Color mixing illumination light unit and system using same

An illumination light unit includes at least two light sources that generate light at different wavelengths. The illumination light unit also includes a reflecting cavity having one or more reflectors and a controlled transmission mirror disposed at an output of the reflecting cavity. The controlled transmission mirror includes an input coupling element, an output coupling element and a first multilayer reflector disposed between the input and output coupling elements. At least some of the light from the light sources is reflected within the reflecting cavity by the one or more reflectors and is mixed. Light passes out of the reflecting cavity through the controlled transmission mirror. The illumination light unit may be used for illumination purposes, or as part of a backlight for illuminating a display.

RELATED APPLICATIONS

This application is related to the following applications, all of which are incorporated herein by reference: U.S. patent application Ser. No. 11/166,723, titled “OPTICAL ELEMENT FOR LATERAL LIGHT SPREADING IN BACK-LIT DISPLAYS AND SYSTEM USING SAME”, filed on even date herewith and having U.S. patent application Ser. No. 11/167,003, titled “OPTICAL ELEMENT FOR LATERAL LIGHT SPREADING IN EDGE-LIT DISPLAYS AND SYSTEM USING SAME”, filed on even date herewith and having U.S. patent application Ser. No. 11/167,001, titled “ILLUMINATION ELEMENT AND SYSTEM USING SAME”, filed on even date herewith and having and U.S. patent application Ser. No. 11/167,019, titled “POLARIZATION SENSITIVE ILLUMINATION ELEMENT AND SYSTEM USING SAME”, filed on even date herewith and having.

FIELD OF THE INVENTION

The invention relates to optical lighting and displays, and more particularly to signs and display systems that are illuminated by backlights.

BACKGROUND

Liquid crystal displays (LCDs) are optical displays used in devices such as laptop computers, hand-held calculators, digital watches and televisions. Some LCDs, for example in laptop computers, cell phones and certain LCD monitors and LCD televisions (LCD-TVs), are illuminated from behind using a backlight that has a number of light sources positioned to the side of the display panel. The light is guided from the light sources using a light guide that is positioned behind the display. The light guide is typically configured to extract the light from the light guide and to direct the light towards the display panel. This arrangement is commonly referred to as an edge-lit display, and is often used in applications where the display is not too large and/or the displayed image does not have to be very bright. For example, most computer monitors are viewed from a close distance, and so do not have to be as bright as an equivalently sized television display, which is typically viewed from a greater distance.

In larger, or brighter displays, the backlight tends to employ light sources positioned directly behind the display panel. One reason for this is that the light power requirements to achieve a certain level of display brightness increase with the square of the display size. Since the available real estate for locating light sources along the side of the display only increases linearly with display size, there comes a point where the light sources have to be placed behind the panel rather than to the side in order to achieve the desired level of brightness.

One important aspect of the backlight is that the light illuminating the display panel should be uniformly bright. Illuminance uniformity is particularly a problem when the light sources used are point sources, for example are light emitting diodes (LEDs). In such cases the backlight is required to spread the light across the display panel so that the displayed image has no dark areas. In addition, in some applications the display panel is illuminated with light from a number of different LEDs that produce light of different colors. It is important in these situations that the light from the different LEDs be mixed so that the color, as well as the brightness, are uniform across the displayed image.

SUMMARY OF THE INVENTION

One embodiment of the invention is directed to an optical system that has an image-forming panel having an illumination side and a backlight unit disposed to the illumination side of the image-forming panel. The backlight unit includes at least first and second light sources, the first light source producing light at a first wavelength and the second light source producing light at a second wavelength different from the first wavelength. The backlight unit also includes a reflecting cavity having at least one reflecting surface and a controlled transmission mirror. The light from the first and second light sources is reflected within the reflecting cavity. The controlled transmission mirror has an input coupling element, an output coupling element and a first multilayer reflector between the input and output coupling elements. The first multilayer reflector is reflective for normally incident light from the first and second light sources. The input coupling element redirects at least some of the light propagating from the first and second light sources in a direction substantially perpendicular to the first multilayer reflector into a direction that is transmitted through the first multilayer reflector.

Another embodiment of the invention is directed to an illumination light unit that has at least a first light source capable of generating illumination light at a first wavelength and a second light source capable of generating illumination light at a second wavelength different from the first wavelength. The illumination light unit also includes a reflecting cavity having one or more reflectors and a controlled transmission mirror disposed at an output of the reflecting cavity. The controlled transmission mirror includes an input coupling element, an output coupling element and a first multilayer reflector disposed between the input and output coupling elements. At least some of the illumination light from the first and second light sources is reflected within the reflecting cavity by the one or more reflectors and is transmitted out of the reflecting cavity through the controlled transmission mirror.

The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The following figures and detailed description more particularly exemplify these embodiments.

DETAILED DESCRIPTION

The present invention is applicable to illuminated signs and displays, such as liquid crystal displays (LCDs, or LC displays), and is applicable to displays that are illuminated using light sources positioned directly behind the display panel, known as direct-lit displays, and to displays that are illuminated using light sources positioned to the side of the display panel, known as edge-lit displays. The invention is believed to be particularly useful for displays that are illuminated by light sources of different colors. The invention is believed also to be applicable to systems that provide space lighting.

A schematic exploded view of an exemplary embodiment of a direct-lit display system100is presented inFIG. 1A. Such a display system100may be used, for example, in an LCD monitor or LCD-TV. In this exemplary embodiment, the device100uses a liquid crystal (LC) display panel102, which typically comprises a layer of LC104disposed between panel plates106. The plates106are often formed of glass, and may include electrode structures and alignment layers on their inner surfaces for controlling the orientation of the liquid crystals in the LC layer104. The electrode structures are commonly arranged so as to define LC panel pixels, areas of the LC layer where the orientation of the liquid crystals can be controlled independently of adjacent pixels. A color filter may also be included with one or more of the plates106for imposing color on the displayed image.

An upper absorbing polarizer108is positioned above the LC layer104and a lower absorbing polarizer110is positioned below the LC layer104. In the illustrated embodiment, the upper and lower absorbing polarizers108,110are located outside the display panel102. The absorbing polarizers108,110and the display panel102, in combination, control the transmission of light from a backlight112through the display panel102to the viewer. In some exemplary embodiments, when a pixel of the LC layer104is not activated, it does not change the polarization of light passing therethrough. Accordingly, light that passes through the lower absorbing polarizer110is absorbed by the upper absorbing polarizer108, when the absorbing polarizers108,110are aligned perpendicularly. When the pixel is activated, on the other hand, the polarization of the light passing therethrough is rotated, so that at least some of the light that is transmitted through the lower absorbing polarizer110is also transmitted through the upper absorbing polarizer108. Selective activation of the different pixels of the LC layer104, for example using a controller113, results in the light passing out of the display100at certain desired locations, thus forming an image seen by the viewer. The controller113may include, for example, a computer or a television controller that receives and displays television images. One or more optional layers109may be provided over the upper absorbing polarizer108, for example to provide mechanical and/or environmental protection to the display surface. In one exemplary embodiment, the layer109may include a hardcoat over the absorbing polarizer108.

Some types of LC displays may operate in a manner different from that described above and, therefore, differ in detail from the described system. For example, the absorbing polarizers may be aligned parallel and the LC panel may rotate the polarization of the light when in an unactivated state. Regardless, the basic structure of such displays remains similar to that described above.

The backlight112generates illumination light and directs the light towards the back of the LC panel102. The backlight112comprises a reflecting cavity114that contains a number of light sources116for generating the light, or that receives light from the light sources116. The light sources116may be, for example, light emitting diodes (LEDs), organic LEDs. (OLEDs), or may be other types of light sources.

