JP7714972B2 - Light modulation device and projection display device - Google Patents

Description

The present invention relates to a light modulation device and a projection display device.

Projection display devices use a light modulation device to modulate illumination light according to the image to be projected, thereby enlarging and projecting the desired image onto a screen. The light modulation device includes a liquid crystal display element as one of its components. The liquid crystal display element has pixel electrodes, a common electrode positioned opposite the pixel electrodes, and liquid crystal sandwiched between the pixel electrodes and the common electrode.

The optical modulator applies a DC voltage, known as Vcom, to the common electrode of the liquid crystal display element. The optical modulator applies an AC voltage that inverts at frame or line intervals between the pixel electrode and the common electrode, optically modulating the incident illumination light. During this process, asymmetry can occur between the positive and negative polarities of the AC voltage applied between the pixel electrode and the common electrode due to variations in the characteristics of the transistors that drive the pixel electrodes, variations in the electro-optical properties of the liquid crystal material, variations in the alignment conditions of the liquid crystal, impurities in the liquid crystal, and differences in the materials used for the pixel electrode and the common electrode.

When asymmetry occurs between the positive and negative polarities of the AC voltage, Vcom deviates from the central voltage at which the AC voltage is inverted, and a DC voltage component is applied to the LCD display element, causing flicker in the projected image. When flicker occurs in the projected image, contrast decreases and the display gradation becomes narrower. In addition, because a DC voltage component is applied to the LCD display element, ionic substances in the liquid crystal are attracted to one of the electrodes, causing image burn-in and reducing the reliability of the LCD display element's operation.

Patent Documents 1 and 2 describe suppressing flicker by using an optical sensor to detect light flicker caused by flicker and adjusting Vcom.

JP 2009-193044 A JP 2011-2745 A

The configuration described in Patent Document 1 requires the projection of an image with a specific pattern to correct deviations from the Vcom inversion center voltage. The configuration described in Patent Document 1 cannot correct deviations in Vcom when projecting a normal image based on an input image signal. The configuration described in Patent Document 2 does not require the projection of an image with a specific pattern, but requires double-speed conversion processing in which the frame rate is doubled and each frame is read out twice in succession. The configuration described in Patent Document 2 detects and corrects deviations in Vcom by setting the polarity to positive polarity on the first readout of each frame and negative polarity on the second readout.

In recent years, projection display devices have increasingly adopted a frame rate of 120 Hz for input image signals, double the conventional 60 Hz, in order to improve resolution in the time direction. If the configuration described in Patent Document 2 is adopted when the input image signal frame rate is 120 Hz, the frame rate must be increased to 240 Hz. Operating a projection display device at a frame rate of 240 Hz would result in a significant increase in circuit size and cost, making it practically impossible to adopt the configuration described in Patent Document 2.

The present invention aims to provide an optical modulation device and a projection display device that can detect and correct deviations in common electrode voltage without the need to project a specific pattern image or perform double-speed conversion of the frame rate of the input image signal to correct deviations in common electrode voltage.

The present invention relates to a liquid crystal display element having a pixel electrode, a common electrode to which a common electrode voltage is applied, and a liquid crystal sandwiched between the pixel electrode and the common electrode, which optically modulates incident first polarized light, which is linearly polarized in one direction, based on an input image signal and emits the modulated light; a reflective polarizer that transmits the first polarized light and reflects second polarized light, which is linearly polarized in one direction perpendicular to the first polarized light, of the modulated light emitted from the liquid crystal display element; and a polarizer that transmits the second polarized light reflected by the reflective polarizer and reflects the first polarized light. a transmission polarizer; an optical sensor that receives the first polarized light reflected by the transmission polarizer and outputs a detection signal; a liquid crystal drive circuit that inverts the polarity of the input image signal between positive and negative polarities with respect to the common electrode voltage for each vertical period and supplies the inverted signal to the pixel electrodes; a positive-side peak absolute value that is an absolute value of the peak of the detection signal output from the optical sensor during a vertical period in which the liquid crystal drive circuit supplies the input image signal to the pixel electrodes with a positive polarity; An optical modulation device is provided that includes: a detection circuit that calculates the difference between the negative polarity peak absolute value, which is the absolute value of the peak of the detection signal output from the optical sensor during the vertical period being supplied, and generates a first flag as a primary flag if the difference is zero, a second flag as a primary flag if the difference is positive, and a third flag as a primary flag if the difference is negative; a selection circuit that compares two adjacent frames of the input image signal to determine whether the two frames are substantially the same image, outputs the primary flag as a secondary flag if it is determined that the two images are substantially the same, and replaces the second and third flags with the first flag and outputs the resulting secondary flag if it is determined that the two images are not substantially the same; and a correction circuit that does not correct the common electrode voltage if the secondary flag is the first flag, corrects the common electrode voltage to lower it if the secondary flag is the second flag, and corrects the common electrode voltage to higher if the secondary flag is the third flag.

The present invention provides a projection display device comprising: a dichroic mirror that separates a white light beam emitted from a light source into randomly polarized red, green, and blue light beams; a polarization conversion element that aligns the randomly polarized red, green, and blue light beams into a first polarized light that is linearly polarized in one direction and emits the light; the above-mentioned light modulation device provided in the optical path of the red, green, and blue light beams aligned to the first polarized light; a color synthesis prism that color-combines the red, green, and blue light beams that have passed through the transmissive polarizer in the light modulation device into a second polarized light that is linearly polarized in one direction perpendicular to the first polarized light; and a projection lens that enlarges and projects the combined light color-combined by the color synthesis prism.

