Television Camera for Endoscopes Provided With an Optical Low-Pass Filter

(a) Field of the Invention:

The present invention relates to a television camera, and more particularly it relates to an improvement of the television camera which can be used suitably with an endoscope or like devices.

(b) Description of the Prior Art:

For example, endoscopes include a non-flexible endoscope which uses a relay lens system serving as the optical system intended to lead, to an ocular unit or eyepiece, the image formed by the objective lens, and a fiberscope using an image guide fiber bundle. Arrangement is provided so that, in case an image obtained by such an endoscope as mentioned above is to be observed through a TV monitor, an image pickup is performed by a television camera which is attached to the eyepiece of the endoscope. In such a case, however, there have to be prepared on hand various types of adapters for selective use with non-flexible endoscopes and fiberscopes, or for respective types of fiberscopes for replacement to be suitable for respective fiberscopes, which have been quite troublesome to operate. Also, in television cameras in general, the arrangement of picture elements of an image pickup device, scanning lines, color separation stripe filter, etc. form regular lattices, so that in case an object to be photographed contains regular lattice portions, there develops moire on the TV monitor screen due to interference, so that arrangement has been provided usually to provide an optical low-pass filter between the photographic lens and the image-forming surface to thereby prevent the development of moire. It should be noted here, however, that this low-pass filter is selected so as to be effective for general photography by the television camera, i.e. for photography of landscapes, persons and the like, so that in case a photography is performed by attaching a TV camera to a fiberscope having a structure that the core portion of the image guide fiber bundle is of a regular arrangment, there has been the inconvenience that, because the exit end face of the image guide fiber bundle has a regularity, it has not been possible to sufficiently prevent the occurrence of moire phenomenon.

For the reasons mentioned above, it has been the practice in the past to use an adapter which has, incorporated therein, only a photographic lens for such an apparatus as a non-flexible endoscope, and also to use, in case of a fiberscope, an adapter in which are incorporated a photographic lens and a low-pass filter intended to eliminate moire. Besides, in view of the fact that different moires apt to develop depending on the size of the respective constituent fibers of the image guide fiber bundle or on the type of the ocular, there has to be prepared, on hand, adapters in which are incorporated different low-pass filters, respectively, for each type of fiberscopes. Thus, there has been the problem that the prior art devices are very poor and troublesome inoperability.

Also, if it is made possible to switch one filter to another to meet various circumstances of use not only in endoscopes but also in television cameras and in electronic still-cameras in general, it is possible to photograph an object always with a highest degree of resolution, and thus this would be convenient.

It is, therefore, the primary object of the present invention to provide a television camera which is of the arrangement to effectively eliminate moire without requiring the preparation, for handy use, of a number of different kinds of adapters.

According to the present invention, this object is attained by the arrangement that a plurality of optical low-pass filters having mutually different optical characteristics, for example, spatial frequency responses, are provided within a camera body so that either one of them is selectively placed in the photographic optical path by manual operation externally of the camera body, or by the arrangement that a plurality of optical low-pass filters having mutually different optical characteristics are arranged on the photographic optical path within the body of camera so that at least one of these optical low-pass filters is rotated around the optical axis by external manual operation.

Another object of the present invention is to construct the television camera of this type in a compact size as much as possible.

Still another object of the present invention is to provide a television camera which provides an image of a high resolving power without intensifying moire elimination effect more than necessary.

Yet another object of the present invention is to provide a television camera which can be suitably used for various types of endoscopes.

These and other objects of the present invention will become more apparent during the course of the following detailed description and appended claims.

FIG. 1 is a diagrammatic illustration showing an example of the state of use of an embodiment of the television camera according to the present invention;

FIGS. 2A, 2B and 2C are diagrammatic illustrations showing the directions of double refraction of the three crystal plates constituting an optical low-pass filter;

FIG. 3 is a diagrammatic illustration showing the orientation of the crystal axis of a crystal plate;

FIGS. 4 and 5 are a sectional view and a pespective view showing a concrete structural example of the low-pass filter changeover mechanism;

FIG. 6 is a diagrammatic general illustration showing a second embodiment of the television camera according to the present invention;

FIG. 7 is a diagrammatic front view of the turret plate of the embodiment shown in FIG. 6;

FIG. 8 is a diagrammatic perspective view showing a concrete example of the turret plate;

