Patent Publication Number: US-2021176441-A1

Title: Display apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2019-220991, filed Dec. 6, 2019, the contents of which are incorporated herein by reference in their entirety. 
     BACKGROUND 
     1. Field of the Invention 
     The present disclosure relates to a display apparatus. 
     2. Description of the Related Art 
     Display apparatuses are known to project images on a screen while bidimensionally scanning with laser light. In such a display apparatus, for example, a microelectromechanical systems (MEMS) mirror that reflects the laser light is driven and thus directions in which the laser light is reflected are varied sequentially. In such a manner, the display apparatus bidimensionally scans with laser light. 
     Some display apparatuses make brightness correction or shade correction. In some cases, correction functions differ according to each display apparatus. Many display apparatuses provide the brightness correction and contrast correction. The brightness correction is achieved by, for example, varying an amount of light from a light source. For the contrast correction, for example, shade level correction is performed such that shade levels of pixels above and below a target pixel are further emphasized. 
     In both cases of the brightness correction and the contrast correction, for example, shade level tables preset in some patterns, or shade level conversion formulas defined in some patterns are stored in the display apparatus. Some display apparatuses have functions of compensating for their own characteristics that are provided based on gamma correction. Characteristic curves defined based on the gamma correction are preset in several patterns (See Japanese Unexamined Patent Application Publication No. 2002-365568, which is referred to as Patent document 1). 
     SUMMARY 
     The present disclosure provides a display apparatus capable of correcting variations in injection current-light output (I-L) characteristics of a laser. 
     The present disclosure provides a display apparatus for displaying video made by laser light, the display apparatus including at least one laser of which the light output varies in accordance with current, a storage unit configured to store at least one conversion table for correcting current-light output characteristics of the at least one laser so as to approximate to desired characteristics, and a control unit configured to retrieve the conversion table from the storage unit to cause the laser to emit light based on data converted using the conversion table. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating an example of a display apparatus according to the present embodiment; 
         FIG. 2  is a plan view illustrating an example of a light scanning unit included in the display apparatus; 
         FIGS. 3A and 3B  are diagrams (first part) illustrating an example of the appearance of the display apparatus according to the present embodiment; 
         FIGS. 4A and 4B  are diagrams (second part) illustrating an example of the appearance of the display apparatus according to the present embodiment; 
         FIG. 5  is a diagram illustrating an example of I-L characteristics of a laser diode; 
         FIGS. 6A and 6B  are diagrams for describing distorted shade levels; 
         FIG. 7  is a diagram illustrating an example of temperature characteristics of the laser diode; 
         FIG. 8  is a diagram illustrating an example of the I-L characteristics (output shade levels with respect to input shade levels); 
         FIG. 9  is a diagram illustrating an example of a conversion table; 
         FIG. 10  is a diagram illustrating an example of arrangement of a photosensor with respect to a screen; and 
         FIG. 11  is a diagram illustrating an example of the configuration of the photosensor. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereafter, one or more embodiments will be described with reference to the drawings. In each drawing, the same components are indicated by the same numerals and duplicate descriptions for the same components may be omitted. 
       FIG. 1  is a block diagram illustrating an example of a display apparatus according to the present embodiment.  FIG. 2  is a plan view illustrating an example of a light scanning unit included in the display apparatus.  FIGS. 3A to 4B  are diagrams illustrating an example of the appearance of the display apparatus according to the present embodiment. 
     (Schematic Configuration of Display Apparatus) 
     Hereafter, the schematic configuration of a display apparatus  1  will be described with reference to  FIGS. 1 to 4B . The display apparatus  1  includes, as main components, circuitry  10 , a light source  20 , a light scanning unit  30 , an optics unit  40 , a screen  50 , and a photosensor  60 , and these components are accommodated in a casing  100 . The display apparatus  1  is an apparatus that displays video made by laser light. For example, the display apparatus  1  is a laser scanning projector. 
