Abstract: An atmospheric distortion compensator comprising a disc and a rotator. The disc, which comprises a phase-modifying structure, is rotationally balanced about a center point. The rotator is mechanically coupled to the disc's center point and configured to spin the disc about an axis. When spinning, the disc is configured to control a property of a beam, which is propagating in parallel to the axis and impinging on the disc. By so doing, scintillation effects within an electro-optical field caused by propagation of the beam within a heterogeneous medium are reduced.

Abstract: An optical device and an eye-tracking system to suppress a rainbow effect are provided. The optical device includes a grating. The grating includes at least one substrate and a grating structure coupled to the at least one substrate. The grating structure is configured to diffract an infrared light beam and transmit a visible light beam with a diffraction efficiency less than a predetermined threshold.

Abstract: A method is provided. The method comprises: receiving incident light, from an object surface, on a top surface of a holographic optical field flattener (HOFF); transforming direction of light, with a hologram, if the light is incident on a portion of the HOFF at an angle equal to a non-zero field angle of the portion; and emitting transformed light from a bottom surface of the HOFF.

Abstract: A laser printing system includes a laser configured to produce a beam of light modulated according to image data input to the laser printing system, a photoreceptor drum including a photoconductive layer disposed along an outer peripheral surface of the photoreceptor drum, and a non-mechanical beam steerer configured for receiving the modulated light beam from the laser and steering the light beam in a scanning motion back and forth across the photoconductive layer of the photoreceptor drum. The laser printing system also includes a printer controller configured to structure the image data input to the laser printing system, and control an amount of electrical current flowing through portions of the non-mechanical beam steerer to change an effective index of refraction of the non-mechanical beam steerer and steer the modulated light beam in the scanning motion.

Abstract: A scanning unit includes a light source controllable to emit a light beam; a scanning mirror having a plurality of reflective surfaces, the scanning mirror receiving the light beam from the light source and deflecting at least portions of the light beam along a scan direction; and a collimator lens disposed between the light source and the scanning mirror, the collimator lens having a light incident surface that is spherical and a light exit surface that is aspheric such that the light beam, after passing through the collimator lens, is diverged by the collimator lens so as to be incident on at least two reflective surfaces of the scanning mirror.

Abstract: An image reading apparatus comprises a housing to block ambient light and including a rectangular opening on a bottom, an image capturing unit to capture an image of a medium in the opening, and a plurality of light sources disposed in the housing and emitting light to irradiate different areas of the opening. The housing includes four cover portions corresponding to sides of the rectangular opening. The four cover portions include a pair of inclined-cover portions which are switched between an open state and a folded state and a pair of connecting-cover portions which connect the two inclined-cover portions. The two connecting-cover portions are connected to the inclined-cover portions in a transformable manner, and when the two inclined-cover portions are in the folded state, the connecting-cover portions enter gap between the inclined-cover portions by being transformed with respect to the inclined-cover portions.

Abstract: An optical element for use in an illumination optical unit of an EUV microlithography projection exposure apparatus includes a plurality of reflective facet elements. Each reflective facet element has at least one reflective surface. In this case, at least one facet element is arranged in a manner rotatable about a rotation axis. The rotation axis intersects the at least one reflective surface of the facet element. With such an optical element, it is possible to alter the direction and/or the intensity of at least part of the illumination radiation within the illumination optical unit in a simple manner.

Abstract: An image forming apparatus includes: a light source; a photosensitive member; a brushless motor including a stator and a rotor; a rotary polygon mirror rotated by the brushless motor; an energization switching unit that turns on/off energizations of the coils; a voltage detecting unit that outputs a detection signal based on induced voltages generated in coils of the stator by rotation of the rotor; and a motor controlling unit that controls the turning on/off of the energizations by the energization switching unit based on the detection signal. In a non-image forming period after one image forming operation, the motor controlling unit performs a low-speed process where the motor controlling unit maintains a rotation speed of the brushless motor at a speed, which is lower than a speed in the image forming operation, and at which the induced voltages are detectable by the voltage detecting unit.

Abstract: A virtual image display device with an optical waveguide to guide, by internal total reflection, parallel pencil groups meeting a condition of internal total reflection, a first reflection volume hologram grating to diffract and reflect the parallel pencil groups incident upon the optical waveguide from outside and traveling in different directions as they are so as to meet the condition of internal total reflection inside the optical waveguide and a second reflection volume hologram grating to project the parallel pencil groups guided by internal total reflection inside the optical waveguide as they are from the optical waveguide by diffraction and reflection thereof so as to depart from the condition of internal total reflection inside the optical waveguide.