FIGS. 9A, 9B and 10A, 10B are diagrammatic illustrations showing other structural examples of optical low-pass filters;

FIG. 11 is a diagrammatic illustration showing a third embodiment of the television camera according to the present invention;

FIGS. 12A and 12B are diagrammatic illustrations showing the directions of double refraction of the crystal plates of FIG. 11;

FIG. 13 is a diagrammatic sectional view of an essential portion showing a fourth embodiment of the television camera according to the present invention;

FIG. 14 is a diagrammatic illustration showing the orientations of the crystal axes of the crystal plates constituting the filter used in the embodiment of FIG. 13;

FIGS. 15 to 17 are diagrammatic illustrations, respectively, showing the action of separating the light by the crystal plates;

FIG. 18 is a diagrammatic illustration showing the image of the end face of the image guide;

FIG. 19 is a diagrammatic illustration showing the relationship between the orientations of the crystal axes of the first crystal plate and the image of the end face of the image guide;

FIG. 20 is a diagrammatic illustration showing the orientations of other crystal axes which the first to the third crystal plates area able to possess;

FIG. 21 is a diagrammatic illustration showing the state of separated light in FIG. 20;

FIG. 22 is a diagrammatic illustration showing the angle .theta. of the crystal axis of the third crystal plate;

FIG. 23 is a chart of MTF curve of the double refraction filter; and

FIGS. 24 and 25 are diagrammatic representations showing modified examples of the embodiment shown in FIG. 11.

The present invention will hereunder be described in further detail based on the embodiments shown in the drawings.

In FIG. 1, reference numeral 1 represents a flexible endoscope which is constructed in the form of, for example, fiberscope; 2 an adapter attached to the ocular unit of the endoscope 1, wherein a photogaphic lens 3 is incorporated; 4 a television camera body removably mounted rearwardly of the adapter 2, and this television camera body is provided therein with a beam splitter 6 having a half-mirror for splitting the path of light to a finder optical system 5 comprising a mirror 5a, a relay lens 5b and an ocular 5c, and to a photographing optical system, an image pickup device 7, an attachment plate 8 provided between said beam splitter 6 and said image pickup device 7, and optical low-pass filters 9 and 10 attached to said attachment plate 8 for being movable in the direction indicated by an arrow. It should be noted here that the low-pass filter 9 is comprised of three crystal or birefringent plates 9a, 9b and 9c arranged in this order starting with the beam splitter 6 side. The respective crystal plates are formed in such a way as shown in FIGS. 2A, 2B and 2C when viewed on the image pickup device 7 side, i.e. these crystal plates 9a and 9c perform splitting of a light bundle in the directions of arrows shown in FIGS. 2A and 2C due to double refraction, and have such a crystal axis orientation .theta. as shown in FIG. 3 about the normal of the surface of the crystal plate, wherein the amount of double refraction becomes maximum at .theta..apprxeq.45.degree. when the crystal plates have a same thickness. That is, in order to obtain a same amount of double refraction, the crystal plates will have a smallest thickness when the orientation of crystal axis is .theta..apprxeq.45.degree. and this is advantageous and useful in making the television camera compact in size. The crystal plate 9b is a 1/4 wave plate. also, the optical low-pass filter 10 is made thinner than the optical low-pass filter 9, so that said low-pass filter 10 has a cemented glass plate (transparent plate) 10a for having a same optical path as that of the low-pass filter 9. FIGS. 4 and 5 show a concrete structural example of the low-pass filter changeover mechanism, in which reference numeral 11 represents an attachment frame for holding the low-pass filters 9 and 10, and this attachement frame is arranged to be movable to insure the respective low-pass filters 9 and 10 to be allowed to be selectively placed into the photographic optical path between the attachment plate 8 and a guide frame 12 having an L-shaped cross section and attached to said attachement plate 8, and also said attachment frame 11 has a pin 11a. Numeral 13 represents a lever secured to an axis 14 which, in turn, is rotatably supported on said attachement plate 8. This lever 13 has a bifurcated end portion 13a which loosely nips the pin 11a of the attachment frame 11, and also has a stopper pin 13b set on the other end portion of this lever for restricting the rotation of the lever 13 as said stopper pin 13b engages in a slot 8a formed through said attachment plate 8. Numeral 15 denotes a ball fit in a hole 13c formed through said lever 13 and urged toward the left side in FIG. 4 by a spring 16, and is arranged to act in such a way that, in the state that the stopper pin 13b is brought into contact with either one marginal end of the slot 8a of the attachment plate 8 allowing either the low-pass filter 9 or 10 to be placed in the photographic optical path, the ball 15 drops into a click hole 8b or 8c both of which are formed through the attachement plate 8 to thereby anchor the lever 13. Numeral 17 represents a side wall of the television camera body. An end of the axis 14 passes through this side wall 17 to protrude beyond thereof to the outside thereof, and it has an operating lever 18 for causing rotation of the lever 13. It should be noted here that a color separation prism not shown may be mounted on the guide frame 12.