The optical modulation device and projection display device of the present invention can detect and correct deviations in the common electrode voltage without the need to project a specific pattern image or perform double-speed conversion of the frame rate of the input image signal to correct deviations in the common electrode voltage.

1 is a diagram showing a configuration of a projection display device according to an embodiment; 1 is a partial side view of a polarization conversion element included in a projection display device according to an embodiment. 2 is an enlarged view of the portion of FIG. 1 surrounded by the dashed line and indicated by reference numeral 50. FIG. FIG. 1 is a conceptual perspective view showing a wire grid polarizer used as a reflective polarizing plate included in a projection display device according to an embodiment. FIG. 2 is a partial enlarged view showing a light beam incident on an optical sensor included in the optical modulation device according to the embodiment. 1 is a partially enlarged view showing an optical sensor included in an optical modulation device according to an embodiment, the optical sensor being disposed at a position where it is not affected by a light beam that should not be incident on the optical sensor. 1 is a diagram illustrating a schematic configuration of an optical modulation device according to an embodiment. FIG. 2 is a block diagram showing a detailed configuration example of a detection circuit, a selection circuit, and a correction circuit included in the optical modulation device of the embodiment. 10 is a flowchart illustrating a process executed by an optical modulation device according to an embodiment. 4 is a timing chart showing the operation of the projection display device according to the embodiment. 4 is a timing chart showing the operation of the projection display device according to the embodiment. 4 is a timing chart showing the operation of the projection display device according to the embodiment.

An optical modulation device and a projection display device according to one embodiment will now be described with reference to the accompanying drawings. The projection display device 1 shown in Figure 1 is configured as follows. In Figure 1, a light source 2 having a xenon lamp 2a and a concave mirror 2b emits high-intensity white light. An infrared transmission filter 3 is positioned at a 45-degree angle on the optical path of the white light. The infrared transmission filter 3 transmits infrared light with wavelengths of approximately 700 nm or greater and reflects light beams in other wavelength bands. This removes infrared light with wavelengths of approximately 700 nm or greater.

Light beams in wavelength bands other than the infrared band have their optical path bent 90 degrees by the infrared transmission filter 3 and enter the ultraviolet reflection filter 4. The ultraviolet reflection filter 4 reflects ultraviolet light with wavelengths of around 400 nm or less and transmits light beams in other wavelength bands. This removes ultraviolet light with wavelengths of around 400 nm or less. The white light from which the infrared and ultraviolet light have been removed enters the dichroic mirror 5, which is tilted 45 degrees relative to the optical path. The dichroic mirror 5 reflects blue light (hereinafter referred to as B light) and transmits red-green light (hereinafter referred to as RG light), thereby separating the incident white light into B light and RG light.

Arranged on the optical path of the B light reflected by the dichroic mirror 5 are a reflecting mirror 6, a first fly-eye lens 7b for B light, a second fly-eye lens 8b for B light, a polarization conversion element 9b, a condenser lens 10b, a field lens 11b, a reflective polarizer 13b, and a reflective liquid crystal display element 12b for B light, in this order. The reflecting mirror 6 is tilted at 45 degrees with respect to the optical path of the B light emitted from the dichroic mirror 5, and bends the optical path of the B light by 90 degrees. The reflective liquid crystal display element 12b and the reflective liquid crystal display elements 12r and 12g, described below, are examples of liquid crystal display elements.

Because the cross section of the white light emitted from the concave mirror 2b of the light source 2 is circular, the cross section of the light beam reflected by the reflecting mirror 6 is also circular. In order to efficiently illuminate the circular cross section light beam reflected by the reflecting mirror 6 onto the effective pixel area of the rectangular reflective LCD element 12b, the circular light beam must be converted into a rectangular light beam.

The first fly-eye lens 7b has a configuration in which small rectangular convex lenses are arranged in a matrix. The second fly-eye lens 8b also has a configuration in which small rectangular convex lenses are arranged in a matrix. Each convex lens of the second fly-eye lens 8b is positioned at the focal position of each convex lens of the first fly-eye lens 7b. The first fly-eye lens 7b and the second fly-eye lens 8b match the shape of the B light beam to the shape of the reflective LCD display element 12b, and split the incident beam into multiple rectangular partial beams in order to uniform the illuminance distribution on the reflective LCD display element 12b.

The partial beams of B light emerging from the second fly-eye lens 8b are randomly polarized light with inconsistent polarization. The polarization conversion element 9b converts the multiple randomly polarized partial beams into multiple partial beams of linearly polarized light in one direction. The polarization conversion element 9b is configured as shown in Figure 2.

As shown in Figure 2, polarization separation surfaces 92, shown by dashed lines, and reflection surfaces 93, shown by solid lines, are alternately formed inside the flat polarizing beam splitter array 91 in the direction along the surface. The polarization separation surfaces 92 and reflection surfaces 93 are formed at a 45-degree angle with respect to the plane of incidence of light on the polarization conversion element 9b. The polarization separation surface 92 transmits P-polarized light (first polarization) and reflects S-polarized light (second polarization). The S-polarized light has its direction of travel bent 90 degrees by the polarization separation surface 92 and heads toward the reflection surface 93. The reflection surface 93 reflects the incident S-polarized light and bends its direction of travel by 90 degrees.

A birefringent half-wave plate 94 is attached to the area sandwiched between the polarization separation surface 92 and the reflection surface 93 and which emits S-polarized light reflected by the reflection surface 93. The half-wave plate 94 is not attached to the area sandwiched between the polarization separation surface 92 and the reflection surface 93 and which emits P-polarized light transmitted through the polarization separation surface 92. The half-wave plate 94 rotates the polarization direction of the incident S-polarized light by 90 degrees and emits P-polarized light. In this way, the polarization conversion element 9b aligns the incident randomly polarized light into P-polarized light, which is linearly polarized light in one direction, and emits it.