Abstract: Provided is a planar lighting apparatus performing a display (i) without luminance nonuniformity, (ii) with a specified luminance distribution in a scanning direction and (iii) having a high luminance and a large area. Also provided is a liquid crystal display device using the planar lighting apparatus. In the plan lighting apparatus, a scanning light quantity of a scanning light 13 to a light guide section 15 is controlled by a controller 20, so at to emit an outgoing light 16 from a principal surface 17 with a specified luminance distribution in a scanning direction 21 of the scanning light 13.

Abstract: There is provided an optical multiplexer including: a substrate having a plurality of beam transmitting portions; and diffraction gratings diffracting beams irradiated to the beam transmitting portions at different angles, each diffraction grating being formed in the corresponding beam transmitting portion. Accordingly, it is possible to reduce an alignment error and manufacturing cost and to reduce the entire size of a recording apparatus or a reproducing apparatus.

Abstract: An electrooptic device includes first insulating layer between a semiconductor layer of a transistor and scanning line. The first insulating layer has a contact hole for electrically connecting the gate electrode and the scanning line, the contact hole having a first portion located beside the semiconductor layer of the corresponding transistor in plan view and that extends in the direction in which one of the scanning lines or the data lines extend and a second portion that overlaps with part of the corresponding data line and that extends in a direction in which the other of the scanning lines data lines extend.

Abstract: A virtual image display device with an optical waveguide to guide, by internal total reflection, parallel pencil groups meeting a condition of internal total reflection, a first reflection volume hologram grating to diffract and reflect the parallel pencil groups incident upon the optical waveguide from outside and traveling in different directions as they are so as to meet the condition of internal total reflection inside the optical waveguide and a second reflection volume hologram grating to project the parallel pencil groups guided by internal total reflection inside the optical waveguide as they are from the optical waveguide by diffraction and reflection thereof so as to depart from the condition of internal total reflection inside the optical waveguide.

Abstract: A light source system and a display apparatus comprising the light source system are provided. The display apparatus comprises the light source system and an imaging system. The light source system is configured to provide a light for imaging. The light source system comprises a mirror wheel and at least two light source modules, configured to provide at least two light beams respectively. These light beams are lit up according to a predetermined sequence. After being reflected by or directly passing through the mirror wheel, the light beams form a continuous light, traveling into the imaging system for imaging on a screen.

Abstract: A method and apparatus that validates a hologram by scanning the hologram at multiple angles is disclosed. The method may include scanning a document containing the hologram at a first angle to create a first image, scanning the document containing the hologram at a second angle to create a second image, determining whether the hologram is valid, and sending the validity determination to a user interface for display to a user.

Abstract: Double Bragg grating scanner receivers (DBS) and methods thereof based on spinning high-efficiency transmitting or reflecting holograms in photo-thermo-refractive (PTRG) glass provides unlimited field of view while incident angle for all components do not exceed approximately 45 degrees. The devices and methods are highly tolerable to high power laser radiation and has no restriction for the use in any laser systems working in visible and near IR spectral regions. Rate of scanning by DBS is higher compared to known mechanical scanners because the use of thin glass plates with recorded holograms and spinning instead of rocking. The components described herein are holographic optical devices (reflecting and transmitting volume gratings) for visible and near IR spectral regions with absolute diffraction efficiency exceeding approximately 95% and high thermal, optical and mechanical stability.

Abstract: A deflection device includes a tabular object for transmitting or reflecting an electromagnetic wave, a drive unit for driving the tabular object so as to rotate or perform a translation motion, and an electromagnetic wave irradiation unit for irradiating the tabular object with an electromagnetic wave so that an irradiation area extending in a direction intersecting a direction of the rotation or translation motion of the tabular object is formed.

Abstract: A portable data storage cartridge comprising a housing, a first information storage medium removeably disposed within the housing, a holographic data storage medium disposed within the housing, wherein the housing is formed to include an aperture, and wherein the holographic data storage medium is disposed adjacent the aperture.

Abstract: A holographic disc recording medium records a hologram over the entire holographic disc recording medium by using a conical reference beam for recording. A hologram recorded image includes recorded images of a plurality of data pages including a plurality of pixels arranged in an approximately rectangular array. A cylindrical reference beam for reading having coherent light having an approximately cylindrical wavefront shape with a small curvature is used to minimize the mismatch in wavefront shapes between the reference beam for recording and reference beam for reading.

