Patent Publication Number: US-2007110383-A1

Title: Light integrator having two adhesives

Description:
RELATED APPLICATIONS  
      U.S. patent application Ser. No. 10/887,052, filed Jul. 8, 2004 and U.S. Provisional Patent Application Ser. No. 60/486,521, filed Jul. 11, 2003 are incorporated herein by reference and benefit there from is hereby claimed. 
    
    
     TECHNICAL FIELD  
      The present invention relates to a light handling device that includes a plurality of separate substrate walls.  
     BACKGROUND OF THE INVENTION  
      A typical light integrator includes reflective walls that transmit light in a hollow area of the tunnel via multiple reflections to the output. A homogeneous light distribution is achieved at the output because of the multiple reflections. In particular, the separate walls are assembled together in such a way that a tunnel is formed. The walls can be reflective before being assembled or can be made reflective after the assembly step. Such a light integrator can be used in a light engine for an image projection apparatus. One important parameter of such a light engine is the brightness of the projector. In order to realize bright images strong light sources need to be used. The sources itself as well as the elements in the optical path need to be able to withstand high intensities without being destroyed. Especially the light integrator needs to be able to withstand high amount of light intensity because all of the illumination light needs to transmit through the light integrator and the cross-section of such a light integrator is relative small. Even a small amount of absorption within the light integrator, which normally cannot be avoided, leads to high temperatures. Temperatures equal to or higher than 150° C. can be reached in such applications. Therefore, the light integrator has to be temperature resistant.  
      A known method for producing such an optical element is to glue the separate planar substrate walls together at the non-optically active surfaces. Typically a UV curing adhesive is used, since this is a fast setting/curing adhesive and therefore allows an easy and inexpensive manufacturing process. In order to perform the assembly, the separate substrate walls are mechanically fixed and the adhesive is applied at the surfaces of the substrates that are not relevant for the optical performance. The mechanical fixation during the gluing guarantees that the relative positions of the substrates are maintained during the complete curing process. After the curing process, when the mechanical strength of the adhesive connection is sufficient in order to stabilize the light integrator, the mechanical fixation is released. If a UV curing adhesive is used, the necessary mechanical strength of the light integrator is reached in a very short time. Typical curing times for commercial available UV curing adhesives are in the range of 5 seconds to 60 seconds. Unfortunately, assemblies based on UV curing adhesives are not temperature resistant for the light intensities as they are more and more used today in image projection systems and therefore they cannot be used for these applications.  
      In order to realize more temperature resistant assemblies, other types than UV curing adhesives can be used, such as one or two component epoxies, silicone based adhesives, ceramic epoxies, and inorganic adhesives (cements). Of particular interest are ceramic epoxies and inorganic adhesives despite there brittleness. The brittleness of these adhesives gives the impression that they are not suitable for high temperature applications where also severe temperature cycles can occur. However, the coefficient of thermal expansion (CTE) matches closely the CTE of the substrates typically used for light integrators. They provide in general a temperature stable connection between the single substrates. Typical curing schemes for above-mentioned adhesives are exposure to high temperature, humidity or chemical reaction, or initiated by mixing of two components. Unfortunately the curing time of such adhesives is quite long and the time the assembly needs to stay in the mechanical fixation for stabilization increases dramatically. Typical curing times for the above mentioned adhesives are in the range from 1 hour to 24 hours. From a manufacturing point of view such a production method is therefore very expensive and not interesting.  
      Another approach to realize high temperature resistant optical assemblies is described in WO 01/14923 A1. In this approach a shrink tube is used to fix the single substrates mechanically. Another possibility would be to use one or several adhesive bands wrapped around the assembly, as described in DE 202 17 720. Unfortunately for this method in order to prevent the assembly from collapsing a special shape of the edges of the single substrates is required. Because of this special shape, the integrator cannot collapse and the single substrates are pressed together with a shrinkable tube. It is clear that such a solution has the disadvantage that the mechanical treatment of the edges is difficult and expensive.  
      Starting from the disadvantages of the methods of the prior art it is the goal of the present invention to provide a possibility to realize temperature stable assemblies not comprising the disadvantages of the prior art such as difficult and/or expensive production processes not suitable for cost efficient mass production.  
     SUMMARY OF THE INVENTION  
      The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.  
      In accordance with one aspect, the present invention provides a light handling device tolerant of elevated heat and heating cycles. The device includes a plurality of separate walls bounding a surrounded area, the walls reflecting light transmitted through the bounded area. The device includes a first composition adhesive engaged with at least two of the walls for securing the two walls together, and a second, different composition adhesive engaged with the same at least two of the walls for securing the two walls together. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The foregoing and other features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings, in which:  
       FIG. 1  is a schematic side view illustration of an example light engine or image projector that may include structure in accordance with the present invention;  
       FIGS. 2A-2D  are schematic end face illustrations that show steps of assembly of a light integrator;  
       FIG. 3  is a schematic perspective illustration showing two different adhesives applied to two walls of a light integrator to secure the walls in accordance with one aspect of the present invention;  
       FIG. 4  is a schematic end face illustration of a light integrator with at least a first adhesive and a first mechanical device, which includes a bracket and a biasing spring, for securing the walls; and  
       FIG. 5  is a schematic end face illustration of a light integrator with at least a first adhesive and a second mechanical device. 
