While LCD displays offer a compact, lightweight alternative to CRT monitors, there are many applications for which LCD displays are not satisfactory due to a low level of brightness, or more properly, luminance. The transmissive LCD used in conventional laptop computer displays is a type of backlit display, having a light-providing surface positioned behind the LCD for directing light outwards, towards the LCD. The light-providing surface itself provides illumination that is essentially Lambertian, that is, having an essentially constant luminance over a broad range of angles. With the goal of increasing on-axis and near-axis luminance, a number of brightness enhancement films have been proposed for redirecting a portion of this light having Lambertian distribution toward normal, relative to the display surface. Among proposed solutions for brightness or luminance enhancement for use with LCD displays and with other types of backlit display types are the following:
U.S. Pat. No. 5,592,332 (Nishio et al.) discloses the use of two crossed lenticular lens surfaces for adjusting the angular range of light in an LCD display apparatus;
U.S. Pat. No. 5,611,611 (Ogino et al.) discloses a rear projection display using a combination of Fresnel and lenticular lens sheets for obtaining the desired light divergence and luminance;
U.S. Pat. No. 6,111,696 (Allen et al.) discloses a brightness enhancement film for a display or lighting fixture. With the optical film disclosed in the '696 patent, the surface facing the illumination source is smooth; the opposite surface has a series of structures, such as triangular prisms, for redirecting the illumination angle. The film disclosed in the '696 patent refracts off-axis light to provide a degree of correction for directing light at narrower angles. However, this film design works best for redirecting off-axis light; incident light that is normal to the film surface may be reflected back toward the source, rather than transmitted;
U.S. Pat. No. 5,629,784 (Abileah et al.) discloses various embodiments in which a prism sheet is employed for enhancing brightness, contrast ratio, and color uniformity of an LCD display of the reflective type. In an embodiment disclosed in the '784 patent, the brightness enhancement film similar to that of the Allen et al. '696 patent is arranged with its structured surface facing the source of reflected light for providing improved luminance as well as reduced ambient light effects. Because this component is used with a reflective imaging device, the prism sheet of the '784 disclosure is placed between the viewer and the LCD surface, rather than in the position used for transmissive LCD systems (that is, between the light source and the LCD);
U.S. Patent Application Publication No. 2001/0053075 (Parker et al.) discloses various types of surface structures used in light redirection films for LCD displays, including prisms and other structures;
U.S. Pat. No. 5,887,964 (Higuchi et al.) discloses a transparent prism sheet having extended prism structures along each surface for improved back-light propagation and luminance in an LCD display. As is noted with respect to the Allen et al. '696 patent mentioned above, much of the on-axis light is reflected rather than transmitted with this arrangement. Relative to the light source, the orientation of the prism sheet in the Higuchi et al. '964 disclosure is reversed from that used in the '696 disclosure. The arrangement shown in the '964 disclosure is usable only for small, hand-held displays and does not use a Lambertian light source;
U.S. Pat. No. 6,356,391 (Gardiner et al.) discloses a pair of optical turning films for redirecting light in an LCD display, using an array of prisms, where the prisms can have different dimensions;
U.S. Pat. No. 6,280,063 (Fong et al.) discloses a brightness enhancement film with prism structures on one side of the film having blunted or rounded peaks;
U.S. Pat. No. 6,277,471 (Tang) discloses a brightness enhancement film having a plurality of generally triangular prism structures having curved facets;
U.S. Pat. No. 5,917,664 (O'Neill et al.) discloses a brightness enhancement film having “soft” cutoff angles in comparison with conventional film types, thereby mitigating the luminance change as viewing angle increases;
U.S. Pat. No. 5,839,823 (Hou et al.) discloses an illumination system with light recycling for a non-Lambertian source, using an array of microprisms; and,
U.S. Pat. No. 5,396,350 (Beeson et al.) discloses a backlight apparatus with light recycling features, employing an array of microprisms in contact with a light source for light redirection in illumination apparatus where heat may be a problem and where a relatively non-uniform light output is acceptable.
While conventional approaches, such as those noted in the disclosures mentioned hereinabove, provide some measure of brightness enhancement at low viewing angles, these approaches have some shortcomings. Some of the solutions noted above are more effective for redistributing light over a preferred range of angles rather than for redirecting light toward normal for best on-axis viewing. Conventional brightness enhancement film solutions have a directional bias, working best for redirecting light in one direction. For example, a brightness enhancement film may redirect the light path in a width direction relative to the display surface, but have little or no effect on light in the orthogonal length direction. As a result, multiple orthogonally crossed sheets must be overlaid in order to redirect light in different directions, typically used for redirecting light in both horizontal and vertical directions with respect to the display surface. Necessarily, this type of approach is somewhat a compromise; such an approach is not optimal for light in directions diagonal to the two orthogonal axes.