     The circuitry  10  is circuitry that controls the light source  20  and the light scanning unit  30 . For example, the circuitry  10  can include a system controller  11 , a central processing unit (CPU)  12 , buffer circuitry  13 , mirror drive circuitry  14 , laser drive circuitry  15 , a memory  16  (e.g., flash memory), and the like. A video signal is input to the system controller  11  from outside the display apparatus  1 . 
     The light source  20  includes a laser diode (LD) module  21  and a dimming filter  24 . 
     The LD module  21  includes lasers  211 R,  211 G, and  2118  of which light output varies in accordance to injection current. The LD module  21  also includes a light amount-detecting sensor  215  that monitors an amount of light immediately emitted from each of the lasers  211 R,  211 G, and  211 B. The LD module  21  further includes a temperature sensor  217  and the like, and the temperature sensor  217  monitors a temperature of a given laser immediately after each laser emits light. 
     The laser  211 R is, for example, a red semiconductor laser and can emit light of a wavelength λR (e.g., 640 nm). The laser  211 G is, for example, a green semiconductor laser and can emit light of a wavelength λG (e.g., 530 nm). The laser  211 B is, for example, a blue semiconductor laser and can emit light of a wavelength λB (e.g., 445 nm). For example, a photodiode or the like can be used as the light amount-detecting sensor  215 . For example, a thermistor can be used as the temperature sensor  217   
     The light scanning unit  30  employs, for example, microelectromechanical systems (MEMS) that cause a mirror  310  to be driven by one or more piezoelectric elements. The mirror  310  serves as a scanning unit that reflects light (synthesized light) emitted from the lasers  211 R,  211 G, and  211 E and bidimensionally scans with the emitted light horizontally and vertically, in accordance with an input video signal, to thereby form an image on the screen  50 . 
     Specifically, as illustrated in  FIG. 2 , the mirror  310  is supported at opposite sides of the mirror  310 , by torsion beams  331  and  332  that serve as an axis. Drive beams  351  and  352  are paired so as to interpose the mirror  310  in a direction perpendicular to the torsion beams  331  and  332 . Piezoelectric elements formed on respective surfaces of the drive beams  351  and  352  cause the mirror  310  to oscillate about the axis that is served by the torsion beams  331  and  332 . In the following, the direction in which the mirror  310  oscillates about the axis that is served by the torsion beams  331  and  332  is referred to as a horizontal direction. For example, resonant vibrations are used in horizontal driving performed through the drive beams  351  and  352 . In such a case, the mirror  310  can be driven at a high speed. A horizontal deflection sensor  391  is a sensor that detects the inclination of the mirror  310  relative to the horizontal direction, in a state in which the mirror  310  oscillates in the horizontal direction. 
     Drive beams  371  and  372  are paired outside of the drive beams  351  and  352 . The drive beams  371  and  372  are each formed by coupling multiple cantilevers in a meandering shape, and each cantilever has a surface on which a piezoelectric element is formed. The piezoelectric elements of the surfaces of the drive beams  371  and  372  can cause the mirror  310  to oscillate in a vertical direction that is a direction perpendicular to the horizontal direction. Each of vertical deflection sensors  395  and  396  is a sensor that detects the inclination of the mirror  310  relative to the vertical direction, in a state in which the mirror  310  oscillates in the vertical direction. Note that in a unit  150  (see  FIG. 3B ), the light scanning unit  30  and other components such as the drive circuitry are provided in, for example, a ceramic package, and are covered by a ceramic cover. 
     Note that a longitudinal (axial) direction of each of the torsion beams  331  and  332  is inclined about 5 degrees relative to (100)-face identified based on crystal orientations. Also, a longitudinal direction of the cantilevers constituting each of the drive beams  371  and  372  is set parallel to the longitudinal direction (axial direction) of each of the torsion beams  331  and  332 . Likewise, the longitudinal direction of the cantilevers constituting each of the drive beams  371  and  372  is inclined about 5° relative to the (100)-face identified based on crystal orientations. 
