JPH0289017A - Image pickup system - Google Patents

Abstract

PURPOSE:To keep an incident angle of a principal ray to an image surface constant even if a position of an exit pupil is varied by providing a visual field lens whose focal distance is variable in the vicinity of the image surface of an image forming lens provided with a brightness diaphragm. CONSTITUTION:In the rear of a concave lens 11, a brightness diaphragm 12, an intermediate lens 13, a focusing lens 14 being movable in the optical axis direction are placed in order and they constitute an image forming lens 15. The focusing lens 14 is moved forward and backward by drawing it by a nearby operating part through a wire 17, etc., and in its rear, a variable focus visual field lens 18 consisting of a liquid crystal lens, and an incident end face of an image guide fiber bundle 19 are placed. Accordingly, focusing is executed by moving forward and backward the focusing lens 14 in two stages of a far point side and a near point side by aligning it with whether an object is far or near, and in this case, a position (exit pupil position) of an image of the brightness diaphragm 12 of the image forming lens 15 is fluctuated, but by following it up, a focal distance of the visual field lens 18 is varied. In such a way, even if the exit pupil position is varied, an incident angle to the image surface can be kept constant.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention is an I-optimum imaging system equipped with a field lens with a variable focal length, and is particularly suitable for color TV cameras and fiberscopes. Regarding.

[Conventional technology]

The following phenomenon occurs in the single most image system of a color TV camera.

+11 As shown in Fig. 15, the angle of incidence of light from the imaging lens l to the solid-state image sensor 2 increases as it moves away from the optical axis, and the reflection on the surface of the image sensor 2 increases accordingly. Even if the brightness is uniform, the image signal obtained from the first imaging element 2 becomes smaller as it goes from the center to the periphery.

(2) As shown in FIG. 16, if the color filter array 3 provided in front of the solid-state image sensor 2 and the light-receiving surface of the hot-water image element 2 are apart, some picture elements 2a However, a portion of the light transmitted through the filter 3a corresponding to the adjacent picture element 2a is mixed in, causing color crosstalk, and a clear color image cannot be obtained.

(3) When the color filter array 3 is an interference filter, the spectral transmittance varies depending on the incident angle of light;
The color tone changes between the center and the periphery of the screen.

Therefore, in order to solve these problems, a field lens is installed near the image plane so that the light is incident perpendicularly to the image plane at both the center and the periphery of the screen (in other words, all principal rays are aligned with the optical axis). An imaging system is known (Japanese Unexamined Patent Publication No. 58-223969) in which the image guide fiber bundle is parallel to its axis and transmits light parallel to its axis with the least amount of loss. , it is desirable that the light be incident perpendicularly to the end face.

[Problem to be solved by the invention]

However, when the imaging lens 1 is moved back and forth along the optical axis 9 for focusing, the diaphragm also moves, and as a result, the distance between the exit pupil and the field lens changes. There is a problem in that the angle of incidence of the principal ray on the image plane changes depending on the back and forth movement of the image plane. Furthermore, when the imaging lens l is a zoom lens, the same problem occurs because the exit pupil moves in response to zooming.

In short, when using an imaging lens in which the position of the exit pupil is not fixed, there is a problem that the angle of incidence of the principal ray on the image plane is not constant, which is undesirable.

SUMMARY OF THE INVENTION In view of the above-mentioned problems, it is an object of the present invention to provide an imaging system that can maintain a constant angle of incidence of a principal ray onto an image plane even if the exit pupil position changes.

[Means for Solving the Problems] The imaging system according to the present invention is characterized in that a field lens with a variable focal length is provided near the image plane of an imaging lens equipped with an aperture diaphragm.

Here, a variable focus lens is one in which the focal length of the lens element itself changes. Such lenses include those that change the focal length by changing the refractive index due to the electro-optic effect (a phenomenon in which the refractive index changes when an electric field is applied) of lenses formed of liquid crystal 2 ferroelectric crystals, etc., and lenses that are formed of transparent resin. There are known lenses that change the focal length by applying an external force to the lens and causing it to elastically deform.

(Function) According to such a configuration, by changing the focal length of the field lens according to a change in the position of the exit pupil, the exit angle of light from the field lens can always be kept constant.

〔Example〕

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

FIG. 1 shows a first embodiment of the present invention, which is configured as an imaging optical system for an endoscope. A concave lens 11 that also serves as a cover glass is provided on the end face of the endoscope tip FS.
are provided, and an aperture stop 12 . is provided in order behind the concave lens ll. An intermediate lens 13 and a focusing lens 14 movable in the optical axis direction are arranged, and these are imaging lenses! A polarizing plate 16 is arranged between the concave lens 11 and the aperture stop 12. The focusing lens 14 can be moved back and forth by pulling it with a hand control unit via a wire 17 or the like, and behind it there is a variable focal field lens 1B made of a liquid crystal lens, and an incident end surface of the image guide fiber bundle 19 ( image plane) is arranged. Variable focus field lens 1
8, as shown in FIG. 2(A), at least one of the two pieces is curved glass, transparent #Ii with acrylic ring;
20.

