Liquid crystal cells – elements and systems – Liquid crystal system – Variable or rotatable retarder used with other retarders to...
Reexamination Certificate
1999-02-02
2002-09-17
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Liquid crystal system
Variable or rotatable retarder used with other retarders to...
C349S119000
Reexamination Certificate
active
06452646
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a device for selectively transforming partially polarized light and in particular to a retarder stack for transforming at least partially polarized light for input into various optical devices such as electro-optic modulators, magneto-optic modulators or other optical components.
2. Background of the Related Art
The manipulation or transformation of polarized or partially polarized light is essential in a wide variety of optical systems. Especially with the onset of integrated optics and the processing of optical signals, it is necessary to predictably manipulate the polarization of light before that light proceeds to the next stage in an optical system.
Many optical systems require input light be completely or nearly completely polarized and have a known bandwidth and polarization such as input light from lasers or light emitting diodes (LEDs). It is important, however, to be able to selectively transform portions of relatively wide bandwidth light for eventual use in optical systems. For example, it is desirable to be able to selectively shift a certain band of frequencies within a wide band of frequencies comprising white light.
Color display and color filters are examples of optical systems which utilize wide bandwidth light such as white light to function.
Color Displays
Color display is generally provided by spatial or temporal multiplexing of the additive primary colors, red, green and blue. In a spatial multiplexed display, each color pixel is divided into three subpixels, one for each primary color. Ideally the pixels are small enough compared to the viewing distance from the eye that the colors are spatially integrated into a single full-color image. As a result of subdividing each pixel, the spatial resolution of the display is reduced by a factor of at least three. In temporal multiplexing, colors are sequentially switched between the three primary colors, and if the switching rate is fast enough the eye temporally integrates the three images to form a single full-color image. In both cases, the color filter is typically combined in series with a binary or display capable of generating a gray scale which is spatially aligned and temporally synchronized with the color filter to modulate the intensity of each color. To display white with spatial multiplexing, all three subpixels simultaneously transmit a primary; with temporal multiplexing the three primaries are sequentially transmitted. In either case, at best only one third of the input intensity can be displayed.
In subtractive display, color is produced by stacking three monochrome displays (for example Plummer, U.S. Pat. No. 4,416,514 and Conner et al., U.S. Pat. No. 5,124,818). Polarization components are placed between each display panel, such that each panel ideally independently controls the transmission of an additive primary color. Subtractive displays have the advantage that every pixel is a three-color pixel and that the display does not, in principle, suffer the throughput loss associated with spatial or temporal multiplexing. However, previous implementations generally could not completely independently modulate each color. Additionally, they utilized pleochroic dye polarizers as the only color selective polarization components between each display panel. Due to the poor performance of pleochroic dye polarizers, including poor color contrast, high insertion loss and shallow transition slopes, the benefits of subtractive displays have not before been realized.
Color Filters
There are two basic classes of liquid crystal color switching filters: polarization interferences filters (PIFs) and switched-polarizer-filters (SPFs). The basic unit of an SPF is a stage, consisting of a color polarizer and a two-state neutral polarization switch. This class is intrinsically binary tunable, such that each filter stage permits switching between two colors. Stages are cascaded in order to provide additional output colors. Color polarizers used in SPFs include single retardation films on neutral linear polarizers and pleochroic color polarizing filters. The polarization switch can be a liquid crystal (LC) polarization switch preceding a static polarization analyzer. The switch optimally provides neutral polarization switching. The chromatic nature of the active element degrades performance and is ideally suppressed in an SPF.
Shutters based on color polarizer consisting of a neutral-polarizer followed by a single retarder are well reported in the art (for example in U.S. Pat. No. 4,002,081 to Hilsum, U.S. Pat. No. 4,091,808 to Scheffer and U.S. Pat. No. 4,232,948 to Shanks). While the polarizer/retarder structure can be described as a complementary color polarizer in the sense that it is possible to produce two distinct hues by rotating the polarizer through 90 degrees, using this type of color polarizer in an SPF does not result in saturated colors.
Shutters based on pleochroic color polarizers are also well reported (for example in U.S. Pat. No. 4,582,396 to Bos, U.S. Pat. No. 4,416,514 to Plummer, U.S. Pat. No. 4,758,818 to Vatne and U.S. Pat. No. 5,347,378 to Handschy). Pleochroic color polarizers are films that function as linear polarizers in specific wavelength bands. They are formed by doping a polymer with long-chain pleochroic dyes. Incident white light polarized along one axis is fully transmitted, but is selectively absorbed along the orthogonal axis. For instance, a cyan color polarizer functions as a linear polarizer by absorbing the red along one axis. A color polarizer that passes a primary color (either additive or subtractive) along each axis can be formed as a composite consisting of two films with crossed axes. Colors are typically selected using crossed complementary color (e.g. red/cyan) polarizer films coupled with a switchable polarizer. A full-color device can comprise five polarizing films (one neutral), and two switching means. The resulting structures provide poor overall peak transmission.
Polarization Interference Filters
The simplest PIFs are essentially two-bean interferometers, where a uniaxial material induces a phase shift between orthogonally polarized field components. Color is generated by interfering these components with an analyzing polarizer. Color switching is accomplished by changing the phase shift between the arms. The most rudimentary color switches comprise a single variable retarding means between neutral polarizers. Single stage devices can also incorporate passive bias retarders with variable birefringence devices. However, these single stage PIFs are incapable of providing saturated color.
PIFS often comprise cascaded filter units in a Lyot structure, each performing a distinct filtering operation to achieve improved selectivity. A polarization analyzer is required between each phase retarder, reducing transmission. Though adequate color saturation is obtained, multiple-stage birefringent filters are by definition incapable of functioning as color polarizer. This is quite simply because color polarizers must transmit both orthogonal polarizations, which does not permit internal polarizers.
Tuning is accomplished by varying the retardance of active elements in each stage, maintaining specific relationships between retardances, in order to shift the pass-band. PIFs use LC elements as variable retarders in order to shift the transmission spectrum. As such, in contrast to SPF, the chromaticity of the active element retardance is not only acceptable, it is often an integral aspect of the design. In PIF designs, an analyzing polarizer is a static component and tuning is accomplished by changing the retardance of the filter elements. When multiple active stages are used, the retardances are typically changed in unison to shift the pass-band, while maintaining the basic design. Variable birefringence PIFs can be tuned to provide peak transmission at any wavelength. By contrast, SPFs do not provide tunable color.
olc filters (
olc (1965), J. Opt. Soc. Am. 55:621) provides high finesse spectra usi
Johnson Kristina M.
Sharp Gary D.
ColorLink, Inc.
Fleshner & Kim LLP
Parker Kenneth
LandOfFree
Optical retarder stack formed of multiple retarder sheets does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical retarder stack formed of multiple retarder sheets, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical retarder stack formed of multiple retarder sheets will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2833178