The polarization conversion element 9b and the polarization conversion element 9rg, which will be described later, may convert the incident randomly polarized light into S-polarized light, which is linearly polarized light in one direction, and then emit the light. When the polarization conversion element 9b and the polarization conversion element 9rg, which will be described later, emit S-polarized light, the light modulation devices for each color, which will be described later, are configured to correspond to the incident S-polarized light. In this case, the first polarized light is S-polarized light, and the second polarized light is P-polarized light.

The condenser lens 10b combines multiple partial beams of linearly polarized light (P polarization) emitted from the polarization conversion element 9b and emits a single combined beam. The field lens 11b converts the incident linearly polarized beam into telecentric illumination light of B light.

Figure 3 shows an enlarged view of the area indicated by the reference numeral 50 in Figure 1, surrounded by a dashed line. In Figures 1 and 3, rectangular openings are formed on three sides of the approximately triangular prism-shaped support 20b, and a reflective liquid crystal display element 12b, a reflective polarizer 13b, and a transmissive polarizer 15b are fixed to the inner surfaces of the three sides, covering each opening. The reflective polarizer 13b is fixed to the side of the support 20b facing the field lens 11b, and the B light emitted from the field lens 11b is incident on the reflective polarizer 13b. The reflective polarizer 13b is inclined at a 45-degree angle with respect to the optical path of the B light. A heat sink 19b for heat dissipation is fixed to the outer surface of the side to which the reflective liquid crystal display element 12b is fixed.

As shown in Figure 4, a wire grid polarizer is used as the reflective polarizer 13b. The reflective polarizer 13b, which is a wire grid polarizer, has a reflective surface in which metal wires 132, such as aluminum, are formed in a regular stripe pattern with a pitch of, for example, about 140 nm on a polarizer optical glass plate 131. The reflective polarizer 13b has the property of transmitting the polarized light component of the incident light beam that is perpendicular to the striped metal wires 132 (here, P-polarized light) and reflecting the polarized light component that is parallel to the metal wires 132 (here, S-polarized light).

Using a wire grid polarizer as the reflective polarizer 13b allows the reflective polarizer 13b to be made lighter. Because wire grid polarizers do not easily absorb incident light, using a wire grid polarizer as the reflective polarizer 13b can reduce degradation in the quality of the projected image caused by birefringence in the glass due to heat generation.

Returning to Figure 1 or 3, reflective polarizer 13b transmits the B light emitted from field lens 11b. The B light that passes through reflective polarizer 13b enters reflective liquid crystal display element 12b. Reflective liquid crystal display element 12b modulates the light based on the B signal from the RGB signals that make up the input image signal, and reflects and emits the B light (S-polarized) whose polarization state has been changed by the optical modulation as modulated light. Reflective polarizer 13b reflects the B light reflected by reflective liquid crystal display element 12b. The B light reflected by reflective polarizer 13b passes through transmissive polarizer 15b and enters cross dichroic prism 16. Cross dichroic prism 16 is an example of a color synthesis prism.

Transmissive polarizer 15b and transmissive polarizers 15g and 15r, which will be described later, are provided to improve the degree of polarization of S-polarized light and thereby improve contrast by reflecting and removing unwanted polarized light (P-polarized light) contained in the incident B light.

In FIG. 1, the optical path of the RG light transmitted through the dichroic mirror 5 is arranged in this order: a first fly-eye lens 7rg for RG light, a second fly-eye lens 8rg for RG light, a polarization conversion element 9rg, a condenser lens 10rg, and a dichroic mirror 14. The first fly-eye lens 7rg, the second fly-eye lens 8rg, the polarization conversion element 9rg, and the condenser lens 10rg have the same configurations and functions as the first fly-eye lens 7b, the second fly-eye lens 8b, the polarization conversion element 9b, and the condenser lens 10b, respectively.

The dichroic mirror 14 reflects the green light (hereinafter referred to as G light) of the RG light and transmits the red light (hereinafter referred to as R light), thereby separating the incident RG light into R light and G light.

A field lens 11r, a reflective polarizer 13r, and a reflective LCD display element 12r for R light are arranged in this order on the optical path of the R light that has passed through the dichroic mirror 14. The field lens 11r converts the incident linearly polarized light beam into telecentric illumination light for R light.

Rectangular openings are formed on three sides of the approximately triangular prism-shaped support 20r, and a reflective LCD element 12r, a reflective polarizer 13r, and a transmissive polarizer 15r are fixed to the inner surfaces of the three sides so as to cover each opening.

Reflective polarizer 13r is fixed to the side of support 20r facing field lens 11r, and receives the R light emitted from field lens 11r. Reflective polarizer 13r is inclined at a 45-degree angle with respect to the optical path of the R light. A heat sink 19r for heat dissipation is fixed to the outer surface of the side to which reflective LCD element 12r is fixed.

Reflective polarizer 13r transmits the R light emitted from field lens 11r. The R light that passes through reflective polarizer 13r enters reflective liquid crystal display element 12r. Reflective liquid crystal display element 12r modulates the R signal from the RGB signals that make up the input image signal, and reflects the R light (S-polarized) whose polarization state has been changed by the optical modulation, and emits it as modulated light. Reflective polarizer 13r reflects the R light reflected by reflective liquid crystal display element 12r. The R light reflected by reflective polarizer 13r passes through transmissive polarizer 15r and enters cross dichroic prism 16.

