Patent Publication Number: US-9405179-B2

Title: Projector

Description:
This is a Continuation application of application Ser. No. 13/306,218 filed Nov. 29, 2011. The disclosure of the prior application is hereby incorporate by reference herein in its entirety. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present invention relates to projectors. 
     2. Related Art 
     A projector using a discharge lamp such as a high-pressure mercury lamp or a metal halide lamp has been put to practical use. As such a projector, in JP-A-2003-102030, for example, a projector having a unit for varying the intensity of a light source in accordance with a color separation unit or the like in synchronization with a picture signal is disclosed. However, JP-A-2009-237302 describes that if the intensity of the light source is simply varied, an electrode of the discharge lamp will wear out significantly. 
     Moreover, in recent years, a projector that outputs a stereoscopic picture by using a discharge lamp such as a high-pressure mercury lamp or a metal halide lamp has been put to practical use. 
     One of the methods to output a stereoscopic picture is a method by which switching between a picture for the right eye and a picture for the left eye is performed and the picture for the right eye and the picture for the left eye are alternately output (for example, active shutter glasses technology such as “XPAND beyond cinema” (a trademark of X6D Limited)). In this method, the right eye is made to view the picture for the right eye and the left eye is made to view the picture for the left eye by using active shutter glasses etc. synchronized with a picture signal, whereby the picture is made to appear stereoscopically by using parallax between what the right and left eyes see. 
     When a stereoscopic picture is projected by the method by which a picture for the right eye and a picture for the left eye are alternately output, the amount of light that enters the right eye and the left eye is less than half of the amount of light that enters the right eye and the left eye when an existing two-dimensional picture is projected. Moreover, when crosstalk by which the picture for the right eye enters the left eye or the picture for the left eye enters the right eye occurs, the observer loses the sense of viewing the stereoscopic picture. Therefore, it is necessary to provide a period in which both active shutters are closed. As a result, when a stereoscopic picture is projected by the method by which a picture for the right eye and a picture for the left eye are alternately output, the picture appears darker than when an existing two-dimensional picture is projected. To make the picture appear bright, the drive power can be simply increased. However, this increases the power consumption of the projector or accelerates the deterioration of peripheral parts associated with the increase in drive power. 
     Moreover, if control to reduce the brightness of the discharge lamp in a period in which both active shutters are closed and increase the brightness of the discharge lamp in a period in which one of the active shutters is opened is performed to make the picture appear bright, the temperature of the electrode of the discharge lamp is reduced in a period in which the brightness of the discharge lamp is reduced, resulting in insufficient fusibility of the tip of the electrode. This may cause deformation of the electrode. When the electrode is deformed, there is a possibility of, for example, the occurrence of flicker. Therefore, special consideration is needed to prevent deformation of the electrode. 
     SUMMARY 
     An advantage of some aspects of the invention is to provide a projector that can project a picture in such a way that the picture appears bright while preventing deformation of an electrode. 
     An aspect of the invention is directed to a projector that outputs a first picture and a second picture alternately while performing switching between the first picture and the second picture with given switching timing, the projector including: a discharge lamp including a first electrode and a second electrode; a discharge lamp driving section that supplies, to the discharge lamp, a drive current that drives the discharge lamp; and a control section that controls the discharge lamp driving section, wherein a period sandwiched between a time at which switching is performed and the next time at which switching is performed starts with a first period and ends with a second period, the control section performs first control in a first control period which continues at least for a period sandwiched between a time at which switching is performed and the next time at which switching is performed and performs second control in a second control period which continues at least for a period sandwiched between a time at which switching is performed and the next time at which switching is performed, the second control period different from the first control period, in the first control and the second control, the control section controls the discharge lamp driving section so that the absolute value of the drive current becomes relatively small in the first period and becomes relatively large in the second period, in the first control, in the first period, the control section controls the discharge lamp driving section so that the discharge lamp driving section supplies an alternating current to the discharge lamp as the drive current, and, in the second period, the control section performs first control processing by which the control section controls the discharge lamp driving section so that the discharge lamp driving section supplies, to the discharge lamp, the drive current by which the energy provided to the first electrode in the second period becomes greater than the energy provided to the second electrode in the second period, and, in the second control, in the first period, the control section controls the discharge lamp driving section so that the discharge lamp driving section supplies an alternating current to the discharge lamp as the drive current, and, in the second period, the control section performs second control processing by which the control section controls the discharge lamp driving section so that the discharge lamp driving section supplies, to the discharge lamp, the drive current by which the energy provided to the second electrode in the second period becomes greater than the energy provided to the first electrode in the second period. 
     According to the aspect of the invention, since the energy provided to the first electrode becomes greater in the first control processing and the energy provided to the second electrode becomes greater in the second control processing, the fusibility of the electrode is increased. This makes it possible to prevent deformation of the electrode. 
     Moreover, according to the aspect of the invention, since the control section controls the discharge lamp driving section so that the absolute value of the drive current becomes relatively small in the first period and becomes relatively large in the second period, it is possible to realize a projector that can project a picture in such a way that the picture appears bright. 
     In the projector, the control section may perform the second control in a period which is at least part of a period sandwiched between the first control periods which are next to each other in terms of time. 
     As a result, it is possible to keep a heat load balance between the first electrode and the second electrode of the discharge lamp. This makes it possible to prevent only one of the electrodes of the discharge lamp from wearing out. 
     In the projector, the control section may perform third control in a third control period which continues at least for a period sandwiched between a time at which switching is performed and the next time at which switching is performed, the third control period different from the first control period and the second control period, and, in the third control, the control section may control the discharge lamp driving section so that the absolute value of the drive current becomes relatively small in the first period and becomes relatively large in the second period, the control section may control the discharge lamp driving section so that the discharge lamp driving section supplies, to the discharge lamp, the drive current by which the difference between the energy provided to the first electrode and the energy provided to the second electrode in the second period is smaller than the differences in the first control and the second control, and the control section may perform at least one of the first control and the second control in a period sandwiched between the third control periods which are next to each other in terms of time. 
     As a result, it is possible to prevent harmful effects such as blackening caused by too high fusibility of the electrode due to the first control and the second control. 
     The projector may include a state detecting section detecting a deteriorating state of the discharge lamp, and the control section may shorten the third control period with the progress of the deteriorating state. 
     The state detecting section may detect, as a value indicating the degree of the deteriorating state, for example, a drive voltage of the discharge lamp, a temporal change in the drive voltage of the discharge lamp, the amount of light of the discharge lamp, a temporal change in the amount of light of the discharge lamp, accumulated lighting time of the discharge lamp, or the like. 
     When the deteriorating state progresses, the fusibility of the electrode is reduced. Therefore, by shortening the third control period with the progress of the deteriorating state, it is possible to increase the fusibility of the electrode and prevent deformation of the electrode. 
     The projector may include a state detecting section detecting a deteriorating state of the discharge lamp, and the control section may lengthen the first control period and the second control period with the progress of the deteriorating state. When the deteriorating state progresses, the fusibility of the electrode is reduced. Therefore, by lengthening the first control period and the second control period with the progress of the deteriorating state, it is possible to increase the fusibility of the electrode and prevent deformation of the electrode. 
     In the projector, in the first control processing, the control section may control the discharge lamp driving section so that the discharge lamp driving section supplies, to the discharge lamp, a direct current by which the first electrode becomes a positive electrode as the drive current, and, in the second control processing, the control section may control the discharge lamp driving section so that the discharge lamp driving section supplies, to the discharge lamp, a direct current by which the second electrode becomes a positive electrode as the drive current. 
     As a result, it is possible to realize the first control processing and the second control processing with simple control. 
     In the projector, in the first control processing, the control section may control the discharge lamp driving section so that the discharge lamp driving section supplies, to the discharge lamp, an alternating current by which the time in which the first electrode is a positive electrode, the time in one period, is longer than the time in which the second electrode is a positive electrode as the drive current, and, in the second control processing, the control section may control the discharge lamp driving section so that the discharge lamp driving section supplies, to the discharge lamp, an alternating current by which the time in which the second electrode is a positive electrode, the time in one period, is longer than the time in which the first electrode is a positive electrode as the drive current. 
     As a result, it is possible to realize the first control processing and the second control processing with simple control. 
     In the projector, in the first control processing, the control section may control the discharge lamp driving section so as to include a period in which the discharge lamp driving section supplies an alternating current as the drive current and a period in which the discharge lamp driving section supplies, to the discharge lamp, a direct current by which the first electrode becomes a positive electrode as the drive current, and, in the second control processing, the control section may control the discharge lamp driving section so as to include a period in which the discharge lamp driving section supplies an alternating current as the drive current and a period in which the discharge lamp driving section supplies, to the discharge lamp, a direct current by which the second electrode becomes a positive electrode as the drive current. 
     As a result, it is possible to realize the first control processing and the second control processing with simple control. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIG. 1  is an explanatory diagram showing an optical system of a projector according to a first embodiment. 
         FIG. 2  is an explanatory diagram showing the configuration of a light source device. 
         FIG. 3  is a diagram showing an example of a circuit configuration of the projector according to the first embodiment. 
         FIG. 4  is a diagram showing an example of a circuit configuration of a discharge lamp lighting device. 
         FIG. 5  is a diagram for explaining another configuration example of a control section. 
         FIGS. 6A to 6D  are explanatory diagrams showing the relationship between the polarity of a drive current which is supplied to a discharge lamp and the temperature of an electrode. 
         FIG. 7  is a diagram for explaining a first period, a second period, and switching timing. 
         FIG. 8  is a diagram for explaining the relationship between switching timing and a first control period and a second control period. 
