US20090179837A1

US20090179837A1 - Display system comprising a mirror device with micromirrors controlled to operate in intermediate oscillating state

Info

Publication number: US20090179837A1
Application number: US12/074,093
Authority: US
Inventors: Taro Endo; Yoshihiro Maeda; Kazuma Arai; Fusao Ishii
Original assignee: Olympus Corp; Silicon Quest KK
Current assignee: Olympus Corp; Silicon Quest KK
Priority date: 2007-03-02
Filing date: 2008-03-01
Publication date: 2009-07-16
Also published as: US20080246783A1; WO2008109050A1; US7961161B2

Abstract

A display system includes a spatial light modulator for displaying an image by the modulation state of a plurality of micromirrors, and a control device for controlling the spatial light modulator. The control device includes a data conversion device for converting all or a part of the digital image data input to the display system into non-binary data, and a modulation-control device for generating a modulation control signal for micromirrors depending on the non-binary data, and a modulation-control device for controlling the spatial light modulator. The modulation state of the micromirrors by the modulation control signal includes modulation by oscillation of the micromirrors. The modulation-control device controls a voltage value of the modulation control signal to be applied to the driving electrode of the micromirror such that the amplitude of the oscillation can be smaller than the maximum amplitude of the micromirrors in the modulation by the oscillation of the micromirrors.

Description

BACKGROUND

1. Field of Invention
The present invention relates to a display technique, an efficient technique applied to a display system for representing gray scale using a display device having a plurality of micromirrors.
2. Prior Art
As disclosed by the patent document 1, there has been a well known technique of displaying a projected picture by performing pulse width modulation (PWM) control based on the digital picture data using a display device such as a DMD (digital mirror device) etc. having a plurality of micromirrors.
That is, optical modulation is performed depending on the digital picture data by balancing the incoming light from a light source to each micromirror between two states, that is, an ON state in which the incoming light is reflected toward a projective optical system and an OFF state in which the incoming light deviates from the projective optical system.
In this case, the brightness of each pixel of a projected picture depends on the total length of time in which each micromirror stays in the ON state in each frame period of the picture. Therefore, there has been the technological problem of an increasing amount of digital picture data to be processed in one frame period, and a higher speed of modulation-controlling a mirror into the ON state in order to represent more delicate gray scale.
Therefore, to represent delicate gray scale without increasing the speed of modulation-controlling a micromirror into the ON state, it is necessary to increase or decrease the quantity of light of a light source in addition to the modulation-control by balancing the micromirror in the DMD as disclosed by the patent document 2, thereby complicating the controlling operation.
[Patent Document 1] U.S. Pat. No. 5,287,096
[Patent Document 2] U.S. Pat. No. 5,589,852

SUMMARY

An advantage of the present invention is to realize more delicate gray scale in the picture display using a spatial light modulation element to display a picture depending on the modulation state of a plurality of micromirrors without increasing the speed of modulation-controlling the micromirrors into the ON state.
Another advantage of the present invention is to realize more delicate gray scale in the picture display using a spatial light modulation element to display a picture depending on the modulation state of a plurality of micromirrors without complicated control of the quantity of light of a light source or an additional circuit.
The present invention provides a display technology for realizing the intermediate oscillation by applying a plurality of voltages to a driving electrode of micromirror, or applying an offset voltage to the potential of the micromirror.
The first aspect of the present invention is a display system including: a light source; a spatial light modulation element having a plurality of micromirrors and forming an image to be displayed from the light from the light source by modulating the plurality of micromirrors; and a control device for controlling the spatial light modulation element. With the configuration, the control device includes a modulation-control device for generating a modulation control signal for the plurality of micromirrors depending on the digital picture data input to the display system, and controlling the spatial light modulation element; the modulation state of the micromirrors by the modulation control signal includes the modulation by the oscillation of the micromirrors; and the modulation-control device controls a voltage value of the modulation control signal to be applied to a driving electrode of the micromirror so that the amplitude of the oscillation can be equal to or smaller than the maximum amplitude of the micromirrors in the modulation by the oscillation of the micromirrors, wherein a number of said voltage value is at least three.
The second aspect of the present invention is based on the display system according to the first aspect. The control device includes a data conversion device for converting a part or all of the digital picture data input to the display system into non-binary data; and the modulation-control device generates a modulation control signal of the micromirrors depending on the non-binary data, and controls the spatial light modulation element.
The third aspect of the present invention is based on the display system according to the first aspect. In the display system, the modulation control signal controls an offset voltage to be applied to said micromirror.
The fourth aspect of the present invention is based on the display system according to the third aspect. In the display system, the offset voltages are a plurality of different voltage values.
The fifth aspect of the present invention is based on the display system according to the first aspect. In the display system, a time duration for applying the voltage value to a driving electrode is shorter than a quarter of the free oscillation period of the micromirrors.
The sixth aspect of the present invention is based on the display system according to the first aspect. In the display system, a time duration for applying the voltage value to a driving electrode is shorter than a quarter of a least significant bit (LSB) period for control of the micromirrors.
The seventh aspect of the present invention is based on the display system according to the first aspect. In the display system, the oscillation having the amplitude includes free oscillation which decreases its amplitude with time.
The eighth aspect of the present invention is based on the display system according to the first aspect. In the display system, a time duration in which the oscillation is repeated is longer than one oscillation period of the micromirrors.
The ninth aspect of the present invention is based on the display system according to the first aspect. In the display system, a number of times at which the oscillation of the micromirrors is repeated in a time duration is two times or more in one frame period of the digital image data.
The tenth aspect of the present invention is based on the display system according to the first aspect. In the display system, the modulation-control device generates a modulation control signal to perform modulation by the oscillation of the micromirrors based on at least 1-bit data other than an MSB in a plurality of bits forming the digital picture data input to the display system.
The eleventh aspect of the present invention is based on the display system according to the first aspect. In the display system, an oscillating state having an amplitude equal to or smaller than the maximum amplitude of the micromirrors is controlled to turn from an ON state of the micromirrors.
The twelfth aspect of the present invention is based on the display system according to the first aspect. In the display system, an oscillating state having an amplitude equal to or smaller than the maximum amplitude of the micromirrors is controlled to turn from an OFF state of the micromirrors.
The thirteenth aspect of the present invention is based on the display system according to the first aspect. In the display system, an oscillating state having an amplitude equal to or smaller than the maximum amplitude of the micromirrors is controlled to turn to an ON state of the micromirrors.
The fourteenth aspect of the present invention is based on the display system according to the first aspect. In the display system, an oscillating state having an amplitude equal to or smaller than the maximum amplitude of the micromirrors is controlled to turn to an OFF state of the micromirrors.

