This invention relates to the field of display systems, more particularly to color field sequential display systems.
Micromechanical devices are small structures typically fabricated on a semiconductor wafer using techniques such as optical lithography, doping, metal sputtering, oxide deposition, and plasma etching which have been developed for the fabrication of integrated circuits.
Digital micromirror devices (DMDs), sometimes referred to as deformable micromirror devices, are a type of micromechanical device. Other types of micromechanical devices include accelerometers, pressure and flow sensors, gears and motors. While some micromechanical devices, such as pressure sensors, flow sensors, and DMDs have found commercial success, other types have not yet been commercially viable.
Digital micromirror devices are primarily used in optical display systems. In display systems, the DMD is a light modulator that uses digital image data to modulate a beam of light by selectively reflecting portions of the beam of light to a display screen. While analog modes of operation are possiblexe2x80x94that is modes in which the mirror deflection is a function of the input data or bias voltagexe2x80x94DMDs typically operate in a digital bistable mode of operation in which the mirror is fully deflected at all times regardless of the image data applied to the mirror.
DMD-based display systems operating in the bistable mode require pulse-width modulation to create intermediate tones. Typical pulse width modulation schemes divide a frame period into binary bit periods. Each image data bit in the input data word controls the operation of the mirror during one bit period. Thus, if the bit is active, the mirror is turned on during the bit period and light from a light source is directed to the image plane during the bit period, If the image data bit is not active, the mirror is turned off during the bit period and light form the light source is directed away from the image plane during the bit period. The human eye, or other photoreceptor, integrates the energy directed to each pixel to create the perception of intermediate intensity levels.
Typical binary pulse width modulation system divide the larger bit periods into two or more bit-splits which are distributed throughout the frame period. Spreading the contribution of the large data bits throughout the frame period eliminates some of the artifacts created by the binary pulse width modulator schemes.
Full color images are created in one of two ways. One method uses three DMDs in parallel to produce three primary color images. The three primary color images are superimposed to form a single full-color image. The primary drawback of this method is cost. Not only do the three DMDs raise the cost of the system, they require the use of large, expensive, precision color prisms to accurately separate and recombine the light to and from the three DMDs. The large prism assembly requires the use of a large projection lens. Three-chip systems also require expensive alignment procedures to converge the images from the three separate modulators.
A second method uses one DMD and a spinning color filter wheel or drum. As each of the three primary color filters held by the wheel passes through a beam of light, the beam of light is temporally segmented into three sequential primary color light beams. Each temporal segment of the light beam is used to sequentially form three single-color images. If the three images are formed in rapid succession, the human eye perceives a single full-color image. Using three sequential primary color images to form a full color image is called field sequential display.
This field sequential method also has many drawbacks. While a single DMD is less expensive than the three used by the other method, a single DMD system requires a color wheel and motor, as well as additional data memory to store the two colors that are not being displayed. The color wheel can be noisy, especially when driven very fast. Color wheels also take time to synchronize to the incoming video signal, which can be annoying when xe2x80x9cchannel-surfing.xe2x80x9d
Single-chip systems are inherently less bright than a three-chip system using the same light source since at any given time only one-third of the available light is being used. The image brightness is also reduced because the modulators must be turned off when the color wheel is transitioning between two filters, commonly called the spoke time. Single DMD systems also create color separation artifacts when displaying motion since the image moves between the color fields. The use of a color wheel also limits the effective pixel depth to about 8 bits, since the frame period is divided into three color periods and the minimum cycle time for a DMD is about 20 xcexcS. To match the picture quality of the traditional cathode ray tube (CRT) a DMD-based system must be capable of displaying the equivalent of 10 bits per color at a frame rate of 72 Hz.
What is needed is a method and system for obtaining the benefits of a field sequential system while overcoming or mitigating the traditional drawbacks of systems that use a color wheel to divide the white light beam into three sequential primary color beams.
Objects and advantages will be obvious, and will in part appear hereinafter and will be accomplished by the present invention which provides a method and system for electronically switching colors in a field sequential display system. According to one embodiment of the disclosed system, a sequential color display system is provided. The display system comprises: a white light source for emitting a beam of light along a first light path, a first set of dichroic filters on the first light path operable to separate the beam of light into three primary color light beams traveling along three primary color light paths, three spatial light modulators, each on one of the three primary color light paths and operable to selectively transmit portions of said primary color light beams, a second set of dichroic filters positioned to combine the selectively reflected portions of the primary color light beam into a recombined light beam, and a projection spatial light modulator positioned to receive the recombined light beam and to selectively transmit portions of the recombined light beam to an image plane in response to image data signals received from a controller. The set of three spatial light modulators sequentially provide a primary color to the projection spatial light modulator.
The present invention provides the technical advantage of producing a sequential color image without the use of a color wheel. The display system allows near-instantaneous synchronization with a received image signal since there is no color wheel to spin up or down, and can alter the color sequences or duration as needed to optimally display a received signal. The spatial light modulators used to switch the colors can have defects without having the defects noticeably affect the projected image. Using a spatial light modulator to create the primary color sequence allows the intensity of the light to be adjusted to improve bit depth. For example, a given modulator may turn off a portion of its elements to reduce the illumination intensity during a given period. The illumination modulators can also be adjusted to change the color temperature of the projected image.