JPH0380677A - Image pickup device - Google Patents

Abstract

PURPOSE:To obtain a change in an MTF non rotationally symmetric with respect to an optical axis by making one lens face of an objective lens eccentric with respect to the optical axis of the objective lens so as to decrease the MTF. CONSTITUTION:An objective lens 6 of a TV camera 2 consists of a convex lens 10, a concave lens 11 and a convex lens 12 in the order of the object and a face 11a of the concave lens 11 is made eccentric by DELTA with respect to the optical axis of the objective lens 6. Then a uniform coma aberration is caused to the image forming face of the objective lens 6, that is, the entire photodetection face of a solid-state image sensor 7 as the aberration of the eccentric system to reduce the high frequency spatial frequency response (MTF). Thus, the change in the MTF due to the change in the diaphragm diameter is little to obtain a change in the MTF in non rotationally symmetry with respect to the optical axis.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention uses an image receiving means such as an image guide fiber bundle or a solid-state image pickup device that spatially samples an object image, so that moiré and aliasing are avoided.

The present invention relates to an imaging device equipped with an imaging optical system in which cross color etc. are likely to occur.

[Conventional technology]

A well-known example of an imaging device having this kind of conventional moiré removal means is one using an optical low-pass filter made of a birefringent plate such as a quartz plate.

Further, as an attempt to remove moiré using a type of soft focus lens, there is a method described in, for example, US Pat. No. 4,720,637.

Further, as an apparatus for removing moiré by generating spherical aberration, there is a method described in Japanese Patent Laid-Open No. 1-151880.

[Problem to be solved by the invention]

However, devices using quartz plates or the like have the drawbacks of being expensive and large in size. Also, in order to change the MTF (spatial frequency response), it is necessary to change the thickness of the plate, which is time-consuming and requires polishing plates with slightly different thicknesses. The drawback was that it was difficult to change.

On the other hand, the lens system described in U.S. Pat. By increasing the lens diameter, the MTF of the lens system itself for high spatial frequencies is lowered, and the absolute value of spherical aberration due to higher-order aberrations remains approximately constant regardless of the size of the diaphragm aperture. Although the MTF of the lens system is kept constant regardless of changes in the aperture diameter, it has the disadvantage that it is difficult to manufacture because it uses an aspherical surface.

Furthermore, the lens described in JP-A-1-151880 removes moiré by increasing the spherical aberration and lowering the MTF of the lens system itself without using an aspherical surface (using only a spherical lens). , this has a disadvantage that the amount of aberration changes with NA due to the small amount of high-order aberrations, so changing the aperture diameter changes the MTF of the lens system, making it easy to change the MTF. Also, the MTF is rotationally symmetrical about the optical axis.
There is also the problem that this is not sufficient to remove moiré from a solid-state image sensor, which typically has different horizontal and vertical fundamental spatial frequencies.

In view of the above-mentioned problems, the present invention is compact, low cost, easy to change MTF characteristics, and has the ability to reduce MTF by changing the aperture diameter.
It is an object of the present invention to provide an imaging device that can achieve changes in MTF that are rotationally asymmetric with respect to the optical axis with little change in MTF.

[Means and effects for solving the problems] An imaging device according to the present invention includes an objective lens that forms an object image, and image receiving means that spatially samples the object image, such as an image guide fiber bundle or a solid-state image sensor. In the imaging device,
Moiré and the like are removed by decentering the lens surface of at least one of the lenses constituting the objective lens with respect to the optical axis of the objective lens to reduce the spatial frequency response.

Here, eccentricity includes both parallel deviation and inclination.

It is generally known that when the lens surface in a lens system is decentered with respect to the optical axis of the lens system, various aberrations called eccentric aberrations (--like coma, tilt of the image plane, eccentric distortion) occur. It is being Uniform coma
, Kubo 1) "Optics" by Kou, published by Iwanami Shoten, Chapter 3) occurs uniformly over the entire screen, and it is possible to blur the point image without distortion of the image or deviation of the image point in the optical axis direction. , this aberration can reduce the MTF of the lens system. Therefore, if such an eccentric system is adopted as an optical system for forming an object-to-image on a solid-state image sensor, etc., the conventionally used optical low-pass filter consisting of a crystal plate or the like can be completely eliminated or Even if some parts are omitted, false signals such as moiré can be effectively removed, which is extremely advantageous for downsizing the optical system. In addition, the manufacturing cost of eccentric systems is lower than that of quartz plates, etc., and the MTF characteristics are easy to change.