A field lens 11g, a reflective polarizing plate 13g, and a reflective LCD display element 12g for G light are arranged in this order on the optical path of the G light reflected by the dichroic mirror 14. The field lens 11g converts the incident linearly polarized light beam into telecentric illumination light of G light.

Rectangular openings are formed on three sides of the approximately triangular prism-shaped support 20g, and a reflective LCD element 12g, a reflective polarizer 13g, and a transmissive polarizer 15g are fixed to the inner surfaces of the three sides so as to cover each opening.

Reflective polarizer 13g is fixed to the side of support 20g facing field lens 11g, and receives the G light emitted from field lens 11g. Reflective polarizer 13g is inclined at a 45-degree angle with respect to the optical path of the G light. A heat sink 19g for heat dissipation is fixed to the outer surface of the side to which reflective LCD element 12g is fixed.

Reflective polarizing plate 13g transmits the G light emitted from field lens 11g. The G light that passes through reflective polarizing plate 13g enters reflective liquid crystal display element 12g. Reflective liquid crystal display element 12g modulates the G light based on the G signal from the RGB signals that make up the input image signal, and reflects the G light (S-polarized) whose polarization state has been changed by the optical modulation, and emits it as modulated light. Reflective polarizing plate 13g reflects the G light reflected by reflective liquid crystal display element 12g. The G light reflected by reflective liquid crystal display element 12g passes through transmissive polarizing plate 15g and enters cross dichroic prism 16.

The cross dichroic prism 16 combines the R, G, and B light beams incident on the three sides and emits the combined light from the exit surface 16a. The projection lens 17 enlarges and projects the combined light emitted from the exit surface 16a onto the screen 18.

As shown in Figure 1, optical sensors 21r, 21g, and 21b are disposed on the outside of supports 20r, 20g, and 20b, respectively, on the optical path of unwanted polarized light reflected by transmissive polarizers 15r, 15g, and 15b. Optical sensors 21r, 21g, and 21b convert the incident light into an electrical signal according to the amount of light. Optical sensors 21r, 21g, and 21b convert the amount of light into a voltage, current, frequency, or the like, and output it as a detection signal.

Using Figures 5 and 6, we will explain where optical sensors 21r, 21g, and 21b are located near support bodies 20r, 20g, and 20b, respectively. Here, we will explain the position of optical sensor 21b relative to support body 20b as a representative example.

In Figure 5, P-polarized B light beam Ha emerging from condenser lens 10b is refracted by field lens 11b to match the size of the display area (effective pixel area) of reflective LCD element 12b and enters reflective polarizer 13b. P-polarized light beam Hb that passes through reflective polarizer 13b is optically modulated by reflective LCD element 12b, and a portion of it becomes S-polarized, is reflected, and re-enters reflective polarizer 13b. S-polarized light beam Hb is reflected by reflective polarizer 13b and enters transmissive polarizer 15b as light beam Hc.

Transmissive polarizer 15b transmits S-polarized light and reflects unwanted P-polarized light. Optical sensor 21b is positioned on the P-polarized light beam Hd, indicated by the dashed line, reflected by transmissive polarizer 15b.

As shown in Figure 6, the reflective polarizing plate 13b does not transmit 100% of the incident P-polarized B light beam Ha, but rather reflects a small percentage of the P-polarized light and also reflects the small amount of S-polarized light contained in the incident light beam Ha. Therefore, a light beam He, indicated by the dashed line and containing both P-polarized and S-polarized light, is emitted from the surface of the reflective polarizing plate 13b facing the field lens 11b toward the outside of the support 20b. This light beam He is not optically modulated by the reflective LCD element 12b, and is therefore not affected by flicker caused by the reflective LCD element 12b. In addition, the light beam He has a greater amount of light than the light beam Hd that is reflected by the transmissive polarizing plate 15b and is affected by flicker caused by the reflective LCD element 12b.

For this reason, the optical sensor 21b must be positioned above the light beam Hd reflected by the transmissive polarizer 15b, avoiding the light beam He reflected by the reflective polarizer 13b, and in a position where it is not affected by the light beam He.

In this way, the optical sensors 21r, 21g, and 21b are positioned above the light beam Hd reflected by the transmissive polarizers 15r, 15g, and 15b, and are not affected by the light beam He reflected by the reflective polarizers 13r, 13g, and 13b.

Although only one optical sensor 21r, 21g, and 21b are shown in FIG. 1, multiple optical sensors 21r, 21g, and 21b may be arranged to receive a light beam Hd that includes reflected light modulated by multiple pixels in the effective pixel area of the reflective liquid crystal display elements 12r, 12g, and 12b. It is preferable that the multiple optical sensors 21r, 21g, and 21b be arranged in two orthogonal directions so as to receive a light beam Hd that includes reflected light modulated by a rectangular area consisting of multiple horizontal and vertical pixels in the effective pixel area.

Each of the multiple optical sensors 21r, 21g, and 21b has a single light receiving element. Alternatively, a single optical sensor 21r, 21g, or 21b may have multiple light receiving elements. The multiple light receiving elements receive reflected light modulated by multiple pixels in the reflective liquid crystal display elements 12r, 12g, and 12b.

The correction operation of the common electrode voltage (hereinafter referred to as Vcom) will be explained in detail using Figures 7 to 9. As shown in Figure 7, the optical modulation device 60 for B light includes a reflective liquid crystal display element 12b, a reflective polarizer 13b, a transmissive polarizer 15b, an optical sensor 21b, a liquid crystal drive circuit 22b, a detection circuit 23b, a selection circuit 24b, and a correction circuit 25b. The reflective liquid crystal display element 12b has a pixel electrode 121, a common electrode 122, and liquid crystal 123 sealed between the pixel electrode 121 and the common electrode 122.

