Television – Video display – Color sequential
Reexamination Certificate
2001-08-10
2004-06-22
Hsia, Sherrie (Department: 2614)
Television
Video display
Color sequential
C348S750000, C348S757000
Reexamination Certificate
active
06753931
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to generating color images from pulsed light sources, and in particular it relates to the sequencing of pulses for generating color images in laser-based image display systems.
BACKGROUND OF THE INVENTION
In modern society information is frequently communicated to an audience by displaying it on a display device. For example, images and text are often shared by displaying them on a monitor such as a cathode ray tube (CRT) or projecting them on a projection screen. More recently, passive and active matrix display systems with flat screens have been widely employed for the same purposes. CRT technology is well-understood and CRTs are used extensively in displaying color images. Unfortunately, CRTs are limited in their ability to display large and detailed images. At close viewing distances pixel size and resolution place a bound on the achievable image contrast and detail. In addition, CRTs generate low frequency electromagnetic fields and X-rays, which are dangerous to viewers located close to the CRT unit. Thus, CRT technology remains confined to devices such as televisions, as these are viewed by users from larger distances.
Matrix displays are typically back-lit by a single incandescent white light source to generate the primary colors—red, green and blue (RGB)—that illuminate a liquid crystal display (LCD) panel. In an active display, the LCD panel has RGB pixels which are independently modulated by the LCD selection matrices that also generate the mastering. Although these projectors have fair resolution, there are other unavoidable problems related to this scheme. The incandescent white light source has a relatively short operating life and generates relatively large amounts of heat. The LCD devices cannot be manufactured without some minimum number of defects that, in turn, manifest themselves as permanent image artifacts on the screen regardless of the graphic or video source. Using LCD devices to generate the raster introduces a fixed and permanent resolution to the display device, making it very difficult to adapt the electronics to accept other resolutions for display of graphics and text information. Furthermore, the light intensity levels of LCD displays are low, rendering them generally unsuitable for viewing under adverse lighting conditions, e.g., outdoors.
Brighter video projectors have been constructed using lasers. Typically, the green and blue beams are generated by argon ion gas lasers that directly emit green and blue light and the red beam is usually generated by a liquid dye laser (pumped with part of the high power blue and green lasers). Laser-based display systems produce brighter images than non-laser based systems, they can achieve close to 100% color saturation, and they also exhibit pixel size stability. Unfortunately, such laser-based display systems have low light generation efficiencies and generate a high amount of waste heat. In addition, the lasers are large, the scanning systems are cumbersome and the resulting devices are too expensive for most common applications.
Several laser display systems have been proposed to address the above-mentioned limitations. U.S. Pat. No. 5,740,190 to Moulton teaches a three-color coherent light system adapted for image display purposes. This system employs a laser source and a frequency doubling crystal to generate green light at 523.5 nm. Moulton's system also generates blue light at 455 nm and red light at 618 nm by relying on frequency doubling and the nonlinear process of optical parametric oscillation. U.S. Pat. No. 5,534,950 to Hargis et al. describes using a microlaser and/or diode laser array for producing an image to be projected. The system includes three linear laser arrays, one red, one green and one blue, each individually addressable laser being powered and modulated in accordance with the input image signal. The image is produced line-by-line with the aid of a scanning mirror. In order to reduce the number of lasers required, a color laser display system described in U.S. Pat. No. 5,828,424 to Wallenstein et al. and in U.S. Pat. No. 6,233,025 B1 to Wallenstein employs nonlinear frequency conversion of light from a single pulsed laser source to produce the three fundamental colors necessary for operating the display.
U.S. Pat. Nos. 5,614,961 and 5,920,361 to Gibeau et al. also discuss methods and apparatus for image projection using the primary colors produced by laser arrays. They teach three linear laser arrays to generate a number of beamlets of the three fundamental colors. Each of the beamlets is individually modulated in luminance according to a specific encoding scheme representing the video image to be produced on the viewing screen. In some cases the fundamental colors are derived from lasers operating at twice the desired wavelength with the aid of nonlinear frequency conversion processes such as second harmonic generation (SHG).
Gibeau et al. recognize that the intensity of the beams can be adjusted by pulse width modulation (PWM). This technique involves varying the number of pulses (duty cycle) during each pixel time such that the average power delivered to any diode over the pixel time will correspond to light from the diode at specific intensity. For example, for maximum intensity the pulse is kept on during the entire duration of the pixel time. For ½ intensity the beam contains a number of pulses whose total duration adds up to ½ of the pixel time. In fact, Gibeau et al. teach that various amplitudes and duty cycles can be used to obtain the desired average power during each pixel time.
Minich et al. also recognize that proper pulsing of lasers in displays is important. In U.S. Pat. No. 5,700,076 they teach a projection light source which has a red laser for producing red high intensity light, a green laser for producing a green high intensity light and a blue laser for producing blue high intensity light. Each of the lasers is switched between ON and OFF states, and in this way the lasers are made to generate sequential mono-colored pulses of light. A single light valve is used to combine the three colors and filter them to produce the image for projection. Each mono-colored pulse is generated at its maximum luminosity level. To increase operation efficiency, each one of the lasers is controlled individually and sequentially by a computer to cause them to be deactivated at a near ON output luminosity for a short OFF period. The lasers are pulsed ON and OFF during the same frame time of the projection system such that each one is preferably on for one third of a frame interval. This means the average power consumed by the lasers is only approximately one-third of the peak power. Minich et al. also teaches that the lasers can be pulsed on for shorter periods of time.
Unfortunately, the prior art does not provide for efficient, low-cost and high power laser display systems using pulsed or continuous-wave (cw) RGB light and sharing light modulators (i.e., no separate modulators assigned to controlling red, green and blue light). Specifically, in systems using three cw lasers for producing RGB light each of the sources has to be off for about ⅔ of the time for performing time-multiplexing with one light modulator. This means that the lasers have to be driven at three times higher output power to generate the same color brightness as they produce when on all the time. The cost of a cw laser usually scales with its peak power, so using three cw lasers each of which has triple the power required is unattractive. Especially red cw diode lasers are expensive and have limited power. Thus, it would be advantageous to operate RGB display systems using red cw diode lasers in a time-multiplexing mode where the red cw diode laser is on for more than ⅓ of the time. This type of time-multiplexing would reduce the peak power requirements for the red cw diode laser. None of the above systems can be used to generate appropriately pulsed and sequenced RGB light or pulsed and cw light with average output power le
Kane Thomas J.
Kmetec Jeffrey D.
Hsia Sherrie
Lightwave Electronics
Lumen Intellectual Property Services
LandOfFree
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