The first embodiment of the present invention is constructed as described above. Therefore, let us here assume that, from the state in which the ball 15 engages in the click hole 8c as shown in FIG. 5, and in which the stopper pin 13b is in contact with a marginal end of the slot 8a of the attachment plate 8, and in which, accordingly, the optical low-pass filter 10 is placed in the photographic optical path, the operating lever 18 is rotated in the direction of the arrow shown in FIG. 5 to cause the stopper pin 13b of the lever 13 to be brought into contact with the other marginal end of the slot 8a of the attachment plate 8. Whereupon, the ball 15 engages in the click hole 8b, causing the optical low-pass filter 9 to be placed in the photographic optical path. When, from the above-mentioned state, the operating lever 18 is rotated in the reverse direction, the mechanism is returned to the illustrated state, i.e. it is set back to the state in which the low-pass filter 10 is placed in the photographic optical path. Thus, by rotating the operating lever 18, two kinds of low-pass filters 9 and 10 can be selected arbitrarily externally of the camera body in accordance with the type of the adapter 2 which is to be attached thereto.

FIGS. 6 and 7 show a second embodiment of the present invention. Reference numeral 20 represents a turret plate provided with four phase type optical low-pass filters 21, 22, 23 and 24 having mutually different moire eliminating effects, respectively. This turret plate is arranged so that, by rotation thereof, these respective low-pass filters 21, 22, 23 and 24 can be selectively placed in the photographic optical path. These respective low-pass filters are produced by irregularly providing, onto each of these glass plates (transparent plates), dots of film by vapor deposition for causing a phase difference of about .pi.. The sum area of these dot portions of vapor-deposition films occupies about one half of the area of the glass plates. The larger the size of the dots is, the smaller becomes the moire elimination effect, and the smaller the size of the dots is, the greater becomes the moire elimination effect. By selecting the size of these dots so as to meet the non-flexible endoscope as well as a plurality of endoscopes in which the diameter of fibers cnstituting the image of the exit end face of the image guide to be formed on the image pickup device 7 by the photographing lens 3 is different, it is possible to obtain an image of a high resolution without requiring to intensifying the moire eliminating effect more than necessary. Here, the reason why the size of the dots is selected so as to meet the diameter of the fibers constituting the image of the exit end face of the image guide of the endoscope is because the intensity of moire varies depending on the diameter of the fibers. FIG. 8 shows an actual structural example of the turret plate 20. The center of the turret plate 20 is rotatably secured to the television camera body 4. On the circumference of the turret plate 20 are formed notches 20a, 20b, 20c and 20d at positions corresponding to the optical low-pass filters 21, 22, 23 and 24, respectively. Arrangement is provided so that a foremost end portion 25a of a click spring 25 which is attached to the television camera body 4 is engageable with the notches 20a, 20b, 20c and 20d, respectively. It should be noted here that a portion of the circumference of the turret plate 20 protrudes to the outside of the television camera body. According to this structure, that portion of the turret plate 20 protruding to the outside of the television camera body 4 is operated manually to cause the engagement of the click spring 25 selectively with either one of the notches 20a, 20b, 20c and 20d so that a desired optical low-pass filter 21, 22, 23 or 24 can be selectively placed in the photographic optical path.

