Abstract: Disclosed is a diffractive optical element which includes a diffractive grating pattern on a base plate. The diffractive grating pattern includes a plurality of phase gratings arranged in parallel lines extending along a predetermined direction to cause diffraction of an incident beam. Each of the plurality of phase gratings has an asymmetrical phase pattern. There is a phase gap, &Dgr;P, representing a phase difference (in radians) between an end point and a beginning point of each phase pattern. The phase gap, &Dgr;P, is substantially equal for each of the plurality of phase gratings and satisfies the relationship. 0.7&pgr;<|&Dgr;P|<1.2&pgr;.

Abstract: A surface pattern comprises microscopically fine relief structures that diffract visible light. When the surface pattern is illuminated perpendicularly with white light, the surface pattern appears with bright and dark regions from a first viewing direction. The length and/or position and/or number of bright and dark regions changes as the viewing angle changes. Preferably, the contour of the surface pattern is selected so that the length or position of the bright regions changes markedly when the viewing angle changes.

Abstract: An optical element includes an incidence surface, one or more reflecting surfaces reflecting light from the incident surface, an emergence surface, and off-axial curved surfaces causing the light to emerge from the emergence surface. At least one of the incidence surface, the emergence surface and the one or more reflecting surfaces is a surface having diffracting action. The surface having the diffracting action is a curved surface.

Abstract: An optical system includes a diffractive optical element having a diffraction grating provided, on a lens surface having curvature, in a concentric-circles shape rotationally-symmetrical with respect to an optical axis. A sign of the curvature of the lens surface having the diffraction grating provided thereon is the same as a sign of a focal length, in a design wavelength, of a system composed of, in the optical system, a surface disposed nearest to an object side to a surface disposed immediately before the lens surface having the diffraction grating provided thereon, and is different from a sign of a distance from the optical axis to a position where a center ray of an off-axial light flux enters the lens surface having the diffraction grating provided thereon. Further, an apex of an imaginary cone formed by extending a non-effective surface of the diffraction grating is located adjacent to the center of curvature of the lens surface having the diffraction grating provided thereon.

Abstract: A diffractor for electromagnetic radiation is based on a pseudo-spherical stepped geometry designed under the constant step width conditions. The diffractor consists of a few small pseudo-spherical curved dispersive elements (oriented crystal surfaces or gratings) that are located on a focal circle. The location on the focal circle of each element is made to guarantee the same Bragg angle for the incident radiation. Thus a diffractor is an array of diffracting elements (“steps”), each one contributing to the total solid angle of the diffractor, that increase the spectral output of the device without decreasing the resolution.

Abstract: A delineator easily recognized by the driver even when attached at a moderate curve of a road includes colorless transparent reflectors attached inclined by a prescribed angle with respect to an aluminum base and amber transparent reflectors attached inclined in the opposite direction. Each reflector has a reflex reflecting element, an optical axis of which is inclined in the same direction as the corresponding reflector. Thus light beams entering in a direction parallel to the surface on which the delineator is attached can also be reflected.

Abstract: A method of fabricating a diffraction grating by utilizing a single substrate comprises the steps of forming a photosensitive material layer and a light transmission reducing film having a predetermined pattern integrally with each other on the substrate, exposing the photosensitive material layer by exposure irradiation light via the light transmission reducing film, and developing the photosensitive material layer after exposure. It is composed so that the direction of exposure and the direction of development are opposite to each other. It is possible to fabricate a diffraction grating in which each grating is formed on a predetermined substrate at a predetermined pitch and a root portion in a cross-section of each diffraction grating is constricted. In this way, it is possible to reduce or eliminate interfaces, so that the generation of noise light can be effectively suppressed and a diffraction grating having a high diffraction efficiency can be made.

Abstract: A diffractor for electromagnetic radiation with a spherical stepped geometry constructed under the constant step width conditions (here defined as pseudo-spherical geometry). The diffractor consists of a plurality of spherically curved dispersive elements (oriented monocrystals, crystal surfaces or gratings) that are located on a focal circle. The location on the focal circle of each element is made to guarantee the same Bragg angle for the incident radiation. Thus a diffractor is an array of diffracting elements (“steps”) each one contributing to the total solid angle of the diffractor, that increase the spectral output of the device without decreasing the resolution. Because the steps are spherically curved, they are curved also in the direction perpendicular to the focal circle in order to satisfy Bragg's law for diffraction over a maximum area of the diffractor.