         FIG. 9A  is a timing chart showing a waveform example in first control, and  FIG. 9B  is a timing chart showing a waveform example in second control. 
         FIG. 10A  is a timing chart showing another waveform example in the first control, and  FIG. 10B  is a timing chart showing another waveform example in the second control. 
         FIG. 11A  is a timing chart showing still another waveform example in the first control, and  FIG. 11B  is a timing chart showing still another waveform example in the second control. 
         FIG. 12  is a diagram for explaining the relationship between switching timing and a first control period, a second control period, and a third control period. 
         FIG. 13  is a timing chart showing a waveform example in third control. 
         FIG. 14  is a flowchart showing a control example of a projector of a third embodiment. 
         FIG. 15  is a diagram showing an example of the correspondence between drive conditions. 
         FIG. 16  is a diagram showing an example of the correspondence between drive conditions. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, a preferred embodiment of the invention will be described in detail by using the drawings. It should be understood that the embodiment described below is not meant to limit unduly the scope of the invention claimed in the appended claims in any way, and all the configurations described below are not always necessary requirements of the invention. 
     1. Projector According to First Embodiment 
     1-1. Optical System of the Projector 
       FIG. 1  is an explanatory diagram showing an optical system of a projector  500  according to a first embodiment. The projector  500  has a light source device  200 , a parallelizing lens  305 , an illumination system  310 , a color separation system  320 , three liquid crystal light valves  330 R,  330 G, and  330 B, across dichroic prism  340 , and a projection system  350 . 
     The light source device  200  has a light source unit  210  and a discharge lamp lighting device  10 . The light source unit  210  has a main reflection mirror  112 , a sub-reflection mirror  50  (which will be described later), and a discharge lamp  90 . The discharge lamp lighting device  10  supplies power to the discharge lamp  90  and turns on the discharge lamp  90 . The main reflection mirror  112  reflects the light emitted from the discharge lamp  90  in a direction of radiation D. The direction of radiation D is parallel to an optical axis AX. The light from the light source unit  210  passes through the parallelizing lens  305  and enters the illumination system  310 . The parallelizing lens  305  parallelizes the light from the light source unit  210 . 
     The illumination system  310  makes the illuminance of the light from the light source device  200  uniform in the liquid crystal light valves  330 R,  330 G, and  330 B. Moreover, the illumination system  310  makes the light from the light source device  200  have one polarization direction to make effective use of the light from the light source device  200  in the liquid crystal light valves  330 R,  330 G, and  330 B. The light whose illuminance distribution and polarization direction have been adjusted enters the color separation system  320 . The color separation system  320  separates the incident light into three colored lights: a red (R) light, a green (G) light, and a blue (B) light. The three colored lights are modulated by the liquid crystal light valves  330 R,  330 G, and  330 B, each being related to a corresponding one of the three colors. The liquid crystal light valves  330 R,  330 G, and  330 B include liquid crystal panels  560 R,  560 G, and  560 E (which will be described later) and polarizers (not shown) which are disposed on light incident-sides and light-exiting sides of the liquid crystal panels  560 R,  560 G, and  560 B. The three modulated colored lights are combined by the cross dichroic prism  340 . The combined light enters the projection system  350 . The projection system  350  projects the incident light onto an unillustrated screen. As a result, an image is displayed on the screen. 
     Incidentally, as the configurations of the parallelizing lens  305 , the illumination system  310 , the color separation system  320 , the cross dichroic prism  340 , and the projection system  350 , various well-known configurations can be adopted. 
       FIG. 2  is an explanatory diagram showing the configuration of the light source device  200 . The light source device  200  has the light source unit  210  and the discharge lamp lighting device  10 . In the drawing, a sectional view of the light source unit  210  is shown. The light source unit  210  has the main reflection mirror  112 , the discharge lamp  90 , and the sub-reflection mirror  50 . 
     The discharge lamp  90  has a rod-like shape extending from a first end  90   e   1  to a second end  90   e   2  in the direction of radiation D. The material of the discharge lamp  90  is, for example, a light-transmissive material such as silica glass. The discharge lamp  90  has a spherical bulge in the center thereof. Inside the spherical bulge, a discharge space  91  is formed. In the discharge space  91 , gas which is a discharge medium containing mercury, a rare gas, a metal halide compound, and the like is encapsulated. 
     The discharge lamp  90  includes a first electrode  92  and a second electrode  93 . In an example shown in  FIG. 2 , the first electrode  92  and the second electrode  93  are provided so as to jut into the discharge space  91 . The first electrode  92  is disposed on the side of the discharge space  91  which is closer to the first end  90   e   1 , and the second electrode  93  is disposed on the side of the discharge space  91  which is closer to the second end  90   e   2 . The first electrode  92  and the second electrode  93  have a rod-like shape extending along the optical axis AX. In the discharge space  91 , the electrode tips (also referred to as the “discharge ends”) of the first electrode and the second electrode  93  face each other with a predetermined distance kept between them. Incidentally, the material of the first electrode  92  and the second electrode  93  is, for example, metal such as tungsten. 
     At the first end  90   e   1  of the discharge lamp  90 , a first terminal  536  is provided. The first terminal  536  and the first electrode  92  are electrically connected by a conductive member  534  passing through the discharge lamp  90 . Similarly, at the second end  90   e   2  of the discharge lamp  90 , a second terminal  546  is provided. The second terminal  546  and the second electrode  93  are electrically connected by a conductive member  544  passing through the discharge lamp  90 . The material of the first terminal  536  and the second terminal  546  is, for example, metal such as tungsten. Moreover, as the conductive members  534  and  544 , molybdenum foil, for example, is used. The first terminal  536  and the second terminal  546  are connected to the discharge lamp lighting device  10 . The discharge lamp lighting device  10  supplies, to the first terminal  536  and the second terminal  546 , a drive current that drives the discharge lamp  90 . As a result, arc discharge occurs between the first electrode  92  and the second electrode  93 . As indicated by dashed arrows, the light (the discharge light) generated by the arc discharge is emitted in all directions from the discharge position. 
     To the first end  90   e   1  of the discharge lamp  90 , the main reflection mirror  112  is fixed by a fixing member  114 . The shape of a reflecting surface (a surface on the side facing the discharge lamp  90 ) of the main reflection mirror  112  is spheroidal. The main reflection mirror  112  reflects the discharge light in the direction of radiation D. Incidentally, the shape of the reflecting surface of the main reflection mirror  112  is not limited to a spheroidal shape, and various shapes by which the discharge light is reflected in the direction of radiation D can be adopted. For example, a rotated parabola-like shape may be adopted. In this case, the main reflection mirror  112  can convert the discharge light into a light which is nearly parallel to the optical axis AX. Therefore, it is possible to omit the parallelizing lens  305 . On the side of the discharge lamp  90  which is closer to the second end  90   e   2 , the sub-reflection mirror  50  is fixed by a fixing member  522 . A reflecting surface (a surface on the side facing the discharge lamp  90 ) of the sub-reflection mirror  50  has a spherical shape surrounding the side of the discharge space  91  which is closer to the second end  90   e   2 . The sub-reflection mirror  50  reflects the discharge light toward the main reflection mirror  112 . This makes it possible to increase the usage efficiency of the light emitted from the discharge space  91 . 
     Incidentally, as the material of the fixing members  114  and  522 , any heat-resistant material (for example, an inorganic adhesive) that is resistant to heat generated by the discharge lamp  90  can be adopted. Moreover, the method for fixing the placement of the main reflection mirror  112 , the sub-reflection mirror  50 , and the discharge lamp  90  is not limited to the method by which the main reflection mirror  112  and the sub-reflection mirror  50  are fixed to the discharge lamp  90 , and any method can be adopted. For example, the discharge lamp  90  and the main reflection mirror  112  may be independently fixed to a housing (not shown) of the projector. The same goes for the sub-reflection mirror  50 . 
     1-2. Circuit Configuration of the Projector 
       FIG. 3  is a diagram showing an example of a circuit configuration of the projector according to the first embodiment. In addition to the optical systems described earlier, the projector  500  may include an image signal converting section  510 , a direct-current power supply device  80 , the discharge lamp lighting device  10 , the discharge lamp  90 , the liquid crystal panels  560 R,  560 G, and  560 B, an image processing apparatus  570 , and a CPU (central processing unit)  580 . Moreover, the projector can also be configured as a projector system  400  including the projector  500  and active shutter glasses  410 . 
     The image signal converting section  510  converts an image signal  502  (such as a luminance-color difference signal or an analog RGB signal) input from the outside into a digital RGB signal with a predetermined word length, generates image signals  512 R,  512 G, and  512 B, and supplies the signals to the image processing apparatus  570 . Moreover, when a picture signal in which a picture alternates between a first picture and a second picture with given switching timing is input as the image signal  502 , the image signal converting section  510  supplies a synchronizing signal  514  to the CPU  580  based on the switching timing with which switching between the first picture and the second picture is performed. 
     The image processing apparatus  570  performs image processing on the three image signals  512 R,  512 G, and  512 B, and supplies, to the liquid crystal panels  560 R,  560 G, and  560 B, driving signals  572 R,  572 G, and  572 B for driving the liquid crystal panels  560 R,  560 G, and  560 B. 
     The direct-current power supply device  80  converts an alternating voltage which is supplied from an external alternating-current power supply  600  into a constant direct voltage, and supplies the direct voltage to the image signal converting section  510  and the image processing apparatus  570  on the secondary side of a transformer (which is not shown, but is included in the direct-current power supply device  80 ) and the discharge lamp lighting device  10  on the primary side of the transformer. 
