Back-lit displays with high illumination uniformity

A directly illuminated display unit has a display panel and one or more light sources disposed behind the display panel. A diffuser is disposed between the one or more light sources and the display panel, and a light diverting layer is disposed between the one or more light sources and the diffuser. The light diverting layer has a first light-diverting surface facing the one or more light sources and a second light diverting surface facing the display panel. The light diverting layer diverts light passing from the one or more light sources to the diffuser, thus improving the uniformity of the light in the display unit.

RELATED APPLICATIONS

This application is related to the following U.S. Patent Applications, filed on even day herewith and which are incorporated by reference: “Back-Lit Displays with High Illumination Uniformity”, Ser. No. 11/560,234; “Back-Lit Displays with High Illumination Uniformity”, Ser. No. 11/560,271; “Back-Lit Displays with High Illumination Uniformity”, and “Back-Lit Displays with High Illumination Uniformity”, Ser. No. 11/560,250.

FIELD OF THE INVENTION

The invention relates to optical displays, and more particularly to liquid crystal displays (LCDs) that are directly illuminated by light sources from behind, such as may be used in LCD monitors and LCD televisions.

BACKGROUND

Some display systems, for example liquid crystal displays (LCDs), are illuminated from behind. Such displays find widespread application in many devices such as laptop computers, hand-held calculators, digital watches, televisions and the like. Some backlit displays include a light source that is located to the side of the display, with a light guide positioned to guide the light from the light source to the back of the display panel. Other backlit displays, for example some LCD monitors and LCD televisions (LCD-TVs), are directly illuminated from behind using a number of light sources positioned behind the display panel. This latter arrangement is increasingly common with larger displays because the light power requirements, needed to achieve a certain level of display brightness, increase with the square of the display size, whereas the available real estate for locating light sources along the side of the display only increases linearly with display size. In addition, some display applications, such as LCD-TVs, require that the display be bright enough to be viewed from a greater distance than other applications. In addition, the viewing angle requirements for LCD-TVs are generally different from those for LCD monitors and hand-held devices.

Many LCD monitors and LCD-TVs are illuminated from behind by a number of cold cathode fluorescent lamps (CCFLs). These light sources are linear and stretch across the full width of the display, with the result that the back of the display is illuminated by a series of bright stripes separated by darker regions. Such an illumination profile is not desirable, and so a diffuser plate is typically used to smooth the illumination profile at the back of the LCD device.

A diffuse reflector is used behind the lamps to direct light towards the viewer, with the lamps being positioned between the reflector and the diffuser. The separation between the diffuse reflector and the diffuser is limited by the desired brightness uniformity of the light emitted from the diffuser. If the separation is too small, then the illuminance becomes less uniform, thus spoiling the image viewed by the viewer. This comes about because there is insufficient space for the light to spread uniformly between the lamps.

SUMMARY OF THE INVENTION

One embodiment of the invention is directed to a directly illuminated display unit having a display panel and one or more light sources disposed behind the display panel. The one or more light sources are capable of producing illumination light. A diffuser is disposed between the one or more light sources and the display panel. A light diverting layer is disposed between the one or more light sources and the diffuser. The light diverting layer comprises a first light-diverting surface facing the one or more light sources. The first light diverting surface diverts light normally incident on the light diverting layer primarily in a first diverting plane orthogonal to the light diverting layer. The light diverting layer further comprises a second light diverting surface facing the diffuser. The second light diverting layer is configured so as to preferably divert light propagating within the light diverting layer in a direction perpendicular to the light diverting layer into a second diverting plane non-parallel to the first light diverting plane.

Another embodiment of the invention is directed to a directly illuminated display unit having a display panel and one or more light sources disposed behind the display panel, the one or more light sources being capable of producing illumination light. A diffuser is disposed between the one or more light sources and the display panel. A light diverting layer is disposed between the one or more light sources and the diffuser. The light diverting layer comprises a first light-diverting surface facing the one or more light sources, and a second light diverting surface facing the display panel, at least a first portion of the light from the one or more light sources that propagates within the light diverting layer in a direction substantially perpendicular to the light diverting layer substantially being transmitted through a flat portion of the second light diverting surface and at least a second portion of the light from the one or more light sources that propagates within the light diverting layer in a direction substantially perpendicular to the light diverting layer being totally internally reflected at a sloped portion of the second light diverting surface.

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

DETAILED DESCRIPTION

The present invention is applicable to display panels, such as liquid crystal displays (LCDs, or LC displays), and is particularly applicable to LCDs that are directly illuminated from behind, for example as are used in LCD monitors and LCD televisions (LCD-TVs). More specifically, the invention is directed to the management of light generated by a direct-lit backlight for illuminating an LC display. An arrangement of light management films is typically positioned between the backlight and the display panel itself. The arrangement of light management films, which may be laminated together or may be free standing, typically includes a diffuser layer and at least one brightness enhancement film having a prismatically structured surface.