Abstract: A display apparatus of the present invention includes: an illumination optical system for emitting illumination light; a reflective display panel divided into a plurality of pixels, illuminated by the illumination light from the illumination optical system, and selectively reflecting the incident illumination light pixel by pixel as projection light; a projection optical system for projecting image information the reflective display panel onto a projected surface by transmitting the projection light reflected at the reflective display panel; and a reflection angle converting optical system disposed ahead of the reflective display panel, changing the angle of incidence of the illumination light on the reflective display panel, and adjusting the sum of the angle of incidence of the illumination light on the reflective display panel and the angle of reflection of the projection light at the reflective display panel.

Abstract: A two-focal-point pickup device capable of suppressing the generation of glitch or bump in reproduced signal. The optical pickup device irradiates a light beam emitted from a light source onto an optical recording medium to form a light spot on a track of a recording surface of the optical recording medium to read the optical recording medium while controlling the position of the light spot relative to the track of the recording surface. The optical pickup device includes a converging lens section coaxially disposed in an optical axis of the light beam to converge the light beam onto the recording surface; and a hologram lens section coaxially disposed in the optical axis of the light beam to diffract the light beam.

Abstract: A two-focal-point pickup device capable of suppressing the generation of glitch or bump in reproduced signal. The optical pickup device irradiates a light beam emitted from a light source onto an optical recording medium to form a light spot on a track of a recording surface of the optical recording medium to read the optical recording medium while controlling the position of the light spot relative to the track of the recording surface. The optical pickup device includes a converging lens section coaxially disposed in an optical axis of the light beam to converge the light beam onto the recording surface; and a hologram lens section coaxially disposed in the optical axis of the light beam to diffract the light beam.

Abstract: A diffractive optical element including a stack of first, second and third optical regions or a stack of first, second, third and fourth optical regions, a first relief pattern formed between the first and second optical regions, and a second relief pattern formed between the second and third or between the third and fourth optical regions. The first and second relief patterns have substantially identical pitch distributions and are substantially aligned in a direction of an optical axis. Depths of the first and second relief patterns are set such that a wavelength dependency of diffraction efficiency can be decreased over a wavelength range to be used. It is possible to manufacture in a simple manner at a low cost a diffractive optical element in which undesired flares and ghosts are suppressed.

Abstract: There is provided an eyepiece lens system for viewing an image formed by an objective lens system. The eyepiece lens system includes a first refraction lens having a positive power, a second refraction lens having a positive power and at least one diffraction lens having a phase grating structure. The first and second lenses are arranged on an eye side with respect to a plane on which the image is formed.

Abstract: A beam splitting element includes a transparent substrate and a diffractive grating formed on the substrate for dividing an incident beam into a plurality of diffracted beams. The diffractive grating has a plurality of identically formed belt-shaped phase patterns that are arranged in parallel lines at equal pitches. The phase pattern has a sectional shape in a plane perpendicular to the parallel lines that causes non-linear phase difference to the beam passing therethrough within one pitch. Further, the adjacent phase patterns are connected without phase gaps at the boundaries therebetween.

Abstract: Disclosed is a scanning optical system including a light source that emits a light beam, a deflector that deflects a light beam emitted from the light source to scan in a predetermined direction, an scanning lens system that allows the light beam deflected by the deflector to pass through and converges the light beam on a surface to be scanned, and a diffracting element having a diffracting surface that functions to correct a chromatic aberration of the scanning lens. The diffracting element is arranged between the deflector and the surface to be scanned.

Abstract: An optical viewfinder system comprising two optical elements, and a field stop or a reticle, to provide an image of a scene to a viewer, and further comprising a diffractive optical element to provide a clear, stationary image of the field stop or reticle to the viewer.

Abstract: A two-focal-point pickup device capable of suppressing the generation of glitch or bump in reproduced signal. The optical pickup device irradiates a light beam emitted from a light source onto an optical recording medium to form a light spot on a track of a recording surface of the optical recording medium to read the optical recording medium while controlling the position of the light spot relative to the track of the recording surface. The optical pickup device includes a converging lens section coaxially disposed in an optical axis of the light beam to converge the light beam onto the recording surface; and a hologram lens section coaxially disposed in the optical axis of the light beam to diffract the light beam.

