WO1991004632A1 - Optical projection system with electronic sector erase system - Google Patents

Abstract

An optical projection system which in the preferred implementation reflects illumination from a source (13) off of an electro-optic imaging member (12) in the shape of a plate. The imaging member is an electro-optic ceramic plate, such as PLZT, which is substantially transparent to visible light when not affected by an electric field or electron beam. The imaging member is bonded to the front face-plate of a cathode-ray tube (35) which includes an electron beam generator (30), a controller (26) for modulating the intensity of the electron beam (36) with an image and a raster scan generator (34, 37, etc.) for scanning the beam across the front face of the tube. At spots where the electron beam (36) strikes the electro-optic plate (12), the opalescence of the spot is changed in accordance with the scattering effect of the crystal and in dependence upon the intensity of the beam thereby writing a physical nonvolatile image in the imaging member. In operation, an image is written on the plate and displayed by illuminating the plate with the source. The reflection of the source illumination is modulated by the physical image area thereby projecting the image for display. The image is changed a group of lines at a time by first erasing the lines with a selected strip-like element and then by rewriting the blank lines with the electron beam.

Description

OPTICAL PROJECTION SYSTEM WITH ELECTRONIC SECTOR ERASE SYSTEM

Cross Reference to Related Application

The present invention is a continuation-in-part of U.S. Application Serial No. 317,429, filed March 1, 1989 and entitled "Optical Projection System" by Iben Browning.

Field of the Invention

The invention pertains generally to optical projection systems with nonvolatile imaging members and more particularly to such projection systems which use an electro-optical element for an image storage medium and which further include a plurality of spaced sector electrodes, each of which is adapted to produce a partial erasure of the image on the storage medium and each of which can be selectively operated for erasure.

Background of the Invention

In conventional video systems such as television, a video image is produced by scanning an electron beam in a raster format across the face of a tube having phosphors which glow with either a white light or color, depending upon the system. Because the phosphors only glow for a predetermined amount of time, the images are relatively volatile and must be frequently rewritten.

In the most common example, an NTSC television format includes a raster scan of 525 horizontal lines per video frame.

To give the illusion of a single image, many of these frames are combined sequentially in time thereby allowing the eye to integrate the combination and perceive moving images on a screen. Usually, the frame rate is 30 frames per second, with two interlaced fields forming a frame. The interlace technique, where one half of the 525 horizontal lines is shown in one field followed by the other half of the horizontal lines in the next field, is to reduce the phenomenon of flicker. Flicker is caused by the on/off nature of the video signals and produces an annoying artifact at the field frequency.

It is known that increasing the frame rate will reduce the perception of flicker, but such requires additional bandwidth to transmit the signal and additional equipment to decode and display the video signals. It is known that much of the bandwidth of a television signal, as much as 60%, is required for this flicker fusion due to the high repetition frame rate.

An image which did not flicker would eliminate the requirement for these high speed repetitive showings of the image because there would be no need for flicker fusion. Much of the time the image shown in a video environment does not change significantly from frame to frame. It is only when an element of the image is moving does the frame itself substantially change. However, the frame rate for motion fusion is much less than flicker fusion. The motion fusion rate can be as low as 11 frames per second, and thus a system which imaged without flicker could be designed to operate at this lower repetition rate. This would reduce the bandwidth for transmitting video signals with present signal formats by as much as 60%. However, a flickerless video system envisions a nonvolatile screen display other than the present phosphor display screens.

The video technology is advancing to where large screen optical projection systems are rivaling commercial movie theaters. These large screen optical projection systems are more common today and generally include a video monitor with a projection system to receive an image from a cathode ray tube screen and to magnify it by optical means onto a larger screen. However, these systems are today somewhat limited in final screen size because of cost and efficiency. To build a cathode ray tube for the initial image which is bright enough, or with enough image definition, to permit significant magnification without substantial reduction in image quality is extremely difficult and expensive. Further, the conventional cathode ray tube is inherently inefficient for such projection because of the way the image is formed. The phosphors of a cathode ray tube screen emit scattered, incoherent light. It is difficult to capture much of the energy from this type of image for projection. The low energy capture of the initial image creates a consequent reduction in the brightness of the final image. There has in the past been no relatively inexpensive optical projection system for magnifying and projecting a video image from a cathode ray tube to provide a bright, high quality image of movie screen size.

However, the distribution of video images and their initial formation are much easier with cathode ray tube technology than with the image medium of film. The distribution of video images by means of over-the-air transmission, video disk or cassette is well developed and the technology is highly efficient. The imaging of a cathode ray tube screen where monochrome or color images can be made by a raster scan of phosphors on the display screen of the tube is also relatively efficient and much more effective than transmitting illumination through a celluloid based film. The production of video programs on film requires a complex distribution system and creation process. It is much easier to make, combine and edit a video tape or disc than it is to perform the same operations for a film of comparable duration. Moreover, video special effects can be more easily incorporated into video tape or disc than on film.

To solve these major problems, an advantageous optical projection system which incorporates the efficient reproduction, editing and transmission video technology of the cathode ray tube while replacing the inefficient display, projection and magnification technology of the phosphor screen display has been recently developed. The optical projection system includes a display screen which is nonvolatile and thereby allows the system to operate at the motion fusion rate without flicker. The system includes as an image medium an electro-optic ceramic, particularly PLZT (Pb_x La_y (Zr₂Ti)0₃), one of the newest classes of electro-optic materials. The optical projection system produces an image by scattering light either transmitted through the image medium or reflected from it. This optical projection system is more fully described in the above-referenced patent application Serial No. 317,429, entitled "Optical Projection System" by Iben Browning, which is commonly assigned with the present application. The disclosure of Browning is hereby expressly incorporated by reference.

Browning describes several preferred embodiments which use an erasure system which is shown operating on a line-by-line basis. While advantageous, such erasure system is somewhat more complex than necessary for many applications. When using a line- by-line erasure system, one is normally constrained to a particular scanning pattern with a set number of lines. It would be advantageous in such systems to devise an erasure system which would be flexible enough to use with several scan formats, such as the 525 line format of the NTSC system used in North America or the 625 line format of the PAL and SECAM systems used in Europe and South America, without having to provide a specially designed display configuration for each format. Additionally, Browning illustrates a line-by-line erasure system where the electron beam of the cathode ray tube is used to close a plurality of photo-responsive switches between respective erasure electrodes and a bias voltage which provides charge to the electrodes for the erasure. While highly advantageous and cost effective for a specific scanning pattern, this structure does not provide much flexibility in the timing of the erasure mechanism as it is predetermined by the structure of the tube when it is built. Further, the timing must be coordinated with the deflection of the electron beam which means non-standard scanning signals have to be applied to the deflection electronics. It would be advantageous to generate standard synchronizing and deflection signals to the deflection electronics of a cathode ray tube and decouple the erasure control signals. It would further be advantageous to provide a flexible or variable timing system which could be used for different television formats and electrode patterns.

Another criterion for a flexible erasure system for an optical projection system would be the ability to be modifiable to a number of different erasure electrode configurations. For a raster scan format, the number of horizontal lines erased by a single electrode is a design choice while the total number of electrodes is set by the video format which is chosen. For graphic or symbol displays the number of electrodes will be determined by the size and shape of the symbols or elements used in the display. It would be of significant import to be able to electronically address the electrodes for selective or random access erasure.

_-uτnmaτ^»y of the Invention

Therefore, it is an object of the invention to provide an improved optical projection system with a selective and flexible erasure system.

It is another object of the invention to provide an improved optical projection system which uses a nonvolatile but erasable imaging member of the electro-optic type. It is yet another object of the invention to provide an optical projection system which use a television type raster format but which does not exhibit flicker.

Still another object of the invention is to provide for the selective erasure of sectors of an imaging member of an optical projection system.

According to one embodiment of the invention, a monochromatic optical projection system includes an electro-optic imaging member generally in the shape of a thin plate or screen. An illumination source is provided which produces visible illumination that can either be reflected off of or transmitted through the imaging member. The imaging member forms an image by varying the opalescence of various areas of the member when irradiated by the source of visible illumination.

In the illustrated implementation, the imaging member forms the front face of a cathode-ray tube and is an electro-optic ceramic, PLZT (lead-lanthanum-zirconate-titanate) , plate which is substantially transparent when not affected by an electric field or an electron beam. A system control includes an electron beam generator, a controller for modulating the intensity of the electron beam and a scan generator for scanning the beam across the front face of the tube in a raster format. At spots where the electron beam strikes the PLZT plate, the opalescence of the spot is changed in accordance with the scattering effect of the PLZT crystal and in dependence upon the intensity of the beam thereby writing a pixel of an image in the plate. The imaging member has a transparent electrode on the front face of the plate and an opposing electrode on the back face of the plate.

In a preferred configuration, the electrode on the back face of the plate is formed of a multiplicity of strip-like elements which are arranged in substantially parallel rows to form corresponding horizontal sectors similar to a raster. The strip- like elements are mirrored on one side to reflect the illumination of the source and are at least partially transparent to the electron beam to allow writing of the image. The image formed in the domains of the imaging member is nonvolatile until erased. The application of a bias voltage between the front face electrode and one of the back electrode elements erases the image written by the electron beam for that particular strip or sector of horizontal lines.

Each strip-like element forms a sector of the image to be erased. In the illustrated embodiment, the sectors are groups of horizontal scan lines. Preferably, the group of horizontal scan lines is an integer number of scan lines evenly divisible into the number of scan lines of the raster format being imaged. There is, however, no requirement that an integer number be used, but such does simplify the timing of the erasure cycles. Each sector will cover a group of lines entirely without any overlap or partial line erasure problem which also significantly reduces the complexity of timing for the erasures. Advantageously, because a raster scan moves vertically during a scanning at the vertical deflection rate, the sector electrodes can be inclined at substantially the same small angle. With this shape and size of sector, a repeating erasure cycle in synchronism with the raster signal can be used for erasure.

In operation, an image is written and stored on the imaging member and contemporaneously displayed by illuminating the plate with visible radiation from the illumination source. The source illumination is modulated by the stored image thereby projecting the image for display. The image can be changed, an electrode or sector at a time, by first erasing a sector of horizontal lines with a selected strip-like element and then by rewriting the blank lines with the electron beam.

The scanning, erasure, and display of an image is provided by a system initial. The system control receives a video signal and processes the signal into a modified video signal suited for display by the imaging member of the optical projection system. Included in the modified video signal are synchronizing signals for the deflection system. The synchronizing signals include conventionally shaped horizontal blanking and synch signals and vertical blanking and synch signals. The sector electrodes are selected and erased by an erasure control just prior to being written. The erasure control accomplishes this task by controlling a plurality of solid state switches to apply an erasure voltage to a selected strip-like electrode based upon the timing of the raster scan. One implementation of the erasure control includes an erasure control processor controlling a shift register with its stage outputs connected to individual sector electrode switches. Advantageously, the erasure control processor communicates with the shift register through a reduced number of signal lines so the control circuitry can be remotely located from the erase circuitry and electrodes. A "clear" signal from the microprocessor to the shift register clears the stages. A "start" bit signal combined with a "shift" signal from the microprocessor causes the first sector to be erased. The shift signal, which is generated synchronously with the horizontal synch signal of the modified video signal, causes the start bit to ripple through the shift register a stage at a time closing a sector electrode switch and erasing the respective sector just prior to its being rewritten by the electron beam.

The preferred implementation for the erasure control includes a detection circuit to strip the synch pulses from the modified video signal and provide interrupts to the erasure control processor. The erasure control counts horizontal synch pulses to determine the position of the electron beam in the raster scan of the imaging member. When the horizontal synch pulse just prior to the start of a selected sector is detected, a delay counter is counted down to a duration which is just enough time to erase a sector and allow it to discharge before the scan beam begins rewriting it. The feature provides the minimum amount of blanking for the image.

Therefore, an optical projection system providing the aforementioned objects has been illustrated by the preferred implementation. The projection system of the invention significantly reduces the complexity of conventional cathode-ray tube structures with corresponding benefits in manufacturing ease and longevity of use.

Brief Description of the Drawings

These and other objects, features and aspects of the invention will be better understood and more fully described upon reading of the following detailed description in conjunction with the appended drawings wherein:

FIG. 1 is a cross-sectional system block diagram of a reflective implementation of a preferred embodiment of an optical projection system constructed in accordance with the invention;

FIG. 2 is an enlarged cross-sectional view of the imaging member of the optical projection system illustrated in FIG. 1;

FIG. 3 is a back view of the imaging member illustrated in FIG. 2 disclosing the configuration of the sector electrodes, and the mounting of one part of the sector electrode control;

FIG. 4 is an enlarged fragmented cross-sectional view of the circled area of FIG. 2;

FIG. 5 is a functional block diagram of the part of the sector electrode control illustrated in FIG 3;

FIG. 6 is a partially pictorial and partially system block diagram of the connection of the system control and the sector electrode control;

FIG. 7 is a pictorial representation of a standard interlaced scanning pattern for an NTSC format television signal; FIGS. 8A - 8D are pictorial representations of the timing and synchronizing signals of an NTSC format television signal; FIG. 9 is a pictorial representation of a sequence of three frames of an NTSC format television signal in time and its modification to the format of the VIDEO OUT signal used by the invention; FIGS. 10A - 10D are pictorial representations of the waveforms for the signals of the sector electrode control relative to the synchronizing signals of the VIDEO OUT signal; and

FIG. 11 is a detailed flow chart of the sector electrode control program of the auxiliary microprocessor illustrated in FIG. 6.

Detailed Description of the Preferred Embodiment

With reference now to FIG. 1, there is shown an optical projection system 8 constructed in accordance with the invention. The preferred embodiment illustrated is a monochromatic reflection implementation and includes an imaging assembly 10 for storing an image in a nonvolatile manner. The image is in the form of various spatial patterns of opalescence of an electro- optic medium and its formation will be more fully described hereinafter. The back (left side) of the imaging assembly 10 includes an imaging member 12 flexibly bonded thereon which has a mirror surface. When irradiated with illumination from an illumination source 13, the imaging member 12 reflects the light and forms an image by scattering the light according to the physical image modulation stored in the member. The illumination source 13 can concentrate its output with a parabolic or elliptical mirror 13a which directs more light toward the imaging member 12.

The illumination source 13 can take the form of many types of radiation sources but preferably, for an image in the visible spectrum, is a conventional high wattage projection bulb of the Xenon type. This type of illumination source produces an intense white light that can be easily modulated by the imaging member 12. The illumination from the source 13 is focused by a generally convex condensing lens 14 and reflected by a mirror 15. The reflection produces illumination by the light source 13 on the mirror surface of the imaging member 12. The reflection from the mirror 15 travels to the back of imaging member 12 through a field lens 17 and an optical flat 19. The illumination reflected off the back of the imaging member 12 is modulated by the stored image by scattering in accordance with the spatial variations of the physical image areas therein. The specularly reflected, modulated image is focused onto a collecting lens 18 and thereafter passes through a projection lens 20 which is at its focal length from, and projects upon a display screen 21. Lenses 17 and 18 are at their focal length from each other so that the reflected light from imaging member 12 emits parallel light from lens 18.

The projection system 8 is enclosed in a housing 16 which retains the light which is scattered and not collected by lens 18. The imaging member 12 produces two images by modulation of the reflected illumination, one a scattered image of the physical modulation of the stored image and the other, a specular image, which is a negative of the scattered image. While either image could be collected and projected, preferably the specular image is utilized because substantially all of the energy in that image can be collected and projected by the inexpensive optics disclosed.

The imaging member 12 is attached flexibly to the optical flat 19 which forms the face plate of a cathode-ray tube 32 having a vacuum envelope 35. The cathode-ray tube 32 has a means, electron gun 30, for generating an electron beam 36 which strikes the imaging member 12. A system control 26 modulates the intensity of the electron beam 36 by means of an intensity control signal I_c. The intensity control signal I_c contains image information obtained in a conventional manner from an input video signal. The system control 26 additionally includes a means for scanning the beam 36 across on the imaging member 12. Preferably, this is implemented by conventional x-y deflection plates 34 and a focusing ring 37 which are regulated by deflection control signals D_x, D_y and a focus control signal F_c. The system control 26 provides control for a power supply 22 and a temperature control 24 for the imaging assembly 10. The power supply 22 provides a DC bias voltage V_b across the imaging member 12 in particular image areas where sector electrode patterns have been placed for selective erasure of the image. Electrode control signals S_β are generated by the system control 26 to determine the selective erasure of each sector. The temperature control 24 is provided to control the temperature of the imaging member 12 to an operating point where the electron beam 36 can write the spatial image on the imaging assembly 12 and the bias voltage Vb can erase it. The system control 26 further provides for the control of the illumination source 13 and an optical window 11 with an illumination control signal L_c.

The temperature controller 24 receives a temperature signal T_a indicative of the actual temperature of the imaging member 12 from a temperature sensor 43 (FIG. 3) affixed to the back of the optical flat 19. The temperature controller 24 regulates the temperature by varying a current signal T_c supplied to a resistive heating element 50 (FIG. 2) on the front face of the optical flat 19. Because the optical flat 19 is relatively large compared to the imaging member 12 and made of a substantially insulative material (optical grade glass) , which does not gain or lose temperature quickly, it acts as a heat sink and maintains the imaging member 12 at a substantially constant temperature.

It should be understood that by the use of different electro-optic materials, or different formulations of the present electro-optic material, the temperature controller may be unnecessary. In fact, the optical flat 19, in some situations, can be used to transfer heat away from the electro-optic material and should thus be a good heat conductor.

The imaging assembly 10 and imaging member 12 will now be more fully described with respect to FIGS. 2-4. In FIG. 2, an enlarged cross-sectional side view of the imaging assembly 10 of FIG. 1 illustrates its structure. The imaging assembly 10 comprises from front to back (right to left as shown in the figure) the field lens 17, the transparent resistive heating electrode 50, the optical-flat 19, and the imaging member 12.

The imaging member 12 comprises from front to back a transparent front electrode 52, an imaging plate 54, and a back electrode 56.

The back electrode 56 is reflective or mirror-like on the side facing the imaging plate 54 and at least partially transparent to the electron beam 36 from the other side. The front and back electrodes 52, 56 cover substantially the entire surface of the imaging plate 54 and form between them a frame imaging area.

The imaging plate 54 is formed from an electro-optic material, preferably ceramic PLZT, and is extremely thin, approximately 3-10 mils, when compared to the optical-flat 19 which operates as a structural support for the imaging member 12. The imaging member 12 is attached to the optical-flat 19 by flexible bonding. When imaged or erased, the polarization of the grain domains in the electro-optic material causes a change in their physical dimensions, thus producing stresses in the material. If not flexibly bonded to its support, these cyclic stresses could in time cause a mechanical failure of the imaging member 12. There are known techniques in the prior art for the flexible bonding of electro-optic materials to support surfaces to prevent such failures and any of these techniques can be used. These stresses in the electro-optic material because of grain domain polarization increase with thickness, which is the reason for the use of a very thin imaging plate. Both of these techniques are used in the present implementation to increase the longevity of the imaging member 12 of the optical projection system 8.

In general, the temperature of the imaging plate 54 is regulated by the temperature control 24 to a condition where the material is transparent. The electron beam 36 varies this transparent condition at spots (pixels) where it strikes the imaging member 12 by forming charge areas in the back electrode 56 with consequent localized electric fields which reorient the charges of the grain domains in the plate 54 to provide various degrees of opalescence. The rotation of the domains and amount of image variation is proportional to the intensity of the electron beam and the localized field. The term opalescent describes the degree by which light is scattered by the grain domains of the imaging member. The term is used whether the light is transmitted through the member or reflected back through the member. When the PLZT or other electro-optic material is in its transparent state, there is little or negligible scattering and absorption of the illumination. When the material is in its opalescent state, there is a maximum scattering of the illumination, though not 100%, and still negligible absorption. Varying degrees of the opalescent state can be formed by varying the polarization of the grain domains from the minimum scattering state to the maximum scattering state within the bounds of the material characteristics. The bounds of these states are variable to some degree by the variance of other parameters such as material composition, operating temperature, residual domain stress, bias fields, etc.

The bias voltage V_b of the power supply, which is on the order of +300 to +600 D.C. volts, supplies a sufficient reverse bias voltage to rotate the domain polarity back to where the material is transparent. This phenomenon thereby permits the storing of image in the imaging member 12 by writing it with the electron beam and then by selectively erasing the written image with the bias voltage in various sectors. The front electrode 52 is preferably maintained at relative ground potential to accelerate the electrons from gun 30 toward the imaging plate 54. Because the electro-optic material can be written with lower energy electrons, the deflection and focusing means are less complex than conventional phosphor screen cathode-ray tubes. However, because of the small tube diameter, generally in the order of x2¹¹ - 3¹¹, the deflection angle does not have to be as great as in regular tubes and can be controlled more precisely. The simple construction of the cathode-ray tube lends itself to inexpensive manufacturing processes and an increased useful lifetime. FIG. 3 illustrates a preferred electrode configuration for the back electrode 56 in the present implementation. The configuration has an electrode pattern imprinted on the surface of the imaging plate 54 with a multiplicity of elongated strip¬ like sector electrodes 64 which extends across the distance from one edge of the imaging area of the imaging plate 54 to the other. The strip-like elements 64 are connected electrically to the bias voltage V_b by switching elements of an integrated circuit (IC) 100. The switching elements are used to selectively connect the strip-like elements to the bias potential. In this manner, any of the strip-like elements 64 is independently switchable between a no voltage potential and the potential of the bias voltage V_b. The IC 100 has an individual connection lead to each sector electrode and control input leads of a reduced number to permit remote control of the switching.

The shape, and size of a sector electrode 64 is preferably that of the area or sector of the image that it will erase. In a raster scan frame imaging system, preferably the electrodes will be approximately the length of a horizontal scan line and several times its width. Each electrode 64 is screened or vapor deposited onto the imaging plate 54 with an attached electrode lead connecting them to the respective outputs of the application specific integrated circuit (ASIC) 100. The ASIC 100 is preferably a gate array which has been configured by custom masking to selectively control the application of the bias voltage V_b to the electrodes. The ASIC 100 could also be configured from a full custom part, standard cells, or be a hybrid circuit. In the illustrated embodiment, the ASIC 100 is mounted on the optical flat 19 and connects to a sealed integral connector embedded in the CRT envelope by four separate signal leads which connect the electrode selection signals Se and the bias voltage V_b from the system control 26 and power supply 22, respectively.

The transparent electrodes for this embodiment can be made from many materials, but preferably such are formed from indium tin oxide which exhibits excellent optical transparency and is compatible with the PLZT imaging plate.

The electro-optic material of the imaging plate can be many non-volatile materials including those which can be imaged and erased according to the methods of the invention. Preferably, electro-optic ceramics operated in the scattering mode are used because most of the energy from the specular image can be recovered and they can be used in the transmissive or reflective modes as taught by the invention.

As a working example, lead-lanthanum-zirconate-titanate (PLZT) formed from a mixture of 7.6-7.8 parts La by weight, 70 parts Zr0₃ by weight, and 30 parts Ti0₃ by weight can be used because of its optical scattering characteristics. Compositions which increase scattering and provide high resolution are preferred. Sintering of the components produces a granular ceramic which has a grain structure surrounded by a matrix. The material has a phase transition at about 120"C and is transparent at this condition. In operation, the temperature controller 24 heats the imaging member to transparency and a potential, V , just below the transition voltage of the imaging plate 54 is applied between the front electrode 52 and the back electrode 56 by power supply 22. The imaging member 12 is then completely erased by controlling switching elements 64 to erase the imaging plate 54. This initializes the imaging plate 54 to a transparent, no image mode. After completion of the erasure, an image can be scanned onto the horizontal lines on the imaging plate by the electron beam 36 in a conventional manner, i.e., by a raster scan technique which sweeps the beam horizontally across the imaging plate to write one line of an image, retraces and then writes the next horizontal line. The process continues until the image raster is complete and an entire image has been written on the imaging plate 54. Because the image is nonvolatile, there is no need to update it as often as with a conventional phosphor screen type cathode- ray tube. Further, because the image does not flicker an entire frame may be written with one raster scan instead of interlacing two fields. The frame rate at which the image is updated can be as low as the rate of change in the image motion. Normally, the motion fusion rate for a video image is understood to be approximately 12 frames/sec. In addition, it is not necessary to erase the entire image and then rewrite it because the image is nonvolatile. Preferably, just before a group or sector of particular lines in the image is to be written in the raster, the strip-like element which correspond thereto is connected to the bias voltage V_b to erase those lines of the frame. The lines are rewritten by the electron beam 36 and the next strip-like electrode 64 of the back electrode 56 is connected to erase the next group of lines.

The invention erases a sector of the image for each selection of a strip-like electrode 64. By a sector what is meant for the illustrated embodiment is the area on the imaging plate which covers two or more horizontal scan lines of the electron beam 36. Preferably, a sector will be some multiple, but integer, number of horizontal scan lines where the integer will divide evenly into the number of horizontal lines for a field or frame of an image. The embodiment shown produces a raster format and advantageously shows erase electrodes of a particular size, shape, and number adapted for that format. It is evident that a sector could be any shape and any size, or arranged in any manner, suitable for erasing a particular image area. The term sector is meant to encompass all such shapes, sizes, numbers and arrangements of the erase electrodes on the imaging member.

In the illustrated embodiment, an NTSC formatted signal of 525 scan lines/frame has been modified to slow the frame rate to 12 frames/sec. Therefore, the preferable sector integer for this system would be one evenly divisible into 525 such a 5, 15, 25, 75 yielding 105, 75, 21, and 7 sector electrodes, respectively. Normally, the number of sectors should exceed approximately 10 so that not more than approximately 10% of an image is erased at any one time but should not be so multiplied as to increase expense unnecessarily. At 100 sectors per frame, 99% of the image will be constantly illuminated at any one time, which is deemed sufficient for most purposes. Therefore, the invention preferably will have the number of sectors for a common raster chosen approximately between 10 and 100. The image quality for this type of system is not only a function of the percent of the image frame erased at any one time but also the frame rate and the amount of time between the erasure and the rewriting of a sector. The frame rate in the present system is set at 12 frames/sec. to reduce bandwidth to a minimum while still performing motion fusion. Thus, the time between the erasure and rewriting any one sector should be reduced to a minimum in this system to provide maximum quality. Of course, any frame rate in excess of 11-12 frames/sec. is acceptable for motion fusion and could be used in particular circumstances to yield the desired results.

This object is accomplished in the present system by timing the raster sweep to determine a point just prior to the time the election beams 36 is about to start writing the sector. An erasure of a sector is made just before the beam begins its sweep over the sector to reduce any delay between the erasure of a sector and its rewriting. Because a horizontal synch pulse is a convenient timing point in the raster scan video signal and a horizontal line approximately 64_/ι/sec. in length for a NTSC signal, the horizontal synch pulse of the line prior to a sector is used to provide a starting point for the erasure cycle.

This signal is used to start a delay counter which holds the difference between a horizontal line time duration and the erasure duration of a particular sector. When the counter time expires, an erasure mechanism is activated to erase the sector and turn off just before the election beam sweep begins to rewrite the sector. By varying the delay count a convenient means for varying the erasure cycle time is provided. For smaller sectors, other landmarks in a conventional video signal, such as the horizontal blanking pulse of the first line of the sector can be used. Any landmark in the video signal prior to the active video of the particular sector can be used.

This technique of line erasure and rewrite advantageously ensures that between 90-99% of the image, is always viewable on the imaging plate 54 at any one time. The full erasure of an image and its rewrite would create a flicker in the video image which could be corrected only by techniques such as interlacing two related fields to form a frame and providing a frame rate far in excess of the motion fusion rate. The present invention reduces the necessary frame rate to 12 frames/sec, just in excess of the motion fusion rate, from the normal 30 frames/sec. This slower frame rate means a 60% reduction in signal bandwidth of the transmitted video signal without the necessity of a high framing rate to achieve flicker fusion.

A functional block diagram of a preferred implementation of the ASIC 100 forming one part of the erase control is illustrated in FIG. 5. The ASIC 100 comprises a plurality of three terminal solid state switches 102 and a shift register 104. Each of the switches 104 has its switch terminals connected between one of the respective sector electrodes 64 and the bias supply V_b. The control terminal of each switch 104 is connected to one of the outputs for the stages Q_n of the shift register 104. A particular switch closes in response to a "one" state from the output of an associated shift register stage and opens in response to a "zero" state of the stage.

The shift register 104 has the same number of stages Q_n as there are sector electrodes and provides a non-inverted pulse output for each stage to drive the control terminal of an associated switch. The shift register 104 includes three control inputs including a clock input, CLK connected to a shift signal, SHIFT; a reset input, RST connected to a clear signal, CLR; and a data in input, D_in connected to a start bit signal, SBIT. The SHIFT signal causes the contents of the shift register 104 to shift its contents one stage higher than the previous state of the register. The CLR signal causes a reset of all stage states to a cleared or "zero" logic level. The SBIT signal loads a logic "one" or a "zero" in the first stage of the register depending upon its level. In operation, the system control 26 generates the three signals SHIFT, CLR, and SBIT to control selective erasures of the imaging member 12.

Before the start of the raster scan of a frame, the signal CLR will be used to reset all states of the register to logical zero. At this time, all of the switches 102 will be open and all of the sector electrodes at a zero potential. Prior to the time of the scan of the first line of the first sector, the state of the SBIT signal is changed from a "zero" to a "one". Synchronously with the start of the erasure period of the first sector, the SHIFT signal is generated to clock the logical one state of the SBIT signal into the first stage of register 104. This produces a high level pulse output from the first stage, closing switch 104 and erasing the first sector of the image.

Sequentially the start bit is shifted from stage to stage with the SHIFT signal thereby closing a specific switch and erasing a specific sector at the correct time. The cycle can be continued from frame to frame by feeding back the start bit from the last stage D_out to the input D_in and by clocking the register 104 with the SHIFT signal. The register 104 at any time can be cleared and restarted using the three signals described hereinabove. In addition, the entire image can be erased by increasing the clock frequency of the SHIFT signal while blanking the video signal, possibly either during horizontal or vertical retrace.

While the control circuit has been shown advantageously as including a shift register, other implementations could reasonably be used. For example, a counter with its output connected to a count-to-line decoder could be used in the manner taught. Each decoder line would be used to open and close a respective switch. In a preferred embodiment, the counter would increment sequentially to a next sector preferably some amount of time prior to the writing of an addressed sector. In addition, the counter can be programmable to give a great deal of flexibility in randomly addressing the electrode sectors.

FIG. 6 discloses a functional block diagram of a portion of the system control 26. In general, an external video signal is digitized by an analog to digital converter 80 under the control of a video processor 82. The external video signal can be supplied from any number of sources such as over-the-air, from a memory, from a video tape or disc, etc. The video processor 82 inputs and stores the video signal in the form of timed digital samples. During the time the video signal is stored in a memory (not shown) of the video processor 82 the signal can be reformatted and processed in a number of different ways. In the preferred implementation, a 30 frame/sec. , 525 line, 2 field NTSC video signal is reformatted into a 12 frame/sec. , 525 line, single frame VIDEO OUT signal. Other processing of the video signal can be accomplished for time base correction, color correction, image enhancement, etc.

The video processor 82 generates the reformatted and processed signal as a timed series of digital samples which are output to digital to analog converter 84. The samples are reconverted to the analog form of the VIDEO OUT signal which is applied to the scanning and deflection means and to the intensity control. The VIDEO OUT signal includes the standard horizontal and vertical synchronizing signals of a NTSC format signal with double the number of horizontal lines. There is no interlace between frames and the frame rate has been slowed to 12 frames/sec. An alternative to the video processor 82 would be to demodulate an over-the-air signal with a reduced bandwidth having 12 frames/sec.

The VIDEO OUT signal is detected by a synch detector 86 that strips the horizontal and vertical synchronizing pulses from the modified video signal. These signals are timing indications of the position of the electron beam on the frame area. The horizontal and vertical synch signals are applied as interrupts to an auxiliary processor 88 which includes a program to control the selection and erasure of the sector electrodes 64.

The auxiliary processor 88 may have multiple tasks and the interrupt method is a facile manner of controlling a real time event such as the erasure of a particular sector electrode, while keeping the processing time to a minimum. The sector electrode control of the auxiliary processor 88 generates three control signals to the ASIC 100 including the start bit signal SBIT, the reset signal CLR, and the shift signal SHIFT. These signals are output from three pins of an output port of the auxiliary processor 88 to a connector 90. A terminal of the connector 90 also connects to the bias power supply 22. The bias voltage and the three control signals are carried to the ASIC 100 over a cable 92 which is double male ended. One end of the cable plugs into the connector 90 and the other into an integral connector 94 embedded in the envelope of projection tube 32. This allows control of any of the sector electrodes with a minimum of transmission cabling. This configuration for the control processes aids in the separation between the high frequency video circuitry which must be shielded and the relatively lower frequency control signals which do not need as much interference protection. The high frequency circuitry can thus be contained remotely from the screen of the projection tube 32 while maintaining selective control of the electrodes.

The illustration in FIG. 7 shows the conventional interlaced scanning pattern of a NTSC raster. The raster format contains 525 horizontal lines which are made up of a field A of 262.5 lines and a field B of 262.5 lines. These two fields are interlaced and sequential in time to become a frame. The timing of the frames are illustrated in FIG. 9. FIGS 8A - 8D illustrate the composite make up of the video raster including the timing of the horizontal and vertical blanking and synchronizing pulse. FIG. 8A is field A timing while FIG. 8B is field B timing. FIG. 8C is horizonal synch timing while FIG. 8D is vertical synch timing. The frame rate of the video signal is 30 frames/sec. where a Field A and Field B make up a frame. In the illustration, six fields of three frames are illustrated. By modifying the frame rate to 12 frames/sec, a video signal having a motion fusion but without needing the wide bandwidth of the standard NTSC can be produced. The video processor of the present system reads the NTSC signal into a frame memory and assembles the VIDEO OUT signal at a slower rate. The implementation uses one out of five fields of the NTSC signal where Field A comes from frame 1 and Field B comes from frame 3. This slows the frame rate down to 12 frames/sec. where each frame has 525 lines.

After the assembly of a frame in the frame store of the video processor 82, it is output digitally and converted to the analog video signal VIDEO OUT. The synchronizing signals from the VIDEO OUT signal are used to synchronize the timing for the sector electrode control of the auxiliary processor. The auxiliary processor 88 times the VIDEO OUT signal by detecting the horizonal synch signal HSYNC and the vertical synch signal VSYNC with the detector 86. These signals produce interrupts to the auxiliary processor to call the timing program of the sector electrode control program which generates the timing signals SHIFT, CLR, and SBIT. A system flow chart for the sector electrode control program is more fully illustrated in the Fig. 11 and will be explained with reference to that figure and FIGS. 10A - 10E which illustrate timing diagrams of the VIDEO OUT signal and erasure cycle for one of the sector electrodes.

FIG. 10A is a timing waveform diagram of two horizontal lines of the VIDEO OUT signal and includes the two horizontal synch pulses 200, 202 separating a line of video signal 204. Each horizontal synch pulse is positioned in the horizontal blanking pulse 206 and separates the blanking pulse into a front porch 208 and a back porch 210. The VIDEO OUT signal includes 525 of such horizontal lines which are separated by a vertical blanking interval to allow vertical retrace and a vertical synch pulse. The detector circuitry 86 detects the horizontal and vertical synchronizing pulses and applies them to the auxiliary microprocessor 88 as interrupts.

The sector electrode control program shown in FIG. 11 in detailed flow chart form is interrupt driven and entered any time a vertical or horizontal synch pulse is detected. In block A10 the program initially begins a determination of whether the detected pulse is a vertical synch pulse and in block A12 whether it is a horizontal synch pulse. If the interrupt is a horizontal synch pulse, a register used as a horizontal line counter is incremented in block A14. This indicates the leading edge of the synch pulse, labeled 212 in FIG. 10A.

If the synch pulse indicates the beginning of the first line of the video frame, as detected in block A16, then a path through block A20 is taken by the program. Block A20 sets the start bit SBIT which is communicated to the sector electrode control circuitry over the cable 92. This action is illustrated in FIG. 10B which shows the start bit signal SBIT making a transition to a high logic level synchronously with the leading edge 212 of the synchronizing pulse 200. The program then advances to block A24 where a preloaded delay register is decremented. After each decrementation, a loop is formed through block A26 to determine if the contents of the register are zero and, if not, then the loop continues. When the contents of the register equal zero, the predetermined delay between the start of the synchronizing pulse 212 and the beginning of the erasure cycle 214 has been counte . The program then begins the start of an erasure cycle for the first sector by setting the shift signal to a high logic level in block A28. This high logic level signal which is communicated to circuitry is illustrated in FIG. IOC. When this signal is a high logic level, the register 104 shifts in the start bit and causes the first stage to begin the erase cycle of the first sector by closing its associated switch. This action is illustrated by the high level output Q_n for stage n in FIG. 10D. After a predetermined period of time, the erasure signal Q_n makes a transition to a low logic level at 218 which allows a settling period before the beginning of the writing of the sector at 216. The voltage of a sector electrode is illustrated in FIG. 10E where the voltage begins at zero volts and makes a charging transition up to the bias voltage Vb with a time constant t^ and then at the end of the signal Q_n produces a relaxation time constant t before the scan of the sector electrode that begins at 216.

Returning again to FIG. 11, after the shift signal is set in block A28, the delay register is reloaded with a constant equal to the delay time in block A30 before the program exits. For horizontal synch pulses which are detected which do not start the first line, the program path is to block A18 where the start bit signal is reset, for example, on the next horizontal synch pulse 202 after the initial synch pulse 200. Thereafter, the program determines whether the line number stored in the line counter is the horizontal synchronizing pulse just prior to one of the sector beginnings in block A22. If the line number is not one of those which should begin an erase cycle, then the shift signal is reset in block A32 before the program exits. However, if the line number is just prior to a sector erase cycle, then the program exits to the loop beginning in block A24 where the delay counter provides the correct timing before the erasure cycle. The cycle is started by shifting the erasure bit one stage to begin an erasure of the associated sector.

Alternatively, when the program begins its path through blocks A10 and A12, if the signal is neither a vertical synch pulse nor a horizontal synch pulse, then it is an extraneous interrupt which should not cause any action and the program immediately exits. When the program detects a vertical synchronizing pulse in block A10 then, a loop is entered to reinitialize the program for its next cycle through a subsequent frame. Initially, the line counter is reset to zero in block A34 and the clear signal set in block A36 to zero out all the stages in the shift register. A delay loop in block A38 and A40 are then entered to allow the clear signal to cycle through to the register and allow the stages of the register sufficient settling time. Thereafter, the clear signal is brought to a low logic level in block A42 and the clear delay constant reloaded into the delay register in block A44. The program thereafter exits to the instruction which it was about to execute prior to the interrupt.

The system provides a convenient method and apparatus for erasing the sector electrodes with the minimum amount of erasure time. The system program is extremely flexible in that it allows for a variable erasure cycle depending upon the programmable delay counter constant. This allows the same program to be used without significant change for any number of sector electrodes and for any video format. Further, the erasure cycle for a particular line number is initiated from a landmark of the video signal itself and thus can be adapted to any standard or future video format. Thus, the system illustrated is easily adaptable to the modified video format illustrated for the 525 line, single frame video out signal, a standard 525 line, two field NTSC signal, or a 625 line PAL or SECAM formatted video signal.

While the preferred embodiments of the invention have been illustrated, it will be obvious to those skilled in the art that various modifications and changes may be made thereto without departing from the spirit and scope of the invention as hereinafter defined in the appended claims. For example, the color embodiments of the projection system shown in Browning can be easily achieved by using the three tube three color format shown therein with the present system.

Claims

WHAT IS CLAIMED IS:

1. An optical projection system for displaying a visible image on a screen, said system comprising: a reflective imaging member of the electro-optic type in the shape of a surface including a multiplicity of strip-like reflecting elements arranged thereon in the shape of said surface to form a mirror; an illumination source for irradiating said reflective imaging member with radiation of an intensity and wavelength which will pass through the member and will be reflected from the mirror of said reflective imaging member in a visible manner; means for writing an image in said reflective imaging member which changes the opalescence of certain areas of the member such that the areas modulate the scattering of said illuminating radiation being reflected from said mirror; means for selectively controlling said strip-like elements with an electrical control signal to modify the portion of the image written by said image writing means which corresponds to the area of an individual element; and means for displaying the reflected image from said reflective imaging on the screen.

2. An optical projection system as set forth in Claim 1 wherein said reflective imaging member comprises: a thin plate of electro-optic material which is substantially transparent or opalescent when not influenced by an electrical field and which can be made opalescent or transparent, respectively in varying degrees by applying an electrical field, said plate having a illumination receiving side and a reflecting side; a transparent electrode on said receiving side; said strip-like elements being formed on said reflecting side and forming a complementary electrode to said transparent electrode on said receiving side; and wherein said means for controlling said elements applies a voltage across said transparent electrode and selectable elements of said multiplicity of strip-like elements to modify the opalescence of the area of said plate between the selected element and the transparent electrode.

3. An optical projection system as set forth in claim 2 wherein said means for writing an image includes; an electron beam generator which generates an electron beam which impinges on said area of said plate to change its opalescence in those areas; means for scanning the electron beam of said beam generator over said plate in a controlled fashion; and means for modulating the intensity of said electron beam with image information such that the degree of opalescence of the areas of said plate impinged on by said beam corresponds to the intensity.

4. An optical projection system as set forth in Claim 3 wherein said scanning means includes: means to scan said electron beam in a raster pattern corresponding to said arrangement of said strip-like elements.

5. An optical projection system for displaying a visible image on a screen, said system comprising: an imaging member of the electro-optic type in the shape of thin plate including a multiplicity of strip-like erasure elements arranged thereon in the shape of an image frame area; said erasure elements being substantially a horizontal scan line length in length and two or more horizontal scan line widths in width; an illumination source for irradiating said imaging member with radiation of an intensity and wavelength which produces a visible image; means for scanning an electron beam in a raster pattern over the image frame area of said imaging member to write an image, said imaging member being written by changing the opalescence of the areas scanned as a function of the electron beam intensity to modulate the scattering of said illuminating radiation; means for selectively controlling individual strip¬ like elements to erase a sector of the image written by said scanning means which corresponds to the area of the element; and means for collecting the secular image from said imaging member and displaying it on the screen.

6. An optical projection system as set forth in Claim 5 wherein said selective erasure means includes: a plurality of solid state switches, each having a control terminal and two switch terminals connected by a switch wherein a control signal applied to the control terminal will cause the closing of a switch and the application of an erasure voltage to the selected element; a sector erase control having a plurality of outputs corresponding to the control terminals of said plurality of switches and adapted to selectively operate said switches by generating respective control signals.

7. An optical projection system as set forth in Claim 6 where said sector erase control includes: means for receiving an indication of the position of the electron beam scan on said image frame area; and means for sequentially changing the outputs of said sector control based upon the beam position indication, such that a sector of said image frame area is erased by an electrode just prior to being written by said electron beam.

8. An optical projection system as set forth in Claim 7 wherein said sector erase control includes: a shift register which is clocked by a shift signal to sequentially shift a closure state from one stage to the next stage; said closure state being output from a stage to become a control signal to close one of said switches.