The liquid crystal drive circuit 22b inverts the polarity of the input image signal (R signal) between positive and negative polarities every vertical period, centered around Vcom, and supplies it to the pixel electrode 121. The detection circuit 23b detects the state of Vcom based on the detection signal from the optical sensor 21b and various drive signals output from the liquid crystal drive circuit 22b. As described below, the detection circuit 23b generates a primary flag based on the detection result of the Vcom state. As described below, the selection circuit 24b outputs a secondary flag. The correction circuit 25b corrects Vcom in accordance with the secondary flag and supplies the corrected Vcom to the common electrode 122 of the reflective liquid crystal display element 12b.

The reflective LCD display element 12r for R light is also provided with an R light optical modulation device 60 that includes a liquid crystal drive circuit 22r, a detection circuit 23r, a selection circuit 24r, and a correction circuit 25r, similar to the liquid crystal drive circuit 22b, the detection circuit 23b, the selection circuit 24b, and the correction circuit 25b.The reflective LCD display element 12g for G light is also provided with a G light optical modulation device 60 that includes a liquid crystal drive circuit 22g, a detection circuit 23g, a selection circuit 24g, and a correction circuit 25g, similar to the liquid crystal drive circuit 22b, the detection circuit 23b, the selection circuit 24b, and the correction circuit 25b.

Figure 8 shows a specific example configuration of detection circuits 23r, 23g, 23b, correction circuits 25r, 25g, 25b, and selection circuits 24r, 24g, 24b. Detection circuits 23r, 23g, 23b include an AD converter 231, a timing generator 232, a positive polarity data hold circuit 233, a negative polarity data hold circuit 234, a subtractor 235, and a flag generator 236. Selection circuits 24r, 24g, 24b include a frame memory 241, a comparator 242, and a flag selector 243. Correction circuits 25r, 25g, 25b include a data hold circuit 251, an arithmetic unit 252, and a DA converter 253.

The operation of the detection circuit 23b, selection circuit 24b, and correction circuit 25b will be described below with reference to the flowcharts shown in Figures 8 and 9. The operation of the detection circuit 23b is as follows: When the power to the projection display device 1 is turned on, the projection display device 1 starts the process shown in Figure 9. The AD converter 231 AD converts the detection signal output by the optical sensor 21b (step S1 in Figure 9). The digital detection signal is supplied to the positive polarity data hold circuit 233 and the negative polarity data hold circuit 234.

The liquid crystal drive circuit 22b inverts the polarity of the input image signal based on horizontal and vertical scanning signals synchronized with the input image signal and a polarity switching signal that alternates between high and low every vertical period. The liquid crystal drive circuit 22b supplies the polarity switching signal and horizontal and vertical synchronization signals to the timing generator 232. The timing generator 232 supplies positive and negative trigger signals to the positive data hold circuit 233 and negative data hold circuit 234, respectively.

The positive data hold circuit 233 and the negative data hold circuit 234 each store the detection signal supplied from the AD converter 231. The positive data hold circuit 233 detects positive peaks from the stored detection signal during the vertical period in which the liquid crystal drive circuit 22b supplies positive input image signals to the pixel electrodes 121, and outputs a positive peak absolute value, which is the absolute value of the peak (step S2 in Figure 9). The negative data hold circuit 234 detects positive peaks from the stored detection signal during the vertical period in which the liquid crystal drive circuit 22b supplies negative input image signals to the pixel electrodes 121, and outputs a negative peak absolute value, which is the absolute value of the peak (step S3 in Figure 9).

The subtractor 235 calculates the difference between the positive polarity side peak absolute value and the negative polarity side peak absolute value (step S3). As an example, the subtractor 235 subtracts the negative polarity side peak absolute value from the positive polarity side peak absolute value. If Vcom is higher than the inversion center voltage, when a positive polarity side projection image is displayed, the amount of S-polarized light will be less than the specified amount and the amount of P-polarized light will be more than the specified amount. Conversely, when a negative polarity side projection image is displayed, the amount of S-polarized light will be more than the specified amount and the amount of P-polarized light will be less than the specified amount.

Since optical sensor 21b detects P-polarized light, which is not used to form a projected image, a positive difference obtained by subtracting the negative peak absolute value from the positive peak absolute value means that Vcom is too high. Conversely, a negative difference means that Vcom is too low.

The flag generator 236 generates, for example, a 2-bit flag according to the difference calculated by the subtractor 235. The flag generator 236 is supplied with a 2-frame timing signal that goes high every 2 frames, and generates and outputs a flag when the 2-frame timing signal goes high.

If the difference output by the subtractor 235 is 0, the flag generator 236 generates the first flag "00" as the primary flag; if the difference is positive, the flag generator 236 generates the second flag "11" as the primary flag; and if the difference is negative, the flag generator 236 generates the third flag "01" as the primary flag (steps S4 to S8 in Figure 9). The first to third flags "00", "11", and "01" of the primary flags represent the decimal values "0", "3", and "1", respectively. The flag generator 236 outputs a primary flag that is one of the first to third flags "00", "11", and "01" every two frames.

The operation of the selection circuit 24b is as follows. An input image signal F(n) is input to the selection circuit 24b. The frames of the input image signal progress as follows: ...F(n-1), F(n), F(n+1)... The input image signal F(n) can sometimes refer to a series of frames collectively.

Frame memory 241 delays the input image signal F(n) by one frame period and outputs the previous frame's image signal F(n-1). The frames output from frame memory 241 progress as follows: F(n-2), F(n-1), F(n), etc. Similarly, image signal F(n-1) can refer collectively to a series of frames delayed by one frame.

