Abstract: An array of graduated prisms disposed in a circle for rotating an incident image, the prisms are disposed with their end faces at the periphery of the circle and their apices at the center. The end faces are all oriented in the same direction either clockwise or counterclockwise, depending on the direction of rotation.

Abstract: A compact optical imaging system that transmits light received from an object located at an image plane to an exit pupil to form an image comprises a first prism, a second prism, a converging reflective element, and a converging transmissive element. The first prism includes a first face, a second face, and a third face that faces the image plane. The included angle between the first face and the second face of the first prism is .alpha.. The second prism includes a first face facing the exit pupil and substantially parallel to the first face of the first prism, and a second face adjacent the second face of the first prism. The converging reflective element is adjacent the first face of the first prism. The converging transmissive element is on or adjacent the first face of the second prism. The included angle .alpha.

Abstract: Illuminated instruments in aircraft cockpits often create reflections from the canopy which interfere with the pilots vision. A wedge of optical material positioned adjacent the instrument retracts the light rays such that the canopy reflections are deflected away from the eyes of the pilot.

Abstract: A beam shaping optical system which is provided with a first wedge prism having two refractive surfaces defining a first principal section, and a second wedge prism having two refractive surfaces defining a second principal section that is parallel to the first principal section, wherein a reference ray of light forms incident angles at each of the refractive surfaces of the prisms satisfying the following condition (1): ##EQU1##

Abstract: Optical components that are designed to be coupled together in modular fashion to form an optical network. The components are shaped so that a plurality of components can be coupled together to form a light cube. The components are provided with mating means to permit easy coupling. The preferred form of component is a right-angled triangular prism that can be coupled together with a similar prism to form a cube.

Abstract: A color-mixing lens for use in a concentrator system and methods of manufacture and operation thereof. The color-mixing lens includes: (1) a light-transmissive substrate that receives broad spectrum light from a source, (2) a first plurality of prisms, located on the substrate, that refract and chromatically disperse the light received therein toward a first plurality of locations on an active region of a target cell, and (3) a second plurality of prisms located on the substrate that refract and chromatically disperse the light received therein toward a second plurality of locations on the active region. Relative dimensions of the first and second pluralities of prisms are preselected to cause the chromatically-dispersed light to mix and thereby increase a power output of the target cell by reducing inter-junction currents therein.

Abstract: A DMD illumination coupler, and a method of improving the wavefront quality of a transmitted illumination beam over the prior art using the invented DMD illumination coupler are disclosed. The invented DMD illumination coupler comprises an angularly selective reflective thin film. The invented DMD illumination coupler has the characteristics of high optical quality and cost effectiveness. The application of the invented DMD illumination coupler in holographic systems is also discussed.

Abstract: An optical system comprises a pentagonal roof prism and a transparent plate. A rear surface of the pentagonal roof prism is provided with a triangular recessed portion. The wall of the recessed portion is frosted. The transparent plate is a triangular plate, slightly smaller than that of the recessed portion, and is applied to the wall of the recessed portion. The obverse face of the transparent plate is a transparent or polished surface. The transparent plate is applied to the recessed portion with adhesive having photo-transmissibility. It is preferable that the adhesive does not shrink when hardening.

Abstract: High radiation concentration photovoltaic cell system including at least one optical structure in the form of a three-dimensional body having a first surface adapted to receive thereon photovoltaic cells and a second surface to be, at least indirectly, exposed to light radiation. The body of the optical structure has formed between its first and second surfaces a two-dimensional array of contiguous light radiation concentrators each in the form of an individual prismatic body portion. Each prismatic body portion tapers in two dimensions toward said first surface along the entire length of the concentrator. Each concentrator defines on the first surface a photovoltaic cell-attaching area to be aligned with an active portion of a single photovoltaic cell to be attached thereto.

Abstract: A solar energy conversion system, in which two separated arrays of refracting elements disperse incident sunlight and concentrate the sunlight onto solar energy converters, such that each converter receives a narrow portion of the broad solar spectrum and thereby operates at higher efficiency.

