WO1997022834A1

WO1997022834A1 - Illumination assembly for providing a polarized light source

Info

Publication number: WO1997022834A1
Application number: PCT/US1996/020306
Authority: WO
Inventors: Scott M. Zimmerman; Hefen Lin; Ivan Steiner; Haiji Yuan; Karl W. Beeson
Original assignee: Alliedsignal Inc.
Priority date: 1995-12-21
Filing date: 1996-12-20
Publication date: 1997-06-26
Also published as: TW323339B

Abstract

An optical illumination system comprising a waveguide (16) that accepts light generated by a light source (14) and transmits the light via total internal reflection. Attached on one surface of the waveguide is an array of microprisms (28) and a polarizing film (44) disposed between the waveguide (16) and microprisms (28). The polarizing film (44) transmits light polarized in a first polarization state and reflects light polarized in a second polarization state. The reflected light is converted to light of the first polarization state to allow the light to transmit through the polarizing film (44).

Description

ILLUMINATION ASSEMBLY FOR PROVIDING

A POLARIZED LIGHT SOURCE

Cross Reference to Related Applications

This application is a continuation-in-part of U.S. Serial No. 08/242,525 filed on May 13, 1994, entitled Illumination System Employing an Array of Microprisms, now U.S. Patent No. 5,428,468 which is a continuation-in-part of U S. Serial No.

08/149,219, filed on November 5, 1993, entitled Backlighting Apparatus Employing an Array of Microprisms, now U.S. Patent No. 5,396,350, each of which is incorporated by reference herein.

Field ofthe Invention

This invention relates generally to an iUumination assembly that provides a polarized light source, and more particularly to a specifically designed backlight structure capable of providing a polarized collimated light source in two orthogonal viewing angles especially suitable for use with liquid crystal displays or any other appUcation that requires a polarized light source.

Background ofthe Invention

Liquid crystal displays (LCDs) are commonly used in portable computer systems, televisions other electronic devices and infoπnation display systems. An exemplary LCD display is graphically illustrated in Fig. 1. The overall display is made up of some type of backlighting apparatus, generically designated as 50, an input light polarizing means 52, a liquid crystal light modulator 54, an optional output light polarizing means 56 and a display means 58. A liquid crystal light modulator is essentially a light valve which allows transmission of light in a first state and blocks transmission of light in a second state. Backlighting the LCD has become the most popular source of light in personal computer systems because ofthe improved contrast ratios and brightness_. However, conventional monochrome LCDs displays are only approximately twelve percent transmissive and color LCDs displays are only approximately two percent transmissive. Therefore, large amounts of light are necessary to provide a visible display. This poses a problem from not only an efficiency standpoint, but also, and more importantly, from a power and space requirement, especially for portable LCDs.

A particular shortcoming associated with LCDs that contributes to overall system inefficiency is that LCDs require polarized light. Most light polarizers are inefficient. Nearly sixty percent ofthe light source is absorbed by the polarizers. This absoφtion limits the brightness ofthe display and contributes to the overall power inefficiency ofthe LCD.

Accordingly, there exists a need in the optical and illumination arts to provide an efifective illumination apparatus that delivers a polarized collimated light source to increase brightness and overall light transmission efficiency.

Summary ofthe Invention

The present invention is directed to an optical illumination system which provides either separately or in combination a non-diffuse or a substantially collimated polarized light source (hereinafter referred to as a spatially-directed polarized light source). Additionally, this invention is directed to any lighting application that requires a low profile spatially-directed polarized light source. In an LCD application, the invention may be used as a backlight that delivers a substantially collimated and polarized light source. The advantage is that minimal light is absorbed by the input polarizer since the light is already substantially polarized in the proper orientation, or even more preferably, an input polarizer is not used.

The optical illumination system comprises a light transmitting means having a generally planar top surface and bottom surface and opposing sides disposed between said top and bottom surfaces; means for directing light into said light transmitting means; a reflecting means optically coupled to said top surface for removing and redirecting the light from the light transmitting means; a polarizing means disposed between said top surface and said reflecting means for transmitting light polarized in a first polarization state out of said light transmitting means. Preferably, the remaining light, polarized in a second polarization state and not transmitted by said polarized means, is recycled within said transmitting means and converted to the first polarization state or randomly polarized to allow light now polarized in the first polarization state to be transmitted by the polarizing means. The cycles continues until substantially all ofthe input light is either transmitted by the polarizing means as a polarized light source or is eliminated from the optical system by various losses such as scattering or absoφtion.