As disclosed in the patent literature listed above, brightness enhancement articles have been proposed with various types of refractive surface structures formed on the top surface of a substrate material, including arrangements employing a plurality of protruding prism shapes, both as matrices of separate prism structures and as elongated prism structures, with the apex of prisms both facing toward and facing away from the light source. For the most part, these solutions still exhibit directional bias, requiring the use of multiple sheets in practical applications.
One problem with existing backlight systems for portable equipment relates to the need to redirect light from a compact source, such as a CCFL bulb, uniformly over a two-dimensional surface, wherein the light source is positioned at one edge of the two-dimensional surface. Conventional light guide panels perform this function using printed dot patterns or etched surfaces in combination with a diffusion film to diffuse light from the edge source and provide uniform backlight illumination. Directional turning films, such as that provided with the HSOT (Highly Scattering Optical Transmission) light guide panel available from Clarex, Inc., provide an improved solution for providing a uniform backlight of this type, without the need for diffusion films or for dot printing in manufacture. HSOT light guide panels and other types of directional turning films use arrays of prism structures, in various combinations, to redirect light from a light guiding plate toward normal, relative to the two-dimensional surface.
Referring to FIG. 1, the overall function of a light guiding plate 10 is shown. Light from a light source 12 is incident at an input surface 18 and passes into light guiding plate 10, which is typically wedge-shaped as shown. The light propagates within light guiding plate 10 until Total Internal Reflection (TIR) conditions are frustrated and then, possibly reflected from a reflective surface 42, exits light guiding plate at an output surface 16. This light then goes to a turning film 22 and is directed to illuminate a light-gating device 20 such as an LCD or other two-dimensional backlit component.
For distributing the light along a two-dimensional surface, light guiding plate 10 and its support components are typically designed to provide both redirection of the light and some amount of collimation that reduces divergence of the beam angle. For example, U.S. Pat. No. 5,854,872 entitled “Divergent Angle Rotator System and Method for Collimating Light Beams” to Tai discloses a light guiding plate that uses an array of elongated microprisms to redirect and collimate light from one or more light sources. In the device disclosed in the Tai '872 patent, the light guiding plate has a first set of prism structures on the light output side elongated in one direction to provide collimation and a second set of prism structures on the opposing side elongated in the orthogonal direction and providing collimation and TIR reflection. Referring to FIG. 2, the arrangement of crossed prism structures is shown for a light guiding plate 10 of this type. Turning film 22 on output surface 16 provides redirection of light toward the normal viewing axis V. The light output from light guiding plate 10 can have a relatively high off-axis angle. Bottom prisms 24 provide collimation in the orthogonal direction. The orthogonal relationship of prism structures on top and bottom surfaces provides a controllable degree of collimation of the output light.
There are a number of variations applied to the basic arrangement represented in FIG. 2. For example, U.S. Pat. No. 6,576,887 entitled “Light Guide for use with Backlit Display” to Whitney et al. discloses a light guide optimized for uniformity, in which structures on the output surface of a turning film 22 may be randomly distributed to provide a more uniform output. U.S. Pat. No. 6,707,611 entitled “Optical Film with Variable Angle Prisms” to Gardiner et al. discloses adaptation of an optical turning film with an arrangement that reduces perceived ripple.
Referring to FIGS. 3A and 3B, there is shown, from two different, approximately orthogonal perspectives, the overall function of a prism 60 for redirecting light, such as would be performed by a conventional turning film. Light entering prism 60 at an incident point P1 is refracted and reflected from a point P2 on a bottom facet 62 by Total Internal Reflection (TIR), then escapes prism 60 at a point P3. It is significant to note that prism 60 provides this turning function effectively in one direction, the Y-direction in the coordinate system shown in FIGS. 3A and 3B. In an orthogonal direction, the X-direction of FIGS. 3A and 3B, prism 60 is ineffective for light collection, resulting in a large divergence along the X-direction. For this reason, as FIG. 2 and as both Tai '872 and Whitney et al. '887 disclosures cited above show, two separate orthogonally disposed sets of prisms are employed in conventional solutions for light redirection. One set of prisms provides collimation of the light, the other set provides beam redirection for a direction. While this arrangement is workable, it can be appreciated that there would be cost and size advantages to an illumination solution that provides both collimation and beam redirection in a single optical component, particularly where such a solution provides improved on-axis luminance. With increased demands for more compact packaging of electronic display apparatus and for improved brightness, and with little promise of dramatic improvement to existing light-scattering approaches for backlight delivery, there is a compelling need to seek out unconventional solutions for light redirection along a two-dimensional surface, particularly well suited to backlighting applications.