Abstract: Disclosed is a diffractive optical element which includes a diffractive grating pattern on a base plate. The diffractive grating pattern includes a plurality of phase gratings arranged in parallel lines extending along a predetermined direction to cause diffraction of an incident beam. Each of the plurality of phase gratings has an asymmetrical phase pattern. There is a phase gap, .DELTA.P, representing a phase difference (in radians) between an end point and a beginning point of each phase pattern. The phase gap, .DELTA.P, is substantially equal for each of the plurality of phase gratings and satisfies the relationship0.7.pi.<.vertline..DELTA.P.vertline.<1.2 .pi..

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: A diffractive optical element for diffracting non-collimated light includes a base surface and a periodic structure formed on the base surface and having a plurality of diffractive surfaces and a plurality of boundary surfaces. The boundary surfaces are separated into plural zones, and in each zone, the boundary surface is placed substantially parallel to a main portion of a light beam impinging on that zone.

Abstract: A kinoform optical element (KOE) has different surface relief heights in different zones of the kinoform surface, in order to optimize the first order diffraction efficiency in each zone. By increasing the first order diffraction efficiency, noise as well as ghost image problems may be reduced and contrast and resolution may be enhanced. In an exemplary embodiment, each zone, or even different areas of the same zone, has a non-constant step height that is preferably defined by a predetermined relationship between an optimal step height and an associated predominant angle of incidence associated with that particular zone or area. In the case of an optical system having a relatively large aperture and a relatively narrow field of view, each point of the KOE sees the entire image; the associated predominant angle of incidence at that point may be defined by a so-called "marginal" ray from the center of the entire image space (i.e., on the optical axis and at the effective "middle" of the usable depth of field).

Abstract: Diffractive indicia for a surface (1) comprising a plurality of small separate diffractive elements (2) and means for a adhering the diffractive elements to the surface. Each diffractive element has a diffractive surface relief structure and is not separately resolvable to the human eye. The appearance of the diffractive indicia, when applied to the surface, changes when the viewing angle and/or angle of illumination relative to the surface changes. In preferred arrangements, the diffractive indicia, are incorporated in an ink or a transfer medium. A palette of diffractive indicia with different characteristic colours and other optical properties may be provided.

Abstract: A magnifier lens comprises three lens elements. More specifically, from a front, eye side to a rear, object side there are a front positive lens element, a middle, negative power lens element and a rear, meniscus lens element. The front, positive power lens element has at least one aspheric surface. The front and the middle lens elements, in combination, contain at least one diffractive surface. The rear, meniscus lens element has front and rear refractive surfaces, both of which are concave toward the object side. The rear surface is positioned within 5 mm of an object to be viewed from the eye side. This rear surface is an aspheric surface of negative power that corrects field curvature.

Abstract: A diffractive optical element including a first layer made of a first optical material having high refractive index and low dispersion, and a second layer made of a second optical material having low refractive index and high dispersion. The first and second layers are stacked together. The outer surfaces of the first and second layers are substantially smooth, while the connecting surfaces of the first and second layers are substantially contoured to follow a relief pattern. To satisfy a preferred operating wavelength range of 400 nm to 700 nm, a glass selected from the group consisting of BSM7, BSM81, BSL7, BAL5, BAL50, BAL22, LAL11, LAL12, LAL18, YGH51, LAH57 and LAH75 may be used as the first optical material, and a silicate glass containing certain amounts (e.g., 2 to 25 mol %) of Tl.sub.2 O may be used as the second optical material.

Abstract: A waveguide input/output device for input and output of laser light L into a thin-film waveguide 2 via an input grating or prism 7 and an output or emission grating 8. The height of the emission grating 8 is gradually increased in a travel direction of a light advancing through the waveguide and either the height of the input grating 7 or the height of the output grating 8 is gradually increased in the direction of propagation of the service elastic waves in the waveguide 3 generated by a transducer element 3.

