Illumination systems

Abstract: An optical illumination system (10, 60, 70) producing a light beam (28) having uniform illumination over a rectangular or non-symmetrical area. An area array of light emitting elements (24) generate light which is condensed to a single focal point F. A holographic diffuser (16) diffuses this light, and a cylindrical lens (18) vertically compresses the light. The light output from the condensing lens is very near the sum of the light output from the individual light emitting elements. With any attenuation in light intensity from the light emitting elements (24), the entire light beam (28) output is reduced uniformly, with no localized image plane degradation being generated. The present invention finds use in printing engines, such as those for xerographic printers.

Description: This invention relates generally to illumination systems, and may be applied, for example, to an image display system such as a xerographic printer. Semiconductor spatial light modulators (SLM's) are one viable solution to realizing high quality, affordable xerographic printers. One promising SLM technology suitable for both printers and displays is the deformable mirror device or digital micromirror device (collectively DMD) manufactured by Texas Instruments Incorporated of Dallas Texas. The DMD is a monolithic semiconductor device having a linear or area array of bi-stable movable micromirrors fabricated over an array of corresponding addressing memory cells. One embodiment of a xerographic printer implementing a tungsten light source focused via optics on an imaging DMD mirror array is disclosed in U.S. Patent 5,041,851 to Nelson, entitled "Spatial Light Modulator Printer and Method of Operation", assigned to the same assignee as the present application. In a xerographic printer implementing an imaging DMD spatial light modulator, it is desired to uniformly illuminate the elongated DMD mirror array (typically about 7 inches in length) with a homogeneous light source such that each pixel mirror of the array modulates a uniform intensity portion of light. This is necessary because the DMD mirror array modulates this light to expose a light sensitive rotating organic printing drum, whereby the intensity and duration of the modulated light directed thereon determines the relative exposure of the charged drum. The exposed portion of the drum comprises a latent image, wherein a quantity of toner will adhere to the drum image, this toner then being transferred to a printing medium such as paper, and fused thereon using heat. It is also necessary that the energy of the light directed upon the DMD mirror array be of sufficient flux per unit area to fully expose the rotating printing drum to obtain a dark image. If insufficient light energy is modulated and directed to the drum by the DMD mirror array, the printing drum may not be fully exposed, thus degrading the contrast of the image printed on a printing medium. U.S. Patent 5,159,485 to Nelson, entitled "System and Method for Uniformity of Illumination for Tungsten Light", assigned to the same assignee of the present invention, discloses an anamorphic optical path arranged such that the vertical component of the source light is compressed to match the physical shape of the DMD mirror array. The embodiment disclosed dramatically increases the optical efficiency of the system, whereby light energy is compressed to irradiate the DMD mirror array more intensely from a given light source, such as a tungsten lamp. U.S. Patent 5,151,718 to Nelson, entitled "System and Method for Solid State Illumination for DMD Devices", also assigned to the same assignee of the present invention, discloses an array of LED emitters constructed to efficiently replace the conventional tungsten source lamp. The LED array is geometrically configured, and can be electrically operated by strobing to vary the brightness of light to individual mirror pixels to achieve gray scale imaging, and reduce fuzzy line images. Each of the LED's in the array can be provided with a lens to help columnate the light through optics and onto the DMD mirror array. Using LED's , light is efficiently directed and focused onto the DMD mirror array, with little light being wasted and directed elsewhere. Less optical energy is required of the light source compared to a conventional tungsten lamp to illuminate the DMD mirror array with a particular light intensity. The LED's can be quickly turned on and off, thereby providing the ability to modulate the light energy directed upon the DMD mirror array, and consequently, helps achieve gray scale printing. For instance, during a given line print cycle, the LED can be on for 50% of the cycle time to irradiate the DMD array with half the light energy available for that particular time interval. The alignment of the optics is critical to ensure that the energy of each LED is directed upon the DMD mirror array. That is to say, the LED array may not produce sufficient and uniform light energy should one LED fail or have a reduced output. U.S. Patent 5,105,207 to Nelson, entitled "System and Method for Achieving Gray Scale DMD Operation", assigned to the same assignee as the present invention discloses a system for enhancing resolution of a xerographic process by submodulation of each individual pixel. The submodulation is achieved by anamorphically reducing the square pixel presentation of light rays to a rectangle having a number of controllable