    
    
     DESCRIPTION OF EXAMPLE EMBODIMENTS  
       FIG. 1  shows a typical use of a light integrator  208  in an image apparatus  200 . In one specific example, the image apparatus  200  is a light engine or image projector. It is to be appreciated that the present invention may be embodied within the light integrator  208 .  
      Although the present invention can be utilized within the light integrator  208 , it should be appreciated that the shown examples of the light integrator  208  and the image apparatus  200  are not limitations on the present invention. It should be understood that the present invention could be utilized within other light integrators that are otherwise used. For example, any suitable size, shape, or type of elements or materials could be used in conjunction with the present invention.  
      Turning back to the shown example, the light integrator  208  receives light from a light source  203 . The light emitted by the source  203  is collected by a reflector  202  and focused (i.e., focused light  206 ) into the entrance of the light integrator  208 . In the shown example, a lens  204  provides for the focusing of the light from the source  203  into the light integrator  208 . The goal of the reflector  202  and the lens  204  is to focus all light emitted by the source  203  into the entrance of the light integrator  208 . Very high energy densities occur within the light integrator  208  because of the concentration of the focused light  206  within the small cross-section of the light integrator  208 . It is to be noted that a high energy density is associated with a high temperature.  
      During passage through the light integrator  208 , the light rays undergo multiple reflections, which cause a homogeneous light distribution at the exit of the light integrator  208 . The homogeneous light at this exit is imaged onto a light valve  212  by means of a relay optic  210 . At the light valve  212  the light is modulated according to the image information. This modulation can be angular modulation if for example DMD panels are used, or polarization modulation if for example LCD based light valves are used. This modulation is translated into an intensity modulation by either the relay optic  210  and/or additional relay optics (not shown). In order to produce an image, the surface of the light valve  212  is often imaged onto a screen with the help of projection optics (the screen and projection optics are not shown). For some applications, a color-sequential method that employs only one light valve is used. In this case a color wheel and/or an optical switch is positioned in the light path somewhere between the light source  203  and the light valve  212 . In other applications, two or more light valves are used and the light is split into a corresponding number of color channels.  
      A major performance criterion of the image apparatus is the brightness of the projected image. To increase the brightness, the power of the light source  203  is correspondingly increased. This leads to higher light densities and therefore higher temperatures in the light integrator  208 . This requires assembly methods of the light integrator  208  that can withstand permanently high temperatures and temperature cycles.  
       FIGS. 2A-2D  show steps of a first example of assembly steps for producing a light integrator  300 . In short summary,  FIGS. 2A-2D  show steps of a method that includes (a) arrangement of substrate walls into jigs, (b) positioning according to desired shape, (c) application of glue, and (d) removing from the mechanical frame.  
      As shown in  FIG. 2A , highly reflective separate substrate walls  304 A,  304 B,  306 A, and  306 B are placed and aligned into mechanical jigs  302 A,  302 B,  303 A, and  303 B. In one example, the walls have a reflective coating on an inwardly directed side. Within the shown example, each wall (e.g.,  304 A) within each opposed pair of walls (e.g.,  304 A and  304 B) is similar to the opposed wall with regard to engagement with the other pair of walls (e.g.,  306 A and  306 B).  
      The walls  304 A,  304 B,  306 A, and  306 B are held in place on the jigs  02 A,  302 B,  303 A, and  303 B, respectively, with the aid of vacuum. With the jigs  302 A,  302 B,  303 A, and  303 B engaging only the outer side of the walls  304 A,  304 B,  306 A, and  306 B, damage to the reflective coating on the inwardly directed side is prevented. The next step is to move the jigs  302 A,  302 B,  303 A, and  303 B inwards such that the walls  306 A and  306 B press against the walls  304  and  304 B (see  FIG. 2B ). The jigs  302 A,  302 B,  303 A, and  303 B move the sidewalls into place in such a way that the requested dimensions of the finished light integrator  300  are obtained. To achieve the required tolerances of typically 0.1 mm or less, the jigs  302 A,  302 B,  303 A, and  303 B have to be very precise. This applies as well to the mechanism (not shown), which moves the jigs  302 A,  302 B,  303 A, and  303 B. Such a precise mechanism is very expensive and therefore short cycle times are desirable.  
      With the walls  304 A,  304 B,  306 A, and  306 B in their final positions (as shown in  FIG. 2B ), the walls have to be kept in this position during application of a fast-set adhesive  308  ( FIG. 2C ). It is to be appreciated that any suitable fast-set adhesive  308  may be used. Within this step of the shown example, the fast-set adhesive  308  is applied with the aid of dispensers (not shown) and is applied along at least a portion of the intersection edges of the walls  304 A,  304 B,  306 A, and  306 B (i.e., exterior corners of the light integrator  300  being produced).  
      After application, the fast-set adhesive  308  must be allowed to set (e.g., cured to the point of retention). In the case of a UV curing adhesive, the adhesive  308  is exposed to intensive UV light for a sufficient time (e.g., 5 to 60 seconds). The set or cured adhesive  308  now maintains the structure of the light integrator  300  and the jigs  302 A,  302 B,  303 A, and  303 B are released (see  FIG. 2D ). The final-form light integrator  300  is removed out of the jigs  302 A,  302 B,  303 A, and  303 B and passed forward for further processing. The jigs  302 A,  302 B,  303 A, and  303 B are now ready to process the next light integrator.  
      It is to be appreciated that the fast-set adhesive  308  is selected for the feature of having a fast set time. Other characteristics of the fast-set adhesive, such as heat resistively, may be of secondary consideration as will be understood upon further appreciation of the remaining steps of the present invention. Specifically, according to the present invention, after the steps of fixing the walls  304 A,  304 B,  306 A, and  306 B in place with a fast-set adhesive, as shown in  FIGS. 2A-2D , a second step is performed which is a reinforcement step.  
      In accordance with an aspect of the present invention, the reinforcement step includes the application of a second adhesive  410  that ensures, for example, the durability in high temperature environments. The use of the second adhesive  410  is shown in the example of  FIG. 3 . Only two intersecting walls  404  and  406  (e.g., forming one corner of a light integrator) are shown. The walls  404  and  406  correspond to any of the intersecting pairs of walls (e.g.,  304 A and  306 A) discussed with regard to the steps shown in  FIGS. 2A-2D . However, it is to be appreciated that the other intersections of walls of the light integrator will be similarly adhered.  FIG. 3  shows a fast-set adhesive  408  (which corresponds to the fast-set adhesive  308  shown within  FIGS. 2A-2D ) and the second, reinforcing adhesive  410 . The setting or curing time of the second, reinforcing adhesive  410  is not critical since no expensive jigs are required to maintain the final shape of the light integrator. Therefore, the second, reinforcing adhesive  410  may have a setting or curing time in the range of 1 minute to 24 hours.  
      With regard to adhesives, one or two component epoxies, silicone based adhesives, ceramic epoxies, and inorganic adhesives (cements) have relatively good performance in applications where parts are exposed to high temperatures and/or temperature cycles as compared to UV curing adhesives. Thus, such example materials may be used. In one example, the first adhesive has a composition that results in a first strength rating and the second adhesive has a composition that results in a second, different strength rating, with the second strength rating being greater than the first strength rating. In another example, the first adhesive has a composition that results in a first tolerance of elevated heat and the second adhesive has a composition that results in a second, different tolerance of elevated heat, with the second tolerance of elevated heat being greater than the first tolerance of elevated heat. In yet another example, the first adhesive has a composition that results in a first tolerance of heating cycles and the second adhesive has a composition that results in a second, different tolerance of heating cycles, the second tolerance of heating cycles being greater than the first tolerance of heating cycles. In one specific example, the first, fast-set (fast-curing) adhesive is a commercially available product: DYMAX 4-20623 (UV curing glue). In another specific example, the second, slower setting (slower-curing) adhesive is a commercially available product: EMERSON ECCOBOND 104 (heat curing).  
      There are different possibilities of geometrical shapes for the applied adhesives. For example, the fast-set adhesive  408  for fixation can be provided as glue spots. Within a further example, the glue spots can be applied in the middle of the light integrator or toward both ends. The second, reinforcing adhesive  410  can be applied between the glue spots or along the whole edge of the light integrator. Within another example, the fast-set (fixation) adhesive can also be applied over the whole length of the light integrator and the second, reinforcing (temperature durable) adhesive is applied on top of the fast-set adhesive. Various other schemes for the application of a fast-set adhesive and a second, reinforcing adhesive are contemplated. Thus, the second adhesive can overlay some or all, (i.e., at least a portion) of the first adhesive.  
      The stability of the light integrator can be further improved by shaping in addition the edges of the walls. Such shaping as for example forming steps in at least some of the edges can provide by itself mechanical stability to the light tunnel geometry. With the fast fixation step this stability can be fixed and the light integrator can be easily handled. With the reinforcement step provided by the second adhesive, the light integrator becomes temperature stable. Because of the prefixing by the fast-set adhesive (e.g., a UV curable adhesive), the edges of the walls need not to be shaped in a special form to prevent the light integrator from collapsing during curing. If a planar edge surface is preferred, the edge could have a final roughness in order to increase the bonding ability of the applied adhesive(s).  
