Microdisplays are becoming increasingly popular as low cost, low power consumption, yet high resolution replacements for traditional information display components such as cathode-ray tubes. Their small size allows integration into hand-held products, such as camcorders and digital still cameras, while their high resolution capabilities promote usefulness in projection applications, such as televisions and business projectors.
The usefulness of microdisplays in projection applications depends greatly on the system's ability to project a sufficiently bright image. However, certain types of microdisplays, liquid crystal microdisplays in particular, require optical elements such as polarizers or diffusers that reduce the amount of light that reaches the viewing area. Moreover, the algorithms used to operate microdisplays to produce images also may result in wasted light, as will be explained immediately below.
Field-sequential color microdisplays produce color images by dividing an image frame into color segments. An image frame is a period of time during which the information necessary to produce a single image is displayed on the display device. A color segment is a portion of a frame during which the image information for a single color is displayed while the display is illuminated with that single color of light. Field-sequential color displays can be contrasted with non-sequential color systems, which usually combine three different color images simultaneously. By using the three primary colors, red, green and blue (RGB), in sequence, a field-sequential color display is capable of producing images from a palette of many colors. The size of the color palette is further increased by adding grayscale.
Grayscale refers to “shading” colors—varying the amount of each primary color included in the image—thus increasing the number of combined colors the system is capable of producing. Field-sequential color systems produce grayscale in one of several ways. One method is to vary the intensity of light either reflected by or transmitted through the device. A second method is to vary the duration of time that the light is reflected or transmitted. Methods for producing grayscale in microdisplays are well known. For example, U.S. Pat. No. 5,748,164, issued May 5, 1998, entitled Active Matrix Liquid Crystal Image Generator, describes various methods for producing gray-scale images in field-sequential color microdisplays, which patent is incorporated herein by reference in its entirety.
In prior art methods of producing gray-scale images in field-sequential color systems, light may be underutilized. For instance, because image information cannot be written to the entire display as fast as the light source can be switched to a different color, color transitions can result in image artifacts unless ameliorative steps are taken. One common ameliorative step is to make the display dark during the time that the light source is switched to a different color. Unfortunately, any light emitted while the display is dark is essentially wasted.
Light may also be wasted even if image information can be written to the entire display as fast as, or faster than, the light source can be switched to a different color. Most methods of switching a light source between colors do so in a finite amount of time, producing intermediate states of illumination that are either of a different color or a different intensity from the desired pure colors before and after the transition. For example, a color wheel that transitions between red and green will produce yellow light during the transition. A liquid crystal color switch may avoid such intermediate colors but will then produce intermediate light of varying intensity. In both cases the intermediate light is generally considered unusable directly by the display element in a field sequential color system designed to operate with primary colors of a single intensity. Again, a common ameliorative step is to make the display dark during the transition.
Light is also wasted in liquid crystal display systems that use a compensator cell while DC balancing the liquid crystal material. Compensators are more fully explained in U.S. Pat. No. 6,075,577, issued Jun. 13, 2000, entitled Continuously Viewable, DC Field-Balanced, Reflective Ferroelectric Liquid Crystal Image Generator, which patent is incorporated herein by reference in its entirety. DC balancing is desirable to prevent image sticking or image retention. It is achieved by inverting the electrical polarity sense of the pixel drive voltage. In the case of liquid crystal displays incorporating polarity responsive liquid crystal materials such as ferroelectric liquid crystals, reversing the electrical polarity sense of the pixel drive inverts the appearance of the image. In such cases, a compensator may be used to allow the inverted image to be displayed without affecting the appearance of the image at the image area. However, as with changing the color of the light source, the compensator can be switched much faster than data can be written over the entire display. Thus, to avoid image artifacts, one prior art solution is to make the display dark during a compensator transition.
A number of additional factors similar to those briefly discussed above also result in inefficient use of the available light in field-sequential color display systems. It is against this backdrop and a desire to solve the problems of the prior art, including a desire to increase the brightness of field-sequential color displays, that the present invention has been developed.