Comparator 242 compares input image signal F(n) with image signal F(n-1), and if the pixel values at the same pixel positions match for a specified percentage or more of all pixels in one frame, it outputs a "1" indicating a match as the comparison result signal; otherwise, it outputs a "0" indicating a mismatch. The specified percentage can be set to an appropriate value, such as 90%, for example.

If pixel values at the same pixel positions match for a specified percentage or more of all pixels in one frame, it means that two adjacent frames in the input image signal F(n) contain substantially the same image. Comparator 242 determines whether the two adjacent frames contain substantially the same image (step S9 in Figure 9). If comparator 242 determines that the two adjacent frames contain substantially the same image, it outputs a "1" as the comparison result signal, and if it determines that the two adjacent frames do not contain substantially the same image, it outputs a "0" as the comparison result signal.

The accuracy of the comparator 242's determination of match/mismatch may be based on the level of Vcom adjustment accuracy. If Vcom adjustment accuracy does not need to be very high, the comparator 242 may simply determine match/mismatch. For example, if each pixel data is 12 bits, the most significant 10 bits may be used to determine whether the pixel values at the same pixel position in the input image signal F(n) and the image signal F(n-1) match.

Furthermore, the comparator 242 does not have to determine whether the pixel values match for all pixels in a frame, but may instead determine whether the pixel values match for some pixels within the frame. The comparator 242 may determine whether the pixel values match for some pixels corresponding to the spatial range in which the optical sensor 21b detects the luminous flux Hd. The comparator 242 only needs to determine whether the pixel values match so as to ensure that the adjustment accuracy of Vcom is at or above a predetermined accuracy.

If the comparison result signal supplied from the comparator 242 is "1" indicating a match, the flag selector 243 outputs the primary flag output from the flag generator 236 as the secondary flag (step S10 in Figure 9). If the comparison result signal is "0" indicating a mismatch, the flag selector 243 outputs a different flag as the secondary flag depending on which of the first to third flags the primary flag is.

If the primary flag is the first flag "00", the flag selector 243 outputs the first flag "00" as is as the secondary flag. If the primary flag is the second flag "11" or the third flag "01", the flag selector 243 replaces the second flag "11" or the third flag "01" with the first flag "00" and outputs it as the secondary flag (step S11 in Figure 9). In other words, if the comparison result signal is "0", indicating a mismatch, the flag selector 243 fixedly outputs the first flag "00" as the secondary flag, regardless of the primary flag output by the flag generator 236.

The first flag "00" indicates that Vcom is at the inversion center voltage and there is no need to correct Vcom. The second flag "11" indicates that Vcom is higher than the inversion center voltage and Vcom needs to be corrected lower. The third flag "01" indicates that Vcom is lower than the inversion center voltage and Vcom needs to be corrected higher. If two adjacent frames contain different images, the determination result of whether Vcom is higher or lower than the inversion center voltage may be incorrect. When the comparison result signals do not match, the second flag "11" or third flag "01" is replaced with the first flag "00" to avoid correcting Vcom when there is a possibility that the determination result may be incorrect.

The operation of the correction circuit 25b is as follows. The secondary flag output by the selection circuit 24b is input to the calculator 252. The data hold circuit 251 holds the initial Vcom value, which is a digital value equivalent to the inversion center voltage of Vcom. As mentioned above, even if Vcom is set based on the initial Vcom value, various conditions, such as variations in the characteristics of the pixel electrode driving transistors, may cause asymmetry between the positive and negative polarities of the AC voltage, making it necessary to correct Vcom. Note that it is not necessary to hold the initial Vcom value in the data hold circuit 251. Even if the initial Vcom value is not held in the data hold circuit 251, the Vcom correction value held in the data hold circuit 251 is updated in sequence, and the optimal Vcom correction value can be obtained after a predetermined time.

When the 2-frame timing signal goes high, the data hold circuit 251 reads the stored value. The calculator 252 reads the Vcom initial value from the data hold circuit 251. If the secondary flag is the first flag "00", the calculator 252 does not correct the Vcom initial value, stores it in the data hold circuit 251, and supplies it to the DA converter 253.

When the Vcom initial value is read from the data hold circuit 251, the calculator 252 subtracts one bit from the Vcom initial value if the secondary flag is the second flag "11", stores the resulting Vcom correction value in the data hold circuit 251, and supplies it to the DA converter 253 (step S12 in Figure 9). When the Vcom initial value is read from the data hold circuit 251, the calculator 252 adds one bit to the Vcom initial value if the secondary flag is the third flag "01", stores the resulting Vcom correction value in the data hold circuit 251, and supplies it to the DA converter 253 (step S12 in Figure 9).

Similarly, after the Vcom correction value is stored in the data hold circuit 251, the calculator 252 reads the Vcom correction value from the data hold circuit 251. If the secondary flag is the second flag "11", the calculator 252 subtracts one bit from the Vcom correction value to create a new Vcom correction value, stores the new Vcom correction value in the data hold circuit 251, and supplies the new Vcom correction value to the DA converter 253 (step S12 in FIG. 9). The calculator 252 reads the Vcom correction value from the data hold circuit 251. If the secondary flag is the third flag "01", the calculator 252 adds one bit to the Vcom correction value to create a new Vcom correction value, stores the new Vcom correction value in the data hold circuit 251, and supplies the new Vcom correction value to the DA converter 253 (step S12 in FIG. 9).

The data hold circuit 251 and the calculator 252 repeatedly update the Vcom correction value, thereby supplying the optimal Vcom correction value to the DA converter 253. The DA converter 253 converts the input Vcom correction value from digital to analog and applies the analog value Vcom to the common electrode 122 (step S13 in Figure 9).