FIGS. 9A to 10B show other examples of optical low-pass filters which can be used in the above-described embodiment. That is to say, FIGS. 9A and 9B represent an example which is intended to use selectively two crystal or birefringent plates 41, 41 having a same thickness but having crystal axis orientations .theta..sub.R and .theta..sub.F about the optical axis, respectively, to vary the intensity of double refraction. As the .theta..sub.R and .theta..sub.F depart more from 45.degree., the amount of double refraction decreases accordingly, causing progressive reduction of moire elimination effect. FIGS. 10A and 10B represent another example which is intended to use selectively a filter 31 which is formed by a single crystal or birefringent plate, and a filter 32 which consists of a first optical low-pass filter as designated by 9 in FIG. 1 and a second optical low-pass filter as designated by 21 in FIG. 7 disposed in front of the first low-pass filter. As the moire elimination effect is large when the filter 32 consisting of a phase type optical low-pass filter and crystal or birefringent plates as shown in FIG. 10B is inserted in the photogaphing light path, if the filter 31 shown in FIG. 10A and the filter 32 shown in FIG. 10B are selectively used, the moire elimination effect will be able to be varied to a great extent.

FIG. 11 shows an embodiment which is of the construction such that, between a beam splitter 6 and an image pickup device 7, there are disposed a fixed crystal or birefringent plate 41, a movable filter 42 which can be selectively placed in the photographic optical path, and a glass plate (transparent plate) 43. The filter 42 is comprised of a 1/4 wave plate 42a and a crystal or birefringent plate 42b. The 1/4 wave plate 42a is disposed in such an orientation that the beam of light emitting from the crystal plate 42b is rendered to a substantially circularly polarized light beam. The directions of separation of the light bundle due to double refraction of the crystal plates 41 and 42b are as shown in FIGS. 12A and 12B. According to this arrangement, while the filter 42 is being placed in the photographic optical path, the crystal plate 41 overlaps the filter 42, so that this embodiment functions in the same way as does the filter 9 of the first embodiment, whereas when the glass plate 43 is inserted in the photographic optical path, only the crystal plate 41 functions as the optical low-pass filter. Here, the glass plate 43 is arranged so that its refractive index and thickness are selected to insure that its equivalent air optical path is equal to that of the filter 42.

FIG. 13 shows a filter 45 of crystal or birefringent plates which can be provided on a turrent plate 20 in place of the phase type optical low-pass filters 21 to 24 which are provided on the turret plate 20 in the second embodiment. This filter 45 consists of three crystal or birefringent plates 46, 47 and 48 which are superimposed one upon another in the direction of the optical axis. The respective crystal plates are arranged in such a way that those crystal axes of the second and third crystal plates 47 and 48 lying within the planes crossing the optical axis at right angle assume substantially 45.degree. and 60.degree., respectively, relative to the crystal axis of the first crystal plate 46, and that thus the light beam separation pattern rendered by the filter 45 acquires a substantial resemblance with the dispositional pattern of the fibers locating at the end face of the image guide.

According to the filter 45 thus constructed, the incident light is divided first into two beams of light having an equal intensity by the first crystal plate 46 within a plane normal to the optical axis (FIG. 15), and then they are divided into four light beams of an equal intensity by the second crystal plate 47 (FIG. 16), and further they are divided into eight beams of light of non-equal intensities by the third crystal plate 48. In FIGS. 15 to 17, the short line passing through each black dot indicates the direction of polarization of the light beam. Also, letters a, b and c each represents the amount of separation of light beam done by the first to third crystal plates 46, 47 and 48, respectively. Also, the intensity of the light beam indicated by the double circles of FIG. 17 is to be understood as being by far the greater than the intensity of the light beam indicated by the black dot, with the ratio being about 14:1. Also, the letter d indicates the distance between the light beams marked by double circles. That is, the light beam separation pattern is formed at the sites of the double circles. The second crystal plate 48 is provided in order to take the balancing of the amounts of light of the two polarized light components (represented by double circles). The displacement of light beam by an amount "b" as shown in FIG. 16 is not important. Accordingly, the thickness of the second crystal plate 47 can be selected within a range of not being too thick, but it should be noted that this second crystal plate 47 desirably has a relatively small thickness. The reason why the crystal axis of the third crystal plate 48 assumes 60.degree. relative to the crystal axis of the first crystal plate 46 is because of the following considerations. That is, because the segments of lines connecting the centers of those adjacently located fibers of the image 49 of the image guide at its end face assume about 60.degree. relative to each other as shown in FIG. 18, it is intended to make the light beam separation pattern have a substantial resemblance with the dispositional pattern of the fibers located at the end face of the image guide.