In addition, --like coma increases or decreases in proportion to the square of the entrance pupil diameter, so it causes spherical aberration and lowers the MTF (spherical aberration changes in proportion to the cube of the NA). ), it is advantageous in that the MTF changes less with respect to changes in the aperture diameter. Moreover, the −-like coma realizes an MTF that is rotationally asymmetric with respect to the optical axis.

〔Example〕

Hereinafter, the present invention will be described in detail based on an illustrated embodiment.

FIG. 1 shows a TV equipped with an embodiment of an imaging device according to the present invention.
FIG. 2 is a diagram showing a state in which a camera is attached to an eyepiece of a fiberscope, and schematically shows the area after the eyepiece. In the figure, a fiberscope I includes an image guide fiber bundle 3 and an eyepiece 4. Further, 5 is the exit pupil of the fiberscope l. The TV camera 2 attached to the eyepiece includes an objective lens 6 and a solid-state image sensor 7 arranged obliquely with respect to the optical axis of the objective lens 6. The light of the object image transmitted to the exit end face of the image guide fiber bundle 3 is turned into a substantially parallel light beam by the eyepiece 4 and exits the fiberscope 1, and then is directed onto the solid-state image sensor 7 by the objective lens 6 provided in the TV camera 2. An object image, that is, an image of the exit end face of the image guide fiber bundle 3 is formed.

The signal representing the object image converted into an electrical signal by the solid-state image sensor 7 is supplied to the camera control unit 8, undergoes predetermined signal processing, and is displayed on the monitor TV 9.

Here, the objective lens 6 of the TV camera 2 is a positive lens 10. Negative lens 11. Although it is composed of a positive lens 12, the object side surface 11a of the negative lens 11 is eccentric by Δ with respect to the optical axis of the objective lens 6.

If this is done, --like coma aberration will occur as decentering system aberration on the entire image forming surface of the objective lens 6, that is, the light receiving surface of the solid-state image sensor 7, and the high frequency MTF will decrease, resulting in an optical low-pass filter. A similar effect can be obtained.

FIG. 2 is a diagram schematically showing the point spread intensity distribution on the light-receiving surface of the solid-state image sensor 7, and shows that the point spread intensity distribution is not circular but has a trailing shape (not shown) peculiar to coma aberration. This indicates that it will be star-shaped. Therefore, in a state where a large amount of coma aberration occurs, the MTF of the optical system is not omnidirectionally symmetric (rotationally symmetric with respect to the optical axis) but has directionality. In this example, since the top has a downward tail, the MTF in the y direction decreases at a lower frequency than in the x direction.

Figure 3 shows the state of convergence of the light rays near the image plane in this embodiment, and shows that the upper ray intersects with the optical axis in front of the Gaussian image plane, and the lower ray intersects with the optical axis at the rear. There is.

FIG. 4 schematically shows the MTF for an object on the optical axis of this embodiment, where the solid line shows the y direction, the broken line shows the X direction, and the dashed line shows the change in MTF when there is no eccentricity. According to this, it can be seen that this eccentric system functions as an optical low-pass filter with a cutoff of approximately uo in the y direction. Furthermore, since the MTF characteristics are rotationally asymmetric with respect to the optical axis, moiré caused by interference between the fiber arrangement on the exit end face of the image guide fiber bundle 3 and the pixel arrangement of the solid-state image sensor 7 can be suppressed by reducing the resolving power to the minimum necessary. It can be removed with . Further, if the amount of eccentricity Δ is continuously changed, the MTF characteristic changes continuously from the dashed line in the figure toward the short wavelength side, so the MTF characteristic can be changed freely.

In addition, as a general property of a --like coma, in addition to the property that it occurs uniformly over the entire image forming surface, it also changes in proportion to the square of the entrance pupil diameter. An eccentric system is more advantageous than a soft focus system that uses spherical aberration, which changes in proportion to the cube of the change, because the dependence of the MTF on the entrance pupil diameter is smaller, that is, there is less change.

Further, --like coma tends to occur on refractive surfaces or reflective surfaces that are concentric with the diaphragm (the center of curvature of the surface is on the diaphragm side).