Abstract: A beam processing system comprises a rotary disk mounted thereon with processing objects, a controller for controlling a reciprocating drive mechanism, and a beam width measuring unit for measuring a beam width of a processing beam. The controller sets an inner and an outer overscan position depending on a measured value of the beam width. The controller, based on the number of rotation of the rotary disk per unit time, a scan speed and the number of reciprocating scan times, a reversal start timing of the rotary disk at at least one of the inner and the outer overscan positions, and the measured value, controls the reciprocating drive mechanism so as to ensure an overlap region between a last and a current processing beam irradiation region on each of the processing objects, the overlap region overlapping at least half of the last processing beam irradiation region.

Abstract: A first holographic data storage device has a first set of holograms stored thereon. A second holographic data storage device has a second set of holograms stored thereon. An opaque layer is disposed between and attached to one side of the first and the second holographic data storage devices. In the case of double reflective diffractive recording, the opaque layer is not necessary.

Abstract: An optical scanning unit with a rotatable hologram disc, including a light source, a rotatable hologram disc provided with a plurality of holograms, the holograms being formed concentrically along a radial direction to diffract a beam emitted from the light source by multiple steps, a reflecting member that reflects the beam emitted from the light source and diffracted by one of the holograms into another hologram, and a fixed hologram correcting the diffracted beam incident through the rotatable hologram disc to focus it on a scanning object. In the optical scanning unit, the light passes through the plurality of holograms formed along a radial direction of the rotatable hologram disc so that a scan width can be extended without reducing a scan speed of the optical scanning unit.

Abstract: The present invention is directed to an improvement in a diffractive display suitable for presenting graphic and the like displays. Broadly, a novel embodiment is realized from a holographic diffraction pattern carried by a magnet or element and an electrically energizable coil magnetically coupled with said magnet that is energizable for movement of the magnet. Rotation of the holographic diffraction pattern generates a display using the diffracted light from the holographic diffraction grating. Another novel embodiment is realized from a faceted rotatable element (FRE) having an array of facets each bearing a diffraction grating and a source energizable for rotation of the FRE from a resting station to a viewing station. Rotation of the FRE generates a display using the diffracted light from the diffraction gratings.

Abstract: An optical crossbar switch system includes: a mask comprising multiple patterns, wherein at least one of the multiple patterns is a diffractive pattern; and a spatial light modulator positioned to receive at least one optical input beam and selectively couple at least a portion of each of the at least one input beam to one of the patterns of the mask, each coupled portion defining an intermediate beam, wherein each pattern, when selected by the spatial light modulator, is configured to redirect the intermediate beam to one of N targets, where N is an integer greater than two. Further, a method for selectively coupling at least one input beam to one of multiple output channels includes: selectively coupling at least a portion of each of the at least one input beam to one of multiple patterns on a mask; and diffracting at least one of the coupled portions from the mask to one of the output channels.

Abstract: A hologram scanner is constructed with a driving source for providing a rotational force, a deflection disk installed at the rotation shaft of the driving source for forming a scanning line, the deflection disk having a plurality of sectors where a hologram pattern for diffracting and deflecting incident light is formed, a TE polarized light emitting device, arranged to face one side of the deflection disk, for emitting TE polarized light so that major axis of an elliptical spot formed at a predetermined position on the deflection disk by the incident light is perpendicular to the radius vector of the deflection disk passing through the spot having an elliptical cross-section and a TE polarization mode, and an optical path altering device for altering the proceeding path of incident light, so that a scanning line formed by the rotation of the deflection disk proceeds to a photoreceptor medium.

Abstract: A holographic laser scanning system for scanning bar code symbols. The system comprises a holographic scanning disc having a plurality of holographic optical elements disposed thereon for scanning the plurality of laser beams directed at the same incident angle. Each holographic optical element as substantially the same Bragg angle. The parabolic light reflective surfaces are disposed beneath the holographic scanning discs. Each parabolic light reflective surface has an optical axis disposed off the Bragg angle of the holographic optical elements, for focusing towards a focal point above the holographic scanning disc, the reflected light rays collected by each holographic optical element. A photodetector is disposed at the focal point of the corresponding parabolic light reflective surface above the holographic scanning disc, and is radially aligned with the optical axis of one parabolic light reflective surface.

