Patent Publication Number: US-2020301137-A1

Title: Optical scanner, display system, and mobile object

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This patent application is based on and claims priority pursuant to 35 U.S.C. § 119(a) to Japanese Patent Application No. 2019-052115, filed on Mar. 20, 2019, in the Japan Patent Office, the entire disclosure of which is hereby incorporated by reference herein. 
     BACKGROUND 
     Technical Field 
     The present disclosure relates to an optical scanner, a display system, and a mobile object. 
     Related Art 
     In the related art, two-dimensional scanning devices are known that includes an oscillating mirror used to scan the light flux emitted from a light source in the first and second directions and form a two-dimensional scanning area. 
     SUMMARY 
     In view of the above, it is an object to provide an optical scanner that substantially prevents a decrease in detection accuracy due to the changes in the environment or the changes over time, a display system incorporating the optical scanner, and a mobile object incorporating the display system. 
     In one aspect of this disclosure, there is provided an improved optical scanner including a light source; a light deflector configured to deflect light emitted from the light source to scan in a main scanning direction and a sub-scanning direction perpendicular to the main scanning direction; a photosensor having a detection field, configured to detect the light scanning the detection field; and processing circuitry. The processing circuitry is configured to control the light source to emit light to scan an irradiation area; and shift the irradiation area between a first position overlapping with the detection field and a second position other than the first position in the sub-scanning direction. 
     In another aspect of this disclosure, there is provided an improved display system including the above-described optical scanner; an imaging optical system configured to reflect the light that has been deflected by the light deflector to scan the screen and projected by the screen; and a reflector configured to reflect the light reflected from the imaging optical system so as to form a virtual image. 
     In still another aspect of this disclosure, there is provided an improved mobile object including the above-described display system. The reflector is a windshield of the mobile object. 
     The embodiments of the present disclosure provide an optical scanner that substantially prevents a decrease in detection accuracy due to the changes in the environment or the changes over time, a display system incorporating the optical scanner, and a mobile object incorporating the display system. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       A more complete appreciation of the disclosure and many of the attendant advantages and features thereof can be readily obtained and understood from the following detailed description with reference to the accompanying drawings, wherein: 
         FIG. 1  is an illustration of an example configuration of a display system according to a first embodiment of the present disclosure; 
         FIG. 2  is a block diagram of an example hardware configuration of a display device according to the first embodiment; 
         FIG. 3  is a diagram illustrating a specific configuration of a light-source device according to the first embodiment; 
         FIG. 4  is an illustration of a specific configuration of a light deflector according to the first embodiment; 
         FIG. 5  is an illustration of an example of a specific configuration of a screen according to the first embodiment; 
         FIGS. 6A and 6B  are illustrations for describing the differences in action caused by the differences of the diameter of incident light flux and the lens diameter in a microlens array; 
         FIG. 7  is an illustration for describing the relation of a mirror and a scanning range of the light deflector; 
         FIG. 8  is an illustration of an example of a trajectory of a scanning line when two-dimensional scanning is performed, according to an embodiment of the present disclosure; 
         FIGS. 9A and 9B  are illustrations for describing the shapes of screens; 
         FIGS. 10A, 10B, and 10C  are illustrations for describing a light receiver; 
         FIG. 11  is an illustration of an augmented reality (AR) overlaid image displayed; 
         FIGS. 12A and 12B  are illustrations for describing a drive voltage and a deflection angle sensitivity of the light deflector; 
         FIG. 13  is an illustration of a configuration for correcting an image according to the first embodiment. 
         FIG. 14  is a front view of a holder of a screen; 
         FIG. 15  is a perspective view of the holder in  FIG. 14 ; 
         FIGS. 16A, 16B, and 16C  are illustrations for describing detection of a scanning position; 
         FIG. 17  is a block diagram of a functional configuration of a controller according to the first embodiment; 
         FIG. 18  is a flowchart of image position adjustment processing and image size adjustment processing; 
         FIGS. 19A, 19B, and 19C  are illustrations for describing a method of detecting an initial value; 
         FIGS. 20A, 20B, and 20C  are illustrations for describing a method of detecting the amount of misalignment; 
         FIGS. 21A, 21B, 21C, and 21D  each is a graph of a detection signal of irradiation light received by the light receiver; 
         FIG. 22  is a graph of scanning angles and drive voltage in the sub-scanning direction according to an embodiment of the present disclosure; 
         FIG. 23  is a diagram illustrating a configuration according to a variation of the embodiment in  FIG. 13 ; 
         FIG. 24  is a graph of scanning angles and drive voltage in the sub-scanning angle according to the variation in  FIG. 23 ; 
         FIGS. 25A, 25B, and 25C  each is a diagram illustrating a method of detecting an initial value according to a variation of the embodiment illustrated in  FIGS. 19A, 19B, and 19C ; 
         FIGS. 26A, 26B, and 26C  each is a diagram illustrating a method of detecting the amount of misalignment according to a variation of the embodiment illustrated in  FIGS. 20A, 20B, and 20C ; 
         FIGS. 27A, 27B, and 27C  each is a diagram illustrating a method of detecting an initial value according to a second variation of the embodiment illustrated in  FIGS. 19A, 19B , and  19 C; 
         FIGS. 28A, 28B, and 28C  each is a diagram illustrating a method for detecting the amount of misalignment according to a second variation of the embodiment illustrated in  FIGS. 20A, 20B, and 20C ; 
         FIG. 29  is a graph of the relation between the position and amount of shift in the sub-scanning direction according to the second variations illustrated in  FIGS. 27A to 27C and 28A to 28C ; 
         FIG. 30  is an illustration of a configuration for correcting an image according to a second embodiment; 
         FIGS. 31A and 31B  are illustrations for describing a correction of an image size; 
         FIGS. 32A and 32B  are illustrations for describing a correction of an image position; 
         FIG. 33  is a graph indicating the relation of a parameter K and a position of the edge of the light receiver in the sub-scanning direction; 
         FIG. 34  is an illustration of the position of the light receiver according to a first variation of the second embodiment; 
         FIG. 35  is an illustration of the position of the light receiver according to a second variation of the second embodiment; 
         FIG. 36  is a flowchart for describing an example of a control of correction of an image size; 
         FIG. 37  is an illustration of a configuration for correcting an image according to a third embodiment; 
         FIG. 38  is an illustration of examples of a display image according to a calculation formula of an image position shift amount; 
         FIGS. 39A and 39B  are illustrations of a configuration for correcting an image according to a fourth embodiment; 
         FIG. 40  is an illustration of a display image area according to the second embodiment; and 
         FIG. 41  is an illustration of a display image area according to the third embodiment; and 
         FIG. 42  is an illustration of a display image area according to the fourth embodiment; and 
         FIG. 43  is a table for comparing the display image areas according to the second to fourth embodiments. 
     
    
    
