Liquid crystal image projection system

Abstract: A liquid crystal image projection system projects an enlargement of an image formed in an active matrix liquid crystal panel (17) onto a screen (19) by using a lamp (15) and a projector lens (18). The orientation of the liquid crystal panel is made such that a molecular axis vector (37) directing from an input light side end to an output light side end of each liquid crystal molecule in the middle of the liquid crystal layer has a vector component opposite to a scanning direction of the panel, and a convergent light is entered into the liquid crystal panel. The system can solve the problem of nonuniformity of the signal voltage-transmittance characteristic of the entire screen.

Description: This invention relates to a liquid crystal image projection system for modulating a light beam by an optical image formed in a liquid crystal panel, and projecting it on a screen by means of a projection lens. In order to obtain a television picture on a large screen, a method of passing a light beam emitted from a light source through a small-sized light valve having formed therein an optical image corresponding to a video signal, and projecting an enlargement of this optical image onto the screen is known. Recently, the method of using liquid crystal as the light valve is noticed. For example, Morozumi et al. proposed, in "LCD Full-Color video Projector," SID 86 Digest, p. 375, a projection type display device using an active matrix type liquid crystal light valve. An example of the basic conventional structure of such liquid crystal light valve optical device is shown in Fig. 11. The light emitted from a lamp 1 is converted into nearly parallel light by a light collecting device 2, and enters a projector lens 4 through a liquid crystal panel 3. In the liquid crystal panel, an optical image is formed corresponding to the video signal as the change in the transmittance, and this optical image is magnified and projected on a screen 5 through the projector lens 4. The liquid crystal panel 3 is formed by adhering two glass substrates 6, 7 by applying a sealant 8 on the periphery, filling the internal enclosed space with a liquid crystal material 9 in twisted nematic (TN) mode, and disposing polarizers 10, 11 outside the glass substrates 6, 7. By applying an electric field to the liquid crystal layer of the liquid crystal panel 3, the transmittance of the liquid crystal panel 3 can be controlled. As one of the methods for driving the liquid crystal panel, the active matrix system is known, in which switching elements such as thin film transistors (TFTs) and nonlinear elements are connected to the pixel electrodes. Fig. 12 shows an equivalent circuit of the active matrix liquid crystal panel using TFTS. Near the intersections of scanning electrodes X₁ X₂, ..., Xn and signal electrodes Y₁ , Y₂, ..., Ym formed in matrix, TFTs 12 are formed as switching elements, and the gate and source of each TFT are connected to the corresponding scanning electrode and signal electrode. Each pixel electrode is connected to the drain of a corresponding TFT. The liquid crystal layer of one pixel may be regarded as a capacitor. The scanning electrodes and signal electrodes are connected to a scanning circuit 13 and a signal supply circuit 14, respectively. The scanning circuit 13 is of linear sequential type, and sequentially scans the scanning electrodes X₁, X₂, ..., Xn, to simultaneously turn on all TFTS connected to each scanning electrode and turn off all other TFTS. Synchronizing with this scanning, signals are applied to the pixel electrodes connected to the TFTs in ON state through the signal electrodes Y₁, Y₂, ..., Ym from the signal supply circuit 14. While the TFTs are OFF, the pixel electrodes are maintained at respective constant voltages. Thus, in the active matrix system, using multiple scanning electrodes, images of high quality can be formed, so that the liquid crystal image projection system is considered ideal for obtaining television images on a large screen. However, the active matrix system involves the following problems. When a direct current is continuously applied to the liquid crystal material, electric and optical characteristics deteriorate. Thus, generally the pixels are driven by an AC voltage so that DC components may not be applied to the liquid crystal materials. The simplest method is to invert the polarity of the signal voltage in every scanning period. In the active matrix system, since the OFF resistance of the switching elements is not perfectly infinite, when the polarity of the signal voltage is inverted in every specific period, the absolute value of the voltage to be held by each pixel is lowered along with the passing of the time, and the effective voltage applied to each pixel becomes smaller than the absolute value of the signal voltage. By entering parallel light into the liquid crystal panel and applying identical signal voltages to the pixels, the brightness of the projected image varies along the scanning