Abstract:
A lens assembly especially adapted for mounting in an environment subject to variable temperatures, such as a projection television set, including an automatic thermal focus adjustment. A lens mount is formed from a material having a first coefficient of thermal expansion and carries at least one lens. A focus mount is coupled to the lens mount and is formed from a material having a second coefficient of thermal expansion different from the first coefficient of thermal expansion. Adjustment and locking structure couples the lens mount to the focus mount and allows the lens mount to be axially adjusted relative to the focus mount and then locked in position. In use, the relative axial positions of the lens mount and focus mount automatically change to move the lens in response to a temperature change in the environment of use and after being locked in position with the adjustment and locking structure.

Description:
This application claims the benefit under 35 U.S.C. §120 of Provisional Application Ser. No. 60/264,483, filed Jan. 26, 2001 and currently pending. The disclosure of that provisional application is hereby fully incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to optical lens assemblies and, more particularly, to lens assemblies used in projection televisions and other optical lens assemblies used in environments which are subject to variable temperatures. 
     BACKGROUND OF THE INVENTION 
     A projection television set typically includes three cathode ray tubes (CRTs), corresponding to the primary colors, red, blue and green. A projection lens assembly uses a plurality lens to magnify the image appearing on the CRT faceplate and project that image onto a much larger viewing screen. CRTs used in projection televisions typically have a diameter of 3 to 9 inches. The image projected onto the screen generally has a size ranging from 40 to 60 inches or larger measured diagonally. 
     Each of the CRTs must provide maximum brightness or light intensity and, to facilitate this objective, each CRT operates at maximum power to produce maximum light output at the faceplate while still maintaining color balance. As a result, the CRTs generate considerable heat within the enclosure of the projection television set. It is not uncommon for portions of the projection television lens assemblies to be elevated by 40° C. to 45° C. or more above room temperature. 
     Each CRT has an associated magnifying lens system mounted adjacent to the CRT faceplate. Usually, the lens assembly is formed with at least one “A” lens element, at least one “B” lens element and at least one “C” lens element. Regardless of the number of lens elements, these are generally referred to in the art as “A”, “B” and “C” lens groups. That is, each “group” may be comprised of one or more lens elements. The “B” lens group usually includes a lens formed of glass, while the “A” and “C” lens groups may be formed of plastic. However, it should be understood that each group may comprise one or more lenses formed of glass and one or more lenses formed of plastic. Alternatively, the lens assemblies may comprise all glass lenses or all plastic lenses. Due to the heat generated by the CRTs, the plastic lenses will distort or otherwise undergo changes in optical properties. This is particularly true of the “C” lens which is mounted closest to the associated CRT. Also, the refractive index of liquid such as ethylene glycol used to cool the lens assemblies will also change due to temperature changes. Due to the temperature induced optical property changes such as these, the focus of the lens system can change. More specifically, as the temperature of the lens assembly changes over several hours of continuous operation, the picture displayed on the television screen could become blurred as a result of the defocusing effect of the increased temperature. This blurring effect can be more noticeable on high definition television (HDTV) sets due to their higher resolution capability. 
     Some temperature correction systems rely on one or more temperature sensors within a projection television cabinet and an automatic focusing mechanism which refocuses the lens assemblies based on feedback from the temperature sensor(s). Drawbacks to such systems include the expense and the relative difficulty in ensuring that each lens assembly is corrected in an effective and independent manner. One simpler manner of addressing the problems of heat-induced lens distortion is described in U.S. Pat. No. 6,104,554, assigned to the assignee of the present invention, and the disclosure of which is hereby fully incorporated by reference herein. The preferred embodiment shown and described in U.S. Pat. No. 6,104,554 utilizes a plurality of bars which thermally expand as the lens assembly is heated by the associated CRT. The bars are mechanically coupled to a lens cell carrying the “A” and “B” lens elements. A “C” lens element is positioned closely adjacent to the CRT and distorts away from the “A” and “B” lens elements. The bars undergo a similar heat induced distortion or expansion and thereby move the “A” and “B” lens elements correspondingly in a direction toward the “C” element. This maintains proper spacing between the “A” and “B” elements and the “C” element as the temperature in the interior of the television set increases. Likewise, when the television set cools down, the “C” element and the bars will return or retract to their original positions to maintain proper focus when the television set is turned on again. 