A schematic exploded view of an exemplary embodiment of a direct-lit display device100is presented inFIG. 1. Such a display device100may be used, for example, in an LCD monitor or LCD-TV. The display device100may be based on the use of an LC 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 areas. A color filter may also be included with one or more of the plates106for imposing color on the image displayed.

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 polarizers are located outside the LC panel102. The absorbing polarizers108,110and the LC panel102in combination control the transmission of light from the backlight112through the display100to the viewer. For example, the absorbing polarizers108,110may be arranged with their transmission axes perpendicular. In an unactivated state, a pixel of the LC layer104may 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 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 by a controller114, results in the light passing out of the display at certain desired locations, thus forming an image seen by the viewer. The controller may 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.

It will be appreciated that some type of LC displays may operate in a manner different from that described above. 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 backlight112includes a number of light sources116that generate the light that illuminates the LC panel102. The light sources116used in a LCD-TV or LCD monitor are often linear, cold cathode, fluorescent tubes that extend along the height of the display device100. Other types of light sources may be used, however, such as filament or arc lamps, light emitting diodes (LEDs), flat fluorescent panels or external fluorescent lamps. This list of light sources is not intended to be limiting or exhaustive, but only exemplary.

The backlight112may also include a reflector118for reflecting light propagating downwards from the light sources116, in a direction away from the LC panel102. The reflector118may also be useful for recycling light within the display device100, as is explained below. The reflector118may be a specular reflector or may be a diffuse reflector. One example of a specular reflector that may be used as the reflector118is Vikuiti™ Enhanced Specular Reflection (ESR) film available from 3M Company, St. Paul, Minn. Examples of suitable diffuse reflectors include polymers, such as PET, PC, PP, PS loaded with diffusely reflective particles, such as titanium dioxide, barium sulphate, calcium carbonate or 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.

An arrangement120of light management films, which may also be referred to as a light management unit, is positioned between the backlight112and the LC panel102. The light management films affect the light propagating from backlight112so as to improve the operation of the display device100. For example, the arrangement120of light management films may include a diffuser plate122. The diffuser plate122is used to diffuse the light received from the light sources, which results in an increase in the uniformity of the illumination light incident on the LC panel102. Consequently, this results in an image perceived by the viewer that is more uniformly bright. In some embodiments the diffuser plate122may be formed as a layer that contains bulk diffusing particles. In some embodiments, the diffuser plate may be attached to another layer in the arrangement of light management films120or may be omitted.

The light management unit120may also include a reflective polarizer124. The light sources116typically produce unpolarized light but the lower absorbing polarizer110only transmits a single polarization state, and so about half of the light generated by the light sources116is not transmitted through to the LC layer104. The reflecting polarizer124, however, may be used to reflect the light that would otherwise be absorbed in the lower absorbing polarizer, and so this light may be recycled by reflection between the reflecting polarizer124and the reflector118. At least some of the light reflected by the reflecting polarizer124may be depolarized, and subsequently returned to the reflecting polarizer124in a polarization state that is transmitted through the reflecting polarizer124and the lower absorbing polarizer110to the LC layer104. In this manner, the reflecting polarizer124may be used to increase the fraction of light emitted by the light sources116that reaches the LC layer104, 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-D440 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. co-owned 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 polarizer useful in connection with the present invention include those described in, for example, 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.

In some embodiments, the reflective polarizer126may provide diffusion, for example with a diffusing surface facing the backlight112. In other embodiments, the reflective polarizer126may be provided with a brightness enhancing surface that increases the gain of the light that passes through the reflective polarizer126. For example, the upper surface of the reflective polarizer126may be provided with a prismatic brightness enhancing surface or with a gain diffusing surface. Brightness enhancing surfaces are discussed in greater detail below. In other embodiments, the reflective polarizer may be provided with a diffusing feature, such as a diffusing surface or volume, on the side facing the backlight112and with a brightness enhancing feature, such as a prismatic surface or gain diffusing surface, on the side facing the LC panel102.

A polarization control layer126may be provided in some exemplary embodiments, for example between the diffuser plate122and the reflective polarizer124. Examples of polarization control layer126include a quarter wave retarding layer and a polarization rotating layer, such as a liquid crystal polarization rotating layer. A polarization control layer126may be used to change the polarization of light that is reflected from the reflective polarizer124so that an increased fraction of the recycled light is transmitted through the reflective polarizer124.

The arrangement120of light management layers may also include one or more brightness enhancing layers. A brightness enhancing layer is one that includes a surface structure that redirects off-axis light in a direction closer to the axis132of the display. This increases the amount of light propagating on-axis through the LC layer104, thus increasing the brightness of the image seen by the viewer. One example is a prismatic brightness enhancing layer, which has a number of prismatic ridges that redirect the illumination light, through 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.

