System for backlighting LCD matrices

Abstract: For generating collimated light for illuminating LCD matrices, the backlighting system (10) includes a plurality of lenses (24) arranged in an array on one side of a planar substrate (20), a plurality of luminescent phosphor dots (28) located on the opposite side of the substrate (20) from the lenses (24), each at the focal points of the lens, and a means (30) for stimulating the phosphor dots (28) to emit light. The phosphor dots (28) may be responsive to ultraviolet light in which case a fluorescent lamp (30) adapted for emitting ultraviolet light may be used to stimulate the dots (28) to emit visible light. The visible light is then collimated by the lenses (24) and directed for backlighting a display matrix.

Description: This invention relates to an apparatus for back lighting an LCD matrix comprising: a two dimensional array of luminescent phosphor dots positioned on one face of a planar substrate; a corresponding planar two dimensional array of lenses; and means for stimulating the phosphor dots to emit visible light, the arrangement being such that each lens collects light emitted by a respective one of the phosphor dots. In Patent Abstracts of Japan, volume 14, no. 313 (P-1072), 5th July 1990 and JP-A-02 101 430, an apparatus of the kind defined hereinbefore is described in which a plano-convex microlens is positioned on top of each phosphor dot of a dot matrix formed on the top surface of a transparent base plate or substrate. The microlens is used as a light diffusing means and a light shielding partition is provided between the microlens and each neighbouring microlens to prevent mixing of the different colours of light, the phosphor dots being arranged in a red, green, blue colour matrix. Proper backlighting is essential to assure good performance of projection type liquid crystal display (LCD) matrices. The light provided by an LCD backlighting system should be orthogonal to the plane of the LCD matrix in order to maximize the efficiency of the crystals in modulating the light to form images. Furthermore, in many applications such as aircraft instrumentation backlighting systems must be as compact as possible since the available space is very limited. According to one aspect of the invention an apparatus of the kind defined hereinbefore at the beginning is characterised in that the lenses are provided on the other face of the substrate, the phosphor dots are respectively positioned at the focal points of the lenses, and each phosphor dot is such that light collected therefrom by the respective lens and directed by that lens towards the LCD matrix overlaps light directed by another of the lenses towards the LCD matrix. According to another aspect of the invention there is provided a method of illuminating an LCD matrix including a plurality of LCD pixels, comprising the steps of: stimulating a set of luminescent dots which are arranged into a two dimensional array on one face of a planar substrate; collecting light emitted by the dots using a set of lenses which are arranged into a two dimensional array and are positioned so as to place the said dots adjacent respective ones of the lenses; and projecting the light emitted by the said dots and collected by the lenses onto the liquid crystal pixels in the LCD matrix in order to backlight the matrix, Characterised by so selecting the thickness of the substrate and the size of the phosphor dots that each phosphor dot is at the focal point of a respective one of the lenses and light collected from the phosphor dot by the respective lens and directed thereby towards the LCD matrix overlaps light directed by another of the lenses towards the LCD matrix, the lenses being provided on the other face of the substrate. Preferred embodiments of the invention provide: a backlighting system for use in conjunction with LCD matrices which generates collimated light and has a compact size capable of fitting along the backside of the LCD matrix; a backlighting system for use in conjunction with LCD matrices which is as planar as possible having a very limited depth as compared with its width and height; a backlighting system for use in conjunction with LCD matrices which supplies light having a uniform intensity distribution across the matrix. Preferably, an apparatus embodying the present invention constitutes an apparatus for backlighting an LCD matrix with collimated light which includes a plurality of plano-convex lenses, a plurality of luminescent phosphor dots and a mechanism for stimulating the dots to emit visible light. The plano-convex lenses are arranged into a two dimensional array on one side of a planar substrate while the phosphor dots are positioned on the opposite side of the substrate at the focal points of the lenses in a corresponding array in which one phosphor dot is provided for each lens. The phosphor dots comprise a photoluminescent material which can be stimulated by ultraviolet light which can be provided by a fluorescent lamp. In operation, ultraviolet light stimulates the phosphor dots to emit visible light a substantial amount of which is directed toward the lenses. Since the dots are in proximity to the focal points of the lenses the lenses act to collimate the light in a direction orthogonal to the plane of the substrate on which the lenses are mounted. The lenses comprise plano-convex lenses which are constructed so as to be internally integrated with the planar substrate whereby the lenses array can be readily manufactured. The invention will now be described by way of example with reference to the accompanying drawings, in which:- FIGURE 1 is a diagrammatic illustration of the use of an embodiment of the present invention with an LCD matrix and a light diffuser. FIGURE 2A and 2B are cross sectional and plan views of the light collimation component of a preferred embodiment of the present invention, showing a lens array and corresponding phosphor dot array constructed on a planar substrate. FIGURE 3 is a cross sectional view showing the operation of a single phosphor dot and lens unit in response to light from an ultraviolet source. FIGURE 4 is a diagrammatic plan view of a special fluorescent lamp which would provide an effective source of ultraviolet light for use in an embodiment of the present invention. FIGURE 5 is a cross sectional view of another embodiment of the present invention in which the phosphor dots are stimulated to emit light by being bombarded by electrons generated from a filament and multiplied in number by a microchannel plate. FIGURES 6A and 6B are a cross sectional view of the lens array in conjunction with an LCD matrix, and a plan view of the areas illuminated by the lenses on the matrix illustrating the proper determination of the spacing between the lenses in lens array. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIGURE 1, the present invention provides an apparatus 10 for backlighting an LCD matrix 12 made up of a multiplicity of LCD pixels which may be individually controlled to form images. The backlighting apparatus 10 projects collimated light 11 onto the backside of the matrix 12 which falls on the rear surface of the matrix 12 at right angles (i.e. orthogonally) to the plane defined by the matrix. The light 13 which passes through the matrix 12 is intercepted by a diffuser 14 in close proximity to the front side of the matrix 12. The diffuser 14 diffuses the light 13 from the matrix 12 in order to enhance the viewing of images formed on the matrix 12 at off-axis or oblique angles since the light 15 emitted by the diffuser 18 is no longer collimated. Referring now to FIGURES 2A and 2B, the light collimating component 20 is shown as including a large number of separate lenses 24 which are arranged on the front side of a substrate 26 in a two dimensional array 22 so as to be equal spaced apart by a distance s which should be as small as possible consistent with other requirements in order to maximize the light output of the device . The lenses 24 are characterized by a plano-convex structure and are integrally constructed as a part of the planar substrate 26. The plano-convex lens 24 may be formed on the planar substrate 26 by selective exposure of glass which is dimensional sensitive to ultraviolet light. For example, the lenses may be formed in accordance with the techniques used by Corning, Inc. of Corning, New York in the manufacture of Foto-Form® type lenses. Alternately, the lenses 24 forming the array 22 may be molded out of plastic having suitable light transmissive and refractive characteristics. A set of luminescent phosphor dots 28 are deposited on the back side of the substrate 26 opposite the lenses 24 in proximity to the focal points of the lenses. The phosphor dots 28 form a two dimensional array on the backside of the substrate 26 corresponding on a one-to-one basis to the lenses 24. The dots 28 have a diameter d and are constructed of a phosphor which may be stimulated to emit visible light by ultraviolet radiation in accordance with well known principles. The substrate 26 has a width w which is dimensioned so as to locate the phosphor dots 28 at the focal points of the lenses 24 when the dots are centrally positioned with respect to the optical axes of the lenses 24. Referring now to FIGURE 3, ultraviolet (UV) light 40 generated by UV source 30 such as a fluorescent lamp is incident upon the phosphor dot 28 and stimulates the emission of visible light 42 from the dot 28 which projects outward in all directions but most importantly in the direction of the lens 24. The light 42 which enters the lens 24 is collimated by the lens in order to generate collimated light 44. The light collimating component 20 should preferrably include opaque shielding areas 32 extending between the lenses 24 for blocking the emission of off-axis light as illustrated by the light ray 46. The collimation component 20 may also include reflective strips 34 on the rear surface of the substrate 26 for reflecting ultraviolet light which intersects the component 20 in areas between the phosphor dots 28 as illustrated by the light ray 48 back to the UV source 30 where it may be returned to the component 20 in order to improve the efficiency of the UV source 30. The phosphor dots 28 should be sized to have a diameter d in accordance with the angle α by which the light 44 may be allowed to diverge from the normal from the plane defined by the substrate 26 (and the LCD matrix). The diameter d of the phosphor dots may in fact be determined in accordance the following equation: d = 2 f sin α Where: d = diameter phosphor dot f = focal length of lenses α = divergence angle In a typical application the divergence angle might be expected to be on the order of seven degrees and therefore the diameter of the phosphor dots d may be expected to be approximately .24 times the focal length f of the lenses 24. Referring now to FIGURE 4, a preferred source 30 of ultraviolet for stimulating the phosphor dots 28 may comprise a fluorescent lamp including a face plate 50 having a special labyrinth like configuration defined by the dividing walls 52 which force the arc of the lamp to follow a serpentine path 54 between the filaments 56. The use of a single fluorescent lamp having a single arc path 54 provides electrical efficiency and simplicity to the UV source 30. Referring now to FIGURE 5, in an alternative embodiment the phosphor dots 28 may be constructed of a cathodoluminescent material responsive to simulation by energized electrons. A filament 60 may be heated and used to generate a supply of electrons which are directed by an electric field toward and into a microchannel plate 62 where they are accelerated down the channels in the plate and generate more electrons as they ricochet off the walls of channels. The microchannel plate 62 generates a copious supply of energized electrons which are projected toward the phosphor dots 28 and which stimulate the phosphor dots to emit visible light which can then in turn be collimated by the lenses 24. The filament 60, microchannel plate 62 and cathodoluminescent phosphor dots 28 provide an alternative technique for generating the light to be collimated by the lenses 24. Referring now to FIGURE 6A and 6B, the distance p between the lenses 24 and the LCD matrix 12 should be determined so as to allow the light supplied by the individual lenses 24 to overlap as shown in FIGURE 6B. The distance p may be determined in accordance with the following equation: p = 2 s/(2 sin α) Where: p = lens/display distance s = lens spacing α = divergence angle. As shown in FIGURE 6A, given a pre-determined angle α with the normal to the surface of the light collimating component 20, light rays 70 and 72 from the lenses 24 will overlap given a sufficient distance p. In fact, if the distance p is calculated in accordance with the equation given above the circles of light such as circle 74 supplied by the individual lenses 24 will overlap so that diagonally adjacent lenses will provide circles of light 74 which are approximately tangent to one another at the point 76 where they intersect. In a typical application given a divergence angle of approximately the 7 degrees, the distance p necessary to ensure overlap of the circles 74 so that light is provided to every part of the rear surface of the LCD matrix 12 would be approximately 12 times the spacing s of the lenses 24.