In one embodiment, a light source is positioned adjacent to a light accepting surface ofthe light transmitting means. The light transmitting means may be any structure that transmits light via reflection, such as a light pipe, light wedge, waveguide or any other structure known to those skilled in the art. Preferably the light transmitting means comprises a waveguide that accepts the light generated by the light source and transports the light via total intemal reflection (TIR). On one surface ofthe waveguide is a light polarizing means upon which is attached an array of microprisms. Each microprism comprises a light input and a light output surface wherein the input surface is optically coupled to the light polarizing means. The microprisms further comprise four sidewalls. At least one ofthe four sidewalls is angled in such a way that light rays traveling through the waveguide are captured by the microprisms, redirected by the angled sidewall(s), reflect through the microprisms via TIR and emerge from the microprisms as a spatially-directed polarized light source. A spatially-directed polarized light source is meant to include a substantially collimated polarized light source in a direction substantially peφendicular to the light output surface or a polarized light source directed at an angle with respect to the normal ofthe light output surface. In an alternate embodiment, an array of microlenses is operatively disposed adjacent to the light output surface ofthe microprisms. The microlenses are formed with the proper curvature and positioned so that the light emanating from each microprism is direαed to at least one corresponding microlens. The light transmits through the microlenses and emerges as a substantially more collimated polarized light source.

Brief Description Of The Drawings

FIGURE 1 is a representative schematic of a prior art LCD in combination with a backlight; FIGURE 2 is a graph ofthe refection coefficient of p-polarized and s- polarized light;

FIGURE 3 is a representation ofthe illumination system ofthe present invention;

FIGURE 4a is a preferred embodiment ofthe reflecting means in conjunction with the present invention;

FIGURE 4b is one embodiment of the polarizing film;

FIGURE 4c is an alternate embodiment ofthe polarizing film;

FIGURE 4d is a further embodiment ofthe polarizing film,

FIGURE 5a is the preferred embodiment ofthe reflecting means in conjunction with the present invention

FIGURE 5b is a alternate view ofthe preferred reflecting means;

FIGURE 6 is an elevation view ofthe preferred reflecting means in combination with an array of microlenses;

FIGURE 7 is a specific embodiment of the polarizing film of FIG. 4b and; FIGURES 8a-b are graphs ofthe reflectance of p-polarized and s-polarized light ofthe polarizing film of FIG. 7.

Detailed Description ofthe Preferred Embodiments The invention will be disclosed in the form of a backlight assembly for an

LCD display for illustrative puφoses; the invention, however, is applicable wherever a low profile spatially-directed polarized light source is required.

In one embodiment of this invention utilizing linear polarization, it is possible to make a polarizing backlight by forming a multilayer coating on the surface of a waveguide. The coating should have alternating layers ofhigh and low index material. The coating utilizes the optical property that when light strikes an interface at some incident angle other than normal incidence, p-polarized light (linearly polarized light which has the polarization vector in the plane containing the incident, reflected and transmitted rays) has a different reflection coefficient (Fresnel coefficient) than s-polarized light (linearly polarized light which has the polarization vector peφendicular to the plane containing the incident, reflected and refracted rays). There is an angle (Brewster's Angle) at which the reflection coefficient for p- polarized light goes to zero. Brewster's Angle is defined as

r θs _B-_«™ _c = tan -1¹ ( ,— ⁿ² s )

where n₂ is the refractive index for the second material and ni is the refractive index for the first material (the material containing the incident ray). The plot ofthe reflection coefficients for all angles is shown qualitatively in Fig. 2.

For an interface between TiO₂ (assume n₂ ~ 2.6) and SiO₂ (ni ~ 1.46), Brewster's angle is approximately 61°. This is a typical angle for light traveling in a waveguide. The polarizing effect for one TiO₂/SiO₂ bilayer is approximately 5-