DRAWINGS

FIG. 1A shows a prior art illustrating the basic principle of a projection display using a micromirror device;

FIG. 1B shows a prior art illustrating the basic principle of a micromirror device used for a projection display;

FIG. 1C shows an example of the driving circuit of prior arts;

FIG. 1D shows the scheme of binary pulse width modulation (binary PWM) of conventional digital micromirrors to generate gray scale;

FIG. 2 shows the concept of an example of the configuration of the display system according to an embodiment of the present invention;

FIG. 3 is a block diagram showing an example of the configuration of the spatial light modulation element forming the display system according to an embodiment of the present invention;

FIG. 4A shows the concept of an example of the configuration of the pixel unit forming the spatial light modulation element according to an embodiment of the present invention;

FIG. 4B shows the concept of an example of a variation of the pixel unit forming the spatial light modulation element according to an embodiment of the present invention;

FIG. 5 is a diagram showing an example of the pulse width modulation (PWM) using binary data;

FIG. 6 is a diagram showing an example of converting binary data into non-binary data;

FIG. 7 is a diagram showing an example of converting a part of binary data into non-binary data;

FIG. 8 is a diagram showing an example of converting binary data into non-binary data in a display system a an embodiment of the present invention;

FIG. 9A is an explanatory view showing the ON state of the micromirrors;

FIG. 9B is a diagram showing the voltage waveform for realizing the ON state of the micromirrors;

FIG. 10A is an explanatory view showing the OFF state of the micromirrors;

FIG. 10B is a diagram showing the voltage waveform for realizing the OFF state of the micromirrors;

FIG. 11A is an explanatory view showing the oscillating state of the micromirrors;

FIG. 11B is a diagram showing the voltage waveform for realizing the oscillating state of the micromirrors;

FIG. 12 is a diagram showing an embodiment of the oscillating state of the micromirrors in the display system according to an embodiment of the present invention;

FIG. 13 is a diagram showing an embodiment of the oscillating state of the micromirrors in the display system according to an embodiment of the present invention;

FIG. 14 is a diagram showing the principle of the improved gray scale by the oscillating state of the micromirrors in the display system according to an embodiment of the present invention;

FIG. 15 is a diagram illustrating the improved gray scale by a combination of the ON state of the micromirrors and the oscillating state by the display system according to an embodiment of the present invention;

FIG. 16 is a diagram illustrating the improved gray scale by a combination of the ON state of the micromirrors and the oscillating state by the display system according to an embodiment of the present invention;

FIG. 17 is a diagram illustrating the improved gray scale by a combination of the ON state of the micromirrors and the oscillating state by the display system according to an embodiment of the present invention; and