     The discharge lamp lighting device  10  forms a discharge path by generating a high voltage between the electrodes of the discharge lamp  90  at the time of startup and thereby producing a dielectric breakdown, and then supplies a drive current I by which the discharge lamp  90  maintains the discharge. 
     The liquid crystal panels  560 R,  560 G, and  560 B each modulate the intensity of the colored light which enters each liquid crystal panel via the optical systems described earlier based on the driving signals  572 R,  572 G, and  572 B. 
     The CPU  580  controls the operation of the projector after the projector is turned on until it is turned off. For example, the CPU  580  may output, to the discharge lamp lighting device  10 , an instruction to turn on the discharge lamp or an instruction to turnoff the discharge lamp via a communication signal  582 . Moreover, the CPU  580  may receive lighting information of the discharge lamp  90  from the discharge lamp lighting device  10  via a communication signal  584 . Furthermore, based on the synchronizing signal  514 , the CPU  580  may output, to the active shutter glasses  410 , a control signal  586  for controlling the active shutter glasses  410  in synchronization with the image signal  502  via a wired or wireless communication unit. 
     The active shutter glasses  410  may include a right shutter  412  and a left shutter  414 . The right shutter  412  and the left shutter  414  are controlled so as to be opened and closed based on the control signal  586 . When the user wears the active shutter glasses  410 , the right eye&#39;s-side field of view can be blocked as a result of the right shutter  412  being closed. Moreover, when the user wears the active shutter glasses  410 , the left eye&#39;s-side field of view can be blocked as a result of the left shutter  414  being closed. The right shutter  412  and the left shutter  414  may be formed as a liquid crystal shutter, for example. 
     1-3. Configuration of the Discharge Lamp Lighting Device 
       FIG. 4  is a diagram showing an example of a circuit configuration of the discharge lamp lighting device  10 . 
     The discharge lamp lighting device  10  includes a power control circuit  20 . The power control circuit  20  generates drive power which is supplied to the discharge lamp  90 . In the first embodiment, the power control circuit  20  is formed as a step-down chopper circuit that uses a direct-current power supply  80  as an input and outputs a direct current Id by stepping down the input voltage. 
     The power control circuit  20  can be configured so as to include a switch element  21 , a diode  22 , a coil  23 , and a capacitor  24 . The switch element  21  can be formed of a transistor, for example. In the first embodiment, one end of the switch element  21  is connected to a positive voltage side of the direct-current power supply  80 , and the other end is connected to a cathode terminal of the diode  22  and one end of the coil  23 . Moreover, to the other end of the coil  23 , one end of the capacitor  24  is connected, and the other end of the capacitor  24  is connected to an anode terminal of the diode  22  and a negative voltage side of the direct-current power supply  80 . A current control signal is input to a control terminal of the switch element  21  from a control section  40  (which will be described later), whereby on/off of the switch element  21  is controlled. As the current control signal, a PWM (pulse width modulation) control signal, for example, may be used. 
     Here, when the switch element  21  turns on, a current flows through the coil  23  and energy is accumulated in the coil  23 . Then, when the switch element  21  turns off, the energy accumulated in the coil  23  is released in a path passing through the capacitor  24  and the diode  22 . As a result, the direct current Id commensurate with the proportion of the amount of time the switch element  21  is on is generated. 
     The discharge lamp lighting device  10  includes a polarity reversal circuit  30 . The polarity reversal circuit  30  receives, as an input, the direct current Id output from the power control circuit  20  and reverses the polarity with given timing, and thereby generates and outputs the drive current I which is a direct current that continues only for a controlled time or an alternating current of any frequency. In the first embodiment, the polarity reversal circuit  30  is formed as an inverter bridge circuit (a full-bridge circuit). 
     The polarity reversal circuit  30  includes, for example, a first switch element  31 , a second switch element  32 , a third switch element  33 , and a fourth switch element  34  which are formed of a transistor or the like, and is formed by connecting, in parallel with each other, the first switch element  31  and the second switch element  32  which are connected in series and the third switch element  33  and the fourth switch element  34  which are connected in series. To the control terminals of the first switch element  31 , the second switch element  32 , the third switch element  33 , and the fourth switch element  34 , a polarity reversal control signal is input from the control section  40 , and on/off of the first switch element  31 , the second switch element  32 , the third switch element  33 , and the fourth switch element  34  is controlled based on the polarity reversal control signal. 
     The polarity reversal circuit  30  alternately reverses the polarity of the direct current Id which is output from the power control circuit  20  by alternately turning on/off the first switch element  31  and the fourth switch element  34  and the second switch element  32  and the third switch element  33 , and generates and outputs the drive current I which is a direct current that continues only for a controlled time or an alternating current of a controlled frequency from a common junction point of the first switch element  31  and the second switch element  32  and a common junction point of the third switch element  33  and the fourth switch element  34 . 
     That is, control is performed so that, when the first switch element  31  and the fourth switch element  34  are on, the second switch element  32  and the third switch element  33  are turned off, and, when the first switch element  31  and the fourth switch element  34  are off, the second switch element  32  and the third switch element  33  are turned on. Therefore, when the first switch element  31  and the fourth switch element  34  are on, the drive current I flowing from one end of the capacitor  24  to the fourth switch element  34  via the first switch element  31  and the discharge lamp  90  in this order is generated. Moreover, when the second switch element  32  and the third switch element  33  are on, the drive current I flowing from one end of the capacitor  24  to the second switch element  32  via the third switch element  33  and the discharge lamp  90  in this order is generated. 
     In the first embodiment, the power control circuit  20  and the polarity reversal circuit  30  collectively correspond to a discharge lamp driving section  230 . That is, the discharge lamp driving section  230  supplies, to the discharge lamp  90 , the drive current I that drives the discharge lamp  90 . 
     The discharge lamp lighting device  10  includes the control section  40 . The control section  40  controls the discharge lamp driving section  230 . In an example shown in  FIG. 4 , the control section  40  controls the power control circuit  20  and the polarity reversal circuit  30  and thereby controls the holding time for which the drive current I with the same polarity continues and the current value, frequency, etc. of the drive current I. The control section  40  performs polarity polarity reversal control on the polarity reversal circuit  30  so as to control the holding time for which the drive current I with the same polarity continues and the frequency etc. of the drive current I by polarity reversal timing of the drive current I. Moreover, the control section  40  performs current control on the power control circuit  20  so as to control the current value of the direct current Id which is output from the power control circuit  20 . 
     The configuration of the control section  40  is not limited to a particular configuration. In the first embodiment, the control section  40  includes a system controller  41 , a power control circuit controller  42 , and a polarity reversal circuit controller  43 . Incidentally, part or all of the control section  40  may be formed of a semiconductor integrated circuit. The system controller  41  controls the power control circuit controller  42  and the polarity reversal circuit controller  43  and thereby controls the power control circuit  20  and the polarity reversal circuit  30 . The system controller  41  may control the power control circuit controller  42  and the polarity reversal circuit controller  43  based on a drive voltage Vla which is detected by a voltage detecting section  60 , which will be described later, provided in the discharge lamp lighting device  10  and the drive current I. 
     In the first embodiment, the system controller  41  includes a storing section  44 . Incidentally, the storing section  44  may be provided independently of the system controller  41 . 
     The system controller  41  may control the power control circuit  20  and the polarity reversal circuit  30  based on the information stored in the storing section  44 . In the storing section  44 , for example, the information on drive parameters such as the holding time for which the drive current I with the same polarity continues and the current value, frequency, waveform, modulation pattern, etc. of the drive current I may be stored. The power control circuit controller  42  controls the power control circuit  20  by outputting the current control signal to the power control circuit  20  based on a control signal from the system controller  41 . 
     The polarity reversal circuit controller  43  controls the polarity reversal circuit  30  by outputting the polarity reversal control signal to the polarity reversal circuit  30  based on the control signal from the system controller  41 . Incidentally, the control section  40  can perform, by a dedicated circuit, the above-described control and various kinds of control of processing which will be described later, but the control section  40  can also perform various kinds of control of the processing by functioning as a computer as a result of a CPU (central processing unit), for example, executing a control program product stored in the storing section  44  or the like.  FIG. 5  is a diagram for explaining another configuration example of the control section  40 . As shown in  FIG. 5 , the control section  40  may be configured so as to function, by the control program product, as a current control unit  40 - 1  that controls the power control circuit  20  and a polarity reversal control unit  40 - 2  that controls the polarity reversal circuit  30 . 
     Moreover, in the example shown in  FIG. 4 , the control section  40  is configured as part of the discharge lamp lighting device  10 ; however, the control section  40  may also be configured so that the CPU  580  shoulders part of the function of the control section  40 . 
     The discharge lamp lighting device  10  may include an operation detecting section. The operation detecting section may include, for example, the voltage detecting section  60  that detects the drive voltage Vla of the discharge lamp  90  and outputs drive voltage information or a current detecting section that detects the drive current I and outputs drive current information. In this embodiment, the voltage detecting section  60  includes first and second resistances  61  and  62 . 
     The voltage detecting section  60  corresponds to a state detecting section in the invention. That is, the state detecting section (the voltage detecting section  60 ) detects the drive voltage Vla as a value indicating the degree of deterioration of the electrode. 
     In the first embodiment, the voltage detecting section  60  detects the drive voltage Vla based on a voltage obtained by voltage dividing performed by the first resistance  61  and the second resistance  62  which are connected in series and are connected in parallel with the discharge lamp  90 . Moreover, in the first embodiment, the current detecting section detects the drive current I based on a voltage which is generated in a third resistance  63  connected in series with the discharge lamp  90 . 
     The discharge lamp lighting device  10  may include an ignitor circuit  70 . The ignitor circuit  70  operates only when the discharge lamp  90  is turned on and supplies, between the electrodes of the discharge lamp  90  (between the first electrode  92  and the second electrode  93 ), a high voltage (a voltage which is higher than the voltage observed when the discharge lamp  90  is in a normal on state) necessary for forming a discharge path by producing a dielectric breakdown between the electrodes of the discharge lamp  90  (between the first electrode  92  and the second electrode  93 ) when the discharge lamp  90  is turned on. In the first embodiment, the ignitor circuit  70  is connected in parallel with the discharge lamp  90 . 
     1-4. Relationship Between the Polarity of the Drive Current and the Temperature of the Electrode 
       FIGS. 6A to 6D  are explanatory diagrams showing the relationship between the polarity of the drive current I which is supplied to the discharge lamp  90  and the temperature of the electrode.  FIGS. 6A and 6B  show the operation states of the first electrode  92  and the second electrode  93 . In the drawings, the tip portions of the first electrode  92  and the second electrode  93  are shown. At the tips of the first electrode  92  and the second electrode  93 , projections  552   p  and  562   p  are provided. The discharge which occurs between the first electrode  92  and the second electrode  93  mainly occurs between the projection  552   p  and the projection  562   p . In this embodiment, it is possible to prevent the movement of the discharge positions (the arc positions) in the first electrode  92  and the second electrode  93  as compared to when no projection is provided. However, such projections may be omitted. 
       FIG. 6A  shows a first polarity state P1 in which the first electrode  92  operates as a positive electrode and the second electrode  93  operates as a negative electrode. In the first polarity state P1, electrons move from the second electrode (the negative electrode) to the first electrode  92  (the positive electrode) by discharge. The electrons are released from the negative electrode (the second electrode  93 ). The electrons released from the negative electrode (the second electrode  93 ) collide with the tip of the positive electrode (the first electrode  92 ). Heat is generated by this collision, and the temperature of the tip (the projection  552   p ) of the positive electrode (the first electrode  92 ) rises. 
       FIG. 6B  shows a second polarity state P2 in which the first electrode  92  operates as a negative electrode and the second electrode  93  operates as a positive electrode. In the second polarity state P2, unlike the first polarity state P1, the electrons move from the first electrode  92  to the second electrode  93 . As a result, the temperature of the tip (the projection  562   p ) of the second electrode  93  rises. 
     As described above, the temperature of the positive electrode is more likely to increase than the negative electrode. Here, if a state in which the temperature of one electrode is higher than the temperature of the other electrode continues, various malfunctions can occur. For example, when the tip of the high-temperature electrode is excessively melted, an unintended electrode deformation can appear. As a result, the arc length may deviate from an appropriate value. Moreover, when the tip of the low-temperature electrode is melted inadequately, minutes projections and depressions which have appeared in the tip can remain without being melted. As a result, so-called arc jump (a phenomenon in which the arc position becomes unstable and moves) may occur. 
     As a technique to prevent such malfunctions, AC drive by which the polarity of each electrode is repeatedly changed can be used.  FIG. 6C  is a timing chart showing an example of the drive current I which is supplied to the discharge lamp  90  ( FIG. 2 ). The horizontal axis represents time T and the vertical axis represents the current value of the drive current I. The drive current I represents a current flowing through the discharge lamp  90 . The positive value represents the first polarity state P1 and the negative value represents the second polarity state P2. In an example shown in  FIG. 6C , a rectangular wave alternating current is used as the drive current I. In addition, in the example shown in  FIG. 6C , the first polarity state P1 and the second polarity state P2 are repeated alternately. Here, a first polarity interval Tp represents the time for which the first polarity state P1 continues, and a second polarity interval Tn represents the time for which the second polarity state P2 continues. Moreover, in the example shown in  FIG. 6C , the average current value of the first polarity interval Tp is Im 1 , and the average current value of the second polarity interval Tn is −Im 2 . Incidentally, the frequency of the drive current I appropriate for the driving of the discharge lamp  90  can be experimentally determined in accordance with the characteristics of the discharge lamp  90  (for example, a value in the 30 Hz-to-1 kHz range is adopted). The other values Im 1 , −Im 2 , Tp, and Tn can also be experimentally determined. 
       FIG. 6D  is a timing chart showing a change in the temperature of the first electrode  92 . The horizontal axis represents time T and the vertical axis represents the temperature H. In the first polarity state P1, the temperature H of the first electrode  92  rises; in the second polarity state P2, the temperature H of the first electrode  92  falls. Moreover, since the first polarity state P1 and the second polarity state P2 are repeated, the temperature H varies periodically between the minimum value Hmin and the maximum value Hmax. Though not shown in the drawing, the temperature of the second electrode  93  varies in a phase opposite to that of the temperature H of the first electrode  92 . That is, in the first polarity state P1, the temperature of the second electrode  93  falls; in the second polarity state P2, the temperature of the second electrode  93  rises. 
     In the first polarity state P1, the tip of the first electrode  92  (the projection  552   p ) is melted, whereby the tip of the first electrode  92  (the projection  552   p ) becomes smooth. This makes it possible to prevent the movement of the discharge position in the first electrode  92 . Moreover, since the temperature of the tip of the second electrode  93  (the projection  562   p ) falls, the second electrode  93  (the projection  562   p ) is prevented from being excessively melted. This makes it possible to prevent unintended electrode deformation. In the second polarity state P2, the situations of the first electrode  92  and the second electrode  93  are opposite to those described above. Therefore, by repeating the two states P1 and P2, it is possible to prevent the malfunctions in the first electrode  92  and the second electrode  93 . 
     Here, when the waveform of the drive current I is symmetrical, that is, when the waveform of the drive current I meets the conditions that “|Im 1 =|=|−Im 2 | and Tp=Tn”, the first electrode  92  and the second electrode  93  are the same in condition of the supplied power. Therefore, when the first electrode  92  and the second electrode  93  are the same in thermal condition (the temperature&#39;s readiness to rise or fall), it is estimated that the temperature difference between the first electrode  92  and the second electrode  93  becomes smaller. 
     Moreover, when a wide range of the electrode is excessively heated (when an arc spot (a hot spot on the surface of the electrode associated with arc discharge) becomes larger), the electrode loses its shape due to excessive melting. On the other hand, when the electrode is excessively cooled (when the arc spot becomes smaller), it becomes impossible to melt the tip of the electrode adequately, whereby the tip cannot be restored to a smooth state, that is, the tip of the electrode tends to be deformed. Therefore, when a state in which energy is uniformly supplied to the electrodes continues, the tips (the projection  552   p  and the projection  562   p ) of the electrodes tend to be deformed into unintended shapes. 
     1-5. Example of Control of the Drive Current 
     Next, a specific example of control of the drive current I in the projector  500  according to the first embodiment will be described. 
       FIG. 7  is a diagram for explaining a first period, a second period, and switching timing. In  FIG. 7 , from above, the contents of the driving signals  572 R,  572 G, and  572 B, the open and closed state of the right shutter  412 , the open and closed state of the left shutter  414 , and the temporal relationship between the first period and the second period and switching timing are shown. The horizontal axis of  FIG. 7  represents time. Hereinafter, an example in which the observer is made to view a display picture stereoscopically by using the first picture and the second picture as a picture for the right eye and a picture for the left eye, respectively, will be described. In an example shown in  FIG. 7 , the driving signals  572 R,  572 G, and  572 B are driving signals corresponding to the picture for the right eye as the first picture from time t1 to time t3, the picture for the left eye as the second picture from time t3 to time t5, the picture for the right eye as the first picture from time t5 to time t7, and the picture for the left eye as the second picture from time t7 to time t9. Therefore, in the example shown in  FIG. 7 , the projector  500  outputs the picture for the right eye as the first picture and the picture for the left eye as the second picture alternately while performing switching between them by using time t1, time t3, time t5, time t7, and time t9 as time at which switching is performed. 
     A period sandwiched between a time at which switching is performed and the next time at which switching is performed starts with the first period and ends with the second period. In the example shown in  FIG. 7 , for example, a period sandwiched between time t1 and time t3, at which switching is performed, starts with the first period from time t1 to time t2 and ends with the second period from time t2 to time t3. The same goes for a period sandwiched between time t3 and time t5, at which switching is performed, a period sandwiched between time t5 and time t7, at which switching is performed, and a period sandwiched between time t7 and time t9, at which switching is performed. Incidentally, in the example shown in  FIG. 7 , the first period and the second period have the same length; however, the length of the first period and the length of the second period can be appropriately set as needed. Moreover, a third period may exist between the first period and the second period. In the third period, control which is different from the control of the drive current I in the first period and the second period, which will be described later, may be performed. 
     The right shutter  412  is in an open state in a period which is at least part of the period in which the driving signals  572 R,  572 G, and  572 B corresponding to the picture for the right eye as the first picture are input to the liquid crystal panels  560 R,  560 G, and  560 B. In the example shown in  FIG. 7 , the right shutter  412  is in a closed state from time t1 to time t2 and is in an open state from time t2 to time t3. Moreover, in the example shown in  FIG. 7 , in the period in which the driving signals  572 R,  572 G, and  572 B corresponding to the picture for the left eye as the second picture are input to the liquid crystal panels  560 R,  560 G, and  560 B, the right shutter  412  starts to close from time t3, is completely closed between time t3 and time t4, and is in a closed state from time t4 to time t5. A change in the open and closed state of the right shutter  412  from time t5 to time t9 is the same as a change in the open and closed state from time t1 to time t5. 
     The left shutter  414  is in an open state in a period which is at least part of the period in which the driving signals  572 R,  572 G, and  572 B corresponding to the picture for the left eye as the second picture are input to the liquid crystal panels  560 R,  560 G, and  560 B. In the example shown in  FIG. 7 , the left shutter  414  is in a closed state from time t3 to time t4 and is in an open state from time t4 to time t5. Moreover, in the example shown in  FIG. 7 , in the period in which the driving signals  572 R,  572 G, and  572 B corresponding to the picture for the right eye as the first picture are input to the liquid crystal panels  560 R,  560 G, and  560 B, the left shutter  414  starts to close from time t1, is completely closed between time t1 and time t2, and is in a closed state from time t2 to time t3. A change in the open and closed state of the left shutter  414  from time t5 to time t9 is the same as a change in the open and closed state from time t1 to time t5. 
     In the example shown in  FIG. 7 , in the period in which the driving signals  572 R,  572 G, and  572 B corresponding to the picture for the right eye as the first picture are input to the liquid crystal panels  560 R,  560 G, and  560 B, a period in which the right shutter  412  is closed corresponds to the first period and a period in which the right shutter  412  is open corresponds to the second period. Moreover, in the example shown in  FIG. 7 , in the period in which the driving signals  572 R,  572 G, and  5723  corresponding to the picture for the left eye as the second picture are input to the liquid crystal panels  560 R,  560 G, and  560 B, a period in which the left shutter  414  is closed corresponds to the first period and a period in which the left shutter  414  is open corresponds to the second period. Furthermore, in the example shown in  FIG. 7 , in the first period, a period in which both the right shutter  412  and the left shutter  414  are closed exists. 
       FIG. 8  is a diagram for explaining the relationship between switching timing and a first control period and a second control period. The horizontal axis of  FIG. 8  represents time. Moreover, times t11 to t27 are times at which switching is performed. The first control period is a period in which the control section  40  performs first control, and the second control period is a period in which the control section  40  performs second control. 
     In the projector  500  according to the first embodiment, the control section  40  performs the first control in the first control period which continues at least for a period sandwiched between a time at which switching is performed and the next time at which switching is performed, and performs the second control in the second control period which continues at least for a period sandwiched between a time at which switching is performed and the next time at which switching is performed, the second control period different from the first control period. In an example shown in  FIG. 8 , the first control and the second control continue for a period which is four times longer than the period sandwiched between a time at which switching is performed and the next time at which switching is performed. For example, in  FIG. 8 , an initial first control period continues for a period from time t11 which is a time at which switching is performed to time t15 which is a time at which switching is performed. Moreover, in  FIG. 8 , an initial second control period continues for a period from time t15 which is a time at which switching is performed to time t19 which is a time at which switching is performed. Incidentally, the length of time for which the first control continues and the length of time for which the second control continues can be appropriately set in accordance with the specifications of the discharge lamp  90 . 
       FIG. 9A  is a timing chart showing a waveform example in the first control, and  FIG. 9B  is a timing chart showing a waveform example in the second control. The horizontal axes of  FIGS. 9A and 9B  represent time and the vertical axes represent the current value of the drive current I. Moreover, in  FIGS. 9A and 9B , the drive current I when the first electrode  92  is a positive electrode is shown as a positive value, and the drive current I when the second electrode  93  is a positive electrode is shown as a negative value. Furthermore, in the following description, the polarity of the drive current I when the first electrode  92  is a positive electrode is expressed as a first polarity, and the polarity of the drive current I when the second electrode  93  is a positive electrode is expressed as a second polarity. 
     In the projector  500  according to the first embodiment, in the first control and the second control, the control section  40  controls the discharge lamp driving section  230  so that the absolute value of the drive current I in the first period becomes relatively small as compared to that in the second period and the absolute value of the drive current I in the second period becomes relatively large as compared to that in the first period. 
     In an example shown in  FIG. 9A , the absolute value of the current value of the drive current I is I 1  in the first period and I 2  in the second period. Moreover, in the example shown in  FIG. 9A , I 1 &lt;I 2 . Therefore, in the first control, the absolute value of the drive current I is relatively small in the first period and relatively large in the second period. 
     Similarly, in an example shown in  FIG. 9B , the absolute value of the current value of the drive current I is I 1  in the first period and I 2  in the second period. Moreover, in the example shown in  FIG. 9B , I 1 &lt;I 2 . Therefore, in the second control, the absolute value of the drive current I is relatively small in the first period and relatively large in the second period. In the examples shown in  FIGS. 9A and 9B , the absolute value of the drive current I in the first period and the absolute value of the drive current I in the second period are constant in each period; however, the invention is not limited thereto. For example, when the absolute value of the drive current I in the first period and the absolute value of the drive current I in the second period vary in each period, the control section  40  may control the discharge lamp driving section  230  so that the average value of the absolute values of the drive current I in each period becomes relatively small in the first period and relatively large in the second period. Moreover, for example, when the absolute value of the drive current I in the first period and the absolute value of the drive current I in the second period vary in each period, the control section  40  may control the discharge lamp driving section  230  so as to take the minimum value of the absolute value of the drive current I, in the first period and take the maximum value of the absolute value of the drive current I in the second period. In the projector  500  according to the first embodiment, in the first control, in the first period, the control section  40  controls the discharge lamp driving section  230  so that the discharge lamp driving section  230  supplies an alternating current to the discharge lamp  90  as the drive current I. The alternating current is a current with alternating first polarity and second polarity. In the example shown in  FIG. 9A , the control section  40  controls the discharge lamp driving section  230  so that the discharge lamp driving section  230  generates an alternating current corresponding to one period by reversing the polarity with the absolute value of the current value of the drive current I kept constant in the first period and supplies the alternating current to the discharge lamp  90  as the drive current I. Moreover, in the example shown in  FIG. 9A , an alternating current of four periods is supplied to the discharge lamp  90  as the drive current I in one first period. The frequency of the alternating current can be experimentally determined in accordance with the characteristics of the discharge lamp  90 . For example, the frequency of the alternating current may be selected in the 30 Hz-to-1 kHz range. In the projector  500  according to the first embodiment, in the first control, in the second period, the control section  40  performs first control processing by which the control section  40  controls the discharge lamp driving section  230  so that the discharge lamp driving section  230  supplies, to the discharge lamp  90 , the drive current I by which the energy provided to the first electrode  92  in the second period becomes greater than the energy provided to the second electrode  93  in the second period. 
     In the example shown in  FIG. 9A , in the first control processing, the control section  40  controls the discharge lamp driving section  230  so that the discharge lamp driving section  230  supplies, to the discharge lamp  90 , a direct current by which the first electrode becomes a positive electrode as the drive current I. The direct current by which the first electrode becomes a positive electrode is a current that starts with the first polarity and is formed of a current with the first polarity. In the example shown in  FIG. 9A , the control section  40  controls the discharge lamp driving section  230  so that the discharge lamp driving section  230  supplies, to the discharge lamp  90 , a direct current which is a current obtained by making the absolute value of the current value of the drive current I constant in the second period as the drive current I. This makes it possible to realize the first control processing with simple control. 
     In the projector  500  according to the first embodiment, in the second control, in the first period, the control section  40  controls the discharge lamp driving section  230  so that the discharge lamp driving section  230  supplies, to the discharge lamp  90 , an alternating current as the drive current I. In the example shown in  FIG. 9B , the control section  40  controls the discharge lamp driving section  230  so that the discharge lamp driving section  230  generates an alternating current corresponding to one period by reversing the polarity with the absolute value of the current value of the drive current I kept constant in the first period and supplies the alternating current to the discharge lamp  90  as the drive current I. Moreover, in the example shown in  FIG. 9B , an alternating current of four periods is supplied to the discharge lamp  90  as the drive current I in one first period. The frequency of the alternating current can be experimentally determined in accordance with the characteristics of the discharge lamp  90 . For example, the frequency of the alternating current may be selected in the 30 Hz-to-1 kHz range. 
     In the projector  500  according to the first embodiment, in the second period, the control section  40  performs second control processing by which the control section  40  controls the discharge lamp driving section  230  so that the discharge lamp driving section  230  supplies, to the discharge lamp  90 , the drive current I by which the energy provided to the second electrode  93  in the second period becomes greater than the energy provided to the first electrode  92  in the second period. In the example, shown in  FIG. 93 , in the second control processing, the control section  40  controls the discharge lamp driving section  230  so that the discharge lamp driving section  230  supplies, to the discharge lamp  90 , a direct current by which the second electrode becomes a positive electrode as the drive current I. The direct current by which the second electrode becomes a positive electrode is a current that starts with the second polarity and is formed of a current with the second polarity. In the example shown in  FIG. 9B , the control section  40  controls the discharge lamp driving section  230  so that the discharge lamp driving section  230  supplies, to the discharge lamp  90 , a direct current which is a current obtained by making the absolute value of the current value of the drive current I constant in the second period as the drive current I. This makes it possible to realize the second control processing with simple control. 
     With the projector  500  according to the first embodiment, since the energy provided to the first electrode  92  becomes greater in the first control processing, the fusibility of the first electrode  92  is increased. Therefore, it is possible to prevent deformation of the first electrode  92 . Moreover, with the projector  500  according to the first embodiment, since the energy provided to the second electrode  93  becomes greater in the second control processing, the fusibility of the second electrode  93  is increased. Therefore, it is possible to prevent deformation of the second electrode  93 . As a result, by performing the first control processing and the second control processing, it is possible to prevent deformation of the first electrode  92  and the second electrode  93 . 
     Moreover, with the projector  500  according to the first embodiment, the control section  40  controls the discharge lamp driving section  230  so that the absolute value of the drive current I becomes minimum in the first period and becomes maximum in the second period. As a result, when the projector  500  is driven with the average drive power throughout the first period and the second period kept constant, the projector  500  can project a darker picture than that projected when the projector  500  is driven by the average drive power in the first period and can project a brighter picture than that projected when the projector  500  is driven by the average drive power in the second period. In the first period, since there is a period in which both the right shutter  412  and the left shutter  414  are closed, even when the projected picture is dark, the picture quality is less likely to be affected. On the other hand, in the second period, any one of the right shutter  412  and the left shutter  414  is in an open state, which makes it possible to make the picture projected so that it is viewed by the user appear brighter than when the projector  500  is driven by the average drive power. In this way, it is possible to realize a projector that can project a picture in such a way that the picture appears bright. Moreover, by projecting a picture in such a way that the picture appears dark in the first period, it is possible to prevent the occurrence of crosstalk. Furthermore, since the need for increasing the average drive power to make the picture appear bright is lessened, it is possible to reduce the power consumption of the projector. This makes it possible to reduce deterioration of peripheral parts associated with the increase in average drive power. 
     In the projector  500  according to the first embodiment, the control section  40  may perform second control in a period which is at least part of the period sandwiched between the first control periods which are next to each other in terms of time. In the example shown in  FIG. 8 , the first control period from time t11 to time t15 and the first control period from time t19 to time t23 are next to each other in terms of time. A period from time t15 to time t19, the period sandwiched between the two first control periods, is the second control period. That is, in the example shown in  FIG. 8 , the entire period sandwiched between the first control periods which are next to each other in terms of time is the second control period. Therefore, in the example shown in  FIG. 8 , the first control period and the second control period are repeated alternately. By performing the second control in a period which is at least part of a period sandwiched between the first control periods which are next to each other in terms of time, it is possible to keep a heat load balance between the first electrode  92  and the second electrode  93  of the discharge lamp  90 . This makes it possible to prevent only one of the electrodes of the discharge lamp from wearing out. 
     1-6. Modified Example 1 
       FIG. 10A  is a timing chart showing another waveform example in the first control, and  FIG. 10B  is a timing chart showing another waveform example in the second control. The horizontal axes of  FIGS. 10A and 10B  represent time and the vertical axes represent the current value of the drive current I. Moreover, in  FIGS. 10A and 10B , the drive current I when the first electrode  92  is a positive electrode is shown as a positive value, and the drive current I when the second electrode  93  is a positive electrode is shown as a negative value. 
     The waveform examples shown in  FIGS. 9A and 9B  differ from the waveform examples shown in  FIGS. 10A and 10B  in the waveform in the second period. That is, the first embodiment differs from Modified Example 1 in the contents of the first control processing and the second control processing, and the first embodiment and Modified Example 1 are the same in other configurations. Therefore, in the following description, only the contents of the first control processing and the second control processing will be described. 
     In an example shown in  FIG. 10A , in the first control processing (the first control in the second period), the control section  40  controls the discharge lamp driving section  230  so that the discharge lamp driving section  230  supplies, to the discharge lamp  90 , an alternating current by which the time in which the first electrode  92  is a positive electrode, the time in one period, is longer than the time in which the second electrode  93  is a positive electrode as the drive current I. In the example shown in  FIG. 10A , the control section  40  controls the discharge lamp driving section  230  so that the discharge lamp driving section  230  generates an alternating current corresponding to one period by reversing the polarity with the absolute value of the current value of the drive current I kept constant in the second period and supplies the alternating current to the discharge lamp  90  as the drive current I. Moreover, the control section  40  controls the discharge lamp driving section  230  so that the discharge lamp driving section  230  reverses the polarity of the drive current I with such timing that the time for which the first polarity continues is longer than the time for which the second polarity continues. This makes it possible to realize the first control processing with simple control. 
     Moreover, in an example shown in  FIG. 10B , in the second control processing (the second control in the second period), the control section  40  controls the discharge lamp driving section  230  so that the discharge lamp driving section  230  supplies, to the discharge lamp  90 , an alternating current by which the time in which the second electrode  93  is a positive electrode, the time in one period, is longer than the time in which the first electrode  92  is a positive electrode as the drive current I. In the example shown in  FIG. 10B , the control section  40  controls the discharge lamp driving section  230  so that the discharge lamp driving section  230  generates an alternating current corresponding to one period by reversing the polarity with the absolute value of the current value of the drive current I kept constant in the second period and supplies the alternating current to the discharge lamp  90  as the drive current I. Moreover, the control section  40  controls the discharge lamp driving section  230  so that the discharge lamp driving section  230  reverses the polarity of the drive current I with such timing that the time for which the second polarity continues is longer than the time for which the first polarity continues. This makes it possible to realize the second control processing with simple control. 
     According to Modified Example 1, since the energy provided to the first electrode  92  becomes greater in the first control processing, the fusibility of the first electrode  92  is increased. Therefore, it is possible to prevent deformation of the first electrode  92 . Moreover, according to Modified Example 1, since the energy provided to the second electrode becomes greater in the second control processing, the fusibility of the second electrode  93  is increased. Therefore, it is possible to prevent deformation of the second electrode  93 . As a result, by performing the first control processing and the second control processing, it is possible to prevent deformation of the first electrode  92  and the second electrode  93 . 
     1-7. Modified Example 2 
       FIG. 1A  is a timing chart showing still another waveform example in the first control, and  FIG. 11B  is a timing chart showing still another waveform example in the second control. The horizontal axes of  FIGS. 11A and 11B  represent time and the vertical axes represent the current value of the drive current I. Moreover, in  FIGS. 11A and 11B , the drive current I when the first electrode  92  is a positive electrode is shown as a positive value, and the drive current I when the second electrode  93  is a positive electrode is shown as a negative value. 
     The waveform examples shown in  FIGS. 9A and 9B  differ from the waveform examples shown in  FIGS. 11A and 11B  in the waveform in the second period. That is, the first embodiment differs from Modified Example 2 in the contents of the first control processing and the second control processing, and the first embodiment and Modified Example 2 are the same in other configurations. Therefore, in the following description, only the contents of the first control processing and the second control processing will be described. 
     In an example shown in  FIG. 11A , in the first control processing (the first control in the second period), the control section  40  controls the discharge lamp driving section  230  so as to include a period in which the discharge lamp driving section  230  supplies an alternating current as the drive current I and a period in which the discharge lamp driving section  230  supplies, to the discharge lamp  90 , a direct current by which the first electrode  92  becomes a positive electrode as the drive current I. In the example shown in  FIG. 11A , in the period in which the discharge lamp driving section  230  supplies an alternating current, the control section  40  controls the discharge lamp driving section  230  so that the discharge lamp driving section  230  generates an alternating current corresponding to one period by reversing the polarity with the absolute value of the current value of the drive current I kept constant in the second period and supplies the alternating current to the discharge lamp  90  as the drive current I. Moreover, in the example shown in  FIG. 11A , in the period in which the discharge lamp driving section  230  supplies a direct current, the control section  40  controls the discharge lamp driving section  230  so that the discharge lamp driving section  230  supplies, to the discharge lamp  90 , a direct current which is a current obtained by making the absolute value of the current value of the drive current I constant in the second period as the drive current I. This makes it possible to realize the first control processing with simple control. 
     In an example shown in  FIG. 11B , in the second control processing (the first control in the second period), the control section  40  controls the discharge lamp driving section  230  so as to include a period in which the discharge lamp driving section  230  supplies an alternating current as the drive current I and a period in which the discharge lamp driving section  230  supplies, to the discharge lamp  90 , a direct current by which the second electrode  93  becomes a positive electrode as the drive current I. In the example shown in  FIG. 11B , in the period in which the discharge lamp driving section  230  supplies an alternating current, the control section  40  controls the discharge lamp driving section  230  so that the discharge lamp driving section  230  generates an alternating current corresponding to one period by reversing the polarity with the absolute value of the current value of the drive current I kept constant in the second period and supplies the alternating current to the discharge lamp  90  as the drive current I. Moreover, in the example shown in  FIG. 11B , in the period in which the discharge lamp driving section  230  supplies a direct current, the control section  40  controls the discharge lamp driving section  230  so that the discharge lamp driving section  230  supplies, to the discharge lamp  90 , a direct current which is a current obtained by making the absolute value of the current value of the drive current I constant in the second period as the drive current I. This makes it possible to realize the second control processing with simple control. Incidentally, in the examples shown in  FIGS. 11A and 11B , the control by which an alternating current is supplied at the start of the second period and a direct current is supplied at the end of the second period is performed; however, an alternating current and a direct current may be supplied in any order. According to Modified Example 2, since the energy provided to the first electrode  92  becomes greater in the first control processing, the fusibility of the first electrode  92  increased. Therefore, it is possible to prevent deformation of the first electrode  92 . Moreover, according to Modified Example 2, since the energy provided to the second electrode becomes greater in the second control processing, the fusibility of the second electrode  93  is increased. Therefore, it is possible to prevent deformation of the second electrode  93 . As a result, by performing the first control processing and the second control processing, it is possible to prevent deformation of the first electrode  92  and the second electrode  93 . 
     2. Projector According to Second Embodiment 
     Next, a projector  500  according to a second embodiment will be described. The configurations of the optical systems and the circuits of the projector  500  according to the second embodiment are the same as those of the projector  500  according to the first embodiment. Therefore, in the following description, a specific example of control of the drive current I in the projector  500  according to the second embodiment will be described. Incidentally, the contents of the driving signals  572 R,  572 G, and  572 B, the open and closed state of the right shutter  412 , the open and closed state of the left shutter  414 , and the temporal relationship between the first period and the second period and switching timing have already been described by using  FIG. 7 . 
       FIG. 12  is a diagram for explaining the relationship between switching timing and a first control period, a second control period, and a third control period. The horizontal axis of  FIG. 12  represents time. Moreover, times t11 to t27 are times at which switching is performed. The first control period is a period in which the control section  40  performs first control, the second control period is a period in which the control section  40  performs second control, and the third control period is a period in which the control section  40  performs third control. Incidentally, the contents of the first control and the second control are the same as those of the first embodiment, and therefore a detailed description thereof is omitted. 
     In the projector  500  according to the second embodiment, the control section  40  performs the third control in a third control period which continues at least for a period sandwiched between a time at which switching is performed and the next time at which switching is performed, the third control period different from the first control period and the second control period. In an example shown in  FIG. 12 , the first control, the second control, and the third control continue for a period which is twice as long as the period sandwiched between a time at which switching is performed and the next time at which switching is performed. For example, in  FIG. 12 , an initial first control period continues for a period from time t13 which is a time at which switching is performed to time t15 which is a time at which switching is performed. Moreover, in  FIG. 12 , an initial second control period continues for a period from time t17 which is a time at which switching is performed to time t19 which is a time at which switching is performed. Furthermore, in  FIG. 12 , an initial third control period continues for a period from time t11 which is a time at which switching is performed to time t13 which is a time at which switching is performed. Incidentally, the length of time for which the first control continues, the length of time for which the second control continues, and the length of time for which the third control continues can be appropriately set in accordance with the specifications of the discharge lamp  90 .  FIG. 13  is a timing chart showing a waveform example in the third control. The horizontal axis of  FIG. 13  represents time and the vertical axis represents the current value of the drive current I. Moreover, in  FIG. 13 , the drive current I when the first electrode  92  is a positive electrode is shown as a positive value, and the drive current I when the second electrode  93  is a positive electrode is shown as a negative value. 
     In the projector  500  according to the second embodiment, in the third control, the control section  40  controls the discharge lamp driving section  230  so that the absolute value of the drive current I in the first period becomes relatively small as compared to that in the second period and the absolute value of the drive current I in the second period becomes relatively large as compared to that in the first period. 
     In an example shown in  FIG. 13 , the absolute value of the current value of the drive current I is I 1  in the first period and I 2  in the second period. Moreover, in the example shown in  FIG. 13 , I 1 &lt;I 2 . Therefore, in the third control, the absolute value of the drive current I is relatively small in the first period and relatively large in the second period. 
     Incidentally, in the example shown in  FIG. 13 , the absolute value of the drive current I in the first period and the absolute value of the drive current I in the second period are constant in each period; however, the invention is not limited thereto. For example, when the absolute value of the drive current I in the first period and the absolute value of the drive current I in the second period vary in each period, the control section  40  may control the discharge lamp driving section  230  so that the average value of the absolute value of the drive current I in each period becomes relatively small in the first period and relatively large in the second period. Moreover, for example, when the absolute value of the drive current I in the first period and the absolute value of the drive current I in the second period vary in each period, the control section  40  may control the discharge lamp driving section  230  so as to take the minimum value of the absolute value of the drive current I in the first period and take the maximum value of the absolute value of the drive current I in the second period. In the projector  500  according to the second embodiment, in the third control, in the first period, the control section  40  may control the discharge lamp driving section  230  so that the discharge lamp driving section  230  supplies an alternating current to the discharge lamp  90  as the drive current I. In the example shown in  FIG. 13 , the control section  40  controls the discharge lamp driving section  230  so that the discharge lamp driving section  230  generates an alternating current corresponding to one period by reversing the polarity with the absolute value of the current value of the drive current I kept constant in the first period and supplies the alternating current to the discharge lamp  90  as the drive current I. Moreover, in the example shown in  FIG. 13 , an alternating current of four periods is supplied to the discharge lamp  90  as the drive current I in one first period. The frequency of the alternating current can be experimentally determined in accordance with the characteristics of the discharge lamp  90 . For example, the frequency of the alternating current may be selected in the 30 Hz-to-1 kHz range. 
     Incidentally, in the projector  500  according to the second embodiment, in the third control, in the first period, the control section  40  may control the discharge lamp driving section  230  so that the discharge lamp driving section  230  supplies a direct current to the discharge lamp  90  as the drive current I. In this case, the control section  40  may control the discharge lamp driving section  230  so that, for example, the discharge lamp driving section  230  supplies, to the discharge lamp  90 , a direct current as the drive current I, the direct current having reverse polarity in two first periods which sandwich one second period in terms of time. This makes it possible to keep a heat load balance between the electrodes of the discharge lamp. As a result, it is possible to prevent only one of the electrodes of the discharge lamp from wearing out. 
     In the projector  500  according to the second embodiment, the control section  40  controls the discharge lamp driving section  230  so that the discharge lamp driving section  230  supplies, to the discharge lamp  90 , a drive current I by which the difference between the energy provided to the first electrode  92  in the second period and the energy provided to the second electrode  93  in the second period in the third control is smaller than those in the first control and the second control. That is, the difference between the energy provided to the first electrode  92  in the second period and the energy provided to the second electrode  93  in the second period in the third control is smaller than the difference between the energy provided to the first electrode  92  in the second period and the energy provided to the second electrode  93  in the second period in the first control and is smaller than the difference between the energy provided to the first electrode  92  in the second period and the energy provided to the second electrode  93  in the second period in the second control. 
     In the example shown in  FIG. 13 , in the projector  500  according to the second embodiment, in the third control, in the second period, the control section  40  controls the discharge lamp driving section  230  so that the discharge lamp driving section  230  supplies an alternating current to the discharge lamp  90  as the drive current I. In the example shown in  FIG. 13 , the control section  40  controls the discharge lamp driving section  230  so that the discharge lamp driving section  230  generates an alternating current corresponding to one period by reversing the polarity with the absolute value of the current value of the drive current I kept constant in the second period and supplies the alternating current to the discharge lamp  90  as the drive current I. Moreover, in the example shown in  FIG. 13 , the control section  40  controls the discharge lamp driving section  230  so that the discharge lamp driving section  230  reverses the polarity of the drive current I with such timing that the length of time for which the first polarity continues is the same as the length of time for which the second polarity continues. In the example shown in  FIG. 13 , an alternating current of two periods is supplied to the discharge lamp  90  as the drive current I in one second period. 
     Incidentally, in the projector  500  according to the second embodiment, in the second period of the third control, the control section  40  may control the discharge lamp driving section  230  so that the discharge lamp driving section  230  reverses the polarity of the drive current I with such timing that the length of time for which the first polarity continues is different from the length of time for which the second polarity continues. In this case, the control section  40  may control the discharge lamp driving section  230  so that the discharge lamp driving section  230  reverses the polarity of the drive current T in such a way that the difference between the length of time for which the first polarity continues and the length of time for which the second polarity continues in the second period of the third control becomes smaller than the difference between the length of time for which the first polarity continues and the length of time for which the second polarity continues in the first control and the second control. As described above, in the third control, the control section  40  controls the discharge lamp driving section  230  so that the discharge lamp driving section  230  supplies, to the discharge lamp  90 , the drive current I that keeps a heat load balance between the electrodes of the discharge lamp  90  as a whole (the drive current I that hardly suffers from a lack of heat load balance) as compared to the first control and the second control. 
     In the projector  500  according to the second embodiment, the control section  40  may perform at least one of the first control and the second control in a period sandwiched between the third control periods which are next to each other in terms of time. In the example shown in  FIG. 12 , the third control period from time t11 to time t13 and the third control period from time t15 to time t17 are next to each other in terms of time. A period from time t13 to time t15, the period sandwiched between the two third control periods, is a first control period. Moreover, in the example shown in  FIG. 12 , the third control period from time t15 to time t17 and the third control period from time t19 to time t21 are next to each other in terms of time. A period from time t17 to time t19, the period sandwiched between the two third control periods, is a second control period. 
     As described above, as a result of the control section  40  performing at least one of the first control and the second control in a period sandwiched between the third control periods which are next to each other in terms of time, it is possible to prevent harmful effects such as blackening caused by too high fusibility of the electrode due to the first control and the second control. 
     Moreover, as shown in  FIG. 12 , also in the second embodiment, as in the first embodiment, the first control is performed in the first control period and the second control is performed in the second control period which is at least part of the period sandwiched between the first control periods which are next to each other in terms of time. As a result, it is possible to keep a heat load balance between the first electrode  92  and the second electrode  93  of the discharge lamp  90 . This makes it possible to prevent only one of the electrodes of the discharge lamp from wearing out. 
     3. Projector According to Third Embodiment 
     Next, a projector  500  according to a third embodiment will be described. The configurations of the optical systems and the circuits of the projector  500  according to the third embodiment are the same as those of the projector  500  according to the first embodiment. Therefore, in the following description, a specific example of control of the drive current z in the projector  500  according to the third embodiment will be described. Moreover, the contents of the first control, the second control, and the third control are the same as those of the first embodiment and the second embodiment, and therefore a detailed description thereof is omitted. 
     The projector  500  according to the third embodiment includes a state detecting section (a voltage detecting section  60 ) that detects a deteriorating state of the discharge lamp  90 , and the control section  40  shortens the third control period with the progress of the deteriorating state. 
     The state detecting section may detect, as a value indicating the degree of the deteriorating state, for example, the drive voltage Vla of the discharge lamp  90 , a temporal change in the drive voltage Vla of the discharge lamp  90 , the amount of light of the discharge lamp  90 , a temporal change in the amount of light of the discharge lamp  90 , accumulated lighting time of the discharge lamp  90 , or the like. In the third embodiment, the voltage detecting section  60  (the state detecting section) detects the drive voltage Vla of the discharge lamp  90  as the deteriorating state of the discharge lamp  90 . 
       FIG. 14  is a flowchart showing a control example of the projector of the third embodiment. In the flowchart shown in  FIG. 14 , the control which is performed after the discharge lamp  90  is stably turned on until it is turned off is shown. First, the voltage detecting section  60  detects the drive voltage Vla (step S 100 ). Next, the control section  40  selects a drive condition corresponding to the drive voltage Vla detected in step S 100  from the correspondence stored in the storing section  44  (step S 102 ). 
       FIG. 15  is a diagram showing an example of the correspondence between drive conditions. In an example shown in  FIG. 15 , the first control period and the second control period have a constant length and are three times longer than a period sandwiched between, a time at which switching is performed and the next time at which switching is performed. Moreover, the length of the third control period becomes shorter as the drive voltage Vla becomes greater (as the deteriorating state of the discharge lamp  90  progresses). In the example shown in  FIG. 15 , the third control period is three times longer than the period sandwiched between a time at which switching is performed and the next time at which switching is performed when the drive voltage Vla is less than 80 V, is twice as long as the period sandwiched between a time at which switching is performed and the next time at which switching is performed when the drive voltage Vla is 80 V or more but less than 95 V, and is as long as the period sandwiched between a time at which switching is performed and the next time at which switching is performed when the drive voltage Vla is 95 V or more. 
     After the drive condition is selected in step S 102  of  FIG. 14 , the control section  40  determines whether or not there is a need to change the drive condition (step S 104 ). If the control section  40  determines that there is a need to change the drive condition (YES in step S 104 ), the control section  40  changes the drive condition to the drive condition selected in step S 102  and drives the discharge lamp  90  (step S 106 ). If the control section  40  determines that there is no need to change the drive condition (NO in step S 104 ), the control section  40  continues to drive the discharge lamp  90  in a previous drive condition. 
     When NO is chosen in step S 104  or after step S 106  is performed, the control section  40  determines whether or not an instruction to turn off the discharge lamp  90  is provided (step S 108 ). If the control section  40  determines that an instruction to turn off the discharge lamp  90  is provided (YES in step S 108 ), the control section  40  ends lighting of the discharge lamp  90  (turns off the discharge lamp  90 ). If the control section  40  determines that an instruction to turn off the discharge lamp  90  is not provided (NO in step S 108 ), the control section  40  repeats the control from steps S 100  to S 108  until an instruction to turn off the discharge lamp  90  is provided. 
     When the deteriorating state of the first electrode  92  and the second electrode  93  of the discharge lamp  90  progresses, the distance between the first electrode  92  and the second electrode  93  (the distance between the electrodes) is increased. When the distance between the electrodes is increased, the drive voltage via rises. That is, the drive voltage Vla rises with the progress of the deteriorating state. 
     Therefore, in the projector  500  of the third embodiment, the third control period is shortened with a rise in the drive voltage Vla (with the progress of the deteriorating state). When the deteriorating state of the discharge lamp  90  progresses, the fusibility of the electrode is reduced. Thus, by increasing the frequency of the first control and the second control by shortening the third control period with the progress of the deteriorating state of the discharge lamp  90 , it is possible to increase the fusibility of the electrode and prevent deformation of the electrode. Moreover, when the deteriorating state of the discharge lamp  90  does not progress, by not increasing the fusibility of the electrode more than necessary, it is possible to prevent harmful effects such as blackening caused by too high fusibility of the electrode. 
     4. Projector According to Fourth Embodiment 
     Next, a projector  500  according to a fourth embodiment will be described. The configurations of the optical systems and the circuits of the projector  500  according to the fourth embodiment are the same as those of the projector  500  according to the first embodiment. Therefore, in the following description, a specific example of control of the drive current I in the projector  500  according to the fourth embodiment will be described. Moreover, the contents of the first control, the second control, and the third control are the same as those of the first embodiment and the second embodiment, and therefore a detailed description thereof is omitted. 
     The projector  500  according to the fourth embodiment includes a state detecting section (a voltage detecting section  60 ) that detects a deteriorating state of the discharge lamp  90 . The control section  40  lengthens the first control period and the second control period with the progress of the deteriorating state. 
     The state detecting section may detect, as a value indicating the degree of the deteriorating state, for example, the drive voltage Vla of the discharge lamp  90 , a temporal change in the drive voltage Vla of the discharge lamp  90 , the amount of light of the discharge lamp  90 , a temporal change in the amount of light of the discharge lamp  90 , accumulated lighting time of the discharge lamp  90 , or the like. In the fourth embodiment, the voltage detecting section  60  (the state detecting section) detects the drive voltage Vla of the discharge lamp  90  as the deteriorating state of the discharge lamp  90 . 
     A control flow in the fourth embodiment is the same as the control flow in the third embodiment described by using  FIG. 14 , and differs therefrom only in the correspondence between the drive conditions selected in step S 102 . 
       FIG. 16  is a diagram showing an example of the correspondence between drive conditions. In an example shown in  FIG. 16 , the third control period has a constant length and is three times longer than a period sandwiched between a time at which switching is performed and the next time at which switching is performed. Moreover, the lengths of the first control period and the second control period become longer as the drive voltage via becomes greater (as the deteriorating state of the discharge lamp  90  progresses). In the example shown in  FIG. 16 , the first control period and the second control period are as long as the period sandwiched between a time at which switching is performed and the next time at which switching is performed when the drive voltage Vla is less than 80 V, are twice as long as the period sandwiched between a time at which switching is performed and the next time at which switching is performed when the drive voltage Vla is 80 V or more but less than 95 V, and are three times longer than the period sandwiched between a time at which switching is performed and the next time at which switching is performed when the drive voltage Vla is 95 V or more. 
     When the deteriorating state of the first electrode  92  and the second electrode  93  of the discharge lamp  90  progresses, the distance between the first electrode  92  and the second electrode  93  (the distance between the electrodes) is increased. When the distance between the electrodes is increased, the drive voltage Vla rises. That is, the drive voltage Vla rises with the progress of the deteriorating state. 
     Therefore, in the projector  500  of the fourth embodiment, the first control period and the second control period are lengthened with a rise in the drive voltage Vla (with the progress of the deteriorating state). When the deteriorating state of the discharge lamp  90  progresses, the fusibility of the electrode is reduced. Thus, by lengthening the first control period and the second control period with the progress of the deteriorating state of the discharge lamp  90 , it is possible to increase the fusibility of the electrode and prevent deformation of the electrode. Moreover, when the deteriorating state of the discharge lamp  90  does not progress, by not increasing the fusibility of the electrode more than necessary, it is possible to prevent harmful effects such as blackening caused by too high fusibility of the electrode. Incidentally, in the third embodiment, an example in which the third control period is shortened as the deteriorating state of the discharge lamp  90  progresses has been described, and, in the fourth embodiment, an example in which the first control period and the second control period are lengthened as the deteriorating state of the discharge lamp  90  progresses has been described. However, these examples can be combined. 
     In the embodiments described above, a configuration in which the projector  500  makes the observer view a display picture stereoscopically by using a first picture and a second picture as a picture for the left eye and a picture for the right eye, respectively, has been adopted; however, the invention is not limited thereto. For example, as a projector, a configuration in which a first picture and a second picture are provided as pictures with different contents and different observers are made to identify visually two display pictures (the first picture and the second picture) in such a way that one observer views one of the display pictures and the other observer views the other of the display pictures may be adopted. 
     When such a configuration is adopted, as the active shutter glasses, it is necessary simply to provide two types of glasses: glasses having, on the right and left eyeglasses, shutters that operate in the same way as the right shutter  412  described earlier and glasses having, on the right and left eyeglasses, shutters that operate in the same way as the left shutter  414  described earlier. 
     In the embodiments described above, a description has been given by taking up a projector using three liquid crystal panels as an example; however, the invention is not limited thereto. The invention can also be applied to a projector using one, two, or four or more liquid crystal panels. 
     In the embodiments described above, a description has been given by taking up a transmissive projector as an example; however, the invention is not limited thereto. The invention can also be applied to a reflective projector. Here, “transmissive” means that, like a transmissive liquid crystal panel or the like, an electrooptic modulating device as a light modulating unit transmits light, and “reflective” means that, like a reflective liquid crystal panel, a micromirror light modulating device, or the like, an electrooptic modulating device as a light modulating unit reflects light. As the micromirror light modulating device, a DMD (Digital Micromirror Device, which is a trademark of Texas Instruments Inc.), for example, can be used. The same effects as those of the transmissive projector can also be obtained when the invention is applied to the reflective projector. 
     The invention can be applied to a front projection projector which projects a projection image from the observer&#39;s side and a rear projection projector which projects a projection image from the side opposite to the observer&#39;s side. 
     It is to be understood that the invention is not limited to the embodiments described above and various modifications are possible within the scope of the subject matter of the invention. 
     The invention includes a configuration which is substantially identical to the configuration described in the embodiment (for example, a configuration having the same function, method, and result as those of the configuration described in the embodiment or a configuration having the same objective and effects as those of the configuration described in the embodiment). Moreover, the invention includes a configuration in which a nonessential portion of the configuration described in the embodiment is replaced with another portion. Furthermore, the invention includes a configuration that can obtain the same effects as those of the configuration described in the embodiment or achieve the same objective as that of the configuration described in the embodiment. In addition, the invention includes a configuration which is obtained by adding a publicly-known technique to the configuration described in the embodiment. The entire disclosure of Japanese Patent Application No. 2010-278936, filed Dec. 15, 2010 is expressly incorporated by reference herein.