10%. To get a larger effect, one can stack many layers of TiO₂ and SiOj on top of each other. This multilayer stack is fabricated onto one surface ofthe waveguide. In another embodiment of this invention utilizing circular polarization, it is possible to make a polarizing backlight by fabricating a layer of cholesteric liquid crystals on one surface of a light guide. It is well known that planar oriented cholesteric liquid crystals reflect the component of incident light which is circularly polarized with the same handedness as the cholesteric helix and transmit the other component. The reflection for normal incidence peaks at λo = n P, where n = (n, + no)/2 and is the average refractive index expressed in terms ofthe extraordinary refractive index n« and the ordinary refractive index no and P is the pitch ofthe cholesteric liquid crystal helix. The bandwidth ofthe reflection is Δλ=λoΔn/n, where Δn = (n. - n_<,)is the birefringence. Stable cholesteric liquid crystal polymer films can be produced for example by using in-situ photopoiymerization of chiral liquid crystal diacrylates. Furthermore, cholesteric polymer films with a pitch gradient can be made to cover the whole spectrum of visible light and wide incident angles. As an example, a 15μm thick cholesteric polymer film with P = 240 nm on one side ofthe film and linearly increased to 410 nm on the other side ofthe film works very well as a broadband and wide angle reflective cholesteric polarizer as disclosed by D. J. Broer et al., in Proceedings of Asia Display '95, p. 735 (1995).

An illumination system according to the present invention is shown in Fig. 3 It should be appreciated that all drawings are merely representations ofthe structure; the actual and relative dimensions will be different. It should also be appreciated that the elements shown in the drawings may not necessarily be contiguous, but rather merely adjacent.

The illumination system 12 contains a light source 14 and a light guide 16. Light guide 16 may be any structure that transmits light via reflection, such as a light pipe, light wedge, waveguide or any other structure known to those skilled in the art. Light guide 16 is transparent to light within the wavelength range from about 400 to about 700 nm. The index of refraction ofthe light guide 16 may range from about 1.40 to about 1.65. The most preferred index of refraction is from about 1.45 to about 1.60. The light guide 16 may be made from any transparent solid material. Preferred materials include transparent polymers, glass and fused silica. Desired characteristics of these materials include mechanical and optical stability at typical operation temperatures ofthe device. Most preferred materials are glass, acrylic, polycarbonate and polyester. Fabricated on one surface of light guide 16 is a polarizer film 44 that is constructed to transmit light of a first polarization state and reflect light of a second polarization state. Optically coupled to polarizer film 44 is a reflector means 18 that has a light input surface that captures light transmitted by polarizer film 44 and one or more surfaces that redirect the light and outputs the light as a spatially- directed polarized light source. Exemplary reflector means are disclosed in U.S. Patent No. 5,161,041, Japanese Publication Nos. 5-45505, 5-89827, 5-333334 and 5-127159. Preferably, reflector means is that disclosed in U.S. Patent Nos. 5,396,350 to Beeson et al. and 5,428,468 to Zimmerman et al.

Figures 4a-d and Fig. 5 illustrate the invention in combination with preferred reflector means 18 for illustration puφoses only, and are not intended to limit the scope ofthe invention. Referring to Fig. 4a, reflecting means 18 comprises an array of microprisms 28 as disclosed in U.S. Patent No. 5,396,350. Light rays reflect through light guide 16 via TIR and light rays of one polarization state transmit through polarizer film 44 and can enter microprisms 28 by way of light mput surfaces 30, reflect off sidewalls 33 and exit microprisms 28 through the light output surfaces 32 as a spatially-directed polarized light source.

On the bottom surface of light guide 16 is a polarization conversion layer 48. Polarization conversion layer 48 can convert a first polarization state into a second polarization state and vice versa. One example of a polarization conversion layer is a birefnngent layer which can be designed to change: (a) a first linear polarization state into a second linear polarization state; (b) a first circular polarization state into a second circular polarization state; (c) a linear polarization state into a circular or elliptical polarization state; or (d) a circular or elliptical polarization state into a linear polarization state. The thickness of a birefnngent layer is typically given in terms ofthe wavelength ofthe light (measured in the birefnngent layer) being converted from one polarization state to another polarization state. For example, if the wavelength of light is 500 nm and the birefringence ofthe birefringent layer material is Δn=0.10, then a polarization conversion 1/4-wave layer would be 500/(4*0.10) nm or 1250 nm thick. This assumes that light is traveling in a direction peφendicular to the plane polarization conversion layer 48. If the light is traveling at an angle ψ relative to that peφendicular direction, then one must multiply by a correction term equal to cosψ in order to obtain an equivalent 1/4-wave thickness. In the examples which follow, we will assume that 1/8-wave, 1/4- wave and 1/2 -wave thicknesses refer to the equivalent thicknesses corrected for the average angle of incidence ψ_iV, ofthe light in that particular example.