A prismatic brightness enhancing layer typically provides optical gain in one dimension. A second brightness enhancing layer128bmay also be included in the arrangement120of light management layers, in which a prismatic brightness enhancing layer is arranged with its prismatic structure oriented orthogonally to the prismatic structure of the first brightness enhancing layer128a. Such a configuration provides an increase in the optical gain of the display unit in two dimensions. In the illustrated embodiment, the brightness enhancing layers128a,128bare be positioned between the backlight112and the reflective polarizer124. In other embodiments, the brightness enhancing layers128aand128bmay be disposed between the reflective polarizer124and the LC panel102.

Another type of brightness enhancing layer128athat may be used to increase the on-axis brightness of the light passing through the display is a gain diffusing layer. One example of a gain diffuser layer is a layer that is provided with an arrangement of elements that act as lenses on its upper surface. At least some of the light that passes out of the gain diffuser layer128athat would otherwise propagate at a relative large angle to the axis132of the display is redirected by the elements on the layer surface to propagate in a direction more parallel to the axis132. More than one gain diffusing brightness enhancing layers128amay be used. For example two or three gain diffusing layers128a,128bmay be used. In addition, one or more gain diffusing layers128amay be used along with one or more prismatic brightness enhancing films128b. In such a case, the gain diffusing films128aand prismatic brightness enhancing layers128bmay be placed in any desired order within the arrangement of light management films120. One example of a gain diffuser layer that may be used in a display is a type BS-42 film available from Keiwa Inc., Osaka, Japan.

The different layers in the light management unit may be free standing. In other embodiments, two or more of the layers in the light management unit may be laminated together, for example as discussed in co-owned U.S. patent applications Ser. No. 10/966,610, incorporated herein by reference. In other exemplary embodiments, the light management unit may include two subassemblies separated by a gap, for example as described in co-owned U.S. patent application Ser. No. 10/965,937, incorporated herein by reference.

Conventionally, the spacing between the light sources116and the diffuser layer122, the spacing between adjacent light sources116and the diffuser transmission are significant factors considered in designing the display for a given value of brightness and uniformity of illumination. Generally, a strong diffuser, i.e. a diffuser that diffuses a higher fraction of the incident light, will improve the uniformity but will also result in reduced brightness, because the high diffusing level is accompanied by strong back diffusion and a concomitant increase in losses.

Under normal diffusion conditions, the variations in brightness seen across a screen are characterized by brightness maxima located above the light sources, and brightness minima located between the light sources. An enhanced uniformity film (EUF)130may be positioned between the light sources130and the diffuser layer122to reduce the nonuniformity in the illumination of the display panel102. Each face of the EUF130, namely the side facing towards the light sources116and the side facing towards the display panel102, may be a light-diverting surface. The light diverting surfaces are formed by a number of light diverting elements that refractively divert light passing from one side of the EUF130to another in a manner that reduces the illumination non-uniformity. The light diverting elements comprise a portion of the EUF surface that is non-parallel to the plane of the EUF130.

One particular exemplary embodiment of EUF200is schematically illustrated inFIG. 2. The EUF200comprises a first light diverting surface202that includes first light diverting elements204. In this particular embodiment, the light diverting elements204are formed as ribs across the surface of the EUF200and have a triangular cross-section. The second light diverting surface206also includes ribbed light diverting elements208that have a triangular cross-section. In this configuration of EUF200, the light diverting elements204and208are relatively oriented so that light210incident on the EUF200in a direction parallel to the z-axis from below is diverted in the x-z plane by the second light diverting surface206. On exiting the EUF200, light propagating within the EUF200parallel to the z-axis is diverted in the y-z plane by the first light diverting surface202. Thus, since light normally incident on the film200is diverted in a plane parallel to the x-z plane, the elements204may be said to form a light diverting plane that is parallel to the x-z direction. Likewise, since light propagating within the film parallel to the z-axis is diverted in the y-z plane, the elements208may be said to form a light diverting plane that is parallel to the y-z direction. In this configuration, the light diverting planes arising from the light diverting elements204and208are perpendicular to each other. In other configurations, the light diverting planes may be non-parallel without being perpendicular.

In some configurations, the light diverting elements of the upper or lower side may divert light in more than one direction. In such a case, the light diverting plane is taken to mean that plane which constitutes the direction where the diversion is greatest.

In some embodiments, the EUF may itself be formed of diffusive material, for example a polymer matrix containing bulk diffusing particles. The diffusing particles may extend throughout the EUF, or may be absent from parts of the EUF such as the light diverting elements. Where the EUF is diffusive, the arrangement of light management films need not include an additional diffuser layer between the EUF and the display panel, although an additional diffuser layer may be present.

The light diverting surfaces may include light diverting elements of different shapes and may also include various portions that lie parallel to the EUF. Some additional exemplary embodiments of EUF are schematically illustrated inFIGS. 3A and 3B. InFIG. 3A, the EUF300has a first light diverting surface302that includes light diverting elements304having a triangular cross-sectional shape. This figure also shows α, the apex angle of a light diverting element304. In this particular embodiment, there is a flat region306between adjacent light diverting elements304where the film surface is parallel to the plane of the EUF300. The width of the flat region306is shown as “w”. The lower light diverting surface308may have the same shape as the first light diverting surface302, or may have a different shape.