     The accompanying drawings are intended to depict embodiments of the present invention and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. 
     DETAILED DESCRIPTION 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. 
     In describing embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that have a similar function, operate in a similar manner, and achieve a similar result. 
     Embodiments of the present disclosure are described with reference to the drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted. 
       FIG. 1  is an illustration of an example configuration of a display system according to a first embodiment of the present disclosure. In the display system  1 , the viewer  3  can visually identify a display image as the projection light PL that is projected from a display device  100  is projected onto a transmissive reflector. The display image is an image that is superimposed and displayed as a virtual image  45  on the field of view of the viewer  3 . For example, the display system  1  is provided for a mobile object such as a vehicle, an aircraft, and a ship, or an immobile object such as a maneuvering simulation system, and a home-theater system. In the present embodiment, cases in which the display system  1  is provided on a vehicle as an example of a mobile object is described. However, no limitation is intended thereby, and the type of usage of the display system  1  is not limited to the present embodiment. 
     For example, the display system  1  is mounted in a vehicle, and makes navigation information visible to the viewer  3  (i.e., the driver) through a windshield  50  of the vehicle. The navigation information includes, for example, the information about the speed of the vehicle, the course information, the distance to a destination, the name of the current place, the presence or position of an object ahead of the vehicle, a traffic sign indicating, for example, speed limit, and traffic congestion, and aids the driving of the vehicle. In such cases, the windshield  50  serves as a transmissive reflector that transmits a portion of the incident light and reflects at least some of the remaining incident light. The distance between the location of the eyepoint of the viewer  3  and the windshield  50  is about several tens of centimeters (cm) to one meter (m). 
     The display system  1  includes the display device  100 , an extraneous light sensor  20 , and a windshield  50 . For example, the display device  100  is a heads-up display (HUD) provided for a vehicle as an example of the mobile object. The display device  100  may be arranged at any desired position in conformity with the interior design of the vehicle. For example, the display device  100  according to the present embodiment may be disposed under a dashboard  200  of the vehicle or built into the dashboard of the vehicle. 
     Moreover, the display device  100  includes an image forming unit  10 , a free-form surface mirror  30 , and a housing  90 . The image forming unit  10 , which is an example of an optical scanner, includes a light-source device  11 , a unit housing  12 , a light deflector  13 , a mirror  14 , and a screen  15 . The extraneous light sensor  20  is a sensing device that is disposed to detect, for example, the illuminance as the intensity of extraneous light of the display system  1 . As illustrated in  FIG. 1 , for example, the extraneous light sensor  20  is arranged near the windshield  50 . 
     The light-source device  11  that is an example of a light source is a device that emits the laser beams emitted from a light source to an area outside the device. For example, the light-source device  11  may emit laser beams in which three-color laser beams of red, green, and blue (RGB) are combined. The laser beams that are emitted from the light-source device  11  are guided to the reflection plane of the light deflector  13 . For example, the light-source device  11  has a semiconductor light-emitting element such as a laser diode (LD) that serves as a light source. However, no limitation is intended thereby, and the light source may be a semiconductor light-emitting element such as a light-emitting diode (LED). 
     The light deflector  13  as an example of a light deflector uses, for example, a micro-electromechanical systems (MEMS) to change the directions of travel of the laser beams. For example, the light deflector  13  is configured by a scanner such as a mirror system composed of one minute MEMS mirror that pivots around two axes orthogonal to each other or two MEMS mirrors that pivot or rotates around one axis. The laser beams that are emitted from the light deflector  13  scans the mirror  14 . The light deflector  13  is not limited to a MEMS mirror, but may be configured by a polygon mirror or the like. 
     For example, the mirror  14  is a concave mirror, and reflects the laser beams, which are deflected by the light deflector  13  and scan the reflection plane of the mirror  14 , towards the screen  15 . 
     A two-dimensional intermediate image (image light) is formed on the screen  15 , which serves as an image forming unit, as the laser beams reflected by the reflection plane of the mirror  14  scan the surface of the screen  15 . Note also that the screen  15  serves as a divergent part through which the scanned laser beams diverge at a predetermined divergence angle. For example, the screen  15  may consist of an exit pupil expander (EPE), and may be configured by a transmissive optical element such as a microlens array (MLA) or diffuser panel that diffuses light. Alternatively, the screen  15  may be configured by a reflective optical element such as a micromirror array that diffuses light. 
     The light-source device  11 , the light deflector  13 , the mirror  14 , and the screen  15  are stored in the unit housing  12 , and serve as a part of the image forming unit  10 . The screen  15  is not covered by the unit housing  12  in its entirety such that the diverging light diverging through the screen  15  can be emitted outside the image forming unit  10 , but is partially held by the unit housing  12 . The unit housing  12  may be a single unit of three-dimensional object, or may be configured by a combination of a plurality of members. As an example configuration or structure in which a combination of a plurality of members are combined, the unit housing  12  may be configured by a combination of a plurality of members including the light-source device  11 , the light deflector  13 , the mirror  14 , a three-dimensional object that covers the optical path in its entirety, and a holder or the like that holds the screen  15 . 
     The virtual image  45  is a magnified view of the intermediate image that is formed on the screen  15 , and such a virtual image is achieved as the laser beams (light flux) that are the light diverging through the screen  15  are projected onto the free-form surface mirror  30  and the windshield  50 . The free-form surface mirror  30  is designed and arranged so as to cancel, for example, the inclination of the image, the distortion of the image, and the displacements of the image, which are caused by the bent shape of the windshield  50 . The free-form surface mirror  30  may be arranged in a pivotable manner around a rotation axis  301 . For example, the rotation axis  301  passes through the center of gravity of the free-form surface mirror  30 , and the free-form surface mirror  30  is rotated on a straight line parallel to the direction orthogonal to the sheet of  FIG. 1  to change the position at which the virtual image  45  is displayed in the up-and-down directions on the sheet of  FIG. 1 . Due to such a configuration, the free-form surface mirror  30  can adjust the reflection direction of the laser beams (light flux) emitted from the screen  15  to change the position at which the virtual image  45  is displayed according to the positions of the eyes of the viewer (driver)  3 . 
     The free-form surface mirror  30 , which is an example of an imaging optical system, reflects the diverging light to project the projection light PL in order to form a virtual image with the light diverging through the screen  15 . Due to this configuration, the free-form surface mirror  30  is designed using, for example, a commercially available optical design simulation software, such that the free-form surface mirror  30  has a certain level of light-gathering power to achieve a desired image-forming position of the virtual image  45 . In the display device  100 , the light-gathering power of the free-form surface mirror  30  is designed such that the virtual image  45  is displayed at a position away from the location of the eyepoint of the viewer  3  in the depth direction by, for example, at least 1 m and equal to or shorter than 30 m (preferably, equal to or shorter than 10 m). 
     The imaging optical system is satisfactory as long as it includes at least one light-concentrating element that has a light-concentrating function. Such a light-concentrating element that has a light-concentrating function is not limited to a free-form surface mirror like the free-form surface mirror  30 , and may be, for example, a concave mirror, a curved-surface mirror, and a Fresnel reflector element. For example, such a light-concentrating element is formed by performing sputtering or vapor deposition on a thin metal film such as of aluminum (Al) and silver (Ag) with high reflectivity. Due to such a configuration, the utilization efficiency of the light incident on a light-concentrating element as the projection light PL can be maximized, and a virtual image with high brightness can be obtained. 
     The projection light PL that is reflected by the free-form surface mirror  30  is projected outside the display device  100  from a slit formed on the housing  90 , and is incident on the windshield  50 . As illustrated in  FIG. 1 , a hole H is formed on the housing  90  by a hole surrounding area  901 . The hole surrounding area  901  is a part of the housing  90  around the hole H. The position and size of the hole H is determined depending on the size of the hole surrounding area  901  and the position at which the hole surrounding area  901  is arranged. In order to prevent a foreign substance from entering the housing  90  through the hole H, a dustproof window  40  is arranged so as to close the hole H. In particular, it is desired that the dustproof window  40  be made of a material through which the projection light PL can pass through. 
     The windshield  50  is an example of a reflector and serves as a transmissive reflector that transmits some of the laser beams (bundle of laser beams) and reflects at least some of the remaining laser beams (partial reflection). The windshield  50  may serve as a semitransparent mirror through which the observer  3  visually recognizes the virtual image  45  and the scenery ahead of the mobile object (vehicle). The virtual image  45  is an image to be displayed that is visually recognized by the viewer  3 , including vehicle-related information (e.g., speed and travel distance), navigation information (e.g., route guidance and traffic information), and warning information (e.g., collision warning). For example, the transmissive reflector may be another windshield arranged in addition to the windshield  50 . 
     The virtual image  45  may be displayed so as to be superimposed on the scenery ahead of the windshield  50 . The windshield  50  is not flat but is curved. For this reason, the position at which the virtual image  45  is formed is determined by the curved surface of the free-form surface mirror  30  and the windshield  50 . In some embodiments, the windshield  50  may be a semitransparent mirror (combiner) that serves as a separate transmissive reflector having a partial reflection function. 
     The plane of the free-form surface mirror  30  is designed and shaped so as to reduce the optical distortion that occurs on the windshield  50 . The light beams that are incident on the free-form surface mirror  30  are reflected by the free-form surface mirror  30  according to the shape of the plane of the free-form surface mirror  30 . The reflected bundles of laser beams (light flux) are then incident on the windshield  50 , and reach at least one eyepoint within an eye-lip area including at least the eye-lip center (i.e., the reference eyepoint). The bundles of laser beams that are incident on the windshield  50  are reflected according to the shape of the surface of the windshield  50 . 
     Due to such a configuration as above, the laser beams (light flux) that are emitted from the screen  15  are projected towards the free-form surface mirror  30 . The projection light that is concentrated by the free-form surface mirror  30  passes through the hole H of the housing  90  and is projected towards the windshield  50 , and is reflected by the windshield  50 . Accordingly, the observer  3  can visually recognize the virtual image  45 , i.e., the magnified image of the intermediate image formed on the screen  15 , due to the light reflected by the windshield  50 . 
     A method of projecting an image using the display device  100  may be implemented by a panel system or a laser scanning system. In the panel system, an intermediate image is formed by an imaging device such as a liquid crystal panel, a digital micromirror device (DMD) panel (digital mirror device panel), or a vacuum fluorescent display (VFD). In the laser scanning system, an intermediate image is formed by scanning the laser beams emitted from the light-source device  11 , using an optical scanner. 
     The display device  100  according to the first embodiment of the present disclosure adopts the laser scanning system. In particular, in the laser scanning system, since emitting/non-emitting can be assigned to each pixel, in general, a high-contrast image can be formed. In some alternative embodiments, the panel system may be adopted as the projection system in the display device  100 . 
       FIG. 2  is a block diagram of a hardware configuration of the display device according to the first embodiment. The hardware configuration illustrated in  FIG. 2  may be adopted in common among the embodiments of the present disclosure. Alternatively, some components or elements may be added to or deleted from the hardware configuration of  FIG. 2 . 
     The display device  100  includes a controller  17  that controls the operation of the display device  100 . For example, the controller  17  is a circuit board or integrated circuit (IC) chip mounted inside the display device  100 . The controller  17  includes a field-programmable gate array (FPGA)  1001 , a central processing unit (CPU)  1002 , a read only memory (ROM)  1003 , a random access memory (RAM)  1004 , an interface (UF)  1005 , a data bus line  1006 , a laser diode (LD) driver  1008 , a micro-electromechanical systems (MEMS) controller  1010 , and a motor driver  1012 . The controller  17  is an example of a control unit. 
     The FPGA  1001  is an integrated circuit whose setting can be changed by a designer of the display device  100 . The LD driver  1008 , the MEMS controller  1010 , and the motor driver  1012  generate a drive signal according to the control signal output from the FPGA  1001 . The CPU  1002  is an integrated circuit that controls the entirety of the display device  100 . The ROM  1003  is a storage device that stores a computer executable program for controlling the CPU  1002 . The RAM  1004  is a storage device that serves as a work area of the CPU  1002 . 
     The interface  1005  communicates with an external device. For example, the interface  1005  is coupled to the controller area network (CAN) of a vehicle Moreover, the interface  1005  is coupled to, for example, the extraneous light sensor  20 . The extraneous light sensor  20  sends the sensing data (i.e., the intensity of the extraneous light) to the controller  17  through the interface  1005 . Note also that the sensor that is coupled to the interface  1005  is not limited to the extraneous light sensor  20 , and other various kinds of sensors for acquiring the internal and external information of the vehicle may be coupled to the interface  1005 . 
     The LD driver  1008  is a circuit that generates a drive signal for driving the light-source device  11 . For example, the LD driver  1008  generates a drive signal for a semiconductor light-emitting element LD that configures a part of the light-source device  11 . The MEMS controller  1010  is a circuit that generates a drive signal for driving the light deflector  13 . For example, the MEMS controller  1010  generates a drive signal for driving the MEMS that is a device for moving a scanning mirror that configures a part of the light deflector  13 . The motor driver  1012  is a circuit that generates a drive signal for driving various kinds of motors. For example, the motor driver  1012  generates a drive signal for driving the motor  1011  that rotates the rotation axis  301  of the free-form surface mirror  30 . 
       FIG. 3  is a diagram illustrating a specific configuration of the light-source device  11  according to the first embodiment of the present disclosure. The light-source device  11  includes light-source elements  111 R,  111 G, and  111 B (these light-source elements may be referred to simply as a light-source element  111  in the following description when it is not necessary to distinguish each of the light-source elements), coupling lenses (collimators)  112 R,  112 G, and  112 B, apertures  113 R,  113 G, and  113 B, combiners  114 ,  115 , and  116 , an optical branching element  117 , a lens, and a photo sensor. 
     For example, each of the light-source elements  111 R,  111 G, and  111 B of three colors (red, green, and blue (RGB)) is a laser diode (LD) having a single or a plurality of light-emitting points. Each of the light-source elements  111 R,  111 G, and  111 B emits laser beams whose light intensity depends on the amount of changes in the drive current that is supplied to each of the light-source elements. The light-source elements  111 R,  111 G, and  111 B emit bundles of laser beams (light flux) having different wavelengths λR, λG, and λB, respectively. For example, λR=640 nanometers (nm), λG=530 nm, and λB=445 nm. 
     The emitted bundles of laser beams (light flux) are coupled by the coupling lenses  112 R,  112 G, and  112 B, respectively. The coupled bundles of laser beams (light flux) are shaped by the apertures  113 R,  113 G, and  113 B, respectively. The shape of the apertures  113 R,  113 G, and  113 B may be various kinds of shape such as a circle, an ellipse, a rectangle, and a square depending on, for example, certain predetermined conditions such as the divergence angle of the bundles of laser beams (light flux). 