Another example of a polarization conversion layer 48 is a metallic mirror layer. Light striking a metallic mirror undergoes a phase shift of 180°. Because of this, a metallic mirror can convert right-circularly-polarized light into left-circularly- polarized light and vice versa.

Unpolarized light can be represented as containing equal proportions of two beams with different polarization states. The two states can either be represented as two linear polarization states oriented at right angles to each other or the two states can be represented as right-circularly-polarized and left-circularly-polarized states. In the embodiments of this invention illustrated in Figs. 4a and 4b, polarization film 44 is designed to transmit one particular linear polarization state. In Fig. 4a, unpolarized light ray 50 is emitted by light source 14 and travels in light guide 16 via TIR. The unpolarized light is illustrated as having two equal and peφendicular linear polarizations. The polarization vector parallel to the plane of the incident and reflected beams at the position ofthe polarization film 44 is designated as p- polarized and the polarization vector peφendicular to the plane of the incident and reflected beams is designated as s-polarized. In this example, polarizer film 44 is a multilayer stack of appropriate layers of different indexes of refraction ni and n₂ as shown in Fig 4b which is fabricated onto surface 17 Examples of materials that can make up film 44 are disclosed in U. S Patent 4,974,219 to Korth and include titanium dioxide (TiO₂) and silicon dioxide (SiO₂) Optically coupled to film 44 is reflecting means 18 Film 44 transmits a first linearly-polarized light component, here, the p-polarized vector, and reflects the second linearly-polarized light component, here, the s-polarized state The p-polarized light can transmit through reflecting means 18 and exit as a spatially-directed polarized light source The s- polarized light continues to transmit within the light guide 16 via TIR and is designated as ray 150 until it strikes polarization conversion layer 48 covering all or selected portions ofthe bottom surface of light guide 16 If polarization conversion layer 48 is a birefnngent 1/8-wave layer, ray 150 will pass downward through layer 48, be reflected off the bottom surface of layer 48 by TIR since the bottom surface is in contact with air, pass through layer 48 a second time traveling in the upward direction and then return to light guide 16. Since the light ray is retarded by a total of 1/4 wave by passing twice through birefringent layer 48, s-polarized ray 150 will be converted to circularly polarized ray 250 which is functionally equivalent to the ray being composed of equal parts of s-polarized and p-polarized light or to being randomly polarized. Ray 250 is shown in Fig 4a as being composed of equal parts of p-polarized and s-polarized light. Ray 250 can pass through polarization film 44 and again be split into its s-polarization component and p-polarization component where the s-polarization component will be reflected by polarization film 44 and the p-polarization component can be transmitted to reflecting means 18 and emerge from reflecting means 18 as spatially-directed polarized light.