     It will be appreciated that such temperature induced defocussing can occur in various embodiments, including those which are subject to variations in temperature in either direction. Despite the improvements made in this area, there is a continuing need for lens assemblies which address the effect of temperature induced focusing problems while, for example, reducing the cost and complexity associated with manufacturing the lens assemblies. 
     SUMMARY OF THE INVENTION 
     Generally, the present invention provides a lens assembly adapted to be mounted in an environment subject to temperature change and providing for at least partial focus correction through an automatic relative movement of a direct or indirect lens mounting component by thermal expansion. A lens mount carries at least one lens and is coupled to a focus mount. Adjustment and locking structure is provided for allowing the lens mount to be axially adjusted relative to the focus mount and then locked into position. The lens mount and focus mount are formed of materials having different coefficients of thermal expansion (CTEs). Depending on the application, the material forming the focus mount can have a CTE which is lower than the CTE of the material forming the lens mount or, in other applications, the reverse situation may apply. In each case, when the lens mount and focus mount are subject to a temperature change, for example, when heated by a light source or when subjected to temperature change by some other environmental condition, the axial position of the lens mount will automatically change relative to the focus mount to move the lens axially after the two mounts are locked in position with the adjustment and locking structure. The automatic axial movement may be toward another lens or optical element or away from another lens or optical element depending on the application requirements. Automatic corrective movement will also take place through contraction in an environment that relatively changes from a heated state to a cooled state. 
     More specifically relative to the projection television industry, in a first embodiment “A” and “B” lens elements are carried by the lens mount and are axially adjusted and then mechanically fixed relative to a “C” lens element. At elevated temperatures, the lens mount will expand at a higher rate than the focus mount to move at least the “A” lens element toward the “C” element. This ensures that the proper lens spacing is maintained, as set during the initial adjustment, as the optical properties of the “C” element change in the heated environment. 
     In this embodiment, the focus mount and lens mount are cylindrical members and a mechanical fastener assembly is provided between these two members to allow the focus mount to be rotationally and axially adjusted relative to the focus mount. Once this manual adjustment is made, the focus mount is fixed relative to the lens mount and, in use, the automatic adjustment provided by the different CTEs of the two mounts takes over and automatically compensates for heat induced changes in optical properties. The manual adjustment is provided by a slot and threaded fastener assembly located approximately midway along the length of the focus mount and lens mount assembly. When fixed, this fastener assembly therefore provides a location from which the lens mount expands in opposite directions. The greatest expansion takes place toward the CRT since the temperature is highest in this region. Thus, at least the “B” lens group is mounted in the region of the lens mount positioned between the fastener assembly and the “C” element and CRT. Alternatively, both the “A” and “B” lens groups may be positioned in this region such that they move essentially in unison toward the “C” lens group. 
     The preferred material for the focus mount in the first embodiment is a 10% glass filled polycarbonate having a coefficient of thermal expansion of 3.22×10 −5  cm/cm/C. The preferred material for the lens mount is unfilled polycarbonate having a coefficient of thermal expansion of 6.75×10 −5 cm/cm/C. It will, however, be understood that other materials having different CTEs may be substituted while still falling within the spirit and scope of this invention. For example, although two plastic materials are used in this embodiment, metals may be substituted for one or both of these materials while achieving similar results. 
     In a second embodiment, the lens mount is formed of aluminum, while the focus mount is formed of plastic. In this embodiment the automatic corrective movement causes the lens or lenses carried by the lens mount to be moved away from another light transmissive or light generating component. 
     Additional objectives, advantages and features of the invention will become more readily apparent to those of ordinary skill in the art upon review of the following detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a review view in elevation showing a lens assembly constructed in accordance with the preferred embodiment. 
     FIG. 2 is a side elevational view of the lens and CRT assembly. 
     FIG. 3 is a cross sectional view taken along line  3 — 3  of FIG.  1 . 
     FIG. 4 is a cross sectional view similar to FIG. 3, but illustrating the lens assembly automatically corrected under the effect of heat induced distortion. 
     FIG. 5A is a longitudinal axial cross section of an alternative lens assembly constructed in accordance with the invention. 
     FIG. 5B is a cross sectional view similar to FIG. 5A, but illustrating the lens assembly automatically corrected under the effect of heat induced distortion. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring generally to FIGS. 1 and 2, lens assembly  10  is constructed in accordance with a preferred embodiment and use of the invention in a projection television, although it will be understood that many other configurations and uses may be utilized within the spirit and scope of the invention. The lens assembly  10  is specifically suitable for use in projection televisions sets having at least one light source that generates heat. For example, many projection televisions use a CRT  12  for each of the primary colors red, green or blue. One of the advantages of the present invention over those, for example, that use temperature sensors in the television cabinet is that each CRT  12  will be thermally controlled in a more independent manner. This can be helpful because each CRT  12  tends to operate at a different temperature and therefore may need a different amount of correction. Temperature sensor based systems may only use an average temperature within the cabinet and, therefore, may overcorrect or undercorrect one or more of the lens assemblies. 