Abstract: An optical element has a small reflection loss of light on a boundary surface between a photonic crystal and the normal medium. The optical element includes: a normal medium region having a first refractive index; a photonic crystal region having a refractive index which is changed periodically depending upon a position thereof, the photonic crystal region having as an averaged refractive index a second refractive index different from the first refractive index; and an intermediate region interposed between the normal medium region and the photonic crystal region, the intermediate region having a refractive index which is gradually changed from the first refractive index to the second refractive index.

Abstract: A two-dimensional optical switch (10) includes a hermetically sealed housing (41) cooperable with several input fibers (11-18) and several output fibers (21-28). A digital micromirror device (71) is disposed within the housing, and can selectively effect travel of radiation between each input fiber and any one of the output fibers. The housing includes two lens sections (56, 57), which each have a platelike support portion (106), and several lens portions (111-118) of approximately semi-spherical shape that project from the inner side of the support portion. Each of the input fibers is fixedly secured to an outer side of one lens section in alignment with a respective lens portion thereon, and each of the output fibers is fixedly secured to the outside of the other lens section in alignment with a respective lens portion thereon.

Abstract: An optical scanning unit with a rotatable hologram disc, including a light source, a rotatable hologram disc provided with a plurality of holograms, the holograms being formed concentrically along a radial direction to diffract a beam emitted from the light source by multiple steps, a reflecting member that reflects the beam emitted from the light source and diffracted by one of the holograms into another hologram, and a fixed hologram correcting the diffracted beam incident through the rotatable hologram disc to focus it on a scanning object. In the optical scanning unit, the light passes through the plurality of holograms formed along a radial direction of the rotatable hologram disc so that a scan width can be extended without reducing a scan speed of the optical scanning unit.

Abstract: A scanner includes a laser for projecting a laser beam at a facet having a blazed diffractive grating thereon. The facet is rotated relative to the laser to traverse the laser beam across the grating to diffract the laser beam into a scan line. The blazed diffractive grating may be readily manufactured using injection molding or photolithograhic manufacturing techniques.

Abstract: A beam scanning system including: a light source; a deflection disc rotatably installed over the light sources and having a hologram pattern on each of the upper and lower surfaces thereof, for diffracting beams emitted from the light source; and a plurality of mirrors for reflecting beams that have been diffracted by the deflection disc, to change the diffracted paths of the beams. Each of the upper and lower hologram patterns is formed with a low density relative to a one-sided hologram pattern.

Abstract: A diffractive deflection beam scanning system including: a plurality of light sources; a deflection disc rotatably mounted over the light sources, having patterns capable of diffracting and deflecting beams emitted from the light sources; a plurality of first reflecting mirrors for changing the traveling paths of beams diffracted and deflected by the deflection disc; a plurality of second reflecting mirrors for reflecting the beams reflected by the first reflecting mirrors in a beam scan direction, wherein the light sources are disposed in a straight line along a bisecting line of the deflection disc, perpendicular to the beam scan direction, and the second reflecting mirrors are arranged over the center of the deflection disc at different heights.

Abstract: A multi-laser scanning unit (LSU) in which scanning lines incident parallel onto a photoreceptor have the same scanning direction. The multi-laser scanning unit includes: a driving source for providing a rotatory force; a deflection disk installed around a rotary shaft of the driving source having a plurality of sectors each with a hologram pattern for diffracting and deflecting incident light, for scanning light through rotation; a plurality of light sources installed facing one surface of the deflection disk for irradiating a plurality of light lines onto predetermined points of the deflection disk; and a light path changing portion for changing traveling paths of the plurality of scanning lines formed by the rotation of the deflection disk, wherein the plurality of light sources are arranged such that two incident points which are the farthest from each other among the plurality of incident points formed on the deflection disk form 180° with the center point of the deflection disk.

Abstract: A laser scanning unit (LSU) in a structure capable of radiating heat generated by a light source. The LSU includes at least one light source, a driving source for providing rotational force, and a deflection disk installed on a rotary shaft of the driving source, for diffracting and deflecting light incident from the light sources. The deflection disk has at least one radiating portion which is depressed in and/or protrudes from the surface of the deflection disk facing the light source and induces air flow by contact with air, such that the heat generated by the light sources is radiated by the air flow induced due to the radiating portion.