In another embodiment of this invention, polarization conversion layer 48 covering all or selected portions ofthe bottom surface of waveguide 16 is constructed from birefringent material to form a 1/4- wave layer. In this example, s- polarized ray 150 will pass downward through layer 48, be reflected off the bottom surface of layer 48 by TIR since the bottom surface is in contact with air, pass through layer 48 a second time traveling in the upward direction and then return to light guide 16. Since the light ray experiences 1/2 wave retardation by passing twice through layer 48, s-polarized ray 150 will be converted into p-polarized light. The p-polarized light can then pass through polarization layer 44 and reflecting means 18 and emerge from reflecting means 18 as spatially-directed polarized light. In still another embodiment of this invention, illustrated in Figs. 4a and 4c, polarizing film 44 comprises of at least layer 45 of a circular polarizing material such as a cholesteric liquid crystal which transmits a first state of circular polarization (for example, right-circularly-polarized light) and reflects a second state of circular polarization (in this example, left-circularly-polarized light). Light rays ofthe first circular polarization state can transmit through polarizer film 44 and can enter microprisms 28 of reflecting means 18 by way of light input surfaces 30, reflect off sidewalls 33 and exit microprisms 28 through the light output surfaces 32 as a spatially-directed circularly polarized light source. Optionally, polarization film 44 also comprises a 1/4-wave polarization conversion layer 46 in addition to circular polarizing layer 45. Light of the first circular polarization state that is transmitted by layer 45 then passes through the 1/4-wave birefringent polarization conversion layer 46 which converts the circularly polarized light to linearly polarized light. Linearly polarized light transmitted through 1/4- wave layer 46 can enter microprisms 28 by way of light input surfaces 30, reflect off sidewalls 33 and exit microprisms 28 through the light output surfaces 32 as spatially-directed linearly polarized light. In this example, polarization conversion layer 46 is in optical contact with circular polarizing layer 45. Optionally, reflecting means 18 may be placed between circular polarizing layer 45 and polarization conversion layer 46 so that layers 45 and 46 are not in physical contact. Light ofthe second circular polarization state that is initially reflected off polarization film 44 will travel across the light guide 16 and interact with polarization conversion layer 48. In one embodiment of this invention, polarization conversion layer 48 is a birefringent 1/4 wave layer so that light ofthe second circular polarization state will pass downward through layer 48, reflect off the bottom surface of layer 48 by TIR, pass through layer 48 a second time traveling in the upward direction and then return to light guide 16 Since the light ray ofthe second circular polarization experiences 1/2 wave retardation by passing twice through layer 48, the light is converted to the first circular polarization state (right circularly polarized light in this example) and can now pass through polarization layer 44 and reflecting means 18 and emerge from reflecting means 18 as spatially-directed polarized light. In another embodiment of this invention, polarization conversion layer 48 is a metallic mirror. A metallic mirror changes the phase of reflected light by 180° relative to incident light. Therefore, a metallic mirror will change light of a second circular polarization state into light of a first circular polarization state. In this embodiment, light ofthe second circular polarization state (left-circularly-polarized light in this example) that is initially reflected off polarization film 44 will travel across the light guide 16 and interact with polarization conversion layer 48. The metallic mirror converts the light ofthe second circular polarization state to the first circular polarization state (right circularly polarized light in this example) and that light can now pass through polarization layer 44 and reflecting means 18 and emerge from reflecting means 18 as spatially-directed polarized light.

In a further embodiment of this invention illustrated by Figs. 4a and 4d, polarization conversion layer 48 is eliminated and polarizing film 44 is comprised of at least a layer consisting of alternating regions 47a and 47b of circularly polarizing materials such as various cholesteric liquid crystals. Regions 47a are designed to transmit a first circular polarization state (for example, right-circularly-polarized light) and reflect a second polarization state (in this example, left-circularly- polarized light). Light of a first circular polarization state that is transmitted by region 47a can enter of reflecting means 18 and emerge from reflecting means 18 as spatially-directed circularly polarized light. Regions 47b are designed to work in the opposite manner as regions 47a. In other words, regions 47b transmit a second circular polarization state and reflect a first circular polarization state. Light of a second circular polarization state that is transmitted by region 47b can enter reflecting means 18 and emerge from reflecting means 18 as spatially-directed circularly polarized light. In this example, light of a second polarization state that is reflected by a region 47a will travel through the light guide 16 by TIR and will eventually interact with a region 47b where it can be transmitted to reflecting means 18 and emerge from reflecting means 18 as spatially-directed circularly polarized light. Light of a first polarization state that is reflected by a region 47b will travel through the light guide 16 by TIR and will eventually interact with a region 47a where it can be transmitted to reflecting means 18 and emerge from reflecting means 18 as spatially-directed circularly polarized light. In the above example, light exiting portions of reflecting means 18 in contact with regions 47a will have a first circular polarization state. Light exiting portions of reflecting means 18 in contact with regions 47b will have a second circular polarization state. If one desires that light exiting both regions have the same circular polarization state or if one desires that light exiting both regions have the same linear polarization state, then polarization layer 44 must be comprised of polarization conversion regions 49a or polarization conversion regions 49b or both 49a and 49b in addition to circular polarization regions 47a and 47b In one embodiment of this invention, polarization layer 44 is comprised of circular polarization regions 47a and 47b and polarization conversion regions 49b which are birefringent 1/2-wave layers. Light exiting reflecting means 18 in contact with circular polarization regions 47a will have a first polarization state. Light transmitted through circular polarization regions 47b will have a second polarization state. This second polarization state will be converted to a first polarization state by birefringent polarization conversion regions 49b and will emerge from reflecting means 18 in a first polarization state, the same polarization state as the portions of reflecting means 18 that are in contact with circular polarization regions 47a. Similarly, one could make a polarized source by having polarization layer 44 comprised of polarization regions 47a and 47b and 1/2 -wave birefringent polarization conversion regions 49a. The output of reflecting means 18 will then be entirely the second circular polarization state.

