Abstract: A high quality image forming apparatus capable of outputting a quality image at a high speed is provided, with which no unevenness is generated on an image and a fog or a change in density is less likely to occur. An exposing unit is operated by an over-filled scanner method for exposure by a rotational polygon mirror that reflects lights on a surface narrower than an incident beam width, and forms an electrostatic latent image by back area exposure for exposing a non-image part.

Abstract: The present invention aims at providing an optical unit, its manufacturing method, and electrophotographic device that can easily and inexpensively achieve a reliable latent image formation on a photosensitive drum, and can form a high-quality image. To realize the above aims, a lens barrel and a collimating lens are bonded only at a single spot, and the bonding orientation is aligned approximately with a main scanning direction. Thereby displacement of the collimating lens caused by a thermal expansion of the adhesive is restricted to the main scanning direction as adjustable by other components.

Abstract: A light modulating or switching array (10) having a plurality of discrete protrusions (16) formed of electro-optic material, each of which is electrically and optically isolated from each other. The protrusions (16) have defined a top face (20), a bottom face (30), first and second side faces (22, 24), and front and back faces (26, 28). There are a plurality of electrodes (34) associated with each of the protrusions (16), these electrodes (34) being capable of inducing an electric field in the electro-optic material for independently modulating a plurality of light beams which are incident upon one of the faces (20, 22, 24, 26, 28, 30) of the protrusions (16). The electro-optic material may be of PLZT, or a member of any of the groups of electro-optic crystals, polycrystalline electro-optic ceramics, electro-optic semiconductors, electro-optic glasses and electro-optically active polymers.

Abstract: A scanning optical device to be used for an image forming apparatus such as a laser beam printer and a digital copying machine comprises a light source such as a semiconductor laser, an optical deflector such as a rotary polygon mirror, a first optical system for leading the light beam emitted from the light source to the optical deflector, and a second optical system for focussing the light beam deflected by the optical deflector onto a surface to be scanned. The first optical system is adapted to make the light beam strike a deflection surface of the optical deflector with a width greater than that of the deflection surface in the main-scanning direction. The second optical system has an optical element provided on at least an optical surface thereof with an anti-reflection film having a film thickness so adapted as to maximize its light transmittance for the angle of incidence of the most off-axis ray of the light beam deflected by the optical deflector and striking the optical surface.

Abstract: A porous photosensitive unit has a transparent conductive layer formed on a surface of a transparent substrate. A photoconductive layer is formed on a surface of the transparent conductive layer. A porous insulating layer formed on a surface of the photoconductive layer has a plurality of holes for holding conductive color particles. The plurality of holes includes a first hole and the adjacent second and third holes. An upper or screen electrode is formed on a surface of the porous insulating layer except where the holes are formed. The photosensitive unit includes an optical arrangement in which, when a light source emits light to cause conductive color particles to fly out of the first hole only, the light exposes a region, within the photoconductive layer, which substantially coextends with a surface portion of the photoconductive layer that is exposed by the first hole.

Abstract: An optical scanning apparatus includes a laser unit formed by integrating a semiconductor laser and collimator lens, an incident optical system for making a light beam emerging from the laser unit strike an optical deflector while keeping the light beam wider than the width of a deflecting surface of the optical deflector in the main scanning direction, and an imaging optical system for forming the light beam reflected/deflected by the optical deflector into an image on a scanned surface. The laser unit is shifted by a shift adjusting means in a predetermined direction with respect to the optical axis of the incident optical system so as to make an illuminance distribution of scanning lines on the scanned surface become substantially symmetrical about the scanning central axis.

Abstract: A method of aligning print images of an electrophotographic machine on a print medium includes providing a plurality of rotating photoconductive drums in the electrophotographic machine. Each of a plurality of laser beams is reflected off of a respective one of a plurality of reflectors. Each reflector rotates relative to the respective laser beam to thereby scan the laser beam across a respective one of the photoconductive drums to produce a scan line in a scan direction. The relative rotation between the laser beams and the respective reflectors is cyclically repeated to thereby produce a plurality of substantially parallel scan lines on each photoconductive drum. Each scan line is displaced from an adjacent scan line by a scan line separation distance in a process direction perpendicular to the scan direction. Toner is applied to each scan line on the photoconductive drums. A moving toner transfer medium contacts each photoconductive drum at a respective toner transfer nip.