     The lens assembly  10  includes an outer, generally cylindrical focus mount  14 . The lens assembly includes a “C” lens element  16  referred to in the art as a field flattener lens. The focus mount  14  is mounted within the interior of the television set (not shown) through the use of a mounting member or coupler  17  and is secured to the coupler  17  by suitable fasteners  18  and flange elements  20 . The field flattener lens or “C” element  16  is fixed between the focus mount  14  and the coupler  17  and is located in a centered position by pins  22 . The “C” lens element  16  is formed from a clear plastic as is conventional in the art and has a convex surface  16   a  facing the CRT  12 . 
     As further shown in FIG. 2, the CRT  12  is mounted against coupler  17  by a bracket  23  and spring-loaded fastener assemblies  24 . O-rings  25 ,  26  are respectively positioned between a flange portion  16   b  of “C” element  16  and the coupler  17  and between the CRT  12  and the coupler  17 . This forms a space  27  between the “C” element  16  and the CRT  12  for receiving a liquid coolant as is conventional. This coolant will also generally undergo a change in its refractive index when subjected to a temperature change. The present invention corrects for this type of defocussing effect as well. 
     Referring to FIGS. 3 and 4, a lens mount  28  carries three lenses including an “A” element  30  and a pair of “B” elements  32 ,  34 . The “B” element  32  which is closest to the “C” element  16  is formed from clear plastic as is the “A” element  30 . The larger, centrally located “B” element  34  is formed from glass and provides the majority of the positive magnifying power to the lens assembly  10 . Referring back to FIG. 2, it will be appreciated that the CRT  12  directs light initially through the “C” lens element  16  and then through the respective “B” elements  32 ,  34  and finally the “A” element  30  before the light and resulting image is received by the television screen (not shown) positioned on a side of the lens assembly  10  opposite to the CRT  12 . 
     The lens mount  28 , including the “A” and “B” lens elements  30 ,  32 ,  34  may be manually adjusted in an axial manner in left and right directions as viewed in FIGS. 3 and 4 relative to the “C” lens element  16 . This is an initial adjustment typically made at the factory during the manufacturing process. To facilitate this adjustment, a fastener assembly  40  comprising an externally threaded fastener  42  received in an internally threaded insert  43  and further received in a nut assembly  44   a ,  44   b  with a lock washer  46  therebetween extends through a slot  50  in the focus mount  14 . An upper insert  43  does not receive any fastener, but simply acts as a guide member during rotational adjustment. The nut assembly  44   a ,  44   b  is loosened allowing the focus mount  14  to rotate relative to the lens mount  28 . As shown in FIG. 2, the slot  50  extends at a nonperpendicular angle relative to the axis  52  (FIG. 2) of the lens assembly  10 . Therefore, as the focus mount  14  is rotated, the fastener insert  43  moves along the axis  52  and thereby moves the lens mount  28  and lenses  30 ,  32 ,  34  along the axis  52  either toward or away from the CRT  12  depending on the direction of rotation. Once the proper focus has been set in this manner, the nut assembly  44   a ,  44   b  is tightened and the distance between the respective “A” and “B” lenses  30 ,  32 ,  34  relative to the “C” lens  16  is fixed. 