direction. This is called the brightness gradient. The brightness gradient may be improved to a certain extent by modulating the signal voltage in the scanning period. But, since the service range of the signal voltage-transmittance characteristics becomes narrow, gradation characteristics will not be sufficient. Further, if the brightness gradient is too large, correction is impossible. It is hence a primary object of this invention to present a liquid crystal image projection system of high picture quality by solving the problem of the brightness gradient. According to the present invention there is provided a liquid crystal image projection system comprising: an active matrix liquid crystal panel comprising first and second transparent substrates in spaced opposed relation to each other and spaced a distance from each other; a layer of liquid crystal material contained between said first and second transparent substrates, one of said first and second transparent substrates having a common electrode thereon; a plurality of pixel electrodes on the other of said transparent substrates arranged in a matrix; a plurality of switching elements connected to the plurality of pixel electrodes, respectively; a plurality of signal electrodes arranged in columns for supplying signal voltages to the plurality of pixel electrodes through the plurality of switching elements; and a plurality of scanning electrodes arranged in rows for selectively activating the plurality of switching elements on a row by row basis; drive means responsive to an image signal for driving the liquid crystal panel by supplying the signal voltages to the plurality of signal electrodes and scanning the plurality of scanning electrodes sequentially along the rows of scanning electrodes in one scanning direction thereby to form an optical image in the liquid crystal panel; light directing means for directing a light emitted from a light source into the liquid crystal panel through the first transparent substrate for being modulated by the optical image formed in the liquid crystal panel and being outputted through the second transparent substrate as a modulated light; and projector means for projecting the modulated light from the liquid crystal onto a screen to form an enlargement of the image on the screen, characterized in that said light directing means includes converging means for converging the light emitted from the light source into a convergent light and directing the convergent light into the liquid crystal panel through the first transparent substrate for being modulated by the optical image formed in the liquid crystal panel and being outputted through the second transparent substrate as said modulated light, and in that said liquid crystal material has each of the liquid crystal molecules thereof in the middle of the distance between said first and second transparent substrates oriented such that the angles between a molecular axis of each liquid crystal molecule and the direction of the convergent light ray passing therethrough are the same at the middle of the distance from the first transparent substrate to the second transparent substrate when the signal voltages applied to all of the pixel electrodes are the same. In this arrangement in which the molecular axis vector of each liquid crystal molecule disposed at the middle of the liquid crystal layer has a vector component which is opposite to the scanning direction and the convergent light enters the liquid crystal panel, when the effective voltages of all pixels are identical, an angle formed between the molecular axis vector of each liquid crystal molecule at the middle of the liquid crystal layer and the light ray running direction becomes smaller in the scanning direction, so that the optical anisotropy of the pixels becomes smaller in the scanning direction. If the OFF resistance of the switching elements for driving the pixels is not perfectly infinite, when identical signal voltage are applied to all pixels, the effective voltages applied to the pixels become smaller in the scanning direction. As the effective voltage becomes larger, the optical anisotropy becomes smaller. In this way, the action due to OFF imperfection of the switching elements and the action due to difference in the angle formed by the molecular axis vector and the light ray running direction work in mutually canceling directions. Therefore, when applying identical signal voltages to all pixels, the change of the optical anisotropy of pixels in the scanning direction is smaller in the convergent light entrance as compared with the parallel light entrance. As a result, the problem of non-uniformity of the signal voltage-transmittance characteristic of the entire screen can be improved. These and other features and effects of this invention will be better understood and appreciated from the following detailed description taken in conjunction with the accompanying drawings in which: Fig. 1 is a schematic structural diagram showing a liquid crystal image