segments within each square pixel scanned line. A conventional tungsten lamp is incorporated in this embodiment. An illumination system for a virtual display system comprising a light source consisting of a red, a green and a blue light emitting element to produce a full three-coulour virtual image, and an anamorphic condensor lens is known from EP-A-0 547 493. The virtual display system further comprises at least one area array deformable mirror device and at least one lens for creating a virtual image of each area array deformable mirror device. US-A-4 893 223 discloses illumination devices for an inspection system which holds the article to be inspected in a particular location and having light emitting diodes disposed in a part spherical array to cause light to converge upon a restricted field of view in which the article is located. It is desirable to provide a low cost, high intensity optical system whereby the elongated DMD mirror array can be uniformly illuminated with high intensity light. Moreover, it is desirable to provide a high intensity light source which can be modulated in intensity to effect gray scale printing. The optical system should be easy to align, whereby any degradation in the light source would be uniformly presented to the DMD array, and would not noticeably degrade the printing quality of the xerographic printer. Respective aspects of the present invention provide illumination systems as set forth in claims 1 and 13. At least in preferred embodiments, the invention achieves technical advantages as an illumination system by implementing an array of light emitting elements each focused to a focal point, with optics positioned proximate the focal point generating a light beam having a uniform distribution of light. In embodiments of the invention, the reduction or loss of light from one or more of the light emitting elements generates a uniform reduction in the beam light output and there is no localized change in image plane intensity. The light output of the light emitting elements is effectively mixed, whereby the light output of the illumination system is near the sum of the light output from the individual light emitting elements. A condenser lens is positioned proximate the area array of light emitting elements to focus the light therefrom to the single focal point. An illumination system embodying the present invention is specified in claim 1. The optics preferably comprises a light integrator, with a holographic diffuser provided proximate the focal point to uniformly diffuse the light before it is passed to an anamorphic lens. The focal point is preferably on a central axis defined by the area array of light emitting elements. The condenser lens is preferably comprised of an aspheric lens, but could also comprise of fresnel lens, a conical light integrator, or an equivalent thereof. The area array of light emitting elements may be arranged together on a concave orientation. A housing is preferably utilized having a plurality of recesses. One of the light emitting elements is mounted within each of the recesses, these recesses collimating the light emitted from the light emitting element and directing the light to the condenser lens. Preferably, the light emitting elements comprise of LEDs. Illumination systems embodying the invention provide a uniform intensity, homogenized beam of light illuminating a spatial light modulator having a rectangular or non-symmetrical area. Embodiments of the invention are relatively low cost and easy to manufacture and align. Embodiments of the invention will now be described, by way of example, with reference to the drawings in which: Figure 1 is a perspective view of an illumination system according to a preferred embodiment of the invention, including an area array of light emitting elements with the light output of each element being focused to a focal point proximate a holographic diffuser; Figure 2 is an optical diagram illustrating an aspheric condenser lens positioned proximate the area array of light emitting elements and focusing the light output therefrom to a single focal point proximate the holographic diffuser, with the diffused light being collimated by a cylindrical lens for illuminating a spatial light modulator; Figure 3 is a perspective view of the housing and an array of parallel recesses extending therethrough, each recess holding an associated LED and collimating the light therefrom; Figure 4 is an optical diagram of an alternative preferred embodiment illustrating a fresnel lens condensing the light from the array of light emitting elements to a single focal point; Figure 5 is an optical diagram of another alternative embodiment of the present invention illustrating a conical light integrator utilized as a condensing lens for focusing the light from the array of light emitting elements to a single focal point; Figure 6 is a perspective view of another alternative embodiment illustrating a concave light unit; and Figure 7 is an optical diagram of the embodiment of Figure 6, illustrating the compact illumination system. Referring now to Figure 1, there is shown a perspective view of an optical illumination system 10 uniformly illuminating a rectangular spatial light modulator 12, which may be comprised of a digital micromirror device (DMD) such as that manufactured by Texas Instruments Incorporated, or other suitable spatial light modulator. Illumination system 10 is seen to include a light source unit