In this way, the correction circuit 25b does not correct Vcom if the secondary flag is the first flag "00", corrects Vcom to be lower if the secondary flag is the second flag "11", and corrects Vcom to be higher if the secondary flag is the third flag "01". The correction circuit 25b can always apply Vcom that matches the inversion center voltage to the common electrode 122.

If the power supply to the projection display device 1 is turned off in step S14 of FIG. 9, the projection display device 1 ends the processing shown in FIG. 9. If the power supply to the projection display device 1 is not turned off, the projection display device 1 repeats the processing from step S1 onwards. The correction circuit 25b repeats the operation of correcting Vcom every two frames.

The lower bits of the first flag "00", the second flag "11", and the third flag "01" represent the number of bits to be added to or subtracted from the Vcom correction value (or Vcom initial value), with a higher-order bit of "1" indicating subtraction and a higher-order bit of "0" indicating addition. Calculator 252 can be configured with a digital circuit in which the higher-order and lower-order bits of the secondary flags are defined in this way. If calculator 252 is configured with such a digital circuit, even if "10", which represents the undefined decimal number "2", is mistakenly input, Vcom will not be corrected, and malfunctions can be avoided.

In FIG. 8, the timing generator 232 generates positive and negative trigger signals during the blanking period of the input image signal F(n) based on the horizontal and vertical synchronization signals, and supplies these signals to the positive data hold circuit 233 and the negative data hold circuit 234.

The optical sensor 21b is preferably positioned to receive modulated reflected light from pixels at the start or end of vertical scanning on the reflective LCD elements 12r, 12g, and 12b. This allows for detection of Vcom deviations at the end of one frame in which all pixels are displayed as positive or negative.

The operation of the projection display device 1 (light modulation device 60) will be explained using the timing charts shown in Figures 10A to 10C. In Figure 10A, (a) conceptually shows the waveforms of the input image signals F(n), F(n+1), etc., (b) shows the input image signals F(n), F(n+1), etc., and (c) shows the image signals F(n-1), F(n), etc. from one frame ago. The frame rate of the input image signals F(n), F(n+1), etc. is 120 Hz, so the period of one frame is 1/120 seconds.

(d) of Figure 10A shows the comparison results C(n), C(n+2), etc. of each pixel in two adjacent frames by comparator 242. (e) of Figure 10A shows comparison result signals 242(n-2), 242(n), 242(n+2), etc., which indicate whether or not the pixel values match for at least a specified percentage of all pixels in one frame. Based on the comparison results C(n), C(n+2), etc. obtained during one frame period, comparator 242 outputs comparison result signals 242(n), 242(n+2), etc., which are either "1" indicating a match or "0" indicating a mismatch, for the next two frames.

As shown in (f) of Figure 10B, liquid crystal drive circuits 22r, 22g, and 22b supply a polarity switching signal that alternates between high and low to timing generator 232. Liquid crystal drive circuits 22r, 22g, and 22b invert the polarity of input image signals F(n), F(n+1), etc. between positive and negative every vertical period, centered around Vcom, and supply them to pixel electrodes 121. Therefore, as shown in (g) of Figure 10B, the liquid crystal drive voltage alternates between positive and negative polarity. When the polarity switching signal is high, the input image signal is +F(n), +F(n+2), etc.; when the polarity switching signal is low, the input image signal is -F(n+1), -F(n+3), etc.

In Figure 10B, (h) indicates the vertical synchronization signal, (i) indicates the positive polarity trigger signal supplied by the timing generator 232 to the positive polarity data hold circuit 233, and (j) indicates the negative polarity trigger signal supplied by the timing generator 232 to the negative polarity data hold circuit 234. The positive polarity and negative polarity trigger signals go high at the beginning of each frame every two frames.

As shown in (k) of Figure 10C, the positive data hold circuit 233 outputs positive peak absolute values 233(n), 233(n+2), etc. every two frames. As shown in (m) of Figure 10C, the negative data hold circuit 234 outputs negative peak absolute values 234(n), 234(n+2), etc. every two frames. The timing at which the negative peak absolute values 234(n), 234(n+2), etc. are output is delayed by one frame period from the timing at which the positive peak absolute values 233(n), 233(n+2), etc. are output. Note that Figure 10C shows the same positive and negative trigger signals as Figure 10B (i) and (j).

As shown in (n) of Figure 10C, the subtractor 235 outputs difference signals 235(n), 235(n+2), etc. between the positive and negative peak absolute values. As shown in (o) of Figure 10C, the flag selector 243 outputs secondary flags 243(n), 243(n+2), etc.

As shown in (q) of Figure 10C, the data hold circuit 251 outputs Vcom correction values 251(n-2), 251(n), 251(n+2), etc. each time the two-frame timing signal shown in (p) goes high. As shown in (r) of Figure 10C, the calculator 252 outputs new Vcom correction values 252(n), 252(n+2), 252(n+4), etc. each time the two-frame timing signal goes high.

Vcom correction value 252(n) is Vcom correction value 251(n-2) + secondary flag 243(n). Vcom correction value 252(n+2) is Vcom correction value 251(n) + secondary flag 243(n+2). Vcom correction value 252(n+4) is Vcom correction value 251(n+2) + secondary flag 243(n+4). However, if secondary flag 243(n), 243(n+2), etc. is the first flag "00", calculator 252 does not perform addition or subtraction processing; if the second flag is "11", calculator 252 performs subtraction processing; and if the third flag is "01", calculator 252 performs addition processing.

