Visual display system

Abstract: A visual display system in which light from a light source is modulated by a space light modulator having a plurality of controllable elements arranged in a line and the modulated light is scanned by an optical scanning device to display an image. An exposure control apparatus having a lens, a photoelectric conversion device, and a space light modulator, wherein the space light modulator is operated to vary the amount of light to be supplied to the photoelectric conversion device, thereby controlling the exposure for the image focused by the lens.

Description: The present invention relates to a visual display system for displaying a video signal or image data in the form of a visible image. Description of Related Art In recent years, demand has been increasing for larger-area and/or higher-definition screens than the screens offered by conventional display devices such as cathode ray tubes (CRTs) or liquid crystal displays (LCDs). Such demand has been further increased by consumers' demand for large-screen televisions, coupled with the demand from many people who want to enjoy shows on a large screen or who need to view screens generated by a computer in conferences or the like. LCDs are suitable for use with small-size computer systems and terminal devices, particularly laptop and portable computers; LCDs for such applications are constructed using liquid crystal cells, each individual cell corresponding to one pixel on the screen. LCDs are sensitive to temperature, are difficult to construct in large size, have a slow response, and require external illumination for viewing. The resolution of the LCD is limited by the complexity of its driving system, the result of which is that the size of the LCD generally increases with increasing resolution. This means an inevitable increase in the size of the projection optical system used, and hence an increase in the cost of a high-definition system. Another problem is that light passed through the LCD (or reflected from it) is polarized, as a result of which the sensation of brightness varies nonlinearly with the distance from the center of the field of view (e.g. off the axis). The most widely used display system is the CRT. However, the CRT has various shortcomings, the first one being the cost. The high cost is due to the difficulty involved in the production of large-size display tubes and the requirement of large volumes of raw materials (particularly glass) for construction. As a result, the display becomes very heavy, is not easy to transport, and therefore, is not suitable for use in a small-size display system. The resolution is also a problem with the CRT. Another shortcoming of such conventional display systems is that they are essentially an analog type. Therefore, when displaying information is stored in digital form in a computer, the information must first be converted into a form compatible with the analog raster scanning for displaying of the CRT. Japanese Patent Application Laid-Open No.3-40693 discloses a visual display system using a space light modulator to overcome the above enumerated shortcomings. The system disclosed therein will be described below as a prior art example that most closely relates to the present invention and is illustrated in Figures 1 and 2 of the accompanying drawings. Fig.1 shows the construction of a two-dimensional digital visual display system comprising an image generating system 501 and a display screen 502. Light emitted from a light source 510 is collected by a mirror 511 which reflects the light into a lens 512. The lenses 512, 513, and 514 together form a beam collimator which acts to collimate the incident light 509 into a columnar light 508, the light energy being thus intensified to enhance the overall efficiency of the system. A folding mirror 520 reflects the column of light 508 through an optical path 507 into a space light modulator 515. The space light modulator 515 selectively redirects portions of the light, passed through the optical path 507, toward a magnifying lens 505 to form an image on the display screen 502. The space light modulator 515 used in this example is one generally known as a deformable mirror device. The space light modulator 515 has a surface 516 onto which the light from the optical path 507 is projected. The surface 516 contains a plurality of switchable elements 517 which can be controlled so as to redirect the incident light toward the magnifying lens 505. When one element 517 is placed in a certain orientation, a portion of light passed through the optical path 507 is redirected into an optical path 506 toward the magnifying lens 505. The portion of the light is magnified or enlarged by the magnifying lens 505 and projected through an optical path 504 onto the display screen 502 to illuminate a pixel 503. A computer 519 controls the operation of the space light modulator 515 via a bus 518 so that portions of the light from the optical path 507 are selectively redirected toward the display screen 502 to form an image thereon. The bus 518 carries necessary control signals and image information from the computer 519 to the space light modulator 515. The computer 519 is, for example, a digital signal processor. Fig.2 shows the configuration of an electronic optical apparatus used for the above described visual display system. As shown, a