This is because when a non-concentric surface is decentered, high-order eccentric coma aberration occurs due to the asymmetry of refraction between the upper and lower rays, and the amount of coma generated within the imaging plane increases with the image height. This is because it changes with the Therefore, as a surface to be decentered, the three aberrations of the second-order eccentric system, namely --like coma, and the tilt of the image plane (=
It is better to select a surface that has less occurrence of image plane inclination and eccentric distortion among eccentric aberrations (eccentric field hawk curvature, eccentric astigmatism) and eccentric distortion, and has a large occurrence of --like coma.

In this embodiment, in addition to the --like coma due to eccentricity of the surface 11a (Fig. 1), a tilt of the image plane (unilateral blur) occurs, but this tilt of the image plane is linear with respect to the image height. Since it changes, the first
If the light-receiving surface of the solid-state image sensor 7 is tilted with respect to the optical axis as shown in the figure, the one-sided blur can be removed.

Next, a numerical example of the optical system of the imaging device of the present invention will be shown.

This lens has a configuration as shown in FIG. 5, and its lens data is shown in Table 1 below.

Table 1 f=loo, F/8,0, ω=7.1'63.5
794 d, =3.7099 n, =1.78472 v,
=25.71r 2=-87,8271 d2=2.1823 r+=■(Aperture) d+'' 1.0912 r, =-41,0842 d, =2.1823 n2=1.68893 v2=
31.08r s =33.6665 d i ” 14.8397 h =■ do =2.1823 ns =1.59270 νh
= 35.29r, = 35.6982 dy = 10.9116 n + = 1.640
00 v 4 = 60.09r, = -35,6982 d, = 3.2735 r 9 = ■ d = = 30.5523 n 5 = 1.51
633 v s = 64.150 BJ = -100
0, If (=13.0939.5K=104.588
.

SA=0.216. MST=0.6
19 However, f is the focal length of the entire system, F/ is the F number, ω is the half angle of view, rl is the radius of curvature of the first surface, and dl is the distance between the first surface and the (i-th
+1) The distance from the surface, n, is the refractive index of the optical element on the fourth surface,
ν is the Atsube number of the fifth element, OBJ is the object position, I
H is the image height, SK is the back focus, SA is the spherical aberration coefficient of the entire system, and M S 'r is the eccentric comatic aberration coefficient of the entire system.

In this case, the eighth surface r8 is inclined by 15'. When the cemented lens is decentered in this way, it is convenient because the amount of eccentricity can be adjusted during processing or the amount of eccentricity can be measured with a centering microscope or the like during assembly.

Table 2 below is an eccentricity correction coefficient table for this numerical example.

Table 2 M.S. O,00474 -0,00256 0,01904 -0,00748 -0,03057 0,02563 0,02762 0,02821 -0,01284 o, ooooo.

0100466 0.00449 0.04126 0.03986 0.00275 o, ooooo.

O,00243 o, ooooo.

DM O,00942 0,00510 0,01381 -0,00543 -0,01505 0,01264 0,02102 0,02146 0,00822 o, ooooo.

O,00398 0,00383 0,00539 0,00506 0,00526 o, ooooo.

O,00451 o, ooooo.

However, MS is the --like coma defined in Fig. 3, DM is the inclination (meridian direction) of the image plane, E is 1' when each surface is eccentric (tilted), and D is 0. This shows when each OI surface is eccentric (parallel misalignment).

In this numerical example, although the eighth surface r8 faces in a direction concentric with the aperture, it can be seen that the amount of --like coma MS generated is large and the image plane tilt DM is small. Further, in the solid-state image sensor 7, the value of DM caused by the tilt of the eighth surface r8 is -0.
,00539 The inclination DS of the image plane in the perpendicular direction is generally shown in Table 2.
The average of the inclinations of the image plane in the meridian direction and the direction perpendicular to it is 1/3 of DM, so the inclination θ of the solid-state image sensor 7 (Fig. 5) is θ=21.2' It becomes X2/3・14.1'. For practical purposes, it is acceptable to some extent.

FIGS. 6(A) and 6(B) show the MTF characteristics of this numerical example. FIGS. 6A and 6B show the MTF on the light-receiving surface of the solid-state image sensor 7 for an object on the optical axis when the eighth surface r8 is not eccentric and when it is 15' eccentric, respectively. Further, the solid line indicates MTF in the X direction, and the dotted line indicates MTF in the y direction. Further, DF is the distance from the final surface of the lens system (including the glass block) to the light receiving surface of the solid-state image sensor 7.