generally shown at 14 including an area array of light emitting elements, a holographic diffuser 16, a cylindrical lens 18, and an aspheric lens 20. Lens 20 is shown as a solid aspheric element, but may also be a fresnel lens or the like which may be preferable because of its relatively thin profile. An area array of light emitting elements 24, which may each be comprised of an LED, are positioned within recesses 26 opening through light unit 14 as shown. The light output of each of the light emitting elements 24 is focused to a common focal point F, with the holographic diffuser 16 positioned at focal point F for diffusing the light. The diffused light is condensed in the vertical direction by the cylindrical lens 18, with the uniform intensity, condensed light, generally shown at 28 being directed upon the rectangular spatial light modulator 12 by the aspheric lens 20. The uniform-intensity light beam 30 illuminating spatial light modulator 12 is modulated by spatial light modulator 12, such as disclosed in commonly assigned US Patent 5,467,146, to form a light image. This light image is directed into the focal point of a projector lens 32, such as a F/5.6 lens, and focused upon the surface of an image plane 34, such as a rotating organic xerographic photoreceptor drum. For additional teaching of using a DMD spatial light modulator in a xerographic printer and some applications of embodiments of the invention, reference is made to commonly assigned US Patent 5,467,146, as well as U.S. Patent 5,461,411 entitled "Process and Architecture for Digital Micromirror Printer" and U.S. Patent 5,461,410 entitled "Gray Scale Printing Using Spatial Light Modulators". Turning now to Figure 2, an optical schematic diagram of illumination system 10 is shown. Light unit 14 is seen to comprise a tubular cylindrical housing 40 encasing and securing an aspheric condensing lens 42 therewithin. Unit 14 is also seen to include an illuminator housing 46. Housing 46 is generally circular and has a disc-like shape with the plurality of parallel recesses 26 bored therethrough, as shown. One LED 24 is secured within each one of the recesses 26 at an entrance end thereof, with the light output of each LED 24 being collimated by the respective recess 26 toward the aspheric condensing lens 42. LEDs 24 may be secured within unit 46 by adhesive, with unit 46 secured within housing 40 using adhesive or welding. As shown, the aspheric lens 42 condenses the parallel beams of light received from each of the LEDs 24, and focuses the light therefrom to a single focal point F. Focal point F is along a common axis 50 defined by housing 46, aspheric lens 42, and the area array of LEDs 24. Holographic diffuser 16 is positioned at focal point F and diffuses the focused light thereat, this diffused light 52 being transmitted to the cylindrical lens 18, as shown. A technical advantage of embodiments of the invention is that any decrease or loss in light output of any one or several LEDs 24 will result in a uniform decrease in the light intensity of beam 52 transmitted to the cylindrical lens 18. That is to say, there is no localized image plane degradation when any on-axis or off-axis LED attenuations are produced. With LED light attenuation, the entire image plane degrades uniformly. This is especially desirable when illuminating the elongated rectangular spatial light modulator 12 which requires uniform illumination to effectively illuminate image plane 34. Referring to Figure 3, a perspective view of illumination unit 14 is shown to illustrate the generally circular area array of LEDs 24 within housing unit 46. The LEDs 24 are shown to be uniformly positioned about the central axis 50 so as to generally uniformly irradiate the aspheric condensing lens 42. However, other uniform geometric distributions of light emitting elements 24 could be utilized as well, such as a hexagonal array of LEDs 24. Referring now to Figure 4, there is shown an illumination system 60 according to an alternative preferred embodiment of the present invention. Illumination system 60 is identical to illumination system 10 as shown in Figure 1 and Figure 2, with the exception that a fresnel lens 62 is utilized within a light unit 64 in place of the condensing lens 42 shown in Figure 2. Fresnel lens 62 receives the parallel beams of incident light from each of the LEDs 24 and focuses the light to the single focal point F. The advantage of implementing a fresnel lens 62 in place of lens 42 is its reduced cost and reduced thickness. Referring now to Figure 5, there is shown an illumination system 70 according to yet another alternative embodiment of the present invention. Illumination system 70 is also substantially identical to system 10 shown in Figure 1 and Figure 2, with the exception that a conical lens 72 is utilized in light unit 74 in place of the aspheric condensing lens 42. Conical lens 72 also focuses the parallel beams of incident light from the array of LEDs 24 to the single focal point F, as shown. The tips of LED's 24 abut the incident surface of integrator 72 to directly couple the light into the light integrator. If desired, LED's without spherical lenses, such as shown at 76, can have the flat tips thereof abutting the flat surface of the integrator and be affixed thereto using lens-grade epoxy so that the LED is actually a physical part of the integrator. This helps couple the light energy directly into the light integrator. Diffuser 16 is