FIG. 18 is a diagram illustrating the improved gray scale by a combination of the ON state of the micromirrors and the oscillating state by the display system according to an embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the present invention are described below in detail with reference to the attached drawings.
FIG. 2 shows the concept of an example of the configuration of the display system according to an embodiment of the present invention. FIG. 3 is a block diagram showing an example of the configuration of the spatial light modulation element forming the display system according to an embodiment of the present invention. FIGS. 4A and 4B show the concept of an example of the configuration of a pixel unit 211 forming the spatial light modulation element according to an embodiment of the present invention.
A display system 100 according to the embodiments of the present invention includes a spatial light modulation element 200, a control device 300, a light source 510, and a projective optical system 520.
As shown in FIGS. 3 and 4 etc., the spatial light modulation element 200 includes a pixel array 210, a column driver 220, a row driver 230, and an external interface unit 240.
In the pixel array 210, a plurality of pixel units 211 are arranged in grid form at each intersection of a bit line 221 vertically extending from the column driver 220 and a word line 231 horizontally extending from the row driver 230.
As illustrated in FIGS. 9A, 10A, and 11A, each pixel unit 211 is provided with a micromirror 212 supported as freely tilted on a substrate 214 through a hinge 213.
On the substrate 214, an OFF electrode 215 and an OFF stopper 215 a, and an ON electrode 216 and an ON stopper 216 a are symmetrically arranged about the hinge 213 provided with a hinge electrode 213 a.
The OFF electrode 215 pulls the micromirror 212 with a Coulomb force by applying predetermined potential, and tilts the micromirror 212 until it touches the OFF stopper 215 a. Thus, incoming light 311 entering the micromirror 212 is reflected toward the optical path in the OFF position deviated from the optical axis of a projective optical system 130.
The ON electrode 216 pulls the micromirror 212 with the Coulomb force by applying predetermined potential, and tilts the micromirror 212 until it touches the ON stopper 216 a. Thus, the incoming light 311 entering the micromirror 212 is reflected toward the optical path in the ON position matching the optical axis.
An OFF capacitor 215 b is connected to the OFF electrode 215, and the OFF capacitor 215 b is connected to a bit line 221-1 through a gate transistor 215 c.
An ON capacitor 216 b is connected to the ON electrode 216. The ON capacitor 216 b is connected to the bit line 221-2 through a gate transistor 216 c.
The word line 231 controls the opening and closing operations of the gate transistor 215 c and the gate transistor 216 c.
That is, a horizontal row of the pixel unit 211 connected to any word line 231 is simultaneously selected, and the charge/discharge of electric charge with respect to the OFF capacitor 215 b and the ON capacitor 216 b is controlled by the bit line 221-1 and the bit line 221-2, thereby individually controlling the ON/OFF of the micromirror 212 in each pixel unit 211 in the horizontal row.
FIG. 4B shows the concept of an example of a variation of the pixel unit illustrated in FIG. 4A.
In the pixel unit 211A of an example of a variation shown in FIG. 4B, the hinge 213 (hinge electrode 213 a) that supports the micromirror 212 is connected to a mirror potential control line 232 so that the potential of the micromirror 212 can be externally controlled, which is different from the configuration shown in FIG. 4A.
Therefore, in the pixel unit 211A in a variation example shown in FIG. 4B, by controlling the combination of the voltage applied from the bit line 221-1 and the bit line 221-2 to the OFF electrode 215 and the ON electrode 216 and the voltage level and the application timing of the voltage applied from the mirror potential control line 232 to the micromirror 212, the tilt angle and the tilt speed of the micromirror 212 can be optionally controlled.
For example, by maintaining constant timing of applying a voltage to the OFF electrode 215 and the ON electrode 216, and changing the value of the voltage applied from the mirror potential control line 232 to the 215 and the ON electrode 216, and further to the micromirror 212, the level of the amplitude of the intermediate oscillation between the ON state and the OFF state of the micromirror 212 can be changed.
The external interface unit 240 illustrated in FIG. 3 includes a timing controller 241 and a parallel/serial interface 242. The timing controller 241 selects the pixel unit 211 of the horizontal row by the word line 231 based on a scanning timing control signal 432 output from a selector 324.
The parallel/serial interface 242 provides a modulation control signal 440 for the column driver 220.
The light source 510 irradiates the spatial light modulation element 200 with incoming light 511, the light is reflected as reflected light 512 by each micromirror 212, and the reflected light 512 in the optical path through the 520 is projected as projected light 513 on the screen etc. not shown in the attached drawings.
As shown in FIG. 2 as a conceptual illustration, the control device 300 according to the present embodiment for controlling the spatial light modulation element 200 is provided with a data splitter 310 and a data converter 320.
