Patent Publication Number: US-2013241971-A1

Title: Display device

Description:
TECHNICAL FIELD 
     The present invention relates to a display device, and in particular to, a display device which displays an image by scanning light emitted from a laser light source. 
     BACKGROUND ART 
     In the past, there have been proposed various types of head-up display devices for vehicles, which project a display image on a semitransparent plate such as a windshield of the vehicle or a combiner, and display a virtual image. Patent Literature 1 discloses an example of such a head-up display device. 
     A head-up display device  1  for the vehicles is installed in a dashboard of a vehicle. Display light L which the head-up display device  1  for the vehicles projects is reflected to the vehicle driver  3  by a windshield  2 , and the vehicle driver  3  can visually identify a virtual image V in being superimposed on a landscape (see  FIG. 1 ). 
     In such a head-up display device  1  for the vehicles, use of a semiconductor laser as a light source has been proposed. Patent Literature 2 discloses an example of such a head-up display device. 
     Such a head-up display device  1  for the vehicles is provided with a semiconductor laser, a scanning system, and a screen, and is configured to generate a display image by scanning a laser light emitted from the semiconductor laser on the screen with the scanning system. 
     However, a threshold current value of a light source changes due to outside air temperature changes and heat generated upon emission of light from a light source. When a threshold current value of a light source changes, current-light output characteristics of a light source change, and a problem that the brightness of an image to display is not stable occurs. 
     To solve the above problem, Patent Literature 3 discloses a technology as a means to keep a fixed output level of a light source, in which a photodetector is provided in an image display device, a light output of a light source is detected with a photodetector by changing a current value supplied to a light source at more than two points, a bias current supplied to a light source is set by computing current-light output characteristics (e.g., a threshold current value and a quantum efficiency) of a light source based on the detection result, and a pulse current corresponding to the current-light output characteristics of a light source are supplied to a light source. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: Japanese Published Unexamined Application No. 5-193400 
         Patent Literature 2: Japanese Published Unexamined Application No. 7-270711 
         Patent Literature 3: Japanese Published Unexamined Application No. 2009-244797 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     However, in the image display device of Patent Literature 3, a light output obtained when a laser driving current lower than a threshold current value is applied is ignored, a light output is detected by changing a laser driving current at more than two points over a threshold current value, and current-light output characteristics are computed. 
     On the other hand, in the head-up display device  1  for the vehicles, it is necessary to set a large dimming ratio by largely changing display brightness in accordance with outside light intensity. Several thousand to several ten thousand cd/m 2  or higher is required at a maximum, and several cd/m 2  is required at a minimum. To attain such a minimum brightness, it is necessary to adjust a laser driving current to lower than a threshold current value and to use also light that is not oscillated as a laser. Thus, it is necessary to drive a laser light source at a predetermined current value based on current-light output characteristics when a current value is lower than a threshold value. 
     Therefore, when the method of Patent Literature 3 is applied to the head-up display device  1  for the vehicles, a threshold current value cannot be exactly specified, current-light output characteristics when a current value is lower than a threshold value are unknown, display brightness becomes unstable particularly in a low brightness condition, and a problem that a desired display color cannot be obtained occurs. 
     Means for Solving the Problem 
     A display device according to the present invention comprising a laser light source to emit laser light, a scanning means to scan the laser light on a screen, an output control means to control an output of the laser light source, and a laser light detector to detect a light intensity of the laser light, wherein the output control means causes the laser light source to emit light for image forming based on image information when a position of scanning the laser light with the scanning means is within a display area, and causes the laser light source to emit light for detection when the scanning position is a predetermined position out of a display area, and the output control means changes a driving current of the laser light source at a first current value lower than a threshold current value and at second and third current values higher than or equal to a threshold current value when a position of scanning with the scanning means is out of a display area, and drives the laser light source with a driving current obtained by computing current-light output characteristics based on a light intensity detected with the laser light detector. 
     Further, the display device according to the present invention, the current-light output characteristics have a first straight line with a linearity connecting a zero point and a light output value detected at the first current value, and a second straight line with a linearity connecting light output values detected at the second current value and at the third current value, assumes an intersection point of the first straight line and the second straight line to be a bending point, and takes the bending point as a threshold current value. 
     Furthermore, the display device according to the present invention is provided with an outside light detector to detect outside light intensity. 
     Moreover, the display device according to the present invention, the output control means drives the laser light source at a driving current obtained from the results of computing the current-light output characteristics, so as to obtain a predetermined light output value based on the outside light intensity detected with the outside light detector. 
     Effect of the Invention 
     Stable light output can be obtained even in a low brightness condition, and display is possible in a desired display color. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an explanatory drawing of a virtual image according to an embodiment of the present invention. 
         FIG. 2  is a sectional view of a display device according to the above embodiment. 
         FIG. 3  is a conceptual diagram of a synthetic laser light generation means according to the above embodiment. 
         FIG. 4  is a graph showing current value-light output value characteristics of a semiconductor laser according to the above embodiment. 
         FIG. 5  is a graph showing temperature characteristics of current value-light output value characteristics of a semiconductor laser according to the above embodiment. 
         FIG. 6  is an explanatory diagram of electrical configuration according to the above embodiment. 
         FIG. 7  is a conceptual diagram showing current value-light output characteristics of red laser light according to the above embodiment. 
         FIG. 8  is a conceptual diagram showing current value-light output characteristics of green laser light according to the above embodiment. 
         FIG. 9  is a conceptual diagram showing current value-light output characteristics of blue laser light according to the above embodiment. 
         FIG. 10  is an explanatory diagram showing a position of a color sensor according to the above embodiment. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, an explanation will be given of an embodiment, in which the present invention is applied to a head-up display device  1 . 
     A head-up display device  1  is provided in a dashboard of a vehicle  2 , and is configured to reflect display light L showing a generated display image D on a windshield  3 , and permits a vehicle driver  4  to visually identify a virtual image V of a display image D showing vehicle information. A vehicle driver  4  can visually identify a virtual image V without turning eyes away from the front. 
     As shown in  FIG. 2 , a head-up display device  1  comprises a synthetic laser light generator (a laser light source)  10 , a MEMS scanner (a scanning means)  20 , a transmissive screen (a screen)  30 , a color sensor (a laser light detector)  40 , a reflecting unit  50 , a light sensor (an outside light detector)  60 , and a housing  70 . 
     The synthetic laser light generator  10  synthesizes three color laser light R, G, B, and emits one synthetic laser light C. The synthetic laser light generator  10  comprises laser diodes  11   r ,  11   g ,  11   b , condensing optical units  12   r ,  12   g ,  12   b , and dichroic mirrors  14  and  15  (see  FIG. 3 ). 
     The condensing optical units  12   r ,  12   g ,  12   b  condense the laser light R, G, B emitted from the diodes  11   r ,  11   g ,  11   b , respectively. The condensing optical unit  12   r  is disposed on the way of the optical path of the laser light R emitted from the laser diode  11   r , and converges the laser light R to form convergent light. The same occurs between the condensing optical unit  12   g  and the laser diode  11   g , and between the condensing optical unit  12   b  and laser diode  11   b.    
     The dichroic mirrors  14  and  15  are comprised of mirrors with a thin or multilayer dielectric film formed on the surface, and are configured to reflect or transmit the laser light R, G, B emitted from the laser diodes  11   r ,  11   g ,  11   b , and to synthesize the laser light R, G, B into one synthetic laser light C. 
     The dichroic mirror  14  is located in the advancing directions of the laser light R and B from the condensing optical unit  12   r  and the condensing optical unit  12   b , and is arranged at a certain angle with respect to the advancing direction of each laser light R and B, thereby transmitting the laser light B and reflecting the laser light R. In other words, the dichroic mirror  14  synthesizes the laser light R and laser light B. 
     The dichroic mirror  15  is located in the light advancing directions of the condensing optical unit  12   g  and the dichroic mirror  14 , and arranged at a certain angle with respect to each light advancing direction, thereby transmitting the synthesized laser light R and B and reflecting the laser light G. In other words, the dichroic mirror  15  further synthesizes the laser light R and B with the laser light G. 
     As described above, the laser light R, G, B are synthesized into one synthetic laser light C, and emitted from the synthetic laser light generator  10 . As each of the laser diodes  11   r ,  11   g  and  11   b  is arranged in being adjusted, a polarizing direction of each synthesized laser light R, G, B is identical, and is determined in consideration of the polarization-dependence of the reflectivity of the window shield  3 . The dichroic mirrors  14  and  15  synthesize also LED light, and the synthesized light is emitted from the synthetic laser light generator  10 . 
     The MEMS (MicroElectroMedical System) scanner  20  scans the synthetic laser light C emitted from the synthetic laser light generator  10 , and generates a display image D on the transmissive screen  30 . 
     The transmissive screen  30  is comprised of a diffusing plate, a holographic diffuser, and a microlens array, etc, and is configured to receive the synthetic laser light C scanned with the MEMS scanner  20  on the lower surface, and to display a display image D on the upper surface. The color sensor  40  is provided on the lower surface of the transmissive screen  30 . 
     The reflecting unit  50  is an optical system provided in the optical path between the transmissive screen  30  and the window shield  3 , so that a display image D displayed on the upper surface of the transmissive screen  30  is formed as a virtual image V at a desired position and in a desired size. The reflecting unit  50  comprises a plane mirror  51 , and a magnifying mirror  52 . 
     The plane mirror  51  is a plain total reflection mirror or the like, and is located at a position to receive display light L showing a display image displayed on the transmissive screen  30 , and reflects the display light L toward the magnifying mirror  52 . 
     The magnifying mirror  52  is a concave mirror or the like, and reflects the display light L reflected from the plane mirror  51  on its concave surface, and transmits the reflected light toward the window shield  3 . A virtual image V thereby formed has a size magnified from the display image D displayed on the transmissive screen  30 . 
     A housing  70  is made of hard resign or the like, and is box shaped having a window part  71  of predetermined size in the upper part. The housing  70  contains in place the synthetic laser light generator  10 , a MEMS scanner  20 , a color sensor  40 , a transmissive screen  30 , a reflecting unit  50 , and a light sensor  60 . The light sensor  60  is located under the window part  71 . 
     The window part  71  is made of translucent resin such as acrylic, curved, and fixed to an opening of the housing  70  by welding or the like. The window part  71  transmits the light reflected from the magnifying mirror  52 . The light sensor  60  is provided under the window part  71 . 
     Next, the laser diodes  11   r ,  11   g  and  11   b  of the synthetic laser light generator  10  will be described in detail based on  FIG. 4  and  FIG. 5 . The laser diode  11   r  emits red laser light R, the laser diode  11   g  emits green laser light G, and the laser diode  11   b  emits blue laser light B, respectively. The laser diodes  11   r ,  11   g  and  11   b  emit nonlaser light at a driving current lower than a threshold current value ITH, and emits laser light R, G, B at a driving current higher than or equal to a threshold current value ITH (see  FIG. 4 ). 
     The current-light output characteristics of the laser diodes  11   r ,  11   g  and  11   b  are different in each region of a current lower than and higher than or equal to a threshold current value ITH, but are linear in both regions. The reason why such a threshold current value ITH is required is that the energy to reverse electron distribution in an active layer and the amount of amplified light are required to be greater than the amount of loss. 
     The laser light R, G, B are oscillated by induced emission at a threshold current value ITH or higher, and nonlaser light is emitted by natural emission at a current lower than a threshold current value ITH. Thus, in the nonlaser light lower than the threshold current value ITH, a wavelength bandwidth is magnified by several nm compared with the laser light R, G, B. However, as a main wavelength and chromaticity of emission color rarely change near the threshold current value ITH, it is unnecessary to change a mixing ratio of the laser light R, G, B at the threshold current value ITH or higher to obtain a desired color even at lower than the threshold current value ITH. 
     Further, as shown in  FIG. 5 , in the current-light output characteristics of the laser diodes  11   r ,  11   g  and  11   b , the threshold current value ITH rises with a temperature rise, and over the threshold current value ITH, the characteristic deterioration by the drop of the efficiency comes to be seen. Thus, to attain a desired brightness without changing a color even when the temperature changes, it is necessary to appropriately change the current values of the laser diodes  11   r ,  11   g  and  11   b  to prevent a change of light output value. 
     The laser diodes  11   r ,  11   g  and  11   b  are arranged to have identical polarizing directions (electric field oscillation directions) of the laser light R, G, B emitted from these laser diodes and finally passed through or reflected by the dichroic mirror  15 . 
     Next, an electrical configuration of the head-up display device  1  is explained with reference to  FIG. 6 . 
     The color sensor  40  detects the light intensity of the laser light R, G, B, and outputs analog data of the light intensity to a microcomputer  81 . The light sensor  60  detects outside light intensity, and outputs analog data of the light intensity to the microcomputer  81 . 
     The microcomputer  81  controls various operations of the head-up display device  1 . The microcomputer  81  is supplied with image data for displaying a display image D as a video signal  10  by LVDS (LowVoltageDifferentialSignal) communication or the like. The microcomputer  81  previously stores position data of the color sensor  40  located at a predetermined position. 
     The microcomputer  81  receives a video signal  100 , generates control data to attain the intensity required by the video signal  100 , and drives the laser diodes  11   r ,  11   g ,  11   b  through an output control unit  82 , thereby emitting a synthetic laser light C from the synthetic laser light generator  10  to the MEMS scanner  20 . 
     The above-mentioned control data means data to adjust the light intensity of laser light R, G, B emitted from the laser diodes  11   r ,  11   g ,  11   b  to the intensity level required by the video signal  100 , based on digital data of a laser light intensity received from the color sensor  40 . 
     The microcomputer  81  drives the MEMS scanner  20  through a MEMS driver  90 . The microcomputer  81  drives the laser diodes  11   r ,  11   g ,  11   b  according to a predetermined timing based on the previously stored position data, and enters obtained laser light into the color sensor  40 . The output control unit  82  is supplied with control data from the microcomputer  81 , and controls the outputs of the laser diodes  11   r ,  11   g ,  11   b  based on the supplied control data. The control unit (output control means)  80  comprises the microcomputer  81 , and the output control unit  82 . 
     The operation of the head-up display device  1  configured as described above can be briefly explained. (1) Under the control of the control unit  80 , the synthetic laser light generator  10  emits a synthetic laser light C, and the MEMS scanner  20  scans the received synthetic laser light C toward the transmissive screen  30 , thereby creating a display image D. (2) Display light L showing a display image D displayed on the transmissive screen  30  is reflected by the reflecting unit  50 , and the reflected light is emitted toward the window shield  3 . (3) The display light L showing the display image D emitted from the head-up display device  1  is reflected by the window shield  3 , whereby a virtual image V of the display image D is formed in front of the window shield  3 . In this manner, the head-up display device  1  enables a driver  4  to visually identify the display image D as a virtual image V. 
     Then, based on  FIG. 7  to  FIG. 10 , an explanation is given of a method of detecting the laser light intensity and a method of controlling an output when displaying a display image D. 
       FIG. 10  is a diagram showing a screen  30  viewed from the scanner  20 . A display area E of the screen  30  is smaller than a scannable range of the MEMS scanner  20 . Because, the MEMS scanner  20  oscillates its reflection surface by resonance, and the reflection surface operation speed slows down or completely stops near a point of turning a scanning direction. Further, when the reflection surface operation speed of the MEMS scanner  20  extremely changes with respect to a dot clock, the display image D becomes distorted, and the resolution declines. 
     A display area E cannot be used when detecting the laser light intensity, the color sensor  40  is placed out of the display area E. In other words, as the color sensor  40  is placed in a nondisplay area out of the display area E that the MEMS scanner  20  can scan, laser intensity detection is possible without affecting a display image D. 
     The microcomputer  81  specifies a scanning position of laser light R, G, B according to a synchronizing signal of the MEMS scanner  20 , records the position information of the previously installed color sensor  40 , and drives the laser diodes  11   r ,  11   g ,  11   b  through the output control unit  82  at timing that a scanning position overlaps the position of the color sensor  40 . 
     When driving laser diodes  11   r ,  11   g ,  11   b  at the above timing for each frame, the microcomputer  81  changes a light output and laser diodes  11   r ,  11   g ,  11   b  to drive. For example, the microcomputer  81  drives the laser diode  11   r  at a current value IF 1  for a first frame, drives the laser diode  11   r  at a current value IF 2  for a second frame, and drives the laser diode  11   r  at a current value IF 3  for a third frame. At this time, the color sensor  40  detects a light output value PO 1 , a light output value PO 2 , and a light output value PO 3  corresponding to the current values, and outputs them to the microcomputer  81 . 
     The current values IF 1 , IF 2 , IF 3  are determined by average current-light output characteristics of the laser diode  11   r  to use. The current value IF 1  is lower than the threshold current value ITH 1 , the current value IF 2  is higher than or equal to the threshold current value ITH 1 , and the current value IF 3  is higher than or equal to the current value IF 2 . However, the current values IF 1 , IF 2 , IF 3  are required to be lower or higher than or equal to the threshold current value ITH 1 , even when the threshold current value ITH 1  changes due to individual difference or temperature characteristics of the laser diode  11   r . For example, the current value IF 2  is higher than or equal to the threshold current value ITH 1 , but is required to be kept at the threshold current value ITH 1  or higher even when the laser diode  11   r  rises to a high temperature and the threshold current value ITH 1  increases. To satisfy the above conditions, the current values IF 1 , IF 2 , IF 3  may be changed in accordance with the temperature of the laser diode  11   r.    
     The microcomputer  81  computes current-light output characteristics from three light output values PO 1 , PO 2 , PO 3  of the laser diodes supplied from the color sensor  40 . The computed current-light output characteristics are comprised of a first straight line M 1  and a second straight line M 2 , and can be expressed in a graph in which the threshold current value ITH 1  represents a bending point. The first straight line M 1  is a straight line connecting an origin point (0, 0) and (the current value IF 1 , and the light output value PO 1 ). 
     Assuming the current value to be x and the light output value to be y, the first straight line M 1  can be expressed by an equation 1. 
     
       
         
           
             
               
                 
                   
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     Compute the intersection point of the first straight line M 1  and the second straight line M 2 , that is the threshold current value ITH 1 , and regard the first straight line M 1  as current-light output characteristics of the laser diode  11   r , when the computed value is lower than the threshold current value ITH 1 , and regard the second straight line M 2  as current-light output characteristics of the laser diode  11   r , when the computed value is higher than or equal to the threshold current value ITH 1 . 
     Similar to the laser diode  11   r , the laser diode  11   g  is driven at current values IF 4  and IF 5  for frames  4  to  6 , and the laser diode  11   b  is driven at current values IF 7 , IF 8 , IF 9  for frames  7  to  9 . The color sensor  40  detects light output values PO 4 , PO 5 , PO 6 , PO 7 , PO 8 , PO 9  corresponding to the current values, and outputs them to the microcomputer  81 . The current-light output characteristics of the laser diodes  11   g  and  11   b  consist of first straight lines M 3  and M 5 , and second straight lines M 4  and M 6 , which are adjoined at the threshold current values ITH 2  and ITH 3 . 
     The microcomputer  81  drives the laser diodes  11   r ,  11   g ,  11   b  at the current values obtained from the computed results via the output control unit  82 , in the next frame that the computation is finished. In particular, when the outside light intensity detected with the light sensor  60  is applied to the microcomputer  81 , the microcomputer  81  sets a display brightness level suitable for the applied outside light intensity level, and determines a driving current value from the above current-light output characteristics to obtain the brightness level. The output control unit  82  drives the laser diodes  11   r ,  11   g ,  11   b  at the respective driving current values outputted from the microcomputer  81 . 
     A period to keep the computed current-light output characteristics is optional, and can be changed according to environments and purposes. After a defined period expires, the light intensity of each diode is detected again, and the computation cycle is repeated. 
     The head-up display device  1  according to the present invention periodically computes current-light output characteristics of laser diodes  11   r ,  11   g ,  11   b , and determines the driving current values of the laser diodes  11   r ,  11   g ,  11   b  based on the computed results. Thus, as current-light output characteristics corresponding to each temperature is referred to, even when the temperatures of the laser diodes  11   r ,  11   g ,  11   b  change, the brightness and color are rarely changed. Further, as threshold current values ITH 1 , ITH 2 , ITH 3  are specified, and current-light output characteristics when a current value is lower than a threshold value are also computed, the laser diodes  11   r ,  11   g ,  11   b  can be stably driven even in a low brightness condition, and a large dimming ratio can be set. 
     Though one laser light intensity is detected for one frame in the embodiment, it is permitted to detect three laser light intensities for each frame, by driving the laser diodes  11   r ,  11   g ,  11   b  at the current value IF 1 , at the current value IF 4 , and at the current value IF 7 , respectively, for a first frame, at the current value IF 2 , at the current value IF 5 , and at the current value  8 , respectively, for a second frame, and at the current value IF 3 , at the current value IF 6 , and at the current value IF 9 , respectively, for a third frame. 
     INDUSTRIAL APPLICABILITY 
     The present invention relates to a display device for vehicles, which is mounted on a movable body such as an automobile, and is applicable as a display device for projecting a display image on a vehicle windshield or the like, and displaying a virtual image. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
         
           
               1  . . . Head-up display device for vehicles 
               2  . . . Vehicle 
               3  . . . Windshield 
               4  . . . Vehicle driver 
               10  . . . Synthetic laser light generator (Laser light source) 
               11   r  . . . Laser diode 
               11   g  . . . Laser diode 
               11   b  . . . Laser diode 
               12   r  . . . Condensing optical unit 
               12   g  . . . Condensing optical unit 
               12   b  . . . Condensing optical unit 
               14  . . . Dichroic mirror 
               15  . . . Dichroic mirror 
               20  . . . MEMS scanner (Scanning means) 
               30  . . . Transmissive screen (Screen) 
               40  . . . Color sensor (Laser light detector) 
               50  . . . Reflecting unit 
               51  . . . Plane mirror 
               52  . . . Magnifying mirror 
               60  . . . Light sensor (Outside light detector) 
               70  . . . Housing 
               71  . . . Window part 
               80  . . . Control unit (Output control means) 
               81  . . . Microcomputer 
               90  . . . MEMS driver 
               100  . . . Video signal 
             V . . . Virtual image 
             L . . . Display light 
             R . . . Laser light 
             G . . . Laser light 
             B . . . Laser light 
             C . . . Synthetic laser light