A transparent electrode 21 and an alignment film 22 are respectively coated on mutually opposing surfaces of the liquid crystal 20, and a nematic liquid crystal 23 is sealed in a convex lens-shaped cavity (cell) formed by the opposing surfaces. Variable focus field lens 1
The focusing lens 1 is attached to the transparent electrode 21.
An AC power source P is connected through a switch SW that turns ON or OFF in response to a change in the position of 4.
In the state shown in A), the SW is OFF and no voltage is applied. In this state, the molecular arrangement of the liquid crystal 23 is as shown in Figure 2 (
As shown in B), the molecules have a homogeneous arrangement in which the direction in which the long axes of the molecules are arranged (rubbing direction) coincides with the polarization direction of the polarizing plate 16.

Therefore, the refractive index of the liquid crystal 23 becomes a high refractive index state with respect to the incident light that has passed through the polarizing plate 16, and the field lens 18
focal length becomes shorter. Furthermore, when the SW is ON and a voltage is applied, the molecular arrangement of the liquid crystal 23 becomes a homeotropic arrangement, that is, an arrangement in which the long axis direction of the molecules is orthogonal to the polarization direction of the polarizing plate 16, so that the liquid crystal 23 refracts the incident light. The focal length of the field lens 18 becomes longer.

Further, an eyepiece lens 24 is arranged behind the exit end face of the image guide fiber bundle 19, and together with the above members, constitutes an observation optical system. Also, light a25. Light guide fiber bundle 26.
An illumination optical system consisting of an illumination lens 27 is arranged.

Since this embodiment is constructed as described above, in FIG.
1ffi, it becomes linearly polarized light that vibrates in the vertical direction (direction parallel to the plane of the paper), and is transmitted to the intermediate lens 13, the focusing lens 14. It passes through a field lens 18 and is imaged onto the incident end face of an image guide fiber bundle 19. and,
Focusing is performed by moving the focusing lens 14 back and forth in two steps, one to the far point and one to the near point, according to the distance of the object. At this time, the brightness aperture 1 of the imaging lens 15 is adjusted.
Although the position of the image 2 (exit pupil position) changes, the focal length of the field lens 18 is changed accordingly, so that the front focal position is always made to coincide with the image position of the aperture diaphragm 12. If this is done, the chief ray passing through the image position will always be parallel to the optical axis due to the field lens 18 and will be incident perpendicularly to the incident end surface of the image guide fiber bundle 19. Therefore, the image guide fiber bundle 1
When transmitting light using 9, the loss of 1 tJl of light is the least.

In the above embodiment, when focusing is performed by continuously moving the focusing lens 14 back and forth, as shown in FIG. The voltage applied to the variable focus field lens 18 may be continuously changed by using a variable resistor VR whose value changes.
As shown in the figure, the variable focus field lens 18 may be constructed from a Fresnel lens.

FIG. 5 shows a second embodiment, which is a field lens 1
In addition to 8, the imaging lens 15 is also composed of a similar liquid crystal lens, and can be moved back and forth in the optical axis direction for focusing.
), when the imaging lens 15 is located on the rear side and has a large refractive power, the refractive power of the field lens 18 is increased, and as shown in FIG. 5(B), when the imaging lens 15 is located on the front side and when the refractive power is small, the refractive power of the field lens 18 is made small,
The front focal position of the field lens 18 is always made to coincide with the exit pupil position of the imaging lens 5.

FIG. 6 shows a third embodiment, which is shown in FIG. 6(A).
As shown in FIG. 1, the entrance end surface of the image guide fiber bundle 19 is formed into a concave surface, and the field lens 18, which is a liquid crystal lens, is constructed by using a transparent plate 20 on the entrance end surface. Here, at the input end surface of the image guide fiber bundle 19, the direction in which the fibers are arranged is as shown in FIG.
), there are directions of <60', so a better image can be obtained by aligning the rubbing with one of these directions and orienting the liquid crystal molecules in that direction.

FIGS. 7(A) and 7(B) respectively show a fourth embodiment and a modification thereof, in which a variable focus field lens 18 is arranged in front of a solid-state image sensor 29 equipped with a color filter array 28. In this way, the light can be filtered through a color filter.
Since the light is always perpendicularly incident on the array 28 and the solid-state image sensor 29, color starch talk is prevented and a clear color image can be obtained. In the case of FIG. 7(A), there is an aperture 12 inside the imaging lens 15, and in the case of FIG. 7(B), there is an aperture 12 inside the imaging lens 15.
There is an aperture 12 in front of the.

FIGS. 8(A) and 8(B) respectively show the fifth embodiment and its modification, and these have the same structure as the fourth embodiment, with two crystal plates 30 and 30 sandwiched between them. An optical low-pass filter 32 consisting of a single λ/4 plate 31 is added to prevent moiré. Note that a depolarization plate may be used instead of the λ/4 plate 31.