Abstract: For the purpose of providing a solar module which has a fixed type solar concentrator of high converging magnification, a solar cell 12 is installed on the bottom face of an extension 30 which is extended further from the apex of a V shape formed by a pair of prisms having a refractive index larger than that of air, and a mirror surface 18 is formed on the back side of an incident surface 20 on which sunlight 10 falls. The mirror surface 18 and the incident surface 20 are formed in such a manner that their distance widens toward the apex of the V shape. The sunlight 10 incident on the prisms 16 performs reflection on the mirror surface 18 and total internal reflection on the incident surface 20, respectively. After repeating such reflection, the sunlight 10 reaches the bottom of the extension 30, where it emerges as outgoing light 22 to the solar cell 12.

Abstract: A prism has a notch at a lower end of a third reflection plane of a pentagonal roof prism. The notch has a frosted wall, on which a transparent plate is adhered with adhesive. A miniature prism is adhered to the transparent plate with adhesive. The adhesive has photo-transmissibility, so that a transparent plane is formed between the pentagonal prism, the transparent plate and the miniature prism. Information displayed by an indication device is directed to an eyepiece optical system, via a mirror and the miniature prism.

Abstract: A transmission type optical deflector wherein a prism holder is supported by a yoke member to rotate about a principal axis through an elastic device. A wedge-shaped transmission prism is provided on the prism holder to transmit and refract light. The prism holder and yoke member are respectively provided with a coil and a permanent magnet secured thereto, to produce a rotational motion of the prism holder in the forward and reverse directions about the principal axis due to an electromagnetic function. The transmission prism is partly cut away at the thick portion thereof so that the center of gravity of the transmission prism is identical to the principal axis in a section in which the principal axis appears as a point and the transmission prism appears as a wedge-shape.

Abstract: An injection-molded roof prism has a roof surface. A positioning portion is provided for a positioning in a direction vertical to both a ridgeline formed at the roof surface and an optical axis bent at the ridgeline. The positioning portion is formed together with the ridgeline by using the same molding plate.

Abstract: A color display including a non-polarized light source, at least one polarizing beam-splitter receiving light from the non-polarized light source, at least one selectably actuable polarization rotating light valve having thereon impinging light from the polarizing beam splitter and operating in a reflective mode, the light valve having a plurality of separately controlled light valve regions which correspond to different colors, a display screen including a plurality of differently colored light impingement regions and imaging optics for directing light reflected from the at least one light valve via the at least one beam-splitter to the display screen such that light from individual ones of the plurality of separately controlled light valve regions which correspond to a given color impinges on correspondingly colored light impingement regions of the display screen.

Abstract: The present invention consists of two right angle prisms, one of which is coated with a wedged layer of low index of refraction transparent material such as magnesium fluoride. The coating varies in thickness, linearly, for example, from about 50 to 700 nanometers, for example. The prisms are cemented together forming a rectangular cube beamsplitter. The beamsplitter is then mounted onto a stage so that the plane of the cemented surfaces is parallel to the axis of translation. As one translates the cube along its axis, the ratio of reflectance to transmittance varies according to the thickness or other factors.

Abstract: Compositions and methods for replication are described. A method includes: providing a first submaster having a first topography; providing a carrier having a first side and a second side; coating the first side with a first resin having a first viscosity; pressing the first topography against the first resin; replicating the first topography in the first resin as a first replica surface; increasing the first viscosity; releasing the first replica surface from the first topography; providing a second submaster having a second topography; coating the second side with a second resin having a second viscosity; pressing the second topography against the second resin; replicating the second topography in the second resin as a second replica surface; increasing the second viscosity; and releasing the second replica surface from the second topography. The compositions and methods provide advantages in that surface topography is replicated with enhanced overall cost effectiveness.

Abstract: The present invention includes a structured optical film with variable pitch peaks and/or grooves to reduce the visibility of moire interference patterns and optical displays incorporating one or more layers of the film. The pitch variations can be over groups of adjacent peaks and/or valleys or between adjacent pairs of peaks and/or valleys.

Abstract: A brightness enhancement film has a plurality of linear prisms. The prisms are disposed in pairs, each pair having first and second prisms and each prism has a prism angle and a valley angle. Either the prism angles or the valley angles, but not both, are equal.

Abstract: To generate a stereoscopic image of an object (41), two light beams (151, 161) unparallel to each other are used to back light a portion (43) of the object (41). The two light beams (151, 161) are deflected to form two deflected light beams (153, 163) substantially parallel to each other. The deflected light beams (153, 163) form a stereoscopic image of the portion (43) of the object (41). A camera (46) records the stereoscopic image of the object (41). The signal from the camera (46) is processed by a vision computer (48) to reconstruct the stereoscopic image and determine whether the object (41) meets design specifications.