Claims: An apparatus for back lighting an LCD matrix comprising: a two dimensional array of luminescent phosphor dots (28) positioned on one face of a planar substrate (26); a corresponding planar two dimensional array of lenses (24); and means (30) for stimulating the phosphor dots (28) to emit visible light, the arrangement being such that each lens (24) collects light emitted by a respective one of the phosphor dots (28), characterised in that the lenses (24) are provided on the other face of the substrate (26), the phosphor dots (28) are respectively positioned at the focal points of the lenses (24), and each phosphor dot (28) is such that light collected therefrom by the respective lens (24) and directed by that lens (24) towards the LCD matrix (12) overlaps light directed by another of the lenses (24) towards the LCD matrix (12). An apparatus according to claim 1, characterised in that the distance p from the array of lenses (24) to the LCD matrix (12) is given by the equation: P = 2 s/(2 sin α) where s is the lens spacing, and α is the divergence angle. An apparatus according to claim 1, characterised in that the distance (p) between the array of lenses and the LCD matrix (12) is approximately equal to twelve times the spacing (s) between the lenses (24). An apparatus according to any preceding claim, characterised in that the phosphor dots (28) are cathodoluminescent and the means for stimulating the phosphor dots (28) includes a filament (60) adapted for generating free electrons and a microchannel plate (62) for generating and directing electrons toward the phosphor dots (28). An apparatus according to any one of claims 1 to 3, characterised in that the phosphor dots (28) are photoluminescent and the means for stimulating the phosphor dots (28) includes a fluorescent lamp (30) adapted for generating ultraviolet light. An apparatus according to any preceding claim, characterised in that the diameters (d) of the phosphor dots (28) are approximately equal to .24 times the focal length (f) of the lenses (24). An apparatus according to any preceding claim, characterised in that the areas (32) between the lenses (24) are opaque in order to limit the production of off-axis light. An apparatus according to any preceding claim, characterised in that the lenses (24) comprise plano-convex lenses which are integrally constructed with the substrate (26). A method of illuminating an LCD matrix including a plurality of LCD pixels, comprising the steps of: stimulating a set of luminescent dots (28) which are arranged into a two dimensional array on one face of a planar substrate (26); collecting light emitted by the dots (28) using a set of lenses (24) which are arranged into a two dimensional array and are positioned so as to place the said dots (28) adjacent respective ones of the lenses (24) ; and projecting the light emitted by the said dots (28) and collected by the lenses (24) onto the liquid crystal pixels in the LCD matrix (12) in order to backlight the matrix (12), Characterised by so selecting the thickness of the substrate (26) and the size of the phosphor dots (28) that each phosphor dot (28) is at the focal point of a respective one of the lenses (24) and light collected from the phosphor dot (28) by the respective lens (24) and directed thereby towards the LCD matrix (12) overlaps light directed by another of the lenses (24) towards the LCD matrix (12), the lenses (24) being provided on the other face of the substrate (26). A method according to claim 9, characterised in that the said selected thickness and size are such that, at the LCD matrix (12), light emanating from one phosphor dot (28) in a group of four adjacent dots (28) overlaps light emanating from two others of the four dots (28) of the group. A method according to claim 9 or 10, characterised in that the step of stimulating the phosphor dots (28) includes illuminating the phosphor dots (28) with ultraviolet light from a fluorescent lamp (30). A method according to claim 9, characterised by diffusing (14) the light passed through the pixels in the LCD matrix (12) in order to enhance the quality of the images presented by the matrix (12) at oblique viewing angles.

Inventors: BOTTORF SCOTT A, BOTTORF, SCOTT A.
Applicants: ROCKWELL INTERNATIONAL CORP, ROCKWELL INTERNATIONAL CORPORATION
Application year: 1992
Main classification: G02F1
IPC codes: G02F1