     As further shown in FIG. 4, after the television set has been in use for a continuous period of time, heat from the CRT  12  will cause the “C” lens element  16  to distort from the position shown in dash-dot lines to the position shown, in exaggerated form, in solid lines. That is, the “C” lens element  16  will deform or distort in a direction toward the CRT  12 . Thus, if the “B” lens elements  32 ,  34  remained in the position shown in FIG. 3, the respective distances L 1 , L 2  between the “B” lens elements  32 ,  34  and the “C” lens element  16  would be different than the distances initially set through the use of fastener assembly  40  to obtain proper focus. The projected image on the screen may therefore become blurred. To compensate for the effects of heat induced distortion or other optical property changes of the “C” lens element  16 , the lens mount  28  is formed from a material having a different coefficient of thermal expansion (CTE) than the focus mount  14 . In particular, the portion of the lens mount  28  holding the “B” lens elements  32 ,  34  moves to the right as shown in FIG. 4 so that the L 1  and L 2  distances are maintained as consistent as possible with those originally set as shown in FIG.  3 . Thus, the portion of the lens mount  28  to the right of fastener assembly  40 , as shown in FIG. 4, thermally expands and moves to the right and carries lens elements  32 ,  34  from the positions shown in dash-dot lines to the positions shown in solid lines. In the preferred embodiment, the lens mount  28  is formed from a plastic material having a CTE of 6.75×10 −5  cm/cm/C, while the focus mount has a lower CTE of 3.22×10 −5  cm/cm/C. Most preferably, the focus mount  14  is formed from a 10% glass filled polycarbonate, while the lens mount  28  is formed from unfilled polycarbonate. The refocusing distance ranges from around 0.10 mm to about 0.15 mm for the projection television lens assembly  10  shown assuming a temperature in the region of the “C” lens element  16  of about 60° C.-65° C. when the television set has been in continuous operation. 
     FIGS. 5A and 5B illustrate one of many possible alternative embodiments of the invention in the form of a lens assembly  100 . Lens assembly  100  includes an outer focus mount  102  rigidly affixed to a suitable support structure  104  and an inner lens mount  106 . Focus mount  102  and lens mount  106  are each preferably cylindrical, but more generally tubular in shape, and extend along a longitudinal axis  108 . Lens mount  106  is received within focus mount  102  and is secured thereto by threads  110  which allow adjustment between the relative axial positions of focus mount  102  and lens mount  106 . This moves lens mount  106  toward and away from a light emitting, or generating component such as a prism assembly  112  schematically shown in FIGS. 5A and 5B. The initial focus position of lens mount  106  is locked into place using a locking ring  114  which bears against an end  102   a  of focus mount  102  and threads onto lens mount  106 . A series of lens elements  116 ,  118   a ,  118   b  are secured within lens mount  106  and are designed and configured to achieve any desired light or image transmission purpose. The configuration, design construction material, and number of lens elements  116 ,  118   a ,  118   b  shown therefore are not to be taken in any limiting sense. An outer “A” lens element  118   a  and an outer “B” lens element  118   b  may be formed of a plastic material, such as acrylic. The remaining “A” and “B” lens elements  116  may be formed from glass. Suitable retaining elements  120  are used to secure the “A” and “B” lens elements  116 ,  118   a ,  118   b  in place within lens mount  106 . This lens assembly  100 , like the first embodiment, is illustrative only and may be changed in terms of configuration and materials of construction according to the needs of a particular application. 
     FIG. 5A illustrates the unheated state of lens assembly and respective distances L 1 , L 2 , L 3 . FIG. 5B illustrates the heated condition (for example, 60° C.-65° C.) in which “B” lens element  118   b  has distorted (in exaggerated form) outwardly toward light transmitting or generating element  112 . Under typical circumstances, this would change the distance L 1  and thereby potentially distort an image being projected through lens assembly  100 . In accordance with the invention, however, lens mount  106  is formed of a material, such as aluminum, having a CTE less than the material forming focus mount  102 . In this example, focus mount  102  may be formed from a plastic material such as Cycolac KJW available from General Electric Plastics, Pittsfield, Mass., and having a CTE of 11.2×10 −5  cm/cm/C. Aluminum has a CTE of 2.4×10 −5  cm/cm/C. Thus, focus mount  102  will expand from support structure  104  to a length L 2 ′ from its original length L 2 , as shown in exaggerated form in FIG.  5 B. This will carry lens mount  106  to the left as viewed in FIG. 5B such that distance L 3  becomes a shorter distance L 3 ′. The result is that the distance between the outer surface of “B” lens element  118   b  and light transmitting or generating element  112  preferably stays the same or changes only slightly. That is, L 1  equals or approximately equals L 1 ′. It will be appreciated that under heated conditions, any movement of lens  118   b  to the left, as viewed in FIG. 5B, will be beneficial to the ultimate image transmission through lens assembly  100  even if the original L 1  distance is not fully achieved. 
     While the present invention has been illustrated by a description of a preferred embodiment and while this embodiment has been described in some detail, it is not the intention of the Applicants to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The various features of the invention may be used alone or in numerous combinations depending on the needs and preferences of the user. This has been a description of the present invention, along with the preferred methods of practicing the present invention as currently known. However, the invention itself should only be defined by the appended claims, wherein