In another embodiment of this invention, linear polarized light can be produced by having regions 47a designed to transmit a first circular polarization state, regions 47b designed to transmit a second circular polarization state, and polarization conversion regions 49a and 49b be 1/4-wave layers of birefringent material where the optical axis ofthe birefringent polarization conversion material in regions 49b is rotated 90° relative to the optical axis ofthe birefringent polarization conversion material in regions 49a. The light transmitted from polarization conversion regions 49a and 49b will then be linearly polarized and in the same polarization state. This light will be transmitted to reflecting means 18 and emerge from reflecting means 18 as spatially-directed linearly polarized light.

In the previous examples related to Figs. 4a and 4d, birefringent polarization conversion regions 49a and 49b are in optical contact with circular polarization regions 47a and 47b and are located between circular polarizing regions 47a or 47b and reflecting means 18. In another embodiment of this invention, birefringent polarization conversion regions 49a and 49b are not in optical contact with circular polarization regions 47a or 47b. In this embodiment, reflecting means 18 is located between the circular polarization regions 47a and 47b and the birefringent polarization conversion regions 49a and 49b.

Several ofthe embodiments above had a polarization conversion layer 48 located on the surface ofthe light guide opposite the polarizing layer 44. Alternatively, polarization conversion layer 48 could be placed between light guide 16 and polarizing layer 44. As a further alternative, light guide 16 may itself be made from a birefringent material so that one state of polarized light could be converted to another state of polarized light or could be randomly polarized as it travels through light guide 16.

Figs. 5 and 5a illustrate an alternate embodiment of microprisms identified here as 90 and disclosed in U.S. Patent No. 5,428,468. In this embodiment, a second state of polarized light may be continuously recycled within light guide 16 to become randomly polarized to then allow the first state of polarized light to transmit through film 44. This embodiment is useful for a single lamp embodiment and is also useful for an illumination system that comprises two opposing light sources 14 and 14 A. Each light source may act as a diffiiser element to randomly polarize reflected polarized light rays.

In this embodiment sidewalls 96 and 98 are tilted at angles φ*, and φ₂, respectively, and sidewalls 97 and 99 form tilt angles θi, and θ₂. Tilt angle φ* may equal angle φ₂ and angle θi may equal 02,but equality ofthe angles is not necessary. The desired values of angle φ range from about 15 to 50 degrees. The desired values of tilt angle θ range from about 0 degrees to about 25 degrees. More preferred values for tilt angle θ range from about 2 degrees to about 20 degrees.

Light of one polarization state may be randomly polarized using the techniques discussed above. Alternatively, one ofthe light sources 14 or 14A may be replaced with a reflecting means such as an eighth or quarter wave plate and a dilfijse or specular reflector. The light ofthe second polarization state not transmitted by film 44 is changed to the first polarization state by the reflecting means or becomes randomly polarized and is recycled within the wave guide 16 so that light rays now polarized in the first polarization state will transmit through polarizing film 44 to again leave behind light ofthe second polarization state for recycling. The cycles continues until substantially all ofthe input light is transmitted by film 44 or is lost from the optical system by scattering or absoφtion.

In an alternate embodiment, reflecting means 18 further comprises an array of microlenses 80 as shown in Fig. 6. The microlenses 80 are disposed in close proximity to microprisms 28 and 90. If the microlenses 80 are fabricated by photopolymerization, they are preferably made from the same monomers as those previously disclosed for the microprisms and have a index of refraction equal to or substantially equal to the index of refraction ofthe microprisms. However, any transparent material may be used, as for example, those materials previously discussed. Preferably, for every microprism there exists at least one corresponding microlens 80 that aligns with the output surface of each microprism.

A spacer 82 separates the microlenses 80 and the microprisms. The thickness of spacer 82 is optimized to cause light from the microprisms to be collimated by microlenses 80. Spacer 82 may be made from any transparent material. Preferred materials include transparent polymers, glass and fused silica. Preferably spacer 82 has an index of refraction equal to or substantially equal to the index of refraction ofthe microprisms and the microlenses 80. Desired characteristics of these materials include mechanical and optical stability at typical operation temperatures ofthe device. Most preferred materials are glass, acrylic, polycarbonate and polyester.