Abstract: A holographic scanner for scanning a surface with a light beam is provided. The holographic scanner includes a light source for emitting light used as the light beam, a front hologram for diffracting the light emitted from the light source to produce a first diffracted beam, a rotary hologram disk having a disk plane and rotatable around an axis thereof vertical to the disk plane, the rotary hologram disk including a plurality of scanning holograms arranged on the disk plane in a circumferential direction, the scanning holograms diffracting the first diffracted beam to produce a second diffracted beam, and a rear hologram for diffracting the second diffracted beam to produce a third diffracted beam, the third diffracted beam being used as the light beam for illuminating the scanning plane.

Abstract: An optical modulator (19) utilizes a variable beam splitter arrangement having two separate plano-periodic cylindrical lens arrays (20 and 22) arranged as a sandwiched air-spaced assembly with the plano surfaces facing outwardly from the assembly. The assembly further includes a means (26) for controllably translating or moving one of the lenses relative to the other. The periodic array is formed as alternating concave and convex cylindrical structures. In a nominal setting, the optical axes of the concave structures of one of the lens arrays are aligned with the optical axes of the convex structures of the other so that the assembly acts as an afocal window having little impact when placed in an optical system. To adjust the beam splitting properties, one lens array is slightly translated with respect to the other to cause controlled misalignment of the two lens array surfaces. The present invention is particularly suited for tuning the point spread function of an electronic imaging system.

Abstract: White light is input to light diffracting means so composed as to have a structure including macro prisms combined with a lenticular structure and include diffraction gratings at a microscopic level. As the height (h(x)) of unit steps of said diffraction grating is modulated at every period of arranged pixels, the output diffracted light is decomposed into three primary colors, and guided with highly accurate matching to the three respective primary color pixels of a liquid crystal panel means. In the pitch modulation type diffraction grating of the prior art, it was basically impossible to avoid the defect that the converged positions of a red light ray and a blue light ray deviate about 30%.

Abstract: A mother layer made of a thermoplastic resin is formed on the substrate. The mother layer is etched to make a grating layer. The etching process is stopped so that an offset layer remains under the grating layer. The grating layer and the offset layer are baked to be molten. The baking process is stopped when a phase grating defined by a surface having a sinusoidal shape is formed.

Abstract: A diffractive optical device includes a substrate for allowing transmission therethrough of light to be diffracted; and a grating section located on the substrate and including a plurality of grating elements each having multiple discrete phase levels. The plurality of grating elements are arranged at different grating periods in different areas of a surface of the substrate and have the phase levels in different numbers in accordance with the grating period.

Abstract: In one embodiment a diffractive element, having at least one surface formed as a plurality of blazed diffractive features that are curved and decentered to focus and chromatically disperse incident light into Red Green and Blue spots. An array of the diffractive elements is disclosed for separating incident light from a color image to form Red, Green, and Blue, separations of the image without the use of filters.

Abstract: A focal plate for use in camera and a method of manufacturing the same are disclosed. The focal plate comprises an optical plate, a plurality of lens-like uneven portions regularly formed on the surface of the optical material, and a plurality of fine rugged portions formed on each uneven portion, and is manufactured by forming a pyramid or prism relief on a metal plate through mechanical working, subjecting it to an electroplating to form a honeycomb pattern composed of hexagonal lens faces containing fine rugged portions therein, and then transferring the honeycomb pattern onto a surface of an optical material.

Abstract: An optical transmissive component (1) is described, which component has an entrance surface (4) and an exit surface (5) for optical radiation (20), in which one of the surfaces (4; 5) is provided with an anti-reflection grating (10; 15). By providing a second surface, (5; 4) with a second anti-reflection grating and by ensuring that the grating strips (11) of the first grating (10) extend essentially perpendicularly to those (16) of the second grating (15) in corresponding areas of the first and the second surface (4; 5), it is prevented that the component is birefringent.

Abstract: A display having a diffraction grating comprises a substrate, a plurality of diffraction grating cells formed over a surface of the substrate and comprising a diffraction grating having curve portions arranged in a parallel shift relation, and a light-shutting unit including light-shutting elements having an opening, wherein a length of the diffraction grating in a direction perpendicular to that shift direction of the curve portions is made substantially equal to a length of the opening of the light-shutting element in the light-shutting unit, a diffraction grating element in the diffraction grating is repeated at least once in a direction perpendicular to the shift direction of the curve portions to provide the above cell, a length of the cell in the direction perpendicular to the shift direction of the curve portions is made substantially equal to a pitch length of the light-shutting element in the light-shutting unit, and the light-shutting unit is arranged on an illumination light entry side of the diffr