The reflecting cavity114may include a base reflector118that reflects light propagating downwards from the light sources116in a direction away from the display panel102. The base reflector118may also be useful for recycling light within the display device100, as is explained below. The base reflector118may be a specular reflector or may be a diffuse reflector. One example of a specular reflector that may be used as the base reflector118is Vikuiti™ Enhanced Specular Reflection (ESR) film available from 3M Company, St. Paul, Minn. Examples of suitable diffuse reflectors include polymers, such as polyethylene terephthalate (PET), polycarbonate (PC), polypropylene, polystyrene and the like, loaded with diffusely reflective particles, such as titanium dioxide, barium sulphate, calcium carbonate and the like. Other examples of diffuse reflectors, including microporous materials and fibril-containing materials, are discussed in co-owned U.S. Patent Application Publication 2003/0118805 A1, incorporated herein by reference.

The reflecting cavity114also includes a controlled transmission mirror120disposed between the light sources116and the display panel102. The term cavity is used for an arrangement of the controlled transmission mirror and at least one reflecting surface, where at least some of the light is able to reflect back and forth between the controlled transmission mirror and the reflecting surface.

The controlled transmission mirror120reflects some of the light within the reflecting cavity114and also encourages light to spread laterally within the cavity114. The lateral light spreading aids in making the intensity profile of the light that exits the controlled transmission mirror120uniform, so that the viewer sees a more uniformly illuminated image. In addition, where different light sources116produce light of different colors, the lateral spreading of the light results in a more complete mixing of the different colors, and so the viewer sees an image of a more uniform color. The operation of the controlled transmission mirror120is discussed in more detail below.

The cavity114may also be provided with reflecting walls122. The reflecting walls122may be formed, for example, of the same specular or diffuse reflecting material as is used for the base reflector118, or of some other type of reflecting material.

An arrangement of light management layers124may also be positioned between the cavity114and the display panel102. The light management layers124affect the light propagating from the cavity114so as to improve the operation of the display device100. For example, the light management layers124may include a reflective polarizer126. This may be advantageous because the light sources116typically produce unpolarized light, whereas the lower absorbing polarizer110only transmits a single polarization state. Therefore, about half of the light generated by the light sources116is not suitable for transmission through to the LC layer104. The reflecting polarizer126, however, may be used to reflect the light that would otherwise be absorbed in the lower absorbing polarizer110, and so this light may be recycled by reflection between the reflecting polarizer126and the cavity114. The light reflected by the reflecting polarizing126may be subsequently reflected by the controlled transmission mirror120or the light may re-enter the cavity114and be reflected by the base reflector118. At least some of the light reflected by the reflecting polarizer126may be depolarized and subsequently returned to the reflecting polarizer126in a polarization state that is transmitted through the reflecting polarizer126and the lower absorbing polarizer110to the display panel102. In this manner, the reflecting polarizer126may be used to increase the fraction of light emitted by the light sources116that reaches the display panel102, and so the image produced by the display device100is brighter.

Any suitable type of reflective polarizer may be used, for example, multilayer optical film (MOF) reflective polarizers, diffusely reflective polarizing film (DRPF), such as continuous/disperse phase polarizers, wire grid reflective polarizers or cholesteric reflective polarizers.

Both the MOF and continuous/disperse phase reflective polarizers rely on the difference in refractive index between at least two materials, usually polymeric materials, to selectively reflect light of one polarization state while transmitting light in an orthogonal polarization state. Some examples of MOF reflective polarizers are described in co-owned U.S. Pat. No. 5,882,774, incorporated herein by reference. Commercially available examples of MOF reflective polarizers include Vikuiti™ DBEF-D200 and DBEF-D400 multilayer reflective polarizers that include diffusive surfaces, available from 3M Company, St. Paul, Minn.

Examples of DRPF useful in connection with the present invention include continuous/disperse phase reflective polarizers as described in co-owned U.S. Pat. No. 5,825,543, incorporated herein by reference, and diffusely reflecting multilayer polarizers as described in, e.g., U.S. Pat. No. 5,867,316, also incorporated herein by reference. Other suitable types of DRPF are described in U.S. Pat. No. 5,751,388.

Some examples of wire grid polarizers useful in connection with the present invention include those described in U.S. Pat. No. 6,122,103. Wire grid polarizers are commercially available from, inter alia, Moxtek Inc., Orem, Utah.

Some examples of cholesteric polarizers useful in connection with the present invention include those described in, e.g., U.S. Pat. No. 5,793,456, and U.S. Patent Publication No. 2002/0159019. Cholesteric polarizers are often provided along with a quarter wave retarding layer on the output side, so that the light transmitted through the cholesteric polarizer is converted to linear polarization.

A polarization mixing layer128may be placed between the cavity114and the reflecting polarizer126to aid in mixing the polarization of the light reflected by the reflecting polarizer126. For example, the polarization mixing layer128may be a birefringent layer such as a quarter-wave retarding layer.

The light management layers124may also include one or more prismatic brightness enhancing layers130a,130b.A prismatic brightness enhancing layer is one that includes a surface structure that redirects off-axis light into a propagation direction closer to the axis of the display. This controls the viewing angle of the illumination light passing through the LC panel102, typically increasing the amount of light propagating on-axis through the LC panel102. Consequently, the on-axis brightness of the image seen by the viewer is increased.

One example of a prismatic brightness enhancing layer has a number of prismatic ridges that redirect the illumination light, through a combination of refraction and reflection. Examples of prismatic brightness enhancing layers that may be used in the display device include the Vikuiti™ BEFII and BEFIII family of prismatic films available from 3M Company, St. Paul, Minn., including BEFII 90/24, BEFII 90/50, BEFIIIM 90/50, and BEFIIIT. It is possible that only one brightness enhancing layer is used, although it is also possible to use two brightness enhancing layers130a,130b,with their prismatic structures oriented at about 90° to each other. This crossed configuration provides control of the viewing angle of the illumination light in two dimensions, the horizontal and vertical viewing angles.

An exemplary embodiment of a display device150that includes an edge-lit display is schematically illustrated inFIG. 1B. In this embodiment, the backlight112includes a light guide152and one or more illumination light units154that generate the illumination light and direct the illumination light into the light guide152. The illumination light units154include a number of light sources116to generate the illumination light. The light sources116may be extended light sources that emit light over an extended length. One example of an extended light source is a cold cathode, fluorescent tube. The light sources116may also be effective point light sources, for example light emitting diodes (LEDs). Other types of light sources may also be used. This list of light sources is not intended to be limiting or exhaustive, but only exemplary.

The illumination light unit154may include a reflecting cavity that is used to collect and direct light from the light sources116to the lightguide152. The lightguide152guides illumination light from the light sources116to an area behind the display panel102, and directs the light to the display panel102. The light guide152may receive illumination light through a single edge, or through multiple edges. In other embodiments, not illustrated, the light may be coupled into the light guide152through a light coupling mechanism other than the edge of the light guide152. A base reflector156may be positioned on the other side of the light guide152from the display panel102. The light guide152may include light extraction features153that are used to extract the light from the lightguide152for illuminating the display panel102. For example, the light extraction features153may comprise diffusing spots on the surface of the light guide152that direct light either directly towards the display panel102or towards the base reflector156. Other approaches may be used to extract the light from the light guide152.