     The optics unit  40  is an optical system for irradiating the screen  50  with light used when the light scanning unit  30  scans. The optics unit  40  can include a reflective mirror  41 , a reflective mirror  42 , a reflective mirror  43 , a concave mirror  44 , and the like, as illustrated in  FIG. 3B  or the like. The light entering the optics unit  40  from the light scanning unit  30  is substantially collimated by the concave mirror  44  and then is focused onto the screen  50 . As a result, an image is formed on the screen  50  in accordance with the video signal. The screen  50  preferably includes a function (such as a microlens array) of eliminating noise, which is called speckles, appearing in a grained pattern in the image. 
     The photosensor  60  is provided with respect to the screen  50  and detects laser light delivered to the photosensor  60  in each of a forward pathway (first direction) and a backward pathway (second direction) with respect to horizontal scanning that is performed with laser light. For example, a photodiode or the like can be used as the photosensor  60 . 
     (Schematic Operation of Display Apparatus) 
     Hereafter, the schematic operation of the display apparatus  1  will be described. For example, the system controller  11  can adjust a deflection angle of the mirror  310 . The system controller  11  uses the buffer circuitry  13  to monitor the inclination of the mirror  310  relative to each of the horizontal direction and the vertical direction, and the monitored inclination of the mirror  310  is obtained by one or more given sensors among the horizontal deflection sensor  391  and the vertical deflection sensors  395  and  396 . Further, the system controller  11  can supply an angle control signal to the mirror drive circuitry  14 . The mirror drive circuitry  14  supplies a predetermined drive signal to each of the drive beams  351  and  352  and the drive beams  371  and  372 , based on the angle control signal from the system controller  11 . The mirror drive circuitry  14  can cause the mirror  310  to be driven to a predetermined angle. In other words, the mirror  310  can scan at the predetermined angle. 
     The system controller  11  can also supply, for example, a digital video signal to the laser drive circuitry  15 . The laser drive circuitry  15  applies a predetermined current to each of the lasers  211 R,  211 G, and  211 B based on the video signal from the system controller  11 . In such a configuration, the lasers  211 R,  211 G, and  211 B respectively emit red, green, and blue lights each of which is modulated in accordance with the video signal. By synthesizing these lights, a color image can be formed. 
     The CPU  12  monitors, for example, an amount of light emitted from each of the lasers  211 R,  211 G, and  2118 , based on the output of the light amount-detecting sensor  215 . The CPU  12  can supply a light-amount control signal to the LD module  21  accordingly. For each of the lasers  211 R,  211 G, and  211 B, a current control is performed based on the light amount-control signal from the CPU  12 , so that the output of a given laser is adjusted to a predetermined output (light amount). 
     Note that the light amount-detecting sensor  215  can include three sensors that independently detect amounts of light emitted from the lasers  211 R,  211 G, and  211 B. Alternatively, the light amount-detecting sensor  215  may include merely one sensor. In this case, the lasers  211 R,  211 G, and  211 B emit light in sequence and then the one sensor sequentially detects the light emitted from the lasers  211 R,  211 G, and  211 B. With such a configuration, the amount of light emitted from each of the lasers  211 R,  211 G, and  211 B can be adjusted. 
     The CPU  12  monitors the temperature of each of the lasers  211 R,  211 G, and  211 B based on the output of the temperature sensor  217 . Also, the CPU  12  can supply, to the LD module  21 , an appropriate light amount-control signal corresponding to the monitored temperature, based on the output of the temperature sensor  217 . 
     The light of respective wavelengths emitted from the lasers  211 R,  211 G, and  211 B is synthesized by a dichroic mirror or the like. Next, the synthesized light is dimmed by a dimming filter  24  to a predetermined light amount, and then the dimmed light enters the mirror  310 . The mirror  310  bidimensionally scans with the emitted light. The light used when scanned is delivered to the screen  50  through the optics unit  40  to therefore form a two-dimensional image on the screen  50 . Note that a function provided by the photosensor  60  will be described below. 