Abstract: A laser scanning apparatus including an optical source installed on a main frame, a scan disk which is rotatably installed on the main frame and has diffraction patterns formed thereon for deflecting a beam emitted from the optical source in a predetermined direction to achieve a normal light path, and a means for shielding an abnormal beam that is emitted from the optical source and diffracted or reflected off the normal light path by the scan disk.

Abstract: A multi-laser scanning unit (LSU) in which scanning lines incident parallel onto a photoreceptor have the same scanning direction.

Abstract: A light-beam scanning apparatus includes at least two holograms with an optical path length difference .DELTA..PHI. along a scanning beam light flux. The path length is measured from a light source to a scanning surface in the first hologram, and is represented by .DELTA..PHI.<C(.lambda..sup.2 /D.lambda.) where C is a constant less than 0.5. The path length is related per the above equation to a wavelength, .lambda., at the center of the light source, and a wavelength displacement .DELTA..lambda. measured from the wavelength .lambda. at the center of the light source.

Abstract: In a beam scanning apparatus of an electrophotographic color printer, at least two beam deflectors, each having a deflection disk consisting of at least two sectors and a driving source for rotating the deflection disk, deflect and scan an incident beam. The deflection disk has a pattern formed thereon for diffracting and scanning the incident beam. A plurality of light sources, arranged between the beam deflector and a photoreceptor, emit light to the deflection disk. A beam path changing unit changes a proceeding path of the beam deflected by the respective beam deflectors to proceed toward the photoreceptor. A beam correcting unit, arranged between the respective beam deflectors and the photoreceptor, corrects the input beam having its path changed by the beam path changing unit.

Abstract: A high-resolution light-beam scanning apparatus utilizing only mass-producible holograms instead of utilizing auxiliary optical systems such as optical lenses or a mirror having curvature, and capable of compensating for disadvantages including scanning beam thickening and variation, failure of a rotatable hologram to rotate at a constant velocity, displacement of a scanning beam position in the scanning direction and the cross scanning direction due to a mode hop of a wavelength of a semiconductor laser, and deviation of a base of rotatable hologram from a parallel state. These disadvantages are detrimental to efforts for increasing the resolution of a hologram scanner and lowering the cost thereof.

Abstract: A beam scanning apparatus includes an optical source, a beam deflector having a deflection disk which is comprised of a plurality of sectors patterned to diffract a beam emitted from the optical source and has a reflective layer for reflecting an incident beam formed on the bottom surface of the beam deflector, and a driving source for rotating the deflection disk. A beam corrector is provided for correcting the beam deflected by the beam deflector.

Abstract: A high-resolution light-beam scanning apparatus utilizing only mass-producible holograms instead of utilizing auxiliary optical systems such as optical lenses or a mirror having curvature, and capable of compensating for disadvantages including scanning beam thickening and variation, failure of a rotatable hologram to rotate at a constant velocity, displacement of a scanning beam position in the scanning direction and the cross scanning direction due to a mode hop of a wavelength of a semiconductor laser, and deviation of a base of rotatable hologram from a parallel state. These disadvantages are detrimental to efforts for increasing the resolution of a hologram scanner and lowering the cost thereof. The light beam scanning apparatus includes at least two holograms, one rotatable and one fixed, wherein the fixed hologram plate has a phase distribution .PHI..sub.H represented as a difference obtained when subtracting a reference wave phase .PHI..sub.0 from an object wave phase .PHI..sub.

Abstract: A high-resolution light-beam scanning apparatus utilizing only mass-producible holograms instead of utilizing auxiliary optical systems such as an optical lens or a mirror having curvature, and capable of compensating for disadvantages including scanning beam thickening and variation, failure of a rotatable hologram to rotate at a constant velocity, displacement of a scanning beam position in the scanning direction and the cross scanning direction due to a mode hop of a wavelength of a semiconductor laser, and deviation of a base of a rotatable hologram from a parallel state, which disadvantages are detrimental to efforts for increasing the resolution of a hologram scanner and lowering the cost thereof, the light-beam scanning apparatus being characterized in that provided in the rotatable hologram (1) and the fixed plate (2) arediffraction gratings for minimizing:either a sum total of values obtained by weighting:a square of an optical path length difference between a) an optical path of a light flux measure

Abstract: A multi-beam scanning apparatus includes at least two optical sources, a beam deflector having a deflection disc with a plurality of sectors for diffracting and transmitting incident beams, and a driving source for rotating the deflection disc, for deflecting and projecting beams emitted from the optical sources. A beam corrector installed along a light path, for correcting the beams deflected by the beam deflector.