Arrays of microprisms 28 and 90 and associated microlenses 80 can be manufactured by any number of techniques such as molding, including injection and compression molding, casting, including hot roller pressing casting, photopolymerization within a mold and photopolymerization processes which do not employ a mold. A preferred manufacturing technique would be one that allows the reflecting means 18 which comprises an array of microprisms 28 or 90, an array of microlenses 80 and a spacer 82 to be manufactured as a single integrated unit. An advantage of this technique would be the elimination of alignment errors between the array of microprisms and microlenses if the arrays were manufactured separately and then attached in the relationship described above.

EXAMPLE

A computer modeling program was used to calculate the polarization properties of a multilayer coating comprising alternating layers of titanium dioxide (TiO-*-) and silicon dioxide (SiO₂) on an acrylic substrate (refractive index 1 49). The multilayer coating represented a Fabry-Perot type coating design. The TiO₂ layers were assumed to have an index of refraction of 2.646; the Siθ2 layers were assumed to have an index of refraction of I 453 The thicknesses ofthe SiO₂ layers and the TiO₂ layers and the number of layers in the multilayer coating were varied to optimize the polarizing properties ofthe coating for 450 run, 550 nm and 650 nm light incident on the coating surface at angles ranging from 50° to 70° from normal The design objective was to produce high s-polarized light reflectance and low p- polarized light reflectance for the stated wave lengths. It was found that a seven (7) layer coating composed of three layers of TiO₂ and four layers of SiO₂ has very good polarization selection properties for light incident angles between 50° and 73° Fig 7 illustrates the coating configuration Figs. 8 and 8A graphically illustrate the computed reflectance of p-polarized and s-polarized light rays respectively

It will be understood that the particular embodiments described above are only illustrative ofthe principles ofthe present invention, and that various modifications could be made by those skilled in the art without departing from the scope and spirit ofthe present invention, which is limited only by the claims that follow.

Claims

What is claimed is:

1. An illumination system comprising a light transmitting means having a generally planar first and second surfaces and opposing sides disposed between said first and second surfaces; means for accepting light into said light transmitting means; a reflecting means optically coupled to said first surface for removing and redirecting light from said light transmitting means; a polarizing means disposed between said transmitting means and said reflecting means for transmitting light polarized in a first polarization state from said light transmitting means and reflecting light polarized in a second polarization state; whereby said reflecting means transmits spatially directed polarized light.

2. The illumination system of claim 1 wherein said polarizing means comprises alternating layers ofhigh and low index materials.

3. The illumination system of claim 1 wherein said polarizing means comprises a layer of cholesteric liquid crystals.

4. The illumination system of claim 1 wherein light polarized in a second polarization state and not transmitted by said polarized means, is re-polarized by polarization conversion means whereby light now polarized in said first polarization state is transmitted by said polarizing means.

5. An illumination assembly comprising: (a) a light transmitting means having means for accepting light;

(b) an array of microprisms wherein each microprism comprises:

(i) an input surface optically coupled to said light transmitting means for receiving a portion of light transmitting through said light transmitting means; (ii) a sidewall having an edge defined by said light input surface and positioned for effecting total intemal reflection of a portion of light received by said light input surface,

(c) polarizing means disposed between said light transmitting means and said array of microprisms wherein said polarizing means transmits light polarized in a first polarization state and reflects light polarized in a second polarization state

6 The illumination system of claim 12 further comprising an array of microlenses in optical cooperation with said array of microprisms

7 An illumination assembly comprising

(a) a light transmitting means;

(b) an array of microprisms wherein each microprism comprises (i) a light input surface optically coupled to said light transmitting means for receiving a portion of light transmitting through said light transmitting means;

(ii) a light output surface having a surface area greater than the surface area of said light input surface;

(iii) a first pair of sidewalls disposed between and contiguous with said light input surface and said light output surface and at least one of said sidewalls is positioned for effecting total inte al reflection of a portion of light received by said light input surface,

(iv) a second pair of sidewalls, disposed between and contiguous with said light input surface and said light output surface and at least one of said sidewalls is positioned for effecting total intemal reflection of a portion of light received by said light input surface, and

(c) polarizing means disposed between said light transmitting means and said array of microprisms wherein said polarizing means transmits light polarized in a first polarization state and reflects light polarized in a second polarization state, whereby, said light transmitted through said polarizing means enters said microprisms through said light input surfaces, is redirected by said sidewalls and emerges through said light output surfaces as spatially-directed polarized light.