signal source 640, which is, for example, a TV tuner, is connected to an electronic optical system 644 via a bus 642. The bus 642 leads to an A/D converter 646. An analog signal is fed via the bus 642 to the A/D converter 646 for conversion into a digital code representing the chrominance and luminance information of each pixel of an image. The digital code from the A/D converter 646 is placed on a bus 648 for transfer to a buffer memory 650. The digital code is thus stored in the buffer memory 650. In a different mode, such digital code or information can be loaded into the buffer memory 650 from an external computer or graphics system via a bus 652. The digital code or information represents an image to be displayed. The digital code stored in the buffer memory 650 is transferred to a central processing unit (CPU) 654 via a bus 658. The CPU 654 is connected to a video memory 660 via a bus 658. The CPU 654 decodes the video signal, including the chrominance and luminance signals, contained in the information transferred from the buffer memory 650. The CPU 654 is so programmed as to extract the image from the information and store the information, including the chrominance and luminance signals, into the video memory 660. The image is further processed by the CPU 654 through a command given via a bus 662 or under control of the program contained in the CPU 654. Thereafter, the processed image data is transferred to a shift register for input to an array in the space light modulator, the image data being loaded into row and column decoders. It is also possible to load image information into the video memory 660 from an eternal computer or graphics system via a bus 668. The electronic optical system 644, together with a projection system 672, constitutes an image generating system 674, for which the image generating system 501 of Fig.1 with the space light modulator 515 connected via the bus 518 to the video memory 660 can be substituted. The image stored in the video memory 660 is transferred to the projection system 672 via a bus 670 and is displayed through an optical path 676 on a display screen 678. In the prior art visual display system, however, the space light modulator is a flat surface type wherein numerous mirrors are arranged in minute patterns on a semiconductor surface. The space light modulator of such construction having a plurality of controllable elements arranged on a flat surface has the problem of low yields in semiconductor fabrication due to a large silicon chip area, increasing the cost and reducing the reliability of the visual display system. Another problem is that addressing circuitry for the controllable elements of surface type and circuitry for light modulation, i.e., circuitry for driving the controllable elements, become large and complex, which is a limiting factor in the downsizing of the system. There is the further problem that the design freedom is limited in that the width-to-height ratio (aspect ratio) of the image forming surface is determined by the shape of the space light modulator unless an additional special optical system is provided. According to the present invention there is provided a visual display system comprising: a light source for emitting light along a first optical path; an image forming surface disposed in a second optical path of said light; a space light modulator disposed in said first and second optical paths and composed of a plurality of controllable elements arranged in a line; and an optical scanning device, disposed between said space light modulator and said image forming surface, for scanning the light reflected into said second optical path by said space light modulator. The invention can provide an inexpensive and highly reliable visual display system wherein driving circuitry for controllable elements is simplified. The invention can also permit the aspect ratio of a projection screen to be varied without an addition of special optical systems. In the visual display system of the invention, light from a light source is modulated by a space light modulator having a plurality of controllable elements arranged in a line, and the modulated light is scanned by an optical scanning device to display an image. The reflecting or refracting surfaces of the optical scanning device are movable so that line images reflected from the space light modulator are expanded into a surface image for display. When the optical scanning device is provided with selectivity with respect to the wavelength of light, a color visual display can be achieved. One scanning cycle of the optical scanning device can be made shorter than one cycle period of the critical flicker frequency of the human eye. Either the light source emits a pulse light whose repetition period is equal to the duration of time the optical scanning device scans across the width of the controllable elements of the space light modulator, or a shutter is provided which is adapted to pass light from the light source for that duration of time. Furthermore, the controllable elements of the space light modulator can be controlled more than one time during that duration of time, in synchronism with the operation of the optical scanning device. The invention will be more fully described in the following detailed description of illustrative embodiments with reference to accompanying drawings, in which:- Fig.1 is a schematic diagram illustrating the construction of a prior art visual display system. Fig.2 is a block diagram illustrating the configuration of an electronic optical apparatus for the prior art visual display system. Fig.3 is a schematic diagram illustrating the construction of a visual display system according to the present invention. Fig.4 is a perspective view showing an optical scanning device using a plane reflecting mirror. Fig.5 is a perspective view showing an optical scanning device of triangular columnar shape. Fig.6 is a perspective view showing an optical scanning device of polygonal columnar shape. Fig.7 is a block diagram illustrating the configuration of an electronic optical apparatus for the visual display system of the present invention. Fig.8 is a schematic diagram illustrating an alternative construction of the part of the visual display system of the present invention. Fig.9 is a schematic diagram illustrating a further alternative construction of the visual display system of the present invention. Fig.3 shows the construction of a two-dimensional digital visual display system comprising an image generating system 1 and a display screen 2, according to one embodiment of the invention. The display screen 2 may be constructed from a relatively planar sheet of suitable material, or may be formed in a shape so curved as to converge reflected light in the direction of the observer. Furthermore, the display screen 2 may be formed from a translucent material for rear projection. A light source 3 provides light energy which finally reaches and illuminates the display screen 2. An ordinary light source, such as an incandescent lamp or halogen lamp, is used as the light source 3. Light 7 emitted from the light source 3 is directed toward a lens 4. The lenses 4, 5, and 6 together form a beam columnator which acts to collimate the light 7 into a columnar light 8, thus preventing the dispersion of light and enhancing the overall efficiency of the system. The columnar light 8 is projected onto a space light modulator 9. A different method may be used to direct the converged light energy source to the space light modulator 9. The space light modulator 9 acts to selectively redirect the columnar light 8 toward an optical scanning device 10. The space light modulator 9 used in this embodiment is identical or similar to a deformable mirror device (DMD) as disclosed in U.S. Patent Nos. 4,441,791, 4,710,732, 4,596,992, 4,615,595 and 4,662,746. In this embodiment, the optical scanning device 10 consists of a reflecting mirror of hexagonal columnar shape. The mirror may be either of plane shape, as shown in Fig.4, of triangular columnar shape, as shown in Fig.5, or of polygonal columnar shape, as shown in Fig.6. Fig.4 shows an example in which a plane reflecting mirror is used for the optical scanning device 10. A reflecting mirror 22 is revolved by means of a motor 21. There are two possible types of plane reflecting mirror, a single-sided mirror and a double-sided mirror. Fig. 5 shows an example in which a scanning section of triangular columnar shape is used for the optical scanning device 10. A scanning section 24 is revolved by means of a motor 23. Fig.6 shows an example in which a scanning section of polygonal columnar shape (hexagonal columnar shape) is used for the optical scanning device 10. A scanning section 26 is revolved by means of a motor 25. The optical scanning device 10 does not necessarily have to use a reflecting mirror, but any suitable optical component that reflects light can be used. By revolving the optical scanning device 10, the angle at which to reflect the light oncoming from the space light modulator 9 can be varied; thus, beams of light from the space light modulator 9 each falling as a line of light can be expanded into an image for display on a two-dimensional surface. Each beam of light reflected by the optical scanning device 10 is directed into an optical path 11 and projected through a magnifying lens 12 onto the display screen 2. A computer 13 controls the operation of the space light modulator 9 via a bus 14 so that portions of the light falling through an optical path 16 are selectively redirected toward the optical scanning device 10 to form an image on the display screen 2. This operation will be described in detail later. A motor 15 is provided to revolve the optical scanning device 10, the motor 15 being synchronized with the space light modulator 9 to ensure proper display of an image. The synchronizing operation is controlled by a controller 20. Fig.7 shows an electronic optical apparatus used for the visual display system. As shown, a signal source 201, for example, a movie, is connected to an electronic optical system 216 via a bus 202. The bus 202 leads to an A/D converter 203. An analog signal fed to the A/D converter 203 via the bus 202 is converted by the A/D converter 203 into a digital code representing the address and luminance signals for each pixel of an image. The digital code is outputted on a bus 204. The digital code from the A/D converter 203 is transferred via the bus 204 to a buffer memory 205. The digital code is thus stored in the buffer memory 205. In a different mode, such digital code or information can be loaded into the buffer memory 205 from an external computer or graphics system via a bus 217, for