Patent Application
20120206500
Embodiments of the present invention relate generally to electronic display systems, and more particularly to improving image quality and optical performance in electronic display systems.
Electronic display systems are increasing prevalent in today's society. Common electronic displays include computer monitors, laptop displays, televisions, and projector systems. Additionally, a broad range of multi-function products have at least one electronic display including, for example, hand-held devices, tablet computers, cell-phones, smart-phones, digital still cameras, and camcorders. For all of these types of electronic displays, manufacturers strive to improve the image quality of their displays to make them easier to use under a wide variety of viewing conditions and provide a better overall viewing experience. Improvements in image quality include increasing color depth, brightness, and display contrast ratio. These improvements also include reducing display artifacts such as “image sticking,” motion artifacts, or color artifacts.
A variety of display technologies are available to make electronic displays, including but not limited to liquid crystal displays (LCDs), organic light-emitting diode displays (OLEDs), plasma displays (PDPs), and displays based on micro-electro-mechanical system (MEMS) technology. These technologies typically use an array of pixel electrodes to drive a voltage or a current to a material or a device that either allows light to be transmitted, reflected, or emitted. These display technologies may suffer from a variety of limitations in performance. For example, it may be difficult to achieve a full range of optical states from a pitch-black dark state to high brightness in a bright state. Another problem that may affect various types of displays is “image sticking,” caused by hysteresis in the optical output of the display. The result is an objectionable “ghost” image that persists after the image is changed on the display.
To illustrate how display performance may be limited in a particular technology, a basic understanding of liquid crystal displays is provided, however, it will be appreciated that other display technologies may suffer from similar limitations in performance.
Liquid crystal displays typically drive an electric field across a liquid crystal layer using a pixel electrode and a common electrode. The liquid crystal layer changes the polarization of light passing through the display by way of the director or optic axis of the liquid crystal molecules. When combined with polarizing filters, this effect produces the ability to modulate light. By way of illustration, a transmissive liquid crystal display may have a layer of liquid crystal between crossed polarizing filters. The liquid crystal layer may be designed such that the optic axis of the layer is aligned with a first polarizing filter, generally called the “polarizer,” when no voltage is applied. In this state, light from the polarizer passes through the display with its polarization unchanged and is extinguished by the orthogonal second polarizer, generally called the “analyzer.” This produces a dark state. If an applied voltage field across the liquid crystal layer effectively rotates the optic axis such that light passing through the polarizer is rotated to be in alignment with the analyzer it will be transmitted, producing a bright state. Reflective liquid crystal displays operate in a similar manner but they typically have only one polarizing filter or a polarizing beam splitter that effectively operates as both the polarizer and analyzer.
Grayscale may be generated by modulating the voltage field across the liquid crystal layer to adjust the optic axis in-between a dark state and a bright state to produce an intermediate state corresponding to the desired grayscale. Alternately, pulse width modulation (PWM) may be used to drive the liquid crystal to a bright state for a time period proportional to the desired brightness intensity level. Because the viewer's eye is not fast enough to perceive the PWM waveform of the pixel, the viewer will see a light output level corresponding to the desired brightness intensity level.
To produce full-color images, color filters may be added in a sub-pixel structure, where each sub-pixel typically displays one of the red, green, or blue component image colors. Alternately, a field sequential color operating mode may be used. In this mode, the red, green, and blue component color images are shown in succession, synchronously illuminated with corresponding red, green, and blue light. When these component images are displayed quickly, typically at a higher rate than a standard video frame rate, viewers perceive a full-color image instead of the individual component images. For field sequential color displays, a ferroelectric liquid crystal may be preferred because of its high switching speed. Because ferroelectric liquid crystals (FLCs) tend to prefer to switch to one of two optical states, PWM is generally used with FLCs to create gray scale for each component color. The two optical states are generally selected in FLCs by driving positive and negative voltage fields across the FLC.
Liquid crystal displays may have limitations with regard to the range of optical states that the liquid crystal layer can produce. The range of optical states produced by a liquid crystal display is determined by several factors including the amount which the liquid crystal layer can rotate incoming polarized light. In some liquid crystals this may be determined by a twist in the optic axis through the liquid crystal layer. In FLCs, the range of optical states is determined by an optic axis rotation angle over which the liquid crystal molecules can rotate with respect to the plane of the liquid crystal layer surface. To produce a fully transmissive bright state and fully extinguishing dark state the optic axis rotation angle must be sufficient to rotate light passing through the display in a dark state to be completely orthogonal to the analyzer and in a bright state to be completely parallel to the analyzer.
For a variety of reasons, a liquid crystal layer may not be able to produce a fully transmissive bright state and fully extinguishing dark state. For example, an FLC may have a native limitation in the optic axis rotation angle between the effective optic axis of the bright state and the effective optic axis of the dark state. While increasing the drive voltage tends to increase the optic axis rotation angle, the FLC may be damaged if the voltage is increased beyond some threshold. Additionally, increasing drive voltage potentially requires larger circuits or a more expensive manufacturing process, either of which may be prohibitively expensive.
Liquid crystal displays may also suffer from “image sticking.” In particular, one type of image sticking is believed to be caused by accumulation of charge at the surfaces of the liquid crystal layer in response to applied voltages. The accumulated charge modifies the voltage field across the liquid crystal layer even after the applied voltage is removed or reversed. The result is a residual “ghost” image that persists after the display image has changed and may decay according to a decay time constant in the range of minutes to hours. In general, this type of image sticking may be reduced by ensuring that the time-averaged electric field across the liquid crystal layer is zero, or “DC balanced.” For some types of liquid crystal displays, including ferroelectric liquid crystals, this may require that the inverse or complement of the image be displayed during a period where the display is not illuminated to ensure that the electric field across the liquid crystal layer is DC balanced. However, time periods where the display is not illuminated reduce the overall brightness of the display. Therefore, reducing or eliminating image sticking without decreasing the brightness of liquid crystal displays has traditionally been an unattainable goal for display manufacturers.
The foregoing examples of display technology and the related limitations are intended to be illustrative and not exclusive. Against this background and with a desire to improve on the prior art, embodiments of the present invention have been developed.
Embodiments of the present invention are illustrated in referenced figures of the drawings. It is intended that the embodiments and figures disclosed herein be considered illustrative rather than limiting.
a illustrates pulse width modulated drive waveforms applied to a ferroelectric liquid crystal cell with duty cycles ranging from 10% to 90%.
b illustrates the voltage field across the ferroelectric liquid crystal layer in a ferroelectric liquid crystal cell with an insulating layer, corresponding to the drive waveforms of
a is a graph of bright state and dark state optic axis orientations versus drive waveform duty cycle for ferroelectric cells with and without added ionic conductivity.
b is a graph of optic axis rotation range versus drive waveform duty cycle for ferroelectric cells with and without added ionic conductivity.
a is a graph of transfer functions between a characteristic of an input image related to image brightness and drive field adjustments.
b illustrates a comparison of bright state performance for a ferroelectric liquid crystal display.
a shows a graph of a characteristic of image brightness over time.
b shows example window step voltages over time resulting from a transfer function of a characteristic of image brightness.
Reference will now be made to the accompanying drawings, which assist in illustrating the various pertinent features of embodiments of the present invention. Although embodiments of the present invention will now be described primarily in conjunction with a reflective ferroelectric liquid crystal (FLC) microdisplay, it should be expressly understood that the present invention may be applicable to other liquid crystal display technologies including nematic liquid crystal displays and other display technologies such as plasma display panels (PDPs), micro-electro-mechanical system (MEMS) displays, organic LED (OLED) display panels and microdisplays and/or to other applications where it is desired to increase display brightness and display contrast ratio and reduce objectionable display artifacts. In this regard, the following description of a reflective FLC microdisplay is presented for purposes of illustration and description. Furthermore, the description is not intended to limit the invention to the form disclosed herein. Consequently, variations and modifications commensurate with the following teachings, and skill and knowledge of the relevant art, are within the scope of embodiments of the present invention. The embodiments described herein are further intended to explain and to enable others skilled in the art to utilize the described embodiments, or other embodiments with various modifications required by particular application(s) or use(s) of embodiments of the present invention.
Reflective microdisplay panel 120 may be a reflective liquid crystal microdisplay panel.
Liquid crystal layer 330 may be an FLC layer. Like other liquid crystals, FLCs are composed of elongated electric dipole molecules that may prefer to align themselves generally parallel to each other in one direction, called the director or optic axis of the FLC. When FLCs are placed within parallel substrates, the FLC may form parallel layers of molecules, where the boundaries of each layer are defined by the ends of the FLC molecules. The layers may be oriented within the parallel substrates such that the plane of the layers is orthogonal to the plane of the substrates. The angle of the FLC director relative to the layer normal may be constrained by the molecular properties of the FLC mixture and composition and surface treatment of alignment layers. This angle is generally known as the tilt angle. An electric field applied to the FLC layer applies a torque to the electric dipole of the FLC molecules, allowing the molecules to be rotated around a cone with the layer normal as the axis and conic angle defined by the tilt angle. In this way, the optic axis of the FLC layer may be rotated through positions on the cone surface by applying an electric field across the FLC layer.