As described later, the control device 300 realizes gray scale using the ON/OFF state (ON/OFF modulation) and the oscillating state (oscillation modulation) of the micromirror 212 of the spatial light modulation element 200.
The data splitter 310 has the function of separating a binary picture signal 400 of externally input binary data into separated data 410 for control of the micromirror 212 for ON/OFF modulation and separated data 420 for control of the micromirror 212 for a modulation state, and the function of outputting a synchronization signal 430 for control of the data converter 320.
The data converter 320 includes a first state control unit 321, a second state control unit 322, a timing control unit 323, and a selector 324.
The first state control unit 321 has the function of controlling the micromirror 212 for the ON/OFF state by outputting non-binary data 411 to the spatial light modulation element 200 through the selector 324 based on the separated data 410.
The second state control unit 322 has the function of controlling the micromirror 212 for the oscillating state by outputting non-binary data 421 to the spatial light modulation element 200 through the selector 324 based on the separated data 420.
The timing control unit 323 calculates the time required to placed the micromirror 212 in the ON state and the time (time duration) required to placed the micromirror 212 in the oscillating state in each frame partly forming the binary picture signal 400 with respect to each micromirror 212 configuring a pixel of an image based on the synchronization signal 430 generated by the binary picture signal 400, controls the first state control unit 321 and the second state control unit 322, and outputs a control signal 431 to the selector 324.
The selector 324 switches the output of the non-binary data 411 or the non-binary data 421 to the spatial light modulation element 200 according to the control signal 431, thereby switching the control of the micromirror 212 from the ON/OFF modulation by the first state control unit 321 (non-binary data 411) to the oscillation modulation by the second state control unit 322 (non-binary data 421), or from the oscillation modulation to the ON/OFF modulation.
Described functions of the data splitter 310, the first state controller 321, the second state controller 322, the timing control unit 323 and the selector 324 can be provided with an integrated processor.
Described below are the binary data and the non-binary data with reference to FIGS. 5, 6, 7, and 8.
As shown in FIG. 5, the N bits of the binary data (binary picture signal 400) are data having different weights from the LSB (least significant bit) to the MSB (most significant bit).
When the gray scale is represented by the control of the pulse width modulation (PWM), the weight of each bit indicates a time width for pulse control, that is, the duration of the ON state of each segment (subframe).
The example shown in FIG. 6 is an embodiment of converting all 5 bits of the input binary data into the non-binary data of “weight”=1.
The period of a segment (subframe) of the binary data of all 5 bits is determined by the weight (=1) of the LSB, the data is converted into non-binary data (bit string) for each segment, and transferred to the spatial light modulation element 200.
That is, the frequency of the ON state of the interval of the LSB of the binary data is calculated, and the gray scale is represented to continue the period of the ON state for the bit string.
The example shown in FIG. 7 is an embodiment of converting the 3 internal bits of the binary data into non-binary data. In this example, the quantity of light is modulated (ratio of the quantity of light =½) on the lowest order bit of the binary data using the spatial light modulation element 200 or the light source 510. In this case, the weight of all bits other than the highest order bit of the binary data is set to 2, and the interval of one segment is extended, thereby matching the interval of the segment of the lowest order bit with the intervals of other segments.
Each pixel element (pixel unit 211) of the spatial light modulation element 200 is a micromirror 212 controlled in any of the ON/OFF (positioning) state, the oscillating state, and the intermediate state.
As shown in FIG. 8, in the case of the present embodiment, the ON/OFF (positioning) state is controlled by the non-binary data 411 output from the first state control unit 321, and the oscillating state is controlled by the non-binary data 421 output from the second state control unit 322.
In this case, the quantity of light is modulated by the spatial light modulation element 200 using the non-binary data 421, thereby extending the interval of a segment, and moderating the time request in an arithmetic operation.
Described below is the basic control of the micromirror 212 of the spatial light modulation element 200 according to the present embodiment.
In the following descriptions, Va (1, 0) indicates that a predetermined voltage Va is applied to the OFF electrode 215, and not applied to the ON electrode 216.
Va (0, 1) indicates that no voltage is applied to the OFF electrode 215, and the voltage Va is applied to the ON electrode 216.
Va (0, 0) indicates that no variation Va is applied to the OFF electrode 215 or the ON electrode 216.
Va (1, 1) indicates that the voltage Va is applied to both of the OFF electrode 215 and the ON electrode 216.
FIGS. 9A, 9B, 10A, 10B, 11A, and 11B show basic examples of the configurations of the pixel unit 211 including the micromirror 212, the hinge 213, the OFF electrode 215, and the ON electrode 216, and the state control of controlling the micromirror 212 in the ON state and the oscillating state.