According to this, it can be seen that the MTF decreases in the high frequency region due to the eccentricity of the eighth surface rl, and the characteristics as an optical low-pass filter are exhibited. Also, the 8th side r
Since the eccentricity of lens 8 is as small as 15', it has the advantage that the lens system can be constructed compactly.

Value Example 2 This lens has a configuration as shown in FIG. 7, and its lens data is as shown in Table 3 below.

Table 3 f=46.526. F/7.9, ω=6.1°r,
= 00 (aperture) d, =0.3000 r 2-■ d2=1.0000 n1=1.51633 v
1=64.15r d, =7.0000 r + = 18.0030 d= =2.9300 n2=1.607291/2
=59.38rs ”'-372.7940 d5=6.8900 r e ”-18,1100 da =o, 5soons =1.67270 vs
=32.1Or? = 18.1100 d, = 2.3500 r s = 44.8390 d, = 1.8200 nt = 1.58297 v,
=46,33r 9 =-15,1920 do =1.2000 rho-■ d lo= 13.0000 n + = 1.5
1633 v + =64.15r −■ d,, =2.9000 r I2:00 dl. =1.6000 n7=1.51633
vt = 64.15r ■ = 0BJ = 500 18 = 4.9400, 5K = 16
．． 500I SA+ =0.028, l MS
T l =0.113 However, the meaning of each symbol is as shown in Numerical Example 1
is the same as

In this case, the sixth surface r6 is shifted in the +y direction by 0.15, and the seventh surface r7 is shifted in the -y direction by 0.06695 in parallel to the optical axis. That is, Δ,=0.15, Δ,=−o, 06695
It is. In this way, the reason why the two surfaces are eccentric is that the sixth
The inclination of the image plane caused by the surface r is offset by the eccentricity of the seventh surface r, and both surfaces r6. This is because the eccentricity of ry causes a −-like coma to occur. Therefore, in this example, the solid-state image sensor 7
can be arranged perpendicular to the optical axis.

Table 4 below is an eccentricity correction body number table for this numerical example.

2 3 4 5 6 7 l 9 Table 4 5 O900006 o, ooooo.

O,00006 o, ooooo.

0000232 0.00442 -0.00274 0.00025 0.00526 -0.00995 0.00288 0.00546 0.00185 0.00142 0.00424 M O,00032 o, ooooo.

O,00032 o, ooooo.

O,00604 0,01153 0,00033 0,00003 0,00369 -0,00698 -0,00825 -0,01564 0,00396 0,00304 0,00326 D O,009530,00731r,
E 0600061 -0.001
15D O,000000,00000r,
, E -0,000480,00079o
o, ooooo o, ooooo.

r, E 0,00041 −
0.00061D O,000000,000
00r, j E -0,000390,0
0056D O, 000000, 00000 However, the meaning of each symbol is the same as in Numerical Example 1.

FIGS. 8A and 8B show the MTF on the light-receiving surface of the solid-state image sensor 7 with respect to an object on the optical axis when there is no eccentricity and when the eccentricity is described above, respectively, in this numerical example. Also, the solid line is MT in the X direction.
F, the dotted line indicates the MTF in the y direction.

More generally, if the eccentricity (parallel deviation) of the i-plane is represented by the slope of Δ1 as El, and the corresponding eccentricity is expressed as DMl, DMl as the inclination of the image plane, a --like coma is obtained. The conditions for this occurrence are as follows: M S T =ΣMS Δ +ΣMS ≠ 0 (3). Also, the conditions for canceling the tilt of the image plane are DMT3Σ
DM, ・Δ1+ΣDM, ・El−0・・・(
4) It becomes. The reason why the sum Σ of the eccentric surfaces is taken here is that it is of course possible to eccentricize two or more surfaces.

Furthermore, if the amount of --like coma aberration caused by eccentricity is too small, the MTF cannot be lowered. Also, when the spherical aberration of the non-eccentric system is SA, MST + ≧-
13AI...(5) is required.

Further, the direction of eccentricity and inclination of each surface can be freely selected two-dimensionally within a plane perpendicular to the optical axis. Equation (3), (4
), (5) shows the case in the meridian plane, but if the eccentricity or inclination is included in two or more planes including the optical axis,
It is sufficient to consider equations (31, (4), and (5)) for each plane.

Alternatively, when the direction of eccentricity and inclination of each surface spans a plane containing many optical axes, instead of formula (3), M S
Tmi Σ MS, Δ1 +Σ MS, -E... (6
) can be used. Even in this case, equation (5) can be applied.