     The laser beams (light flux) that are shaped by the apertures  113 R,  113 G, and  113 B are combined by the three combiners  114 ,  115 , and  116 , respectively. The combiners  114 ,  115 , and  116  are plate-like or prismatic dichroic mirrors, and reflect or transmit the laser beams (light flux) therethrough according to the wavelength of the laser beams to combine the laser beams into one bundle of laser beams (light flux) that travels along one optical path. Then, the combined light flux enters the optical branching element  117 . 
     Some of the light that has entered the optical branching element  117  passes through the optical branching element  117 , and different some of the light that has entered the optical branching element  117  is reflected by the optical branching element  117 . In other words, the combined laser beams (light flux) are branched into transmitted light and reflected light by the optical branching element  117 . 
     The transmitted light passes through the lens, and the light deflector  13  is irradiated with the transmitted light. As a result, the transmitted light is used to draw an image or display a virtual image on the screen  15 . In other words, the transmitted light is used as light for projecting an image. 
     On the other hand, the photosensor is irradiated with the reflected light. The photosensor outputs an electrical signal according to the light intensity of the received laser beams. For example, the output electrical signal is output to the FPGA  1001 , and may be used to control the display system  1 . As described above, according to the present embodiment, the reflected light is used as monitoring light that adjusts the intensity of the laser beams or monitoring light that adjusts the color or brightness of the resultant virtual image. 
       FIG. 4  is a diagram illustrating a specific configuration of the light deflector  13  according to the first embodiment of the present disclosure. The light deflector  13  is a MEMS mirror produced by semiconductor processing, and includes a mirror  130 , a serpentine beam  132 , a frame  134 , and a piezoelectric member  136 . The light deflector  13  is an example of a light deflector. 
     The mirror  130  has a reflection plane that reflects the laser beams emitted from the light-source device  11  towards the screen  15  side. In the light deflector  13 , a pair of serpentine beams  132  are formed across the mirror  130 . Each of the pair of serpentine beams  132  has a plurality of turning portions. Each of these turning portions is configured by a first beam  132   a  and a second beam  132   b  that are arranged alternately. Each of the pair of serpentine beams  132  is supported by the frame  134 . The piezoelectric member  136  is disposed such that the first beam  132   a  and the second beam  132   b,  which are adjacent to each other, are coupled to each other. The piezoelectric member  136  applies different levels of voltage to the first beam  132   a  and the second beam  132   b  to bend each of the first beam  132   a  and the second beam  132   b  differently. 
     As a result, the first beam  132   a  and the second beam  132   b,  which are adjacent to each other, bend in different directions. As the bending force is accumulated, the mirror  130  rotates in the vertical direction around the horizontal axis. Due to such a configuration as above, the light deflector  13  can perform optical scanning in the vertical direction at a low voltage. An optical scanning in the horizontal direction around the axis in the vertical direction is implemented by the resonance produced by a torsion bar or the like coupled to the mirror  130 . 
       FIG. 5  is an illustration of an example of a specific configuration of a screen according to the first embodiment. The screen  15  forms an image of the laser light emitted from the LD  1007  that forms a part of the light-source device  11 . The screen  15  serves as a divergent part that diverges the laser beams at a predetermined divergence angle. The screen  15  as illustrated in  FIG. 5  has a microlens-array structure in which a plurality of hexagonal-shaped microlenses  150  are arranged with no gap therebetween. For example, the width of each of the microlenses  150  (the distance between two sides that face each other) is optimized to a ranged from 50 micrometers (μm) to 300 μm. In the present embodiment, the width of each of the microlenses  150  is approximately 200 μm. As the microlenses  150  of the screen  15  have a hexagonal shape, the multiple microlenses  150  can be arrayed with high density. 
     Note that the shape of each microlens  150  is not limited to the hexagon shape. Thus, alternatively, in some embodiments, each microlens  150  has, for example, a rectangular shape, or a triangle shape. In the present embodiment, structure in which the multiple microlenses  150  are arrayed in a regularized manner is described. However, no limitation is intended thereby, and the arrangement of the microlenses  150  is not limited to this structure. For example, the centers of the multiple microlenses  150  may be decentered from each other, and the microlenses  150  may be arranged in an irregular manner. When adopting such an eccentric arrangement, each microlens  150  has a different shape. 
     Alternatively, the height of the vertex in the optical-axis direction may be changed. When the decentering in the direction in which microlenses are arrayed or the shifting in the optical-axis direction is determined on a random basis, for example, the speckles that are caused by the interference of the laser beams that have passed through the boundary between each pair of neighboring microlenses and the moire that is caused by the cyclic array can be reduced. 
     The laser beams that have reached the screen  15  scan the inside of the microlenses  150 , and multiple dots are marked as the laser beams are switched on and off during the scanning. For example, the levels of gradation can be expressed by a combination of on-off control of light. Alternatively, the levels of gradation may be expressed by adjusting the radiation intensity of the laser beams. 
       FIGS. 6A and 6B  are illustrations for describing a difference in action due to the difference in incident-light-beam diameter and lens diameter in a microlens array. As illustrated in  FIG. 6A , the screen  15  is configured by an optical plate  151  in which the multiple microlenses  150  are neatly arranged. When an incident light  152  is scanned on the optical plate  151 , the incident light  152  diverges as passing through the microlenses  150 , and the incident light  152  becomes a diverging light  153 . Due to the structure of the microlenses  150 , the screen  15  can diverge the incident light  152  at a desired divergence angle  154 . The intervals  155  at which the microlenses  150  are arranged is designed to be wider than the diameter  156   a  of the incident light  152 . Accordingly, the screen  15  does not cause interference among the lenses, and does not cause speckles (speckle noise). 
       FIG. 6B  is a diagram illustrating the optical paths of diverging light beams when the diameter  156   b  of the incident light  152  is twice wider than the intervals  155  at which the microlenses  150  are arranged. The incident light  152  is incident on two microlenses  150   a  and  150   b,  and these two microlenses  150   a  and  150   b  produce two diverging light beams  157  and  158 , respectively. In such cases, a light interference might occur because two diverging light beams exist in an area  159 . Such an interference between two diverging light beams (coherent light) is visually recognized as a speckle by an observer. 
     In view of the above circumstances, the intervals  155  at which the microlenses  150  are arranged is designed to be wider than the diameter  156  of the incident light  152  in order to reduce the speckles. A configuration with convex lenses is described above with reference to  FIGS. 6A and 6B . However, no limitation is indicated thereby, and advantageous effects can be expected in a similar manner in a configuration with concave lenses. 
     As described above with reference to  FIG. 5 ,  FIG. 6A , and  FIG. 6B , the screen  15  that is an example of an image forming unit serves as a divergent part through which the scanned laser beams diverge at a predetermined divergence angle. Due to this functionality, the driver (viewer)  3  can recognize an image in the range of the eye box. Thus, even when the driver (viewer)  3  who is seated on the driver&#39;s seat changes the positions of his/her eyes to some extent, his/her visually-recognizable range can be secured. 
     As described above, it is desired that the shape of each one of the microlenses  150  has a certain level of precision such that the light appropriately diverges through the screen  15  provided with the microlenses  150 . Further, preferably, the screen  15  can be mass-produced. For this reason, for example, the screen  15  is molded by resin material. A concrete example of resin that satisfies the reflection property or optical property required for the microlenses  150  may include methacrylic resin, polyolefin resin, polycarbonate, and cyclic polyolefin resin. However, no limitation is intended thereby. 
       FIG. 7  is an illustration for describing the relation of a mirror and a scanning range of the light deflector  13 . The FPGA  1001  of the controller  17  controls the light-emission intensity, the timing of light emission, and the light waveform of the multiple light-source elements in the light-source device  11 . The LD driver  1008  drives the multiple light-source elements of the light-source device  11  to emit laser beams. As illustrated in  FIG. 7 , the laser beams that are emitted from the multiple light-source elements and whose optical paths are combined are two-dimensionally deflected about the α axis and the β axis by the mirror  130  of the light deflector  13 , and the screen  15  is irradiated with the laser beams deflected by the mirror  130 , which serve as scanning beams. In other words, the screen  15  is two-dimensionally scanned by main scanning and sub-scanning by the light deflector  13 . 
     As described with reference to  FIG. 1 , in the present embodiment, the mirror  14  is provided on the optical path between the light deflector  13  and the screen  15 . That is, the scanning light from the light deflector  13  two-dimensionally scans the mirror  14  and the reflected light from the mirror  14  scans the two-dimensionally on the screen  15  as the scanning light. 
     In the present embodiment, the entire area to be scanned by the light deflector  13  may be referred to as a scanning range. The scanning beams scan (two-way scans) the scanning range of the screen  15  in an oscillating manner in the main scanning direction (X-axis direction) at a high frequency of about 20,000 to 40,000 hertz (Hz) while one-way scanning the scanning range of the screen  15  in the sub-scanning direction (Y-axis direction) by a predetermined amount. The scanning in the sub-scanning direction is performed at a frequency of about a few tens of Hz, which is lower than the frequency in the main scanning direction. In other words, the light deflector  13  performs raster scanning on the screen  15 . In this configuration, the display device  100  controls the light emission of the multiple light-source elements according to the scanning position (the position of the scanning beam). Accordingly, an image can be drawn on a pixel-by-pixel basis and a virtual image can be displayed. 
     As described above, a two-dimensional scanning area is formed on the plane including the surface of the screen  15  every time raster scanning is performed one time. In other words, a two-dimensional scanning area is formed on the plane including the surface of the screen  15  for every sub-scanning cycle that is a predetermined cycle. The scanning area that is formed every time raster scanning is performed one time may be referred to as a scanning frame. As described above, the sub-scanning frequency is about a few tens of hertz (Hz). Accordingly, the length of time it takes for a scanning area to be formed on the screen  15  every sub-scanning cycle, i.e., the length of time to scan one scanning frame (one cycle of two-dimensional scanning), is a few tens of millisecond (msec). For example, assuming that the main-scanning cycle and the sub-scanning cycle are 20,000 Hz and 50 Hz, respectively, the length of time to scan one frame in one cycle of two-dimensional scanning is 20 msec with reference to the sub-scanning frequency. 
       FIG. 8  is an illustration of an example of a trajectory of a scanning line when two-dimensional scanning is performed, according to an embodiment of the present disclosure. As illustrated in  FIG. 8 , the screen  15  includes a display image area  15 R 1  and a non-image area  15 R 2 . The display image area  15 R 1  is irradiated with the light that is modulated according to the image information, and an intermediate image is drawn on the display image area  15 R 1 . The non-image area  15 R 2  is a frame-shaped area that surrounds the display image area  15 R 1 . 
     The scanning area  15 R 3  is a combined range of the display image area  15 R 1  and the non-image area  15 R 2  on the screen  15 . In  FIG. 8 , the track of the scanning in the scanning range and a plurality of main scanning lines, which is a linear track of scanning in the main scanning direction, are drawn in the sub-scanning direction, and form a zigzag line. For the sake of explanatory convenience, the number of main-scanning lines in  FIG. 8  is less than the actual number of main-scanning lines. In  FIG. 8 , the track of the scanning is a zigzag line where the ends of the main scanning lines are contiguous to each other. However, no limitation is intended therein. In other words, the main scanning lines may be parallel to each other, and the ends of the main scanning lines may be not continuous. The main scanning lines may be formed by the two-way scanning as illustrated in  FIG. 8 , or may be formed by repeated one-way scanning. 
     Further, the screen  15  includes a detection image field G that includes a light receiver disposed at the edges of the display image area  15 R 1  (a part of the non-image area  15 R 2 ) in the scanning range. In  FIG. 8 , the detection image area G is disposed on the −X and +Y side of the display image area  15 R 1 . More specifically, the detection image area G is disposed at a corner on the +Y side. The light detection sensor  60  is disposed at a position where the scanning light incident on the detection image area G is detected. When the scanning light is detected by the light detection sensor  60 , the light detection sensor  60  outputs a light signal to the FPGA  1001 . The light detection sensor  60  has a light receiver, such as a photodiode, and is fixed and disposed on a part of the screen  15  or a part of the unit housing  12 , for example. The FPGA  1001  detects the operation of the light deflector  13  based on the timing at which the signal is received. Accordingly, the start timing of scanning or the end timing of scanning can be determined. 
     In other words, the track of scanning line where a plurality of main scanning lines are drawn in the sub-scanning direction, which is linear track of scanning in the main scanning direction, is formed in the scanning range. In the display image area  15 R 1  of the scanning range, a display image to be presented to a user is formed by the track of scanning line where a plurality of main scanning lines, which are linear track of scanning in the main scanning direction, are drawn in the sub-scanning direction. For example, the display image is a still image (frame) that makes up the input moving images (video data). In the following description, the area within the display image area  15 R 1  in a scanning frame may be referred to as a scanning-frame image. 
     All the cycles in which a scanning frame and a scanning-frame image are formed corresponds to the sub-scanning cycle. A scanning-frame image is sequentially formed based on the image data for every sub-scanning cycle, and the virtual image  45  is drawn. Due to such a configuration, the viewer  3  who observes the virtual image  45  that is sequentially displayed can visually recognize the virtual image  45  as moving images. 
     As described above, the screen  15  is configured by a transmissive optical element such as the microlens array that diffuses light. However, no limitation is intended therein. In some embodiments, the screen  15  may be a reflective element such as a micromirror array that diffuses light, depending on the design or layout of the display device  100 . Alternatively, in some embodiments, the screen  15  may be a flat plate or curved plate that does not diffuse light. 
     In the embodiments of the present disclosure as described above, the formation of the scanning area  15 R 3  is described on condition that the scanning light that is emitted from the light deflector  13  scans the plane that includes a surface of the screen  15 . However, no limitation is intended therein. For example, when a mirror  14  is disposed on the optical path between the light deflector  13  and the screen  15 , the scanning area  15 R 3  is formed on the mirror  14 . Further, when another type of reflecting mirror is disposed on the optical path between the light deflector  13  and the screen  15 , the scanning area  15 R 3  is formed also on the reflecting mirror or the plane that is scanned by the scanning light reflected by the reflecting mirror. 
     At least, the display image area  15 R 1  is formed on the screen  15  in the end, but it is not necessary for all the scanning area  15 R 3  to reach the plane that includes the surface of the screen  15 . For example, the detection image area G may be designed to be detected by the light detection sensor  60  provided before reaching the plane including the screen  15 . Alternatively, the optical path may be changed before reaching the plane including the screen  15 , and the detection image area G is detected by the light detection sensor  60  disposed in the changed optical path. 
     The shape of the screen  15  and the shape of the display image area  15 R 1  are not limited to the shape as illustrated in  FIG. 8 . The display image area  15 R 1  does not need to be a flat rectangular shape as illustrated in  FIG. 8 . The display image area  15 R 1  may have a curved surface. Alternatively, the display image area  15 R 1  may have a rectangular or polygonal shape different from the rectangular shape of the screen  15 . 
     For example, the shape of the display image area  15 R 1  may be determined by the shape of a portion (i.e., a holder or the like) of the unit housing  12  that holds the screen  15 . In other words, when the screen  15  is held by the unit housing  12  or a holder of the screen  15 , which is a part of the unit housing  12 , so as to cover the non-image area  15 R 2 , the light incident on the held portion is blocked by the unit housing  12  and the free-form surface mirror  30  is not irradiated with the light. Accordingly, the free-form surface mirror  30  is irradiated with the diverging light from the intermediate image of the display image area  15 R 1 . As described above, the virtual image  45  of a desired shape is formed by determining the shape of the display image area  15 R 1 . 