InFIG. 3B, the EUF320has a light diverting surface322that includes light diverting elements324having a truncated triangular cross-sectional shape, with a top flat portion326. In this particular embodiment, there is also a flat region328between adjacent light diverting elements324. The lower light diverting surface330may have the same shape as the first light diverting surface322, or may have a different shape.

Some other exemplary embodiments of EUF are schematically illustrated inFIGS. 4A-4C. InFIG. 4A, the EUF400has a first light diverting surface402that includes light diverting elements404having curved faces406. The second light diverting surface408may have light diverting elements having curved faces although this is not necessary. Likewise, the first light diverting surface need not have curved surfaces while the second light diverting surface does have curved surfaces. InFIG. 4B, the exemplary EUF420has a light diverting surface422with light diverting elements424that have curved faces426and flat portions428. In the illustrated embodiment, the flat portions428are parallel to the plane of the EUF film420. In some embodiments, the light diverting surface422may contain flat portions430between the light diverting elements424. In the illustrated embodiment, the flat portions430are parallel to the plane of the EUF420.

In the exemplary embodiments illustrated inFIGS. 4A and 4B, the curved surfaces of the light diverting elements404,424include a relatively abrupt change in surface gradient that may be considered similar to a mathematical discontinuity. For example, an abrupt change in gradient occurs at point408inFIG. 4A, at the apex of the light diverting member404, and at point432of the light diverting member424inFIG. 4B. These relatively abrupt changes in gradient prevent a single light diverting member from operating as a lens, since a lens requires smooth changes in the gradient across its surface. Thus, the light diverting members404,424do not produce a single focus for parallel light passing therethrough, either a real focus or a virtual focus. It will be appreciated that any of the light diverting surfaces discussed herein may be included on a single-sided EUF, in other words one that has a light diverting surface on only one side of the film, or a two-sided EUF, one that has light diverting surfaces on both sides.

In the exemplary embodiments illustrated inFIGS. 4A and 4B, the light diverting elements404,424may be viewed as protruding from the surface of the LUF400,420. In other embodiments, the light diverting elements may be formed as depressions in the surface of the LUF. One exemplary embodiment of such an LUF440is schematically illustrated inFIG. 4C. In this case, the light diverting surface442is formed with light diverting elements444having surfaces446. In some embodiments, flat areas448may be provided in the depression, and flat areas450may be provided between light diverting elements444. It is unimportant to the invention whether a light diverting surface contains light diverting elements that protrude out of the LUF or into the LUF and, in fact, the two configurations may in some circumstances be understood as being equivalent, with the portion452between two depressed light diverting elements being considered to be a light diverting element that protrudes out from the LUF.

The light diverting elements need not all be of the same height. For example, as is schematically illustrated inFIG. 4D, the light diverting elements464may be of different heights. Also, a single light diverting element may have a height that varies along its length. For example, the light diverting element470on the second light diverting surface468has a height, h, that varies depending on the position along the film460.

Another embodiment of a EUF whose light diverting elements vary in height is schematically illustrated inFIG. 5. The EUF500has a first light diverting surface502whose light diverting elements504are formed as prisms506having undulating ridges508The height of the ridges508varies along the prisms506and the width, w, also varies along the prisms506. This type of surface is described in greater detail in U.S. patent application Ser. No. 11/467,230, incorporated herein by reference. The second light diverting surface510may contain light diverting elements of any desired shape. For example, the second light diverting surface510may include light diverting elements formed as prisms having undulating ridges.

The light diverting elements need not be symmetrical relative to a normal to the EUF. One example of an EUF600having asymmetrical light diverting element602is schematically illustrated inFIG. 6A. In this particular embodiment, the light diverting elements602are formed as prisms having straight sides. At least some of the light diverting elements, for example light diverting elements602aand602bare asymmetrical relative to the axis604drawn normal to the EUF600. The lower light diverting surface606may or may not include asymmetrical light diverting elements.

Another embodiment of an EUF620having asymmetrical light diverting elements622is schematically illustrated inFIG. 6B. At least some of the light diverting elements622have curved sides and are asymmetric relative to the axis624that is normal to the EUF620, for example elements622aand622b.

FIG. 7Aschematically illustrates the use of an EUF with other light management layers704. In the illustrated embodiment, the light management layer704comprises a prismatic brightness enhancing layer. In other embodiments, different types of layer, or additional light management layers, such as a reflective polarizer layer, may be positioned above the diffuser layer702. The EUF710is positioned on the input side of the diffuser layer702. The EUF710has a first light diverting surface712facing the diffuser layer702and a second light diverting surface714facing away from the diffuser layer702. Light708from one or more light sources (not shown) passes through the EUF710to the diffuser layer702and on to the other light management layer or layers704.