Abstract: A laser thermal printing system and method utilizes a blur filter (3) made of one or more pieces of double refractive crystal material to smooth printing or laser spots or fill up gaps in an array of laser spots. Blur filter (3) is disposed between a light source (1) and a media or image plane (100). Light source (1) provides for an original array of laser spots (30) at media plane (100) and blur filter (3) splits an image of each of the original array of laser spots (30) to create a displaced duplicate array of laser spots (31) with improved spot uniformity.

Abstract: In a laser marking method, wherein a liquid crystal mask having thereon a desired pattern to be marked is scanned with a laser beam to mark an object with the pattern by the laser beam passed through the liquid crystal mask, the liquid crystal mask is maintained in a predetermined condition of a temperature higher than the room temperature, a marking operation being carried out by using a laser beam in such a high-temperature condition, thus enabling a speed of response of the liquid crystal element, and a working efficiency concerning the laser marking operation to be improved.

Abstract: A laser printing process using a rotary light sensitive roller electrically charged by points by a laser beam and intended for receiving onto the charged points particles of a toner powder and then transferring the same onto a band to be printed, includes a first step wherein the image to be printed is stored, by a first laser beam coming from a first laser source controlled by a computer, onto a rotary memory roller whose surface is covered with a layer of a material capable of assuming either an amorphous or a crystalline structure, this structure being convertible under the action of a laser beam, and this material showing a high difference in reflecting ability according to whether it assumes the one or the other of its structures, and includes a second step wherein the thus stored image is transferred, by the optical reflection of a second laser beam coming from a second laser source having in its whole a linear character, onto the light sensitive roller intended to receive the toner and to transfer the

Abstract: A solid state laser array is multiplexed using an array of micromirrors to permit high resolution printing in a wide format. Each laser in the laser array and each micromirror in the mirror array is individually controlled. The laser array may be an array of VCSELs produced on a GaAs substrate.

Abstract: A method for printing or exposing photosensitive media is disclosed herein. The method uses standard spatial light modulators with standard addressing circuitry. The data is written to the device for the first row, the photosensitive media is exposed to the light reflected from the device, and the device is turned off. The data from the first row is then written to the second line of the device, and new data is loaded into the first line of the device. The media is again exposed. This is repeated until the entire region of the drum is completely exposed. The device can be repositioned to cover a different region of the drum and the process would be repeated.

Abstract: A print apparatus includes a photoconductor and a mechanical system for moving the photoconductor past a scan line exposure station. The print apparatus includes a signal generator for providing outputs indicative of the movement of the photoconductor. A first comparator produces a first position error signal that is derived from a difference between a reference signal and a position error signal output, such position error signal indicating that the position of the photoconductor differs from a predetermined print position that is determinable with respect to the reference signal. The print apparatus further includes a laser beam scanner and a beam detector for producing a scan position signal. A second comparator is responsive to the scan position signal and the position error signal from the first comparator to produce a beam deflection control signal that is applied to a beam deflector having a mirror attached to a piezoelectric bimorph crystal cantilever element.

Abstract: Correcting color banding problems resulting from facet-to-facet jitter in a color imaging device having a multifaceted polygon are corrected by starting each color separation using the same facet. This facet synchronization has been shown to reduce the objectionable color banding. Imaging offsets that result from either advancing or retarding the exposure of a latent image such that the latent image begins with the same facet as other latent images are compensated for using a rotating cylinder mirror whose rotation is controlled by a piezoelectric element.

Abstract: A technique for marking pixels on workpieces by routing a scanned beam to different marking stations to mark individual pixels on the workpieces. A diffractive scan lens focuses the beam. A mark is formed on a workpiece by producing the mark and curing the mark. Angular position of a scanning mirror in a raster scanner is determined by moving a beam, reflected from the scanner, across rulings on an optical element during scanning. A print head for printing spots on a surface of a workpiece has a walled, internally pressurized chamber and structure for causing an inked web to conform to a contour of the chamber wall and to be pulled along the chamber wall. A print head has a compliant-walled, internally pressurized chamber. A print head has a chamber having a low-coefficient of friction coating. Two workpieces may be marked at two marking stations by two-directional scanning.

Abstract: The invention avoids an increase in the number of parts and components which form an apparatus for forming an image, and hence, prevents a main body of the image forming apparatus from completed in a large size.