example. To enhance the processing capability, the buffer memory 205 may be constructed from a double board or a triple board type memory. Design of a multiple board memory is also known to the industry. The digital code or information represents an image to be displayed. The digital code stored in the buffer memory 205 is transferred to a central processing unit (CPU) 207 via a bus 206. The CPU 207 is, preferably, a digital signal processing unit (DSP). The information processed by the CPU 207 is transferred via a bus 208 to a serial/parallel (S/P) converter 209 where the information is divided per address, the divided information then being transferred to a shift register latch 211 via a bus 210. After all address data are inputted to the shift register latch 211 from the S/P converter 209, the shift register latch 211 transfers each address data to a pulse width generator 213 via a bus 212 in accordance with a synchronizing signal fed from the CPU 207 via a bus 220. The pulse width generator 213 converts each address data, transferred via the bus 212, into a pulse width that matches the contents of the data, and transfers the thus converted address data to a space light modulator 215 via a bus 214 in accordance with the synchronizing signal fed via the bus 220. The pulse width generator 213 can be incorporated into the CPU 207. The space light modulator 9 has a high speed switching capability and can modulate each pixel of a bright image at a data speed 833 times faster than a CFF. When the desired luminance is at a lower level, the operating speed will become much faster. Luminance of different levels can be obtained by dividing one line display period of the optical scanning device 10 by the necessary number of shades and by having the pulse width generator 213 generate a pulse width that matches the desired luminance. The space light modulator 9 operates at a speed much faster than one line display period. Since this speed is faster than the critical flicker frequency of the human eye, the amount of light sent to the image forming surface for a fixed period is recognized as a variation in luminance. As described, when compared with the two-dimensional surface type space light modulator of prior art, the above construction of the invention allows the use of an image data shift register or latch of one-dimensional type, and scanning in the vertical direction can be achieved just by controlling the rotation of the optical scanning device 10, which allows a significant reduction in circuitry. Furthermore, in the prior art surface type space light modulator, the aspect ratio is fixed at the production stage of the space light modulator, but according to the invention, the aspect ratio can be easily varied by varying the rotational speed of the optical scanning device 10. The aspect ratio can be varied without lowering the resolution when the scanning section of the optical scanning device 10 is formed in a shape that allows a wide angle scanning (for example, the number of scanning surfaces is reduced). Modifications of the above embodiment will now be described. For example, a color visual display can be achieved by adding filter means to the optical scanning device 10 of Fig.3. Optical filters each having the property of reflecting light of a particular frequency or frequency band are disposed on the reflecting surfaces of the optical scanning device 10. Such filters may be formed, for example, from coatings of materials each having the property of reflecting or transmitting light of a particular color band. In the case of the optical scanning device 10 having the shape shown in Fig.6, optical filters of red (R), green (G), and blue (B), are placed in a periodic manner. Since the optical scanning device 10 of Fig.6 has six reflecting surfaces, R, G, B, R, G, and B, two full color scannings take place with one revolution of the optical scanning device 10. The following description deals in detail with the operation of this color visual display. Referring to Fig.3, the light 7 emitted from the light source 3 passes through the lenses 4, 5, and 6, and the resulting columnar light 8 hits the space light modulator 9 to which image data are successively fed from the computer 13 via the bus 14. Based on the image data received from the computer 13, the space light modulator 9 is operated to selectively move the controllable elements (pixels) to transmit an optical image through the optical path 16 to the optical scanning device 10. The optical image is projected as a line of light onto one of the reflecting surfaces of the optical scanning device 10, for example, an R surface. As the R surface is turned, such lines of light are expanded into a screen image. When one scanning is completed with the R surface, a single colored image of red has been displayed on the display screen 2. Needless to say, the image data supplied to the space light modulator 9 from the computer 13 for the production of this image is the screen image data of red. When the scanning with the R reflecting surface is completed, the next reflecting surface of the optical scanning device 10, i.e. a G reflecting surface, appears. In a similar manner to that for the above R reflecting surface, scanning is performed with the G reflecting surface to produce a screen image of green, and likewise for the next B reflecting surface. Thus, one