     (Shade Level Control of Laser) 
     A semiconductor emitting device includes two regions being a light emitting diode (LED) emission region where an LED emits light and a laser diode (LD) emission region where an LD emits light. These regions are distinguished from the magnitude of injection current. In general, a region used in a laser diode is the latter region. Injection current-light output characteristics (I-L characteristics) in the LD light emission region are not linear. 
       FIG. 5  is a diagram illustrating an example of the I-L characteristics of the laser diode. In the example in  FIG. 5 , a slope of the I-L characteristics is not linear at each of portions A, B, and C. The portion A is a portion near a threshold current at which the current magnitude is decreased. At the portion B, a current-light output line is bent and kinks appear where a line slope is inverted with respect to a negative slope or positive slope. The C portion relates to a high shade level region where the current magnitude is increased and the slope of the I-L characteristics is decreased and consequently becomes attenuated. 
     In many cases, the I (injection current)-L (light output) characteristics vary from one laser diode to another. In contrast, in the range of from low shade levels to high shade levels, when injection current is increased at regular intervals, the light output is not linear as in the case described in  FIG. 5 . For this reason, when the light output is presented using a color chart, color gradations are not represented properly as illustrated in  FIG. 6B .  FIG. 6A  illustrates expected shade levels. When shade levels are represented with respect to the portions A, B, and C illustrated in  FIG. 5 , gradations are not represented properly, resulting in distorted shade levels as illustrated in  FIG. 6B . 
     An environment in which the display apparatus  1  is used differs depending on an application. There are light environments and dark environments. When an ambient environment is dark, an amount of light of the display apparatus  1  is reduced. In such an environment, when gradations are not represented properly, colors could not be reproduced as suited. 
     Further, the I-L characteristics described above vary with temperature.  FIG. 7  illustrates an example of temperature characteristics of the laser diode. The laser diode has the temperature characteristics illustrated in  FIG. 7 . A linearity and a slope of the I-L characteristics vary depending on temperature. In the example in  FIG. 7 , the I-L characteristics at 25° C. are approximately linear. In contrast, for the I-L characteristics at 60° C., the light output is gradually attenuated in a high shade level region. In other words, the I-L characteristics are relatively linear at low temperatures. In contrast, the I-L characteristics becomes nonlinear at high temperatures and consequently the slope of the I-L characteristics is decreased in the high shade level region. 
     Accordingly, at high temperatures, even when a maximum light amount is maintained constantly under the control of a light amount, a projected image, in which middle shade levels are increased and thus shade levels as a whole cannot be reproduced correctly, appears. For this reason, there are cases where shade levels of output video in accordance with the video signal input to the display apparatus  1  are not be represented as expected. 
     In order to solve the problem caused by non-linearity of the I-L characteristics of the laser diode as described above, for the display apparatus  1 , a shade level control of the laser is performed. 
     Specifically, in the display apparatus  1 , a plurality of conversion tables for correcting I-L characteristics are preliminarily created and stored in a memory  16  that is a storage unit. The CPU  12  selects an appropriate conversion table in accordance with a situation, and retrieves the selected conversion table from the memory  16  to transmit the conversion table to the system controller  11 . The system controller  11  applies the selected conversion table to the video signal input to the display apparatus  1 , and supplies converted data to the laser drive circuitry  15 . 
     Each conversion table stores input shade levels and output shade levels. For the input shade levels, for example, 256 shade levels of a video signal, which are in the range of from minimum shade level 0 to maximum shade level 255, are associated with one-on-one output shade levels. In order to create the conversion table, first, I-L characteristics before correction with respect to each laser (lasers  211 R,  211 G, and  211 B) to be used are acquired. At this time, it is assumed that an amount of light at each of the minimum shade level 0 and the maximum shade level 255 is adjusted to become an expected value. 