Abstract: A polychromatic diffractive lens is usable with broadband or multi-spectral illumination to bring a plurality of spectral components of the illumination to a common focus in space. The lens has a Fresnel zone structure of zones and a profile which provides a phase jump delay at each zone boundary to the illumination of at least one of the spectral components which is greater than one period (wave) of that wavelength, and more particularly, a multiple of 2.pi.p, where p is an integer greater than or equal to two. The parameter p and the width of the zones are selected so that the spectral components are directed, and if desired directed so as to be brought to the common focus with high diffraction efficiency in distinct diffractive orders, the lens thereby being a multi-order diffractive (MOD) lens.

Abstract: A holographic grating element is provided which enables tracing different scan lines. The element is divided into a plurality of sectors. A first grating pattern is provided which diffracts a beam in one direction, and a second grating pattern is next to the first and diffracts the scan beam in the reverse direction. Sectors can be produced with different sector sizes to enable scanning at different rates in the reverse direction, or in successive scans in the same direction. Angle relationships are described to produce diffraction patterns for a desired relationship between disc rotation and the angle of the scan line.

Abstract: A two-dimensional optical low-pass filter includes an optical phase grating having a plurality of unitary grating periods on only one side of a grating plane. Each of the unitary grating periods includes at least two phase retardation structures spaced from each other, thereby providing phase retardation across the grating plane. The two phase retardation structures invariably have at least one non-continuous phase retardation configuration substantially in all directions on the grating plane.

Abstract: A complex lens of one-piece construction has first and second lens surfaces opposite to each other. At least one of the first and second lens surfaces of the complex lens is integrally formed with a diffraction grating. This diffraction grating has a multiplicity of substantially parallel grating grooves each being of a generally triangular, sinusoidal or trapezoidal cross-section. An optical head utilizing the complex lens is also disclosed.

Abstract: A photographing apparatus comprising an optical low-pass filter of phase grating type in which a plurality of prism portions, convex or concave in cross section, are arrayed in succession at a predetermined pitch, the arrangement being such that the aperture center or the optical axis of a photo taking lens is located substantially at an apex position of a concave or convex portion which is formed by a prism portion and its adjacent prism portion.

Abstract: In a wavelength-selective phase-grating type optical low-pass filter including at least a pair of adjacent first and second transparent layers, there is provided at least one adhesive layer through which a pair of adjacent first and second transparent layers stacked on each other in a phase grating form for generating a phase difference distribution on the boundary surface between the pair of adjacent first and second transparent layers. The optical low-pass filter satisfies the following conditions under a condition of 470 nm<.lambda.<580 nm: (a) No (.lambda.)=Ni (.lambda.), (b) .nu.d1<.nu.d2 or .nu.d1>.nu.d2, and (c) 0.9< Np(.lambda.) / No(.lambda.)< 1.1, where .lambda. is a wavelength of light passing through the optical low-pass filter, No (.lambda.) and Ni (.lambda.) are refractive indexes of the first and second transparent layers, respectively, at the wavelength .lambda., .nu.d1 and .nu.d2 are Abbe's numbers of the first and second transparent layers, respectively, No(.lambda.

Abstract: A method of fabricating a diffraction grating comprises the steps of illuminating a first hyperbolic diffraction grating having a spatial frequency of f/2 with a spherical wave from a point light source to produce a diffracted ray, diffracting the diffracted ray from the first hyperbolic diffraction grating with second and third hyperbolic diffraction gratings having a spatial frequency of f, merging the diffracted rays from the first and second hyperbolic diffraction gratings on a photographic plate to record interference fringes. The point light source, the first, second and third diffraction gratings, and the photographic plate are disposed to satisfy the relationship ##EQU1## where d is the distance between the point light source and the first diffraction grating and c is the distance between the first diffraction grating and each of the second and third diffraction gratings and also the distance between each of the second and third diffraction gratings and the photographic plate.