One exemplary embodiment of an illumination light unit199is now described with reference toFIG. 2A. The figure shows part of the light unit199, including some light sources116a,116b.A reflecting cavity118may be formed between at least one reflecting surface202and a controlled transmission mirror200that are arranged so that at least some of the illumination light produced by the sources116a,116bis reflected by both the controlled transmission mirror200and the reflecting surface202. In the illustrated embodiment, the reflecting surface202is positioned behind the light sources116a,116b.

The controlled transmission mirror200comprises a multilayer reflector204that has a reflection spectrum such that at least some of the light generated by the light sources116a,116b,when normally incident on the multilayer reflector204, is reflected.

An input coupling element206is disposed at the lower side of the multilayer reflector204, and an output coupling element208is disposed at the upper side of the multilayer reflector204. The input coupling element206and the output coupling element208are used to change the direction of at least some of the light entering the coupling elements206,208, so as to couple light through the controlled transmission mirror200. Exemplary embodiments of input coupling elements206and output coupling elements208include diffusers, both surface and bulk diffusers, and microreplicated surfaces. Examples of input coupling elements206and output coupling elements208are described in greater detail below. The output coupling element208may be the same type of coupling element as the input coupling element206, for example, the input and output coupling element206,208may both be bulk diffusers, or the output coupling element208may be different from the input coupling element206. The input and output coupling elements206,208may be laminated or otherwise formed integrally with the multilayer reflector204.

The multilayer reflector204is generally constructed of optical repeating units that form the basic building blocks of a dielectric stack. The optical repeating units typically include two or more layers of at least a high and a low refractive index material. A multilayer reflector can be designed, using these building blocks, to reflect infrared, visible or ultraviolet wavelengths and one or both of a given orthogonal pair of polarizations of light. In general, the stack can be constructed to reflect light of a particular wavelength, λ, by controlling the optical thickness of the layers according to the relationship:
λ=(2/M)*Dr,  (1)
where M is an integer representing the order of the reflected light, and Dris the optical thickness of an optical repeating unit. For the first order reflection (M=1), the optical repeating unit has an optical thickness of λ/2. Simple quarter-wave stacks comprise a number of layers that each have an optical thickness of one quarter of the wavelength, λ/4. Broadband reflectors can include multiple quarter-wave stacks tuned to various wavelengths, a stack with a continuous gradation of the layer thickness throughout the stack, or combinations thereof. A multilayer reflector may further include non-optical layers. For example, a coextruded polymeric dielectric reflector may include protective boundary layers and/or skin layers used to facilitate formation of the reflector film and to protect the reflector. Polymeric optical stacks particularly suited to the present invention are described in published PCT Patent Application WO 95/17303, entitled Multilayer Optical Film and U.S. Pat. No. 6,531,230, incorporated herein by reference. In other embodiments, the dielectric stack may be a stack of inorganic materials. Some suitable materials used for the low refractive index material include SiO2, MgF2, CaF2and the like. Some suitable materials used for the high refractive index material include TiO2, Ta2O5, ZnSe and the like. The invention is not limited to quarter-wave stacks, however, and is more generally applicable to any dielectric stack including, for example, computer optimized stacks and random layer thickness stacks.

Reflection by a dielectric stack of light at a particular wavelength is dependent, in part, on the propagation angle through the stack. The multilayer reflector204may be considered as having a reflection band profile (e.g., band center and bandedges) for light propagating in the stack at a particular angle. This band profile changes as the angle of propagation in the stack changes. The propagation angle in the stack is generally a function of the incident angle and the refractive indices of the materials in the stack and the surrounding medium. The wavelength of the bandedge of the reflection band profile changes as the propagation angle in the stack changes. Typically, for the polymeric materials under consideration, the bandedge of the reflector for light at normal incidence shifts to about 80% of its normal incidence value when viewed at grazing incidence in air. This effect is described in greater detail in U.S. Pat. No. 6,208,466, incorporated herein by reference. The bandedge shift may shift considerably further when the light is coupled into the reflector using a medium having a refractive index higher than air. Also, the shift in the bandedge is typically greater for p-polarization light than for s-polarization light.

The angular dependence of the reflection band profile, e.g., bandedge shifting with angle, results from a change in the effective layer thickness. The reflection band shifts towards shorter wavelengths as the angle increases from normal incidence. While the total path length through a given layer increases with angle, the change in band position with angle does not depend on the change in the total path length through a layer with angle, θ, where the angle is measured relative to an axis230perpendicular to the layers of the reflector204. Rather, the band position depends on the difference in path length between light rays reflected from the top and bottom surfaces of a given layer. This path difference decreases with angle of incidence as shown by the familiar formula n.d.cos θ=λ, the wavelength to which a given layer is tuned as a λ/4 thick layer. In the expression, n is the refractive index of the layer material and d is the thickness of the layer.

The above description describes how the bandedge of the reflection band profile changes as a function of angle. As used herein, the term bandedge generally refers to the region where the multilayer reflector changes from substantial reflection to substantial transmission. This region may be fairly sharp and described as a single wavelength. In other cases, the transition between reflection and transmission may be more gradual and may be described in terms of a center wavelength and bandwidth. In either case, however, a substantial difference between reflection and transmission exists on either side of the bandedge.

As light at the particular wavelength propagates in the stack at increasing propagation angles (measured from the axis230normal to the interface of the repeating units), the light approaches the bandedge. In one example, at high enough propagation angles, the stack will become substantially transparent to that particular wavelength of light and the light will transmit through the stack. Thus, for a given wavelength of light, the stack has an associated propagation angle below which the stack substantially reflects the light and another propagation angle, above which the stack substantially transmits the light. Accordingly, in certain multilayer stacks, each wavelength of light may be considered as having a corresponding angle below which substantial reflection occurs and a corresponding angle above which substantial transmission occurs. The sharper the bandedge, the closer these two angles are for the associated wavelength. For the purposes of the present description, the approximation is made that these two angles are the same and have a value of θmin.

The above description describes the manner in which monochromatic light in a given stack shifts from reflection to transmission with increasing angle of propagation. If the stack is illuminated with light having a mixture of components at different wavelengths, the angle, θmin, at which the reflective stack changes from being reflective to transmissive is likely to be different for the different wavelength components. Since the bandedge moves to shorter wavelengths with increasing angle, the value of θminis lower for light at longer wavelengths, potentially allowing more light at longer wavelengths to be transmitted through the multilayer reflector than at shorter wavelengths. In some embodiments it may be desired that the color of the light passing out of the controlled transmission mirror be relatively uniform. One approach to balancing the color is to use an input and output coupling element that couples more light at shorter wavelengths than at longer wavelengths into the controlled transmission mirror.

One example of such a coupling element is a bulk diffuser that contains scattering particles dispersed within a polymer matrix, as is discussed below with regards toFIGS. 3A and 4A. The scattering particles have a refractive index different from the surrounding matrix. The nature of diffusive scattering is that, all else being equal, light at shorter wavelengths is scattered more than light at longer wavelengths.

In addition, the degree of scattering is dependent on the difference between the refractive indices of the particles and the surrounding matrix. If the difference in refractive index is greater at shorter wavelengths, then even more short wavelength light is scattered. In one particular embodiment of a diffusive coupling element, the matrix is formed of biaxially stretched PEN, which has an in-plane refractive index of about 1.75 for red light and about 1.85 for blue light, where the light is s-polarized, i.e., has high dispersion. The in-plane refractive index is the refractive index for light whose electric vector is polarized parallel to the plane of the film. The out-of-plane refractive index, for light polarized parallel to the thickness direction of the film, is about 1.5 The refractive index for p-polarized light is lower than that of the s-polarized light, since the p-polarized light experiences an effective refractive index that is a combination of the in-plane refractive index and the out-of-plane refractive index.