FIG. 9A shows that the micromirror 212 is pulled and tilted from the neutral position into the ON state by applying the predetermined voltage Va only to the ON electrode 216 (Va (0, 1)), and enters the ON state. In the ON state of the micromirror 212, the reflected light 512 passing through the micromirror 212 is captured by the projective optical system 520, and projected as the projected light 513. FIG. 9B shows the quantity of light projected in the ON state.
FIG. 10A shows that the micromirror 212 is pulled and tilted from the neutral position into the OFF state by applying the predetermined voltage Va only to the OFF electrode 215 (Va (1, 0)), and enters the OFF state. In the OFF state of the micromirror 212, the reflected light 512 deviates from the projective optical system 520, and does not become the projected light 513. FIG. 10B shows the quantity of light projected in the OFF state.
FIG. 11A shows an example of performing free oscillation at the maximum amplitude A0 between the tilt position (full ON) in which the micromirror 212 touches the ON electrode 216 and the tilt position (full OFF) in which the micromirror 212 touches the OFF electrode 215 (Va (0, 0)).
The micromirror 212 is irradiated with the incoming light 511 at a predetermined angle. The quantity of light reflected in the ON direction and a part of the quantity of light (quantity of light of the reflected light 512) reflected between the ON direction and the OFF direction enter the projective optical system 520, and is projected as the luminance (projected light 513) of an image. FIG. 11B shows the quantity of light projected in the OFF state.
That is, in the ON state of the micromirror 212 shown in FIG. 9A, substantially all of the reflected optical flux travels in the ON direction in which it is captured by the projective optical system 520, and projected as the projected light 513.
In the OFF state of the micromirror 212 shown in FIG. 10A, the reflected light 512 travels and deviates from the projective optical system 520 in the OFF direction, and there is no light projected as the projected light 513.
In the oscillating state of the micromirror 212 shown in FIG. 11A, a part of the optical flux of the reflected light 512, diffracted light, scattered light, etc. are captured by the projective optical system 520, and projected as the projected light 513.
In the examples shown in FIGS. 9A, 9B, 10A, 10B, 11A, and 11B, the voltage Va represented by two values, that is, 0 and 1, is applied to each of the OFF electrode 215 and the ON electrode 216, but the levels of the Coulomb force generated between the micromirror 212 and the OFF electrode 215 and the ON electrode 216 can be increased by increasing the levels of the value of Va as multiple values, thereby controlling the tilt angle of the micromirror 212 to the subtleties.
Furthermore, in the examples shown in FIGS. 9A, 9B, 10A, 10B, 11A, and 11B, the micromirror 212 (hinge electrode 213 a) is described as ground potential, but the tilt angle of the micromirror 212 can be controlled to the subtleties by applying the offset voltage to the micromirror 212.
In the case of the present embodiment, as described later, the amplitude of the tilt displacement of the micromirror 212 is obtained by generating free oscillation of the amplitude Al and the amplitude A2 smaller than the maximum amplitude AO between the ON and the OFF by applying Va (0, 1), Va (1, 0), and Va (0, 0) in appropriate timing during the tilt displacement of the micromirror 212 between ON and OFF, thereby realizing more subtle gray scale.
A method of displaying a picture using the display system 100 is described below.
When the binary picture signal 400 is input to the control device 300, it is divided into the separated data 410 and the separated data 420.
The first state control unit 321 and the second state control unit 322 calculate the time duration in which the micromirror 212 is placed in the ON state in one frame of a picture with respect to the respectively micromirrors 212 of the spatial light modulation element 200 forming the pixel of a picture depending on the separated data 410 and the separated data 420 of the picture signal, the time duration in which the micromirror 212 is placed in the oscillating state, or the frequency of oscillating the micromirror 212.
The first state control unit 321 and the second state control unit 322 of the control device 300 calculate the time duration in which the micromirror 212 is placed in the ON state, the time duration in which the micromirror 212 is placed in the oscillating state, or the frequency at which the micromirror 212 is oscillated using the ratio of the quantity of light of the projected light 513 obtained by the oscillation in the oscillation time T of a predetermined micromirror 212 and the quantity of light of the projected light 513 obtained by placing the mirror in the ON state in the oscillation time T.
Using the calculated time duration or value of the frequency, the ON/OFF control and the oscillation-control are performed on each micromirror 212 forming one frame of a picture.
Described below is an example of realizing free oscillation in the intermediate position between the ON state and the OFF state in the control device 300 according to the present embodiment.
FIG. 12 shows an example of performing free oscillation with an amplitude A smaller than the maximum amplitude AO in the intermediate state between the ON state and the OFF state from the OFF state. Described are means for realizing the process.