In addition, Δ1. There is an upper limit for E1. If Δ1 is too large, the off-axis luminous flux portion of the light beam will be eclipsed and will not pass through. Therefore, Δ1 has an upper limit, and 1Δ11≦EP
...(7). However, EP is the aperture diameter (radius). In an optical system without an aperture, this can be considered the entrance pupil diameter.

In the case of a spherical surface with radius R, Δ and El have the following relationship.

ΔE=・・・
(8) FIG. 9 shows an optical system of another embodiment, and this is an example in which the lenses 13 and 14 are decentered so that the MTF characteristics change with zooming of the zoom lens. Lenses 15 and 16 move in the optical axis direction during zooming, and constitute a zoom lens.

In this example, a crystal plate (filter) 17 is further added. This is due to the following reason. Although the lens system of the imaging device according to the present invention can reduce the high-frequency MTF over a wide frequency band, the point at which the MTF first becomes O is not very clear, and the MTF decreases gradually. Therefore, the decrease in MTF at low frequencies is somewhat large. On the other hand, in the case of a quartz crystal plate, the drop in MTF of low frequencies is small, but the rise of high frequencies is large. Therefore, if the two are combined as in this example, and the cutoff frequency of the crystal plate (or birefringent filter) is selected to be lower than the low MTF region of the eccentric lens system, the MTF characteristic as shown by the solid line in Figure 1O can be obtained. Thus, it is possible to reduce the drop in the low frequency MTF and reduce the high frequency MTF over a wide frequency band.Although spherical lenses are used in all of the embodiments, it is also possible to tilt the parallel planes. It is also possible to make the aspherical surface eccentric. Further, the aspherical surface is not limited to the optical axis symmetrical surface, but may also be a toric surface.

The imaging device according to the present invention is not limited to a television camera for an endoscope or a television camera for a rigid endoscope, but also an electronic camera.

It can be applied to VTR cameras, television cameras, and even fiberscopes and viewing scopes connected to fiberscopes.

Further, as shown in FIG. 9, the television camera body including the solid-state image pickup device and the lens group including the eccentric lens system may be separated along the dotted line, so that the lens group can be made interchangeable.

〔Effect of the invention〕

As described above, the imaging device according to the present invention is compact, inexpensive, easy to change MTF characteristics, has little change in MTF due to change in aperture diameter, and has an MTF that is non-rotationally symmetrical with respect to the optical axis.
It has a practically important advantage of being able to change the TF.

[Brief explanation of drawings]

FIG. 1 shows a TV equipped with an embodiment of an imaging device according to the present invention.
A diagram showing the camera attached to the eyepiece of a fiberscope, etc., FIG. 2 is a diagram schematically showing the point image intensity distribution on the light receiving surface of the solid-state image sensor 7 of the above embodiment, and FIG. A diagram showing the state of convergence of light rays near the image plane in the above embodiment, and FIG. 4 shows the MT for an object on the optical axis in the above embodiment.
FIG. 5 is a diagram showing the configuration of Numerical Example 1 of the optical system of the imaging device of the present invention, and FIGS. 6(A) and (B) are Numerical Example 1 with no eccentricity and eccentricity, respectively. Figure 7 is a diagram showing the configuration of numerical example 2, and Figures 8 (A) and (B) are diagrams showing the MTF on the light-receiving surface of the solid-state image sensor with respect to an object on the optical axis. A diagram showing the MTF on the light-receiving surface of the solid-state image sensor for an object on the optical axis when there is no eccentricity and when there is eccentricity, FIG. 9 is a diagram showing an optical system of another embodiment, and FIG. 10 is a diagram other than the above. It is a figure which shows the MTF characteristic of the Example. 1...Fiber scope, 2...TV camera 3...Image guide fiber bundle, 4...Eyepiece lens, 5...Exit pupil, 6...Objective lens,
7...Solid-state image sensor, 8...Camera control unit, 9...Monitor TV, 10.12...
...Positive lens, 11...Negative lens, lla...
Surface, 13° 14.15.16...Lens, 17...
...Crystal plate. Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 6 (A) (B) Figure 9 Figure 10

Claims (1)

[Claims] In an imaging device comprising an objective lens that forms an object image, and an image receiving means that spatially samples the object image, such as an image guide fiber bundle or a solid-state image sensor, at least one surface of the lenses constituting the objective lens is An imaging device characterized in that an objective lens is decentered with respect to an optical axis to reduce spatial frequency response.