Abstract: In order to improve the resolution of a LED printing head by a comparatively simple method, optical shutters (28) are provided between two rows of rod lens arrays (26) and LED arrays (18) correspondingly to the rod lens arrays (26). The central optical axes of rod lenses (24) in the rod lens arrays (26) are inclined by an angle of inclination 0 in the opposite directions with respect to the direction of perpendicular lines passing the centers of the light-emitting surfaces of LED's (14), and signal light sent out from the LED's (14) is condensed on a photosensitive surface (20) alternately via predetermined rod lens arrays (26) so that one line is formed on the photosensitive surface (20).

Abstract: A single pass ROS system provides a plurality of latent images which may subsequently be developed in different colors. The ROS units are initially aligned so that each color image is precisely registered at the same relative position along the exposed area of a photoreceptor belt. The alignment is accomplished by forming a pair of opposed alignment apertures in the surface of the belt and detecting coincidence or lack of coincidence of signals generated by the beginning and end of each scan line. The correction is enabled by rotating the output window in the ROS system, a transmissive optical component with no optical power, subsequent to the ROS, to create the required rotation of the projected scan line. Once the initial X and Y-axis alignments are complete, subsequent alignment is maintained through subsequent passes by checking the positions of the previously identified pixels as they advance past additional lead edge apertures formed in the process direction along the belt surface.

Abstract: An image forming apparatus having an exposure optical system provided with an exposure light source in which light-emitting elements are arranged on a base plate to be line-shaped and with a lens array of a distributed refractive index type that converges a beam emitted from the exposure light source on the surface of a photoreceptor and conducts imagewise exposure. The apparatus is configured to satisfy the expression: ##EQU1## In the expression, .DELTA.T1 represents a variation width (K) of the temperature in the vicinity of light-emitting elements, R represents a radius of the photoreceptor mentioned above, Ri represents a length (.mu.m) of each member constituting the exposure optical system in the radial direction of the photoreceptor, .alpha. represents a coefficient of linear thermal expansion (.degree.C..sup.-1) of a transparent base that forms the photoreceptor, .alpha.i represents a coefficient of linear thermal expansion (.degree.C..sup.

Abstract: Light emitted from an array of light-emitting elements in an electrophotographic printer is transmitted through a birefringent plate that deflects light polarized in a first plane but does not deflect light polarized in a second plane perpendicular to the first plane, and through a polarization switching device that is controlled by an electrical signal so as to selectably transmit light polarized in either the first plane or the second plane. The transmitted light illuminates a photosensitive medium. Each light-emitting element can illuminate two dots on the photosensitive medium, depending on the state of the polarization switching device, thereby doubling the dot resolution.

Abstract: The present invention is directed to a precise method of registering a plurality of ROS imagers, so as to form sequential registered images on a photoreceptor belt in a single pass. Pairs of belt holes are formed in the photoreceptor belt, outside the image area and outside of the scan width of the ROS imagers. A pair of light sources are incorporated into the post polygon optics of each ROS imager so as to produce a light spot at the photoreceptor surface which will periodically illuminate the holes on the belt. As the leading edge of the belt holes advances into the beams, detectors placed beneath the belt and beneath the exposure station provide signals representing the exposure level of the light source output. The position of a transmissive optical element with no optical power, subsequent to each of said ROS imagers, is adjusted in response to registration error signals generated by said photodetecting means to adjust the position of said scan lines.

Abstract: A positioning apparatus of an optical element in the form of a thin film includes at least one positioning element having a surface opposed to the optical element without contacting the same, together with a mechanism for moving at least one of the optical element or the positioning element, in a direction parallel to a plane of the thin film. The opposed surface of the positioning element produces a fluid force when relative movement between the optical element and the opposed surface of the positioning element occurs and serves to hold the thin film at a predetermined position. An optical scanner incorporating such a positioning apparatus is also disclosed.

Abstract: An optical scanning apparatus which includes a housing, an optical source which emits an optical beam, a deflector which deflects an optical beam from the optical source, a focusing lens which focuses the optical beam from the deflector, and plural mirrors which reflect the optical beam from the deflector. The plural mirrors include a first mirror which reflects the optical beam from the deflector, a third mirror which reflects the optical beam from the first mirror, and a second mirror which reflects the optical beam from the third mirror. The optical beam is focused upon a photoconductive element by the focusing lens through the plural mirrors on a scanning surface, and at least one of the first mirror and the third mirror is pivotally supported to be adjusted in a sub-scanning direction. Further, the second mirror is slidably supported to be adjusted in a direction normal to a reflecting surface of the second mirror.