     In this description, for ease of calculation, as an example, a given shade level selected from among the range of from the minimum shade level 0 to the maximum shade level 255 is normalized using the maximum shade level 255. For example, when the light output at the maximum shade level 255 is given as Lmax [mW], a normalized light amount In(n) at a given shade level n is determined by the formula In(n)=I(n)×255/Lmax, where, I(n) represents the light output [mW] at the shade level n. In this case, for example, I-L characteristics illustrated in  FIG. 8  are obtained. In  FIG. 8 , a given input shade level corresponds to an I value given from the I-L characteristics, and a given output shade level corresponds to an L value given from the I-L characteristics. 
     Hereafter, a conversion table is created based on input shade levels and output shade levels illustrated in  FIG. 8 . In  FIG. 8 , for example, an input shade level at output shade level 3 is 5, and an input shade level at output shade level 4 is 3. From such relations, as illustrated in  FIG. 9 , in the conversion table, a value at output shade level 3 is set to 5, and a value at output shade level 4 is set to 3. When a suitable input shade level is not available, a value is set by taking into account a value close to a given output shade level. Under such a condition, for example, the conversion table illustrated in  FIG. 9  is obtained. 
     When the I-L characteristics before conversion (output shade levels in  FIG. 8 ) are compared with the I-L characteristics after conversion (output shade levels in  FIG. 9 ), it can be seen that the I-L characteristics after conversion (output shade levels in  FIG. 9 ) approximates to a linearity in comparison to the I-L characteristics before conversion (output shade levels in  FIG. 8 ). Note that a conversion table is created for each of the lasers  211 R,  211 G, and  211 B. 
     When I-L characteristics differ depending on temperature, conversion tables are preliminarily obtained for respective different temperatures, in the same manner as described above. Preferably, a control unit (the system controller  11  and the CPU  12 ) switches the conversion tables in accordance with variation in an ambient temperature detected by the temperature sensor  217 . The temperature set for each of the conversion tables depends on a specification of the display apparatus. For example, the temperature can be selected from among −5° C., 10° C., 25° C., 40° C., 65° C., and the like. 
     As described above, the display apparatus  1  includes a storage unit that stores conversion tables each of which is used to correct for current-light output characteristics of a given laser so as to approximate to desired characteristics. Further, the display apparatus  1  also includes the control unit (the system controller  11  and CPU  12 ) that retrieves a given conversion table from the storage unit and causes a given laser to emit light based on data converted using the conversion table. 
     Each conversion table can be set as a table in which input shade levels correspond to one-on-one out shade levels. There are no restrictions other than upper and lower limits of settable shade levels. When each conversion table is created, first, digital data with multiple shade levels is preliminarily obtained. Then, the conversion table, a value indicating a given shade level to be converted is set so that the digital data with the multiple shade levels approximates to a linearity. 
     With use of such conversion tables, I-L characteristics for which approximation is needed to be performed using a high-degree function, or I-L characteristics for which function approximation is not easily performed, can be easily corrected. For example, even in a case of complex I-L characteristics, such as characteristics for which kinks appear, correction can be made so that the I-L characteristics is linearly approximated. 
     Preferably, multiple conversion tables are each created with respect to an ambient temperature to be stored in the memory  16 . In such a configuration, I-L characteristics can be corrected in accordance with a given ambient temperature. Further, the I-L characteristics can be corrected in accordance with a required maximum light amount. In other words, a projected image with predetermined shade levels can be provided at any time regardless of ambient temperature. Further, a projected image with predetermined shade levels can be provided at any time regardless of whether a decreased light amount or an increased light amount is provided. 
     Even when there are variations in I-L characteristics of individual lasers that are provided in the display apparatus, each conversion table can be created by taking into account the variation with respect to a given laser. Thus, variation in shade levels according to each individual display apparatus can be minimized. 
     In the correction made using the conversion tables described above, a given table is used for correction, and calculation is not performed. In this case, it is advantageous for circuitry or software for performing image processing, from the viewpoints of a space taken up by hardware or load. 