projection system according to an embodiment of the invention; Fig. 2 is a sectional view showing a structure of a liquid crystal panel; Fig. 3 is a schematic plan view showing a structure of a glass substrate on which TFTs are arranged; Fig. 4 is a plan view for explaining the rubbing direction of the orientation film; Fig. 5 is an equivalent circuit diagram for explaining the operation of TFT; Figs. 6 (a), (b) and (c) are voltage waveform diagrams showing an output voltage waveform of the signal supply circuit, an output voltage waveform of the scanning circuit and a voltage waveform at the pixel electrode, respectively; Figs. 7 (a), (b) and (c) are voltage waveform diagrams showing the voltage waveforms at the pixel electrodes in the upper, middle and lower parts of the liquid crystal panel, respectively; Fig. 8 is a perspective view for explaining the directions related to the rubbing direction of the orientation film; Figs. 9 (a) and (b) are schematic views for explaining the state of liquid crystal molecules; Figs. 10 (a) and (b) are illumination distribution diagrams showing the illumination profiles on the screen of a conventional liquid crystal image projection system and the liquid crystal image projection system of the invention, respectively; Fig. 11 is a schematic structural diagram showing the conventional liquid crystal image projection system; and Fig. 12 is an equivalent circuit diagram of an active matrix liquid crystal panel using TFTs. Fig. 1 shows a structure of an optical system in one embodiment of this invention. The light emitted from a lamp 15 is converted into a convergent light by a light collecting device 16. The convergent light from the light collecting device 16 enters a liquid crystal panel 17, and an optical image formed as a change in transmittance in the liquid crystal panel 17 is magnified and projected onto a screen 19 by a projector lens 18. Fig. 2 shows a sectional view of the liquid crystal panel 17. The peripheries of two glass substrates 20, 21 are adhered by using a sealant 22, and the internal enclosed space is filled up with a TN liquid crystal material 23. Polarizers 24, 25 are placed outside the glass substrates 20, 21. On one glass substrate 21, as shown in Fig. 3, scanning electrodes 26 and signal electrodes 27 are formed in matrix, and TFTs 28 using amorphous silicon are formed near the intersections of the scanning electrodes 26 and signal electrodes 27 as switching elements. In the region enclosed by the scanning electrodes 26 and signal electrodes 27, pixel electrodes 29 of indium tin oxide (ITO) thin film are formed. The source, gate and drain of each TFT 28 are connected to the corresponding signal electrode 27, scanning electrode 26 and picture element electrode 29, respectively. On the other glass substrate 20, a common electrode 30 of ITO thin film is formed. Orientation films 31, 32 are formed each at the interface surface of the liquid crystal. The orientation films 31, 32 are rubbed in order to align the molecular axes of the liquid crystal molecules. Referring to Fig. 4, the rubbing directions as seeing the liquid crystal panel 17 from the projector lens 18 side are in a direction A at the input light side and in a direction B at the output light side, supposing the scanning direction to be V. The rubbing direction of the orientation film 31 of the input light side is about 45 degrees to the scanning direction V in the direction from upper right to the lower left side in the drawing, and the rubbing direction of the orientation film 32 at the output light side is nearly at right angle to the rubbing direction of the orientation film 31 at the input light side. As a result, the liquid crystal molecules are arranged so that the molecular axis may be twisted about 90 degrees from the input light side to the output light side. The operation of the system of the invention is described below. First the operation of TFT is explained. Fig. 5 is an equivalent circuit showing a pixel at the portion corresponding to the middle part of the screen, in which the source, gate and drain of TFT 28 are connected to the signal supply circuit 33, scanning circuit 34, and pixel electrode 29. The liquid crystal layer held between the picture element electrode 29 and common electrode 30 forms a capacitor 35. Figs. 6 (a), (b) and (c) show the output voltage waveform of the signal supply circuit 33, the output voltage waveform of the scanning circuit 34 and the voltage waveform of the pixel electrode 29, respectively. The output voltage waveform of the signal supply circuit 33 is +E from time t₁ to time t₃, and -E from time t₃ to time t₅. That is, the polarity is inverted cyclically in the scanning period without changing the absolute value. The scanning circuit 34 generates pulses at specified time t₂, t₄ within the scanning period from the top to the bottom of the screen. The TFT 28 is ON for a short period when the pulse is present, and is OFF in the other period. When the TFT 28 is turned ON at time t₂, since the signal voltage