       FIGS. 9A and 9B  are illustrations for describing the shapes of the screen  15 . The following describes the effect of the difference in the distance between the arrival positions of the light deflector  13  and the screen  15  due to the shape of the screen  15  on the image, with reference to  FIGS. 9A and 9B . 
       FIG. 9A  is an illustration of the case in which the screen  15  is flat.  FIG. 9B  is an illustration of the case in which the screen  15  is curved. More specifically, the screen  15  in  FIG. 9B  has a concave surface facing the mirror  130 . 
     In the case of the flat-screen in  FIG. 9A , the intermediate image  25  is distorted due to the difference in the distance between the arrival positions of the light deflector  13  and the screen  15 . In addition, the beam spot diameter varies depending on the arrival position. When such light beams are formed as the virtual image  45  through the observation optical system constituted by the free-form surface mirror  30 , the windshield  50 , and the like, the image quality might decrease. An optical element for correcting such distortion of images and variation in spot diameter might be disposed between the light deflector  13  and the to-be-scanned surface  22 , but this leads to an increase in HUD size and cost. 
     By contrast, curving the screen  15  as illustrated in  FIG. 9B  can reduce the difference in the above-described distance. Accordingly, the distortion of the intermediate image  25  and the variation of the beam spot diameter can be reduced as compared with the case of the flat screen. Thus, the image quality of the virtual image  45  can be improved without increasing the size of the HUD or increasing the cost. 
     As described with reference to  FIG. 1 , in the present embodiment, the mirror  14  is provided on the optical path between the light deflector  13  and the screen  15 . That is, the scanning light from the light deflector  13  two-dimensionally scans the mirror  14  and the reflected light from the mirror  14  scans the two-dimensionally on the screen  15  as the scanning light. 
     The configuration of the light detection sensor  60  will be described with reference to  FIGS. 10A, 10B, and 10C . As described above, the light detection sensor  60  includes a light receiver  61  (an example of a photosensor) and a signal line  62 . The light receiver  61  is, for example, a photodiode that converts an optical signal into an electric signal. When the laser beam scans the light receiving surface of the light receiver  61 , a pulse signal waveform is obtained. A detection field in which the light receiver  61  detects the irradiation light (the light emitted from the light-source device) is equal to an area of the light receiver  61 , and such a detection field may be smaller than the area of the light receiver  61 . 
       FIG. 10A  is an illustration of the configuration of the light detection sensor  60 . As described above, the light detection sensor  60  includes the light receiver  61  and the signal line  62 . The light receiver  61  is, for example, a photodiode that converts an optical signal into an electric signal. The light receiver  61  has two divided surfaces to receive light, which are light-receiving surfaces  611  and  612 . The light-receiving surface  611  has a smaller width in the main scanning direction than the light-receiving surface  612  does. The signal line  62  has a signal line  621  for outputting a signal from the light-receiving surface  611  and a signal line  622  for outputting a signal from the light-receiving surface  612 . 
       FIG. 10B  is a graph of a signal output when scanning laser light moves in a direction from the light-receiving surface  611  to the light-receiving surface  612 . More specifically,  FIG. 10B  indicates two types of signals output from the light-receiving surfaces  611  and  612  when the scanning is performed in a direction from the light-receiving surface  611  to the light-receiving surface  612  having a larger width in the scanning direction. As illustrated in  FIG. 10B , when the scanning of the laser beam is performed, a pulse signal having a width corresponding to the time it takes for the laser light to pass through each light-receiving surface. Thus, the timing at which light has passed through the light-receiving surface can be detected using the pulse signal. 
       FIG. 10C  is an illustration of a pulse signal waveform obtained based on the intersecting timing of the two signals in  FIG. 10B . More specifically, a signal as illustrated in  FIG. 10C  is output based on the intersecting timing so as to obtain a pulse signal waveform when the laser light scans on the photodiode. 
     The absolute value of an output signal value of the pulse signal changes with the amount of light incident on one photodiode varies. Accordingly, the timing at which the light passes through the light-receiving surface might be shifted at the time of detection even if the light receiver is scanned at the same timing. In view of such a situation, when the fluctuation of the amount of incident light is significant, the intersecting timing of the two signals as illustrated in  FIG. 10C  is used so as to achieve high-accuracy detection. 
     In the above-described embodiment, the case where the light detection sensor provided with two divided light-receiving surfaces is used is described. However, no limitation is intended thereby. In some examples, the light-receiving surface may not be divided. In this case, as illustrated in  FIG. 10B , the point in time at which the output signal value exceeds a predetermined threshold value is detected as the timing at which light has passed through the light-receiving surface. 
     The following describes an example in which the two-dimensional scanning device described above is mounted on a vehicle.  FIG. 11  is an example of a displayed image when the display device  100  is mounted on a vehicle as a HUD. A typical example case where the display device  100  is used is to display navigation information such as the speed, mileage, and destination of a vehicle in the field of view of the viewer  3 . In recent years, for the purpose of enhancing driving safety, there has been a demand for the HUD to superimpose a display image over a real object, that is, provide the AR superimposition. 
     For example, as illustrated in  FIG. 11 , the display AR 1  or AR 2  such as a highlighted display is AR superimposed on the vehicle D 1  and the boundary line D 2 , which are objects to be recognized by the viewer  3  during the driving, by the HUD. This induces the visual recognition of the viewer  3 . For example, when such an AR superimposition is provided, it is more desired that an image be displayed at a proper position with a larger image size in the future. However, during the formation of an image by the two-dimensional scanning of the light deflector  13 , an image size and an image-forming position might fluctuate with the environmental temperature or over time, and thus a desired image cannot be obtained. 
       FIGS. 12A and 12B  are illustrations of the change in the sensitivity of the mirror  130  with respect to the drive voltage over time. 
     When an MEMS mirror is used as the mirror  130  as an example, the deflection angle of the mirror  130  with respect to the drive voltage for driving the MEMS mirror (deflection angle sensitivity with respect to the drive voltage of the MEMS mirror) changes with the environmental temperature and over time. 
     In other words, as illustrated in  FIG. 12A , when the drive voltage for driving the MEMS mirror is constant, the deflection angle sensitivity of the mirror  130  decreases with time t 2  with respect to the initial state t 1 . Accordingly, the size w 2  of the virtual image  45  at time t 2  is smaller than the size w 1  of the virtual image  45  at the initial state t 1  as illustrated in  FIG. 12B . In the present embodiment, the case where the image size in the sub-scanning direction changes is described. However, no limitation is intended therein. The image size in the main scanning direction may change as well. 
     When the fixed state of each light source in the light deflector  13  and the light-source device  11  fluctuates within the two-dimensional scanning plane with the environmental temperature and over time, the image forming position of the intermediate image  25  formed on the screen  15  varies, and the image forming position of the virtual image  45  that is visually recognized by the viewer  3  in the end varies. Further, when the image forming positions of the red, blue, and green colors are independently changed, the images drawn in the respective colors are spatially shifted, so that the colored images are visually recognized. 
       FIG. 13  is an illustration of the configuration for correcting an image according to the first embodiment.  FIG. 13  illustrates a display image area  15 R 1 , non-image areas  15 R 51  and  15 R 52 , the two-dimensional scanning area  15 R 3 , an area  15 R 4  that is optically effective on the screen  15 , the light receivers  61 A and  61 B (an example of first and second photosensors), the first detection image area G 1 , and the second detection image area G 2 . The symbol “L 1 ” denotes the center of the main scanning amplitude of the two-dimensional scanning on the screen  15  by the light deflector  13 , and the symbol “L 2 ” denotes the center of the sub-scanning amplitude of the two-dimensional scanning on the screen  15  by the light deflector  13 . The first and second detection fields in which the light receivers  61 A and  61 B detect the irradiation light are equal to the areas of the first and light receivers  61 A and  61 B, respectively, and such detection fields may be smaller than the areas of the light receivers  61 A and  61 B, respectively. 
     As illustrated in  FIG. 13 , the two-dimensional scanning area (the scanning area)  15 R 3  is extended to the outside of the display image area  15 R 1 , and the light receivers  61 A and  61 B are disposed in the non-image areas  15 R 51  and  15 R 52 , which are scanning ranges outside the display image area  15 R 1 . The light receivers  61 A and  61 B are separated from each other in the main scanning direction. 
     The non-image areas  15 R 51  and  15 R 52  does not overlap with the display image area  15 R 1 , but are arranged so as to overlap with the display image area  15 R 1  along the main scanning direction and the sub-scanning direction. Such an arrangement increases the display image area  15 R 1  as much as possible. 
     Further, the first detection image area G 1  to be illuminated with laser light is provided as a certain area including the lower end of the light receiver  61 A, and the second detection image area G 2  to be illuminated with laser light is provided as a certain area including the lower end of the light receiver  61 B in the sub-scanning direction. 
     The light receivers  61 A and  61 B are arranged at the same position in the sub-scanning direction, and the detection image areas G 1  and G 2  are also arranged at the same position in the sub-scanning direction. 
     Each of the first and second detection image areas G 1  and G 2  has a wider width in the main scanning direction than the width of each of the light receivers  61 A and  61 B in the main scanning direction. Scanning the light-receiving surface of the light receiver  61  in the main scanning direction by the laser light that scans the detection image area G enables the light receiver  61  to detect the scanning timing of the scanning light. Note that the first detection image area G 1  is an example of a first irradiation area, and the second detection image area G 2  is an example of a second irradiation area. 
     As the area on the light receiver  61  is illuminated with laser light, a light signal relating to the scanning state is obtained from the light receiver  61 . Using such a signal regarding the scanning state, the controller  17  controls the driving of the light deflector  13  and the light-emission timing of the light-source device  11  so as to adjust the image forming position and the image size. The signal relating to the scanning state is, for example, a time interval to pass through the light receiver  61  or the number of scanning lines that has passed through the light receiver  61 . 
     At this time, it is desirable that the display image area  15 R 1  be apart from the first and the second detection image areas G 1  and G 2  by a predetermined interval so as to prevent leakage of light from the first and second detection image areas G 1  and G 2  to the display image area  15 R 1 . Further, it is also desirable that the size of the display image area  15 R 1  be smaller than or equal to the size of an optical effective area  15 R 4  of the screen  15 . When there is no need to distinguish the first detection image area G 1  and the second detection image area G 2 , the first detection image area G 1  and the second detection image area G 2  are sometimes collectively referred to as the detection image area G. 
     Note that the detection image (an image to be used for detection) formed in the detection image area G may be changed according to what is to be controlled. Further, the detection image may be formed at a predetermined scanning frame interval, or may be formed when a specific condition is satisfied. Further, the detection image may be formed with all of the light sources turned on, or with a specific light source turned on. Alternatively, any light source may be selected to be turned on for each scanning frame. 
       FIGS. 14 and 15  are illustrations of an example of a screen holder.  FIG. 14  is a plan view of a holder  121  as a part of the unit housing  12 , and  FIG. 15  is a perspective view as seen from the opposite side of  FIG. 14 . 
     As illustrated in  FIG. 14 , the holder  121  is a holder for holding the screen  15  indicated by the dotted line. The light detection sensors  60 A and  60 B are provided in a part of the holder. The light detection sensors  60 A and  60 B includes light receivers  61 A and  61 B, and the light receivers  61 A and  61 B are provided at positions of the detection images G 1  and G 2 . In  FIG. 15 , scanning light is emitted through an opening  122  in a direction from the rear side to the front side of the drawing sheet in which  FIG. 15  is drawn. Thus, the shape of the display image area  15 R 1  is determined according to, for example, the shape of the opening  122 . 
       FIG. 16A  is an illustration of raster scan being performed on the light receiver  61 .  FIG. 16B  is an illustration for describing the changes in the scanning position in the sub-scanning direction over time during the two-dimensionally scanning.  FIG. 16C  is an illustration of light signals obtained from the light receiver  61  through the second detection image area G 2 . 
     As illustrated in  FIG. 16A , the raster scan is performed on the light receiver  61 , and light signals are obtained as illustrated in  FIG. 16C  every time the scanning light passes through the light receiver  61 . Accordingly, by counting the number of light signals obtained during the time period between the times to scan the detection image area G, the number of scanning lines that has passed through the light receiver  61  is counted. Adjustment of an image in the sub-scanning direction is performed based on the number of scanning lines (of light received by the light receiver) detected from the illuminated areas of the first and second detection image areas G 1  and G 2 . 
     Although not described in detail herein, since the timing at which the laser light passes through the light receiver  61  can be detected, the image size and image position in the main scanning direction are controlled, and the drive parameter of the two-dimensional deflection element involved in driving in the main scanning direction is controlled. 
       FIG. 17  is a block diagram of a functional configuration of the controller  17  according to an embodiment. The controller  17  as a control unit includes an image adjuster  171 , an image input unit  172 , an image processor  173 , and a detection image generation unit  174 . For example, each functional configuration of the controller  17  is implemented by some of the elements illustrated in  FIG. 2 . In particular, the controller  17  may be implemented by the processing performed by the CPU  1002 , the ROM  1003 , the FPGA  1001 , the LD driver  1008 , the MEMS controller  1010 , and the motor driver  1012 , as well as a computer executable program stored in the ROM  1003 . 
     The image adjuster  171  calculates a change in the image forming position and a change in the image size from the initial state based on the light signal output from the light receiver  61 . 
     When the amount of change in the image forming position exceeds a predetermined value (amount), the adjustment signal generation unit  1711  sends an adjustment signal to a light source control unit  1712  so as to adjust the light emission timing of the light-source device  11  thus to adjust the image formation position. 
     Further, when the amount of change in the image size exceeds a predetermined value (amount), the adjustment signal generation unit  1711  sends an adjustment signal to a light deflector control unit  1713  so as to adjust a deflection angle of the light deflector  13  thus to adjust the image size. 
     The light source control unit  1712  controls the driving of the light-source device  11  based on the drive signal. Then, the light deflector control unit  1713  periodically controls the driving of the light deflector  13  based on the drive signal. 
     The image input unit  172  outputs image data for forming an image to the image processor  173 . The image input unit  172  outputs image data for forming a display image that is to be displayed for the user, to the image processor  173 . When the display device  100  is provided for a vehicle as a HUD, an image to be presented to a user includes, for example, the vehicle-related information (e.g., speed and travel distance) and external information (for example, position information from the global positioning system (GPS), routing information from a navigation system, or traffic information) of the vehicle received from an external network. However, no limitation is intended thereby, and an image to be presented to a user may be, for example, an image based on an image regenerative signal read from the television (TV), the Internet, or a recording medium. 
     The image processor  173  generates driving data for the light-source device  11  such as the timing at which laser beams are to be emitted and light-emission intensity (power of light emission), based on the generated image data. The generated driving data is output to the light source control unit  1712 . Then, the image processor  173  generates driving data for the light-source device  11  such as the timing at which laser beams are to be emitted and light-emission intensity (power of light emission), based on the image data for forming an image to be presented to the user. The generated driving data is output to the light source control unit  1712 . 
     When the image data output from the image input unit  172  is, for example, moving-image data, the image processor  173  generates driving data for forming a scanning frame image based on each of the frames constituting the moving image, that is, each image corresponding to one screen included in the moving image, so as to sequentially display virtual images  45 . For example, one frame of the moving images may be displayed using the scanning-frame images in two continuous cycles of two-dimensional scanning. 
     The light source control unit  1712  controls the light emission (illumination control) of the light-source device  11  based on the driving data and the adjustment signal from the adjustment signal generation unit  1711 . 
     The detection image generation unit  174  outputs the image data of a detection image to be received by the light receiver  61  to the image processor  173 . 
       FIG. 18  is a flowchart for describing the image position adjustment processing and image size adjustment processing. 
     When the display device  10  is powered on, the controller  17  executes a startup mode to detect the amount of misalignment of an image relative to an initial value obtained in advance before shipment of the product, in step S 101  (S 101 ). The startup mode is a process performed before the regular operation mode. In the regular operation mode, the controller  17  controls the light-source device  11  to emit light based on the image information to form an image within the display image area  15 R 1  on the screen  15  at predetermined frame intervals. In the startup mode, the controller  17  does not form an image on the screen  15  by controlling the light-source device  11  to emit light based on image information. 
     The controller  17  determines whether the amount of misalignment of image detected in step S 101  is greater than or equal to a certain value (S 102 ). When it is determined that the amount of misalignment is greater than or equal to the certain value (Yes, in S 102 ), the controller  17  adjusts the image position (S 103 ). 
     When it is determined that the amount of misalignment is not greater than or equal to the certain value (No, in S 102 ), the controller  17  determines that adjustment of the image position is needed. Then, the process proceeds to the regular operation mode. In the regular operation mode, the controller  17  executes the regular operation at predetermined frame intervals while displaying navigation information such as vehicle speed, mileage, and destination. Such an operation is repeated until the display device  10  is turned off. In the regular operation mode, the processes of steps S 101  to S 103  are not performed. 
     In the regular operation mode, the controller  17  detects the amount of variation in image size relative to the initial value obtained in advance before the shipping of the product (S 104 ). Specifically, the controller  17  detects the amount of fluctuations in the number of scanning lines (light beams) received by the first illuminated area (ΔNA), the amount of fluctuations in the number of scanning lines (light beams) received by the second illuminated area (ΔNB), or the sum of ΔNA and ΔNB. 
     The controller  17  determines whether the amount of variation in the image size detected in step S 104  is greater than or equal to a certain value (S 105 ). When it is determined that the amount of variation in the image size is greater than or equal to the certain value (Yes in S 105 ), the controller  17  adjusts the drive voltage of the mirror  130  of the light deflector  13  so as to adjust the image size (S 106 ). 
     The adjustment of the image size is performed by the controller  17 &#39;s control to maintain the detection value constant, so that the variation in the image size relative to the initial state can be reduced. The image size can be controlled by adjusting the drive voltage applied to the mirror  130 . By arranging the detection image areas G 1  and G 2  as illustrated in  FIG. 13 , the size of the project image can be adjusted with high accuracy regardless of the position conditions of the light receivers. 
     When it is determined that the amount of variation in the image size is not greater than or equal to the certain value (No in S 105 ), the controller  17  determines that adjustment of the image size is not needed. Then, the process proceeds to a next process. 
     The controller  17  checks whether the display device  10  is in use (S 107 ). When it is determined that the display device  10  is in use (Yes in step S 107 ), the process return to step S 104  to perform the process at the predetermined frame intervals. 
     In the present embodiment, since the misalignment of image is detected and corrected at start-up of the device, the adjustment (control) of the image size may be prioritized at the regular operation. In the typical control operation, a process is performed to eliminate those caused by the misalignment of image. However, such a process is not performed in the operation of the present embodiments. For this reason, a higher resolution for the detection of the image size and higher accuracy of the detection are obtained in the present embodiment than those in the typical operation. 
     Further, in the present embodiment, the image size is adjusted in the regular operation mode, and the image position is adjusted in the startup mode that is a timing different from the regular operation mode the image position is adjusted. With this configuration, the size and the position of the projection image can be adjusted with higher accuracy irrespective of the conditions for the position of the light receiver. 
     The adjustment of the image position may be performed in the inactive mode. The inactive mode is a mode in which a process is performed after the regular operation mode, and the controller  17  ceases to control the light-source device  11  to emit light based on image information to form an image on the screen  15 . 
       FIGS. 19A, 19B, and 19C  are illustrations for describing the method of detecting an initial value, which is performed in step S 101  in  FIG. 18 . 
     As illustrated in  FIG. 19A , the controller  17  first controls the light-source device  11  to emit light so that the amplitude of the scanning line  630  of the scanning light deflected by the light deflector  13  in the sub-scanning direction (Y direction in  FIG. 19A ) occurs only near the sub-scanning amplitude center L 2 . Since the controller  17  controls the light-source device  11  to emit light to illuminate the scanning area  15 R 3  as a whole of the scanning line  630 , the detection image area G (irradiation area) to be irradiated with the light from the light-source device  11  is equal to the scanning area  15 R 3 . 
     In this case, the scanning area  15 R 3  is sufficiently small in the sub-scanning direction, to not overlap with the detection field of the light receiver  61 . 
     As illustrated in  FIG. 19B , the controller  17  controls the light-source device  11  to emit light so that the amplitude of the scanning line  630  of the scanning light deflected by the light deflector  13  becomes larger. Since the controller  17  controls the light-source device  11  to emit light to illuminate the scanning area  15 R 3  as a whole of the scanning line  630 , the detection image area G (irradiation area) to be irradiated with the light from the light-source device  11  is equal to the scanning area  15 R 3 . 
     In  FIG. 19B , the scanning area  15 R 3  reaches and overlap with the lower end  611  (an example of a first detection position) of the light receiver  61  (detection field) in the sub-scanning direction at the scanning angle θ 1  in the sub-scanning direction. The controller  17  stores the drive voltage G 1  for the mirror  130  of the light deflector  13  at this state, in the ROM  1003  or the like. 
     As illustrated in  FIG. 19C , the controller  17  controls the light-source device  11  to emit light so that the amplitude of the scanning line  630  deflected by the light deflector  13  in the sub-scanning direction becomes larger. Since the controller  17  controls the light-source device  11  to emit light to illuminate the scanning area  15 R 3  as a whole of the scanning line  630 , the detection image area G (irradiation area) to be irradiated with the light from the light-source device  11  is equal to the scanning area  15 R 3 . 
     In  FIG. 19C , the scanning area  15 R 3  reaches and overlap with the upper end  61   u  (an example of a second detection position) of the light receiver  61  (detection field) in the sub-scanning direction at the scanning angle θu in the sub-scanning direction. The controller  17  stores the drive voltage Gu for the mirror  130  of the light deflector  13  at this state, in the ROM  1003  or the like. 
     In the above-described case, the controller  17  may move or sweep the scanning area  15 R 3  in plural times to change the state in  FIG. 19A  to the state in  FIG. 19B  and to the state  FIG. 19C . 
     The processing sequence is not limited to that order of  FIGS. 19A, 19B, and 19C . The controller  17  may execute the processes illustrated in  FIGS. 19A, 19B, and 19C  in any suitable order. Further, any other processes other than the processes illustrated in  19 A,  19 B, and  19 C may be included in step S 101  in  FIG. 18 . 
       FIGS. 20A, 20B, and 20C  are illustrations for describing a method of detecting the amount of misalignment, which is performed in step S 101  of  FIG. 18 . 
     As illustrated in  FIG. 20A , the controller  17  first controls the light-source device  11  to emit light so that the amplitude of the scanning line  630  of the scanning light deflected by the light deflector  13  in the sub-scanning direction (Y direction in  FIG. 20A ) occurs only near the sub-scanning amplitude center L 2 . 
     In this case, the misalignment ΔY occurs in the sub-scanning direction. That is, the sub-scanning amplitude center L 2  is misaligned by ΔY from the original sub-sub-scanning amplitude center L 20  at the initial state. 
     Since the controller  17  controls the light-source device  11  to emit light to illuminate the scanning area  15 R 3  as a whole of the scanning line  630 , the detection image area G (irradiation area) to be irradiated with the light from the light-source device  11  is equal to the scanning area  15 R 3 . 
     In this case, the scanning area  15 R 3  is sufficiently small in the sub-scanning direction, to not overlap with the detection field of the light receiver  61 . 
     As illustrated in  FIG. 20B , the controller  17  controls the light-source device  11  to emit light so that the amplitude of the scanning line  630  of the scanning light deflected by the light deflector  13  in the sub-scanning direction becomes larger. Since the controller  17  controls the light-source device  11  to emit light to illuminate the scanning area  15 R 3  as a whole of the scanning line  630 , the detection image area G (irradiation area) to be irradiated with the light from the light-source device  11  is equal to the scanning area  15 R 3 . 
     In  FIG. 20B , the scanning area  15 R 3  reaches and overlap with the lower end  611  (an example of a first detection position) of the light receiver  61  (detection field) in the sub-scanning direction at the scanning angle θ 1  in the sub-scanning direction. The controller  17  stores the drive voltage G 1 ′ for the mirror  130  of the light deflector  13  at this state, in the ROM  1003  or the like. 
     As illustrated in  FIG. 20C , the controller  17  controls the light-source device  11  to emit light so that the amplitude of the scanning line  630  of the scanning light deflected by the light deflector  13  in the sub-scanning direction becomes larger. Since the controller  17  controls the light-source device  11  to emit light to illuminate the scanning area  15 R 3  as a whole of the scanning line  630 , the detection image area G (irradiation area) to be irradiated with the light from the light-source device  11  is equal to the scanning area  15 R 3 . 
     In  FIG. 20C , the scanning area  15 R 3  reaches and overlap with the upper end  61   u  (an example of the second detection position) of the light receiver  61  (detection area) in the sub-scanning direction at the scanning angle θu in the sub-scanning direction. The controller  17  stores the drive voltage Gu′  for the mirror  130  of the light deflector  13  at this state, in the ROM  1003  or the like. 
     In the above-described case, the controller  17  may move or sweep the scanning area  15 R 3  in plural times to change the state in  FIG. 20A  to the state in  FIG. 20B  and to the state  FIG. 20C . 
     The processing sequence is not limited to that order of  FIGS. 20A, 20B, and 20C . The controller  17  may execute the processes illustrated in  FIGS. 20A, 20B, and 20C  in any suitable order in step S 101 . Further, any other processes other than the processes illustrated in  20 A,  20 B, and  20 C may be included in step S 101  in  FIG. 18 . 
       FIGS. 21A, 21B, 21C, and 21D  each is a graph of a detection signal of the irradiation light received by the light receiver  61 . 
       FIG. 21A  corresponds to the states of  FIGS. 19A and 20A , in which the amplitude of the scanning line  630  in the sub-scanning direction is sufficiently small, and the light receiver  61  fails to receive the irradiation light. For this reason, as illustrated in the graph of  FIG. 21A , the light receiver  61  does not output any detection signal. 
       FIG. 21B  corresponds to the states of  FIGS. 19B and 20B , in which the amplitude of the scanning line  630  in the sub-scanning direction is greater than the cases of  FIG. 19A  and  FIG. 20A , and the irradiation light is received by the lower end  611  of the detection field of the light receiver  61  in the sub-scanning direction. In this case, the light receiver  61  outputs a detection signal for one scanning line  630 . By receiving the detection signal for the one scanning line  630 , the controller  17  determines that the scanning area  15 R 3  has reached the lower end  611  of the light receiver  61 . 
       FIG. 21C  corresponds to the states of  FIGS. 19C and 20C . The amplitude of the scanning line  630  in the sub-scanning direction is further increased, and the irradiation light is received by the upper end 61μ of the detection field of the light receiver  61  in the sub-scanning direction. In this case, the light receiver  61  outputs detection signals for all the scanning lines  630  that have scanned the entire detection area. Since the scanning line  630  of the scanning light is moved back and forth (round trip) in the sub-scanning direction, the detection signals F received in the first leg of a round trip and the detection signals B received in the second leg of the round trip are chronologically indicated in  FIG. 21C . 
       FIG. 21D  indicates the state in which the amplitude of the scanning line  630  in the sub-scanning direction is further increased, and the irradiation light reaches a portion above the upper end 61μ of the detection field of the light receiver  61  in the sub-scanning direction. Compared to  FIG. 21C , a time with no detection signals occurs between the detection signals F received in the first leg of the round trip and the detection signals B received in the second leg of the round trip in  FIG. 21D  because the irradiation light reaches the portion above the upper end 61μ of the detection field of the light receiver  61 . Based on the difference in the state between  FIG. 21C  and  FIG. 21D  and the time with no detection signals, the controller  17  determines that the scanning area  15 R 3  reaches the upper end 61μ of the light receiver  61 . 
     In the above-described embodiments, the method that determines whether the scanning area reaches the lower end  611  or the upper end  61   u  of the light receiver  61  based on the number of scanning lines  630  output by the light receiver  61  is described. In some examples, such a determination may be made based on the intensity of the detection signal integrated with a predetermined time. In this case, the intensity of the detection signal increases as the time of scanning the light receiver  61  increases. Accordingly, the controller  17  determines that the scanning area reaches the lower end  611  of the light receiver  61  in response to a start of outputting of the intensity of the detection signal, and determines that the scanning area reaches the upper end  61   u  of the light receiver  61  when the intensity of the detection signal reaches a level of saturation to remain at a constant value with no increase in the intensity. 
       FIG. 22  is a diagram for describing the relation of the scanning angle and the drive voltage in the sub-scanning direction. 
     In the graph of  FIG. 22 , an approximate line A indicates the relation of the original scanning angles and the original values of the drive voltage at the initial state as illustrated in  FIGS. 19A, 19B, and 19C , and an approximate line B indicates the relation of the scanning angles and the drive voltage when the misalignment ΔY occurs in the sub-scanning direction as illustrated in  FIGS. 20A, 20B, and 20C . 
     In the approximate line A, as illustrated in  FIGS. 19A to 19C , the drive voltage Gl of the mirror  130  with respect to the scanning angle θl in the sub-scanning direction and the drive voltage Gu of the mirror  130  with respect to the scanning angle θu in the sub-scanning direction are plotted. In the approximate line B, as illustrated in  FIGS. 20A to 20C , the drive voltage Gl′ of the mirror  130  with respect to the scanning angle θl in the sub-scanning direction and the drive voltage Gu′ of the mirror  130  with respect to the scanning angle θu in the sub-scanning direction are plotted. 
     The image formed within the display image area  15 R 1  on the screen  15  with the light emitted from the light-source device  11  based on the image information is misaligned in the sub-scanning direction due to the misalignment with an amount of misalignment ΔY in the sub-scanning direction between the approximate line A and the approximate line B. 
     The amount of misalignment ΔY in the sub-scanning direction is obtained by the following equations: 
       θ l=a×Gl′+ΔY  
 
       θ u=a×Gu′+ΔY  
 
     Accordingly, the following equation is derived from the above equations: 
       Δ Y =( Gl′×θu−Gu′×θl )/( Gl′−Gu ′)
 
     As described above, in the present embodiment, by changing the amplitude of the scanning line  630  in the sub-scanning direction to move the irradiation area between a first position overlapping with the detection field and a second position not overlapping with the detection field (the second position being a position other than the first position overlapping with the detection field), the misalignment with an amount of ΔY of an image due to the changes in the environment or over time can be detected with higher accuracy. 
     Note that the tilt of the approximate line A and the approximate line B, and the symbol “a” in the above-described equations denote the sensitivity of the scanning angle with respect to the drive voltage. 
     In the present embodiment, the controller  17  shifts the image formed within the display image area  15 R 1  on the screen  15  with the light emitted from light-source device  11  based on the image information, back in the opposite direction of the detected misalignment ΔY in the sub-scanning direction, by the amount of misalignment ΔY. This enables a higher accuracy of the misalignment ΔY of an image. 
     In the above description, the detection positions are the lower end  611  and the upper end  61   u  of the light receiver  61 . However, no limitation is intended thereby. In some embodiments, the detection position may be, for example, the end of a light shield that partially shields the light receiver  61 . 
     In the present embodiment, with two detection positions of the lower end  61   l  and the upper end  61   u  of the light receiver provided, the misalignment ΔY can be accurately detected and corrected irrespective of a great change in the sensitivity of the scanning angle with respect to the drive voltage of the mirror  130  between the initial state and the state of the misalignment in the sub-scanning direction. 
     Note that if the change in the sensitivity of the scanning angle with respect to the drive voltage of the mirror  130  is sufficiently small, the detection position may be one of the lower end  61   l  and the upper end  61   u  of the light receiver  61 . 
       FIG. 23  is a diagram illustrating a configuration according to a variation of the embodiment in  FIG. 13 . 
     In the variation in  FIG. 23 , the light receivers  61 A and  61 B are separated from each other in the main scanning direction as in  FIG. 13 . However,  FIG. 23  differs from  FIG. 13  in that the light receivers  61 A and  61 B are misaligned along the sub-scanning direction (Y direction in  FIG. 23 ). 
     Specifically, the upper end of light receiver  61 A is located above the upper end of light receiver  61 B in  FIG. 23 . The upper end of the light receiver  61 B is located above the lower end of the light receiver  61 A in  FIG. 23 . The lower end of the light receiver  61 A is located above the lower end of the light receiver  61 B in  FIG. 23 . 
       FIG. 24  is a graph of scanning angles and drive voltage in the sub-scanning angle according to the variation in  FIG. 23 . 
     In the graph of  FIG. 24 , an approximate line A indicates the relation of the original scanning angles and the original values of the drive voltage at the initial state, and an approximate line B indicates the relation of the scanning angles and the drive voltage when the misalignment ΔY occurs in the sub-scanning direction. 
     In  FIG. 24 , θl 1  denotes a scanning angle when the scanning area  15 R 3  reaches and overlaps with the lower end of the light receiver  61 B in the sub-scanning direction, and θl 2  denotes a scanning angle when the scanning area  15 R 3  reaches and overlaps with the lower end of the light receiver  61 A in the sub-scanning direction. Further, θu 1  denotes a scanning angle when the scanning area  15 R 3  reaches and overlaps with the upper end of the light receiver  61 B in the sub-scanning direction, and θu 2  denotes a scanning angle when the scanning area  15 R 3  reaches and overlaps with the upper end of the light receiver  61 A in the sub-scanning direction. 
     In the approximate line A, the drive voltage Gl 1  of the mirror  130  with respect to the scanning angle θl 1  in the sub-scanning direction, the drive voltage Gl 2  of the mirror  130  with respect to the scanning angle θl 2  in the sub-scanning direction, the drive voltage Gu 1  of the mirror  130  with respect to the scanning angle θu 1  in the sub-scanning direction, and the drive voltage Gu 2  of the mirror  130  with respect to the scanning angle θu 2  in the sub-scanning direction are plotted. 
     In the approximate line B, the drive voltage Gl 1 ′ of the mirror  130  with respect to the scanning angle θl 1  in the sub-scanning direction, the drive voltage Gl 2 ′ of the mirror  130  with respect to the scanning angle θl 2  in the sub-scanning direction, the drive voltage Gu 1 ′ of the mirror  130  with respect to the scanning angle θu 1  in the sub-scanning direction, and the drive voltage Gu 2 ′ of the mirror  130  with respect to the scanning angle θu 2  in the sub-scanning direction are plotted. 
     In the present variation, four detection positions are used: the lower end and the upper end of the light receiver  61 A, and the lower end and the upper end of the light receiver  61 B. Accordingly, the linear approximate lines are obtained based on the detection data from the four detection positions, and based on the approximate lines, the misalignment of an image can be more accurately detected, for example, because even if data from one location is erroneous, other data from the other three locations can cover the error. 
       FIGS. 25A, 25B, and 25C  each is a diagram illustrating a method of detecting an initial value according to a variation of the embodiment illustrated in  FIGS. 19A, 19B, and 19C . 
     In the method of detecting the initial value illustrated in  FIGS. 19A to 19C , the amplitude of the scanning line  630  of scanning light deflected by the light deflector  13  in the sub-scanning direction (Y direction in the drawing) is changed. In the variation illustrated in  25 A to  25 C, the tilt of the reflecting surface of the mirror  130  of the light deflector  13  is changed. 
     As illustrated in  FIG. 25A , the controller  17  first initializes the tilt of the reflecting surface of the mirror  130  so that the normal direction of the reflecting surface of the mirror  130  is along the horizontal direction (Z direction in  FIG. 25A ). In that state, the controller  17  controls the light-source device  11  to emit light so that the amplitude of the scanning line  630  of the scanning light deflected by the light deflector  13  in the sub-scanning direction (the Y direction in  FIG. 25A ) occurs only near the sub-scanning amplitude center L 2 . The amplitude in the sub-scanning direction is set to zero. Since the controller  17  controls the light-source device  11  to emit light to illuminate the scanning area  15 R 3  as a whole of the scanning line  630 , the irradiation area to be irradiated with the light from the light-source device  11  is equal to the scanning area  15 R 3 . 
     In this case, the scanning area  15 R 3  is sufficiently small in the sub-scanning direction, to not overlap with the detection field of the light receiver  61 . 
     As illustrated in  FIG. 25B , the controller  17  controls the light-source device  11  to emit light with the reflecting surface of the mirror  130  tilted upward (Y direction in  FIG. 25B ) without an increase in the amplitude of the scanning line  630  in the sub-scanning direction. Since the controller  17  controls the light-source device  11  to emit light to illuminate the scanning area  15 R 3  as a whole of the scanning line  630 , the irradiation area to be irradiated with the light from the light-source device  11  is equal to the scanning area  15 R 3 . 
     In  FIG. 25B , the scanning area  15 R 3  reaches and overlaps with the lower end  611  (an example of a first detection position) of the light receiver  61  (detection field) in the sub-scanning direction at the scanning angle θ 1  in the sub-scanning direction. The controller  17  stores the drive voltage G 1  for the mirror  130  of the light deflector  13  at this state, in the ROM  1003  or the like. 
     As illustrated in  FIG. 25C , the controller  17  controls the light-source device  11  to emit light with the reflecting surface of the mirror  130  tilted further upward (Y direction in  FIG. 25C ) without an increase in the amplitude of the scanning line  630  in the sub-scanning direction. Since the controller  17  controls the light-source device  11  to emit light to illuminate the scanning area  15 R 3  as a whole of the scanning line  630 , the irradiation area to be irradiated with the light from the light-source device  11  is equal to the scanning area  15 R 3 . 
     In  FIG. 25C , the scanning area  15 R 3  reaches and overlaps with the upper end  61   u  (an example of a second detection position) of the light receiver  61  (detection field) in the sub-scanning direction at the scanning angle θu in the sub-scanning direction. The controller  17  stores the drive voltage Gu for the mirror  130  of the light deflector  13  at this state, in the ROM  1003  or the like. 
     In the above-described case, the controller  17  may move or sweep the scanning area  15 R 3  in plural times to change the state in  FIG. 25A  to the state in  FIG. 25B  and to the state  FIG. 25C . 
     The processing sequence is not limited to that order of  FIGS. 25A, 25B, and 25C . The controller  17  may execute the processes illustrated in  FIGS. 25A, 25B, and 25C  in any suitable order. Further, any other processes other than the processes illustrated in  25 A,  25 B, and  25 C may be included in step S 101  in  FIG. 18 . 
       FIGS. 26A, 26B, and 26C  each is a diagram illustrating a method of detecting the amount of misalignment according to a variation of the embodiment illustrated in  FIG. 20A, 20B, and 20C . 
     In the method of detecting the initial value illustrated in  FIGS. 20A to 20C , the amplitude of the scanning line  630  of scanning light deflected by the light deflector  13  in the sub-scanning direction (Y direction in the drawing) is changed. In the variation illustrated in  26 A to  26 C, the tilt of the reflecting surface of the mirror  130  of the light deflector  13  is changed. 
     As illustrated in  FIG. 26A , the controller  17  first initializes the tilt of the reflecting surface of the mirror  130  so that the normal direction of the reflecting surface of the mirror  130  is along the horizontal direction (Z direction in  FIG. 26A ). In that state, the controller  17  controls the light-source device  11  to emit light so that the amplitude of the scanning line  630  of the scanning light deflected by the light deflector  13  in the sub-scanning direction occurs only near the sub-scanning amplitude center L 2 . 
     In this case, the misalignment ΔY occurs in the sub-scanning direction. That is, the sub-scanning amplitude center L 2  is misaligned by ΔY from the original sub-sub-scanning amplitude center L 20  at the initial state. 
     Since the controller  17  controls the light-source device  11  to emit light to illuminate the scanning area  15 R 3  as a whole of the scanning line  630 , the irradiation area to be irradiated with the light from the light-source device  11  is equal to the scanning area  15 R 3 . 
     In this case, the scanning area  15 R 3  is sufficiently small in the sub-scanning direction, to not overlap with the detection field of the light receiver  61 . 
     As illustrated in  FIG. 26B , the controller  17  controls the light-source device  11  to emit light with the reflecting surface of the mirror  130  tilted upward (Y direction in  FIG. 26B ) without an increase in the amplitude of the scanning line  630  in the sub-scanning direction. Since the controller  17  controls the light-source device  11  to emit light to illuminate the scanning area  15 R 3  as a whole of the scanning line  630 , the irradiation area to be irradiated with the light from the light-source device  11  is equal to the scanning area  15 R 3 . 
     In  FIG. 26B , the scanning area  15 R 3  reaches and overlaps with the lower end  611  (an example of a first detection position) of the light receiver  61  (detection field) in the sub-scanning direction at the scanning angle θl in the sub-scanning direction. The controller  17  stores the drive voltage G 1 ′ for the mirror  130  of the light deflector  13  at this state, in the ROM  1003  or the like. 
     As illustrated in  FIG. 26C , the controller  17  controls the light-source device  11  to emit light with the reflecting surface of the mirror  130  tilted further upward without an increase in the amplitude of the scanning line  630  in the sub-scanning direction. Since the controller  17  controls the light-source device  11  to emit light to illuminate the scanning area  15 R 3  as a whole of the scanning line  630 , the irradiation area to be irradiated with the light from the light-source device  11  is equal to the scanning area  15 R 3 . 
     In  FIG. 26C , the scanning area  15 R 3  reaches and overlaps with the upper end  61   u  (an example of a second detection position) of the light receiver  61  (detection field) in the sub-scanning direction at the scanning angle θu in the sub-scanning direction. The controller  17  stores the drive voltage Gu′ for the mirror  130  of the light deflector  13  at this state, in the ROM  1003  or the like. 
     In the above-described case, the controller  17  may move or sweep the scanning area  15 R 3  in plural times to change the state in  FIG. 26A  to the state in  FIG. 26B  and to the state  FIG. 26C . 
     The processing sequence is not limited to that order of  FIGS. 26A, 26B, and 26C . The controller  17  may execute the processes illustrated in  FIGS. 26A, 26B, and 26C  in any suitable order. Further, any other processes other than the processes illustrated in  26 A,  26 B, and  26 C may be included in step S 101  in  FIG. 18 . 
     In the present variation, by changing the tilt of the reflecting surface of the mirror  130  of the light deflector  13  to move the irradiation area between a first position overlapping with the detection field and a second position not overlapping with the detection field, the misalignment with an amount of ΔY of an image due to the changes in the environment or over time can be detected with higher accuracy. 
       FIGS. 27A, 27B, and 27C  each is a diagram illustrating a method of detecting an initial value according to a second variation of the embodiment illustrated in  FIGS. 19A, 19B , and  19 C. 
     In the method of detecting the initial value illustrated in  FIGS. 19A to 19C , the amplitude of the scanning line  630  of scanning light deflected by the light deflector  13  in the sub-scanning direction (Y direction in the drawing) is changed. Further, in the variation illustrated in  25 A to  25 C, the tilt of the reflecting surface of the mirror  130  of the light deflector  13  is changed. In the present variation illustrated in  FIGS. 27A, 27B, and 27C , the position of the irradiation area to be irradiated with light from the light-source device  11  is changed without any particular control of the scanning line  630 . 
     The controller  17  mechanically controls the position of the mirror  130  of the light deflector  13  to change the position of the irradiation area. In some examples, the position of the light-source device  11  or the position of a component, such as a collimator lens  112 , may be mechanically controlled so as to change the position of the irradiation area. 
     As illustrated in  FIG. 27A , the controller  17  first initializes the tilt of the reflecting surface of the mirror  130  so that the normal direction of the reflecting surface of the mirror  130  is along the horizontal direction (Z direction in  FIG. 27A ). In that state, the controller  17  controls the light-source device  11  to emit light so that the amplitude of the scanning line  630  of the scanning light deflected by the light deflector  13  in the sub-scanning direction (the Y direction in  FIG. 27A ) occurs only near the sub-scanning amplitude center L 2 . The amplitude in the sub-scanning direction is set to zero. Since the controller  17  controls the light-source device  11  to emit light to illuminate the scanning area  15 R 3  as a whole of the scanning line  630 , the irradiation area to be irradiated with the light from the light-source device  11  is equal to the scanning area  15 R 3 . 
     In this case, the scanning area  15 R 3  is sufficiently small in the sub-scanning direction, to not overlap with the detection field of the light receiver  61 . 
     As illustrated in  FIG. 27B , the controller  17  controls the light-source device  11  to emit light with the mirror  130  of the light deflector shifted upward (Y direction in  FIG. 27B ) in the sub-scanning direction, without the changes in the amplitude of the scanning line  630  in the sub-scanning direction and the tilt of the reflecting surface of the mirror  130 . Since the controller  17  controls the light-source device  11  to emit light to illuminate the scanning area  15 R 3  as a whole of the scanning line  630 , the irradiation area to be irradiated with the light from the light-source device  11  is equal to the scanning area  15 R 3 . 
     In  FIG. 27B , the scanning area  15 R 3  reaches and overlaps with the lower end  611  (an example of a first detection position) of the light receiver  61  (detection field) in the sub-scanning direction at the position Yl in the sub-scanning direction. The controller  17  stores the amount of shift G 1  of the mirror  130  of the light deflector  13  at this state, in the ROM  1003  or the like. 
     As illustrated in  FIG. 27C , the controller  17  controls the light-source device  11  to emit light with the mirror  130  of the light deflector shifted further upward in the sub-scanning direction, without the changes in the amplitude of the scanning line  630  in the sub-scanning direction and the tilt of the reflecting surface of the mirror  130 . Since the controller  17  controls the light-source device  11  to emit light to illuminate the scanning area  15 R 3  as a whole of the scanning line  630 , the irradiation area to be irradiated with the light from the light-source device  11  is equal to the scanning area  15 R 3 . 
     In  FIG. 27C , the scanning area  15 R 3  reaches and overlaps with the upper end  61   u  (an example of a second detection position) of the light receiver  61  (detection field) in the sub-scanning direction at the position Yu in the sub-scanning direction. The controller  17  stores the amount of shift Gu of the mirror  130  of the light deflector  13  at this state, in the ROM  1003  or the like. 
     In the above-described case, the controller  17  may move or sweep the scanning area  15 R 3  in plural times to change the state in  FIG. 27A  to the state in  FIG. 27B  and to the state  FIG. 27C . 
     The processing sequence is not limited to that order of  FIGS. 27A, 27B, and 27C . The controller  17  may execute the processes illustrated in  FIGS. 27A, 27B, and 27C  in any suitable order. Further, any other processes other than the processes illustrated in  27 A,  27 B, and  27 C may be included in step S 101  in  FIG. 18 . 
       FIGS. 28A, 28B, and 28C  each is a diagram illustrating a method of detecting the amount of misalignment according to a second variation of the embodiment illustrated in  FIGS. 20A, 20B, and 20C . 
     In the method of detecting the amount of misalignment illustrated in  FIGS. 20A to 20C , the amplitude of the scanning line  630  of scanning light deflected by the light deflector  13  in the sub-scanning direction (Y direction in the drawing) is changed. Further, in the variation illustrated in  26 A to  26 C, the tilt of the reflecting surface of the mirror  130  of the light deflector  13  is changed. In the present variation illustrated in  FIGS. 28A, 28B, and 28C , the position of the irradiation area to be irradiated with light from the light-source device  11  is changed without any particular control of the scanning line  630 . 
     The controller  17  mechanically controls the position of the mirror  130  of the light deflector  13  to change the position of the irradiation area. In some examples, the position of the light-source device  11  or the position of a component, such as a collimator lens  112 , may be mechanically controlled so as to change the position of the irradiation area. 
     As illustrated in  FIG. 28A , the controller  17  first initializes the tilt of the reflecting surface of the mirror  130  so that the normal direction of the reflecting surface of the mirror  130  is along the horizontal direction (Z direction in  FIG. 28A ). In that state, the controller  17  controls the light-source device  11  to emit light so that the amplitude of the scanning line  630  of the scanning light deflected by the light deflector  13  in the sub-scanning direction occurs only near the sub-scanning amplitude center L 2 . 
     In this case, the misalignment ΔY occurs in the sub-scanning direction. That is, the sub-scanning amplitude center L 2  is misaligned by ΔY from the original sub-sub-scanning amplitude center L 2  at the initial state. 
     Since the controller  17  controls the light-source device  11  to emit light to illuminate the scanning area  15 R 3  as a whole of the scanning line  630 , the irradiation area to be irradiated with the light from the light-source device  11  is equal to the scanning area  15 R 3 . 
     In this case, the scanning area  15 R 3  is sufficiently small in the sub-scanning direction, to not overlap with the detection field of the light receiver  61 . 
     As illustrated in  FIG. 28B , the controller  17  controls the light-source device  11  to emit light with the mirror  130  of the light deflector shifted upward (Y direction in  FIG. 28B ) in the sub-scanning direction, without the changes in the amplitude of the scanning line  630  in the sub-scanning direction and the tilt of the reflecting surface of the mirror  130 . Since the controller  17  controls the light-source device  11  to emit light to illuminate the scanning area  15 R 3  as a whole of the scanning line  630 , the irradiation area to be irradiated with the light from the light-source device  11  is equal to the scanning area  15 R 3 . 
     In  FIG. 28B , the scanning area  15 R 3  reaches and overlaps with the lower end  611  (an example of a first detection position) of the light receiver  61  (detection field) in the sub-scanning direction at the position Yl in the sub-scanning direction. The controller  17  stores the amount of shift G 1 ′ of the mirror  130  at this state, in the ROM  1003  or the like. 
     As illustrated in  FIG. 28C , the controller  17  controls the light-source device  11  to emit light with the reflecting surface of the mirror  130  tilted further upward without an increase in the amplitude of the scanning line  630  in the sub-scanning direction. Since the controller  17  controls the light-source device  11  to emit light to illuminate the scanning area  15 R 3  as a whole of the scanning line  630 , the irradiation area to be irradiated with the light from the light-source device  11  is equal to the scanning area  15 R 3 . 
     In  FIG. 28C , the scanning area  15 R 3  reaches and overlaps with the upper end  61   u  (an example of a second detection position) of the light receiver  61  (detection field) in the sub-scanning direction at the position Yu in the sub-scanning direction. The controller  17  stores the amount of shift Gu′ of the mirror  130  of the light deflector  13  at this state, in the ROM  1003  or the like. 
     In the above-described case, the controller  17  may move or sweep the scanning area  15 R 3  in plural times to change the state in  FIG. 28A  to the state in  FIG. 28B  and to the state  FIG. 28C . 
     The processing sequence is not limited to that order of  FIGS. 28A, 28B, and 28C . The controller  17  may execute the processes illustrated in  FIGS. 28A, 28B, and 28C  in any suitable order. Further, any other processes other than the processes illustrated in  28 A,  28 B, and  28 C may be included in step S 101  in  FIG. 18 . 
       FIG. 29  is a graph of the relation between the position and amount of shift in the sub-scanning direction according to the second variation illustrated in  FIGS. 27 and 28 . 
     In the graph of  FIG. 29 , an approximate line A indicates the relation of the original position and the original amount of shift in the sub-scanning direction at the initial state as illustrated in  FIGS. 27A, 27B, and 27C , and an approximate line B indicates the relation of the position and the amount of shift in the sub-scanning direction when the misalignment ΔY occurs in the sub-scanning direction as illustrated in  FIGS. 28A, 28B, and 28C . 
     In the approximate line A, as illustrated in  FIGS. 27A to 27C , the amount of shift Gl of the mirror  130  with respect to the position Yl in the sub-scanning direction and the amount of shift Gu of the mirror  130  with respect to the position Yu in the sub-scanning direction are plotted. In the approximate line B, as illustrated in  FIGS. 28A to 28C , the amount of shift Gl′ of the mirror  130  with respect to the position Yl in the sub-scanning direction and the amount of shift Gu′ of the mirror  130  with respect to the position Yu in the sub-scanning direction are plotted. 
     The image formed within the display image area  15 R 1  on the screen  15  with the light emitted from the light-source device  11  based on the image information is misaligned in the sub-scanning direction due to the misalignment ΔY in the sub-scanning direction between the approximate line A and the approximate line B. 
     The amount of misalignment ΔY in the sub-scanning direction is obtained by the following equations: 
     
       
      
       Yl=a×Gl′+ΔY  
      
     
     
       
      
       Yu=a×Gu′+ΔY  
      
     
     Accordingly, the following equation is derived from the above equations: 
       Δ Y =( Gl′×Yu−Gu′×Yl )/( Gl′−Gu′ )
 
     Note that the tilt of the approximate line A and the approximate line B, and the symbol “a” in the above-described equations denote the sensitivity of the position in the sub-scanning direction with respect to the amount of shift of the mirror  130 . 
     In the present embodiment, the controller  17  shifts the image formed within the display image area  15 R 1  on the screen  15  with the light emitted from light-source device  11  based on the image information, back in the opposite direction of the detected misalignment ΔY in the sub-scanning direction, by the amount of misalignment AY. This enables a higher accuracy of the misalignment ΔY of an image. 
     In the above description, the detection positions are the lower end  611  and the upper end  61   u  of the light receiver  61 . However, no limitation is intended thereby. In some embodiments, the detection position may be, for example, the end of a light shield that partially shields the light receiver  61 . 
     In the present embodiment, two detection positions, the lower end  61   l  and the upper end  61   u , of the light receiver are used. When the sensitivity of the position of the mirror  130  in the sub-scanning direction with respect to the amount of shift of the mirror  130  is absent or sufficiently small between the initial state and the state of the misalignment in the sub-scanning direction, the detection position may be one of the lower end  61   l  and the upper end  61   u  of the light receiver  61 . 
       FIG. 30  is an illustration of a configuration for correcting an image according to the second embodiment. Specifically,  FIG. 30  indicates a display image area  15 R 1 , a non-image area  15 R 2 , light receivers  61 A and  61 B, a first detection image area G 1 , and a second detection image area G 2 , which are in an initial state. The symbol “L 1 ” denotes the center of the main scanning amplitude of the two-dimensional scanning on the screen  15  by the light deflector  13 , and the symbol “L 2 ” denotes the center of the sub-scanning amplitude of the two-dimensional scanning on the screen  15  by the light deflector  13 . 
     As illustrated in  FIG. 30 , the two-dimensional scanning area (the scanning area)  15 R 3  is extended to the outside of the display image area  15 R 1 , and the light receiver  61  (the light receivers  61 A and  61 B) are disposed in the non-image area  15 R 2  that is a scanning area outside the display image area  15 R 1 . Further, the first detection image area G 1  to be illuminated with laser light is provided as a certain area including the lower end of the light receiver  61  (one light receiver, for example,  61 A), and the second detection image area G 2  to be illuminated with laser light is provided as a certain area including the upper end of the light receiver  61  (the other light receiver, for example,  61 B) in the sub-scanning direction. The first and second detection image areas G 1  and G 2  include different positions in the sub-scanning direction. 
     Each of the first and second detection image areas G 1  and G 2  has a wider width in the main scanning direction than the width of the light receiver  61  in the main scanning direction. Scanning the light-receiving surface of the light receiver  61  in the main scanning direction by the laser light that scans the detection image area G enables the light receiver  61  to detect the scanning timing of the scanning light. Note that the first detection image area G 1  is an example of a first area, and the second detection image area G 2  is an example of a second area. 
     As the area on the light receiver  61  is illuminated with laser light, a light signal relating to the scanning state is obtained from the light receiver  61 . Using such a signal regarding the scanning state, the controller  17  controls the driving of the light deflector  13  and the light-emission timing of the light-source device  11  so as to adjust the image forming position and the image size. The signal relating to the scanning state is, for example, a time interval to pass through the light receiver  61  or the number of scanning lines that has passed through the light receiver  61 . 
     At this time, it is desirable that the display image area  15 R 1  be apart from the first and the second detection image areas G 1  and G 2  by a predetermined interval so as to prevent leakage of light from the first and second detection image areas G 1  and G 2  to the display image area  15 R 1 . Further, it is also desirable that the size of the display image area  15 R 1  be smaller than or equal to the size of an optical effective area  15 R 4  of the screen  15 . When there is no need to distinguish the first detection image area G 1  and the second detection image area G 2 , the first detection image area G 1  and the second detection image area G 2  are sometimes collectively referred to as the detection image area G. 
     Note that the detection image (an image to be used for detection) formed in the detection image area G may be changed according to what is to be controlled. Further, the detection image may be formed at a predetermined scanning frame interval, or may be formed when a specific condition is satisfied. Further, the detection image may be formed with all of the light sources turned on, or with a specific light source turned on. Alternatively, any light source may be selected to be turned on for each scanning frame. 
       FIGS. 31A and 31B  are illustrations for describing a change in the number of scanning lines of received light due to a change in the image size in the sub-scanning direction. 
       FIG. 31A  illustrates the display image area  15 R 1 , the two-dimensional scanning area  15 R 3 , the first detection image area G 1  on the light receiving element, and the second detection image area G 2  in the initial state. The initial state refers to a state in which an adjustment is made to obtain a desired image size and a desired image position before shipping the product. Two light receivers  61 A and  61 B are disposed substantially diagonally in one of areas obtained by dividing the two-dimensional scanning area  15 R 3  by the sub-scanning amplitude center L 2 . 
       FIG. 31B  is an illustration of an image size that has been reduced from the initial state in the sub-scanning direction. The image size is reduced in the sub-scanning direction, for example, due to the changes in the sensitivity with respect to the drive voltage of the MEMS mirror over time as described with reference to  FIG. 12A . Similar phenomena can also occur due to the change in sensitivity with temperature or the change in the property of the optical system. 
     As described with reference to  FIG. 31B , the adjustment of an image in the sub-scanning direction is performed based on the number of scanning lines (of received light) detected from the first and second detection image areas G 1  and G 2  illuminated with the light from the light-source device. As the overlapping areas of the light receiver and each of the first detection image area G 1  and the second detection image area G 2  increase, the number of scanning lines of received light increases.  FIGS. 31A and 31B  indicate an overlapping area A of the first detection image area G 1  and the light receiver, an overlapping area B of the second detection image area G 2  and the light receiver, an overlapping area A′ of the first detection image area G 1  and the light receiver when the image size is changed, and an overlapping area B′ of the second detection image area G 2  and the light receiver when the image size is changed. The value obtained by dividing the overlapping area of the illuminated area and the light receiver by the sub-scanning interval of the raster scan on the light receiver is the number of scanning lines received by the light receiver, which are referred to as NA, NB, NA′, NB′, respectively. NA is an example of a predetermined number of scanning lines in the first area. NB is an example of a predetermined number of scanning lines in the second area. 
     For the scanning line obtained from the first detection image area G 1 , ΔNA that is the amount of fluctuations (increase-decrease) in the number of scanning lines from the initial state in  FIG. 31A  to the changed image size in  FIG. 31B  is obtained by subtracting the value of NA from the value of NA′ (ΔNA=NA′−NA). Similarly for the scanning line obtained from the second detection image area G 2 , ΔNB that is the amount of fluctuations (increase-decrease amount) in the number of scanning lines from the initial state in  FIG. 31A  to the changed image size in  FIG. 31B  is obtained by subtracting the value of NB from the value of NB′ (ΔNB=NB′−NB). The adjustment control is performed to maintain the sum of ΔNA and ΔNB (ΔNA+ΔNB) constant, so that the change in the image size from the initial state can be reduced or prevented. The image size can be controlled by adjusting the drive voltage applied to the mirror  130 . ΔNA is an example of the amount of change from a predetermined value (predetermined number) in the first area. ΔNB is an example of the amount of change from a predetermined value (predetermined number) in the second area. 
     The change in the number of scanning lines from the initial state is caused not only by the change in the image size described above but also by the change in the image position described later. However, by performing the adjustment control to maintain a constant total value of ΔNA and ΔNB (≢NA+ΔNB), which are the amount of fluctuations in the number of scanning lines received within the two illuminated areas, respectively, the adverse effect due to the change in the image position can be eliminated, and the change in the image size can be independently detected. For example, it is possible to figure out the occurrence of a shift in the sub-scanning direction based on the change in the number of scanning lines of light received by one illuminated area. However, it is impossible to determine whether such a shift is due to the change in image size or the change in image position, and thus a proper adjustment is difficult to perform. In the present embodiment, the shift in the sub-scanning direction is detected based on the number of scanning lines of light received by each of the two illuminated areas, which are at different locations in the sub-scanning direction. Accordingly, the shift in the sub-scanning direction can be detected at high accuracy and the change from the initial state can be reduced or prevented without using, for example, a detector provided with a high-cost area sensor or a special V-shaped slit. 
       FIGS. 32A and 32B  are illustrations for describing a change in the number of scanning lines of received light due to a change in the image position in the sub-scanning direction. 
     Specifically,  FIG. 32A  indicates a display image area  15 R 1 , a scanning area  15 R 3 , a first detection image area G 1 , and a second detection image area G 2  on the light receivers  61 A and  61 B, which are in an initial state. 
       FIG. 32B  is an illustration of an image position in the sub-scanning direction that has been shifted (changed) in −Y′ direction from the initial state by ΔY. The cause of the change in the image position in the sub-scanning direction is, for example, a change in the position of a component of the optical system. 
     Same as in  FIGS. 31A and 31B ,  FIGS. 32A and 32B  also indicate an overlapping area A of the first detection image area G 1  and an overlapping area B of the second detection image area G 2 , which are in the initial state.  FIGS. 32A and 32B  also indicate the overlapping area A″ of the first detection image area G 1  and the light receiver and the overlapping area B″ of the second detection image area G 2  and the light receiver, which are at the time at which the image position has been changed. 
     As can be seen from  FIGS. 31A, 31B, 32A, and 32B , the number of scanning lines detected by the first detection image area G 1  being illuminated with the light decreases from the number in the initial state when the image size is reduced, and also decreases from the number in the initial state when the image position is shifted in the −Y′ direction. Similarly, the number of scanning lines that is detected by the second detection image area G 2  being illuminated with the light increases from the number in the initial state when the image size is reduced, and also increases from the number in the initial state when the image position is shifted in the −Y′ direction. Under actual use conditions, the change in image size and the change in image position occur at the same timing. Accordingly, the change in image size and the change in mage position need to be detected independently so as to control them with high accuracy. 
     In the following description, the amount of fluctuations (increase-decrease amount) in the number of the scanning lines with the image position changed (shifted) is considered. When the image position is shifted, the display image area  15 R 1 , the scanning area  15 R 3 , and the first detection image area G 1  and the second detection image area G 2  on the light receiver are each shifted in parallel in the −Y′ direction by ΔY. Since the positions of the light receivers  61 A and  61 B are not shifted, the following equations are established: A−ΔY=A″ and B+ΔY=B″ As a result, the following equation is induced: A″−A=(B″−B)×(−1). That is, the amount of fluctuations in the number of scanning lines detected from the first detection image area G 1  being illuminated with the light from the light-source device is equal to the amount of fluctuations in the number of scanning lines detected from the second detection image area G 2  being illuminated with the light from the light-source device. In addition, the signs are opposite to each other. 
     Accordingly, as described above, in controlling the change in the image size, the sum of ΔNA and ΔNB (ΔNA+ΔNB) is adjusted to be maintained constant so that the cause of the change in the image position is eliminated, and the change in the image size is independently detected and controlled. 
     The following describes the independent detection of the change in the image position. The amount of fluctuations in the number of scanning lines detected by the first detection image area G 1  being illuminated with the light from the light-source device is expressed by formula 1 below when a change in image size and a change in image position occur at the same timing. 
       Δ NA=α ( ha )+ΔNY  ( 1 )
 
     where α (ha) is the amount of fluctuations (increase-decrease amount of) in the number of scanning lines when the image size changes, which depends on the heights of the end portions of the light receivers  61 A and  61 B. The heights of the end portions of the light receivers  61 A and  61 B are indicated by symbols hA and hB in  FIG. 32A . Hereinafter, the amount of fluctuations in the number of scanning lines received when the image size changes is represented by α(ha) for the first detection image area G 1  and α(hb) for the second detection image area G 2 . In addition, the ratio of α (hb) with respect to α (ha) is equal to k (α(hb)/α(ha)=k). ΔNY is the number of scanning lines received when the image position is shifted by ΔY. 
     As described above, those caused by the change in image position are eliminated by the sum of ΔNA and ΔNB (ΔNA+ΔNB), which is expressed by formula (2) below. 
       Δ NA+ΔNB=α ( ha )−α( hb )  (2).
 
     Formulae (1) and (2) lead to Formula (3) below: 
       Δ NY={NA −{(Δ NA+ΔNB )/(1− k )}  (3).
 
     When the equation: 1/(1−k)=K is established, formula (3) is represented by formula (4) below: 
       Δ NY=ΔNA ×(1− K )−Δ NB×K   (4).
 
     As described above, using formula (4), the change in the image position is independently detected and controlled based on the change in the number of scanning lines received in the first detection image area G 1  and the change in the number of scanning lines received in the second detection image area G 2 . This enables a change in the image position from the initial state to be substantially prevented. In formulae (3) and (4), k and K are parameters based on the sub-scanning positions of the lower end of the light receiver included in the first detection image area G 1  and the upper end of the light receiver included in the second detection image area G 2 , and these parameters are known parameters. 
       FIG. 33  is a graph indicating the relation of a parameter K and a position of the end portion of the light receiver in the sub-scanning direction. In  FIG. 33 , the horizontal axis denotes the interval in the sub-scanning direction between the lower end of the light receiver included in the first detection image area G 1  and the upper end of the light receiver included in the second detection image area G 2 , and the vertical axis denotes the value of K. As illustrated in  FIG. 33 , as the interval in the sub-scanning direction increases, the value of K decreases. Note that the values of the intervals in the sub-scanning direction indicated by the horizontal axis and the parameters of the vertical axis in  FIG. 33  are merely examples, and no limitation is intended thereby. 
     In the present embodiment, the change in the image position is calculated from the change in the number of scanning lines received in the first detection image area G 1  and the change in the number of scanning lines received in the second detection image area G 2 , using formula (4). When an error occurs in the number of scanning lines of each area, the error in calculation of the change in image position increases with an increase in K. 
     For example, the value of K is  3  (K=3) when the sub-scanning interval between the end portions of the light receivers is as indicated by the dotted line in  FIG. 33 . When an error of one scanning line occurs in the number of scanning lines of ΔNB, the control error is enlarged three times and calculated as the control error. That is, in order to obtain the amount of change in the image position with higher accuracy, it is preferable that the sub-scanning interval between the first detection image area G 1  and the second detection image area G 2  is wider, that is, the sub-scanning interval between the light receivers  61 A and  61 B is wider. Accordingly, the arrangement of the light receivers  61 A and  61 B at substantially diagonal positions on the screen  15  enables a high-accuracy detection. 
       FIG. 34  is a first variation of the first embodiment of the present disclosure. The layout of the light receivers  61 A and  61 B, the first detection image area Gl, and the second detection image area G 2  is not limited to that in  FIG. 30 , but may be a layout as illustrated in  FIG. 34 . In this case, the image size and the image position can be adjusted in the same way as in  FIG. 30 . 
       FIG. 35  is an illustration of a second variation of the second embodiment. Specifically,  FIG. 35  is an illustration of a configuration that includes a different relative position of the light receiver  61 B and the second illumination area (to be illuminated with the light from the light-source device). In the following description, the illuminated area is also referred to as an illumination area that is to be illuminated with light from the light-source device. 
     In the above description, the relative position of the light receivers and the illumination areas are set such that the first detection image area G 1  includes the lower end of the light receiver and the second detection image area G 2  includes the upper end of the light receiver. However, no limitation is intended thereby. As illustrated in  FIG. 35 , the first detection image area G 1  may include the lower end of the light receiver and the second detection image area G 2  may include the lower end of the other light receiver. In such a configuration, different formulae are used for detecting and controlling the change in image size and the image position, but the same concept may be applied. 
     For example, in the case of the change in image size, the adjustment control is performed to maintain a constant difference value (ΔNA−ΔNB) between the amount of fluctuations ΔNA in the number of scanning lines in the first detection image area G 1  and the amount of fluctuations ΔNB in the number of scanning lines in the second detection image area G 2 , so that the change in the image size from the initial state can be reduced or prevented. As is clear from the above description, by using such a formula, the adverse effect due to the change in image position can be eliminated. Further, the detection control of the change in image position may be considered in the same manner as described above. 
     The change in the number of scanning lines from the initial state is caused not only by the change in the image size described above but also by the change in the image position described later. However, by performing the adjustment control to maintain a constant difference value between ΔNA and ΔNB (ΔNA−ΔNB), which are the amount of fluctuations in the number of scanning lines received by the two illumination areas, respectively, the adverse effect due to the change in the image position can be eliminated, and the change in the image size can be independently detected. For example, it is possible to figure out the occurrence of a shift in the sub-scanning direction based on the change in the number of scanning lines of light in one lighting area. However, it is impossible to determine whether such a shift is due to the change in image size or the change in image position, and thus a proper adjustment is difficult to perform. In the present embodiment, the shift in the sub-scanning direction is detected based on the number of scanning lines of light received by each of the two illumination areas at different locations in the sub-scanning direction. Accordingly, the shift in the sub-scanning direction can be detected at high accuracy and the change from the initial state can be reduced or prevented without using, for example, a detector provided with a high-cost area sensor or a special V-shaped slit. 
     In formulae (3) and (4), k and K may be determined based on the sub-scanning positions of the lower end of the light receiver  61 A included in the first detection image area G 1  and the lower end of the light receiver  61 B included in the second detection image area G 2 . Further, in the present embodiment, the light receivers  61 A and  61 B have substantially the same shape and are arranged facing in substantially the same direction in the scanning area. However, no limitation is intended thereby. 
       FIG. 36  is a flowchart for describing an example of a process of adjusting image size. In the flowchart in  FIG. 36 , the position of the scanning light is changed so that the image size is adjusted at constant frame intervals. 
     The light-source device  11  emits laser light to irradiate the first detection image area G 1  and the second detection image area G 2  under the control of the light source control unit  1712  (in step S 11 ) so as to form a detection image (image for detection). 
     Then, the light detection sensor  60  detects the irradiation in step S 11  by using the light receiver  61  and outputs a detection signal. In step S 12  as the detection step, the adjustment signal generation unit  1711  detects the number of scanning lines based on the signal output from the light detection sensor  60  (S 12 ). For example, the number of light signals output from the light receivers  61 A and  61 B within the time during which the first detection image area G 1  and the second detection image area G 2  are scanned is counted, so as to obtain the number of scanning lines. 
     In step S 13  as calculation step, the adjustment signal generation unit  1711  calculates the sum of ΔNA and ΔNB (the value of ΔNA+ΔNB) based on the number of scanning lines obtained from the first detection image area G 1  and the second detection image area G 2  (S 13 ). 
     Further, the adjustment signal generation unit  1711  compares the calculated value of ΔNA+ΔNB with the value of ΔNA and ΔNB in the initial state whose value has been obtained in advance before shipping of product, so as to determine whether the calculated value differs from the value of the initial state by a certain value or more (whether the difference value is more than or equal to a certain value) (in step S 14 ). When the difference value is less than the certain value (No in step S 14 ), the adjustment process for a target frame ends because no adjustment is needed. When the difference value is more than or equal to the certain value (Yes in step S 14 ), the process proceeds to step S 15  to perform the adjustment. 
     Then, in step S 15  as the scanning-position change step, the light deflector control unit  1713  changes the position of the scanning light in the sub-scanning direction to change the image size (S 15 ). For example, the drive voltage is adjusted based on the difference value calculated in step S 14 , and the adjusted voltage is applied to the mirror  130 . For such an adjustment, the relation between ΔNA+ΔNB and the drive voltage may be acquired in advance, and the voltage value may be uniquely determined based on the characteristics of the relation. Alternatively, any desired value for the adjustment step may be set as a maximum voltage value adjustment width. 
     By adjusting the image size at constant frame intervals in such a manner, an appropriate image display is provided. 
       FIG. 37  is an illustration of a configuration for correcting an image according to the third embodiment. In the second embodiment illustrated in  FIG. 30 , the light receiver that receives light of the first detection image area G 1  and the light receiver that receives light of the second detection image area G 2  are separated from each other in the sub-scanning direction. In the third embodiment as illustrated in  FIG. 37 , the light receiver that receives light of the first detection image area G 1  and the light receiver that receives light of the second detection image area G 2  are disposed in substantially horizontal positions in the sub-scanning direction. 
     With such an arrangement, a part of the two-dimensional scanning area  15 R 3 , which is unavailable as the display image area  15 R 1  due to the presence of the light receivers  61  and the second detection image area G 2  on the periphery of the display image area  15 R 1  in the embodiment illustrated in  FIG. 35 , can be used as the display image area  15 R 1 . Thus, a larger image can be displayed in the main scanning direction. 
     In this case as well, the amount of change in the image position is calculated using formula (4) as described above. This calculation can be performed at a higher accuracy with a wider sub-scanning interval between the light receivers. Thus, the case, in which the amount of change in the image position is calculated with a smaller sub-scanning interval as in the third embodiment, is considered below. 
     That is, the calculation of the amount of change in the image position according to the third embodiment is performed using formula (5) or (6) below. 
       Δ NY=ΔNA   (5) or
 
       Δ NY=ΔNB×− 1  (6)
 
     The following describes the difference in display image due to the difference between formulae for calculating the amount of change in the image position to be used for the correction process, using  FIG. 38 .  FIG. 38  is an illustration of a display image in which the text “50 km/h” is displayed at the sub-scanning position of the virtual reference line S within the display image area  15 R 1 .  FIG. 38A  is an illustration of the result of the adjustment of the image position by using formula (4) that includes parameter K, and  FIG. 38B  is an illustration of the result of the adjustment of the image position by using formula (5) that does not include K. 
     In the display image of  FIG. 38A , a display image Pb formed by the blue light source is displayed closest to the reference line S, and the display position of a display image Pr formed by the red light source and the display position of a display image Pg formed by the green light source are different from the position of the display image Pb. That is, the display image in  FIG. 38A  is an image in which colors are shifted from each other (a color-shift image). 
     Such a color-shift image might be generated by an error in calculation due to the parameter K included in formula (4). That is, when ambient light occurs and a signal noise is included in the light signal from the light detection sensor  60  during detection of scanning light emitted from a light source of a certain color, an error might occur in the number of scanning lines detected based on the light signal. For example, in the case where 21 scanning lines are erroneously detected although the actual number of scanning lines is 20, if the amount of change in the image position using formula (4), this erroneous difference by one line is multiplied by K, and accordingly the image position needs to be corrected by K scanning lines. As described above, when a detection error occurs due to, for example, ambient light, by performing the correction using formula (4) that includes parameter K, a color shift image that can be recognized by a person might be generated.  FIG. 38A  indicates the case where the positions of the images formed by the light beams emitted from the light sources of three colors are shifted from each other. For example, when a detection error occurs for one of a plurality of light sources, a color shift occurs only in the image formed by the light from the one light source. 
     By contrast, in  FIG. 38B , the position of the image display is shifted upward in the sub-scanning direction with respect to the reference line S. The display images Pr, Pg, and Pb formed by the light beams from the respective light sources are displayed at the same position. 
     Since formula (4) is not used for the display image in  FIG. 38B , the calculation of the amount of change in the image position is influenced by the amount of change in image size. That is, even if the number of scanning lines actually increases or decreases due to an increase or decrease in the image size, the image position correction is executed as a change in the image position. However, even if the image position is corrected based on such an amount of change influenced by the amount of change in the image size, the positions of the images of three colors are moved (changed) together, and a color shift is less likely to occur. In addition, in the case of  FIG. 38B , a color shift due to the erroneous calculation of parameter K as in  FIG. 38A  is less likely to occur because formula (4) is not used. 
     That is, the cases of  FIG. 38A  and  FIG. 38B  differ from each other in that in  FIG. 38A , a color shift occurs although the amount of change in the image position is accurately calculated with an elimination of the amount of change in the image size whereas in FIG.  38 B, no color shift occurs although the calculated amount of change in the image position is an amount of change in position influenced by the change in the image size as compared to the actual amount of change in position. Accordingly, in consideration of this difference, any desired formula may be selected according to the case. For example, as a human recognition characteristic, the color shift is more likely to be recognized by person than the shift in the image position does. Accordingly, an image in which the color shift is reduced as illustrated in  FIG. 38B  is more prioritized, it is desirable to use formula (5) or formula (6) rather than formula (4). 
     The method using formula (5) or formula (6) may be used in the configuration in which two light receivers are disposed substantially diagonally as illustrated in  FIG. 35 . 
       FIGS. 39A and 39B  are illustrations of a configuration for correcting an image according to the fourth embodiment. In the second embodiment and the third embodiment, two light receivers are provided such that the first illumination area is provided for one light receiver, and the second illumination area is provided for the other light receiver. In the fourth embodiment, only one light receiver  61  is provided such that either one of the first illumination area and the second illumination area is selected to be illuminated with the light at the time of each scanning, and an image adjustment control is performed using the detection result of each illumination. 
       FIG. 39A  indicates a first illumination area including the lower end of the light receiver in the sub-scanning direction in N frame, and  FIG. 39B  indicates a second illumination area including the upper end of the light receiver in the N+1 frame. With this configuration, at least any one of the light receivers  61  performs an image correction, and a larger display image is obtained with a larger two-dimensional scanning area  15 R 3 . 
     Note that the configuration that temporally switches between the first illumination area and the second illuminated area illuminating of the second illumination area to be illuminated with the light from the light-source device can be used in the second embodiment and the third embodiment. In the image display device having the configuration of the embodiment of the present disclosure, light other than the light for projecting a display image might unintentionally leak to the display image area  15 R 1  due to scattering of light on a mirror surface of the mirror  130 . Accordingly, the viewer  3  might visually recognize unwanted light. Temporally switching between the first illumination area and the second illumination area to be illuminated with the light from the light-device source eliminates illumination of other locations other than the image information within one frame, and thus substantially prevents the unintentional leakage of light. Thus, an image with good visibility can be displayed. 
     As an example, a scan frame image may be scanned for one cycle in an image for one screen included in the image information. Alternatively, a scan frame image n and a scan frame image n+1 may be formed from an image for one screen. 
       FIGS. 40 to 42  are illustrations for comparing the sizes of the display image areas according to the second to fourth embodiments. 
       FIGS. 40, 41, and 42  indicate maximum display image areas according to the arrangement of the light receiver  61  of second to fourth embodiments. For example,  FIGS. 30 and 40  indicate the same configurations of the light receivers  61 A and  61 B and the detection image areas G 1  and G 2 , and differ in that the display image area  15 R 1  in  FIG. 30  as a whole is disposed between the light receivers  61 A and  61 B in the main scanning direction. By contrast, the display image area in  FIG. 40 , the maximum main scanning width of the display image area  15 R 1  ranges from the position of the light receiver  61 A to the position of the light receiver  61 B. That is, in the configuration of the second embodiment, the display image area  15 R 1  may have the maximum size as illustrated in  FIG. 40 , and may also be smaller than that in  FIG. 40 . 
       FIG. 43  is a table comparing the sizes of the display image areas  15 R 1  illustrated in  FIGS. 40 to 42 . The comparison of the image sizes is normalized so that each dimension width of the second embodiment is one time, and the minimum sub-scanning width can be expanded to 1.5 times larger by using that of the third embodiment. Further, by using that of the fourth embodiment, the minimum main scanning width can be expanded to 1.3 times larger. In the present disclosure, the two-dimensional scanning area  15 R 3  has the same size in any of the embodiments. As described above, the light receiver  61  and the detection image area G may be selected as appropriate depending on the desired accuracy of the image adjustment and the desired size of the display image area  15 R 1 . 
     In the second and third embodiments, each pair of the light receiver  61 A and the detection image area G 1  and of the light receiver  61 B and the detection image area G 2  is disposed across the display image area  15 R 1  in the main scanning direction. However, no limitation is intended thereby. Each pair may be disposed on the same side with respect to the display image area  15 R 1  in the main scanning direction. 
     Each of the functions of the described embodiments may be implemented by one or more processing circuits or circuitry. Processing circuitry includes a programmed processor, as a processor includes circuitry. A processing circuit also includes devices such as an application specific integrated circuit (ASIC), DSP (digital signal processor), FPGA (field programmable gate array) and conventional circuit components arranged to perform the recited functions. 
     Numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the disclosure of the present disclosure may be practiced otherwise than as specifically described herein. For example, elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims.