Patent Application
20060132472
The present invention relates to display devices, and more particularly to laser-based display devices.
Conventional displays are currently produced in several technology types, including cathode-ray tube (CRT), light emitting diode (LED), liquid crystal displays (LCDs), and projection display systems. CRT displays utilize a vacuum tube and an electron beam source mounted behind a luminescent screen to generate an image. LED displays include an array of light emitting pixels that are individually addressed by an active or passive backplane (addressing circuitry) to generate an image. Projection display systems utilize a projection device that projects an image onto a passive, typically white screen, which is reflected back toward an audience.
Large area display applications (e.g., greater an 60″) are most commonly implemented using projection display technology due to their lower cost and power consumption. CRT and LED displays are typically cost effective to product and operate when relatively small in size, but are typically too heavy and/or require too much power to operate when produced in a large area display format. In contrast, projection display systems are more easily scalable to larger area formats simply by increasing the size of the relatively low-cost, light weight screen, and increasing the size of the image projected on the screen.
Projection displays include arc lamp displays and laser-based projection displays. Early projection display systems used a white light source, such as a xenon arc or halogen lamp, that illuminates one or more light valves or spatial light modulators with appropriate color filtering to form the projected image, thus facilitating the production of relatively inexpensive, scalable, low-power, large area displays. However, such arc lamp projection displays are often criticized because of poor picture sharpness, a small viewing angle, and because the projected picture is readily “washed out” by bright ambient light. More recently, laser-based projection displays have been introduced that operate in a manner similar to arc lamp projection displays, but avoid the picture quality issues by utilizing relatively bright red, green and blue laser beams to generate much higher quality projected images. A fundamental problem with large-area laser-based displays, however, is the laser power that is required to generate a suitable picture. The power required (e.g. >1 W) is well beyond that which is considered safe in consumer applications. In addition, inexpensive lasers with sufficient power are not yet available, especially at the green and blue wavelengths, thus making laser-based displays significantly more expensive than arc lamp displays. Moreover, even high-powered displays become washed out in high ambient light due to their use of white screens (which are used to limit the required laser brightness). Dark or black screens may be used to prevent this wash-out problem, but this only increases the power requirements on the lasers, making the overall display system impractically expensive.
What is needed is a scalable, large area display apparatus that provides a picture equal to or greater than state of the art laser-based projection displays, but is less expensive to produce and operate, and avoids the safety concerns associated with the use of high powered lasers.
The present invention utilizes an emissive (visible light-emitting) screen and a laser addressing system to provide a scalable low-cost display apparatus that solves both the safety and brightness issues associated with conventional laser-based projection displays. The emissive screen includes an array of red, green, and blue pixels that are addressed solely by the laser addressing system (i.e., no active or passive addressing backplane is provided on the emissive screen). Similar to the light amplification techniques utilized in image enhancement (e.g., night vision) systems, each pixel of the emissive screen includes a photon-multiplication device formed by a luminescent pad located near a photocathode. When the laser addressing system transmits the laser beam onto the photocathode of a selected pixel, free electrons are created that are accelerated by an applied electric field from the photocathode to the luminescent pad, thereby causing the luminescent pad to emit visible light with a brightness (energy) that is dependent only on the optical gain of the photon-multiplication device. Because the laser beam is not image-forming in itself (i.e., most of the power used to produce the image is provided by the emissive screen), a single low-power laser (or a small number of parallel lasers nominally the same wavelength or different wavelengths) may be used to generate a color image. Thus, the cost and safety issue related to conventional laser-based displays is addressed by facilitating the use of “safe” (i.e., low power) lasers that generate any visible, near UV or UV wavelength. Moreover, because pixel addressing is performed by scanning and modulating the laser beam using the laser addressing system, the emissive screen does not require an active or passive matrix backplane to address the light-emitting pixels, thus facilitating production of the emissive screen using low-cost screen printing and blanket coating techniques. Accordingly, the present invention facilitates the production of displays including very large (e.g., 60″ or more) emissive screens that both avoid the safety issues associated with conventional laser-based projection displays, and can also be produced at a substantially lower cost than any conventional laser-based, CRT and LED display.
In one embodiment, the emissive screen includes spaced-apart photocathode and photoanode plates that are produced using inexpensive screen-printing or blanket coating techniques. The photocathode plate includes a glass pane with a conductor layer formed on its inside surface, and a photocathode material formed on the conductor layer. The photoanode plate includes a second glass pane having a second conductor layer formed on its inside surface, and a photoanode layer including blue, green, and red luminescent regions printed or otherwise formed on the second conductor layer. In a reflective-type arrangement, the laser beam passes through the photoanode plate to activate a selected photocathode region, and the resulting visible light is emitted back through the photoanode plate (i.e., toward the laser beam source). In a transmissive-type arrangement, the laser beam passes through the backside of the photocathode plate to activate a selected photocathode region, and the resulting visible light is emitted back through the photocathode plate (i.e., toward the laser beam source). In yet another embodiment, the laser beam passes through the back side of the photocathode plate to activate a selected photocathode region, and the resulting visible light is emitted through the photoanode plate (i.e., away from the laser beam source).
In another embodiment, the emissive screen includes pixels having spaced-apart, hexagonal luminescent regions that define central apertures for passing the laser beam to the pixel's photocathode. The apertures facilitate the use of relatively low energy laser beams by facilitating relatively unimpeded passage through the photoanode plate, and also relax the requirements imposed on the scanning system by limiting pixel activation to beam energy that passes through the relatively small apertures. The hexagonal luminescent regions are separated by a black border region that improves contrast, and thus image quality.
In other embodiments, different approaches are disclosed for increasing the spacing between the photoanode and photocathode plates, thereby facilitating the use of higher energy (and higher efficiency) phosphors. In one embodiment, doughnut-shaped (annular) anode electrodes are formed under the hexagonal luminescent regions to focus the freed electons such that they only activate the luminescent region of the addressed pixel. In another embodiment, a “honeycomb” stand-off plate is mounted between the photocathode plate and the photoanode plate. The stand-off plate defines passages that extend between the photocathode region and the luminescent region of an associated pixel, thereby acting as a conduit that directs electrons from the photocathode region to the associated luminescent region.
In another embodiment, inexpensive, molded millichannel plates are utilized to produce the desired photon-multiplication effect. These millichannel plates are similar to MicroChannel Plates (MCPs), which are utilized in second and third generation image enhancement systems to produce higher photon-multiplication. However, MCPs are only available in sizes that are substantially smaller than the large area display format of the emissive screen, and are also too expensive for practical use in such large area applications. The molded millichannel plates are similar to the honeycomped stand-off plates described above, but include channels coated with an electron-producing material, and utilize an applied high voltage potential to facilitate the desired photon-multiplication.
In accordance with another aspect of the present invention, a display apparatus includes a Position Sensitive Detector (PSD) that is provided on or next to the emissive screen, and is utilized to detect and measure the timing and coordinates of the impinging laser beam, and to transmit this timing/location data to the laser scanning/modulating system. The thus-produced closed-loop laser control system avoids the need for precise alignment between the laser addressing system and the emissive screen, and significantly relaxes the specification requirements (and thus the cost) of the scanning/modulating system over that required in an open-loop arrangement, thereby potentially significantly reducing manufacturing costs. In one embodiment, the PSD includes one-dimensional (1D) sensor strips mounted along the vertical edges of the emissive screen to detect a laser pulse generated at the start and end of each scan path. The 1D PSD strips detect the vertical location of the impinging beam at the beginning and end of each scan, for example, by detecting differential currents at each end of the ID PSD strips. Timing and location data generated associated with the detected beam are transmitted by wire or wireless transmission (e.g., infrared) to the laser scanning/modulating system, which uses the data to register (aim) the laser beam and to modulate the laser beam's energy. In addition to the side-located PSD strips, one or more 1D vertical PSD strips may be utilized inside the active display area (e.g., mounted behind the screen). Moreover, in another embodiment, the photocathode or photoanode layers of the emissive screen may be used to provide “free” two-dimensional PSD sheets that can be used to modulate the laser beam, thereby facilitating the use of a low-cost scanning system.
In accordance with yet another aspect of the present invention, ambient light is filtered to prevent generating unintended pixel activation. In one embodiment a filter coating is utilized to generate a high-pass optical filter that only passes light in the wavelength of the selected addressing laser. Another embodiment utilizes a spatial filter that only passes light received from the direction of the laser addressing system. Yet another embodiment utilizes electronic filtering to pass only signals having frequencies characteristic of the addressing laser.
These and other features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings, where:
Emissive screen 110 includes an array of pixels 115 that include a simple photo-multiplier arrangement for emitting visible light in a manner similar to that utilized in so-called night-vision (i.e., image enhancement) systems. Emissive screen 110 includes a photocathode plate 120 and a photoanode plate 130 that are maintained at a high voltage potential during operation, with photocathode plate 120 coupled to a first, relatively low (negative) voltage source V1, and photoanode plate 130 coupled to a second, relatively high (ground or positive) voltage source V2. Both photocathode plate 120 and photoanode plate 130 are planar (flat) glass plates that are maintained in a parallel relationship (i.e., separated by a gap distance G) by appropriate edge structures (not shown), and fabricated such that a vacuum (or low pressure) region 140 is defined between photocathode plate 120 and photoanode plate 130. Photocathode plate 120 includes one or more layers of photocathode material (e.g., magnesium) that may be segmented (as indicated by dashed lines) into an array of photocathode regions 125. Photoanode plate 130 includes a corresponding array of luminescent regions 135, with each luminescent region 135 being spaced from a corresponding photocathode regions 125 by a corresponding portion of vacuum region 140. Each pixel 115 is formed by a photocathode region 125, the corresponding luminescent region 135, and the corresponding portion of vacuum region 140. For example, referring to the upper left portion of
Laser addressing system 150 is similar to laser systems utilized in conventional laser-based displays in that laser system 150 includes a scanning/modulating apparatus 152 that raster scans laser beam 157 in a predetermined two-dimensional pattern across the pixel array of emissive screen 110, and modulates laser beam 157 to selectively transmit high energy pulses to selected pixels 115 of emissive screen 110. In one embodiment, the scanning and modulating functions performed by scanning/modulating apparatus 152 are similar to those performed in conventional laser systems, and electromechanical systems utilized to provide these functions are therefore well known to those skilled in the art. Such systems may be formed, for example, using semiconductor lasers, collimation/focusing optics, two-dimensional (2D) scanning systems, and electronics for laser modulation that are well-known to those skilled in the art. Many implementations of 2D optical scanners are known in the art. One example of a suitable embodiment for a large projection TV type display apparatus might be a small spinning polygon mirror for the fast horizontal direction in combination with a micromachined galvo scanner operated in mechanical resonance for the slow vertical direction. Note that the scanner doesn't require any particularly tight specifications (e.g., linearity, angular accuracy, repeatability, drift, etc.) when a position sensitive device (described in detail below) is utilized to determine the location of the impinging beam. Such a scanner can be considered as the display equivalent of “reflex printing” in xerography, and could provide a very inexpensive type of scanner.
As set forth above, laser beam 157 is not image-forming in itself, as in conventional reflective laser-based projection displays, but is merely used to address (i.e., produce local light emission from) the pixels of emission screen 110. Accordingly, by forming emissive screen 110 to include red, green, and blue pixels (i.e., pixels having luminescent regions formed, for example, by red, green, and blue phosphor material), display apparatus 100 provides a full color display system in which laser addressing system 150 may be implemented using a single laser or small group of parallel lasers having nominally the same (e.g., violet, ultraviolet (UV), near-UV, or visible) wavelength. That is, unlike conventional reflective laser-based projection displays that require the use of red, green and blue lasers to produce a full color image, a single laser wavelength may be used to activate red, green and blue pixels of emissive screen 110, thereby facilitating the use of a substantially lower cost laser system than that used in conventional laser-based systems. Further, the intensity (energy) of the light emitted by emission screen 110 is substantially higher than the incident laser beam (i.e. emissive screen 110 has built-in optical gain). Therefore, according to another aspect of the present invention, display apparatus 100 is able to produce high quality images using a relatively low-power laser (i.e., substantially lower power than that used in reflective-type laser-based displays), thereby avoiding the safety issues associated with conventional laser-based projection systems by facilitating the use of lasers that meet established safety requirements. Thus, safety-rated violet, UV, near-UV and visible lasers may be used to form residential embodiments of display apparatus 100.
According to another aspect of the present invention, by solely utilizing laser addressing system 150 to activate selected pixels, emissive screen 110 may be fabricated using inexpensive, high yield fabrication methods that facilitate scalability. In particular, similar to projection screens, emissive screen 110 does not require an active or passive matrix backplane to address the light-emitting pixels. Accordingly, emissive screen 110 can be produced by screen-printing the luminescent material (e.g., phosphors), and blanket coating all other materials (e.g., photocathode materials, conductive layers, and spacer materials). Thus, the size of emissive screen 110 is not limited by whatever large-area processing equipment is available at the time, thereby avoiding the relatively high costs and low production yield associated with the use of such equipment. The present inventors believe that the absence of any kind of matrixed backplane, active or passive, and the absence of large-area processing lines to be kept up-to-date, might dramatically reduce the cost of emissive screen 110 in comparison to conventional display alternatives. The cost advantage would only get larger for increasing screen sizes. Further, cost efficiencies arise from the ability to use a single laser system to implement displays of several sizes. For example, referring to
Additional features and aspects of display apparatus 100 will now be described with reference to several exemplary embodiments.
Photocathode plate 120A includes a first flat glass pane 122A, a first conductive layer 124A formed on an inside surface of glass pane 122A, and a photocathode layer 125A formed on conductive layer 124A (for descriptive purposes, photocathode layer 125A is indicated by a first region 125A1, a second region 125A2, and a third region 125A3). Photocathode material layer 134A includes, for example, at least one of an alkali glass, a semiconductor material, and a glass doped with at least one of magnesium and aluminum. Note that there may be real or perceived safety issues with scanning violet, UV or near-UV laser light in living rooms, even at low power. If so, it should be possible to use a longer wavelength laser, in the visible, maybe even red wavelengths. Photocathode materials with lower work functions are needed in this case (e.g., potassium (K) or sodium (Na) doped glass, instead of Al or Mg, carbon nanotubes or carbon powder, or materials with even lower work functions, such as diamond like carbon).
Photoanode plate 130A includes a second flat glass pane 132A that is parallel to first glass pane 122A, a second, transparent conductive layer 134A (e.g., indium-tin oxide (ITO)) formed on an inside surface of glass pane 132A, and luminescent regions formed on conductive layer 124A. The luminescent regions include a green region 135A1 that is located opposite to photocathode region 125A1, a blue region 135A2 that is located opposite to photocathode region 125A2, and a red region 135A3 that is located opposite to photocathode region 125A3. These red, green, and blue luminescent regions are formed using fluorescent quantum dot nanoparticles produced, for example, by NanoSys Inc. of Palo Alto, Calif., USA. An inexpensive fabrication method involves using such nanoparticles with clear polymer binder that is screen printed in three passes onto a thin carrier sheet. Similar approaches are possible using phosphors and appropriate dyes or pigments.
As depicted at the upper portion of
Note that the wavelength/color of the visible light emitted by emission screen 110A depends on which luminescent region is “selected”. Those skilled in the art will recognize that selecting the red, green, and blue pixels in an appropriate sequence and frequency will produce a desired color (e.g., simultaneously selecting adjacent red and blue pixels produces an apparently purple dot on the screen surface).
In addition to the projection-like arrangement depicted in
According to another aspect of the present invention, each of luminescent regions 135C1 through 135C10 defines a central, circular aperture 138C for passing the laser beam to a selected pixel's photocathode. As discussed below, the aperture may be covered by a filter material, but at any rate are substantially transparent to the incoming laser beam, thereby facilitating the use of relatively low energy lasers by allowing substantially unimpeded passage of the beam through photoanode plate 130. Note that, in the previous embodiments, the laser beam was required to pass through one or more conductive, luminescent and/or photocathode layers. Apertures 138C also relax the requirements imposed on the laser scanning system by preventing pixel activation unless the laser beam passes through the aperture. For example, when luminescent regions 135C1 through 135C10 have diameters of 0.4 mm, providing an aperture having a diameter of approximately 0.125 mm facilitates the use of an incident laser beam having a diameter up to 0.35 mm without risk of exposing more than one aperture at a time, regardless of spot position. This further relaxes the requirements imposed on the laser scanning system, and one approach might be to slightly overlap the scans to make sure the entire photoanode area is covered.
According to another aspect of the present invention, luminescent regions 135C1 through 135C10 are separated by a black (or other dark color), non-luminescent border region 139C. As discussed above, the “blackness” of such border regions is found to be directly proportional to the contrast, depth and dynamic range of images generated by displays utilizing black pixel borders. By providing emissive screen 100C with sufficiently high optical gain, the problems associated with generating suitable images using black border region 139C are overcome, thus providing a potentially exceptional viewing experience without the need for high powered (and thus dangerous) lasers.
The maximum vacuum region spacing between luminescent region 135C5 and photocathode region 125C5 is limited by the divergence angle of the emitted electrons. In the embodiment of
In accordance with another embodiment of the present invention, the conductive layer formed on the photoanode plate includes a series of doughnut-shaped (annular) anode electrodes (as opposed to a blanket coating) that focuses the freed electrons toward the addressed (targeted) luminescent region, thereby avoiding unwanted activation of adjacent pixels. Referring to the lower left corner of
The embodiments described above have relied on first generation image enhancement technology to provide the photon-multiplication utilized by the various emission screens. The following example illustrates the use of second generation image enhancement technology to generate higher optical gains than that possible using first generation technology. However, the examples disclosed herein are not intended to be limiting, and those skilled in the art will recognize that any suitable image enhancement technology may be beneficially utilized to produce an emission screen in accordance with the present invention.
MCPs for conventional second and third generation image enhancement systems are quite expensive, and only available in small sizes. Such MCPs are typically made of high-efficiency (1000-10000× gain) secondary electron emission alkali glasses with pores in the 10-20 micron diameter range. They are made by pulling a large bundle of alkali glass tubes to thinner and thinner diameters and finally slicing the bundle into plates. More recently, silicon based MCPs have become popular. In the present embodiment, MCP 170 represents a crude version of these high gain MCPs in that MCP 170 provides only modest gain (e.g. order of 10× electron multiplication, in addition to 10× gain from the HV electron acceleration), and much larger hole sizes is all that is needed, but it needs to be inexpensive and scale inexpensively to large areas.
Important material properties for both MCP 170 and stand-off plate 160 (described above) are: (1) no outgassing under vacuum, (2) high electrical resistivity, because of the high voltage across the plate, (3) thermal expansion coefficient matched to the glass used for front & back panels, and (4) reflective sidewalls or light colored light-scattering sidewalls. In addition, the MCP 170 includes secondary electron emission material 176 (i.e., a material such as MgO having high secondary electron emission yield). Many other suitable secondary electron emission materials are known in the art. Finally, both top and bottom of the MCP 170 include metalization 178 for contacting purposes. Metalization 178 typically partially penetrates into the channels, and this is known to be advantageous for electron collimation at the exit side. A high voltage is applied across the top and bottom surface. Tailored conductivity (typically a few hundred MΩ top to bottom) of the plate bulk material (leaded alkali glass) provides a path for supplying the secondary electrons to the sidewall without drawing excessive shunt current. Alternatively, the bulk material is highly insulating, but coated with a conductor with appropriate conductivity prior to coating with the electron emission material. The requirements of no-outgassing (1) and expansion matching (3) point towards glasses and ceramics, and away from polymers. Glasses and ceramics are notoriously difficult to machine, but molding of glass or ceramic for the honeycombs may be an option given that the required hole diameter is relatively large. Sintering glass frits in a “bed-of-nails” mold is a possibility, but a moldable ceramic called Mykroy-Micalex seems particularly promising. The material contains no polymers (mixture of glass frit and mica particles) but can be molded as a plastic. The dimensional tolerances are very tight and the thermal expansion coefficient is well matched. By appropriate selection of the glass frit or by addition of appropriate additive materials in the mix, the electrical resistivity may be controlled to within a range needed for MCP 170. The molded ceramic panes may be used as-is as passive collimator, or coated with electron emission material and metallized for use as a coarse MCP. Thickness of the plates might be in the range of 5-10 mm.
