Patent Publication Number: US-2019197967-A1

Title: Projection-type display device

Description:
TECHNICAL FIELD 
     The present invention relates to a projection-type display device. The invention specifically relates to a projection-type display device capable of adjusting the brightness of an image to be displayed for each section. 
     BACKGROUND ART 
     As one kind of display devices, known is a projection-type display device that projects a generated image onto an arbitrary screen and the like accordingly to display the image. The projection-type display device is, for example, performs spatial light modulation of light of three colors of a red color, a green color, and a blue color corresponding to the primary colors of light using an optical modulation element such as a digital micromirror device (DMD) accordingly to form an image to be projected. 
     The projection-type display device that uses the DMD displays an image by employing what is called a field sequential driving method in which at least one light-emitting element is caused to emit light for each sub-frame obtained by temporally dividing a frame for which an image is formed. The DMD includes a plurality of micro-mirrors corresponding to respective pixels of an image to be displayed, and drives these micro-mirrors individually for each sub-frame so as to be set in one inclined state of two inclined states, so that a full color image is displayed by light that is reflected in a prescribed direction by the micro-mirror, among light emitted by the light-emitting elements. 
     However, as the time being in one inclined state of the two inclined states becomes longer, the micro-mirrors in the DMD are highly probably fixed to the one inclined state, which results in a concern that the lifetime of the DMD may be shortened. In order to solve this problem, for example, a projection-type display device disclosed in PTL 1 has been proposed. 
     In the projection-type display device disclosed in PTL 1, each frame for which an image is formed is divided into two periods of a display period that is a period when at least one element of the plurality of the light-emitting elements is caused to emit light, and a non-display period that is a period when none of the plurality of the light-emitting elements is caused to emit light. The inclined state of the micro-mirror has no influence on the image to be displayed in the non-display period, so that it is possible to set the ratio of the respective inclined states within the non-display period for each of the plurality of micro-mirrors in accordance with the ratio of the respective inclined states within the display period for each of the plurality of micro-mirrors. As a result, it is possible to arbitrarily set the ratio of each inclined state of the two inclined states in the one frame as a whole, with respect to all the micro-mirrors, thereby resolving the concern that the lifetime of the DMD may be shortened. 
     Meanwhile, in a case where such a method is employed, a change in the brightness of an image to be displayed is limited to only a change in the brightness of the entire image by adjusting the light-emitting luminance of the light-emitting element. Therefore, the present inventors have recognized that there is room for improvement in the projection-type display device disclosed in PTL 1, in order to allow the brightness of an image to be displayed for each section to be changed. 
     CITATION LIST 
     Patent Literature 
     PTL 1: JP-A-2014-10417 
     SUMMARY OF INVENTION 
     One object of the invention is to provide a projection-type display device capable of changing the brightness of an image to be displayed for each section. Another object of the invention will be apparent for persons skilled in the art with reference to the aspects, the preferred embodiment, and the attached drawings, which are exemplified below. 
     A first aspect according to the invention relates to a projection-type display device including: a reflection-type spatial optical modulation unit that includes a plurality of movable reflection elements an inclined state of which can be switched to either one of a first inclined state and a second inclined state; 
     a light source unit that includes a plurality of light-emitting elements; and 
     a control unit that controls the reflection-type spatial optical modulation unit and the light source unit such that light constituting an image in accordance with a video signal to be input is projected, 
     in which 
     the control unit 
     divides respective frames of the image into a display period that is a period when at least one element of the plurality of the light-emitting elements is caused to emit light, and a non-display period that is a period when none of the plurality of the light-emitting elements is caused to emit light, 
     further divides, among a plurality of sub-frames that is a plurality of periods and constitutes the frame, each of a plurality of display sub-frames that are the sub-frames included in the display period into a plurality of division display sub-frames, 
     controls the light source unit such that the at least one light-emitting element of the plurality of the light-emitting elements emits light for each display sub-frame, and 
     controls the reflection-type spatial optical modulation unit such that the inclined state of each of the plurality of movable reflection elements is set for each division display sub-frame. 
     In the projection-type display device of the first aspect, the first inclined state or the second inclined state of the movable reflection element can be set for each division display sub-frame, so that it is possible to change the brightness of the image to be displayed for each section. 
     In a second aspect according to the invention, in the first aspect, the movable reflection element in the first inclined state may reflect light of the light-emitting element in a direction where the light is projected as the light constituting the image, 
     the movable reflection element in the second inclined state may reflect light of the light-emitting element in a direction where the light is not projected as the light constituting the image, and 
     the control unit 
     may divide the plurality of the movable reflection elements in the reflection-type spatial optical modulation unit into a plurality of groups, and 
     may determine, among the plurality of the division display sub-frames obtained by each display sub-frame being divided, the division display sub-frame in which the movable reflection element is fixed in the second inclined state, for each of the plurality of the groups of the plurality of the movable reflection elements. 
     In the second aspect, it is possible to change the brightness for each of areas of the image to be displayed corresponding to the plurality of the groups of the movable reflection elements. For example, an area including a specific mark in the displayed image is displayed brighter to allow an attention to be urged to the vehicle driver of the vehicle using the mark. 
     In a third aspect according to the invention, in the first or second aspect, the control unit 
     may determine any one of the plurality of the groups that is capable of being switched to the first inclined state in accordance with a video signal, in all the division display sub-frames. 
     In the third aspect, the control unit determines any one group of the plurality of the groups as an area where the movable reflection element included in the one group is capable of being switched to the first inclined state in accordance with a video signal, in all the division display sub-frames, so that it is possible to prevent the division display sub-frame that does not contribute as an image because although the light source unit is lit, any of the movable reflection elements are fixed to the second inclined state, from being generated. 
     In a fourth aspect according to the invention, in the first or second aspect, the control unit may change the plurality of the groups in accordance with a type of the image to be displayed and elapsed time after the plurality of the groups has been configured. 
     In the fourth aspect, appropriate differences in intensity of the brightness for each region in the displayed image are possible, so that it is possible to precisely notify the vehicle driver of necessary information in accordance with the operation status of the vehicle. 
     In a fifth aspect according to the invention, in the first to fourth aspects, the control unit may control the reflection-type spatial optical modulation unit to allow the inclined state of each of the plurality of the movable reflection elements to be set for each sub-frame included in the non-display period such that a ratio of time when each movable reflection element is in the first inclined state and time when in the second inclined state is constant per one frame or a plurality of the frames. 
     In the fifth aspect, the ratio of the time when each movable reflection element becomes in the first inclined state and the time when in the second inclined state per one frame or per the plurality of the frames becomes a constant ratio to prevent the movable reflection element from being fixed to either one of the first inclined state and the second inclined state. 
     In a sixth aspect according to the invention, in the first to fifth aspects, the control unit may further divide, among the plurality of the sub-frames, each of a plurality of non-display sub-frames that are the sub-frames included in the non-display period into a plurality of divided non-display sub-frames, and 
     the control unit may control the reflection-type spatial optical modulation unit to allow the inclined state of each of the plurality of the movable reflection elements to be set for each divided non-display sub-frame included in the non-display period such that a ratio of time when each movable reflection element is in the first inclined state and time when in the second inclined state is constant per one frame or a plurality of the frames. 
     Each movable reflection element is controlled for each non-display sub-frame to allow the first inclined state or the second inclined state of each movable reflection element in the non-display period to be set more minutely, compared with a case where the non-display sub-frame is not divided into a plurality of divided non-display sub-frames, for example. 
     In a seventh aspect according to the invention, in the first to sixth aspects, the control unit may determine a ratio of periods of the plurality of the division display sub-frames obtained by the display sub-frame being divided to be constant in the plurality of the display sub-frames. 
     In the seventh aspect, the control unit determines the ratio of periods of the plurality of the division display sub-frames in which each display sub-frame is divided so as to be constant in all the display sub-frames to allow the brightness of the displayed image to be changed for each section, in a state where the gradation step of color mixing is kept. 
     In an eighth aspect according to the invention, in the first to sixth aspects, the control unit may arbitrarily determine a ratio of periods of the plurality of the division display sub-frames obtained by the display sub-frame being divided, for each display sub-frame. 
     In the eighth aspect, the control unit arbitrarily determines the ratio of periods of the plurality of the division display sub-frames in which each display sub-frame is divided, for each display sub-frame to allow the brightness of the monochromatic display in the displayed image to be determined for each color. For example, in the displayed image, the monochromatic display of a given color is displayed brighter than the monochromatic display of other colors to allow an attention to be urged to the vehicle driver of the vehicle using the monochromatic display that is displayed brighter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating an example of the configuration of a projection-type display device in the invention. 
         FIG. 2  is a block diagram illustrating an example of the configuration inside a housing illustrated in  FIG. 1 . 
         FIG. 3A  is a diagram illustrating an example of the configuration of a reflection-type spatial optical modulation unit illustrated in  FIG. 1 . 
         FIG. 3B  is a diagram illustrating an examples in which a movable reflection element illustrated in  FIG. 3A  is in a first inclined state. 
         FIG. 3C  is a diagram illustrating an example in which the movable reflection element illustrated in  FIG. 3A  is in a second inclined state. 
         FIG. 4  is a diagram for explaining frames of an image to be displayed, which are divided by a control unit. 
         FIG. 5  is a timing chart diagram illustrating movable reflection elements and a light source unit. 
         FIG. 6  is a timing chart diagram illustrating the movable reflection elements and the light source unit. 
         FIG. 7  is a diagram illustrating an example in which areas are reflected in the reflection-type spatial optical modulation unit illustrated in  FIG. 3A . 
         FIG. 8A  is a graph for explaining a relation between an external illuminance and a light-emitting luminance of the light source unit. 
         FIG. 8B  is a graph for explaining a relation between the temperature of the reflection-type spatial optical modulation unit and the light-emitting luminance of the light source unit. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     A preferred embodiment described below is used for easy understanding of the invention. 
     Accordingly, persons skilled in the art should be noted that the invention is not unduly limited by the embodiment to be described below. 
     An example of an overall configuration of a projection-type display device  10  in the invention will be described.  FIG. 1  illustrates, as one example of the projection-type display device  10 , the projection-type display device (head-up display device)  10  to be mounted onto a vehicle  100 . It should be noted that Fr illustrated in  FIG. 1  represents the front seen from a vehicle driver of the vehicle  100 , and Rr illustrated in  FIG. 1  represents the rear seen from the vehicle driver of the vehicle  100 . 
     Light  90  that configures an image formed by the projection-type display device  10  is reflected by a front window shield  110  and enters a position of a viewpoint of the vehicle driver, so that the vehicle driver of the vehicle  100  can visually identify a virtual image  300  in front of the front window shield  110 . The vehicle driver of the vehicle  100  can visually identify the virtual image  300 , for example, in a state where at least a part of scenery that can be seen over the front window shield  110  and the virtual image  300  are superimposed on each other. 
     The projection-type display device  10  is provided with a reflection-type spatial optical modulation unit  20 , a light source unit  30 , and a control unit  40 . The projection-type display device  10  may be further provided with, for example, a light source unit drive circuit unit  50 , an external illuminance acquisition unit  60 , and a temperature acquisition unit  70 . Moreover, the reflection-type spatial optical modulation unit  20 , the light source unit  30 , the external illuminance acquisition unit  60 , and the temperature acquisition unit  70  are housed in a housing  80 . 
     As the reflection-type spatial optical modulation unit  20 , a digital micromirror device (DMD)  20  is employed. The DMD  20  spatial-light-modulates light emitted by the light source unit  30  under the control by the control unit  40  accordingly to form an image. The DMD  20  is provided with a plurality of micro-mirrors  21  that are movable reflection elements  21 , and a DMD base substrate  26  that is a reflection element drive circuit board  26 . Inclined states of the respective micro-mirrors  21  can be switched such that the respective micro-mirrors  21  are individually set to either one of a first inclined state and a second inclined state depending on a change in potential that is applied thereto by the DMD base substrate  26 . The configuration and the inclined state of the micro-mirrors  21  are described later. 
     The light source unit  30  includes a plurality of light-emitting elements such as light emitting diodes (LEDs) that emit light due to the supply of drive currents from the light source unit drive circuit unit  50 . In order to enable the projection-type display device  10  to form a color image, the light source unit  30  includes, for example, a red LED  31 , a green LED  32 , and a blue LED  33  corresponding to the primary colors of light so as to obtain white light. It should be noted that although an example in which the red LED  31 , the green LED  32 , and the blue LED  33  are employed as light-emitting elements in the light source unit  30  is described in the present embodiment, the light-emitting elements in the light source unit  30  may be, for example, other semiconductor light-emitting elements such as semiconductor lasers. Moreover, the light source unit  30  may include a plurality of LEDs per one color. 
     The light source unit drive circuit unit  50  supplies a drive current that is in accordance with a pulse width modulation (PWM) signal and is received from the control unit  40 , to the light source unit  30 . When the light source unit  30  includes three LEDs, the light source unit drive circuit unit  50  may include three drive circuits such that drive currents can be individually supplied to the three LEDs. 
     The control unit  40  is a micro computer that takes charge of the overall control of the projection-type display device  10 , and includes a processing unit, a storage unit, and an input-output unit, none of which is illustrated. The processing unit includes, for example, a single or a plurality of micro processors, a micro controller, an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), and any other integrated circuit (IC). The storage unit includes, for example, a single or a plurality of memories capable of storing a program and/or data, such as a rewritable random access memory (RAM), a reading dedicated read only memory (ROM), an erasable program reading dedicated electrically erasable programmable read-only memory (EEPROM), and a flash memory that is a nonvolatile memory. The input-output unit includes an input-output port that is directly connected to the reflection-type spatial optical modulation unit  20 , the light source unit drive circuit unit  50 , the external illuminance acquisition unit  60 , the temperature acquisition unit  70 , or other not-illustrated devices. Moreover, The input-output unit includes a communication unit or the like that is connected to a communication line of a not-illustrated network such as a controller area network (CAN), and allow information to be input from a vehicle electronic control unit (ECU)  120  that is connected to the communication line. 
     The external illuminance acquisition unit  60  is configured so as to be capable of acquiring an external illuminance that is an illuminance outside the projection-type display device  10 . In the example illustrated in  FIG. 1 , the external illuminance acquisition unit  60  is an illuminance sensor that is attached to the housing  80  and includes light-receiving elements such as a photo diode, a photo IC diode, and a photo transistor. The external illuminance acquisition unit  60  may be the external illuminance acquisition unit  60  that is dedicated to the projection-type display device  10  and is directly connected to the control unit  40  as illustrated in  FIG. 1 , or may be the shared external illuminance acquisition unit  60  that is directly connected to the control unit  40  or is connected to the control unit  40  via a communication line of a not-illustrated network and is used for other usage purposes. The other usage purposes include, for example, an auto-light system that causes headlights to automatically illuminate in accordance with the brightness of the outside of the vehicle. When the shared external illuminance acquisition unit  60  is employed, the external illuminance acquisition unit  60  may be attached to a vehicle interior side of the front window shield  110 . 
     The temperature acquisition unit  70  includes a temperature sensor including a thermistor and the like that are embedded in a ceramic portion of the DMD base substrate  26 , and an A/D converter. The temperature sensor measures the temperature of the micro-mirrors  21 , and the A/D converter converts analog data output from the temperature sensor into digital data, and outputs temperature data T to the control unit  40 . It should be noted that the A/D converter may be embedded in the control unit  40 . Moreover, the temperature sensor may measure not the temperature of the micro-mirrors  21  but also the temperature inside the housing  80  or in the surrounding of the housing  80  that influences on the temperature of the micro-mirrors  21 . In addition, the temperature sensor may be placed on a control substrate (which is not illustrated) on which the control unit  40  is mounted, and the temperature sensor on the control substrate may remotely measure the temperature of the micro-mirrors  21 . 
       FIG. 2  illustrates an example of a configuration inside the housing  80 . The housing  80  is formed in a box shape with black color hard resin having a light blocking effect. An opening portion  80   a  having a prescribed size is formed on an upper part of the housing  80 . A cover  81  made of a transparent resin is attached over the opening portion  80   a , for example. 
     The housing  80  houses therein the light source unit  30 , an illumination optical system  82 , the DMD  20 , the temperature acquisition unit  70 , a projection optical system  83 , a screen  84 , a plane mirror  85 , a concave mirror  86 , and the like at prescribed positions. The illumination optical system  82  includes, for example, a concave lens, and adjusts light emitted from the light source unit  30  to the size of the DMD  20 . The projection optical system  83  includes, for example, a concave lens or a convex lens, is an optical system for irradiating with high efficiency the screen  84  with light  90  constituting a displayed image  200  projected by the DMD  20 . 
     The screen  84  includes a diffusion plate, a holographic diffuser, a micro lens array, and the like, and is a transmission type screen that receives the light  90  constituting the displayed image  200  projected by the DMD  20  on a lower surface thereof, and forms the displayed image  200  on an upper surface thereof. The plane mirror  85  is obtained, for example, by forming a reflection film on a surface of a base material including a synthesize resin and a glass material by a method such as vapor deposition, and causes the light  90  having passed through the screen  84  and constituting the displayed image  200  to reflect toward the concave mirror  86 . The concave mirror  86  is obtained, for example, by forming a reflection film having a concave face on a surface of a base material including a synthesize resin and a glass material by a method such as vapor deposition, and further reflects the light  90  having been reflected on the plane mirror  85  toward an opening portion  80   a . This allows a user to visually identify the virtual image  300  to be image-formed in a state where the displayed image  200  having been image-formed on the screen  84  is enlarged. 
     Moreover, as illustrated in  FIG. 2 , the external illuminance acquisition unit  60  is attached, for example, to a position on which external light is incident, of an inner side of the cover  81  made of a transparent resin. Moreover, as illustrated in  FIG. 2 , the temperature acquisition unit  70  is attached to a position of the DMD  20  at which the temperature of the micro-mirrors  21  is capable of being measured. 
     With reference to  FIG. 3A ,  FIG. 3B , and  FIG. 3C , an example of the configuration of the DMD  20  and the operation of the micro-mirrors  21  will be described. As illustrated in  FIG. 3A , the DMD  20  includes the plurality of the micro-mirrors  21 , and the DMD base substrate  26  that supports these micro-mirrors  21 . The plurality of the micro-mirrors  21  respectively correspond to pixels of the displayed image  200 . The DMD base substrate  26  includes a complementary metal oxide semiconductor (CMOS) integrated circuit, changes a potential to be applied to each micro-mirror  21  in accordance with a control signal input from the control unit  40 , and switches the inclined states of the respective micro-mirrors  21  individually. 
       FIG. 3B  illustrates the micro-mirror  21  in a first inclined state, and  FIG. 3C  illustrates the micro-mirror  21  in a second inclined state. The micro-mirror  21  generally includes a mirror surface  22 , a hinge  23 , and a base  24  including an electrode and the like, for example. A voltage is applied from the DMD base substrate  26  to the electrode on the base  24  to cause the mirror surface  22  to incline +12 deg or −12 deg, for example, using the hinge  23  as a fulcrum point. The micro-mirror  21  in the first inclined state illustrated in  FIG. 3B  is in a state where the mirror surface  22  is inclined +12 deg, for example, using the hinge  23  as a fulcrum point, and the micro-mirror  21  in the second inclined state illustrated in  FIG. 3C  is in a state where the mirror surface  22  is inclined −12 deg, for example, using the hinge  23  as a fulcrum point. 
     As illustrated in  FIG. 3B , the micro-mirror  21  when being in the first inclined state reflects light (incident light) of the light-emitting elements  31 ,  32 , and  33  that has been passed through the projection optical system  83  and enters, to a direction of the screen  84 . In contrast, as illustrated in  FIG. 3C , the micro-mirror  21  when being in the second inclined state reflects light (incident light) of the light-emitting elements  31 ,  32 , and  33  that has been passed through the projection optical system  83  and enters, to not the direction of the screen  84  but a direction of a not-illustrated light absorber plate. In other words, the micro-mirror  21  when being in the first inclined state is in an inclined state where light (incident light) that enters from the light-emitting elements  31 ,  32 , and  33  is reflected as the light  90  constituting the displayed image  200 . In contrast, the micro-mirror  21  when being in the second inclined state is in an inclined state where light (incident light) that enters from the light-emitting elements  31 ,  32 , and  33  is not reflected as the light  90  constituting the displayed image  200 . It should be noted that when a power supply of the projection-type display device  10  is in an OFF state, each micro-mirror  21  is stopped to be driven in an inclined state (for example 0 deg) that is a midway between the first inclined state and the second inclined state, for example. 
     The DMD  20  can switch each micro-mirror  21  by an order of microseconds ( 1/1000000 seconds) from the first inclined state to the second inclined state, or from the second inclined state to the first inclined state. In this manner, the control unit  40  can control whether light that enters from the light source unit  30  is reflected as the light  90  constituting the displayed image  200  for each one pixel of the displayed image  200 , for each sub-frame SF obtained by a frame F being divided, which is described later and a cycle of the displayed image  200  being displayed. Hereinafter, by the control unit  40 , a control so as to set the micro-mirror  21  in the first inclined state is also called ON control, and a control so as to set the micro-mirror  21  in the second inclined state is also called OFF control. 
     Hereinafter, an operation of the projection-type display device  10  will be described. The control unit  40  receives a video signal for displaying the displayed image  200  from a vehicle ECU  120  via a not-illustrated communication line. The control unit  40  generates illumination control data such that each of the light-emitting elements  31 ,  32 , and  33  in the light source unit  30  emits light at the luminance of the light and the light-emitting timing that are demanded by this video signal, and controls the light source unit drive circuit unit  50  in accordance with this illumination control data accordingly to cause each of the light-emitting elements  31 ,  32 , and  33  in the light source unit  30  to emit light. Simultaneously with this, the control unit  40  generates display control data such that the light  90  constituting the displayed image  200  demanded by the video signal is reflected by the plurality of the micro-mirrors  21  of the DMD  20 , and controls the DMD base substrate  26  in accordance with this display control data to ON control or OFF control each micro-mirror  21 . 
     The projection-type display device  10  displays the displayed image  200  by a field sequential driving method. Here, a general display method of the displayed image  200  by the field sequential driving method will be simply described. 
     Each frame F of the displayed image  200  includes the sub-frames SF that are divided into a plurality of periods. The control unit  40  outputs a PWM signal to each drive circuit of the light source unit drive circuit unit  50  such that the LED of one color among the red LED  31 , the green LED  32 , and the blue LED  33  emits light for each sub-frame SF at the light-emitting luminance and the light-emitting timing in accordance with the illumination control data. Simultaneously with this, the control unit  40  ON controls or OFF controls the plurality of the micro-mirrors  21  of the DMD  20  individually for each sub-frame SF, in accordance with the display control data. In this manner, the control unit  40  controls, for each sub-frame SF, whether light of a red color, a green color, or a blue color is reflected to the screen  84  as the light  90  constituting the displayed image  200 , for each one pixel of the displayed image  200 . As a result, the red color, the green color, and the blue color that are basic colors are additive mixed during the one frame F, so that the displayed image  200  is displayed as a full color image. 
     Subsequently, a display method of the displayed image  200  in the projection-type display device  10  of the invention will be described. As illustrated in  FIG. 4 , the control unit  40  divides each frame F into a display period Fa that is a period when at least one of the red LED  31 , the green LED  32 , and the blue LED  33  is caused to emit light, and a non-display period Fb that is a period when none of the LEDs is caused to emit light. The control unit  40  divides each of a plurality of display sub-frames SFa that is the sub-frame SF included in the display period Fa in the sub-frame SF is further divided into division display sub-frames dSFa that are a plurality of periods. In an example illustrated in  FIG. 4 , each display sub-frame SFa is divided into two periods of a period X and a period Y, and for example, a display sub-frame SFa 1  is divided into a division display sub-frame dSFa 1 -X and a division display sub-frame dSFa 1 -Y. It should be noted that each display sub-frame SFa may be divided into three or more periods of the period X and the period Y, and other periods. 
     The control unit  40  causes one of the red LED  31 , the green LED  32 , and the blue LED  33  to emit light, for each display sub-frame SFa, at the light-emitting luminance and the light-emitting timing in accordance with the illumination control data. Simultaneously with this, the control unit  40  individually ON controls or OFF controls the plurality of the micro-mirrors  21  of the DMD  20 , for each division display sub-frame dSFa, in accordance with the display control data. Here, the control unit  40  causes none of the red LED  31 , the green LED  32 , and the blue LED  33  to emit light, among a plurality of non-display sub-frames SFb that are the sub-frames SF included in the non-display period Fb in the sub-frame SF. 
     With reference to  FIG. 5 , a concrete example will be described.  FIG. 5  illustrates driving states of a micro-mirror  21 - 1 , a micro-mirror  21 - 2 , the red LED  31 , the green LED  32 , and the blue LED  33  in a part of the display period Fa in the given one frame F. It should be noted that the micro-mirror  21 - 1  and the micro-mirror  21 - 2  are the micro-mirrors  21  different from each other. 
       FIG. 5  illustrates, in the display period Fa of the given one frame F, the display sub-frame SFa 1 , a display sub-frame SFa 2 , a display sub-frame SFa 3 , and a display sub-frame SFa 4  (only a start portion). Moreover, each display sub-frame SFa is divided into two periods of a period X and a period Y, and the division ratio between the period X and the period Y is 6:4 in each display sub-frame SFa. 
     In the example illustrated in  FIG. 5 , the red LED  31  emits light in the display sub-frame SFa 1 , the green LED  32  emits light in the display sub-frame SFa 2 , the blue LED  33  emits light in the display sub-frame SFa 3 , and the red LED  31  emits light in the display sub-frame SFa 4 . In other words, only the red LED  31  is in an ON state in the display sub-frame SFa 1 , only the green LED  32  is in an ON state in the display sub-frame SFa 2 , only the blue LED  33  is in an ON state in the display sub-frame SFa 3 , and only the red LED  31  is in an ON state in the display sub-frame SFa 4 . 
     Moreover, in the example illustrated in  FIG. 5 , the micro-mirror  21 - 1  and the micro-mirror  21 - 2  is ON controlled in the display sub-frame SFa 1  and the display sub-frame SFa 3 . Here, the micro-mirror  21 - 1  is ON controlled during both periods of the period X and the period Y, in the display sub-frame SFa 1  and the display sub-frame SFa 3 . In contrast, the micro-mirror  21 - 2  is ON controlled only during the period X, and is OFF controlled during the period Y, in the display sub-frame SFa 1  and the display sub-frame SFa 3 . 
     In this manner, in the frame F illustrated in  FIG. 5 , a pixel of the displayed image  200  corresponding to the micro-mirror  21 - 2  is displayed with color mixing in the same gradation step and the brightness of 6/10, with respect to a pixel of the displayed image  200  corresponding to the micro-mirror  21 - 1 . For example, it is assumed that in a frame F′ that is prior to the frame F illustrated in  FIG. 5  by one frame, the micro-mirror  21 - 1  and the micro-mirror  21 - 2  have been ON controlled (ON control in the division display sub-frames dSFa 1 -X, dSFa 1 -Y, dSFa 3 -X, and dSFa 3 -Y) at the same timing as the micro-mirror  21 - 1  illustrated in  FIG. 5 . In that case, when the frame F illustrated in  FIG. 5  is displayed, of the pixel of the displayed image  200  corresponding to the micro-mirror  21 - 1  and the pixel of the displayed image  200  corresponding to the micro-mirror  21 - 2 , only the brightness of the pixel of the displayed image  200  corresponding to the micro-mirror  21 - 2  becomes dark. 
     As in the foregoing, in the projection-type display device  10  of the invention, the first inclined state or the second inclined state of the micro-mirror  21  can be set for each division display sub-frame dSFa, so that it is possible to change the brightness of the displayed image  200  for each section. 
     Moreover, the control unit  40  determines the ratio of periods of the plurality of the division display sub-frames dSFa obtained by each display sub-frame SFa being divided so as to be constant in all the display sub-frames SFa to allow the brightness of the displayed image  200  to be changed for each section, in a state where the gradation step of color mixing is kept. For example, when the total number of gradation steps of the respective colors in one frame F is 256, the display period Fa in the one frame F includes about eight display sub-frames SFa that allow 8-bit 256-gradation expression for each color. As the micro-mirror  21 - 1  illustrated in  FIG. 5 , the micro-mirror  21  is ON controlled or OFF controlled for each display sub-frame SFa before the division (the period X and the period Y are combined) to allow the gradation step of each color that displaces from 0 to 255 by one to be determined. Moreover, as the micro-mirror  21 - 2  illustrated in  FIG. 5 , the micro-mirror  21  is ON controlled or OFF controlled for each period X of each display sub-frame SFa before the division (OFF control in the periods Y) to allow the gradation step of each color in which the gradation step of each color displaces from 0×X/(X+Y) to 255×X/(X+Y) by X/(X+Y) to be determined. 
     The micro-mirror  21 - 2  illustrated in  FIG. 5  indicates that the micro-mirror  21  is OFF controlled in the division display sub-frame dSFa of the period Y, and is ON controlled or OFF controlled in the division display sub-frame dSFa of the period X, by the control unit  40 . However, conversely, as for the micro-mirror  21 - 2 , the control unit  40  may OFF control in the division display sub-frame dSFa of the period X, and may ON control or OFF control the micro-mirror  21  in the division display sub-frame dSFa of the period Y. In other words, when the division ratio of the period X and the period Y in each display sub-frame SFa is set to 6:4, the control unit  40  can express each pixel of the displayed image  200  by three types of the brightness, including the brightness of 10/10 by performing ON control or OFF control in both of the period X and the period Y, the brightness of 6/10 by performing ON control or OFF control only in the period X and OFF control in the period Y, and the brightness of 4/10 by performing ON control or OFF control only in the period Y and OFF control in the period X. 
     In this manner, in both of the case where the micro-mirror  21  is ON controlled or OFF controlled as the micro-mirror  21 - 1  illustrated in  FIG. 5 , and the case where the micro-mirror  21  is ON controlled or OFF controlled as the micro-mirror  21 - 2  illustrated in  FIG. 5 , the total number of gradation steps of the respective colors in one frame F is 256. Accordingly, the control unit  40  determines the ratio of periods of the plurality of the division display sub-frames dSFa obtained by each display sub-frame SFa being divided so as to be constant (X:Y=6:4 in the example illustrated in  FIG. 5 ) in all the display sub-frames SFa to allow the brightness of the displayed image  200  to be changed for each section, in a state where the gradation step of color mixing is kept. 
     However, the control unit  40  may arbitrarily determine the ratio of periods of the plurality of the division display sub-frames dSFa obtained by each display sub-frame SFa being divided, for each display sub-frame SFa.  FIG. 6  illustrates a part of an example of the display sub-frame SFa in which the red LED  31  is caused to emit light being divided into a plurality of the division display sub-frames dSFa with the ratio of the period X larger than that in the display sub-frame SFa in which the green LED  32  or the blue LED  33  is caused to emit light. In other words, in the example illustrated in  FIG. 6 , the division ratio of the period X and the period Y is 8:2 in the display sub-frame SFa 1  in which the red LED  31  emits light, and the division ratio of the period X and the period Y is 2:8 in the display sub-frame SFa 2  in which the green LED  32  emits light and in the display sub-frame SFa 3  in which the blue LED  33  emits light. 
     In the example illustrated in  FIG. 6 , the micro-mirror  21 - 1  corresponds to a pixel in which the red color is displayed in monochrome in the displayed image  200 , and the micro-mirror  21 - 2  corresponds to a pixel in which the blue color is displayed in monochrome in the displayed image  200 . Moreover, in the example illustrated in  FIG. 6 , the micro-mirror  21 - 1  is ON controlled during the periods X in all the display sub-frames SFa in which the red LED  31  emits light, and the micro-mirror  21 - 2  is ON controlled during the periods X in all the display sub-frames SFa in which the blue LED  33  emits light. 
     As a result, as the display period Fa illustrated in  FIG. 6  as a whole, the brightness of the red color in the pixel corresponding to the micro-mirror  21 - 1  is 8/10 times, compared with a case where each display sub-frame SFa is not divided into the division display sub-frames dSFa. Similarly, as the display period Fa illustrated in  FIG. 6  as a whole, the brightness of the blue color in the pixel corresponding to the micro-mirror  21 - 2  is 2/10 times, compared with a case where each display sub-frame SFa is not divided into the division display sub-frames dSFa. 
     Accordingly, the control unit  40  arbitrarily determines the ratio of periods of the plurality of the division display sub-frames dSFa obtained by each display sub-frame SFa being divided, for each display sub-frame SFa to allow the brightness of the monochromatic display in the displayed image  200  to be determined for each color. For example, in the displayed image  200 , the monochromatic display of a given color is displayed brighter than the monochromatic display of other colors to allow an attention to be urged to the vehicle driver of the vehicle  100  using the monochromatic display that is displayed brighter. 
     The control unit  40  may divide the plurality of the micro-mirrors  21  of the DMD  20  into a plurality of groups (areas).  FIG. 7  illustrates an example in which the plurality of the micro-mirrors  21  of the DMD  20  is divided into a first area  160 - 1  and a second area  160 - 2 . The first area  160 - 1  and the second area  160 - 2  illustrated in  FIG. 7  respectively correspond to areas in the screen  84  illustrated in  FIG. 2 . In this case, the control unit  40  may determine, among the division display sub-frames dSFa that is a plurality of periods obtained by each display sub-frame SFa being divided, the division display sub-frame dSFa in which the ON control of the micro-mirror  21  is prohibited for each of the plurality of the groups (areas) of the plurality of the micro-mirrors  21 . 
     In an example illustrated in  FIG. 7 , each display sub-frame SFa is assumed to be divided into the two division display sub-frames dSFa that are two periods of the period X and the period Y. For example, in a given frame F, the control unit  40  allows the micro-mirrors  21  included in the first area  160 - 1  illustrated in  FIG. 7  to be ON controlled in the two division display sub-frames dSFa that are both of the period of the period X and the period Y in each display sub-frame SFa, in accordance with a video signal. In other words, the control unit  40  allows the micro-mirrors  21  included in the first area  160 - 1  illustrated in  FIG. 7  to be ON controlled, in all the division display sub-frames dSFa in accordance with a video signal (so as to include no division display sub-frame dSFa to be fixedly OFF controlled independent of a video signal). Moreover, in the same frame F, the control unit  40  allows the micro-mirrors  21  included in the second area  160 - 2  illustrated in  FIG. 7  to be ON controlled in only the division display sub-frame dSFa that is the period X in each display sub-frame SFa, in accordance with a video signal. In other words, the control unit  40  certainly OFF controls the micro-mirror  21  included in the second area  160 - 2  illustrated in  FIG. 7 , in the division display sub-frame dSFa that is the period Y, in each display sub-frame SFa, independent of a video signal. 
     With such controls, it is possible to change the brightness for each area in the displayed image  200 . For example, an area including a specific mark (image of a specific type) in the displayed image  200  is displayed brighter to allow an attention to be urged to the vehicle driver of the vehicle  100  using the mark. Moreover, the brightness may be capable of being arbitrarily changed by an operation of the vehicle driver in the displayed image  200  for each area, or the brightness may be arbitrarily changed in the displayed image  200  for each area on the basis of change in operation status of the vehicle input from the vehicle ECU. 
     With that, the control unit  40  determines any one of a plurality of areas as an area where the control is capable of being switched to the ON control in accordance with a video signal in all the division display sub-frames dFa, it is possible to prevent the division display sub-frame dSFa that does not contribute as an image because although the light source unit  30  is lit, any of the micro-mirrors  21  are fixed to an OFF state, from being generated. 
     Moreover, the way to divide areas that differ in the brightness, the arrangement, the size, and the like in the DMD  20  may be changed on the basis of the change in the operation status of the vehicle input from the vehicle ECU. Moreover, the way to divide areas that differ in the brightness, the arrangement, the size, and the like in the DMD  20  may be changed depending on elapsed time after the prescribed areas has been formed. Such controls allow appropriate differences in intensity of the brightness for each region in the displayed image  200 , so that it is possible to precisely notify the vehicle driver of necessary information in accordance with the operation status of the vehicle and the like. 
     Moreover, the control unit  40  ON controls or OFF controls the plurality of the micro-mirrors  21  in each non-display sub-frame SFb that is the sub-frame SF included in the above-mentioned non-display period Fb illustrated in  FIG. 4 . Preferably, the control unit  40  ON controls or OFF controls each micro-mirror  21  in the non-display period Fb such that the ratio of the time when each micro-mirror  21  becomes in the first inclined state and the time when in the second inclined state per one frame F or per the plurality of the frames F becomes a constant ratio. The ratio of the time when each micro-mirror  21  becomes in the first inclined state and the time when in the second inclined state per one frame F or per the plurality of the frames F becomes a constant ratio to prevent the micro-mirror  21  from being fixed to either one of the first inclined state and the second inclined state. 
     More preferably, the control unit  40  ON controls or OFF controls each micro-mirror  21  in the non-display period Fb such that the ratio of the time when each micro-mirror  21  becomes in the first inclined state and the time when in the second inclined state per one frame F or per the plurality of the frames F becomes 50:50. The ratio of the time when each micro-mirror  21  becomes in the first inclined state and the time when in the second inclined state per one frame F or per the plurality of the frames F becomes 50:50 to further prevent the micro-mirror  21  from being fixed to either one of the first inclined state and the second inclined state. 
     Moreover, the control unit  40  may further divide each of the plurality of the non-display sub-frames SFb that is the sub-frame SF included in the non-display period Fb in the sub-frame SF into divided non-display sub-frames dSFb that are a plurality of periods. In this case, the control unit  40  may ON control or OFF control each micro-mirror  21  for each non-display sub-frame dSFb such that the ratio of the time when each micro-mirror  21  becomes in the first inclined state and the time when in the second inclined state per one frame F or per the plurality of the frames F becomes a constant ratio. Each micro-mirror  21  is controlled for each divided non-display sub-frame dSFb in this manner to allow the ON control or the OFF control of each micro-mirror  21  in the non-display period Fb to be set more minutely, compared with a case where the non-display sub-frame SFb is not divided into a plurality of divided non-display sub-frames dSFb, for example. 
     Here, the ratio of the display period Fa and the non-display period Fb in each frame F is not necessarily the same. For example, the ratio of the display period Fa in each frame F is made large to allow the brightness of the entire displayed image  200  to be brighter. The control unit  40  determines the ratio of the display period Fa and the non-display period Fb in each frame F in accordance with external illuminance input from the external illuminance acquisition unit  60 . 
     For example,  FIG. 8A  illustrates a relation between the external illuminance and the ratio of the display period Fa and the non-display period Fb in each frame F. In the graph illustrated in  FIG. 8A , the horizontal axis represents external illuminance L that becomes larger toward the right side, and the longitudinal axis represents the ratio of the display period Fa and the non-display period Fb in each frame F in which the ratio of the display period Fa becomes large toward the upper side. For example, the relation between the external illuminance and the ratio of the display period Fa and the non-display period Fb in each frame F as illustrated in  FIG. 8A  is stored in the storage unit in the control unit  40 . 
     In an example illustrated in  FIG. 8A , the display period Fa:the non-display period Fb=50:50 is obtained when the external illuminance L is a first external illuminance L 1 , the display period Fa:the non-display period Fb=60:40 is obtained when the external illuminance L is a second external illuminance L 2 , and the display period Fa:the non-display period Fb=70:30 is obtained when the external illuminance L is a third external illuminance L 3 . Moreover, in the example illustrated in  FIG. 8A , the display period Fa:the non-display period Fb=70:30 is fixed even when the external illuminance L is larger than the third external illuminance L 3 . This is because the non-display period Fb is secured to implement the ratio of the time when each micro-mirror  21  becomes in the first inclined state and the time when in the second inclined state per one frame F being a constant ratio in all the frames F. 
     It should be noted that the control unit  40  may adjust the electric power to be supplied to the light source unit  30  per one frame F, thereby adjusting the brightness of the entire displayed image  200 . Specifically, the control unit  40  may change the current flowing through the light source unit  30 , thereby adjusting the electric power to be supplied per one frame F, and changing the brightness of the entire displayed image  200 . Alternatively, the control unit  40  may change a period (Duty ratio) when the current is flowed into the light source unit  30  in the sub-frame SF, thereby adjusting the electric power to be supplied per one frame F, and changing the brightness of the entire displayed image  200 . 
     In addition, the control unit  40  may combine the change in the current or the Duty ratio as mentioned above and the ratio of the display period Fa and the non-display period Fb in each frame F, thereby changing the brightness of the entire displayed image  200 . Specifically, for example, the control unit  40  may: when the external illuminance L falls in between the first external illuminance L 1  and the second external illuminance L 2 , keep the display period Fa:the non-display period Fb=70:30 and gradually change the current (or Duty ratio) of the light source unit  30  on the basis of the change in the external illuminance L; when the external illuminance L falls in between the second external illuminance L 2  and the third external illuminance L 3 , may keep the display period Fa:the non-display period Fb=50:50 and gradually change the current (or Duty ratio) of the light source unit  30  on the basis of the change in the external illuminance L; and when the external illuminance L is less than the third external illuminance L 3 , may keep the display period Fa:the non-display period Fb=50:50 and gradually change the current (or Duty ratio) of the light source unit  30  on the basis of the change in the external illuminance L. 
     It should be noted that it is considered that as for the micro-mirror  21 , the micro-mirror  21  is likely to be fixed to either one of the first inclined state and the second inclined state as the temperature of the micro-mirrors  21  becomes high. Therefore, the control unit  40  determines an upper limit of the ratio of the display period Fa in each frame F in accordance with the temperature (the temperature of the micro-mirrors  21 ) input from the temperature acquisition unit  70 . 
     For example,  FIG. 8B  illustrates a relation between the temperature and the allowable ratio of the display period Fa and the non-display period Fb in each frame F. In the graph illustrated in  FIG. 8B , the horizontal axis represents a temperature T in the surrounding or the interior of the projection-type display device  10  that becomes high toward the right side, and the longitudinal axis represents the allowable ratio of the display period Fa and the non-display period Fb in each frame F in which the ratio of the display period Fa becomes large toward the upper side. For example, the relation between the temperature T and the allowable ratio of the display period Fa and the non-display period Fb in each frame F as illustrated in  FIG. 8B  is stored in the storage unit in the control unit  40 . 
     In the example illustrated in  FIG. 8B , when the temperature T is lower than a first temperature T 1 , the display period Fa:the non-display period Fb=70:30 is allowed in each frame F, when the temperature T is equal to or higher than the first temperature T 1  and lower than a second temperature T 2 , the allowable ratio of the display period Fa in each frame F becomes smaller as the temperature T becomes higher than the first temperature T 1 , and when the temperature T is equal to or higher than the second temperature T 2 , the allowable ratio of the display period Fa in each frame F is fixed to 50. In other words, for example, even when the external illuminance L is the third external illuminance L 3 , the display period Fa:the non-display period Fb=50:50 is obtained. It should be noted that when the temperature T is equal to or higher than the third temperature T 3  that is higher than the second temperature T 2 , the ratio of the display period Fa may be set to 0 to stop the display of the displayed image  200 . 
     It should be noted that the ratio of the display period Fa and the non-display period Fb does not gradually change on the basis of the change in the temperature T as illustrated in  FIG. 8B , but may stepwisely change on the basis of the change in the temperature T. Specifically, for example, when the temperature T is lower than a first threshold, the display period Fa:the non-display period Fb=70:30 may be obtained, when the temperature T is equal to or higher than the first threshold and lower than a second threshold that is the temperature higher than the first threshold, the display period Fa:the non-display period Fb=60:40 may be obtained, and when the temperature T is equal to or higher than the second threshold, the display period Fa:the non-display period Fb=50:50 may be obtained. 
     The invention is not limited to the abovementioned exemplary embodiment, and persons skilled in the art could easily change the abovementioned exemplary embodiment in a range within the scope of the claims. 
     INDUSTRIAL APPLICABILITY 
     The invention is preferable as a projection-type display device of vehicle information and road information such as a turn-by-turn display that is mounted on an automobile or a motorcycle, or a moving object provided with an agricultural machine or a construction machine, for example, in terms of the mirror unit. 
     REFERENCE SIGNS LIST 
       10  . . . projection-type display device,  20  . . . reflection-type spatial optical modulation unit, DMD,  21  . . . movable reflection element, micro-mirror,  30  . . . light source unit,  31  . . . red LED,  32  . . . green LED,  33  . . . blue LED,  40  . . . control unit,  50  . . . light source unit drive circuit unit,  60  . . . external illuminance acquisition unit,  70  . . . temperature acquisition unit,  80  . . . housing,  84  . . . screen,  90  . . . light constituting image to be displayed,  100  . . . vehicle,  120  . . . vehicle ECU,  200  . . . displayed image,  300  . . . virtual image of displayed image.