As described above, the light modulation device 60 and the projection display device 1 equipped with the light modulation device 60 do not need to project a specific pattern image to correct Vcom deviation, and can constantly correct Vcom deviation while projecting a normal image based on an input image signal. This suppresses flicker in the projected image and reduces deterioration over time of the liquid crystal display elements (reflective liquid crystal display elements 12r, 12g, 12b). Because flicker in the projected image is suppressed, the projection display device 1 can display high-quality images. Because deterioration over time of the liquid crystal display elements is suppressed, the reliability of the projection display device 1 can be improved.

The optical modulation device 60 and the projection display device 1 equipped with the optical modulation device 60 eliminate the need to double the frame rate of the input image signal. Therefore, even if the frame rate of the input image signal is 120 Hz or higher, Vcom deviation can be corrected without significantly increasing circuit size or cost.

Since optical sensors 21r, 21g, and 21b are installed inside the projection display device 1, the projection display device 1 does not become larger. The presence of optical sensors 21r, 21g, and 21b does not interfere with replacing the projection lens 17. The shadows of optical sensors 21r, 21g, and 21b are not cast on the enlarged image projected onto the screen 18 by the projection lens 17.

In the projection display device 1 shown in Figure 1, a light modulation device 60 having the configuration shown in Figure 8 is provided in the light path of each color, but a light modulation device 60 may be provided in only the light paths of some of the colors. Of the R, G, and B lights, a light modulation device 60 may be provided only in the light path of G light, which is the brightest and most likely to cause noticeable flicker. A light modulation device 60 may be provided only in the light path of B light, which has a short wavelength and is more likely to affect organic materials used in optical components.

The configuration of this embodiment may be used in combination with a conventional configuration in which the optical sensor is located near the projected image displayed on the screen 18 or within the projection lens 17.

The present invention is not limited to the embodiment described above, and various modifications are possible without departing from the spirit of the present invention. The detection circuits 23r, 23g, 23b, the selection circuits 24r, 24g, 24b, and the correction circuits 25r, 25g, 25b may be configured using integrated circuits, or at least some of the circuits may be configured using a microcomputer.

REFERENCE SIGNS LIST 1 projection display device 2 light source 5, 14 dichroic mirror 9r, 9g, 9b polarization conversion element 16 cross dichroic prism 17 projection lens 12r, 12g, 12b reflective liquid crystal display element 13r, 13g, 13b reflective polarizer 15r, 15g, 15b transmissive polarizer 21r, 21g, 21b optical sensor 22r, 22g, 22b liquid crystal drive circuit 23r, 23g, 23b detection circuit 24r, 24g, 24b selection circuit 25r, 25g, 25b correction circuit

Claims (4)

a liquid crystal display element having pixel electrodes, a common electrode to which a common electrode voltage is applied, and liquid crystal sandwiched between the pixel electrodes and the common electrode, which optically modulates incident first polarized light, which is linearly polarized light in one direction, based on an input image signal, and emits the modulated light;
a reflective polarizing plate that transmits the first polarized light and reflects a second polarized light that is linearly polarized in one direction orthogonal to the first polarized light, out of the modulated light emitted from the liquid crystal display element;
a transmission polarizing plate that transmits the second polarized light reflected by the reflective polarizing plate and reflects the first polarized light;
an optical sensor that receives the first polarized light reflected by the transmission polarizing plate and outputs a detection signal;
a liquid crystal drive circuit that inverts the polarity of the input image signal between positive and negative polarities with respect to the common electrode voltage every vertical period and supplies the inverted signal to the pixel electrodes;
a detection circuit that calculates a difference between a positive polarity side peak absolute value, which is the absolute value of the peak of the detection signal output from the optical sensor during each vertical period in which the liquid crystal drive circuit supplies the input image signal to the pixel electrode with positive polarity, and a negative polarity side peak absolute value, which is the absolute value of the peak of the detection signal output from the optical sensor during each vertical period in which the liquid crystal drive circuit supplies the input image signal to the pixel electrode with negative polarity, and generates a first flag as a primary flag if the difference is zero, a second flag as a primary flag if the difference is positive, and a third flag as a primary flag if the difference is negative;
a selection circuit that compares two adjacent frames in the input image signal to determine whether they are substantially the same image, and if it is determined that they are substantially the same image, outputs the primary flag as a secondary flag as is, and if it is determined that they are not substantially the same image, replaces the second and third flags with the first flag and outputs the resulting secondary flag;
a correction circuit that does not correct the common electrode voltage if the secondary flag is the first flag, corrects the common electrode voltage to be lower if the secondary flag is the second flag, and corrects the common electrode voltage to be higher if the secondary flag is the third flag;
An optical modulation device comprising: The optical modulation device of claim 1, wherein the optical sensor is positioned to receive reflected light modulated by a pixel at a position where vertical scanning of the liquid crystal display element starts or ends. The optical modulation device of claim 1 or 2 includes a single optical sensor that receives the first polarized light reflected by the transmissive polarizer with multiple light receiving elements and outputs a detection signal, or multiple optical sensors that receive the first polarized light reflected by the transmissive polarizer with a single light receiving element and output a detection signal. a dichroic mirror that separates a white light beam emitted from a light source into randomly polarized red, green, and blue light beams;
a polarization conversion element that converts randomly polarized red, green, and blue light beams into first polarized light that is linearly polarized in one direction and then emits the first polarized light;
the light modulation device according to any one of claims 1 to 3, which is provided in the optical paths of the red light, green light, and blue light beams aligned to the first polarization;
a color combining prism that combines the red light, green light, and blue light transmitted through the transmission polarizing plate in the light modulation device into second polarized light beams that are linearly polarized in one direction orthogonal to the first polarized light;
a projection lens that enlarges and projects the combined light obtained by the color combining prism;
A projection display device comprising: