The present invention generally relates to full color liquid crystal displays generated by the use of birefringent filters. More particularly, the present invention relates to full color liquid crystal displays using polarization interference filters in a passive matrix addressing scheme.
Traditionally color generation in liquid crystal displays (LCD) is accomplished by the patterning of color filters onto individual pixels. However, the use of color filters results in both a loss of resolution, due to subpixeling, and lower light transmission, due to the absorptive nature of the filters. Other methods of generating color include field sequential color in which the actual color is generated by RGB light emitting diodes (LED) and the liquid crystal (LC) pixel is driven so as to either block or transmit the colored light. This typically requires the LC to be driven at a very high frame rate which in turn calls for a very fast switching LC material and mode. Such materials and modes are difficult to find.
Another method of color generation employs polarization interference filters (PIF). Such filters work by introducing a phase shift between two orthogonally polarized field components by either a static or dynamic optical element such as a uniaxial retarder or electrically driven LC cell. Color is generated by the interference of these two components with an analyzer, and color switching is accomplished by changing the phase shift between the two components by using dynamic optical elements. Since there is no more absorption than the losses associated with polarizer absorption, the PIF can generate color with a higher luminance than absorptive color filters.
The use of retarders between polarizers to function as filters has been studied for several years, starting with the original design for monochromatic imaging by French astronomer Lyot. See I. J. Hodgkinson, Q. H. Wu: Birefringent Thin Films and Polarizing Elements, (World Scientific Press, Singapore 1997) 1st ed., Chap. 4, p. 52, incorporated herein by reference. The Lyot filter design was improved upon by the Solc design, which offered similar filtering characteristics but higher overall transmission. This design is generally disclosed in Ivan Solc, Czechoslovak Journal of Physics, Vol. 10(1), 16-34 (1960), also incorporated herein by reference. The Solc design was then generalized by Harris et al., A Optical Network Synthesis Using Birefringent Crystals, 1. Synthesis of lossless Networks of Equal-Length Crystals, J. Opt Soc. America (1964) 54(10):1267-1279, incorporated herein, describing a synthesis procedure for obtaining various filter designs.
The two original Solc designs are the “fan” and “folded” type. In the fan design, the wavelengths at which the elements have an even number of half waves of retardation pass through an output polarizer oriented parallel to the input polarizer. In the folded design, the wavelengths at which the elements have an odd number of half waves of retardation pass through the output polarizer, which is crossed with respect to the input polarizer. The passband characteristics of the Solc filters are directly related to the number of elements used to create the filter, i.e., the larger the number of elements, the sharper the passband. Another crucial element in the transmission function is the size and frequency of the sidelobes. The Solc filters, as it turns out, are not optimal in design and give passband sidelobes that can be avoided using various synthesis techniques.
The use of PIFs for the generation of color in a display device was shown by Sharp et al., U.S. Pat. No. 5,929,946. Starting from a Solc filter design, the design of a device capable of switching between a single desired color and black or white, depending upon polarizer orientations, was shown. Given that a folded Solc filter will filter the desired wavelengths by rotating them by pi/2, a second active optical element was added to the filter in order to be able to switch between a filtering and non-filtering action. Particularly, a single, switchable half wave plate was placed between the filter and the second half of the filter was repositioned. Proper repositioning required inverting the second half of the filter and rotating it by pi/2, as shown in FIG. 1, and discussed below.
With reference to FIG. 1, a single color generation stage of a display device according to Sharp is designated generally by the numeral 1. Color generation stage 1 includes a first retarder stack 2 (pre filter), a second retarder stack 3 (post filter), which is inverted and rotated by 90° with respect to first retarder stack 2, and an active matrix addressed optical element 4 (liquid crystal cell), particularly, a half wave plate, placed between first and second retarder stacks 2,3. First retarder stack 2 is oriented to rotate a desired wave length of light to an angle of pi/4, with respect to the optic axis of the active matrix addressed optical element 4 (half wave plate). As is generally known, crossed or parallel polarizers (not shown), particularly an input polarizer and an output analyzer, are employed to introduce polarized light to the color generation stage 1 and to analyze the light transmitted through color generation stage 1.
If LC cell 4 is switched so that its net retardation is zero, then the first part of the filter, first retarder stack 2, is crossed with respect to the second part of the filter, second retarder stack 3, as shown in FIG. 1. In this case, the net retardation between the two polarizers is zero and so the display appears black for crossed polarizers and white for parallel polarizers. If LC cell 4 is switched so that it is a half wave plate, then its effect on the filter depends on its orientation in the stack. Given that a folded Solc design rotates the desired wavelength by pi/2, then half the filter will rotate it by pi/4. As shown in FIG. 1, light 5 entering the filter from the left will have the desired wavelength rotated by pi/4 while light entering from the right will be rotated by −pi/4 to 3pi/4. Therefore, for light 5 to pass through the complete filter, LC cell 4 must take light polarized at pi/4 and rotate it to 3pi/4 degrees. A pi/2 rotation is easily accomplished by placing LC cell 4 at pi/4 to the incoming polarization. In this position the LC cell allows the filter to be switched on or off depending on its own state. For intermediate voltages, LC cell 4 acts as a retarder between 0 and pi/2 and so rotates the incoming linearly polarized light to some elliptical state. As a result, the desired wavelength is not completely rotated by the second half of the filter and so suffers absorptive losses at the analyzer. In this manner, intermediate grey scale colors can be created.
The effect of the filter on the rest of the spectra is as follows. In the case of a pure folded Solc filter, the undesired wavelengths entering the filter linearly polarized at 0° are all left unrotated (more so for a multiple plate retarder than a two plate) and are absorbed at the analyzer, which is crossed with respect to the polarizer. For the above design, the placement of the LC cell ensures that the unwanted spectra remains unaffected. While the desired wavelength is rotated, by the first part of the filter, to pi/4, the rest of the spectra remains at 0°. Thus, the undesired wavelengths are at pi/2 to the LC cell optics axis and the LC cell has no effect on them. Passing through the second half of the filter, they remain linearly polarized at 0° and are absorbed by the analyzer.
Therefore, in summary, Sharp design requires the following:                (1) a polarizer providing linearly polarized white light at a known orientation;        (2) a prefilter to rotate desired wavelengths to an angle of pi/4 with respect to the optic axis of the LC cell (or other electro- or magneto-optic modulator) while leaving undesired wavelengths untouched;        (3) an active matrix addressed optical element such as a LC cell (or other electro- or magneto-optic modulator) that rotates linearly polarized light by pi/2, i.e., a half wave plate, and has no effect on light polarized along or orthogonal to its optic axis; and        (4) a postfilter that is a copy of the prefilter except inverted and rotated by pi/2.        
Such a filter design is capable of producing a relatively narrow passband transmission function (depending on the number of retarders making up the pre and post filter) and so can produce a single color or its grey scales from input white light. Such a filter is discussed in S. Saeed, P. J. Bos, Z. Li, SID00 Digest, 830 (2000), incorporated herein by reference. In order to produce more than one color, i.e., red, green, and blue, a stacked approach is taken to provide a full color liquid crystal display, as shown in FIGS. 2 and 3. In the stacked approach, a color generation stage according to FIG. 1 is tuned for each of the three primary colors (designated as 1B, 1G, and 1R) and then stacked one on top of the other. In FIG. 2, like parts as compared to FIG. 1 have received like numerals, with distinctions made between each stage and its elements by employing the letters B, G, and R to indicate blue, green, and red. Only one polarizer 6 and analyzer 7 is used at the two ends, and, in FIGS. 2 and 3, they have transmission axes that are orthogonal to one another, although parallel configuration may alternately be practiced. The stacked design works since each stack only rotates the wavelengths associated with its color, eg., the blue stack is designed to rotate wavelengths 430 nm to 490 nm while the rest of the spectra is left linearly polarized at 0°. Since the analyzer is placed at the end of the stack, this part of the spectra is able to move through the other two stacks, as shown in FIG. 3(B).
In FIG. 3, white polarized light is shown passing through three different stacks of filters, each stack containing three filters individually tuned for red (1R), green (1G), and blue (1B) wavelengths. At (A), the filters are all in an optically inactive mode, and the entire spectrum is blocked by the crossed polarizers. At (B), the blue filter is in an optically active mode, such that the blue spectrum is rotated and the stack ultimately outputs blue light. At (C), all three of the filters, blue (1B), green (1G), and red (1R), are in an optically active mode, and, thus, all three parts of the spectrum, blue, green, and red, are rotated, and the stack ultimately outputs white light.
Since the entire white light spectra is able to move through the three layer stack with each layer only affecting selected wavelengths, the stack can produce red, green, blue and white light or any combination thereof. This however will only be true if the red, green, and blue spectra do not overlap. As mentioned before, the bandwidth of each filter depends on the number of elements making up the filter. If only four retarders are used, for example, then the filter's bandwidth will be quite broad. If this filter was to be placed in the stacked design, it would rotate parts of the undesired spectra along with the desired spectra. For example, if the red filter is broadband, then it will rotate parts of the green spectra also. As a result, if the stack is setup to be in the white state, i.e., all three layers are rotating their selected wavelengths, then the green would be over rotated, and the resulting white output would lack some green. Therefore, in a stack design, it is important to have each filter's bandpass be as narrow as possible so as to prevent the overlap that reduces efficiency.
There are a number of orientations that the retarders making up the filter can have and still provide similar if not identical transfer functions. Synthesis techniques described by Harris et al., cited above, and Buhrer, Appl. Opt. 33, 2249-2254 (1994), are useful for obtaining the best orientations of the filter elements for a required transfer function, and each reference cited is incorporated herein. These synthesis techniques rely on the fact that each element in the filter provides one cosine Fourier component of the desired transmission function. Such synthesis techniques are useful in situations where the active element in the filter operates in the 0 to pi/2 range.
A birefringent filter based LCD employing electrically controlled birefringence (ECB) type cells, which fall into the category of optically simple active elements, was disclosed in the above-referenced Saeed et al. publication. The ECB type cells, however, require a rather expensive drive method called active matrix addressing, which involves the use of a transistor at each pixel edge to provide a switching operation.
A less expensive drive method is passive matrix addressing. A passive matrix addressing design could make use of the electro-optical characteristics of the LC mode and could drive displays without the use of transistors, and, as a result, the display would be cheaper and easier to manufacture. However, the requirement of a sharp electro-optical curve means that LC modes such as the ECB cannot be used in conjunction with passive matrix addressing. Thus, there exists a need in the art for a birefringent filter-based color generation scheme for a passive matrix display device.
In the design of a single stage of a color filter, the incident light can be considered to have two spectral components: a controlled spectral component and an un-controlled spectral component.
One of the required features of the electro-optical element in the color filter of Sharp is that the polarization state of the controlled spectral component of the light is effected by the state of the electro-optical element, but the polarization state of the uncontrolled component is not.
Sharp used an untwisted liquid crystal device for which it is well known that if light is linearly polarized with a particular state (at 45° to the projection of the optic axis on the plane of the device), that the polarization state of the light will be affected by a voltage applied to the device; but if light is polarized parallel to the projection of the optic axis, then the polarization state will not be affected by a voltage applied to the device.
However, untwisted nematic devices typically do not show that appropriate voltage versus polarization state change that is required for a multiplexed passive matrix display, and it is not clear, from Sharp, that it could be possible to use a nematic material of twisted structure. In fact, it is well known that twisted devices do not have the characteristic that a state of linearly polarized light with a wide wavelength spectral band can be unaffected by the state of the device. Therefore, the problem of designing a filter of the type discussed by Sharp, but having a twisted, multiplexable, liquid crystal device, is the focus of this invention.