The particles in the matrix may have a high refractive index, for example titanium dioxide particles have a refractive index of about 2.5. The refractive index of TiO2varies by approximately 0.25 over the range 450 nm-650 nm, which is greater than the approximately 0.1 variation for PEN over a similar wavelength range. Thus, the refractive index difference between the particles and the matrix changes by about 0.15 across the visible spectrum, resulting in increased scattering for the blue light. Consequently, the refractive index difference between the particles and the matrix can vary significantly over the visible spectrum.

Thus, due to the wavelength dependence of the diffusive scattering mechanism and the large difference in the refractive index difference over the visible spectrum, the degree to which blue light is scattered into the multilayer reflector is relatively high, which at least partially compensates for the larger value of θmin, at shorter wavelengths.

Other embodiments of input and output coupling elements, for example those described below with reference toFIGS. 3B-3Dand4B-4D, rely primarily on refractive effects for diverting the light. For example, a coupling element may be provided with a surface structure or holographic features for coupling the light into or out of the multilayer reflector. Normal material dispersion results in greater refractive effects for shorter wavelengths. Therefore, input and output coupling elements that rely on refractive effects may also at least partially compensate for the larger value of θmin, at shorter wavelengths.

Understanding, therefore, that the light entering the controlled transmission mirror may have a wide variation in the value of θmin, the following description refers to only a single value of θmin, for simplicity.

Another effect that the system designer can use to control the amount of light passing through the multilayer reflector is the selection of a Brewster's angle, the angle at which p-polarized light passes through the multilayer reflector without reflective loss. For adjacent isotropic layers1and2in the multilayer reflector, having refractive indices n1and n2respectively, the value of Brewster's angle in layer1, θB, for light passing from layer1to layer2, is given by the expression tan θB=n2/n1. Thus, the particular materials employed in the different layers of the multilayer reflector may be selected to provide a desired value of Brewster's angle.

The existence of the Brewster's angle for a multilayer reflector provides another mechanism for allowing light to pass through the reflector other than relying on the input and output coupling layers to divert the light through large angles. As the angle within the controlled transmission mirror is increased for p-polarized light, the reflection band substantially disappears at Brewster's angle. At angles above the Brewster's angle, the reflection band reappears and continues to shift to shorter wavelengths.

In certain embodiments, it may be possible to set the value of θBfor blue light to be less than θmin, but have θBbe greater than θminfor red light. This configuration may lead to an increased transmission for blue light through the multilayer reflector, which compensates at least in part for the higher value of θminfor shorter wavelength light.

At least some of the light from the light source116apropagates towards the controlled transmission mirror200. A portion of the light, exemplified by light ray210, passes through the input coupling element206and is incident on the multilayer reflector204at an angle greater than θminand is transmitted through the reflector204. Another portion of the light, exemplified by light ray212, is incident at the input coupling element206at an angle less than θmin, but is diverted by the input coupling element206to an angle of at least θmin, and is transmitted through the multilayer reflector204. Another portion of light from the light source116a,exemplified by light ray214, passes through the input coupling element206and is incident at the multilayer reflector204at an angle that is less than θmin. Consequently, light214is reflected by the multilayer reflector204to the reflecting surface202. The light214may be reflected at the reflecting surface202either specularly or diffusely.

In some embodiments it may be desired that the multilayer reflector204is attached to the output coupling element208in a manner that avoids a layer of air, or some other material of a relatively low refractive index, between the multilayer reflector204and output coupling element208. Such close optical coupling between the multilayer reflector204and the output coupling element208reduces the possibility of total internal reflection of light at the multilayer reflector204.

The maximum angle of the light within the controlled transmission mirror, θmax, is determined by the relative refractive indices of the input coupling element206, ni, and the refractive index of the particular layer of the multilayer reflector204, n1, n2, where the subscripts1,2refer to the alternating layers in the multilayer reflector204. Where the input coupling element206is a surface coupling element, the value of niis equal to the refractive index of the material on which the coupling surface is formed. Propagation from the input coupling element206into the multilayer reflector204is subject to Snell's law. The value of θmaxin each alternate layer of the multilayer reflector204is given by the expression:
θmax=sin−1(ni/n1,2).  (2)
where either n1or n2is used. Where ni>n1and ni>n2, then θmaxcan be up to 90°.

The output coupling element208is used to extract at least some of the light out of the illumination light unit199. For example, some of light212may be diffused by the output coupling element208so as to pass out of the controlled transmission mirror120as light220.

Other portions of the light, for example ray222, may not be diverted by the output coupling element208. If light222is incident at the upper surface of the output coupling element208at an angle greater than the critical angle of the output coupling element, θc=sin−1(1/n0), where no is the refractive index of the output coupling element208and the output coupling element208is interfaced with air, then the light222is totally internally reflected within the output coupling element208as light224. The reflected light224may subsequently be totally internally reflected at the lower surface of the input coupling element206. Alternatively, the light224may subsequently be diverted by the input coupling element206and pass out of the controlled transmission mirror200towards the reflecting surface202.

If the light that passes into the multilayer reflector204with an angle of at least θminis incident at the output coupling element208with an angle greater than θc, then that light which is not diverted out of the output coupling element208is typically totally internally reflected within the output coupling element208. If, however, the light that passes into the multilayer reflector204with an angle of at least θminreaches the output coupling element208at a propagation angle less than θcthen a fraction of that light may be transmitted out through the output coupling element208, even without being diverted by the output coupling element208, subject to Fresnel reflection loss at the interface between the output coupling element208and the air. Thus, there are many possibilities for the light to suffer multiple reflections and for its direction to be diverted within the reflecting cavity118. The light may also propagate transversely within the space between the controlled transmission mirror200and the reflecting surface202. These multiple effects combine to increase the likelihood that the light is spread laterally and extracted with to produce a backlight illuminance of more uniform brightness.

Except for the possibility that the multilayer reflector204has a value of Brewster's angle, θB, that is lower than θmin, there is a forbidden angular region for light originating at the light source116a.This forbidden angular region has a half-angle of θmin, and is located above the light source116a.Light cannot pass through the multilayer reflector204within the forbidden angular region. Light232from neighboring light sources116, for example light source116b,however, may be able to escape from the controlled transmission mirror200at a point perpendicularly above light source116a,at the axis230, and so the illumination light unit199is effective at mixing light from different light sources116a,116b.

In view of the description of the controlled transmission mirror200provided above, it can be seen that the function of the input coupling element206is to divert at least some light, that would otherwise be incident at the multilayer reflector204at an angle less than θmin, so as to be incident at the multilayer reflector204at an angle of at least θmin. Also, the function of the output coupling element208is to divert at least some light, that would otherwise be totally internally reflected within the controlled transmission mirror200, so as to pass out of the controlled transmission mirror200.

Another exemplary embodiment of a controlled transmission mirror200is schematically illustrated inFIG. 2B, in which a transparent layer250is disposed between the multilayer reflector204and the output coupling element208. The transparent layer250may be formed of any suitable transparent material, organic or inorganic, for example polymer or glass. Suitable polymer materials may be amorphous or semi-crystalline, and may include homopolymer, copolymer or blends thereof. Example polymer materials include, but are not limited to, amorphous polymers such as poly(carbonate) (PC); poly(styrene) (PS); acrylates, for example acrylic sheets as supplied under the ACRYLITE® brand by Cyro Industries, Rockaway, N.J.; acrylic copolymers such as isooctyl acrylate/acrylic acid; poly(methylmethacrylate) (PMMA); PMMA copolymers; cycloolefins; cylcoolefin copolymers; acrylonitrile butadiene styrene (ABS); styrene acrylonitrile copolymers (SAN); epoxies; poly(vinylcyclohexane); PMMA/poly(vinylfluoride) blends; atactic poly(propylene); poly(phenylene oxide) alloys; styrenic block copolymers; polyimide; polysulfone; poly(vinyl chloride); poly(dimethyl siloxane) (PDMS); polyurethanes; and semicrystalline polymers such as poly(ethylene); poly(propylene); poly(ethylene terephthalate) (PET); poly(carbonate)/aliphatic PET blends; poly(ethylene naphthalate)(PEN); polyamides; ionomers; vinyl acetate/polyethylene copolymers; cellulose acetate; cellulose acetate butyrate; fluoropolymers; poly(styrene)-poly(ethylene) copolymers; PET and PEN copolymers, and clear fiberglass panels. Some of these materials, for example PET, PEN and copolymers thereof, may be oriented so as to change the material refractive index from that of the isotropic material. The transparent layer250may be used to allow more lateral spreading of the light from the light sources116a,116bbefore extracting the light from the controlled transmission mirror200using the output coupling element208.

One or more of the edges of the transparent layer250may be covered by a reflector252. Thus, light254that might otherwise escape from the transparent layer250is reflected back into the transparent layer250and may be extracted from the illumination light unit199as useful illumination light. The reflector256may be any suitable type of reflector, including a multilayer dielectric reflector, a metal coating on the edge of the transparent layer250, a multilayer polymer reflector, a diffuse polymer reflector, or the like. In the illustrated embodiment, the reflector252at the side of the transparent layer250may be also used as a side reflector for the reflecting cavity114, although this is not intended to be a limitation of the invention.

In another exemplary embodiment, schematically illustrated inFIG. 2C, the transparent layer250is disposed between the input coupling element206and the multilayer reflector204.

In some other embodiments, the controlled transmission mirror200may be provided with two multilayer reflectors204,205positioned on either side of the transparent layer250, as is schematically illustrated inFIG. 2D. The multilayer reflectors204,205may have the same value of θmin, although this is not required.

The use of a transparent layer is described further in U.S. patent application Ser. No. 11/166,723, titled “OPTICAL ELEMENT FOR LATERAL LIGHT SPREADING IN BACK-LIT DISPLAYS AND SYSTEM USING SAME” filed on even date herewith, incorporated herein by reference.

Exemplary embodiments of different types of input coupling elements are now discussed with reference toFIGS. 3A-3D. In other exemplary embodiments, not illustrated, a transparent layer may be provided between the multilayer reflector and either of the input and output coupling elements.

InFIG. 3A, an exemplary embodiment of a controlled transmission mirror320comprises an input coupling element326, a multilayer reflector304and an output coupling element308. In this particular embodiment, the input coupling element326is a bulk diffusing layer, comprising diffusing particles326adispersed within a transparent matrix326b.At least some of the light entering the input coupling element326at an angle less than θmin, for example light rays328, is scattered within the input coupling element326at an angle greater than θmin, and is consequently transmitted through the multilayer reflector304. Some light, for example ray330, may not be scattered within the input coupling element326through a sufficient angle to pass through the multilayer reflector304, and is reflected by the multilayer reflector304. Suitable materials for the transparent matrix326binclude, but are not limited to, polymers such as those listed as being suitable for use in a transparent layer above.

The diffusing particles326amay be any type of particle useful for diffusing light, for example transparent particles whose refractive index is different from the surrounding polymer matrix, diffusely reflective particles, or voids or bubbles in the matrix326b.Examples of suitable transparent particles include solid or hollow inorganic particles, for example glass beads or glass shells, solid or hollow polymeric particles, for example solid polymeric spheres or polymeric hollow shells. Examples of suitable diffusely reflecting particles include particles of titanium dioxide (TiO2), calcium carbonate (CaCO3), barium sulphate (BaSO4), magnesium sulphate (MgSO4) and the like. In addition, voids in the matrix426bmay be used for diffusing the light. Such voids may be filled with a gas, for example air or carbon dioxide.

Another exemplary embodiment of a controlled transmission mirror340is schematically illustrated inFIG. 3B, in which the input coupling element346comprises a surface diffuser346a.The surface diffuser346amay be provided on the bottom layer of the multilayer reflector304or on a separate layer attached to the multilayer reflector304. The surface diffuser346amay be molded, impressed, cast or otherwise prepared.

At least some of the light incident at the input coupling element346, for example light rays348, is scattered by the surface diffuser346ato propagate an angle greater than θmin, and is consequently transmitted through the multilayer reflector304. Some light, for example ray350, may not be scattered by the surface diffuser346athrough a sufficient angle to pass through the multilayer reflector304, and is reflected.

Another exemplary embodiment of a controlled transmission mirror unit360is schematically illustrated inFIG. 3C, in which the input coupling element366comprises a microreplicated structure367having facets367aand367b.The structure367may be provided on the bottom layer of the multilayer reflector304or on a separate layer attached to the multilayer reflector304. The structure367is different from the surface diffuser346aofFIG. 3Bin that the surface diffuser346aincludes a mostly random surface structure, whereas the structure367includes more regular structures with defined facets367a,367b.

At least some of the light incident at the input coupling element366, for example rays368incident on facets367a,would not reach the multilayer reflector304at an angle of θminbut for refraction at the facet367a.Accordingly, light rays368are transmitted through the multilayer reflector304. Some light, for example ray370, may be refracted by facet367bto an angle less than θmin, and is, therefore, reflected by the multilayer reflector304.

Another exemplary embodiment of a controlled transmission mirror380is schematically illustrated inFIG. 3D, in which the input coupling element386has surface portions382in optical contact with the multilayer reflector304and other surface portions384that do not make optical contact with the multilayer reflector304, with a gap388being formed between the element386and the multilayer reflector304. The presence of the gap388provides for total internal reflection (TIR) of some of the incident light. This type of coupling element may be referred to as a TIR input coupling element.

At least some of the light incident at the input coupling element386, for example rays390incident on the non-contacting surface portions384would not reach the multilayer reflector304at an angle of θminbut for internal reflection at the surface384. Accordingly, light rays390may be transmitted through the multilayer reflector304. Some light, for example ray392, may be transmitted through the contacting surface portion382to the multilayer reflector304and is incident at the multilayer reflector302at an angle less than θmin. The light392is reflected by the multilayer reflector304.

Other types of TIR coupling elements are described in greater detail in U.S. Pat. No. 5,995,690, incorporated herein by reference.

Other types of input coupling elements may be used in addition to those described in detail here, for example input coupling elements that include a surface or a volume hologram. Also, an input coupling element may combine different approaches for diverting light. For example, an input coupling element may combine a surface treatment, such as a surface structure or surface scattering pattern or surface hologram, with bulk diffusing particles.

It may be desired in some embodiments for the refractive index of the input coupling element and output coupling element to have a relatively high refractive index, for example comparable to or higher than the effective refractive index (the average of the refractive indices of the high index and low index layers) of the multilayer reflector304. A higher refractive index for the input and output coupling elements helps to increase the angle at which light may propagate through the multilayer reflector304, which leads to a greater bandedge shift. This, in turn, may increase the amount of short wavelength light that passes through the controlled transmission mirror, thus making the color of the backlight illumination more uniform. Examples of suitable high refractive index polymer materials that may be used for input and output coupling elements include biaxially stretched PEN and PET which, depending on the amount of stretch, can have in-plane refractive index values of 1.75 and 1.65 respectively for a wavelength of 633 nm.

Commensurate with the choice of materials for the input and output coupling elements, the substrate should be chosen to have an index that does not cause TIR that would block prohibitive amounts of light entering or exiting at large angles. Conversely, a low index for the substrate would result in high angles of propagation in the substrate after injection from the input coupler having a higher index than the substrate. These two effects can be chosen to optimize the performance of the system with respect to color balance and lateral spreading of the light.

Similar approaches may be used for the output coupling element. For example, a controlled transmission mirror unit420is schematically illustrated inFIG. 4Ato have an input coupling element406, a multilayer reflector404and an output coupling element428. In this particular embodiment, the output coupling element428is a bulk diffusing layer, comprising diffusing particles428adispersed within a transparent matrix428b.Suitable materials for use as the diffusing particles428aand the matrix428bare discussed above with respect to the input coupling element326ofFIG. 3A.

At least some of the light entering the output coupling element428from the multilayer reflector404, for example light ray430, may be scattered by the diffusing particles428ain the output coupling element408and is consequently transmitted out of the light output coupling element428. Some light, for example ray432, may not be scattered within the output coupling element428and is incident at the top surface429of the output coupling element428at an incident angle of θ. If the value of θ is equal to or greater than the critical angle, θc, for the material of the matrix428b,then the light432is totally internally reflected at the surface429.

Another exemplary embodiment of controlled transmission reflector440is schematically illustrated inFIG. 4B, in which the output coupling element448comprises a surface diffuser448a.The surface diffuser448amay be provided on the upper surface of the multilayer reflector404or on a separate layer attached to the multilayer reflector404.

Some light propagating within the multilayer reflector404, for example light450, is incident at the surface diffuser448aand is scattered out of the light mixing layer440. Some other light, for example light452, may not be scattered by the surface diffuser448a.Depending on the angle of incidence at the surface diffuser448a,the light452may be totally internally reflected, as illustrated, or some light may be transmitted out of the controlled transmission mirror440while some is reflected back within the multilayer reflector404.

Another exemplary embodiment of controlled transmission mirror460is schematically illustrated inFIG. 4C, in which the output coupling element466comprises a microreplicated structure467having facets467aand467b.The structure467may be provided on a separate layer468attached to the multilayer reflector404, as illustrated, or be integral with the top surface of the multilayer reflector404itself. The structure467is different from the surface diffuser448ain that the surface diffuser includes a mostly random surface structure, whereas the structure467includes more regular structures with defined facets467a,467b.

Some light propagating within the multilayer reflector404, for example light470, is incident at the surface diffuser structure467and is refracted out of the controlled transmission mirror460. Some other light, for example light472, may not be refracted out of the controlled transmission mirror460by the structure467, but may be returned to the multilayer reflector404. The particular range of propagation angles for light to escape from the controlled transmission mirror460is dependent on a number of factors, including at least the refractive indices of the different layers that make up the controlled transmission mirror460and the layer468, as well as the shape of the structure467.

Another exemplary embodiment of a controlled transmission mirror480is schematically illustrated inFIG. 4D, in which the output coupling element486comprises a light coupling tape that has surface portions482in optical contact with the multilayer reflector404and other surface portions484that do not make optical contact with the multilayer reflector404, forming a gap488between the element486and the multilayer reflector404.

At least some of the light incident at the output coupling element486, for example light ray490, is incident at a portion of the multilayer reflector's surface that is not contacted to the output coupling element486, but is adjacent to a gap488, and so the light490is totally internally reflected. Some light, for example ray492, may be transmitted through the contacting surface portion482, and be totally internally reflected at the non-contacting surface portion484, and so is coupled out of the controlled transmission mirror480.

Other types of output coupling elements may be used in addition to those described in detail here. Also, an output coupling element may combine different approaches for diverting light out of the controlled transmission mirror. For example, an output coupling element may combine a surface treatment, such as a surface structure or surface scattering pattern, with bulk diffusing particles.

In some embodiments, the output coupling element may be constructed so that the degree to which light is extracted is uniform across the output coupling element. In other embodiments, the output coupling element may be constructed so that the degree to which light is extracted out of the controlled transmission mirror is not uniform across the output coupling element. For example, in the embodiment of output coupling element428illustrated inFIG. 4A, the density of diffusing particles428amay be varied across the output coupling element428so that a higher fraction of light can be extracted from some portions of the output coupling element428than others. In the illustrated embodiment, the density of diffusing particles428ais higher at the left side of the output coupling element428. Likewise, for the embodiments of controlled transmission mirror440,460,480illustrated inFIGS. 4B-4D, the output coupling elements448,468,488may be shaped or designed so that a higher fraction of light can be extracted from some portions of the output coupling elements448,468,488than from other portions. The provision of non-uniformity in the extraction of the light from the controlled transmission mirror, for example extracting a smaller fraction of light from portions of the controlled transmission mirror that contain more light and extracting a higher fraction of light from portions of the controlled transmission mirror that contain less light, may result in a more uniform brightness profile in the illumination light propagating towards the display panel.

The number of bounces made by light within the controlled transmission mirror, and therefore, the uniformity of the extracted light, may be affected by the reflectivity of both the input coupling element and the output coupling element. The trade-off for uniformity is brightness loss caused by absorption in the input coupling element, the multilayer reflector and the output coupling element. This absorption loss may be reduced by proper choice of materials and material processing conditions.

In some exemplary embodiments, the controlled transmission mirror may be polarization sensitive, so that light in one polarization state is preferentially extracted. A cross-section through one exemplary embodiment of a polarization sensitive controlled transmission mirror520is schematically illustrated inFIG. 5A. The controlled transmission mirror520comprises an optional transparent layer502, a multilayer reflector504, an input coupling element506and a polarization sensitive output coupling element528. A three-dimensional coordinate system is used here to clarify the following description. The axes of the coordinate system have been arbitrarily assigned so that the plane of the controlled transmission mirror520lies parallel to the x-y plane, with the z-axis having a direction through the thickness of the controlled transmission mirror520. The lateral dimension shown inFIG. 5Ais parallel to the x-axis, and the y-direction extends in a direction perpendicular to the drawing.

In some embodiments, the extraction of only one polarization of the light propagating within the controlled transmission mirror520is effected by the output coupling element528containing two materials, for example different polymer phases, at least one of which is birefringent. In the illustrated exemplary embodiment, the output coupling element528has scattering elements528a,formed of a first material, embedded within a continuous matrix528bformed of a second material. The refractive indices for the two materials are substantially matched for light in one polarization state and remain unmatched for light in an orthogonal polarization state. Either or both of the scattering elements528aand the matrix528bmay be birefringent.

If, for example, the refractive indices are substantially matched for light polarized in the x-z plane, and the refractive indices of the first and second materials are n1and n2respectively, the condition holds that n1x≈n1z≈n2x≈n2z, where the subscripts x and z denote the refractive indices for light polarized parallel to the x and z axes respectively. If n1y≠n2y, then light polarized parallel to the y-axis, for example light530, may be scattered within the output coupling element528and pass out of the controlled transmission mirror520. The orthogonally polarized light, for example light ray532, polarized in the x-z plane, remains substantially unscattered on passing within the output coupling element528because the refractive indices for this polarization state are matched. Consequently, if the light532is incident on the top surface529of the output coupling element528at an angle equal to, or greater than, the critical angle, θc, of the continuous phase528b,the light532is totally internally reflected at the surface529, as illustrated.

To ensure that the light extracted from the output coupling element528is well polarized, the matched refractive indices may be preferably matched to within at least ±0.05, and more preferably matched to within ±0.01. This reduces the amount of scatter for one polarization state. The amount by which the light in the y-polarization is scattered is dependent on a number of factors, including the magnitude of the index mismatch, the ratio of one material phase to the other and the domain size of the disperse phase. Preferred ranges for increasing the amount by which the y-polarized light is forward scattered within the output coupling element528include a refractive index difference of at least about 0.05, a particle size in the range of about 0.5 μm to about 20 μm and a particle loading of up to about 10% or more.

Different arrangements of a polarization-sensitive output coupling element are available. For example, in the embodiment of output coupling element548, schematically illustrated inFIG. 5B, the scattering elements548aconstitute a disperse phase of polymeric particles within a continuous matrix548b.Note that this figure shows a cross-sectional view of the output coupling element548in the x-y plane. The birefringent polymer material of the scattering elements548aand/or the matrix548bmay be oriented, for example, by stretching in one or more directions. Disperse phase/continuous phase polarizing elements are described in greater detail in co-owned U.S. Pat. Nos. 5,825,543 and 6,590,705, both of which are incorporated by reference.

Another embodiment of polarization-sensitive output coupling element558is schematically illustrated in cross-section inFIG. 5C. In this embodiment, the scattering elements558aare provided in the form of fibers, for example, polymer fibers or glass fibers, in a matrix558b.The fibers558amay be isotropic while the matrix558bis birefringent, or the fibers558amay be birefringent while the matrix558bis isotropic, or the fibers558aand the matrix558bmay both be birefringent. The scattering of light in the fiber-based, polarization sensitive output coupling element558is dependent, at least in part on the size and shape of the fibers558a,the volume fraction of the fibers558a,the thickness of the output coupling element558, and the degree of orientation, which affects the amount of birefringence. Different types of fibers may be provided as the scattering elements558a.One suitable type of fiber558ais a simple polymer fiber formed of one type of polymer material that may be isotropic or birefringent. Examples of this type of fiber558adisposed in a matrix558bare described in greater detail in U.S. patent application Ser. No. 11/068,159, incorporated herein by reference. Another example of polymer fiber that may be suitable for use in the output coupling element558is a composite polymer fiber, in which a number of scattering fibers formed of one polymer material are disposed in a filler of another polymer material, forming a so-called “islands-in-the-sea” structure. Either or both of the scattering fibers and the filler may be birefringent. The scattering fibers may be formed of a single polymer material or formed with two or more polymer materials, for example a disperse phase in a continuous phase. Composite fibers are described in greater detail in U.S. patent application Ser. Nos. 11/068,157 and 11/068,158, both of which are incorporated by reference.

It will be appreciated that the input coupling element may also be polarization sensitive. For example, where unpolarized light is incident on the controlled transmission mirror, a polarization-sensitive scattering input coupling element may be used to scatter light of one polarization state into the controlled transmission mirror, allowing the light in the orthogonal polarization state to be reflected by the multilayer reflector back to the base reflector. The polarization of the reflected light may then be mixed before returning to the controlled transmission mirror. Thus, the input coupling element may permit light in substantially only one polarization state to enter the controlled transmission mirror. If the different layers of the controlled transmission mirror maintain the polarization of the light, then substantially only one polarization of light may be extracted from the controlled transmission mirror, even if a non-polarization-sensitive output coupling element is used. Both the input and output coupling elements may be polarization sensitive. Any of the polarization sensitive layers used as an output coupling element may also be used as an input coupling element.

In other embodiments of illumination light unit, particularly suitable for quasi-point sources such as LEDs, the light sources may be located within the controlled transmission mirror itself. One exemplary embodiment of such an approach is schematically illustrated in cross-section inFIG. 6A. The controlled transmission mirror620has a transparent layer622, a multilayer reflector624, and an output coupling element628. The lower surface of the transparent layer622is provided with a diverting layer626. Side reflectors632may be provided around the edge of the controlled transmission mirror620. The side reflectors may be used to reflect any light that propagates out of the peripheral edge of the transparent layer622.

The diverting layer626may comprise a transmissive redirecting layer626athat redirects light, for example any of the layers discussed above for use as an input coupling element, including bulk or surface diffusers or a structured surface. The transmissive redirecting layer626amay be used with a base reflector618that reflects the light that has been transmitted through the transmissive redirecting layer626a.The base reflector618may be any suitable type of reflector. The base reflector618may be a specular or a diffuse reflector and may be formed e.g., from a metalized reflector or a MOF reflector. The base reflector618may be attached to the transmissive redirecting layer626a,as illustrated, or may be separate from the transmissive redirecting layer626a.The diverting layer626is not referred to as an input coupling element in this embodiment, however, because it is not used for coupling the light into the controlled transmission mirror620. Different configurations of the diverting layer626are possible. In some exemplary embodiments, for example as is schematically illustrated inFIG. 6B, the diverting layer626may simply comprise a diffuse reflector.

Light sources616, for example LEDs, although other types of light sources may also be used, are arranged so that a light emitting surface616aat least directly faces the transparent layer622, or may even be recessed within the transparent layer622. Thus, the light emitting surface616ais disposed between the diverting layer626and the multilayer reflector624. In this embodiment, light634from the light sources616enters the transparent layer622without being transmitted through the diverting layer626located at the lower surface of the transparent layer622. A refractive index-matching material, for example a gel, may be provided between the light emitting surface616aand the transparent layer622to reduce reflective losses and increase the amount of light coupled into the transparent layer622from the light source616.

The light sources616may be arranged on a carrier617. The carrier617may optionally provide electrical connections to the light sources616and may also optionally provide a thermal pathway for cooling the light sources616.

Even when the light sources616directly inject light into the transparent layer622without passing through an input coupling element, the multilayer reflector624still controls the minimum angle, θmin, at which light propagating within the transparent layer622may exit out of the controlled transmission mirror620. Some light, exemplified by light rays636and638, is emitted into the transparent layer622from the light source616at an angle less than θmin, and is, therefore, reflected by the multilayer reflector624. Some of the reflected light, for example ray636, may be diverted by the diverting layer626before or after incidence at the base reflector618and reflected back into the transparent layer622at an angle greater than θminas ray636a.Consequently, some of the light, e.g., ray636a,is diverted into an angular range that permits subsequent transmission through the multilayer reflector624after only one reflection from the multilayer reflector624. Another portion of the reflected light, for example light ray638, may not be diverted at the diverting layer626and is, therefore, reflected from the base reflector618at an angle that will result in another reflection at the multilayer reflector624.

Some of the light emitted from the light sources616, exemplified by light rays640and642, is emitted into the transparent layer622from the light source616aat an angle equal to or greater than θmin, and is, therefore, transmitted through the multilayer reflector624. Some of the transmitted light, for example ray640, may be diverted by the output coupling element628and transmitted out of the controlled transmission mirror620as light640a.Another portion of the transmitted light, for example ray642, may pass through the output coupling element628without being diverted and, if it is incident at the upper surface628aof the output coupling element628at an angle greater than the critical angle, θc, is totally internally reflected back towards the transparent layer622.

Some of the light644propagating within the transparent layer622may be reflected at the edge reflector632. The edge reflector632may be used to reduce the amount of light escaping from the edge of the transparent layer622, and thus reduces losses.

Another embodiment of a controlled transmission mirror650is schematically illustrated inFIG. 6C, in which the transparent layer652also operates as a diverting layer. In this embodiment, the transparent layer652contains some diffusing particles, so that some of the light passing therethrough is diverted. In one example, light beam654, which propagates from the light source616at an angle less than θminmay be diverted within the transparent layer652so as to be incident on the multilayer reflector624at an angle greater than θmin. In another example, light beam656, which is reflected by the multilayer reflector624, may be diverted within the transparent layer652so as to be reflected by the base reflector618at an angle greater than θmin.

In a direct-lit display, the illumination light unit may be configured as a single panel that is positioned behind the display panel. In another exemplary embodiment, schematically illustrated inFIG. 7, the backlight700may include a number of illumination light units702. In the illustrated embodiment the light units702are configured as bars and each include a number of light sources716a,716b,716cwhich may be located at staggered positions. The illumination light units702may have different shapes. In addition, the light sources716a,716b,716cmay produce light of different colors. For example, some light sources716amay produce red light, while other light sources716bproduce green light and other light sources716cproduce blue light. The differently colored light sources716a,716b,716cmay be arranged so as to increase the degree to which the light of different colors is mixed so as to produce mixed light of a desired color uniformity.

Another embodiment of an illumination light unit800is shown inFIG. 8A, in which a number of light sources806are located at the end810of a reflecting cavity802. In this exemplary embodiment, there is more than one light source806and the cross-sectional shape of the reflecting cavity802is rectangular. The light sources806may each generate light of the same color or of a different color. In the case where different light sources806generate light of different colors, the light from each light source806is mixed in the reflecting cavity802with the light from the other light sources806so that the light emerging from the controlled transmission mirror804may be a mixed color. For example, if there are three light sources806producing red, green and blue light respectively, the light emerging from the controlled emission mirror804may be a white color. The shade of the mixed color output light depends, inter alia, on the relative output powers of the different light sources and on the spectral properties of the controlled transmission mirror804.

The extraction of light through the controlled transmission mirror804may be graded along the length of the controlled transmission mirror804so that the brightness of the light extracted from the illumination light unit800is relatively uniform along its length.

An exemplary embodiment of a backlight820that uses the illumination unit800is schematically illustrated inFIG. 8B. The illumination unit800is at least partially surrounded by a reflector822and is positioned so that the light824emitted from the controlled transmission mirror804is directed towards a lightguide826. An optional brightness enhancing layer828, for example a prismatic brightness enhancing layer, may be positioned between the illumination unit800and the lightguide826. The brightness enhancing layer828reduces the angular spread of the light entering the lightguide826and may promote lateral spreading in the lightguide826. Some of the light, for example ray830, may be reflected by the brightness enhancing layer828. The reflected light830may be redirected back towards the lightguide826by the controlled transmission mirror804or some other reflector in the illumination unit800, or by the reflector822that surrounds the illumination light unit800.

Another embodiment of an illumination light unit900is shown inFIG. 9A, in which light sources906are located on a face908of a reflecting cavity902opposing a controlled transmission mirror904. In this exemplary embodiment, there is more than one light source906and the cross-sectional shape of the reflecting cavity902is rectangular. The light sources906may each generate light of the same color or of different colors. The reflecting cavity902may be used to mix the light from the different light sources906so that the intensity profile of the light output from the controlled transmission mirror904is relatively uniform. Furthermore, in the case where the light sources906produce light of different colors, the different colored light is mixed so that the light emerging from the controlled transmission mirror904is a mixed color. For example, if there are three light sources906producing red, green and blue light respectively, the light emerging from the controlled emission mirror904may be a white color. The light from the light sources906may be mixed within the reflecting cavity902so that the brightness of the light extracted from the illumination light unit may be relatively uniform.

Another embodiment of an illumination unit920is schematically illustrated inFIG. 9B, in which the controlled transmission mirror954is positioned on the top of the reflecting cavity902. Additional light sources906may be placed around the edge of the reflecting cavity902.

Another embodiment of an illumination light unit1000is schematically illustrated inFIGS. 10A and 10B. The unit1000has a reflecting cavity1002that includes a reflector1008and a controlled transmission mirror1004. One or more light sources1006are provided on a base1007. The base1007may be reflective. The base1007may also provide electrical connections for driving the light source1006and provide a heatsink for removing heat from the light source1006.

Light1020from the light sources1006is reflected by the reflector1008towards the controlled transmission mirror1004. The reflector1008may have any suitable shape and may be curved (as illustrated) or flat. If the reflector1008is curved, the curve may be any suitable type of curve, for example, elliptical or parabolic. In the illustrated embodiment, the reflector1008is curved in one dimension. The reflector1008may be any suitable type of reflector, for example a metalized reflector, or a multilayer dielectric reflector, which includes multiple layer polymer film (MOF) reflectors. Light that is transmitted through the controlled transmission mirror1004may be coupled into a light guide1012for back-illuminating a display device. The space1014within the reflecting cavity1002may be filled or may be empty. In embodiments where the space1014is filled, for example, with a transparent optical body, then the reflector1008may be attached to the outer surface of the body. In other embodiments, there is an empty space between the light source1006and the reflector1008. Different configurations of reflective cavities are described further in U.S. patent application Ser. No. 10/701,201 and, incorporated herein by reference.

The light sources1006, for example LEDs, may all produce light of the same color, or different LEDs may produce light of different colors, for example, red, green and blue. In some exemplary embodiments, an optional wavelength converter1022may be used to change the color of at least some of the light1020. For example, where the light1020is blue or ultraviolet, the wavelength converter1022may be used to convert some of the light to green and/or red light1024(dashed lines). A low-pass reflector1026may be positioned between the controlled transmission mirror1004and the wavelength converter1022. The low-pass reflector1024transmits the relatively short wavelength light1020from the light sources1006and reflects light1024afrom the wavelength converter1022towards the light guide1012.

In another embodiment, the controlled transmission mirror1004may use as an output coupling element a diffuser having a matrix loaded with phosphor particles. In such a configuration, some of the light transmitted through the multilayer reflector is converted by the phosphor to light of a different wavelength. Light that is not diffused or converted by the particles may be totally internally reflected by the matrix layer so as to pass back through the multilayer reflector.

Another embodiment of an illumination unit1100that may be used as a backlight for a display device is schematically illustrated inFIG. 11A. In this embodiment, one or more light sources1106are disposed between first and second reflectors1102,1104. In some embodiments, the light sources1106, which may be LEDs, may emit substantially away from the second reflector1102, in which case an optional curved reflector1108may be provided to direct the light1110along the space between the first and second reflectors1102,1104. In other embodiments, not illustrated, the light sources1106may substantially emit light sideways, in a direction along the space between the first and second reflectors1102,1104.

The first and second reflectors1102,1104may be specular reflectors, for example ESR film available from 3M Company, St. Paul, Minn. A folding reflector1112is positioned at each end to fold the light1110into a reflecting cavity formed between the second reflector1104and a controlled transmission mirror1114. The light1110is eventually directed out of the unit1100through the controlled transmission mirror1114. The first reflector1102may be mounted on a base1116that provides electrical power to the light sources1106and may also operate as a thermal sink to remove heat from the light sources1106.

The light sources1106may be arranged in different patterns on the first reflector1102. In the arrangement illustrated inFIG. 11B, which shows a slice through the unit1100between the first and second reflectors1102,1104, the light sources1106are arranged in a linear pattern, with the light being directed towards the edges1120a,1120b.In the arrangement schematically illustrated inFIG. 12C, the light sources1106and reflector1108are arranged in a radial pattern, so that the light is directed radially outwards to the folding reflector1112situated around the periphery of the first reflector1102.

An illumination light unit as described herein is not restricted to use for illuminating a liquid crystal display panel. The illumination light unit may also be used wherever discrete light sources are used to generate light and it is desirable to have uniform illumination out of a panel that includes one of more of the discrete light sources. Thus, the illumination light unit may find use in solid state space lighting applications and in signs, illuminated panels and the like.