- The Timing Method
- The first method is to control the micromirror by applying two voltages, zero and Va, to the electrodes while the micromirror is at GND state or zero volt. As seen 411 in FIG. 12, the voltage Va(0,1) is applied to the OFF electrode 215 and the ON electrode 216 of the micromirror 2l2 in the OFF state.
- At time t1 a, Va(0,0) is applied to the OFF electrode 215 and the ON electrode 216 of the micromirror 212, resulting no Coulomb force between the electrodes and the micromirror.
- By the spring force of the hinge 213, the micromirror 212, which was pulled to the OFF state, is released to move toward the ON state.
- In the period between t1 b and t1 c, Va(0,1) is applied to the OFF electrode 215 and the ON electrode 216 to pull the micromirror 212 toward the OFF state in order to reduce the speed of the micromirror 212 which is moving toward the ON state.
- At time t1 c, before the micromirror 212 is reaching the ON state, Va(0,0) is applied to the OFF electrode 215 and the ON electrode 216 to set Coulomb force between the electrodes and the mirror to zero, and the micromirror 212 starts free oscillation with the amplitude A smaller than the maximum amplitude A0.
- At time t1 d, Va(0,1) is applied to the OFF electrode 215 and the ON electrode 216 of the mircomirror 212, the oscillation of the micromirror 212 is stopped and the mirror is placed in the OFF state.
- The free oscillation period T2, or oscillation modulation period, is set to get desirable gray scale level, from the time required by the number of the free oscillation cycle calculated by the light intensity obtained by one cycle of the free oscillation, or from the time calculated by the light intensity per arbitrary free oscillation period T.
- In this control method, the timing of t1 a, t1 b and t1 c governs the amplitude A or the initial speed of the free oscillation of the micromirror 212.
- It is understood that the oscillation amplitude A can be controlled by setting the timing of t1 a, t1 b and t1 c.
- The period between t1 a to t1 c is shorter than the half of the free oscillation period of the micromirror 212, and shorter than the half of the period required by LSB in the PWM control. Especially, the period between t1 a to t1 b is shorter than the quarter of the free oscillation period of the micromirror 212, and shorter than the quarter of the period required by LSB in the PWM control.
- The time t2 a through time t2 d and the value of the voltage Va are determined by the first state control unit 321 and the second state control unit 322 of the data converter 320.
- In this embodiment, the driving circuit of each electrode is simplified by making the voltage Va common with the voltage value for use in the PWM control (ON state control) of the micromirror. That is, in this case, it is not necessary to have a plurality of driving voltages.
- (2) Multiple Voltage Method
- The acceleration of the micromirror 212 moving between ON state and OFF state is governed by Coulomb force generated by the voltage applied to the electrodes and the micromirror 212.
- The second method is to control the micromirror 212 by use of three voltage value 0, Va and Vb applied to the OFF electrode 215. The micromirror 212 is set to zero volts or in GND state in this embodiment as well.
- As seen 411 in FIG. 12, the voltage Va(0,1) is applied to the OFF electrode 215 and the ON electrode 216 of the micromirror 2l2 in the OFF state.
- At time t1 a, Va(0,0) is applied to the OFF electrode 215 and the ON electrode 216 of the micromirror 212, resulting no Coulomb force between the electrodes and the mirror. By the spring force of the hinge 213, the micromirror 212, which was pulled to the OFF state, is released to move toward the ON state.
- In the period between t1 b and t1 c, Vb(0,1) is applied to the OFF electrode 215 and the ON electrode 216 to pull the micromirror 212 toward the OFF state in order to reduce the speed of the micromirror 212 which is moving toward the ON state.
- Vb is greater than Va and thus the force to reduce the speed of the micromirror 212 is greater.
- At time t1 c, before the micromirror 212 is reaching the ON state, Va(0,0) is applied to the OFF electrode 215 and the ON electrode 216 to set Coulomb force between the electrodes and the mirror to zero, and the micromirror 212 starts free oscillation with the amplitude A2 smaller than the maximum amplitude A0.
- At time t1 d, Va(0,1) is applied to the OFF electrode 215 and the ON electrode 216 of the micromirror 212, the oscillation of the micromirror 212 is stopped and the mirror is placed in the OFF state.
- The free oscillation period T2, or oscillation modulation period, is set to get desirable gray scale level, from the time required by the number of the free oscillation cycle calculated by the light intensity obtained by one cycle of the free oscillation, or from the time calculated by the light intensity per arbitrary free oscillation period T.
- In this control method, the timing of t1 a, t1 b and t1 c are fixed and the voltage applied in the period between t1 b and t1 c governs the amplitude A2 or the initial speed of the free oscillation of the micromirror 212.
- That is, the amplitude A can be controlled by the control voltage without changing the timing of t1 a, t1 b and t1 c.
- It should be understood that it is also possible to obtain the same effect by applying other value than zero volts to the micromirror 212 although above description mentions the method applying voltage to the electrodes.

FIG. 13 shows an example of performing free oscillation in the intermediate state between the ON state and the OFF state from the ON state of a mirror. Described below is means for realizing the oscillation in the control device 300 according to the present embodiment.
As seen 412 in FIG. 13, the voltage Va(1,0) is applied to the OFF electrode 215 and the ON electrode 216 of the micromirror 2l2 in the ON state.
At time t2 a, Va (0, 0) is applied to the OFF electrode 215 and the ON electrode 216 of the micromirror 212, resulting no Coulomb force between the electrodes and the micromirror.
By the spring force of the hinge 213, the micromirror 212, which was pulled to the ON state, is released to move toward the OFF state.
In the period between t2 b and t2 c, Va (1, 0) is applied to the OFF electrode 215 and the ON electrode 216 to pull the micromirror 212 toward the ON state in order to reduce the speed of the micromirror 212 which is moving toward the OFF state.
At time t2 c, before the micromirror 212 is reaching the OFF state, Va(0,0) is applied to the OFF electrode 215 and the ON electrode 216 to set Coulomb force between the electrodes and the mirror to zero, and the micromirror 212 starts free oscillation with the amplitude A smaller than the maximum amplitude A0.
At time t2 d, Va (0, 1) is applied to the OFF electrode 215 and the ON electrode 216 of the micromirror 212, the oscillation of the micromirror 212 is stopped and the mirror is placed in the OFF state.
The free oscillation period T3, or oscillation modulation period, is set to get desirable gray scale level, from the time required by the number of the free oscillation cycle calculated by the light intensity obtained by one cycle of the free oscillation, or from the time calculated by the light intensity per arbitrary free oscillation period T.
In this control method, the timing of t2 a, t2 b and t2 c governs the amplitude A or the initial speed of the free oscillation of the micromirror 212.
It is understood that the oscillation amplitude A can be controlled by setting the timing of t2 a, t2 b and t2 c.
The time t2 a through time t2 d and the value of the voltage Va are determined by the first state control unit 321 and the second state control unit 322 of the data converter 320.
The method to control the amplitude A, smaller than the maximum amplitude A0 by controlling the timing has been described. It is also possible to obtain the same effect by controlling the voltage applied to the electrodes by three or more voltage value, or controlling the offset voltage applied to the micromirror, as described earlier.
The improved gray scale according to the present embodiment is realized as illustrated in FIG. 14. That is, FIG. 14 shows an example of realizing the quantity of light of the projected light 513 obtained in the oscillation time T of the micromirror 212 as indicating about ¼ and ⅛ of the quantity of light obtained by placing the micromirror 212 in the ON state for the same time duration by the control method shown in FIG. 12.
That is, the ¼ of the luminance ratio is realized by setting the amplitude A of the oscillation of the micromirror 212 as the amplitude Al (for example, 50%) with respect to the maximum amplitude A0.
In addition, the ⅛ of the luminance ratio is realized by setting the amplitude A of the oscillation of the micromirror 212 as the amplitude A2 (for example, 25%) with respect to the maximum amplitude A0.
Assuming that the gray scale of 256 levels (8 bits) is represented by changing the time Ton for control of the ON state in one frame of a picture to be displayed, the gray scale of 1024 levels (10 bits) can be represented by combining it with the free oscillation (1st state) of the amplitude A1.
In addition, by combining the ON state, the amplitude A1 (first oscillating state) and the amplitude A2 (second oscillating state) as a free oscillation state, the gray scale of 2048 levels (11 bits) can be generated.
FIGS. 15, 16, 17, and 18 show examples of realizing 2048 levels (11 bits) by combining the ON state, the amplitude A1, and the amplitude A2 as free oscillation.
That is, FIG. 15 shows an example of sequentially and independently executing the oscillating state of the amplitude A1 and the amplitude A2 after the transfer from the ON state to the OFF state of the micromirror 212 in one frame of the binary picture signal 400.
FIG. 16 shows an example of executing the oscillating state of the amplitude A1 after continuously generating an oscillating state of the amplitude A2 after the ON state of the micromirror 212, and temporarily entering the OFF state.
FIG. 17 shows an example of executing the oscillating state of the amplitude A1 temporarily through the OFF state after transferring to the oscillating state of the maximum amplitude A0 after the transfer of the amplitude A2 from the OFF state of the micromirror 212.
FIG. 18 shows an example of executing the oscillating state of the amplitude A2 after temporarily entering the OFF state after the OFF state. of the amplitude A1 without entering the OFF state during the ON state of the micromirror 212.
In any case shown in FIGS. 15 to 18, 2048 levels (11 bits) of the gray scale can be realized by combining the ON state of the micromirror 212 with the free oscillation of the amplitude A1 and the amplitude A2.
In description above, the free oscillation of the amplitude A1 and the amplitude A2 are provided within continuous one frame period. To reduce complication of the operation as controlling oscillation of the micromirrors, frame period can be divided so that each oscillation is performed in a separate period(sub-frame).
Thus, by controlling the micromirror with the amplitude A of the oscillation adjusted, the luminance of 1/n (n is an integer) of the quantity of light Lon obtained by placing the micromirror 212 in the ON state for the same time duration T can be acquired.
To increase the gray scale of the picture to be displayed by combining the ON state and the oscillating state of the micromirror, the mirror can be controlled to obtain the values of n=1.33, n=2, n=3, n=5, and n=10 as the luminance ratio of the ¾ of Lon in addition to the above-mentioned value of n.
The relationship between the luminance Losc obtained by the oscillation-control in one frame period of picture data and the time Tosc in which the oscillation-control is performed can be represented by the following equation.
Losc=Lon×(1/n)×(Tosc/T)
That is, to realize the same luminance Losc, control can be performed by increasing the value of the integer n, extending the modulation time Tosc, or decreasing the value of the integer n to shorten the modulation time Tosc.
Thus, the system of projecting a picture to be displayed using a plurality of spatial light modulation elements can set substantially equal timing of the control time for each of the spatial light modulation elements, thereby reducing the motion artifacts and color artifacts of a picture to be displayed.
Furthermore, it is possible to oscillation-control the micromirror such that the luminance of the projected picture obtained by one oscillation of the micromirror controlled in the oscillation period T1 of the mirror by a control device can be 1/n of the luminance Lon2 of the projected picture obtained in the ON state of the memory controlled in the oscillation period T1.
In this case, the relationship between the luminance Losc obtained by the oscillation-control in a frame period of the picture data and the frequency m at which the oscillation of the micromirror is performed by the oscillation-control can be represented by the following equation.
Losc=Lon2×(1/n)×m
That is, to realize the same luminance Losc, control can be performed by increasing the value of an integer n to increase the frequency m at which the oscillation is performed, or decreasing the value of the integer n to decrease the frequency of the modulation.
Thus, the system of projecting a picture to be displayed using a plurality of spatial light modulation elements can set substantially equal timing of the control time for each of the spatial light modulation elements, thereby reducing the motion artifacts and color artifacts of a picture to be displayed.
As described above, according to the display system 100 of the present embodiment, more delicate gray scale can be realized in the picture display using a spatial light modulation element 200 to display a picture depending on the modulation state of a plurality of micromirrors 212 without increasing the amount of data of the digital picture data (binary picture signal 400).
In addition, without the necessity to perform complicated control such as increase/decrease of the quantity of light of the light source 510 or add an additional circuit, delicate gray scale can be realized on a picture display using the spatial light modulation element 200 for displaying a picture in the modulation state of the plurality of micromirror 212.
More delicate gray scale can be read in the picture display using a spatial light modulation element to display a picture depending on the modulation state of a plurality of micromirrors without increasing the speed of modulation-controlling the micromirrors into the ON state.
In addition, more delicate gray scale can be read in the picture display using a spatial light modulation element to display a picture depending on the modulation state of a plurality of micromirrors without complicated control of, for example, the quantity of light of a light source or an additional circuit.
The present invention is not limited to the configurations according to the above-mentioned embodiments, but various changes can be bade within the gist of the invention.