In accordance with another aspect, display apparatus constructed in accordance with the present invention may utilize an open loop scanning/modulating system (e.g., as depicted in
In accordance with the present embodiment, the PSD mechanism includes vertical, one-dimensional (1D) PSD strips 181 and 182 positioned along the side edges of the active screen area (i.e., the portion of emissive screen 110F formed by photocathode plate 120F and photoanode plate 130F; i.e., the portion defines the array of pixels 115). PSD strips 181 and 182 generate detection signals indicative of the timing and vertical location of laser beam 157 in the manner described below, and these detection signals are provided to a detector circuit 185, which in turn processes the detection signals for transmission to laser addressing system 150F. Referring to
According to another alternative embodiment, the differential currents utilized to locate the laser beam impingement position may also be utilized to determine both the laser beam energy (e.g., by measuring differential currents in the photocathode plate) and the pixel brightness (e.g., by measuring differential currents in the photoanode plate). In this embodiment, the sum of the collected currents in the photoanode plate at any given point in time is a measure of the electrons generated and, therefore, of the brightness of the corresponding pixel. This information can be used to calibrate out pixel non-uniformity, aging effects, or auto-adjust for ambient lighting conditions etc. In a similar manner, the collected currents in the photocathode plate can be used to measure the energy imparted by the laser beam to the emissive screen.
The unintended amplification of photons from ambient light (i.e., optical noise) is another issue that may present a problem to the operation of emissive screens formed in accordance with the present invention. Ambient light may be prevented from significantly effecting the operation of the emissive screen by operating the laser scanning/modulating system in a way that the power density from ambient light is insignificant relative to the time-averaged power density from the focused laser beam. Addressing the ambient light problem in this manner will probably be the main consideration dictating the lower bound on laser power and the upper bound on the photon-multiplier gain. However, although utilizing a relatively high laser power (i.e., relative to ambient light) may solve the ambient light problem, in some high ambient brightness situations, this solution may be unsatisfactory or undesirable due to the safety-related limits on laser brightness. The following paragraphs set forth other possible solutions to this potential problem.
A second solution to the potential ambient light problem utilizes a spatial filter, such as a collimation screen, similar to stand-off plate 160 (described above with reference to
An alternative or complementary approach is to use electrical filtering. The light originating from the laser beam has a characteristic modulation pattern, from the bitmap (source) image data, but particularly from scanning across the fixed grid of UV entry apertures (in the case of the embodiments described above with reference to FIGS. 3 to 7). The laser light that enters an aperture generates electrons that flow through the anode/cathode/power-supply circuit. No electrons are generated or current flows when the laser spot is in between apertures. Hence, the current in the anode/cathode/power-supply circuit shows a characteristic modulation with frequency equal to scan rate divided by aperture spacing. When an electrical bandpass filter centered around this frequency is added to the circuit, only light that shows temporal modulation within the band will be multiplied. (Quasi-) DC ambient light or 50 Hz fluorescent light will not be multiplied; light from the laser will. The modulation frequency from the image data is also within the pass band. This electrical “lock-in” approach would eliminate the need for the optical front filter coating and leave more freedom in laser and photocathode material choice.
A final aspect of the present invention involves maintaining the vacuum gap provided between the photocathode and photoanode plates of the emissive screen. Plasma displays and especially Field Emission Displays (FEDs) use embedded getter materials to help maintain vacuum quality over time. Given the modest gain needed in the emissive screen of the present invention, the inventors assume that the vacuum level requirement of the emissive screen is moderate. That is, unlike FEDs which use emission tips that get very hot and oxidize in the presence of residual oxygen, the emissive screen of the current invention does not use tips that get hot. Further, the small-gap configuration of the FEDs dictate their high current/low energy mode of operation, and the FED phosphors are also known to heat up more, and react more with residual gasses. In contrast, the emissive screens of the present invention, and in particular the “wide gap” embodiments described above with reference to
Although the present invention has been described with respect to certain specific embodiments, it will be clear to those skilled in the art that the inventive features of the present invention are applicable to other embodiments as well, all of which are intended to fall within the scope of the present invention.