is +E, the capacitor 35 is quickly charged, and the voltage of the pixel electrode 29 becomes very close to +E. Since the signal voltage is +E from time t₂ to time t₃, the voltage of the pixel electrode 29 is hardly changed. At time t₃, when the signal voltage is inverted to -E, the potential difference of the signal voltage from the voltage at the pixel electrode 29 becomes 2E, so that the absolute value of the voltage at the pixel electrode 29 decreases from time t₃ to time t₄ depending on a time constant determined by the OFF resistance of TFT 28 and the capacitor 35. Thus, one scanning period is divided into the phase in which the voltage of the pixel electrode 29 and the output voltage of the signal supply circuit 33 are in the same polarity and into the other phase differing in polarity. In the same-polarity phase, the pixel electrode 29 holds the signal voltage, but in the different-polarity phase, the voltage of the pixel electrode 29 changes in the direction to decrease the absolute value with the passing of the time. The voltage waveforms of the pixel electrodes at portions corresponding to the upper part, middle part and lower part of the screen are respectively shown in Figs. 7 (a), (b) and (c). Since the different-polarity phase becomes longer in the scanning direction (from top to bottom of the screen), the effective voltage of the pixel electrode drops along the scanning direction. That is, by the OFF imperfection of TFT, the voltage holding ability is lowered along the scanning direction. Next is explained the action when a convergent light is inputted into the liquid crystal panel. As shown in Fig. 8, the rubbing directions of the orientation films at the input light side and output light side are supposed to be A, B, respectively. Suppose that the direction of dividing the angle formed by the direction A and the opposite direction to the direction B into two equal divisions be C, the direction of the normal of the liquid crystal layer seen in the light beam running direction be N, the direction perpendicular to the direction C and direction N be X, and the plane containing the direction C and direction N be s. Since the liquid crystal molecules are arranged so as to be departed from the glass substrate along the rubbing direction, when the state of the liquid crystal molecules in the nonelectric field is seen from direction X, the molecular axes of all liquid crystal molecules may look parallel as shown in Fig. 9 (a), and the output light side end of each molecule is located at the side in direction C with respect to the normal direction N (i.e. the output light side end of each molecule is located more in direction C than the input light side end of the molecule). The molecular axis of the liquid crystal molecule 36 at the intermediate position of the liquid crystal layer is parallel to plane s. When the state of applying an electric field to the liquid crystal layer is seen from direction X, the direction of the liquid crystal molecular axis approaches the direction of the electric field as shown in Fig. 9 (b), but the output light side end of all liquid crystal molecules is still located at the side in the direction C with respect to the normal direction N. In the liquid crystal molecule 36 at the middle position of the liquid crystal layer in Fig. 9 (a), (b), if the electric field is varied, the molecular axis remains parallel to plane s, and since most optical characteristics are nearly symmetrical to the plane s, qualitatively, the liquid crystal molecules in the middle of the liquid crystal layer may be regarded as average liquid crystal molecules. The vector 37 running from the input light side and to the output light side end along the molecular axis of the liquid crystal molecules 36 at the middle of the liquid crystal layer is called the molecular axis vector. The optical anisotropy becomes larger as the angle ϑ formed by the direction of the molecular axis vector 37 and the light beam running direction 38 becomes wider. In all picture elements, when the tilting of the liquid crystal molecular axis toward the electric field direction is set identical and the convergent light is entered into the liquid crystal panel, the angle ϑ formed by the direction of the molecular axis vector 37 and the light ray running direction 38 becomes wider along direction C, the optical anisotropy increases along direction C. Let us now suppose a case in which the direction C shown in Fig. 8 is opposite to the scanning direction V, where the liquid crystal panel is scanned from the bottom to the top in the figure, and same signal voltages are applied to all pixels. Since the effective voltages of the pixel electrodes become smaller along the scanning direction, it acts to increase the optical anisotropy along the scanning direction. On the other hand, if the effective voltages of the pixel electrodes are the same, the angle ϑ formed by the direction of the molecular axis vector 37 and the light ray running direction 38 becomes smaller in the scanning