Display backlighting apparatus

Abstract: Light emitting devices are combined with non-imaging optical elements to create a backlighting system for liquid-crystal displays (LCDs). A pair of optically isolated sub-systems, a night vision imaging system (NVIS) (14) and a daylight system (12) are combined to provide optimum display legibility in ambient illuminations ranging from bright sunlight to total darkness. The sub-systems utilize non-imaging optics based upon total internal reflection (TIR) and light reflection, both coupled to a light emitting source, such as fluorescent tubes (34) and other linearly shaped light sources. Light reflectors (32) are shaped to concentrate light onto non-imaging optics lens (30) so as to compensate for non-linear characteristics of the TIR lenses.

Description: The present invention relates generally to displays and, more particularly, to means for optimizing display legibility in ambient illuminations ranging in brightness, for example from bright sunlight to total darkness, including use with night vision imaging systems (NVIS). Prior art backlight systems for liquid crystal devices generally do not efficiently collect light. One conventional system utilizes liquid crystal displays (LCDs) in conjunction with a white painted chamber within which light is generated. In this system, a wide angle of uniform light is provided. However, because LCDs have limited viewing angles and when they are placed within the wide angle of uniform light, a substantial amount of backlighting is ineffectual. The result is white light without an image, or interference with the image, with low contrast at the ends of the viewing area. In part to compensate for both loss of light and low contrast, a large number of fluorescent tubes or equivalent light sources are employed. In some prior art systems, a planar mirror may be utilized. However, were a total internal reflecting (TIR) lens to be used, because TIR lenses are non-linear, the TIR lens would tend to direct a greater portion of the light to its centre than to its edges. Therefore, its centre would be more brightly illuminated than its edges. This non-uniform illumination would deleteriously affects the displayed images. Some conventional backlight systems for. liquid crystal devices, such as described in United States Patent 5,211,463, perform satisfactorily but consume considerable power, because the optical lighting system is generally inefficient. European Patent Application number 0540140 discloses a backlighting system having fluorescent tubes for daylight operation and secondary light sources for low light level operation. A reflector block is located behind the fluorescent light tubes. The secondary light sources are located either in the reflector block, or else holes are provided in the reflector block to enable light from the secondary light sources to pass through the reflector block. European Patent Application number 0560614 discloses a backlighting system comprising a plurality of lamps for day use and a night lamp for night use. A wedge-shaped block is provided for the transmission of light from both the day lamps and the night lamps. According to the present invention there is provided a display backlight apparatus for illuminating an object under different ambient light conditions comprising: a daylight illumination section positioned on one side of the object for illuminating the object under daylight ambient light conditions; and a night light illumination section, positioned on the same side of the object, for illuminating the object under low light ambient light conditions, characterized by optical barrier means optically separating said daylight illumination section and said night light illumination section so as to prevent leakage of light therebetween. A pair of optically isolated sub-systems, a night. vision imaging system (NVIS) and a daylight system are combined in a preferred embodiment to provide an apparatus with enhanced display legibility in ambient illuminations ranging from bright sunlight to total darkness. The apparatus may be adapted for operation with night vision goggles. The sub-systems may utilize non-imaging optics based upon total internal reflection (TIR) and light reflection, both coupled to a light emitting source, such as fluorescent tubes and other linearly shaped light sources. The light reflectors may be shaped to concentrate light onto the non-imaging optics so as to compensate for the non-linear characteristics of the TIR lenses. Several advantages are derived from the above. Sufficient background light for the LCDs is provided (1) to reduce power requirements, (2) to achieve a compact package, and (3) to facilitate thermal management by drawing heat from the light sources. Thus, the backlighting system when applied to colour liquid crystal displays can be very efficient. At least 50% of power can be saved as compared to conventional systems. Non-imaging optics have the capability of capturing light emitted from a source very efficiently and collimating it towards the liquid crystal display for day operation and a light waveguide for night operation. The backlight assembly can also be packaged in a very flat profile typically of approximately 12.5 mm (one-half inch) in thickness. Further aspects of the invention are exemplified by the attached claims. For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:- FIG. 1 is perspective view of a preferred embodiment of the present invention; FIG. 2 is a top view of the embodiment depicted in FIG. 1, with a portion of its cover removed to expose its interior; FIG. 3 is an enlarged view of the embodiment shown in FIG. 2 taken along lines 3-3 thereof; and FIG. 4 is a still further enlarged view of one section of the daylight illumination area (equally so for the night