FLCs typically exhibit a preference for the FLC molecules to be in one of two more stable states where the director of the FLC is generally parallel to the substrate surface. While these states are more stable than other positions on the FLC cone, there is a degree of analog response in the FLC optic axis position relative to the orientation of the substrate. Therefore, while a positive voltage field across the FLC layer will tend to switch the FLC molecules to one of the two stable states on the cone defined by the tilt angle, the exact optic axis position varies somewhat with applied voltage.
The electric field across the FLC layer is determined by the voltages of the array of pixel electrodes and the common window electrode 350. The pixel electrodes may switch between a low pixel voltage VPIXL and a high pixel voltage VPIXH, while common window electrode 350 is at an intermediate voltage VWIN. For example, VPIXL may be 0V, while VPIXH may be 5V and VWIN may be 2.5V. In this example, when pixel electrode 321 is at VPIXL, FLC layer 330 has an electric field VFLCL of −2.5V from the pixel electrode 321 to the common window electrode 350. When pixel electrode 321 is at VPIXH, FLC layer 330 has an electric field VFLCH of +2.5V from the pixel electrode 321 to the common window electrode 350. The positive and negative electric fields across FLC layer 330 switch the FLC molecules generally from one side of the FLC cone to the other.
As with other liquid crystals, FLCs exhibit optical birefringence, which causes light polarized parallel to the optic axis to experience a different index of refraction than light polarized perpendicular to the optic axis. Light that is polarized parallel to the optic axis will pass through the FLC layer with its polarization direction unchanged. However, light passing through the FLC layer polarized at an angle to the optic axis will have its polarization rotated by phase retardation. If the FLC layer is of an appropriate thickness, the polarization of light passing through the FLC will be rotated by twice the angle (Θ) of the optic axis to the incident light. Combined with a first polarizing filter, or “polarizer,” and a second polarizing filter, or “analyzer,” the FLC layer can modulate light. With a crossed polarizer and analyzer, this creates a dark optical state when the optic axis of the liquid crystal is parallel to the axis of the polarizer and a bright optical state when the optic axis of the liquid crystal is at an angle to the axis of the polarizer. To achieve the brightest possible bright state, the FLC optic axis would be at a 45 degree angle to the polarizer and induce a 90 degree polarization rotation, which would allow the analyzer to fully transmit all light passed through the polarizer. In reflective microdisplay system 100, polarizing beam splitter 130 operates as both the polarizer and the analyzer, creating a crossed polarizer system.
Microdisplay system 100 may display input images received as input image data that are grayscale images or full-color images. Because FLCs are fast-switching liquid crystals and have two primary stable states, grayscale is most commonly generated using pulse width modulation (PWM). Color may be achieved using field sequential color (FSC) or using color filters over sub-pixels for the individual colors.
FLCs have traditionally required drive waveforms that have a zero time-averaged DC field. During field periods 411, 421, and 431, called balance periods, the pixels may be driven to VPIXH for a time that is complementary to the time that the pixel was driven to VPIXH during the preceding illuminated time period. For example, during balance time period 411, pixel waveform 440 is driven to VPIXH for a time period that is complementary relative to tFIELD of the time period that pixel waveform is driven to VPIXH relative to tFIELD during illumination period 410. This waveform maintains a zero time-averaged DC electric field across the FLC layer over the frame time 400. This drive scheme, called dc-compensation or dc-balancing, prevents charge accumulation at the FLC-alignment layer interfaces.
For a variety of reasons, it may not be possible in a particular display panel configuration to rotate the FLC optic axis through a 45 degree angle from the dark state optic axis to the bright state optic axis. For example, the maximum voltage that may be applied to a pixel electrode for a particular display technology may be limited by the breakdown voltage of the transistors used in active pixel drive circuits. This limited voltage range may not switch the optic axis completely through an optimal 45 degree angle with FLC voltage fields VFLCL and VFLCH.
As described above, FLC layer 330 may have some analog response to increasing the voltage field across the FLC for the bright state and dark state optic axis positions. However, high pixel voltage VPIXH may be constrained by circuit topology or manufacturing process within a certain voltage range. Within this range, electric fields VFLCL=VPIXL−VWIN and VFLCH=VPIXH−VWIN, where VWIN=½(VPIXH−VPIXL), may not rotate the molecules of FLC layer 330 to the optimal 45 degree optic axis rotation range 510.
Increasing drive voltage requires circuits capable of driving the higher voltage. To manufacture a reflective FLC microdisplay at a small pixel pitch it may be advantageous to use a standard integrated circuit process. The range of voltages available for the standard integrated circuit process may be limited by the technology and size of the transistors in the process. For example, in a 0.25 micron CMOS process, the standard voltage level for which the transistors are designed may be 2.5 V. It may be possible to increase the available voltage range by cascoding transistors, however, multiple levels of cascoded transistors increases circuit complexity and therefore circuit and pixel size. It may also be possible to use special transistors of higher voltage for pixel circuits, however, this also increases either circuit and pixel size, or increases processing cost by adding special processing steps, or both. Therefore, increasing pixel voltage will likely increase pixel pitch or manufacturing process cost, which both increase the final cost of the microdisplay panel. Increasing the applied voltage beyond a certain point may also damage the liquid crystal if the increased voltage is constantly applied.
The general solution to an FLC layer with a reduced optic axis rotation range is to rotate optic axis rotation range 520 so that the dark state optic axis is aligned with the polarizer along axis 512. This will produce a fully extinguished dark state. A fully extinguished dark state is important because the contrast ratio of a display is the ratio of the optical throughput of the bright state to the optical throughput of the dark state. Because the dark state is the denominator in the contrast ratio, making the dark state darker by a certain amount has a much larger impact on display contrast than increasing the bright state by the same amount. However, aligning the dark state optic axis of optic axis rotation range 520 with the polarizer axis 512 reduces the maximum brightness of the display further as bright state optic axis 524 will also be rotated towards axis 512, reducing optical throughput in the bright state.
With these problems in mind, video data dependent adjustment of display drive for modifying the optic axis rotation range to improve the optical performance of FLC layer 330 will be described.
The video data dependent adjustment of display drive takes advantage of the response of the viewer's eye to the overall brightness of a particular image. For a substantially dark image, a reduced bright state may not be apparent to the viewer because the viewer's eye will adjust to the overall brightness of the image, making the bright portions of a substantially dark image look brighter. For a substantially bright image, the viewer's eye adjusts to the brightness of the image and it will be harder for the viewer to perceive that dark portions of the image have become brighter. For example, a fully dark-adapted eye may have a sensitivity threshold to grayscale levels several orders of magnitude lower than an eye adapted to bright conditions. Accordingly, video data dependent adjustment of display drive produces brighter images when higher brightness is most important and darker images when it is more important to produce a darker dark state.
The video data dependent adjustment of display drive may be accomplished by changing the voltage of the common window electrode VWIN. In this embodiment, adjustment of display drive may be independent of pixel drive voltages. For example, PWM waveforms between a high pixel voltage VPIXH and a low pixel voltage VPIXL proportional to the image data values of an input image may be used to generate grayscale during illuminated periods for the array of pixels in a display. The pixel drive waveforms may be dc-compensated by providing non-illuminated balance periods that have inverse PWM waveforms with respect to the illumination periods. For a substantially dark image, VWIN may be increased above ½(VPIXH−VPIXL) during an illumination period, which makes VPIXL a more negative voltage and applies a larger electric field driving the FLC molecules towards the polarizer axis 512 in the dark state. For a substantially bright image, VWIN may be decreased below ½(VPIXH−VPIXL) during an illumination period, which makes VPIXH a more positive voltage and applies a larger electric field driving the FLC molecule to rotate away from polarizer axis 512 in the bright state. The common window electrode voltage VWIN may be adjusted in the opposite direction during balance periods to the adjustment during the illumination periods. This adjustment maintains dc-compensation while providing the benefits of a dynamically rotated optic axis rotation range during illumination periods.
Where possible with the drive circuit technology and process, video data dependent adjustment of display drive may also be accomplished by changing VPIXH and VPIXL. For a substantially dark image, VPIXL, may be reduced to create a more negative voltage across the FLC layer for substantially dark pixels. For a substantially bright image, VPIXH may be increased to create a more positive voltage across the FLC layer for substantially bright pixels. Additionally, video data dependent adjustment of display drive may be accomplished with a combination of adjustments of VWIN, VPIXH, and VPIXL. Again, the PWM waveforms of the pixels between voltages VPIXH and VPIXL that provide the grayscale of the pixel according to the pixel data values may remain unchanged in this embodiment.
Video data dependent adjustment of display drive may be accomplished by determining a characteristic related to the brightness of the input image. For example, the characteristic could be determined from the image data values of the input image. The characteristic may include, but is not limited to, parameters such as the average, the minimum, the maximum, the distribution, a histogram, or the standard deviation of the image data values of the input image. The characteristic could be calculated from all image data values of the input image or a subset of the image data values. The characteristic could weight parameters of all component colors equally or give more weight to one component color over others.
Standard video sources provide all component colors, for example red, green, and blue (RGB), for each pixel in an image in raster order. However, to display the image in field sequential color mode, the component colors of the input image are displayed one at a time. Therefore, a display using field sequential color typically must store an entire input image before displaying the image. Using the stored data, such a display may be able to determine the characteristic using more advanced processing of the input image data. For example, the characteristic could be determined from the average brightness of the darkest region of the input image larger than a given size. Other ways of determining the characteristic from the stored input image data that take into account the apparent brightness of the input image to a viewer are possible. For example, the characteristic could be determined from the number of image data values over a particular threshold or the average of the image data values in a region larger than ½, ¼ or ⅛ of the total image area.
A transfer function could be applied between the characteristic and an adjustment of the display drive field. For example, the transfer function could be applied between the characteristic and adjustments of the common window electrode VWIN for an FLC cell. The transfer function could be a linear transfer function between the characteristic and the drive field adjustments. Alternately, the transfer function could compensate for the non-linear response of the optical states due to changes in drive field. For example, the response of the dark state optic axis and the bright state optic axis to changes in liquid crystal drive field may be non-linear. In addition, as illustrated by optical transmission curve 710 in
The transfer function could account for the perceptual response of the viewer to different brightness levels. For example, a perceptual response curve could be determined by experimentally measuring the ability of viewers to perceive changes in grayscale for images of varying average brightness. In an embodiment of the invention, the transfer function compensates for the non-linear nature of the optical response relative to drive field and adjusts the drive fields so that the optical response varies according to a perceptual response curve based on the characteristic. In this embodiment, the display drive fields are adjusted according to a perception-based model.
The transfer function could account for multiple characteristics of the input image to produce drive field adjustments. For example, the transfer function could accept the minimum, average, and maximum brightness of an input image to determine the drive field adjustments. The transfer function could apply equal weights to multiple characteristics of the input image or weight one characteristic more heavily than others in determining the drive field adjustments.
The transfer function could also adjust the optical states based on the characteristics of multiple input images. It may take several seconds for the viewer's eye to adjust from a substantially bright image to a substantially dark image. Therefore, the transfer function could apply a temporal filter to the characteristic from multiple images from a video source. The filter could have an impulse response that is related to the speed with which the viewer's eye adjusts to the relative brightness of the input images. The filter could have a different impulse response time for transitions from darker images to brighter images than the impulse response time for transitions from brighter images to darker images.
As described above, FLCs typically require a zero time-averaged DC field to prevent charge accumulation at the FLC-alignment layer interfaces that contributes to image sticking. With respect to charge accumulation that causes image sticking, the time-constant for charge to accumulate and decay may be in the range of minutes to hours. Using dc-compensated PWM waveforms prevents charge accumulation by ensuring that there is no net DC field across the FLC. However, dc-compensation drive waveforms typically require a balance period for each illuminated period during which the FLC is driven with a complementary waveform. Because the illumination source is turned off during the balance periods, the resulting duty cycle of the illumination source is approximately 50%. This low duty cycle reduces the overall brightness of the display.
Embodiments of the invention contemplate the use of a liquid crystal material, such as an FLC, that has been formed with a base FLC with ions added to dope the base FLC to adjust its conductivity (resistivity) as described in copending U.S. patent application Ser. Nos. 12/794,267 and 13/007,297, the entire contents of which are incorporated herein by reference. In those applications, an FLC cell is disclosed including an FLC layer and an alignment layer, where the alignment layer may act as an insulating layer. In addition, methods and compositions for adjusting the conductivity of the FLC are described including adding ionizable compounds to the base FLC or resistive elements to the FLC.
The material for the alignment layers and the material for the FLC can be selected such that the alignment layer resistance is much greater than that of the FLC. In such cases, the resistance RA of the alignment layer can be set to RA=∞ in equivalent circuit 820, which effectively provides that resistance RA can be omitted from equivalent circuit 820. The alignment layer is generally thin compared to the FLC. For example, the thickness of an alignment layer may typically be 20 nm, while the thickness of the FLC may be 800 nm. Other thicknesses can be used. With such differences in thickness, the capacitance CA of an alignment layer is approximately one to two orders of magnitude larger than capacitance CF of the FLC. Further, after FLC switching events, where the polarization switching current is near zero, the FLC's conductivity is dominated by the motion of ionic charge carriers, which conductivity is represented by RI in the equivalent circuit.
In an embodiment of the invention, an ionically doped FLC cell may be driven with a PWM waveform without dc-compensation. The alignment layers effectively act as an electrical high-pass filter, blocking the DC component to the waveform and passing the high frequency component of the drive waveform to the FLC.
The decay time constant ½RICA could be set by using a selected material of a selected thickness as an alignment layer. For example, a generic polyimide layer of a given thickness could be selected. The decay time constant could be set by manipulating the doping of the FLC to achieve the desired RI. Alternately, for a given FLC having a given RI, the decay time constant can be adjusted by selecting a value of CA to produce the desired decay constant. For example, the desired value of CA may be attained by selecting a particular material for the alignment layer or manipulating the structural characteristics such as alignment layer thickness to achieve a given CA value.
Another consideration for selection of the characteristics of the FLC and alignment layers includes selecting the decay time constant, ½RICA, such that it is substantially longer than the time, tSW, to switch the liquid crystal between display states (e.g. bright to dark, comprising substantially contrasting optical states). Otherwise, the FLC may not switch fully and images may not be displayed. Combining these two factors for an appropriate decay time, the condition, tSW<½RICA<tVISION, can be used to select materials and sizes for the FLC and alignment layers. The switching time of the FLC may be on the order of 50-1000 μs. Preferably, the switching time of the FLC is shorter than the field time. Therefore, the minimum time for the decay time constant ½RICA could be set to be greater than a field time, for example, ⅓, ⅙, 1/9, or 1/12 of the frame time. Depending on the video source, which may have 24, 30, 50, or 60 frames per second, for example, the frame time may be between 1/24 of a second and 1/60 of a second. Therefore the field time may be on the order of 1/720 of a second to 1/72 of a second.
In an example embodiment, the decay time is desired to be in the range tSW<½RICA<tVISION, where tVISION is an acceptable decay time for image sticking. A generic polyimide alignment layer having a thickness of ˜20 nm and a dielectric constant of ˜4 may be used, with capacitance CA of approximately 200 nF/cm2. Using a minimum decay constant time greater than 1/720 s and tVISION= 1/30 s, the value of RI is set to the range 14 kΩ<RI<0.3 MΩ for a cell area of 1 cm2. For a typical FLC layer whose thickness is on the order of 1 μm, the electrical resistivity, ρI, for ionic charge carriers should correspondingly be in the range 140 MΩ·cm<ρI<3 GΩ·cm. In practice, the upper limit for tVISION of 1/30th s may be excessively stringent, i.e., it may be visually acceptable for image sticking to persist for a larger fraction of a second or multiple seconds so that higher electrical resistivities may be acceptable.
While the ionic doping of the FLC layer and use of alignment layers as insulating layers may reduce the persistence of image sticking in the FLC cell, it may have effects on the optic axis rotation range (ΔΘ) of the FLC layer. As shown in
a and 11b illustrate the effect on switching of the FLC optic axis for FLC cells with and without ions added. As shown in
In various embodiments, an FLC cell with ions added such that the decay time is tSW<½RICA<tVISION may be driven with a field sequential color, PWM grayscale waveform that is not fully dc-compensated. For example,
According to embodiments of the invention, video data dependent adjustment of display drive may be used to improve the image quality of FLC displays using doped FLCs driven with PWM waveforms that are not fully dc-compensated. Specifically, the optical states of the FLC may be adjusted depending on the pixel data values in the input image data. If the input image data is substantially dark, the display drive is modified such that the optic axis rotation range (ΔΘ) will rotate to an equilibrium optic axis rotation range (ΔΘD) for dark pixels such that the equilibrium dark state for dark pixels is rotated for improved extinction. If the input image data is substantially bright, the display drive is modified such that the optic axis rotation range (ΔΘ) will rotate to an equilibrium optic axis rotation range (ΔΘB) for bright pixels such that the equilibrium bright state for bright pixels is rotated for improved transmission.
The video data dependent adjustment of display drive for a doped FLC may be accomplished by changing the voltage of the common window electrode VWIN. In this embodiment, adjusting the common window voltage VWIN to adjust the optic axis rotation range may be independent of the pixel drive waveforms.
Other adjustments of common window electrode voltage VWIN using video data dependent adjustment of display drive may also provide advantages. For example, common electrode voltage VWIN may be adjusted only during one or more of the illumination periods 1210, 1220, and 1230 to window illumination step voltage VWSI shown by step voltage 1581 or 1681. Conversely, common electrode voltage VWIN may be adjusted only during one or more balance periods 1240 to window balance step voltage VWSB shown by step voltage 1582 or 1682. Additionally, the adjustment of window step voltages VWSI and VWSB does not need to be equal. For example, the adjustment to VWSB could be greater than the adjustment to VWSI.
Where possible with the drive circuit technology and process, video data dependent adjustment of display drive may also be accomplished by changing VPIXH and VPIXL. For a substantially dark image, VPIXL may be reduced to create a more negative voltage across the FLC layer for substantially dark pixels. For a substantially bright image, VPIXH may be increased to create a more positive voltage across the FLC layer for substantially bright pixels. Additionally, video data dependent adjustment of display drive may be accomplished with a combination of adjustments of VWIN, VPIXH, and VPIXL.
In other embodiments, video data dependent adjustment of display drive contemplates changing the drive field on a pixel-by-pixel basis, independently of other pixels. A pixel adjustment value, either determined by a circuit local to the pixel, or determined by a circuit outside the pixel array and communicated to the pixel, is used to modify the optical states of the pixel based on the effect of the pixel states on the optic axis rotation range of the pixel. For example, a particular pixel could select a high drive voltage VSELPIXH and a low drive voltage VSELPIXL from a range of pixel voltages based on the pixel adjustment value. In this way, as the pixel drive waveform approaches the extremes of duty cycle, the pixel adjustment value compensates for the change in optical states of the FLC for the particular pixel by adjusting the drive field of the pixel.
Video data dependent adjustment of display drive may be accomplished by determining a characteristic related to the brightness of the input image. For example, the characteristic could be determined from the pixel data values of the input image. The characteristic may include, but is not limited to, parameters such as the average, the minimum, the maximum, the distribution, a histogram, or the standard deviation of the pixel data values of the input image. The characteristic could be based on parameters of all pixel data values of the input image or a subset of the pixel data values. The characteristic could weight parameters of all component colors equally or give more weight to one component color over others.
Standard video sources provide all component colors, for example red, green, and blue (RGB), for each pixel in an image in raster order. However, to display the image in field sequential color mode, the component colors of the input image are displayed one at a time. Therefore, a display using field sequential color typically must store an entire input image before displaying the image. Using the stored data, such a display may be able to determine the characteristic using more advanced processing of the input image data. For example, the characteristic could be determined from the average brightness of the darkest region of the input image larger than a given size. Other ways of determining the characteristic from the stored input image data are possible that take into account the apparent brightness of the input image to a viewer.
A transfer function could be applied between the characteristic and adjustment of the pixel drive fields of the display. For example, the transfer function could be applied between the characteristic and adjustments of the common window electrode VWIN for an FLC cell.
The transfer function could be a linear transfer function between the characteristic and the drive field adjustments as shown by illumination window step function 1711 and balance window step function 1712. Alternately, the transfer function could compensate for the non-linear response of the optical states due to change in drive field. For example, the response of the dark state optic axis and the bright state optic axis to changes in liquid crystal drive field may be non-linear. In addition, as illustrated by optical transmission curve 710 in
The transfer function could account for the perceptual response of the viewer to different brightness levels. For example, a perceptual response curve could be determined by experimentally measuring the ability of viewers to perceive changes in grayscale for images of varying average brightness. In an embodiment of the invention, the transfer function compensates for the non-linear response of the optical states to display drive field and adjusts the drive field so that the optical states vary based on the characteristic according to the perceptual response curve. Example non-linear illumination window step function 1721 and balance window step function 1722 may compensate for the non-linear response of optical states due to change in drive field and the non-linear perceptual response of viewers. It will be appreciated that once the perceptual response curve and the non-linear optical response with respect to drive field are determined, the transfer function may be calculated to provide the desired perceptual response curve. In this embodiment, the display drive fields are adjusted according to a perception-based model.
Operation of video data dependent adjustment of display drive using window voltage VWIN with a doped FLC according to an embodiment of the invention is illustrated by considering illumination window step function 1711 and balance window step function 1712 of
It will be appreciated that the zero crossing point for the transfer function may depend on the rotation of the FLC cell relative to the polarizers. For example,
a and 18b illustrate in more detail how the transfer function modifies the window step voltages VWSI and VWSB shown in
The transfer function for a display may be programmable. For example, the transfer function may be stored as a look-up-table (LUT) in non-volatile memory of the display system. The transfer function may be interpolated between the set-points of the LUT. The transfer function may be linearly interpolated between the set-points of the LUT. Alternately, the transfer function may be stored in the display system as a polynomial function or other type of function. The display may calculate the drive field adjustment according to the function and the characteristic of the input image.
The transfer function could account for multiple characteristics of the input image to produce an optical state adjustment. For example, the transfer function could accept the minimum, average, and maximum brightness of an input image to determine the drive field adjustment. The transfer function could apply equal weights to multiple characteristics of the input image or weight one characteristic more heavily than others in determining the drive field adjustment.
The transfer function could also adjust the optical states based on the characteristics of multiple input images. For example, the transfer function could apply a temporal filter to the characteristic from multiple images from a video source. It may take several seconds for the viewer's eye to adjust from a substantially bright image to a substantially dark image. Therefore, the filter could have an impulse response that is related to the speed with which the viewer's eye adjusts to the relative brightness of the input images. The filter could have a different impulse response time for transitions from darker images to brighter images than the impulse response time for transitions from brighter images to darker images.
The filter could have an impulse response that is related to the decay time constant of the FLC. For example, the transfer function could apply a filter which has an impulse response equal to the decay time constant of the FLC. In this example, if the decay time constant of the FLC is set to equal tVISION, where tVISION= 1/30th s, and the video frame rate is 60 frames per second, the transfer function would be set to have an impulse response equal to two frames. This could be implemented with a simple second order finite impulse response filter. The transfer function could account for multiple characteristics from multiple images according to various embodiments.
It will be appreciated that a frame period may be divided into many combinations of illumination periods and balance periods. For a variety of reasons, it may be advantageous to have a color field rate greater than 3× the frame rate. It will also be appreciated that the illumination periods do not have to be equivalent time periods to the balance periods. By way of example, the frame period could be broken up into combinations of illumination periods and balance periods that result in ratios of aggregate illuminated time to aggregate balance time of 6-6, 9-3, or 10-2, respectively. Additionally, the balance periods could be positioned anywhere within the frame period, for example, the balance periods could come before the illumination periods, between the illumination periods, or after the illumination periods.
b shows the improvements in brightness for a doped FLC cell according to various embodiments of the invention. Brightness level bar 1730 shows the brightness for a fully dc-compensated PWM pixel drive waveform with a ratio of aggregate illumination time to aggregate balance time of 6-6 for a particular doped FLC mixture. Bar 1740 shows the normalized brightness of a PWM pixel drive waveform with a ratio of aggregate illumination time to aggregate balance time of 9-3. Bar 1750 shows the normalized brightness of a PWM pixel drive waveform with a ratio of aggregate illumination time to aggregate balance time of 10-2. Bar 1760 shows the normalized brightness of a PWM pixel drive waveform with a ratio of aggregate illumination time to aggregate balance time of 10-2 using video data dependent adjustment of display drive according to embodiments of the invention.
Control circuit block 1920 may include image processing block 1921 and drive field control block 1922. Drive field control block 1922 processes image data to determine a characteristic related to the brightness of the image data values. Drive field control block 1922 may also include a transfer function that adjusts the window electrode voltage 1955 using window electrode driver 1950, which may be a digital to analog converter (DAC) to convert a digital output of drive field control block 1922 to window electrode voltage 1955. The window electrode voltage 1955 is coupled to the common window electrode of the FLC cell by way of a direct connection from the display panel or a connection through a printed circuit board or other package for the display panel.
Display panel backplane 1900 may be designed in accordance with microdisplay architectures described in U.S. patent application Ser. No. 11/969,734, entitled DIGITAL DISPLAY and/or U.S. Pat. No. 7,283,105, entitled MICRODISPLAY AND INTERFACE ON SINGLE CHIP, which describe microdisplay backplanes with integrated frame buffers capable of accepting standard raster-order video signals and displaying in color sequential mode. Alternately display panel backplane 1900 may be designed with a different architecture that accepts input image data and applies a drive field using pixel electrodes. A display system according to an embodiment of the invention could have an external display controller chip that includes portions of the various circuit blocks of display panel backplane 1900.
Another embodiment of the present invention sets the adjustment parameters of video data dependent adjustment of display drive on a device-by-device basis. For example, a reflective microdisplay device with a doped FLC layer may be manufactured according to embodiments of the invention. The FLC may be driven with an unbalanced PWM waveform like those described previously with regard to
It will be appreciated that video data dependent adjustment of display drive may provide advantages in image quality including increased brightness and/or contrast ratio for other liquid crystal display technologies. For example, video data dependent adjustment of display drive may be used with any liquid crystal display technology where the polarization rotation of light passing through the liquid crystal layer is less than fully extinguished in a dark state and/or less than fully transmissive in a bright state. Additionally, video data dependent adjustment of display drive may be applied to applications where liquid crystals materials have optical states that are affected by a time-dependent component of a display drive waveform. In particular, video data dependent adjustment of display drive may be used with other liquid crystals that are doped with ionic compounds to reduce the decay time constant of image sticking.
Additionally, it will be appreciated that video data dependent adjustment of display drive may be applied to other display technologies. For example, video data dependent adjustment of display drive may be applied to any display technology where the optical state switching is constrained by manufacturing or process parameters such that either the dark state is not fully dark or the bright state is not optimally bright under standard driving conditions.
The foregoing description has been presented for purposes of illustration and description. Furthermore, the description is not intended to limit embodiments of the invention to the form disclosed herein. While a number of exemplary aspects and embodiments have been discussed above, those of skill in the art will recognize certain variations, modifications, permutations, additions, and sub-combinations thereof.