9. An optical projection system as set forth in Claim 5 wherein: said strip-like elements are inclined with respect to the horizontal by an amount substantially equal to the vertical deflection rate for a scan line.

10. An optical projection system as set forth in Claim 5 wherein: the width of said strip-like elements is equal to an integer number N of horizontal scan line widths, where N is two or more.

11. An optical projection system as set forth in Claim 10 wherein: the sector width integer N is evenly divisible into the number of horizontal scan lines in the image frame.

12. An optical projection system as set forth in Claim 10 wherein: said sector width integer N is between 10 and 100.

13. An optical projection system as set forth in Claim 11 wherein: said raster scan produces an image frame of 525 lines and wherein: said sector integer N is 7, 15, 21, or 105.

14. An optical projection system as set forth in Claim 11 wherein said raster scan produces an image frame of 625 lines and wherein: said sector integer N is 5, 25, or 121.

15. An optical projection system as set forth in Claim 11 wherein said means for selectively controlling includes: means for detecting the horizontal synchronizing pulse of the horizontal scan line just prior to the first scan line of a selected sector electrode.

16. An optical projection system as set forth in Claim 15 wherein said means for selectively controlling includes: means for initiating an erasure cycle for said selected sector electrode based on said detected synchronizing pulse.

17. An optical projection system as set forth in Claim 16 wherein said erasure cycle initiating means includes: means for counting a delay time between said detected synchronizing pulse and the start of an erasure cycle such that said erasure cycle time is substantially equivalent to the shortest physical erasure time for said selected electrode.

18. An optical projection system as set forth in Claim 17 wherein said erasure cycle initiating means includes: means for closing a switch between said selected sector electrode and voltage source for a predetermined period of time after the imitation of said erasure cycle.

19. An optical projection system as set forth in Claim 18 wherein said erasure cycle initiating means further includes: means for opening said switch after said predetermined closure time to provide a relaxation time period prior to the start of the scanning of said selected sector electrode.

20. An optical projection system for displaying a visible image on a screen, said system comprising: an imaging member of the electro-optic type in the shape of a thin plate; a multiplicity of strip-like erasure elements arranged on one side of said imaging member in the shape of an image frame area, wherein the erasure elements are substantially a horizontal line length in length and two or more horizontal line widths in width; an illumination source for irradiating said imaging member with radiation of an intensity and wavelength which produces a visible image; a system control for generating a video signal for modulating the scanning on electron beam in a raster pattern over the image frame area of said imaging member to write an image, said imaging member being written by changing the opalescence of the area scanned as a function of the electron beam intensity whereby the varying areas of opalescence will modulate the scattering of said illuminating radiation; sector erasure control means for selectively controlling individual strip-like elements to erase a sector of the image written by said system control means which corresponds to the area of the selected element; and means for projecting the image from said imaging member onto the screen.

21. An optical projection system as set forth in Claim 20 wherein said sector erasure control includes: electrode control processor means for determining an indication of the position of the electron beam scan on said image frame area; and means for controlling said sector electrodes so that a sector of said image frame area is erased just prior to being written.

22. An optical projection system as set forth in Claim 21 wherein said means for determining the position of the electrode beam include: means for detecting said video signal including the horizontal synchronizing signal; and means for interrupting said electrode control processor upon detection of said horizontal synchronizing signal.

23. An optical projection system as set forth in Claim 22 wherein said sector electrode controlling means includes: a plurality of solid state switches, each having a control terminal and two switch terminals connected by a switch, wherein a control signal applied to the control terminal will cause a closing of the switch and the application of an erasure potential to the selected sector electrode; and a shift register having a plurality of outputs from stages corresponding to the control terminals of said plurality of switches, said outputs adapted to selectively operate said switches in dependence upon the logic state of the corresponding stage.

24. An optical projection system as set forth in Claim 23 wherein said shift register includes: a clock input which is activated by a logic shift signal to shift the logic state of one stage to the next stage.

25. An optical projection system as set forth in Claim 24 wherein said shift register includes: a data input for setting the logic state of the first stage of said register from a start bit signal.

26. An optical projection system as set forth in Claim 25 wherein said shift register includes: a reset input which is activated by a clear signal to reset the logic state of all stages of said shift register.

27. An optical projection system as set forth in Claim 26 wherein said shift register includes: a data out output for outputting the logic state of the last stage of said shift register; and wherein said data out output is connected to said data in input.

28. An optical projection system as set forth in Claim 22 wherein said sector electrode controlling means include an application specific integrated circuit including: a plurality of solid state switches, each having a control terminal and two switch terminals connected by a switch, wherein a control signal applied to the control terminal will cause a closing of the switch and the application of an erasure potential to the selected sector electrode; and a shift register having a plurality of outputs from stages corresponding to the control terminals of said plurality of switches, said outputs adapted to selectively operate said switches in dependence upon the logic state of the corresponding stage.

29. An imaging member for an optical projection system which is written by an electron beam in a raster format of frames, said imaging member comprising: a thin plate of electro-optic material; a transparent electrode on one side of said plate arranged in the shape of said image frame area; and a multiplicity of transparent strip-like electrodes on the other side of said plate arranged in the shape of said image frame area.

30. An imaging member as set forth in Claim 29 wherein: said electro-optic material is PLZT.

31. An imaging member as set forth in Claim 29 wherein: said strip-like elements are inclined with respect to the horizontal an amount substantially equal to the vertical deflection rate for a horizontal scan line of said raster.

32. An imaging member as set forth in Claim 29 wherein: the length of said strip-like elements is substantially equivalent to a horizontal scan line length; and the width of said strip-like elements is equal to an integer number N of horizontal scan line widths, where N is two or more.

33. An imaging member as set forth in Claim 32 wherein: the sector width integer N is evenly divisible into the number of horizontal scan lines of the image frame area.

34. An imaging member as set forth in Claim 32 wherein: said sector width integer N is between 10 and 100.

35. An imaging member as set forth in Claim 32 wherein said raster scan produces an image frame of 525 horizontal seam lines and wherein: said sector width integer N is 7, 15, 21, or 105.

36. An imaging member as set forth in Claim 32 wherein said raster scan produces an image frame of 625 horizontal scan lines and wherein: said sector width integer N is 5, 25, or 121.