Abstract: A method and apparatus for providing sequential temporal and spatial integration of a collimated non-symmetrical excimer laser beam to optimize the temporal and spatial characteristics of the beam. The temporal integrator comprises a pair of cylindrical lenses spaced along the beam axis by a distance substantially equal to the sum of the focal length of both lenses, and a motor mechanism for rotating the two spaced cylindrical lenses about the beam axis. The spatial beam integrator includes a plurality of prisms distributed about a hollow center, the outlet face of each prism being angled with respect to the body axis of the spatial beam integrator so that portions of the laser beam passing through a given prism are refracted towards the center upon emergence from the outlet face.

Abstract: A projection-type display apparatus in which the entry of light rays to the back side of liquid crystal light valves from dichroic prism that composes optical system light synthesizer is prevented. To this end, a projection display according to the present invention preferably includes a color synthesizer for combining colored and modulated light beams, comprising: a square column dichroic prism composed of four triangle prisms having equal refractive index together whose cross sections are right isosceles triangles such that three light valves are located adjacent to three exterior peripheral surfaces of dichroic prism to receive colored and modulated light beams therefrom and; a filter for absorbing light with a specified wavelength range, leaked from surface of dichroic prism at the opposite side of incident light surface and directed to liquid crystal light valve, the filter being located between liquid crystal light valves and surface of dichroic prism.

Abstract: An endoscope includes a disposable probe and a non-disposable focusing ocular. The probe includes a transfer module assembly including one or more relay modules, and an objective element including a distal glass window, a molded plastic prism and three molded plastic lenses. Each of the relay modules contains an entry glass rod, an intermediate glass rod, and an exit rod, each having flat end surfaces and two identical molded plastic doublets. The elements of the ocular are all of glass and include an axially movable focusing doublet and two non-movable doublets. The disposable probe introduces certain aberrations into the image. The non-disposable focusing ocular corrects these aberrations. A single curved surface is formed on the distal glass window of the probe; all other curved optical surfaces of the disposable probe are formed on molded plastic members, thereby reducing the cost of the probe sufficiently to be cost-effective for single patient, disposable use.

Abstract: A non-deviating prism system (40) and method provides continuously variable dispersion of light with limited deviation. Light is dispersed in a prism arrangement which includes a selected liquid filled in the spaces between adjacent prism elements (41, 51). The indices of refraction and the apex angles of the prisms (41, 51) and the intervening liquid are set to limited deviation according to predetermined relationship. The prisms are mounted near the edges of mutually insertable tubes (62, 64) and a refractive liquid is secured in a containment space bounded by the prism elements (41, 51).

Abstract: An improved optical mounting apparatus permits a single kinematically adjustable surface to be presented in either a horizontal or vertical orientation with respect to a support. A three point fastening system allows cylindrical optical elements of various sizes to be fastened to the kinematically adjustable surface in a horizontal orientation. The three point fastening system is removable so that when the kinematically adjustable surface is placed in a horizontal orientation transparent optical elements such as prisms can be fastened to the surface with an unobstructed optical path for a full 360.degree. around the mounting surface. The apparatus for mounting optical elements comprises a support, base plate, a pivot member, two adjustable projections, a stage plate, and at least three removable fasteners. The base plate includes first and third opposing faces and a second face at right angles to the first and third faces.

Abstract: There is provided an optical apparatus which has excellent assembly performance, can suppress an increase in cost, and attain a decrease in thickness. The optical apparatus includes a board (1) having a photographing optical system and a mechanical system mounted on its surface. A plurality of opening portions (1a, 1b, 1c) are formed in the board (1). The opening portion (1a) is sealed with a sealing member such as a glass member to allow a light beam to pass and prevent dust and the like from entering the storage space for parts between the board (1) and a shield case (49). The photographing optical system mounted on the board (1) includes a stop unit (30) for adjusting the light amount of an object image guided through the opening portion (1a), and prism-like optical members (G1, G2, G3, G4), each consisting of, e.g., a glass or plastic material and having a sculptured surface as a reflecting surface.

Abstract: A light deflection apparatus includes an optical device for deflecting light, and a spherical guide for guiding the light deflected by the optical device.

