a multifaceted lens is used as an optical low-pass filter. In the color camera 2, 16 is the aperture, 17
is a polygonal lens, and 18 is a trichromatic separation prism.

1O and 11 respectively show a cross section and a front view of the polygonal lens 17, and the polygonal lens 17 is made by combining two curved surfaces (aspherical surfaces may be used) having approximate spherical centers separated by Δ. It is something. Here, when each surface of the polygonal curved surface of the polygonal lens 17 is considered as one lens, the paraxial magnification is β1, and the composite paraxial magnification of the lens system 19 (see FIG. 9) after the polygonal curved surface is β2. When, (1) MTF should be set to 0 at a frequency of 21 (-Δ+Δβl)·β2.

In this way, the light beam will be at the imaging surface of the solid-state image sensor IO (-Δ
Separate into two points separated by +Δβ1)·β21.

Note that it is preferable to place the multifaceted lens 17 near the pupil.

This is because the same low-pass characteristic can be maintained over the entire screen.

FIG. 12 shows a third embodiment, in which a part of the imaging lens 5 of the color camera 2 is made into a lens system 20 consisting of an eccentric lens to obtain low-pass characteristics. . Here, in order to make the low-pass characteristic variable, the eccentricity of some lenses in the lens system 20 may be made variable.

Incidentally, the above-mentioned optical low-pass filter 8. Multifaceted lens 17
．． Any decentered lens system 20 can also be used to remove moiré. Here, the reason why the eccentric lens system 20 has a moiré removal effect will be described.

In a lens system, all lenses are normally arranged coaxially (intermittently), and if one or more of the lenses is decentered, aberrations occur accordingly. There are various aberrations caused by eccentricity, including eccentric distortion, eccentric field curvature, and eccentric coma.

Eccentric spherical aberration is the main one. Of these, decentered spherical aberration is also called uniform coma, and causes approximately -like coma-like blur across the entire screen. Therefore, if the eccentricity of the lens occurs so that only eccentric spherical aberration occurs without other eccentric aberrations, a moire removal effect similar to that of a soft focus lens can be expected.

According to the study results of the inventor of the present application, eccentric spherical aberration occurs when a lens surface having a substantially concentric shape is slightly decentered with respect to an aperture diaphragm located near the aperture diaphragm.
It has become clear that other eccentric aberrations can be suppressed to a small extent compared to this. Therefore, by adjusting the amount of eccentricity according to the characteristics of the optical system, the nature of moire, etc., the effect of removing moire can be obtained.

FIG. 13 shows a fourth embodiment, in which the roof angle α is set to 90° as an optical low-pass filter for a color camera.
This uses a roof prism that is made smaller by θ. In the fiberscope I, 21 is an eyepiece l. In the color camera 2, 22 is a roof prism placed near the aperture 16. According to this, the light beam is divided into two points separated by the following distance a on the image plane, and the effect is equivalent to that of a quartz filter that separates the same points.

a=4nl θ l cos ψ−S−β
----(10) Here, n is the refractive index of the roof prism 22, ψ is the angle between the ridgeline of the roof prism 22 and a plane perpendicular to the optical axis, and S is the image obtained by the lens in the front group of the roof prism 22. The distance β from the exit surface of the guide fiber bundle 4 to the image as an object is the lateral magnification of the optical system after the roof prism 22.

FIG. 14 shows a fifth embodiment, in which an imaging lens 23 having strong positive distortion is used as the imaging lens of the color camera 2.

The imaging lens 23 uses an aspherical surface 25 whose refracting effect on the chief ray 24 becomes weaker toward the periphery.

That is, in fiberscopes, rigid endoscopes, etc., strong negative distortion generally occurs in the objective lens, so in order to reduce this in the imaging lens 23, the imaging lens 23 is provided with positive distortion.

In this case, it is not appropriate to place a crystal filter as an optical low-pass filter just in front of the solid-state image sensor IO.

This is because the image of the fibers in the image guide fiber bundle 4 is greatly enlarged around the periphery by the imaging lens 23, so even if a crystal filter is placed just in front of the solid-state image sensor 1O, only a portion of the moiré image can be removed. It is from. Therefore, in this embodiment, the phase filter 26 or the polygonal lens 17 is placed in front of the aspherical surface 25, or the crystal filter 27 is placed in front of the aspherical surface where the light beams from one object point are not parallel. good.

[Effects of the Invention] As described above, the imaging device according to the present invention has the practically important advantage that the obstructive fiber arrangement pattern is not visible when the left TV camera is connected to a thin fiberscope to observe the image. are doing.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of a first embodiment of an imaging device according to the present invention;
Figure 2 is an explanatory diagram of 1 cpd, Figure 3 is M when observed with the naked eye.
A diagram showing changes in TF, FIG. 4 is an enlarged view of a main part of an exit end face image of an image guide fiber bundle, and FIGS. 5 and 6 respectively show the configuration and cross section of an example of the optical low-pass filter of the first embodiment. Figure 7 is a diagram showing the MTF of the optical low-pass filter on a two-dimensional frequency plane, and Figure 8 is a diagram showing the MTF when the optical low-pass filter is rotated around the optical axis on a two-dimensional frequency plane. 9 shows the second embodiment, FIGS. 1O and 11 show the cross section and front view of the polygonal lens of the second embodiment, and FIGS. 12 to 14 show the second embodiment, respectively. It is a figure which shows 3rd thru|or 5th Example. 1...Fiber scope, 2...Color camera, 3...Objective lens, 4...Image guide fiber bundle, 5, 23...Imaging lens, 6, 8...
・・Optical low-pass filter 7・・Infrared light cut filter 9・・・・Mosaic filter IO・
...Solid-state image sensor, 11...Camera control unit, 12...TV monitor 13.14...
... Crystal plate, 15 ... Wave plate, 16 ... Diaphragm, 17 ... Multifaceted lens, 18 ... Three-color separation prism, 19.20 ... Lens system, 21 ...eyepiece, 22 ... roof prism, 24 ... principal ray, 25 ... aspherical surface, 26 ... phase filter 27 ... crystal filter spatial frequency 1 1- Figure 5'j! P6 figure 1 P71! i Spear 8m P9wi U 1P10 Spear 41m

Claims (1)

[Scope of Claims] An image guide fiber bundle, an imaging optical system that forms an image of an exit end face of the image guide fiber bundle, and an imaging element that receives an image of the exit end face, the imaging element being arranged in a direction perpendicular to the imaging element. The sampling frequency is N_y, and if the image sensor also performs sampling in the horizontal direction, the sampling frequency is N_x, and the basic spatial frequencies in the horizontal and vertical directions of the fiber arrangement pattern in the image of the exit end face are respectively When U_x and U_y, U_y<
In an imaging device that satisfies N_y/2 and/or U_x<N_x/2, the spatial frequency response of the imaging optical system is expressed as U_y and/or in the optical path from the exit end face of the image guide fiber bundle to the imaging element. Or, an imaging device characterized by disposing an optical element that makes U_x about 0.5 or less.