In some embodiments, the first light-diverting surface712may be attached to the diffuser layer702, for example through the use of an adhesive. One exemplary embodiment of such an arrangement is schematically illustrated inFIG. 7B, in which parts of the first light diverting surface712penetrate into an adhesive layer722on the lower surface703of the diffuser layer702. In some embodiments, a gap724remains between the adhesive layer722and parts of the surface712. The attachment of structured film surfaces to other layers using adhesives is described in more detail in U.S. Pat. No. 6,846,089, incorporated by reference.

Another exemplary embodiment is schematically illustrated inFIG. 7C, in which the light-diverting surface712contains light diverting elements having portions730that are parallel to the lower surface702aof the diffuser layer702. The light diverting surface712surface may be pressed against the lower surface702aof the diffuser layer702, or may be adhered to the lower surface702a, for example using an adhesive.

Model Examples

An optical ray trace model of a display's illumination unit, having a backlight and a light management unit, was constructed to investigate the optical performance of the illumination unit as a function of various parameters of an EUF. The model illumination unit800, schematically illustrated inFIG. 8, comprised a reflective frame802that defines the edge limits of the light source array cavity804, a back reflector806below the array of lamps808, a diffuser layer810and an EUF812. Unless other wise indicated, the model assumed that the reflector806was a specular reflector. The model assumed that the lamps808each comprised a 38,000 nit elongated source, similar to a cold cathode fluorescent lamp. The lamps808were regularly spaced apart by a center-to-center distance S, the separation between the reflector806and the diffuser layer810was given by D and the separation distance between the lamps808and the reflector806was H. The spacing between lamps808, S, was assumed to be 30 mm, the diameter, 2R, of the lamps was assumed to be 3 mm and the value of D was assumed to be 7 mm. The diffuser layer810was 2 mm thick while the EUF812had a thickness of approximately 0.45 mm and was in contact with the lower surface of the diffuser layer810. There were three bulbs808in the cavity. A brightness enhancing layer814and reflective polarizer layer815were positioned above the diffuser layer810. The brightness enhancing layer814was formed of prismatic ribs oriented parallel to the elongation direction of the bulbs808.

The refractive index of the material used for the EUF was assumed to be 1.586, which corresponds to the value of the refractive index for an epoxy acrylate material, as might be used for the EUF. Other suitable types of materials for an EUF may be used. Example polymer materials include, but are not limited to, poly(carbonate) (PC); syndiotactic and isotactic poly(styrene) (PS); C1-C8 alkyl styrenes; alkyl, aromatic, and aliphatic ring-containing (meth)acrylates, including poly(methylmethacrylate) (PMMA) and PMMA copolymers; ethoxylated and propoxylated (meth)acrylates; multifunctional (meth)acrylates; acrylated epoxies; epoxies; and other ethylenically unsaturated materials; cyclic olefins and cyclic olefinic copolymers; acrylonitrile butadiene styrene (ABS); styrene acrylonitrile copolymers (SAN); epoxies; poly(vinylcyclohexane); PMMA/poly(vinylfluoride) blends; poly(phenylene oxide) alloys; styrenic block copolymers; polyimide; polysulfone; poly(vinyl chloride); poly(dimethyl siloxane) (PDMS); polyurethanes; unsaturated polyesters; poly(ethylene), including low birefringence polyethylene; poly(propylene) (PP); poly(alkane terephthalates), such as poly(ethylene terephthalate) (PET); poly(alkane napthalates), such as poly(ethylene naphthalate)(PEN); polyamide; ionomers; vinyl acetate/polyethylene copolymers; cellulose acetate; cellulose acetate butyrate; fluoropolymers; poly(styrene)-poly(ethylene) copolymers; PET and PEN copolymers, including polyolefinic PET and PEN; and poly(carbonate)/aliphatic PET blends. The term (meth)acrylate is defined as being either the corresponding methacrylate or acrylate compounds.

The uniformity of the light emitted from the diffuser layer810was modeled for various shapes of light diverting surfaces on the EUF. The surfaces of the EUF900were modeled as shown inFIGS. 9A and 9B. The upper light diverting surface902, facing the diffuser layer, had light diverting elements904with curved faces906having a radius of curvature R. The light diverting elements904were assumed to be arranged with a pitch, P. R was a dimensionless number normalized to the pitch, P. Thus, if the radius of curvature is 50 times the pitch, then R has a value of 50. The apex angle of the light diverting elements, E, was defined by the virtual triangle connecting the apex of the light diverting element904with the base corners of the light diverting element904. In some cases the light diverting elements were modeled with a flat tip. The extent, F, of the flat portion908was varied between zero and 0.2P. The apex angle of the light diverting element904having a flat portion was taken to be that which would otherwise have been the apex angle without the flat portion truncating the element904. The lower light diverting surface912is shown inFIG. 9B, with light diverting elements914protruding out from the EUF900.

In all the cases discussed below, the light diverting elements904,914were the same size for each light diverting surface902,912, and had a uniform height. The behavior of the illumination unit was modeled for various values of different parameters using a Monte Carlo method.

EXAMPLES

Modeled Illumination Unit with EUF Below Diffuser Layer

The optical characteristics of illumination units having several combinations of various EUF parameters were modeled. The different ranges of the various EUF parameters are listed in Table I. The term “apex” refers to the apex angle of the light diverting member, the term “flat” refers to the extent of the flat region, F, and the term “R” refers to the radius of curvature of the light diverting elements. The term “bottom” refers to the lower surface of the EUF facing the light sources and the term “top” refers to the upper surface of the EUF facing away from the light sources.

The term “diffuser-g” refers to the Henyey-Greenstein diffusion parameter: a value of g=1 results in all forward scattering and g=−1 is associated with completely backward scattering. A value of g=0 corresponds to uniform scattering in all directions. Values of g used in the modeling were in the range of 0.92-0.955, which corresponds approximately to a single pass transmission through the 2 mm thick diffuser layer in the range 56%-99%. The angular distribution of scattered rays, f(θ) is given by f=(1−g2)/[2(1+g2−2 g cos θ)1.5], where θ is the angle relative to the input direction of the light ray. For these values of g the scattering is highly biased in the forward direction. The Henyey-Greenstein u-factor, describing the inverse of the mean free path for light within the diffuser, was set at 14 mm−1. Thus, the scattering coefficient, C, is an exponential factor given by C=e−ud, where d is the position within the diffuser.

The lamp height, H, refers to the separation between the lamps and the reflector, as shown inFIG. 8. Optical losses were included in the model: the reflector was assumed to reflect 98.5% of the incident light, with the remaining 1.5% being absorbed, and the materials of the optical films were assumed to have an absorption length of 0.003 mm−1. The reflector was modeled as a specular reflector unless otherwise stated. The parameters A, T and R corresponds to the percentages of incident light that are absorbed in the diffuser, transmitted through the diffuser and reflected by the diffuser respectively.

The actual values of the various parameters used for Examples 1A-6A are presented in Table II. Examples 1A-6A are selected from the many different combinations considered, and are exemplary of good performance in terms of overall brightness and uniformity.

The illumination units of examples 1-6 included, in order from the light source, an EUF, a diffuser sheet, a prismatic brightness enhancing layer and a reflective polarizer layer.

Comparative Examples 7-12

Illumination Unit Without EUF

In order to compare the performance of an illumination unit having an EUF against the performance of conventional illumination units, several sets of comparative data were obtained for an illumination unit like that shown inFIG. 8except that the EUF812was omitted. The values of diffuser-g and lamp height used in these comparative examples are presented in Table III.

Examples 7-12 were analyzed under two different conditions, namely i) with the reflector806being a diffuse reflector and ii) with the reflector806being a specular reflector. The diffuse reflector was treated as Lambertian with a reflectance of 97%. These two different conditions are symbolized in the example name with a letter following the example number, the letter “D” representing an example that used the diffuse reflector and the letter “S” representing an example that used the specular reflector. Thus, for example, there are two sets of data for example 7. One set, labeled “7D” represents example 7 where a diffuse reflector was used and “7S” represents example 7 where a specular reflector was used. All other parameters values are the same for both the “S” and “D” examples.

Modeling Results

The model was used to calculate various operating parameters of an illumination unit, including the brightness of the light above the illumination unit and the uniformity in the brightness of the light propagating in a direction perpendicular to the films of the illumination unit.FIG. 10presents a scatter plot showing relative brightness uniformity (in %) plotted against brightness (in nits). The brightness uniformity for an example is presented as a percentage of the brightness for that example. Comparative examples 7S-12S exhibit a slightly higher average brightness than the EUF examples (1-6), with the higher transmission examples having a higher brightness. The brightest EUF example, Example 1 has a brightness approximately 99.5% the brightness of the brightest comparative example, Example 12S while the least bright EUF example, Example 6, still has a brightness of approximately 96% of the brightness of Example 12. Thus, the brightness of the EUF examples 1-6 is not significantly compromised, if at all, relative to the specular reflecting comparative examples 7S-12S. The diffusive comparative examples, 7D-12D, demonstrate a level of brightness that is lower than the EUF examples 1-6.

The brightness uniformity was calculated as a ratio of the standard deviation of the uniformity across the illumination unit divided by the average brightness of light produced by the illumination unit. The resulting values are, therefore, relative uniformity values. The uniformity of the EUF examples 1-6 is significantly better than either set of comparative examples, falling in the range of approximately 0.2%-1%. The uniformity of the specularly reflecting comparative examples 7S-12S is in the range of approximately 3.6%-4%, while the uniformity of the diffusely reflecting examples 7D-12D falls in the range of approximately 6.6%-7.1%. Thus the modeling shows that the presence of an EUF layer can make a significant improvement in the uniformity of the light emitted by the illumination unit, while substantially maintaining the same level of brightness.

A graph showing the brightness as a function of position, for light propagating normally from the illumination unit, is shown inFIG. 11. The graph shows the brightness for all EUF examples, Examples 1-6, and comparative Examples 9S and 9D. The level of brightness in Examples 9D and 9S varies considerably more than the brightness of Examples 1-6.

It will be appreciated that many different EUF parameters may be changed in order to achieve increased EUF performance. EUF performance may be measured by the ability of the EUF to suppress the light intensity peak above the light source, The parameters that may be varied include apex angle, the radius of curvature and the amount of flat space for both the upper and lower diverting surfaces, and also the refractive index of the EUF material.

FIG. 12shows a bar chart that illustrates the uniformity of light propagating out of the illumination unit at various angles for Examples 1-6 and Examples 7S-9S. For each example, the brightness uniformity is provided for light propagating at three angles, namely 0°, 15° and 30°. The results at 0° were also provided inFIG. 10. The results at 15° and 30° show a slight reduction in uniformity with increased angle, although most of the EUF examples still show a uniformity of less than 1% at 30°, with only two examples having a uniformity slightly above 1% at 30°. The brightness uniformity for examples 7S-9S remain in excess of 3.5% for 0°, 15° and 30°.

It is believed that the two-sided EUF functions in the following manner. First, it is useful to refer toFIG. 8, which shows the EUF812having light diverting members812athat are elongated in a direction parallel to the axes of the light sources816(into the plane of the figure). In some exemplary embodiments, this means that the light diverting members812aare ribbed members that are elongated in the same direction as the longitudinal axis of a fluorescent lamp. With S=30 mm, D=7 mm, and H=0.5 mm, then the angle of incidence on the lower light diverting surface of the EUF812is 0° directly above the light source816and 72° at the midpoint, P, between the bulbs.

The top light diverting member812bmay be regarded as being a graduated transmission filter whose transmission depends upon the inclination angle of incident light. The bottom light diverting members812aselect a transmission angle based upon the angle of incidence Θi=a tan((D−d)/x), where the d is the height of the center of the light source816above the reflector806, and x represents the separation between the point of incidence on the lower surface of the EUF and a point directly above the light source on the EUF. Thus, the transmission angle through the bottom light diverting members812ais a function of the distance x from the light source816bulb, which in turn determines the transmission level through the upper light diverting members812b. The EUF812provides a useful tool for controlling the transmission of light as a function of the distance from the light source812, thereby affecting brightness uniformity.

To understand the combined effects of a dual-sided EUF, it is useful to consider the properties of each side separately. The effect of the upper light-diverting surface that faces away from the source of light is considered first. Such a film is like a brightness enhancing film, where the transmission of light is low for normally incident light due to total internal reflection within the prisms of the brightness enhancing film, and is significantly higher for light incident at angles higher than that which permits total internal reflection.FIG. 13Ashows a polar/azimuthal map of transmitted light incident on the piano side of a plano/prism film like a prismatic brightness enhancing film. The prisms were assumed to have an apex angle 90° (F=0, R infinite). In this map, darker shading corresponds to more light being transmitted and white corresponds to zero light transmitted. The white hour-glass-shaped zone along and around the prism axis coincides with the total internal reflection (TIR) zone.

The effect of the light-diverting surface that faces the light sources is described with reference toFIGS. 13B-13D. These figures present polar/azimuth maps of light transmitted through the structured side of a prism/plano film, where the prisms have an apex angle 70°. These figures are better understood with reference toFIG. 13E, which shows a film1300with prisms1302facing a light source (not shown). The prisms1302comprise light diverting members. The prisms are elongated along a direction parallel to the y-direction and the z-axis is perpendicular to the film1300. InFIG. 13B, the plane of incidence of light is 0°, meaning that the light rays are confined to a plane normal to the prism structure and coplanar with the prism axis, i.e. are confined to the y-z plane inFIG. 13E. InFIG. 13Cthe plane of incidence is 70°, so light rays are confined to a plane tilted at700to the normal to the film and coplanar with the prism axis, plane1304. InFIG. 13D, the light rays are confined to a plane tilted800to the normal and coplanar with the prism axis.

A composite polar/azimuth map of the top prism transmission and the three bottom prism incidence plane cases, namely 0°, 70°, and 80° is shown inFIG. 13F. The patterns correspond to the light diverting structures on the two sides of the EUF being oriented perpendicular to each other. In the 0° case, light is diverted away from the center meridian where TIR dominates the interaction with the top prism, hence light emitted directly upward from the bulb is mostly reflected by the EUF. In the 70° and 80° incidence cases, light is diverted toward the center meridian where the top prism TIR zone is narrow (weak reflection), hence light emitted from the lamp at steep angles and incident on the bottom prism structured surface near the midpoint between the bulbs is strongly transmitted. The combination of low transmission for light above the lamp and high transmission for light incident at the EUF at a point between the lamps results in a flattening of the overall illuminance profile, and so the illuminance can become more uniform. InFIG. 13F, there is a graduated scale of transmission for rays incident at angles between 0° and 70°, which strongly selects for light transmission near the midpoint between the bulbs. The graduated transmission scale can be tuned by adjusting the parameters of the lower and upper light diverting members, for example apex angle, facet curvature (radius), flattened or tip radius or prism tip wetout, prism axes orientation, refractive index, canted or variable apex angle prisms, and prism surface texture. The orientation of the top and bottom prism axes, i.e. the direction of the prism ridges, may be varied relative to each other and/or relative to the bulbs.

FIG. 14shows the calculated brightness uniformity as a function of the value D, which affects the thickness of the illumination unit. In each case the value of D was varied over the range 5 mm-11 mm. Curves1401,1402,1403and1405respectively show the results for design like those of Examples 1, 2, 3 and 5. Curve1407S shows the results for a design like that of Example 7S while curve1407D shows the results for Example 7D. The values of the uniformity for D=7 mm are the same as those provided inFIG. 10.

An important parameter for optimizing the performance of an illumination unit is the ratio S/D, the ratio of the inter-lamp spacing to the thickness. In order to reduce the thickness of displays, it is desired that the value of S/D be higher, however, the brightness uniformity should not be compromised. Typically, conventional displays that use fluorescent lamps with a diffuse back reflector806use an S/D value that is less than 2. This is confirmed by the trend of curve1407D inFIG. 14: the brightness uniformity approaches 1% for values of D that are significantly higher than 11 mm. Recent developments using a specular back reflector806have demonstrated that S/D values up to about 3 are possible, while maintaining a brightness uniformity of approximately 1%, thus permitting illumination units to be reduced in thickness. This is described in greater detail in pending U.S. application Ser. No. 11/33,504, incorporated herein by reference.

The trends illustrated byFIG. 14shows that the introduction of the EUF results in the achievement of acceptable brightness with substantially thinner illumination units. In all the designs used for the results shown inFIG. 14, the inter-lamp spacing, S, was 30 mm. Thus, a uniformity of 1% or better was achieved with all EUF designs (curves1401,1402,1403,1405) for values of S/D of about 2.7 or more. In particular, curves1401,1402,1403,1405show a brightness uniformity of less than 1% for values of D down to less than 7 mm, i.e. an S/D ratio of more than 4.3. The exploration of different EUF designs may lead to a system having a uniformity of less than 1% where the value of D is less than 6 mm.

Experimental Results

Experimental measurements of the light produced by a light box both with and without an EUF are shown inFIG. 15. The light box was approximately 33 cm wide and contained8fluorescent lamps on a 35 mm center-to-center spacing. The light box had a depth of 8 mm, and so the S/D ratio had a value of approximately 4.4. In all cases a diffuser layer was present above the lamps. Curve1502shows the measured luminance (in Cd m−2) as function of position across the diffuser layer. This curve shows a significant spatial variation in intensity. Curve1504, which represents a similar measurement that was performed in the presence of a EUF below the diffuser layer, shows that the EUF has a significant effect in making the output intensity more uniform. The EUF used to produce these experimental results was a two-sided EUF, with each surface containing a series of prismatic ribs with no flat areas between the ribs, and no flat areas on the ribs themselves. On the lower light diverting surface, facing the lamps, the prism apex angle was 66° and on the upper light diverting surface, facing the diffuser layer, the prism apex angle was 100°.

The measurements were repeated, but with three light management films disposed above the diffuser. The films were, in order from the diffuser layer upwards, a gain diffuser, a brightness enhancement film and a reflective polarizer film. The gain diffuser was a type BS-42 gain diffuser sheet available from Keiwa Inc, Osaka, Japan. The brightness enhancing film was BEFIII-10T, a prismatic brightness enhancing film available from 3M Company, St. Paul, Minn., and the reflective polarizer layer was DBEF-D400, a multilayer reflective polarizer film also available from 3M Company. The addition of the light management films significantly increased the mount of passing upwards from the light box. The graph shows the luminance as a function of position across the light box both without the EUF (curve1506) and with the EUF (curve1508). The illuminance at the center is generally higher than at the edges, due to boundary conditions. However, the profile measured with the EUF in place is significantly smoother than when the EUF is absent.

It should be understood that light-diverting surfaces may take on many different types of shapes that are not discussed here in detail, including surfaces with light-diverting elements that are random in position, shape, and/or size. In addition, while the exemplary embodiments discussed above are directed to light-diverting surfaces that refractively divert the illumination light, other embodiments may diffract the illumination light, or may divert the illumination light through a combination of refraction and diffraction. The computational results described here show that different types and shapes of light-deviating layer provide the potential to increase illuminance, and reduce the variation in the illuminance, compared with a simple diffuser alone.