Abstract: A prism sheet having a plurality of substantially triangular prism-shaped lens units with their longitudinal axes almost parallel to each other arranged on one side and having the other side roughened, and with a characteristic value of 100 when said roughned side is smooth, the characterstic values are not less than 50 in glossiness, not more than 70 in internal reflective ability, and not more than 70 in friction coefficient when the plane including the prism peaks is rubbed by the roughened surface. The prism sheet of the present invention is therefrom free from interference fringes, substantially safe from scratching and uniform and high in brightness, being thus useful for liquid crystal devices etc.

Abstract: The invention relates to a method of manufacturing prisms, particularly microprisms and beam-splitting prisms, including the following steps: a) producing rectangular plates (1), b) fixing a larger number of these plates at their plate surfaces (2) to form a rectangular block (4), c) processing at least one face (support face 5) of the block at a right angle relative to the plate surfaces, d) rearranging the plates of the blocks so that all plate surfaces are aligned at an angle .alpha..sub.1 and that the sharp edges of the plates are placed in contact with the alignment plate (6), e) cementing of several displaced blocks with their cover faces (11) to a carrier body and grinding and polishing the support face (5), f) multiple repetition, if necessary, of steps d) through e) with different angles adjustments .alpha., g) rearranging the plates of the blocks so that all plate surfaces are arranged at an angle .beta.

Abstract: There is provided an optical apparatus which has excellent assembly performance, can suppress an increase in cost, and attain a decrease in thickness. The optical apparatus includes a board (1) having a photographing optical system and a mechanical system mounted on its surface. A plurality of opening portions (1a, 1b, 1c) are formed in the board (1). The opening portion (1a) is sealed with a sealing member such as a glass member to allow a light beam to pass and prevent dust and the like from entering the storage space for parts between the board (1) and a shield case (49). The photographing optical system mounted on the board (1) includes a stop unit (30) for adjusting the light amount of an object image guided through the opening portion (1a), and prism-like optical members (G1, G2, G3, G4), each consisting of, e.g., a glass or plastic material and having a sculptured surface as a reflecting surface.

Abstract: The invention relates to a stereoendoscope having a sight line folded relative to the tube axis. The endoscope includes a distal front optic having a deflecting prism 4b. The deflecting prism 4b is mounted between a component 4a having a negative refractive power and a proximal component 4c having a positive refractive power. The inlet pupils 11 of the viewing component are imaged demagnified in the reflecting prism 4b by field optics (7, 7', 13) and transmitting optics (8, 8', 14). In this way, the beam can be guided free of vignetting without field cropping for both stereo channels through the endoscope tube and through the reflecting prism 4b even for a large object field angles and without image cropping. The endoscope tube 2 and the viewing component 1 are rotatable with respect to each other about the optical axis 12 within the endoscope tube 2 so that the line of sight can be changed for a viewing component fixed in space. The stereo basis remains for a rotation of the endoscope tube 2 fixed in space.

Abstract: The present invention relates to an optical instrument such as an optical axis deflecting device or an image stabilizing device having an optical member supported for rotation about a predetermined axis and rotated about the predetermined axis to thereby deflect the optical axis of passing light, moving means adapted to move in a direction along a plane inclined by a predetermined angle from a plane perpendicular to the predetermined axis to thereby rotate the optical member about the predetermined axis, and driving means for driving the moving means. With such construction, a driving force generated by the driving means acts on that portion of the optical member which is far from the rotational shaft thereof and as a result, a driving torque for driving the optical member can be enhanced without causing the bulkiness and higher cost of the device and an increase in consumed electric power.

Abstract: An image display apparatus which enables observation of a clear image at a wide field angle and which is extremely small in size and light in weight. The apparatus includes an image display device (6) for displaying an image, and an ocular optical system (7) for leading the image displayed by the image display device (6) to an observer's eyeball (1). The ocular optical system (7) has a first surface 3, a second surface 4, and a third surface 5. A space formed by the three surfaces is filled with a transparent medium having a refractive index larger than 1. Light rays emitted from the image display device (6) pass through the third surface (5) and are internally reflected by the first surface (3) and further internally reflected by the second surface (4), which is decentered and has positive power. The reflected light rays pass through the first surface (3) and are led to the observer's eyeball (1). The angle .theta..sub.

Abstract: An optical system is disclosed for improving the brightness symmetry of a beam emitted from a laser diode where the beam has a large width and a narrow height. The optical system includes a tilt plate for displacing one half of the width of the beam downwardly. A first beam steering prism functions to tilt the remaining, second half of the beam width in a plane parallel to the width dimension so that the second half travels towards the first half. A second beam steering prism is provided to tilt the second half of the beam so that its propagation axis is parallel to the propagation axis of the first half and wherein the second half is stacked above the first half. The optical system functions to improve the brightness symmetry of the beam by a factor of about five. The corrected beam can be used to improve the performance of a solid state laser which is end pumped by a broad area laser diode or a laser diode bar.

Abstract: The prevent invention relates to an optical device capable of varying the angle formed by a pair of optical elements. The optical device comprises a pair of support members having portions for preventing the optical elements from slipping outwardly, and for supporting the optical elements, a connection member for forming a watertight space, the connection member being adhesively secured to the support members and being deformable, and a transparent substance filling the watertight space.

Abstract: Binoculars provided with first and second objective lens barrels respectively having objective lenses at an end, first and second eyepiece lens barrels respectively having eyepiece lenses and adapted to slide respectively on the other end portions of the first and second objective lens barrels, prisms positioned between the objective lenses and the eyepiece lenses and provided respectively in the first and second objective lens barrels, and a focusing mechanism provided with a focusing member and adapted to displace the first and second eyepiece lens barrels in the axial direction by the rotation of the focusing member, wherein each of the prisms is composed of a roof prism having an entrance face for receiving the light beam from the objective lens and an exit face for introducing the received light beam to the eyepiece lens, the exit face being so formed as to be loosely insertable in the eyepiece lens barrel and the periphery of the entrance face being fixed in the objective lens barrel so as that the exit

Abstract: An optical projector is described that orients its optics engine preferably vertically within an enclosure that is higher than it is wide or deep. The invented optics engine uses a prismatic cube for color separation or beam splitting, with the prismatic elements within the cube being secured within a durable frame that mounts the three color light valves, e.g. twisted nematic liquid crystal display (LCD)-type shutters, in a predefined position and orientation relative to the prismatic cube. In order to improve image quality and brightness, especially of the notoriously difficult-to-reproduce green color, red and blue beams are S polarized, as is conventional, but the green beam is P-polarized.

Abstract: A hollow, cylindrical light conduit for transporting light generally longitudinally therethrough and distributing light therealong includes a cylindrical wall member of a transparent material. The wall member has an inner cylindrical smooth surface and an outer cylindrical structured surface having an array of isosceles prisms arranged side by side thereon. The faces of adjacent prisms extend longitudinally along the cylindrical wall member and are aligned at the same desired prism angle relative to one another. The light transporting performance of the light conduit, achieved by total internal reflection, and the light distributing performance of the light conduit, specifically relative to radially aligned, high incident angle longitudinal light rays in planes parallel or near parallel to the prisms, is optimized as a function of the desired prism angle between the faces of adjacent prisms.

Abstract: An optical mount secures an optical component so that its freedom of movement is restricted in all directions. The mount uses a spring retainer assembly to urge the optical component against various faces of the mount. The mount is especially well suited for work with crystals, such as frequency doubling or optical parametric oscillation crystals.

Abstract: An achromatic optical system for beam transformation is used for example with the optical head of an optical disk apparatus. This optical system is arranged in an optical path which is traversed by a beam emitted from a light source to reach a surface to be irradiated. This optical system includes a first prism arranged on the side of the light source and a second prism arranged on the side of the surface to be irradiated. The first and second prisms are constructed in such a manner that each of the prisms has a beam incident angle which is greater than its beam exit angle so as to expand the width of a beam passed therethrough and that with respect to a plurality of transmitted beams of different wavelengths the deviation angle of the beam passed through the optical system on the whole (the angle formed by the incident beam on the first prism and the emerging beam from the second prism) becomes the same for each other.

Abstract: The invention is directed to a confocal incident light microscope for viewing an object. The microscope includes: an illuminating device for transmitting an illuminating beam along an illuminating beam path; a viewing optical assembly for defining a viewing beam path; an optical unit for joining the illuminating beam path and the viewing beam path to form a common beam path; a perforated mask arranged in the common beam path at an angle .delta. to a plane perpendicular to the common beam path; an imaging optic arranged between the perforated mask and the object; a first prism having a first wedge-shaped section arranged in the common beam path between the optical unit and the perforated mask; the first wedge-shaped section defining a wedge angle .alpha.

Abstract: Rotary beamsplitter prism mount providing individually adjustable rotation about three axes where adjustment about one axis does not significantly affect the remaining axis with respect to beam emergence.

Abstract: A double Porro prism having an incident surface, an exit surface, and a plurality of reflecting surfaces for internally reflecting light rays from the incident surface to the exit surface. The plurality of reflecting surfaces having a final reflecting surface that is positioned near the exit surface. The double Porro prism also has a ghost preventing surface that is formed between a ridge of the final reflecting surface and a ridge of the exit surface.

Abstract: An image stabilizer has, in succession from the object side, an objective lens group having at least positive refractive power, variable angle prism having its vertical angle variable, and optical prism, and which satisfies the following condition:-20<.nu..sub.VAP -.nu..sub.RP <40,when .nu..sub.VAP is the Abbe's number of the variable angle prism, and .nu..sub.RP is the mean value of the Abbe's number of the optical prism.

Abstract: A dispersive optical element called a "grism" (50) which has characteristics of both a prism (56) and a grating (62). The grism (50) consists of a prism (56) with a grating (62) disposed adjacent to one surface of the prism (56). Light passing through the grism (50) is dispersed by both the prism (56) and the grating (62). The grating (62) may be attached to either the first or second surface of the prism (56) or may be simply adjacent to the prism (56). The grism (50) has dispersive characteristics such as resolving power that can be optimized in a very flexible manner by choice of both the grating (62) and prism (56) characteristics. For example, the grating (62) may be used to amplify the angular spread introduced by a prism (56). Also different diffractive orders of the grating (62) may be used simultaneously. The result is a device with a very flexible and improved dynamic range of its resolving power, which is particularly useful in applications such as spectrometers.

Abstract: A method and apparatus for providing sequential temporal and spatial integration of a collimated non-symmetrical excimer laser beam to optimize the temporal and spatial characteristics of the beam. The temporal integrator comprises a pair of cylindrical lenses spaced along the beam axis by a distance substantially equal to the sum of the focal length of both lenses, and a motor mechanism for rotating the two spaced cylindrical lenses about the beam axis. The spatial beam integrator includes a plurality of prisms distributed about a hollow center, the outlet face of each prism being angled with respect to the body axis of the spatial beam integrator so that portions of the laser beam passing through a given prism are refracted towards the center upon emergence from the outlet face.

Abstract: Endoscopic instrument having an elongated image guide adapted to be inserted through an opening in the human body. A prism is provided at one end of the image guide for directing images from a predetermined field of view through the image guide and means for. By tilting the prism relative to the image guide the field of view from which images are directed through the image guide is changed to provide continuous scanning for viewing and examining different features of internal cavities and hollow organs of the human body.

Abstract: An optical system is disclosed for improving the brightness symmetry of a beam emitted from a laser diode where the beam has a large width and a narrow height. The optical system includes a tilt plate for displacing one half of the width of the beam downwardly. A first beam steering prism functions to tilt the remaining, second half of the beam width in a plane parallel to the width dimension so that the second half travels towards the first half. A second beam steering prism is provided to tilt the second half of the beam so that its propagation axis is parallel to the propagation axis of the first half and wherein the second half is stacked above the first half. The optical system functions to improve the brightness symmetry of the beam by a factor of about five. The corrected beam can be used to improve the performance of a solid state laser which is end pumped by a broad area laser diode or a laser diode bar.

Abstract: An apochromatic grism (42) includes a first prism (48) with a second prism (50) disposed adjacent to it. A grating (44) is adjacent to the front surface of the first prism (48) and the Abbe number and dispersion coefficient of the grating are chosen such that the primary and secondary angular spread for a given spectral band is approximately zero. The apochromatic grism is a simple and compact device with minimal amounts of residual chromatic and anamorphic aberrations.

Abstract: A non-deviating prism system (40) and method provides continuously variable dispersion of light with limited deviation. Light is dispersed in a prism arrangement which includes a selected liquid filled in the spaces between adjacent prism elements (41, 51). The indices of refraction and the apex angles of the prisms (41, 51) and the intervening liquid are set to limited deviation according to predetermined relationship. The prisms are mounted near the edges of mutually insertable tubes (62, 64) and a refractive liquid is secured in a containment space bounded by the prism elements (41, 51).