direction, which causes the optical anisotropy to decrease along the scanning direction. Therefore, the action due to OFF imperfection of TFT and the action due to the angle formed by the average liquid crystal molecular axis and the light ray running direction are mutually canceled. Fig. 1 shows the state of the liquid crystal molecules when signal voltages having a same value are applied to the pixel electrodes. Supposing the angle formed by the molecular axis vector of the liquid crystal molecule at the middle of the liquid crystal layer and the light ray running direction of the convergent light to be ϑ, it is defined ϑ = ϑ₁ in the upper middle part of the liquid crystal panel, ϑ = ϑ₂ in the center middle part of the panel, and ϑ = ϑ₃ in the lower middle part of the panel. When properly selecting the angle formed by the ray passing through the screen upper middle part and the ray passing through the screen lower middle part so that ϑ₁, ϑ₂ and ϑ₃ become nearly identical, (ϑ₁ = ϑ₂ = ϑ₃), the difference of the optical anisotropy due to location when same signal voltages are applied to the pixels can be remarkably decreased. That is, the problem of the brightness gradient due to OFF imperfection of TFT can be successfully solved. The principal specifications of this embodiment are shown in Table 1. The light collecting device enters the convergent light so as to form a lamp image at the pupil position of the projector lens. Fig. 10 (a) and Fig. 10 (b) show vertical direction illumination distribution diagrams from the upper middle part P through the center middle part O to the lower middle part Q on the screen when same signal voltages are applied to all pixels of the liquid crystal panels in the structures of Fig. 11 and Fig. 1, respectively. In Fig. 11, the projector lens possesses the telecentric performance with F 4.5 and maximum field angle of principal ray at the liquid crystal panel side of ω′ = ± 3°. The angle formed by the ray passing through the upper middle part of the liquid crystal panel and the ray passing through the lower middle part is 4° in the structure in Fig. 11, and 16° in Fig. 1. In Fig. 10 (a), from the screen upper middle part P to the screen lower middle part Q, the illumination L is lowered due to the action of OFF imperfection of TFT. This drop of illumination L may be improved somewhat when the signal voltage is modulated in the scanning period, but the service range of the signal voltage-transmittance characteristic of the liquid crystal panel is narrowed. On the other hand, in Fig. 10 (b), the illumination reaches the maximum at the screen center O, and gradually decreases as going toward the peripheral area of the screen. Between the screen upper middle part P and the screen lower middle part Q, the illumination L is slightly asymmetric, but this degree of illumination change may be sufficiently improved by modulating the signal voltage in the scanning period. Thus, the brightness gradient is improved while sufficiently making use of the range of the signal voltage-transmittance characteristic of the liquid crystal panel. It is because the screen illumination L is determined by L = L₀·η·COS⁴ω that the brightness is lowered from the screen center O to the peripheral area of the screen, where L₀ is the illumination at the screen center, η is the vignetting factor of the projector lens, and ω is the field angle of the principal ray of the screen side of the projector lens. If the maximum picture angle ω′ of the principal ray at the liquid crystal panel side of the projector lens is too large, the symmetricity of the illumination L between the screen upper middle part P and the screen lower middle part Q is largely broken. In Fig. 10, the changes in the illumination in the vertical direction along the middle of the screen is explained, but the same holds true in the entire screen. If the direction C shown in Fig. 8 is not opposite to the scanning direction V, as far as the direction C has a component which is opposite to the scanning direction V, the action due to OFF imperfection of TFT and the action by the angle formed by the average liquid crystal molecular axis and the light ray running direction are cancelled each other. By utilizing this phenomenon, the magnitudes of the both actions can be balanced. In the foregoing embodiment, the orientation process by rubbing is explained, but any other method of orientation may be used as far as the panel can act as a liquid crystal panel. In the above embodiment, the liquid crystal panel of TN mode is mentioned, but the invention is equally applied to other practical liquid crystal panels in different modes. In any cases, it is only enough to pay attention to the molecular axis vector of the liquid crystal molecules in the middle of the liquid crystal layer. In the liquid crystal panel, instead of the TFTs as switching element, nonlinear elements may be also used. The same effects as in the above embodiments may be obtained as far as the liquid crystal panel of the active matrix type is used.