illumination area) shown in FIG. 3, to illustrate how selected rays of light are transmitted directly and by reflection from a source into a collimated pattern. Referring to FIGS. 1-3, an optically enhanced dual liquid crystal display backlight system 10 includes a centrally placed daylight illumination part 12 and a pair of night illumination parts 14 placed at the ends of part 12. Parts 12 and 14 are supported on a base 16 and covered by a waveguide 18. The waveguide terminates at its sides 20 by oppositely directed sloped portions to present a trapezoidal appearance. NVIS illumination part 14 is optically isolated from the day illumination parts 12 by optical barriers 22 to prevent transmission of light therebetween. This ensures that all night illumination is transmitted through NVIS filter 54. Reflectors 24 are positioned at the exterior sides of night illumination parts 14. Daylight illumination part 12 comprises a plurality of internal, contiguously positioned optical cells 26, and night illumination parts 14 each include a side cell 28. As best shown in FIGS. 3 and 4, each internal cell 26 of daylight illumination part 12 includes a total internal reflection (TIR) lens 30, a reflector segment 32, and a fluorescent tube 34. While total internal reflection (TIR) lens 30 is illustrated as comprising several contiguously positioned lenses joined together by a proper light conducting bonding material 37, they may be formed integrally. The choice is one of expense. The number of fluorescent tubes used will depend on the overall size of the LCD. Each internal cell total internal reflection lens 30 has facets 36a and 36b to transmit input light from fluorescent tubes 34 into a collimated output light 42. Facets 36a and 36b are configured to relay and convert reflected and direct light (respectively designated by indicia 38 and 40 in FIG. 4) from fluorescent tube 34 into collimated light 42. The collimated light is directed to wavequide 18 and thence to a liquid crystal device. A typical total internal reflection lens does not evenly transmit light. It concentrates light at its center. Therefore, the light tends to be brighter at its center than at its edges. Internal cell reflector 32 is designed to correct this aberration as immediately discussed below. Reflector segment 32 comprises a pair of total internal reflection lens surfaces 44 of concave configuration joined at their centers by an upstanding section 46. Fluorescent tube 34 or other linearly shaped light source is supported on each center support 46 and at their ends in any convenient manner by a pair of cradles 48 (see FIGS. 1 and 2)(only one being shown in the drawings). The support of tubes 34 along their lengths atop center supports 46 may be augmented by any appropriate means, such as by a gel, to protect the tubes from damage occurring, for example, from shock and vibration. Reflector surfaces 44 of lenses 32 are so curved as to concentrate a greater portion of light at the outer portions (designated by indicium 50) of TIR lens 30, to compensate for any nonlinearities in the total internal reflection lens. Waveguide 18 includes a brightness enhancing film 52 placed atop daylight illumination section 12 and a pair of NVIS optical filters 54 respectively placed atop night illumination sections 14. A diffuser plate 56 is bonded to the top surface of waveguide 18. Each night illumination section 14 includes a partial total internal reflection lens 30a and a reflector segment 32a. Reflector segment has an upstanding section 46a on which a fluorescent tube 34a is supported. Aside from being segmented, each total internal reflection lens 30a and a reflector segment 32a is configured and operates exactly as total internal reflection lens 30 and reflector segment 32 of daylight illumination section 12. In operation, downward rays 38 from tubes 34 and 34a are reflected upwards in a predetermined profile. Direct rays 40 and reflected rays 38 impinge onto the facets of Total Internal Reflecting (TIR) lens 30 and 30a. Tubes 34 in the middle which cover the entire LCD active area are used for daylight applications. Vertical reflectors 24 help to minimize losses. Since a much lower level of illumination is required for night operation, the reduced efficiency in optical transmission is not significant. The rays from these are injected into wave guide 18 and its diffuser plate 56 to illuminate the LCD by scattered light. In Night Vision System Imaging (NVIS) applications, optical filters 54 are placed in the light path to filter out infra-red radiation to meet system requirements. Uniform illumination for day and night operations are achieved differently. TIR lens 30 and 30a by itself collimates the light emitted over the upper 180° arc from the sources very efficiently, the intensity being highest directly above the light source. The main purpose of reflector 32 and 32a, as shown in FIG. 4, is to redirect rays emitted in the lower 180° towards TIR lens 30 and 30a so that nearly all the light generated by the fluorescent tube can be used to illuminate the LCD. Not all of these rays are collimated as some of them (rays 38) enter the opposite facets 36b of the TIR lens compared to the direct rays 40 to direct facets 36a. This is helpful in scattering the rays. A second objective of reflector 32 and 32a, as stated above, is to shape the reflection profile to compensate for the nonlinearities of TIR lens 30 as much as possible. Diffuser 52 then performs the final touch of creating a uniform backlit surface having a dispersion angle matching the viewing angle profile of the LCD. Although the invention has been described with respect to a particular embodiment thereof, it should be realized that various changes and modifications may be made therein without departing from the scope of the invention.