The present invention relates generally to a display for producing modulated light so as to form an image during a frame. The display includes a display panel having a plurality of pixels configured to modulate light in a temporal sequence. The pixels have a plurality of light modulating states, including OFF and ON. The display also includes a light source arrangement that illuminates the display panel, the light source arrangement being configured to selectively emit light of at least one of at least two different colors. The display also includes a data ordering arrangement that is receptive of incoming video data and that generates drive signals that drive the pixels to the light modulating states. The frame is divided time-wise into color segments with only one of the different colors of light being emitted from the light source arrangement during each color segment. The color segments are separated in time by transition periods during which more than one color of light may illuminate the panel. The incoming video data includes information from which the data ordering arrangement determines pixel brightness values for each color segment. The drive signals cause individual pixels to be in the same state during each transition period of a frame. The state of a pixel during the transition periods is determined by the pixel brightness values. Pixels with pixel brightness values above predetermined values for each color segment in a given frame are in a state other than OFF during the transition periods corresponding to the frame.
The number of color segments may be at least three. The light source arrangement may illuminate the panel with red light during one color segment, green light during a different color segment and blue light during a different color segment. The light source arrangement may illuminate the panel with at least two different colors during a transition period. The intensity of the different colors of light illuminating the panel during the transition periods may be adjustable to maintain a stable white point.
The display may also include a temperature sensing arrangement that senses the temperature of the light source arrangement. The intensity of the different colors of light illuminating the panel during the transition periods may be adjustable to maintain a stable white point in response to the temperature of the light source arrangement. The light source arrangement may include color filters. The light source arrangement may include a color wheel. The color wheel may be divided into at least three color areas. The color wheel may include red, green and blue color areas. The color wheel may include at least one broadly-transmissive area that passes a substantial amount of light across the visible light spectrum. The color wheel may include at least one broadly-transmissive area between each color area. The light illuminating the panel during a transition segment may travel through a broadly-transmissive area of the color wheel.
The display may also include a compensator cell having at least two states, one state wherein the compensator has no effect on the appearance to a viewer of individual pixels, and a second state wherein the compensator changes the appearance of pixels by inverting the contrast of the image. The compensator may change from the first state to the second state at least once during the frame. The compensator may change state during the color segment during which the light source is illuminating the display panel with blue light.
The present invention also relates to a display with a display panel. The panel has a plurality of pixels configured to modulate light in a temporal sequence so as to form an image during a frame. The pixels have a plurality of light modulating states, including OFF and ON. The display also includes a light source arrangement that illuminates the display panel. The light source arrangement is configured to selectively emit light of at least one of at least two different colors. The display also includes a data ordering arrangement that is receptive of incoming video data and that generates drive signals that drive the pixels to the light modulating states. The frame is divided time-wise into color segments with only one of the different colors of light being emitted from the light source arrangement during each color segment. The color segments include one or more grayscale periods. The incoming video data includes information from which the data ordering arrangement determines pixel brightness values for each color segment. The state of a pixel during each grayscale period is determined by the pixel brightness values for the corresponding color segment. Pixels with pixel brightness values above predetermined values for each color segment in a given frame are in a state other than OFF during corresponding grayscale periods of each color segment of the frame.
The present invention also relates to a display having a display panel. The panel has a plurality of pixels configured to modulate light in a temporal sequence so as to form an image during a frame. The display also includes a light source arrangement that illuminates the display panel. The light source arrangement is configured to selectively emit light of at least two different colors. The display also includes a compensator cell having at least two states, one state wherein the compensator has no effect on the appearance to a viewer of individual pixels, and a second state wherein the compensator changes the appearance of pixels by inverting the contrast of the image. The frame is divided time-wise into color segments with only one of the different colors of light being emitted from the light source arrangement during each color segment. The light source arrangement illuminates the panel with blue light during at least one color segment. The compensator changes from the first state to the second state at least once during the frame. The compensator may change state during the color segment during which the light source is illuminating the display panel with blue light.
The present invention also relates to a method of increasing the brightness of a display. The method includes providing a display panel having a plurality of pixels. The pixels have a plurality of light modulating states, including ON and OFF. The pixels are configured to modulate light in a temporal sequence so as to form an image during a frame. The method also includes providing a light source arrangement that illuminates the display panel. The light source arrangement is configured to selectively emit light of at least one of at least two different colors. The method also includes providing a data ordering arrangement that is receptive of incoming video data and that generates drive signals that drive the pixels to the light modulating states. The method also includes dividing the frame time-wise into a plurality of color segments. The method also includes inserting transition periods between each color segment. The method further includes determining pixel brightness values for each color segment from the incoming video data. The method also includes generating drive signals that cause individual pixels with pixel brightness values above predetermined values for each color segment in a given frame to be in a state other than OFF during each transition period of the frame. The method also includes illuminating the panel with only one of the different colors of light from the light source arrangement during a color segment. The method also includes illuminating the panel with at least one of the different colors of light from the light source arrangement during a transition period.
The number of color segments may be at least three. The panel may be illuminated with red light during one color segment, green light during a different color segment, and blue light during a different color segment. The panel may be illuminated with at least two different colors of light during a transition period.
The method may also include adjusting the intensity of the different colors of light illuminating the panel during the transition periods to maintain a stable white point. The method may also include providing a temperature sensing arrangement that senses the temperature of the light source arrangement and adjusting the intensity of the different colors of light illuminating the panel during the transition periods to maintain a stable white point in response to the temperature of the light source arrangement.
The light source arrangement may include color filters. The light source arrangement may include a color wheel. The color wheel may be divided into at least three color areas. The color wheel may include red, green and blue color areas. The color wheel may include at least one broadly-transmissive area that passes a substantial amount of light across the visible light spectrum. The color wheel may include at least one broadly-transmissive area between each color area. The light illuminating the panel during a transition segment may travel through a broadly-transmissive area of the color wheel.
The method may also include providing a compensator cell having at least two states, one state wherein the compensator has no effect on the appearance to a viewer of individual pixels, and a second wherein the compensator changes the appearance of pixels by inverting the contrast of the image. The compensator may change from the first state to the second state at least once during the frame. The method may also include changing the compensator state during the color segment during which blue light is illuminating the display panel.
The present invention also relates to a field-sequential grayscale color display system having a display panel. The display panel includes an array of pixels having a plurality of light modulating states. The display system also includes an illumination arrangement that illuminates the pixels. The illumination arrangement includes a color generating arrangement that produces light in at least three different color spectrum ranges. The illumination arrangement sequentially illuminates the panel with light from each one of the at least three different color spectrum ranges at a time during a frame. The light from the at least three different color spectrum ranges produces white light when combined. The display system also includes a pixel driving arrangement that is receptive of incoming video data and that produces grayscale pixel drive signals that drive individual pixels to light modulating states in accordance with a grayscale scheme. The pixel driving arrangement also produces compensation drive signals that cause each pixel to be in the same light modulating state during each of at least three compensation periods. One compensation period corresponds to each color spectrum range of light in the illumination sequence.
An invention is described herein relating to methods and systems for increasing brightness in field-sequential color display systems. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. Based on the following description, however, it will be obvious to one skilled in the art that the present invention may be embodied in a variety of specific configurations. In addition, well-known processes for producing various components and certain well-known optical effects of various optical components will not be described in detail in order not to unnecessarily obscure the present invention.
The present invention applies to many display systems, including, for example, digital micro-mirror devices (DMDs) and liquid crystal devices (LCDs), including nematic, ferroelectric and antiferroelectric LCDs. However, for purposes of illustration, where necessary, the invention will be described herein as embodied in a ferroelectric liquid crystal microdisplay device. As will be readily apparent to one skilled in the art, the present invention should not be considered limited to ferroelectric liquid crystal devices specifically or even liquid crystal devices generally. Further, the present invention is not limited to microdisplays, as the teachings herein apply to most display systems using field-sequential color, as will become apparent in view of this detailed description.
Attention is first directed to
The light source arrangement 38 produces colored light in any of a number of different ways, some of which will be described in more detail hereinafter. For example, the light source arrangement 30 may include light emitting diodes (LEDs) that each emit light of a specific color, in which case the light source arrangement 38 would include at least two such devices to produce the at least two different colors. Alternatively, the light source arrangement may use a color wheel or color filter(s) in combination with a broad spectrum light emitting device. The color wheel or color filter(s) would include filtering components that each pass light of a specific color, in which case the light source arrangement 38 would include a color wheel or color filter(s) having at least two filtering components to produce the at least two colors.
As previously stated, display system 28 includes a data ordering arrangement 34 that receives incoming video data and produces drive signals that cause individual pixels 36 of the pixel array 32 to assume various light modulating states. In binary state display systems, such as DMDs and most ferroelectric LCDs, the pixels typically have two states, ON and OFF. In other systems, such as nematic or antiferroelectric LCDs, the pixels may have any number of light modulating states. Microdisplay systems, such as system 28 of
The number of colors a microdisplay system is capable of producing may be increased beyond the number of different colors produced by the light source arrangement 38 by adding grayscale. Grayscale may be produced using any of a number of well known methods, including amplitude modulation, pulse-width modulation and binary pulse-width modulation. For instance, grayscale may be produced using amplitude modulation by driving the pixels to any light modulating state from fully ON to fully OFF for each color segment. The amplitude of the pixel drive signal determines the intensity of the light reflected by or transmitted through the pixel. Alternatively, grayscale may be produced using pulse-width modulation or binary pulse-width modulation. In both examples of pulse-width-modulated grayscale systems, the pixels are driven to the ON state for only a portion of each color segment. The amount of time the pixel is in the ON state determines the intensity of the light either reflected by or transmitted through the pixel during each color segment. Pulse-width-modulated grayscale may be produced using either analog or binary drive circuitry. In analog drive systems, each pixel typically is held in the ON state continuously during each color segment for a duration corresponding to the desired intensity of the pixel for that color segment. In binary drive systems, each color segment typically is divided into grayscale time periods having different durations corresponding to a binary coded system, and the pixel may be driven to either the ON or OFF state for each grayscale time period. Thus, regardless of the grayscale system used in combination with the present invention, the RGB data produced by the data ordering arrangement 34, represents the grayscale intensity of the pixel in each color segment in each frame. Although the present invention is applicable to display systems that produce grayscale according to any of these methods, as well as various hybrids thereof, for ease of illustration, the invention will be described herein with reference to a binary pulse-width-modulated grayscale system.
In the example of
A microdisplay panel such as panel 30 contains an array of pixels 32. as explained above with reference to
In this diagram, each color segment of each pixel has a grayscale value of 5. A single value has been chosen for purposes of illustration to simplify the example. In a 3-bit binary grayscale system, 5 is represented by bit 2 being ON, bit 1 being OFF and bit 0 being ON.
The pixel in row R1 receives the bit 2 drive signal first, as can be appreciated by observing the timing diagram of
As can be appreciated with reference to
One possible solution to overcome this problem is illustrated in
Attention is now directed to
In addition to not being necessary for all pixels to be OFF during each transition period 52, it is also not necessary for all pixels in the array to display the same data during the color transitions 52, as was the case in the example of
The immediately preceding example of
The transformation of incoming video data to R'G'B'W data may be accomplished according to a number of well known methods. Examples may be found, for instance, in copending U.S. patent application Ser. No. 09/923,920, filed Aug. 17, 2001, entitled, Color-Balanced Enhancement for Display Systems, which application is incorporated herein by reference in its entirety. Other examples may be found in U.S. Pat. No. 6,256,425, issued Jul. 3, 2001, entitled, Adaptive White Light Enhancement for Displays, which patent is incorporated herein by reference in its entirety. The data ordering arrangement essentially evaluates the red, green and blue intensity information for each frame for a given pixel, and, if each color requires sufficient intensity, the data ordering arrangement places some of the intensity from each color in the W period, instead of the respective color segment.
The present invention is not limited to inserting W bits only between color segments; it may be desirable to place W bits in the middle of a color field for various reasons. For instance, some binary pulse-width-modulated grayscale algorithms (especially those involving multiple subpixels) result in “leftover” grayscale time periods when balancing the number of periods in a frame. Heretofore, such periods were written with OFF data, resulting in wasted light. However, in light of the present invention, leftover grayscale periods in RGB field-sequential color systems may be written with W data, which may be either ON or OFF, provided the W data is written to the pixel in each color segment in a frame.
The dark display at color transitions was also used previously to mask other events. For instance, some display systems, ferroelectric liquid crystal display systems in particular, use “compensator” cells to DC-balance the liquid crystal material without blocking the light.
Attention is directed to
The present invention may be embodied in display systems that use any one of a number of different types of light source arrangements. For example, the system may use either color filters or color wheels in a number of advantageous ways. Attention is now directed to
Color filters have a non-negligible switching time to change between the passive, light-blocking state and the active, light-transmitting state. Also, the time the color filter takes to fully transition from the active state to the passive state may be different than the time the color filter takes to transition from the passive state to the active state.
Color filters, as well as other light source arrangements, often have transition times that are affected by their temperature. In such cases, the degree of overlap may be adjusted to maintain the desired color of combined light in response to the temperature. This may be accomplished by using a temperature sensing arrangement 76 in
Thus, the present invention greatly increases the brightness of a display image by more efficiently utilizing light during color transitions. Although this aspect of the present invention has been described with respect to color filters, it should not be considered limited to color filters. Any light source arrangement that includes light source devices having either negligible or non-negligible transition times may be used, and the system would enjoy the benefits of the present invention. For example, discrete light sources with nearly instantaneous ON and OFF times such as light emitting diodes (LEDs) could be used, in which case it may even be beneficial to have all LEDs in an RGB system ON for some portion of each color transition. Thus, any such light source should be considered within the scope of the present invention.
As stated previously, the present invention may include a light source arrangement that utilizes a color wheel. Color wheels are more fully explained in copending U.S. patent application Ser. No. 09/923,920, filed Aug. 17, 2001, entitled, Color-Balanced Enhancement for Display Systems, which application is incorporated herein by reference in its entirety. Attention is directed to
Attention is now directed to
In color wheel systems, it is further possible to vary the brightness of the combined light across all transition segments in a frame in a number of ways. In one example, the transmissivity of the broadly-transmissive area with respect to wavelength may be selected to provide the desired color temperature. However, because this method is fixed at the time of manufacture, it alone does not allow further operational adjustments.
In a second method for adjusting the color temperature, a color wheel system designed according to the present invention may include more than one W bit between each color segment. Such a system is illustrated in
The foregoing description is considered as illustrative only of the principles of the invention. Furthermore, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and process shown and described above. For example, all or part of the teachings of the present invention may be applicable to other types of displays, including without limitation, nematic liquid crystal displays, digital micromirror displays, and others. Lastly, the present invention is also applicable to transmissive as well as reflective display systems. Accordingly, all suitable modifications and equivalents may be regarded as falling within the scope of the invention as defined by the claims that follow.
This application is a continuation of U.S. patent application Ser. No. 09/974,437 filed Oct. 9, 2001, the contents of which are incorporated herein by reference.
Number | Date | Country | |
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Parent | 09974437 | Oct 2001 | US |
Child | 11383718 | May 2006 | US |