Accordingly, if the inter-central distance of respective adjacent fibers of the image 49 of the image guide is assumed here to be P as shown in FIG. 18, the respective thicknesses of the first to third crystal plates are selected by utilizing known formula to acquire the following relationships: ##EQU1## and such crystal plates are disposed between the image pickup device 7 and the adapter 2 as shown in FIG. 13. Whereby, those frequency components causing moire phenomenon with the image pickup device 7 are removed out of the incident light, so that there is obtained a picture which is free of moire. It should be noted here, however, that the orientation of the crystal axis of the first crystal plate 46 is assumed to be parallel with the rectilinear lines connecting the centers of the fibers located at the image 49 of the image guide as shown in FIG. 19. In this way, there is performed the elimination of moire. Since, however, the low-pass filter according to the present invention is constructed by crystal plates, there is a very little loss of low-frequency components, and there hardly arises a degradation of the image quality. Also, the orientations of the crystal axes of the first to third crystal plates 46, 47 and 48 may be as shown in FIG. 20. The light beam separation in this instance will become as shown in FIG. 21. Also, the material of the first to third crystal plates 46, 47 and 48 allows the use of any other substances in place of crystal provided that these other materials possess double refractivity. Next, the above-mentioned Formula (2) is considered by generalizing same. To begin with, if the angle of the crystal axis of the third crystal plate 48 located within the plane crossing the optical axis at right angle relative to the crystal axis of the first crystal plate 46 is designated as .gamma., the distance d in this instance and the .PSI. shown in FIG. 17 (which is the smaller angle of the interior angles of a parallelogram formed by combining the separation of light beams which have relatively higher intensity among eight separation light beams emitted from the third birefringent plate are expressed by the following formula (3): ##EQU2## Therefore, if b, c and .gamma. are selected so as to obtain d=P/2, .PSI.=.+-.60.degree. or .+-.120.degree., the optical low-pass filter which satisfies the above will display a better performance than the optical low-pass filter which satisfies the above formula (2). Practically, it is enough to select a, b and c so as to satisfy the following conditions: ##EQU3## for the d and .PSI. which are defined by formula (3). The extent of the above formula (4) is determined substantially in the following manner. The MTF curve of the double refraction filter takes substantially such a configuration as shown in FIG. 23, and it can be expressed substantially by MTF= cos 1/2.pi.fP. In this case, the cut-off frequency f.sub.c =1/P, so that if the above embodiment is referred to, the relationship of "a" to "d" is exactly a=d=P/2. 1/3P.about.2/3P represents a range that MTF becomes about 1/2 centering around the cut-off frequency f.sub.c. When MTF becomes so small up to this order, moire will disappear on the whole. Also, the condition of .vertline..PSI..vertline. is being selected within the extent in which the value of MTF can eliminate moire to a practically unharmful extent.

FIG. 24 shows the arrangement that, in the embodiment of FIG. 11, the glass 43 is excluded and that, out of the filter 42, the 1/4 wave plate 42a is provided so as to be able to rotate around the optical axis, and that the crystal plate 42b is fixed, and that, by adjusting the rotation of this 1/4 wave plate 42a, the moire elimination effect can be varied. It should be noted here that moire elimination effect can be varied also by the arrangement that the 1/4 wave plate 42a is fixed while the crystal plate 42b is rendered rotatable.

FIG. 25 shows an embodiment wherein phase type optical low-pass filters 50 and 51 are disposed between a beam splitter 6 and an image pickup device 7. At least either one of the low-pass filters 50 and 51 is rotatable about the optical axis, and by adjusting the rotation thereof, the moire elimination effect can be varied. In this case, by arranging the low-pass filters 50 and 51 to be positioned close to each other and by arranging the location of the dots of these two filters so as to be identical in each filter, and by making the total area of the dots portions of these respective filters to be either 25% or 75% of the surface area of these filters, a moire elimination effect similar to that by the filter 21 of FIG. 7 can be obtained when the two low-pass filters 50 and 51 are locally overlapped in a displaced fashion. Also, when the two low-pass filters 50 and 51 are overlapped without any displacement relative to each other, moire elimination effect will almost disappear, and thus, by the rotation of the two low-pass filters 50 and 51, a necessary moire elimination effect can be achieved.

Description has been made above with respect to the television cameras having an optical finder in the above-mentioned embodiments. However, the present invention is not limited to such a television camera, but needless to say the present invention can be applied also to television cameras for endoscope having no optical finder. Further, the present invention can be equally effectively applied also to ordinary television cameras and to electronic still cameras.