preferably a spherical diffuser. An advantage of utilizing a conical lens 72 is the additional integration of the individual beams. In the alternative embodiments shown in Figures 4 and Figure 5, the holographic diffuser 16 is also positioned closely proximate, and preferably at, focal point F, with the cylindrical lens 18 being utilized to vertically compress the transmitted diffused light 52 toward the aspheric lens 20. Aspheric lens 20 serves to further direct the uniform intensity light beam 30 to illuminate the rectangular spatial light modulator 12, as shown in Figure 1. Referring now to Figure 6 and Figure 7 there is shown an illumination system 80 having a concave light unit 82 including a circular area array of LED's 84. A concave housing 86 has an array of openings 88, each opening 88 holding one LED 84. The axis of each opening 88 is oriented such that the light from each LED 84 intersects and is focused at focal point F after being focused by an aspheric condensing lens 90. Lens 90 may be solid, as shown, but may also be a fresnel lens or another lens which condenses the incident beams of light from the concave array of LEDs to point F. Compared to the flat-array light unit 14 of Figure 2, light unit 82 has reduced comatic beam distortion from LEDs at the edges of the array, and is compact in the axial direction. With the concave orientation of light unit 82, use of lens 90 is optional. Embodiments of the invention achieve technical advantages as illumination systems producing a uniform beam of light over a rectangular or non-symmetrical area, such as a spatial light modulator comprising a DMD as shown in Figure 1. Should the light output of one or more of the multiple LEDs 24 be reduced or lost, the entire image plane intensity degrades uniformly. In other words, no local image plane degradation is experienced when any one, or a combination of, on-axis and off-axis LED attenuation occur. The condensing lenses truly "mix" the light received from the array of LEDs 24 and focuses the light to a common focal point F. The light intensity at focal point F is very near the sum of all the LED outputs. Although LEDs and conventional lenses are used for the present embodiments, other sources of radiation and non-imaging optical condensers/integrators can be used. Though embodiments of the invention have been described, many variations and modifications will be apparent to those skilled in the art.

Claims: An illumination system (10; 60; 70; 80), comprising: a light source (14; 64; 74; 82) comprising a generally circular area array of light emitting elements (24; 84); a condenser lens (42; 62; 72; 90) positioned proximate the element array for focussing light from said light emitting elements (24; 84) to a common focal point (F); and optics (16) positioned proximate said focal point (F) for receiving the condensed light and generating a light beam (30) having a uniform distribution of light. The system as specified in claim 1 wherein said optics (16) comprises a light integrator. The system as specified in claim 1 or claim 2 wherein said optics (16) includes a holographic diffuser proximate said focal point (F). The system as specified in any preceding claim wherein said focal point (F) is on a central axis (50) defined by said area array of light emitting elements (24; 84). The system as specified in any preceding claim wherein said condenser lens comprises an aspheric lens (42). The system as specified in any one of claims 1 to 4 wherein said condenser lens comprises a fresnel focusing lens (62). The system as specified in any one of claims 1 to 4 wherein said condenser lens comprises a conical light integrator (72). The system as specified in any preceding claim wherein said area array of light emitting elements (84) are arranged together in a concave orientation. The system as specified in any preceding claim further comprising a housing (40; 46) having a plurality of recesses (26), one said light emitting element (24) being mounted within each said recess (26), such that said recesses (26) collimate light emitted by said light emitting elements (24). The system as specified in any preceding claim wherein said light emitting elements (24; 84) comprise LEDs. The system as specified in any preceding claim further comprising a spatial light modulator (12) for modulating said light beam (30) having a uniform distribution of light. The system as specified in claim 11 further comprising a projector lens (32) for receiving the modulated light from said spatial light modulator (12). An illumination system (80), comprising: a light source (82) comprising a generally circular concave area array of light emitting elements (84) said light emitting elements being oriented such that the light from each element intersects at a common point (F); and optics (16, 18) positioned proximate said common point (F) for receiving said light from said concave area array and generating a light beam having a uniform distribution of light. The system as specified in claim 13, further comprising an aspheric lens (90) positioned between said concave area array of elements (84) and said optics (16, 18). The system as specified in claim 14 further comprising a light diffuser (16) at a focal point (F) of said aspheric lens (90).

Inventors: ANDERSON CHARLES H, ANDERSON, CHARLES H.
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1996
Main classification: G02B26
IPC codes: G02B26, G02B5, G02B27, H04N5, B41J2, H04N1