Abstract: A novel laser printer is disclosed, in which heat generating parts are positively cooled while reducing the noise generated by a blowing mechanism. A rotor of the motor for rotating a scanning mirror of a scanning device is surrounded by a cover. An air flow inlet port and an air flow outlet port are formed on the sides of the cover. The space in the proximity of the air flow inlet port and the air flow outlet port is divided by an air flow separation member. One longitudinal end of the air flow separation member is close to the rotor and the other longitudinal end and the lateral ends thereof are connected to the cover. The air flow separation member partially divides the rotating air flow generated around the rotor in the proximity of the air flow outlet port, and generates the discharge air flow, which in turn is discharged out of the cover. A suction air flow is introduced from the air flow inlet port to the interior of the cover which is now negative in pressure, in the form of the rotating air flow.

Abstract: There is provided an optical filtering system intended for use in a raster scanning arrangement for a document processing system. The document processing system includes a laser beam generating device providing a laser beam and a raster scanning device for transmitting the provided laser beam along a path. The optical filtering system includes a) an optical filter, disposed in the path, for reflecting the laser beam or permitting a portion of the laser beam to be transmitted therethrough, and b) a heating subsystem, thermally communicating with the optical filter, for maintaining the optical filter at a substantially fixed temperature.

Abstract: A raster output scanner (ROS) with a liquid crystal window located at the output of the ROS for correcting the nonuniformity of a light beam generated by the ROS is disclosed. Since the photocells in the liquid crystal can be controlled individually, a variable attenuation to match the curve of the intensity of the light beam, generated by the ROS, can be applied to the light beam. When the light beam with a variable intensity passes through the liquid crystal window of this invention, each cell applies an attenuation to offset the variation in intensity of the light received by that cell. Therefore, the light beam emerging from the liquid crystal window will have a uniform intensity regardless of the variation of the intensity generated by the ROS.

Abstract: The present invention relates to an image forming system comprising an image forming device for forming an image on a sheet; first feeding rollers arranged at an upstream side of the image forming device, for pinching the sheet and for feeding the sheet to the image forming device; and second feeding rollers arranged at a downstream side of the image forming device, for pinching the sheet and for feeding the sheet at a speed faster than that of the first feeding rollers in order to apply a tension force to the sheet in a transverse direction thereof.

Abstract: Image forming apparatus and method for forming half tone images by pulse width modulating multi-valued image signals comprises first and second pattern signal generators which generate pattern signals each having the same period and a phase shifted from each other, first and second pulse width modulators for pulse width modulating said image signals using the first and second pattern signals, respectively, a selector for selecting either of the first and second pulse width modulated signals and a controller for controlling the selector to obtain a screen angle of 45.degree..Further, image forming apparatus and method which realizes a screen angle .theta. wherein tan .theta. is represented by a rational number.

Abstract: A scanner has a light sensing unit, and a reflecting unit which reflects light to the sensing unit either directly or indirectly. The reflecting unit receives light from an object being scanned or from a light source. In the former case, the light is reflected to the sensing unit directly. In the latter case, the light is reflected to the sensing unit indirectly. Thus, the reflecting unit directs light to an object being scanned and the object transmits or reflects the light to the sensing unit. The reflecting unit is made up of a series of micromirrors, and each of the micromirrors corresponds to a respective point of the object being scanned. The micromirrors are independently movable between rest positions in which they reflect light away from the sensing unit and active positions in which they reflect light towards the sensing unit.

Abstract: Raster output scanner assemblies, and systems which use such assemblies, having mirrors mounted on the outside of an outer frame. Slits in the outer frame enable light to reflect from the mirrors. Beneficially, the outside mirror or mirrors are protected from contamination by mounting the primary surface of the mirrors (the surface which reflects light) against the outer frame and by mounting a cover over the mirrors.

Abstract: An optical scanning system includes a first lens for receiving a light beam from a light source and allowing the light beam to pass therethrough, and a polygon mirror having at least one reflection surface for deflecting the light beam from the first lens by reflecting the light beam on the reflection surface thereof. A second lens is provided for receiving the light beam from the polygon mirror and focusing the light beam on a first plane to be scanned. The second lens has a first surface facing toward the polygon mirror and a second surface facing toward the first plane.