Claims: A display backlight apparatus (10) for illuminating a display device under different ambient light conditions comprising: a daylight illumination section (12) positioned on one side of the display device for illuminating the display device under daylight ambient light conditions; and a night light illumination section (14), positioned on the same side of the display device, for illuminating the display device under low light ambient light conditions, characterized by optical barrier means (22) optically separating said daylight illumination section (12) and said night light illumination section (14) so as to prevent leakage of light therebetween. An apparatus according to claim 1 wherein said daylight illumination section (12) includes a plurality of internal, contiguously positioned optical cells (26), each of which is provided with reflector means (44), a total internal reflection lens (30) and a source of light (34). An apparatus according to claim 2, wherein each internal cell reflector means (44) is positioned with respect to said light source (34) so as to direct light therefrom to said total internal reflection lens (30), and each internal cell total internal reflection lens (30) is configured to relay reflected and direct light from said internal cell light source (34) as collimated light. An apparatus according to claim 1, 2 or 3, wherein said night light illumination section (14) comprises at least one night light illumination optical cell (28) positioned adjacent said barrier means (22) and including reflector means (44a), a total internal reflection lens (30a), and a source of light (34a). An apparatus according to claim 4 in which within said night light illumination cell (28) said reflector means (44a) is positioned with respect to said light source (34a) so as to direct light therefrom to said total internal reflection lens (30a), and said total internal reflection lens (30a) is configured to relay reflected and direct light from said light source (34a) as collimated light. An apparatus according to claim 2, 3, 4 or 5 in which the or each reflector means (44, 44a) is configured to shape its or their reflection profile or profiles to compensate for any nonlinearities in its or their respective total internal reflection lens or lenses (30, 30a). An apparatus according to any of claims 2 to 6 in which the or each reflection lens (30, 30a) has facets that enable the collimation of the light. An apparatus according to any of the preceding claims in which said optical barrier means (22) comprises a pair of opaque optical barriers respectively positioned either side of said daylight illumination section (12). An apparatus according to claim 8, wherein said night light illumination section (14) comprises a pair of separate portions arranged adjacent to respective ones of the optical barriers, each portion including at least one night light illumination optical cell (28). A display backlight apparatus according to any of the preceding claims and comprising a waveguide (18) positionable adjacent to said display device and including a brightness enhancing film (52) placed atop said daylight illumination section (12) and a pair of night vision imaging system optical filters (54) respectively placed atop said night illumination sections (14), a diffuser plate (56) contacting the top of said waveguide (18). A display backlight apparatus according to any of the preceding claims for illuminating a liquid crystal device, wherein: said daylight illumination section (12) includes a plurality of internal, contiguously positioned optical cells (26) bordered by a pair of side cells, each said internal cell (26) including a reflector (32), a total internal reflection lens (30) and a fluorescent tube (34), said internal cell reflector (32) having a pair of concave reflectively surfaced portions (44) joined at a central support (46) supporting said fluorescent tube to direct light therefrom to said total internal reflection lens (30), each said concave portion being configured to shape its reflection profile to compensate for any nonlinearities in said total internal reflection lens, and each said internal cell total internal reflection lens (30) having facets (36a, 36b) which are configured to relay reflected and direct light from said internal cell fluorescent tube (34) as collimated light to a liquid crystal device; said optical barrier means comprises a pair of opaque optical barriers (22) respectively positioned against said internal side cells; said night light illumination section (14) includes a pair of night light illumination optical cells respectively having a first side positioned adjacent to said barriers and a second side comprising a side reflector (24), each said night light illumination cell including a reflector portion (32a), a total internal reflection lens portion (30a), and a fluorescent tube (34a), each said night light illumination cell reflector (24) having first and second concave reflectively surfaced portions (44a) joined at a central support (46a) supporting the respective fluorescent tube (34a) to direct light therefrom to said total internal reflection lens (30a), each said concave portion (44a) being configured to shape its reflection profile to compensate for any nonlinearities in said total internal reflection lens, and each said night light illumination cell total internal reflection lens (30a) having facets which are configured to relay reflected and direct light from the respective cell fluorescent tube as collimated light to a liquid crystal device; and the apparatus further comprises a waveguide (18) positionable against the liquid crystal device and a brightness enhancing film (52) placed atop said daylight illumination section (12) and a pair of night vision imaging system optical filters (54) respectively placed atop said night illumination sections (14), and a diffuser plate (56) contacting the top of said waveguide (18). A display backlight apparatus according to claim 10 or 11 in combination with said display device positioned adjacent to the waveguide (18). An apparatus and display device according to claim 12, wherein the display device is a liquid crystal display device.

Inventors: BERNARD EDWIN, BERNARD, EDWIN
Applicants: LITTON SYSTEMS INC, LITTON SYSTEMS, INC.
Application year: 1998
Main classification: G02F1
IPC codes: G09F9, G02F1, F21V8