Abstract: Designs for cylindrical-wave controlling, generating and guiding devices are disclosed, which include multi-cylindrical-layer reflector and anti-reflection for unguided cylindrical waves, and concentric-circle-grating waveguide reflector, directional coupler and distributed feedback structures for guided cylindrical waves. A multi-cylindrical-layer structure takes the form of cylindrical layers wrapped concentrically around a central rod. A grating waveguide structure takes the form of a slab waveguide in which one of the slabs has circular ridge-groove grating. Layer and ridge-groove spacings are critical to the functionality of these structures. Design rules are outlined. These structures are important components for cylindrical wave controls. Designs of narrow-linewidth and high-power surface-emitting semiconductor lasers using the grating reflectors and 90-degree couplers are also disclosed.

Abstract: An optical filter 10 for filtering spatial frequencies from an output of an optical imaging system comprises a substantially transparent, or reflective, optical element 12 having a plurality of light refracting, or reflecting, segments 18 arranged on a surface thereof. Each of the plurality of segments 18 is provided with a predetermined angle of refraction, or reflection, distinct from adjacently located segments 18, wherein the different angles of refraction, or reflection, and the arrangement of the segments 18 on the optical element 12, are determined based on a desired spatial frequency transfer function for the filter 10. The optical spatial filter 10 reduces errors in optically formed images by accurately confining a broadened Point Spread Function (PSF) to a desired spot size.

Abstract: A beam sampler comprises a substrate made of highly transparent fused silica or zinc selenide, both capable of sustaining high power laser beams. The substrate defines an outer surface through which the light beam being sampled propagates. A sinusoidal diffracting relief is etched on this outer surface directly into the light-propagating material of the substrate. When a light beam propagates through the outer surface of the substrate, the three-dimensional diffracting relief extracts from this light beam at least one pair of low power beam samples.

Abstract: Disclosed is apparatus for continuous wideband tuning of lasers, along with means for relating such tuning to a reference wavelength, by maintaining any desired offset from that reference wavelength. Also disclosed are multichannel fiber optics communications networks employing the above means, said networks being self organizing in terms of the wavelengths of their channels.

Abstract: A diffractive mirror has a plurality of diffractive zones at least one of which has an optical step having a height equal to j.lambda./2 and at least one of which has an optical step having a height equal to k.lambda./2 where .lambda. is a design wavelength of the mirror and j and k are unequal nonzero integers. Alternatively stated at least one of the optical steps induces a relative phase shift of 2j.pi. at the image point between two light rays emerging from a point source at the object point and striking the lens immediately on opposite sides of the step and at least one of the steps induces a relative phase shift of 2k.pi. at the image point between two light rays emerging from a point source at the object point and striking the lens immediately on opposite sides of that step.

Abstract: A phase grating optical low-pass filter having wavelength selectivity consisting of a plurality of layers which are the same in refractive index at a certain wavelength and different from one another in refractive index dispersion, and has a configuration for generating a phase difference at a boundary of the layers.

Abstract: An optical connector comprises a ring-shaped waveguide and a straight waveguide for introducing a light beam into the waveguide. These waveguides are arranged on a substrate in a manner such that the gap between their nearest positions is not longer than the wavelength of the light beam. The radius, width, thickness, and refractive index distribution of the ring-shaped waveguide are set so that a light beam with a wavelength .lambda.=2.pi.r/n (r and n are the radius of the ring-shaped waveguide and a certain natural number, respectively) continues to circulate therein. A grating having a cycle of 2.pi.r.lambda./(2.pi.r+.lambda.) or 2.pi.r.lambda./(2.pi.r-.lambda.] is formed on the surface of the ring-shaped waveguide.

Abstract: A projector capable of compactizing the structure and improving the efficiency of light utilization is disclosed. The projector has an illuminating system, a light modulation device and a projection optical system. A first and a second lights of mutually different colors are emitted from the illumination optical system. The light modulating device has a first light modulator for modulating the first light according to a first image signal and a second light modulator for modulating the second light according to a second image signal. Each of the first and second light modulators is provided with a grating, a liquid crystal filled in the grooves of the grating and a driver for driving the molecules of the liquid crystal.

Abstract: A wavelength-selective phase-grating optical low-pass filter has a phase difference structure formed at a boundary face between two layer materials which have different refractive index distributions and an identical refractive index at an intended wavelength. A trapezoid form is adopted at the phase difference section of the phase grating to achieve an excellent MTF characteristic in the condition of: ##EQU1## thereby obtaining a wavelength-selective phase-grating optical low-pass filter capable of adjusting the cutoff characteristic and improving the characteristic in the low-frequency region of the spatial frequency.