Abstract: A method of manufacturing a fixed hologram plate of a light-beam scanning apparatus for diffracting a light incident from a light source portion by a rotatable hologram, scanning by the diffracted light through the rotation of the rotatable hologram, diffracting the resulting light by the fixed hologram plate, and for conducting a light-beam scanning on a scanning surface. The method includes the steps of preparing an interference fringe distribution of the fixed hologram plate by two waves: a first wave having a spherical aberration, a transverse aberration of the first wave is in the Y direction, the transverse wave including an astigmatism and a coma; and a second wave having a spherical aberration and astigmatism, and having a wavelength different from a wavelength of a reconstructing wave that is selected to minimize distortion, wherein a transverse aberration of the second wave is in the X direction.

Abstract: Bow can be minimized by an auxiliary grating in a post deflection path of the diffracted beam from the hologon which is disposed in non-parallel relationship to the plane of rotation of the hologon. The ellipticity of the spot which generates the scan line and intensity variation in the scan beam can be minimized by using a dispersive element, preferably a prism, between the hologon and the bow compensation grating with its dispersion in a direction opposite to the direction of the dispersion of the hologon and of the auxiliary grating. Another grating before the hologon can be used to correct for wavelength shift induced cross-scan error associated with the dispersion in the hologon and a bow compensation grating following the hologon.

Abstract: A high-resolution light-beam scanning apparatus utilizing only mass-producible holograms instead of utilizing auxiliary optical systems, and capable of compensating for disadvantages.

Abstract: A light-beam scanning apparatus including a rotatable hologram and a fixed plate, which are provided with diffraction gratings to minimize: either a sum total of values obtained by weighting where a square of an optical path length difference between an optical path of a light flux measured along a principal axis of a light beam incident on and diffracted by a diffraction grating of a rotatable hologram, and incident on and diffracted by a diffraction grating of a fixed plate so as to conduct a scanning and converging on a scanning point on an image formation surface, and an optical path of a light flux measured along a marginal ray distanced from the principal axis; or an absolute value of the optical path difference thereof.

Abstract: A high-resolution light-beam scanning apparatus utilizing only mass-producible holograms instead of utilizing auxiliary optical systems such as optical lenses or a mirror having curvature, and capable of compensating for disadvantages including scanning beam thickening and variation, failure of a rotatable hologram to rotate at a constant velocity, displacement of a scanning beam position in the scanning direction and the cross scanning direction due to a mode hop of a wavelength of a semiconductor laser, and deviation of a base of rotatable hologram from a parallel state. These disadvantages are detrimental to efforts for increasing the resolution of a hologram scanner and lowering the cost thereof. The light beam scanning apparatus includes at least two holograms with an optical path length difference .DELTA..PHI. along a scanning beam light flux. The path length is measured from a light source to a scanning surface in the first hologram, and is represented by .DELTA..PHI.<C(.lambda..sup.2 /D.lambda.

Abstract: A portable 3D scanning system collects 2D-profile data of objects using a combination of a laser-stripe positioning device and a video camera which detects the images of the laser stripe reflected from the object. The scanning system includes a laser-stripe generator, a video camera, a scanning mirror attached to a continuously rotating motor, an encoder or a photodiode operationally coupled to the motor, and associated electronics. As the rotating, scanning mirror reflects the laser stripe and variably positions the laser stripe across the object, the encoder or the photodiode generates signals indicating the angular position of the mirror. The video images of the reflected laser stripes are stored on a storage medium, while data relating to the angular positions of the laser stripes recorded in the video images are simultaneously stored on a storage medium.

Abstract: A high-resolution light-beam scanning apparatus including a rotatable hologram and a fixed plate which are diffraction gratings for minimizing either a sum total of values obtained by weighting (1): a square of an optical path length difference between a) an optical path of a light flux measured along a principal axis of a light beam incident on and diffracted by a first diffraction grating of the rotatable hologram, and incident on and diffracted by a second diffraction grating of the fixed plate so as to conduct a scanning and converging on an image formation surface scanning point, and b) an optical path of a light flux measured along a marginal ray distanced from the principal axis, or an absolute value of the optical path difference thereof; or (2) a square of a sum obtained by adding an amount of displacement of a light beam convergent on the scanning point, which displacement is measured along the marginal ray distanced from the principal axis of an incident reconstructing light flux with respect to th