Abstract: A laser light radiation device has a light source for emitting laser light which is modulated according to an image signal, a collimator lens for collimating the laser light emitted from the light source, and a polygonal mirror for deflecting the laser beam emerging from the collimator lens. The device further has a diffraction grating in a laser optical path between the collimator lens and the polygonal mirror.

Abstract: In the electrophotographic image recording apparatus, the optical path switching mechanism switches the optical path between an image reading mode and an image recording mode. In the image reading mode, the optical path switching mechanism directs light from a light scanning portion toward the original. Light reflected from the original is guided by a light guiding mechanism to a light receiving element where the light is converted into electrical signals.

Abstract: A copying machine for forming an image by scanning a laser beam which is emitted from an optical system on a photosensitive member. Photosensors are provided at optically equivalent positions to a beginning portion and an end portion of a scanning line on a photosensitive member. A scanning time of a laser beam in the main scanning direction is measured by these photosensors. The measured time and a standard time for a copy magnification set by an operator are compared, and a correction value is calculated, and at the same time, magnification in the main scanning direction is corrected using the correcting value. Also, focusing (adjusting of the beam diameter) is carried out using the photosensors. Each of the photosensors contains a photoelectric transfer element, and a beam which comes through a slit is incident to the element. Each photosensor is provided with two slits. One is perpendicular to the main scanning direction, and the other is inclined to the main scanning direction.

Abstract: In an overfilled polygon ROS architecture, fast scan jitter is reduced by uniformly illuminating the overfilled facets. In one embodiment, the collimator lens is designed with a long focal length to transmit only the more uniform central portion of the Gaussian shaped intensity profile output of a laser diode. In another embodiment, the light level at the polygon facet is made uniform by introducing a variable transmission filter along the optical path.

Abstract: A laser beam printer includes a movable mirror which is rotated by a scan mirror motor. Laser light is alternately deflected toward an information reading station and an information writing station by applying the laser light to the movable mirror from a semiconductor laser. The laser light is switched between writing-use light and reading-use light at the time the movable mirror is rotated through a predetermined angle. In the laser beam printer having the semiconductor laser which is used for both writing and reading information, information is written and read out during one rotation of the movable mirror. It is therefore possible to perform a reading operation and a writing operation simultaneously by simply switching the laser light at the predetermined time depending on the rotation angle of the movable mirror.

Abstract: A device (40) for patterning an imaging member (46) is provided. The device (40) comprises a light source (42) which emits light rays (44). Light rays (44) pass through a collimator (45) to collimate the light rays (48). The light then strikes a spatial light modulator (50) which is controlled by a computer (52) to reflect the light (54). The light passes through an imaging member (56) to demagnify the pattern for striking imaging member (46). Imaging member (46) is thus patterned by changing modulator (50) by computer (52).

Abstract: An electrophotographic recording apparatus and method wherein an electrophotographic recording medium is continuously moved in a first direction relative to a row of light-emitting recording elements such as LEDs. A beam shifter assembly is located within an optical path between the LEDs and the recording medium. The beam shifter assembly is operable in a first mode to pass light from the LEDs without substantial lateral shifting and in a second mode to pass light with substantial shifting of said light in a lateral direction having a directional component parallel to the row of LEDs. A control is provided for controlling the beam shifter assembly in different modes on alternate recording time lines of exposure. This helps minimize in-track artifacts. The apparatus is also operable in a multiaddress mode that is automatically instituted when a higher resolution image scanner is used or when a higher resolution character font library is input for recording.

Abstract: There is disclosed an optic system for illuminating a spatial light modulator array in a xerographic printing process consisting of an array of LED emitters constructed to efficiently replace the conventional tungsten source used in prior art. The array of LED emitters can be geometrically configured or electrically operated by strobing or varying the brightness of individual pixels to compensate for other system optical deficiencies and results in improved printing process. By exposure strobing the LED source correction for fuzzy line edges can be achieved.

Abstract: An optical element for the selective scan line skew correction in a raster output scanner (ROS) of an electrostatographic printer. The optical element includes a chamber having a volume defined by parallel surfaces in substantially parallel planes containing a liquid crystal material having a variable index of refraction as a function of voltage applied thereacross. An inhomogenous electric field is generated through the liquid crystal material to produce a refractive index gradient along substantially perpendicular planes in the optical element. The ROS includes a control and feedback system coupled to a variable voltage source for applying a variable biasing voltage across the optical element to control the variable index of refraction of the liquid crystal material for selectively correcting scan line skew.

Abstract: A method for the exposure of photo-sensitive material to be exposed (1) is performed inside of a casing (2) sealed against light from an external light source. The material to be exposed (1) is placed in a circular arc-shaped plane (3) having a uniform distance to a center longitudinal axis (4). Laser light beams (10), derived from the laser beam (6) of a laser generator (5) and deflected with a rotatable mirror (9), are providing exposure spots on the material to be exposed (1) with point-shaped signals. The recording speed is doubled or, respectively, an image or text composition is processed in half the time previously required by deflecting the light beams (10), exiting in parallel to an axis from the collimator (8), onto at least two mirror faces (12a and 12b), disposed at an angle (11) relative to the center median axis (4).

Abstract: Image forming apparatus and method for forming half tone images by pulse width modulating multi-valued image signals comprises first and second pattern signal generators which generate pattern signals each having the same period and a phase shifted from each other, first and second pulse width modulators for pulse width modulating said image signals using the first and second pattern signals, respectively, a selector for selecting either of the first and second pulse width modulated signals and a controller for controlling the selector to obtain a screen angle of 45.degree..Further, image forming apparatus and method which realizes a screen angle .theta. wherein tan .theta. is represented by a rational number.

Abstract: A copying machine for forming an image by scanning a laser beam which is emitted from an optical system on a photosensitive member. Photosensors are provided at optically equivalent positions to a beginning portion and an end portion of a scanning line on a photosensitive member. A scanning time of a laser beam in the main scanning direction is measured by these photosensors. The measured time and a standard time for a copy magnification set by an operator are compared, and a correction value is calculated, and at the same time, magnification in the main scanning direction is corrected using the correcting value. Also, focusing (adjusting of the beam diameter) is carried out using the photosensors. Each of the photosensors contains a photoelectric transfer element, and a beam which comes through a slit is incident to the element. Each photosensor is provided with two slits. One is perpendicular to the main scanning direction, and the other is inclined to the main scanning direction.

Abstract: A single pass highlight color printer is disclosed which utilizes two pairs of Raster Output Scan (ROS) systems mounted symmetrically about a common polygon motor assembly. Four identical optical systems are thus provided, 180.degree. apart with diode laser outputs directed from a ROS frame parallel and separated to allow the appropriate spacing for xerographic components situated adjacent the xerographic belt. Each ROS unit includes a 90.degree. mirror pair which is mounted on a roof mirror carriage which in turn is connected to a focus carriage via an adjustable guide system. The interaction between the focus carriage and the roof mirror carriage enables a skew alignment and process (beam steering) adjustment to be accomplished with a high degree of precision.

Abstract: A single pass color xerographic printing system with a single polygon, single optical system Raster Output Scanning (ROS) system has a dual wavelength laser diode source for the ROS which images the dual beams at a single station as closely spaced spots on a dual layer photoreceptor with each photoreceptor layer sensitive to or accessible by only one of the two wavelengths.

Abstract: A record head is energized in accordance with received image data to transfer an ink of an ink sheet to a record medium and perform image recording on the record medium while the ink sheet and the record medium are fed in opposite directions. The rotation period of a roller driven during feeding of the record medium or the feed direction of the record medium is detected to detect adhesion between the ink sheet and the record medium. When adhesion between the ink sheet and the record medium is detected, a recording operation is interrupted to feed the ink sheet and the record medium to release adhesion between the ink sheet and the record medium. When the adhesion is released within a predetermined time period, the recording operation is continued. When the adhesion is not released within the predetermined time period, an alarm sound is issued.

Abstract: An optical beam scanning apparatus comprises means of modulating the optical power of a laser beam produced by a laser beam source, means of shaping the laser beam profile, and means of deflecting and focusing the shaped laser beam on a scanning plane. The optical power of the laser beam is varied in synchronism with the variation of the optical power distribution so that the peak level of optical power of the laser beam is kept constant. The optical power of the laser beam on the photosensitive drum is kept constant irrespective of the beam diameter, and printed dots of intended diameters can be produced, whereby fine multi-tone images can be printed stably.