Claims: A liquid crystal image projection system comprising: an active matrix liquid crystal panel (17) comprising first and second transparent substrates (20, 21) in spaced opposed relation to each other and spaced a distance from each other; a layer of liquid crystal material (23) contained between said first and second transparent substrates, one of said first and second transparent substrates having a common electrode (30) thereon; a plurality of pixel electrodes (29) on the other of said transparent substrates arranged in a matrix; a plurality of switching elements (28) connected to the plurality of pixel electrodes, respectively; a plurality of signal electrodes (27) arranged in columns for supplying signal voltages to the plurality of pixel electrodes through the plurality of switching elements; and a plurality of scanning electrodes (26) arranged in rows for selectively activating the plurality of switching elements on a row by row basis; drive means (33, 34) responsive to an image signal for driving the liquid crystal panel by supplying the signal voltages to the plurality of signal electrodes and scanning the plurality of scanning electrodes sequentially along the rows of scanning electrodes in one scanning direction (V) thereby to form an optical image in the liquid crystal panel; light directing means (15, 16) for directing a light emitted from a light source into the liquid crystal panel through the first transparent substrate for being modulated by the optical image formed in the liquid crystal panel and being outputted through the second transparent substrate as a modulated light; and projector means (18) for projecting the modulated light from the liquid crystal onto a screen (19) to form an enlargement of the image on the screen, characterized in that said light directing means (15, 16) includes converging means (16) for converging the light emitted from the light source into a convergent light and directing the convergent light into the liquid crystal panel through the first transparent substrate for being modulated by the optical image formed in the liquid crystal panel and being outputted through the second transparent substrate as said modulated light, and in that said liquid crystal material has each of the liquid crystal molecules (36) thereof in the middle of the distance between said first and second transparent substrates (20, 21) oriented such that the angles between a molecular axis of each liquid crystal molecule and the direction of the convergent light ray passing therethrough are the same at the middle of the distance from the first transparent substrate (20) to the second transparent substrate (21) when the signal voltages applied to all of the pixel electrodes are the same. A system according to claim 1, further comprising first and second orientation films (31, 33) disposed between the liquid crystal material layer (23) and the first transparent substrate (20) and between the liquid crystal material layer (23) and the second transparent substrate (21), respectively, said first and second orientation films having been rubbed in respective rubbing directions (A, B) which are substantially perpendicular to each other and the rubbing direction (A) of the first orientation film (31) being at an angle of substantially 45 degrees to the scanning direction (V). A system according to claim 1 or 2, wherein, with the liquid crystal panel (17) oriented for making the scanning direction (V) vertical, the liquid crystal has, relative to the scanning direction, an upper part above the center of the liquid crystal material layer (23) in the scanning direction and an equal lower part below the center, and an angle formed by a light ray of the convergent light passing through the upper part and a light ray of the convergent light passing through the lower part has a size for causing transmittances of the liquid crystal material layer in the upper and lower parts to be the same when the signal voltages applied to all of the pixel electrodes are the same.

Inventors: MIYATAKE YOSHITO, TANAKA TAKAAKI, MIYATAKE, YOSHITO, TANAKA, TAKAAKI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1989
Main classification: H04N9
IPC codes: G02F1, H04N9, G09F9, H04N5