     In a case of software or correction circuitry, correction made using the conversion tables is achieved. For example, for a given display apparatus, when shade level correction applies only to linear function correction, distortion of I-L characteristics could not be corrected. In this case, in actuality, the software or the correction circuitry would not updated. However, in the case of making correction using a given conversion table, by merely converting a value indicating a given input shade level into a corresponding value in the conversion table, the I-L characteristics can be corrected. 
     Note that conversion tables are preferably created for each laser to be stored in the memory  16 . Preferably, one or more conversion tables for at least one red laser are stored. Where, for I-L characteristics of the red laser, non-linearity is relatively enhanced. 
     A method of correcting the I-L characteristics using conversion tables to approximate a linearity has been described above. However, such a method is not limiting. One or more conversion tables are used to make tone curve correction such that I-L characteristics approximate desired characteristics. For example, an optimal gradation curve defined based on, e.g., gamma correction, can be set using one or more conversion tables. One or more conversion tables are used to be able to be set any contrast. 
     (Detection of Laser Light by Photosensor  60 ) 
     In the display apparatus  1 , the photosensor  60  is provided with respect to the screen  50  to detect laser light delivered to the photosensor  60  in each of the forward pathway (first direction) and backward pathway (second direction) with respect to horizontal scanning that is performed using the laser light. 
       FIG. 10  is a diagram illustrating an example of arrangement of the photosensor in the screen. As illustrated in  FIG. 10 , the photosensor  60  is disposed at a location outside (a blanking region) of a display region  51  of the screen  50  where an image is displayed, and the location is, for example, under the bottom left of the display region  51 . Note that the screen  50  has a region scanned with laser light, and the display region  51  is a region where an image input to the display apparatus  1  is projected. 
     Note that in the example in  FIG. 10 , the photosensor  60  is disposed under the display region  51 , but is not limited to the example described above. When the photosensor  60  is disposed with respect to the blanking region under the display region  51 , the photosensor  60  may be situated under a middle portion of the display region  51  in the horizontal scanning direction. The photosensor  60  may also be disposed with respect to a given blanking region above the display region  51 . The photosensor  60  may be disposed with respect to a given blanking region on a left or right side of the display region  51   
     The photosensor  60  is used for correction of a deflection angle and a phase shift with respect to the horizontal scanning direction, as well as correction of a deflection angle with respect to the vertical scanning direction, where laser light used to scan the blanking region is used as a reference beam L. The photosensor  60  detects multiple reference beams L that are delivered to respective different locations situated with respect to the vertical scanning direction. 
     Note that in the present embodiment, for example, one reference beam L is radiated for each one frame, and the screen  50  is scanned with respect to the horizontal direction. In the present embodiment, for example, the number of reference beams L delivered to the blanking region may be about  20   
       FIG. 11  is a diagram illustrating an example of the configuration of the photosensor. As illustrated in  FIG. 11 , the photosensor  60  includes a photodiode (PD)  61  with a first light reception region, and includes a PD  62  with a second light reception region. In the present embodiment, correction of a deflection angle and phase shift of the mirror  310  with respect to the horizontal scanning direction, as well as correction of a deflection angle of the mirror  310  with respect to the vertical scanning direction, are made based on detected results of laser light at the multiple photodiodes (PDs)  61  and  62 . 
     In the photosensor  60 , the PD  61  and the PD  62  are arranged to overlap a region to be scanned with respect to the vertical scanning direction of the mirror  310 . In other words, the photosensor  60  is a photo detector having the first light reception region and the second light reception region that are arranged at different locations with respect to the vertical scanning direction. 
     Note that in  FIG. 11 , a direction represented by the arrow V is the vertical scanning direction of the mirror  310 , and a direction represented by the arrow H is the horizontal scanning direction of the mirror  310 . 
     The PD  61  and PD  62  are used to correct a deflection angle of the mirror  310  with respect to the vertical scan direction. The PD  62  is used to correct a deflection angle and phase shift of the mirror  310  with respect to the horizontal scanning direction. 