Claims

1. A display system, comprising:

a light source;

a spatial light modulator having a plurality of micromirrors and forming an image to be displayed from a light from said light source; and

a control device controlling said spatial light modulator, wherein

said control device comprising a modulation-control device generating a modulation control signal for said plurality of micromirrors depending on a digital image data input to the display system, and controlling said spatial light modulator;

a modulation state of said micromirrors by said modulation control signal includes modulation by oscillation of said micromirrors; and

said modulation-control device controls a plurality of voltage value to be applied to a driving electrode of said micromirror so that said amplitude of said oscillation can be equal to or smaller than the maximum amplitude of said micromirrors.

wherein a number of said voltage value is at least three.

2. The display system according to claim 1, wherein

said control device comprises a data conversion device converting a part or all of said digital image data into a non-binary data; and

said modulation-control device generates a modulation control signal of said micromirrors depending on said non-binary data, and controls said spatial light modulator.

3. The display system according to claim 1, wherein

said modulation control signal controls an offset voltage to be applied to said micromirror.

4. The display system according to claim 3, wherein

said offset voltages are a plurality of different voltage values.

5. The display system according to claim 1, wherein

a time duration for applying said voltage value to a driving electrode is shorter than a quarter of said oscillation period of said micromirrors.

6. The display system according to claim 1, wherein

a time duration for applying said voltage value to a driving electrode is shorter than a quarter of a least significant bit (LSB) period for control of said micromirrors.

7. The display system according to claim 1, wherein

said oscillation having said amplitude includes free oscillation which decreases its amplitude with time.

8. The display system according to claim 1, wherein

a time duration in which said oscillation is repeated is longer than one oscillation period of said micromirrors.

9. The display system according to claim 1, wherein

a number of times at which said oscillation of said micromirrors is repeated in said time duration is two times or more in one frame period of said digital image data.

10. The display system according to claim 1, wherein

said modulation-control device generates the modulation control signal to perform modulation by said oscillation of the micromirrors based on at least 1-bit data other than a most significant bit (MSB) in a plurality of bits forming said digital image data input to said display system.

11. The display system according to claim 1, wherein

an oscillating state having an amplitude equal to or smaller than the maximum amplitude of said micromirrors is controlled to turn from an ON state of said micromirrors.

12. The display system according to claim 1, wherein

an oscillating state having an amplitude equal to or smaller than the maximum amplitude of said micromirrors is controlled to turn from an OFF state of said micromirrors.

13. The display system according to claim 1, wherein

an oscillating state having an amplitude equal to or smaller than the maximum amplitude of said micromirrors is controlled to turn to an ON state of said micromirrors.

14. The display system according to claim 1, wherein

an oscillating state having an amplitude equal to or smaller than the maximum amplitude of said micromirrors is controlled to turn to an OFF state of said micromirrors.