Optical: systems and elements – Polarization without modulation – Polarization variation over surface of the medium
Reexamination Certificate
1998-05-14
2001-03-27
Schuberg, Darren (Department: 2872)
Optical: systems and elements
Polarization without modulation
Polarization variation over surface of the medium
C359S487030, C349S009000, C353S020000, C362S019000
Reexamination Certificate
active
06208463
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical systems within the visible and near visible spectrums which include a polarizer apparatus for producing a generally polarized light beam from a generally unpolarized source light beam. More particularly, the present invention relates to such optical systems comprised of a plurality of optical elements, one of which consists of a polarizer having a generally parallel arrangement of elongated elements disposed in the source light beam for interacting with the electromagnetic waves of the source light beam to generally (i) transmit or pass light having a polarization oriented perpendicular to the length of the elements, and (ii) reflect light having a polarization oriented parallel with the length of the elements.
2. Prior Art
Polarized light is utilized in various applications such as, for example, liquid crystal displays (LCDs) and projection systems. Liquid crystal displays are commonly used for displays in laptop computers and other information displaying devices such as watches and calculators. Liquid crystal projectors are also used to display information, but project the information or images onto a distant screen. Such projectors usually have their own, powerful light source.
The liquid crystal display devices within these projectors employ polarizer devices in combination with the properties of the liquid crystal elements to selectively transmit or absorb light to produce a pattern of light and dark pixels, creating the desired image. The ability to turn light on or off leads to their common designation as a liquid crystal light valve. They function by taking advantage of the liquid crystal material's ability to rotate the polarization of light when organized and aligned appropriately, and its characteristic that this proper alignment can be altered by an external electric field.
Typically, two polarizer devices are employed, one on each side of the liquid crystal elements, creating a light valve assembly. The purposes of the polarizer devices are to present polarized light to the liquid crystal elements and then to analyze the light passed by the liquid crystal elements and block light of the undesired polarization.
It should be understood that the first polarizer device that presents light to the liquid crystal elements need not be immediately adjacent to the liquid crystal elements. However, it is required that the light arriving at the liquid crystal elements be highly plane polarized in order to present a quality, high-contrast image. Therefore, a polarized light beam generated by a polarizer device some distance from the liquid crystal elements could function as this first polarizer device. Of course, there are other applications for polarized light beams, such as are found in scientific instruments and certain types of illumination systems.
The term “polarized” or “polarized light” refers to a beam of light generally having a single linear, or planar, polarization defined by similarly oriented electromagnetic waves. A natural beam of light, on the other hand, is generally unpolarized, or has a number of planes of polarization defined by the electromagnetic waves emitted by the light source. This natural, or unpolarized, light may be characterized as being composed of two, orthogonal, linear (plane) polarizations.
The electromagnetic waves of a particular polarization, or orientation, may be separated out from the unpolarized source, which contains both the particular polarization and the orthogonal polarization. Devices that separate out a particular polarization are called polarizers and may be used to obtain a beam of light generally having a single polarization, or linearly polarized light.
The concepts of polarized light and certain polarizing devices have existed for over a century. Surprisingly, the most modern and advanced applications of polarized light still employ polarizers that are fundamentally unchanged from those of over 30 years ago. This situation is surprising because the fundamental physical mechanisms by which these polarizers function do not provide ideal polarizers for most applications. The resulting performance limitations seriously constrain optical system design flexibility, optical efficiency, system cost, and over-all performance. The consequences of these limitations have led to numerous attempts to improve polarizer performance in ways that typically compromise performance in one or more characteristics, in order to obtain less restrictive performance in another characteristic.
Examination of the history of polarizers and their use in optical systems to produce polarized light beams shows that the polarizer component is the primary and most significant reason why the use of polarized light beams exhibits one or more of the following characteristics: inefficiency, color-dependent performance variations, a requirement for highly collimated light, and complicated optical systems.
Probably the first polarizer known was a birefringent polarizer formed from a calcite crystal. Birefringent polarizers can now be made from many crystals and also certain stretched polymers. Birefringent polarizers are formed from materials that have a different optical index in one direction compared to another, though the degree of difference in the optical index will vary with the color of the light. This differing optical index can be used to separate beams of one linear polarization from another, though this separation typically consists of a small angular deviation. This narrow separation may require the use of complicated optics. It may also require that the light travel through a significant amount of material or over an extended optical path, leading to a bulky optical element or design. Finally, the narrow separation makes it difficult to use both polarizations, meaning half of the light is usually discarded or wasted through absorption or other means.
Use of a birefringent polarizer is typically characterized by inefficiency, color-dependent performance variations, a requirement for highly collimated light, and complicated optical systems. The bulky optics and extended optical path impose additional performance and design penalties. For these reasons, birefringent polarizers are not commonly used in optical systems such as image projectors.
Another type of polarizer, developed in the 1930s and still the primary polarizer used in laptop computer displays, is the dichroic polarizer. A dichroic polarizer is a polarizer device that absorbs one polarization and passes the other. Many types of dichroic polarizers have been developed, but the most common type consists of a polymer sheet that has been stretched to orient its molecules and then treated with iodine and/or other materials so that the oriented molecules absorb any polarization of one orientation.
The most significant problem with dichroic polarizers is their absorption of light. Typical stretched polymer sheet polarizers absorb essentially all of one polarization and 15% or more of the desired or passed polarization, leading to an inefficient use of light. All polymer polarizers have other problems as well, such as their low tolerance for heat and sensitivity to photon induced chemical changes that cause the material to yellow or become brittle with use and age. These problems become increasingly critical as the brightness of the optical system is increased. The inherent inefficiency of all dichroic polarizers combined with the environmental (heat and light) sensitivity of the most common polymer sheet polarizers leaves much to be desired.
Still another fundamental polarizer technology is the thin-film polarizer. It uses the Brewster's effect in which light striking the surface of glass or another medium at the Brewster's angle (near 45 degrees) is converted into two polarized beams, one transmitted and the other reflected. Polarization of light by use of the Brewster's angle can only be accomplished effectively over a very narrow angular range. An example of this type of polarizer
Gunther John
Hansen Douglas P.
Moxtek
Schuberg Darren
Thorpe North & Western L.L.P.
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