     In the photosensor  60 , the PD  61  and the PD  62  are each arranged to overlap a region corresponding to a pitch Ov with respect to the vertical scanning direction. In the following description, a region of the PD  61  corresponding to the pitch Ov is referred to as an overlap region  611 , and a region of the PD  62  corresponding to the pitch Ov is referred to as an overlap region  621 . 
     Each of the overlap region  611  and overlap region  621  is a light reception region where scanning with respect to the vertical scanning direction is performed using multiple reference beams delivered to different locations of the region. When the overlap regions  611  and  621  are each scanned with one or more given reference beams, the reference beams emitting a given overlap region are detected. 
     When the overlap region  611  of the PD  61  is not scanned and the region  612  is scanned with a given reference beam, the PD  62  does not detect the reference beam. Likewise, when the overlap region  621  of the PD  62  is not scanned and the region  622  is scanned with a given reference beam, the PD  61  does not detect the reference beam. 
     In such a configuration, when a given deflection angle of the mirror  310  with respect to the vertical scanning direction varies, at least any one, among the number of detection times of reference beams in the region  612  of the PD  61 , the number of detection times of reference beams in the overlap regions  611  and  621 , and the number of detection times of reference beams in the region  622 , varies. In view of the point described above, according to the present embodiment, the deflection angle of the mirror  310  with respect to the vertical scanning direction is corrected in accordance with variation in at least one number of detection times described above. 
     Note that in the present embodiment, the overlap region  611  and the overlap region  621  are scanned with multiple reference beams. However, such a manner is not limiting. The number of reference beams used when the overlap region  611  and the overlap region  621  are scanned may be a single, which is not a plurality. 
     Preferably, the light reception regions of the PD  61  and the PD  62  are continuous with respect to the vertical scanning direction. However, these light reception regions may not be continuous. In other words, the PD  61  and the PD  62  may be disposed at different locations with respect to the vertical scanning direction. In this case, the overlap region  611  and the overlap region  621  may not be present in the PD  61  and the PD  62 , respectively. 
     When the light reception regions of the PD  61  and the PD  62  are not continuous with respect to the vertical scanning direction, the deflection angle of the mirror  310  with respect to the vertical scanning direction may be corrected in accordance with variation in at least one among the number of detection times of reference beams in the first light reception region of the PD  61 , and the number of detection times of reference beams in the second light reception region of the PD  62 . 
     The PD  62  is used to enable the deflection angle and phase shift of the mirror to be detected with respect to the horizontal scanning direction. Specifically, the CPU  12  can specify a pixel (hereafter referred to as a “first pixel”) detected, through the PD  62 , in the forward pathway with respect to the horizontal scanning that is performed with laser light. The CPU  12  can also specify a pixel (hereinafter referred to as a “second pixel”) detected through the PD  62 , in the backward pathway with respect to the horizontal scanning that is performed with laser light. Further, the CPU  12  can detect and compensate for a phase shift of laser light with respect to the horizontal scanning direction, as well as variation in a deflection angle of the mirror  310 , based on deviation from a desired value of a given first pixel and deviation from a desired value of a given second pixel. 
     The CPU  12  can specify a count value of the first pixel and a count value of the second pixel, based on a timing at which the PD  62  performs detection, with respect to which pixel number of a given image formed using laser light delivered to the screen  50  is indicated, out of a total number of pixels. Also, when the PD  62  can detect multiple pixels continuously arranged in the horizontal direction, the CPU  12  may specify, as a first pixel or a second pixel, the pixel that photosensor  60  first detects, for example. 
     The CPU  12  may repeatedly acquire a count value of a given first pixel and a count value of a given second pixel (predetermined number of times). 
     For example, in a region outside of the display region corresponding to the PD  62 , horizontal scanning is performed using a reference beam, for each one frame. In this case, the PD  62  detects, for each one frame, a given first pixel and a given second pixel, and can acquire, for each one frame, a count value of the first pixel and a count value of the second pixel. 