In the case of the example shown in FIG. 8(B), since the optical low-pass filter 32 is located in front of the field lens 18, a depolarizing plate 33 and a polarizing plate 34 are placed between the low-pass filter 32 and the field lens 18. is intervening.

FIG. 9 shows a sixth embodiment, in which a television camera 36 having a built-in solid-state image sensor 29 is externally attached to an eyepiece 35 of an endoscope or a rigid scope. In this case, the eyepiece lens 24 and the objective lens 37 constitute an imaging lens, and the exit pupil of the endoscope corresponds to the aperture of the imaging lens. Therefore, since the exit pupil position of the objective lens 37 of the television camera 36 changes due to the exit pupil position differing from endoscope to endoscope, the exit pupil position of the objective lens 37 of the television camera 36 changes. In order to do this, the variable focus field lens 18 is placed in front of the solid-state image sensor 29.

FIG. 10 shows a seventh embodiment, in which an imaging lens is constructed from two convex lenses 38 and 39 made of liquid crystal lenses, a polarizing plate 16, an aperture 12, and a concave lens 40. As shown in FIG. 11(A), the convex lens 38
The refractive power of the convex lens 39 is large (switch SW1 is 0FF) and the refractive power of the convex lens 39 is small (switch SW z is ON).
Therefore, when the exit pupil position of the imaging lens is close to the field lens 18, the refractive power of the field lens 18 is increased (switch SW is set to 0FF), and the convex lens 38 is set as shown in No. 11 [J (B). refractive power is small (switch SWl is O)
N) and the refractive power of the convex lens 39 is large (switch S
When the exit pupil position of the imaging lens is far from the field lens 18 because W8 is 0FF), the field lens 18
The refractive power of the image guide fiber bundle 19 is made small (the switch SW is turned on) so that the light always enters the incident end face of the image guide fiber bundle 19 perpendicularly.

In each of the above embodiments, the focusing lens 14 in the imaging lens 15 may be composed of a variable focal length liquid crystal lens as shown in FIG. Since it can be shared, the loss of light quantity is reduced, which is advantageous. Furthermore, if a λ/2 plate 41 is arranged between the focusing lens 14, which is a liquid crystal lens, and the field lens 18, as shown in FIG. Since the same effect can be obtained, wiring may be easier in mounting. Further, as shown in FIG. 14, the aperture diaphragm 12 has ring-shaped transparent electrodes 21 and 21, and the orientation directions are 90 degrees from each other.
° orientation II! Nematic liquid crystal 42 between 2222
It may also be composed of a liquid crystal cell 43 and two polarizing plates 44 and 45 arranged before and after the liquid crystal cell 43 whose polarization directions are orthogonal to each other. In that case, the polarizing plate 45 is Since it can be shared, the loss of light quantity is reduced, which is advantageous.

〔Effect of the invention〕

As described above, the imaging system according to the present invention has the practically important advantage that the angle of incidence on the image plane can be kept constant even if the exit pupil position changes.

[Brief explanation of drawings]

FIG. 1 is a sectional view of the first embodiment of the imaging system according to the present invention, FIG. 2 is an explanatory diagram of the main parts of the first embodiment, and FIGS. 3 and 4 are modifications of the main parts of the first embodiment, respectively. Figures showing examples, Figures 5 to 9
The figures show the main parts of the second to sixth embodiments, respectively. Figures 1O and 11 show the seventh embodiment and its operation, and Figures 12 to 14 respectively show the above-mentioned components. Figures 15 and 16, which show a modification of the main part of the embodiment, are diagrams showing problems in the conventional example, respectively. 11.40...Concave lens, 12...Aperture diaphragm, 13...Intermediate lens, 14...Focusing lens, 15...Imaging lens, 16.34.44
45...Polarizing plate, 17...Wire 18...
...Variable focus field lens, 19...Image guide fiber bundle, 20...Transparent plate, 21...Transparent electrode, 22...Alignment film, 23.42...Nematic liquid crystal , 24...Eyepiece, 25...Light source, 26...Light guide fiber east, 27...
...Illumination lens, 28...Color filter array, 2
9...Solid-state image sensor, 30...Crystal plate, 31...
...λ/4 plate, 32... Optical low-pass filter 33... Depolarization plate, 35... Eyepiece, 3
6...TV camera, 37...Objective lens, 3
8.39...Convex lens, 41...λ/2 plate, 4
3...Liquid crystal cell. t3 figure spear 4 figures 1-5 refraction n small refraction ten thousand small spears 8v! 1 1F9 picture spear 40 figure 1-fiml t12 figure 1'1311

Claims (1)

[Claims] An imaging system comprising an imaging lens having an aperture stop and a variable focal field lens provided near an imaging plane of the imaging lens.