Polarizing beam splitter

Details Polarizing beam splitter Polarizing beam splitter

2001-06-11

2003-08-26

Adams, Russell (Department: 2851)

Optics: image projectors

Polarizer or interference filter

C353S031000, C353S033000, C359S494010, C359S490020, C359S490020, C359S506000, C359S580000, C359S586000, C428S212000

Reexamination Certificate

active

06609795

ABSTRACT:

TECHNICAL FIELD
The present invention pertains to a polarizing beam splitter useful in, among other applications, a projection system. In particular, the polarizing beam splitter combines a prism of relatively high refractive index with a birefringent multi-layer film. The multi-layer film functions as a polarizer and contains at least two different materials, at least one of which exhibits birefringence after uniaxial orientation. The multi-layer film is selected so as to be stable to near UV and blue light.
BACKGROUND
For projection systems that use reflective liquid crystal display (LCD) imagers, a folded light path where the illuminating light beam and the projected image share the same physical space between a polarizing beam splitter (PBS) and an imager offers a compact design. Most reflective LCD imagers are polarization rotating, i.e., polarized light is either transmitted with its polarization state substantially unmodified for the darkest state or transmitted with its polarization state rotated to provide a desired gray scale. Thus, a polarized light beam is generally used as the input beam. Use of a PBS offers an attractive design because it can function to polarize the input beam and fold the light path.
A PBS is an optical component that splits incident light rays into a first (transmitted) polarization component and a second (reflected) polarization component. One common PBS is the MacNeille polarizer that discriminates between s and p-polarized light as described in U.S. Pat. No. 2,403,731 to MacNeille. In a MacNeille polarizer, the s-polarization is reflected and, over a narrow range of angles near the Brewster angle, the p-polarization is mostly transmitted. The p-component corresponds to light polarized in the plane of incidence. The s-component corresponds to light polarized perpendicular to the plane of incidence. The plane of incidence means a plane defined by a reflected light ray and a normal to the reflecting surface.
Some skilled in the art have devised other types of PBS. For example, U.S. Pat. No. 5,912,762 (Li et al.) discloses a thin film polarizing device that may be used in a PBS. The device has first and second light transmissive substrates in the form of prisms and a plurality of thin film layers disposed between the prisms. The thin film layers comprise high refractive index layers and low refractive index layers, the high refractive index layers having one or more different refractive indices and the low refractive index layers having one or more different refractive indices. The light transmissive substrates have a refractive index greater than the refractive index of each of the low refractive index layers. The prisms are shaped so as to allow incident light to strike upon the thin film layers at a plurality of angles greater than or equal to the critical angle (i.e., the angle that generates total internal reflection conditions) for the highest refractive index of the low refractive index layers. Like the MacNeille polarizer, the polarizer in U.S. Pat. No. 5,912,762 discriminates between s and p-polarized light, although in the latter, s-polarized light is transmitted and p-polarized light is reflected.
As another example, WO 00/70386, in
FIG. 1
, discloses a Cartesian PBS element
50
that includes a multi-layer birefringent film
52
encased in a glass cube
54
, and oriented so as to reflect light incident with x-polarization (i.e., approximately s-polarization). See page 11, lines 9 to 11. The notation in WO 00/70386 publication is different in that y-polarization is said to be approximate to s-polarization. For incident rays of light in a large cone angle, the Cartesian PBS has been demonstrated to provide a higher contrast than a PBS that discriminates only on the basis of s-polarization vs. p-polarization.
The technology discussed thus far, although disclosing useful PBS using multi-layer films, may not be well suited for use in a projection system. In such a system, the PBS typically experiences high intensity of light from a wide range of wavelengths possibly for long periods of time. Although the inorganic based multi-layer films of U.S. Pat. No. 2,403,731 and U.S. Pat. No. 5,912,762 may be stable to high intensity blue light, they have deficiencies in angular performance needed in low f-number systems. What is needed to advance the art is a multi-layer film based PBS that has the durability to withstand the light source and simultaneously to provide contrast for incident light in large cone angles so that the resulting image of a projection system, when viewed by an observer, appears bright, sharp, distinct, and possesses crisp colors.
SUMMARY
Polarizing beamsplitters can be fabricated from birefringent polymeric multi-layer films, as disclosed in U.S. Pat. No. 5,962,114. Although many polymers exhibit a high transparency to visible light, many have strong absorption peaks in the near ultraviolet (UV) region. As a result, an absorption tail may extend into the visible portion of the spectrum. Although the percentage of absorbed light may be low, the absorbed energy in an intense light beam can result in over-heating of the film leading to thermal induced degradation of the polymer, light induced degradation or both. For some high index polymers, the absorption tail in the blue region is strong enough to impart a yellow color to the film. A key parameter in selecting polymers for a stable multi-layer PBS for high intensity projection systems is the proximity of their absorption edges to the visible spectrum.
The present invention provides a PBS that combines at least one high refractive index (i.e., greater than n=1.60) prism with a birefringent multi-layer film (sometimes referred to as “multi-layer film” for convenience). The multi-layer film functions as a polarizer. It contains alternating material layers that are stable when exposed to wavelengths associated with near UV light and blue light. These material layers are chosen based on their absorption spectrum within the visible spectrum and on the location of absorption edges in the UV and infrared (IR).
On the UV end of the spectrum, absorption edges for the material layers in the multi-layer film are preferably at least 40 nm less than, more preferably 50 nm less than, most preferably 60 nm less than the shortest wavelength that illuminates the PBS. For color projection displays, blue light below 420 nm can be rejected without substantially affecting the color balance or brightness of the display. Thus in a preferred embodiment, the shortest wavelength that illuminates the PBS is 420 nm. Depending on the light source, the preferred lower wavelength may be shorter, such as 410 nm, or somewhat higher, such as 430 nm. On the IR end of the spectrum, the absorption edges for the material layers in the multi-layer film are preferably at least 40 nm greater than, more preferably 50 nm greater than, most preferably 60 nm greater than the longest wavelength that illuminates the PBS. These considerations may exclude some combinations of materials that can be oriented to produce a high index difference between them in the x (stretched) direction. Practical processing and environmental stability considerations may restrict the set of available materials to those that have a relatively small refractive index difference (i.e., less than 0.15 &Dgr;n
x
) between them (in the x direction) after orientation.
In this document, the term “about” is presumed to modify each numerical recitation of a property such as, but not limited to, wavelength, refractive index, ratios, weight percentages, mole percentages. For example, a recitation of 500 nm for wavelength means about 500 nm. The term “pass axis” means the optical axis of transmission of the polarizer and corresponds to the y-axis or non-stretch direction of the multi-layer film. The term “extinction axis” means the axis of reflection of the polarizer and corresponds to the x-axis or stretch direction of the multi-layer film.
The term “absorption edge” means generally the wavelength at which the polymeric material becomes substanti

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