     In such a manner, based on deviation from a predetermined value of a given first pixel detected by the PD  62  when horizontal scanning is performed using laser light in the forward pathway with respect to the horizontal scanning, as well as deviation from a predetermined value of a given second pixel detected by the PD  62  when horizontal scanning is performed using laser light in the backward pathway with respect to the horizontal scanning, a phase shift of the laser light with respect to the horizontal scanning direction, and variation in a given deflection angle of the light scanning unit with respect to the horizontal scanning direction can detected and compensated. 
     The PD  62  may include, for example, multiple PDs  623  and  624  that are arranged with respect to the horizontal scanning direction. By arranging the multiple PD  623  and  624  with respect to the horizontal scan direction, detection accuracy of a given position where the reference beam is radiated with respect to the horizontal scanning direction can be improved. For example, in the present embodiment, while a region corresponding to the PD  623  is scanned using a reference beam, a signal corresponding to a light amount of the reference beam is output from the PD  623 . Further, while a region corresponding to the PD  624  is scanned using a reference beam, a signal corresponding to a light amount of the reference beam is output from the PD  624 . 
     In such a configuration, when the signal from the PD  623  is not output and the signal from the PD  624  is output, a reference beam is assumed to be detected and thus the PD  62  may output a signal indicating that it detects the reference beam. 
     When scanning is performed with laser light and the laser light is delivered to a boundary between the PD  621  and the PD  622 , the PD  62  can output a signal indicating that a given first pixel and second pixel are detected. In other words, it can be detected that the reference beam is delivered to the boundary between the PD  623  and the PD  624  of the PD  62 . Thus, detection accuracy of the position where the reference beam is radiated with respect to the horizontal scanning direction can be improved. 
     Here, in a case of the light output at low shade levels, a rate at which a light amount varies with respect to one step in which varying current of the laser driver is set is increased. Thus, a diameter of a given laser spot is varied increasingly. In light of the point described above, when the PD  62  is used to detect a deflection angle and phase shift with respect to the horizontal scanning direction, a target may be the light output in the range of middle shade levels to high shade levels. Note that when an amount of light emitting the PD  62  is increased, a dimming filter or the like may be disposed at a front stage of the PD  62  to reduce the light, in order to adjust light emitting the PD  62  to a desired permitted light amount for the PD  62 . 
     As described above, there is variation in the I-L characteristics of a given laser. For this reason, if any correction is not made, the light output at the middle shade levels varies and a spot diameter of laser that is delivered to the PD  62  varies accordingly. For example, when the light output at the middle shade levels varies to be increased, a diameter of a given laser spot is increased and consequently a predetermined signal is output from the PD  62  at a timing that is earlier than that at a normal state. In contrast, when the light output with respect to the middle shade scale is varied to be decreased, a diameter of a given laser spot is decreased and consequently a predetermined signal is output from the PD  62  at a timing that is later than that at a normal state. As a result, detection accuracy of the reference beam may be reduced. 
     In contrast, in the display apparatus  1 , since the I-L characteristics are corrected by using one or more conversion tables to thereby approximate a linearity, variation in the light output at the middle shade levels can be minimized. Accordingly, variation in a given spot diameter of laser delivered to the PD  62  can be minimized. As a result, detection accuracy of the reference beam can be improved. In other words, with use of the PD  62 , detection accuracy of the deflection angle and the phase shift with respect to the horizontal scanning direction can be improved. 
     The preferred embodiments have been described in detail above. However, these embodiments are not limiting. Various modifications and substitutions to the embodiments described above can be made without departing from a scope described in the present disclosure. 
     For example, the embodiments has been described using an example in which a display apparatus in the present disclosure is applied to a laser scanning projector. However, such a manner is an example and the display apparatus in the present disclosure is applicable to a variety of laser-based apparatuses. Such apparatuses include, for example, a digital light processing (DLP) (which uses a digital micromirror device), a head-up display for an automobile, a laser printer, a laser scanning epilator, a laser head lamp, radio detection and ranging (LiDAR), and the like. 
     The embodiments have been described using an example of three lasers. However, at least one laser may be used. In this case, a monochrome display apparatus can be adopted.