screen image is displayed for each component of R, G, and B. The space light modulator 9 and the optical scanning device 10 are synchronized for operation, and the R, G, and B images are successively presented on the screen at a frequency higher than the critical flicker frequency of the human eye, so that the colors are mixed by the human eye to enable the observer to see a completed color image on the display screen 2. In the foregoing embodiment having the construction shown in Fig.3, it is also possible to achieve a color display by changing the light source 3. More specifically, the light source 3 shown in Fig.3 is replaced by light emitting elements, such as LEDs, that emit lights at the central wavelengths approximately equal to the wavelengths of red, blue, and green colors, respectively. With this arrangement, the light emitting elements, for example, red with 640 nm wavelength, blue with 470nm wavelength, and green with 570 nm wavelength, are successively switched on to emit lights in the respective colors, thereby achieving a color display as by the previously described optical scanning device using white light in conjunction with the optical filters. In the foregoing embodiment, the optical scanning device 10 of Fig.3 constantly revolves at a predetermined speed; when beams of light are constantly supplied from the light source 3, and image data are continuously inputted to the space light modulator 9, the image displayed on the display screen 2 will smear in the vertical direction and cannot be properly recognized. Also, a phenomenon will occur in which the continuity of the image is disrupted at the portions corresponding to the joints between the reflecting surfaces of the polygonal optical scanning device 10. Therefore, the light from the light source 3 is pulsed in order to avoid these problems. First, when one reflecting surface of the optical scanning device 10 comes to an angle at which to start the scanning, the light source 3 turns on a light pulse for the duration of time just equal to one line display period. Next, when the reflecting surface of the optical scanning device 10 is turned to an angle at which to reflect the next line, a light pulse is turned on, again for the duration of time just equal to one line display period. This operation is repeated until the reflecting surface comes to an angle at which to reflect the final line for display. When the scanning of the final line is completed, the light pulse is turned off and remains off until the next reflecting surface of the optical scanning device 10 comes to an angle at which to start the scanning. By repeating the above operations, a proper image can be displayed without causing image lines to smear and without displaying unwanted image portions during the horizontal retrace periods. The emitting operation of such light pulses is controlled by the controller 20. Instead of adapting the light source 3 to emit light pulses, an optical shutter 17 may be provided between the light source 3 and the lens 4, as shown in Fig.8, to attain the same effect as described above. The optical shutter 17 is operated to open for the ON period and close for the OFF period, with the same on/off timing of the light pulse. The actuation of the optical shutter 17 is controlled by the controller 20. In the foregoing embodiment, image line smearing can be prevented by employing, as the motor 15 for controlling the optical scanning device 10, a stepper motor having a step angle equivalent to the angle through which the optical scanning device 10 scans across the width of the controllable elements of the space light modulator 9. In addition, the rotating speed of the optical scanning device 10 needs to be synchronized with the timing of the image data sent from the computer 13 to the space light modulator 9 and the timing of the light pulse or the optical shutter 17. Such synchronizing operation is controlled by the controller 20. In the foregoing embodiment, reflecting mirrors are used in the optical scanning device 10, but instead, a prism may be used. Fig.9 is a schematic diagram illustrating an alternative construction of the embodiment wherein an optical scanning device 18 consisting of a prism is used. The optical scanning device 18 shown is constructed from a hexagonal refracting prism, but it will be appreciated that a triangular prism or lens of the shape shown in Fig.5 or a polygonal prism or lens of the shape shown in Fig.6 may be used to serve the purpose. The optical scanning device 18 is revolved by means of a motor 19. The motor 19 is driven to revolve a scanning section of the optical scanning device 18 to vary the angle of incidence of the image light projected through the optical path 16, thus enabling the scanning to be performed by varying the direction of refraction. Otherwise, the construction of Fig.9 is identical to that of Fig.3; therefore, the same reference numerals are assigned to the same parts, and their descriptions are omitted herein. As described above, according to the visual display system of the invention, since the space light modulator is constructed by arranging the controllable elements in a line, the system is inexpensive to construct, addressing circuitry for the controllable elements and electrical circuitry for light modulation can be simplified, and the aspect ratio can be varied easily.

Claims: A visual display system comprising: a light source (3) for emitting light along a first optical path (8); an image forming surface (2) disposed in a second optical path (11,16) of said light; a space light modulator (9) disposed in said first and second optical paths (8,11,16) and composed of a plurality of controllable elements arranged in a line; and an optical scanning device (10,18), disposed between said space light modulator (9) and said image forming surface (2), for scanning the light reflected into said second optical path (11,16) by said space light modulator (9). A visual display system as set forth in claim 1, wherein said optical scanning device (10,18) is so adapted as to selectively scan the light reflected from said space light modulator (9) according to the wavelength thereof to display a colour image on said image forming surface. A visual display system as set forth in claim 1 or 2, wherein said optical scanning device is a reflecting mirror, a prism or a lens. A visual display system as set forth in claim 1 or 2, wherein said optical scanning device (10,18) has a movable reflecting or refracting surface. A visual display system as set forth in claim 1 or 2, wherein said optical scanning device (10,18) comprises a double-sided reflecting mirror (22) whose reflecting surfaces are turned to scan the light reflected from said space light modulator (9). A visual display system as set forth in claim 1 or 2, wherein said optical scanning device (10,18) comprises a plane prism (18) whose refracting surfaces are turned to scan the light reflected from said space light modulator (9). A visual display system as set forth in claim 1 or 2, wherein said optical scanning device (10,18) is of polygonal columnar shape, with a polygonal cross-section, whose reflecting or refracting surfaces are turned to scan the light reflected from said space light modulator (9). A visual display system as set forth in claim 1 or 2, wherein said optical scanning device (10,18) has a plurality of reflecting or refracting surfaces, each of the reflecting or refracting surfaces having filter characteristic that reflects or refracts light of a particular wavelength or wavelength range. A visual display system as set forth in claim 8, wherein said reflecting or refracting surfaces are periodically provided with said filter characteristic. A visual display system as set forth in any one of the preceding claims, wherein reflecting or refracting surfaces of said optical scanning system complete at least one cycle of rotation within one cycle period of the critical flicker frequency of the human eye. A visual display system as set forth in any one of the preceding claims, wherein said light source (3) comprises a source that provides white light. A visual display system as set forth in any one of claims 1 to 10, wherein said light source (3) comprises at least three light emitting elements that radiate light at the central wavelengths approximately equal to red, blue and green colours, respectively. A visual display system as set forth in any one of the preceding claims, wherein said light source (3) emits a pulse light whose repetition period is equal to the duration of time said optical scanning device (10,18) scans across the width of the controllable elements of said space light modulator (9). A visual display system as set forth in any one of claims 1 to 12, further comprising an optical shutter (17) disposed on the light emitting side of said light source (3), one operation cycle of said optical shutter (17) being equal to the duration of time said optical scanning device (10,18) scans across the width of the controllable elements of said space light modulator (9). A visual display system as set forth in claim 13 or 14, wherein said space light modulator (9) controls said controllable elements more than one time during the period of transmission of the light in the pulse or via the open shutter (17). A visual display system as set forth in claim 13, 14 or 15, further comprising means for synchronizing the rotation of said optical scanning device (10,18) and the emission of said light in the pulse or via the open shutter (17). A visual display system as set forth in any one of the preceding claims, wherein said space light modulator (9) controls said controllable elements more than one time within one cycle period of the critical flicker frequency of the human eye. A visual display system as set forth in any one of the preceding claims, wherein the rotation of said optical scanning device (10,18) is controlled in such a manner that the scanning section thereof is turned incrementally with an angle equivalent to the angle through which to scan across the width of the controllable elements of said space light modulator (9). A visual display system as set forth in any one of the preceding claims, further comprising means for synchronising the rotation of said optical scanning device (10,18) and the control of said controllable elements. A visual display system as set forth in any one of the preceding claims, wherein the scanning time of said optical scanning device (10,18) is variable with respect to the overall length of said controllable elements arranged in a line.

Inventors: NAKAGAKIUCHI SUSUMU, UEDA SHINOBU, NAKAGAKIUCHI, SUSUMU, UEDA, SHINOBU, Nakagakiuchi, Susumu, c/o Mitsubishi Denki K.K., Ueda, Shinobu, c/o MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1992
Main classification: G09G3
IPC codes: H04N5, G09G3