      In addition to the procedure described above there are different additional steps, which may provide another means of securing the walls together that is resistant to degradation caused by elevated heat and heating cycles. For example, the second, reinforcing (temperature durable) adhesive can be applied while the light integrator is still in the jig. The setting or curing for the reinforcing, durable adhesive may be done after the light integrator is removed from the jig. It is also conceivable to apply the fixation adhesive, and before curing it, to apply the reinforcing, durable adhesive. After both adhesives are applied, a pre-curing is done with the light integrator held in the jig and the curing for the reinforcement adhesive is finished upon removal of the light integrator from the jig. A preferred method is to apply both the fixation adhesive and the reinforcing, durable adhesive simultaneously with a dispensing aid, which ensures that both adhesives get in contact with the planar faces of the walls to be joined. It may be required that such a dispensing aid be configured and used such that mixing of the two adhesives is avoided.  
      It is to be appreciated that other structures may also be associated with the light integrator and may be used in conjunction with the use of adhesives. For example, another means of securing the walls together that is resistant to degradation caused by elevated heat and heating cycles includes the use of a bracket on the outside of a light integrator  500 . One specific example of such a bracket is shown in  FIG. 4 . It is to be noted that walls  504 A,  504 B,  506 A, and  506 B generally correspond to the walls  304 A,  304 B,  306 A, and  306 B of the light integrator provided by the steps shown within  FIGS. 2A-2D . In this example, the light integrator  500  is arranged within a first or lower, U-shaped tube  501  of the bracket. Additionally, a second U-shaped tube  503  of the bracket is put on top of the light integrator  500 . The second U-shaped tube  503  is fixed to the first U-shaped tube  501  with a spring-loaded retaining mechanism  510  and  512  associated with the bracket. In the shown example, the spring-loaded retaining mechanism includes a retainer  510  and a spring  512 . The spring-loaded mechanism is preferably applied in such a way that in the light integrator  500  no force is applied to adhesive(s)  508  (which represents at least one of the adhesives  408  and  410  shown within  FIG. 3  and may represent the second adhesive overlying some or all of the first adhesive). The force from the spring-loaded mechanism  510 ,  512  is distributed in the light integrator  500  like in a force fit. Force  514  from one pair of walls  506 A and  506 B is transferred directly to the other pair of walls  504 A and  504 B. In such a configuration, the light integrator  500  cannot collapse.  
      There are different possibilities for the layout of such a bracket and spring loaded mechanism. For example, the light integrator could be glued in the bracket with a temperature-curable adhesive, a high temperature resistant silicone or an organic adhesive. The bracket could cover a whole single substrate wall or only part of it, such as for example only the front and end of the light integrator. If this is the case, the bracket, together with the spring loaded mechanism could additionally provide aperture shields for preventing the light to enter the substrate walls themselves and protect the mirror coating. On the other hand it would be sufficient if the bracket only is in the middle of the light integrator.  
      In another example, another means of securing the walls together that is resistant to degradation caused by elevated heat and heating cycles includes the use of a shrink tube. One specific example is shown in  FIG. 5 . A light integrator  600  has a first pair of substrate walls  602 A and  602 B and a second pair of substrate walls  604 A and  604 B (generally corresponding to the walls  304 A,  304 B,  306 A, and  306 B of the light integrator provided by the steps shown within  FIGS. 2A-2D ). The walls  602 A,  602 B,  604 A, and  604 B are assembled in such a way that they form a hollow rectangle. Adhesive(s)  608  (which represents at least one of the adhesives  408  and  410  shown within  FIG. 3  and may represent the second adhesive overlying some or all of the first adhesive) is/are applied at the outer edges of the contact lines of the walls  602 A,  602 B,  604 A, and  604 B as described above. In order to increase the high temperature durability a shrink tube  610  is applied to the light integrator  600  and is shrunk into form-fitting arrangement around the light integrator  600 .  
      In one specific example, the walls  602 A,  602 B,  604 A, and  604 B are assembled in such a way that the outer surfaces of the walls  602 A and  602 B are within the rectangle described by the outer surfaces of the walls  604 A and  604 B. With such an arrangement, no force is directly applied by the shrink tube  610  to the walls  602 A and  602 B.  
      The examples presented above mainly focus on rectangular-shaped light integrators. However it should be noted that the scope of the present invention is by no means restricted to this application. The same basic ideas could be applied for example to light integrators which do not have a rectangular shape. All kinds of geometries such as a trapezoidal shape or the shape of a parallelepiped could be used. In addition, the light integrators could have a tapered form.  
      More generally, the present invention is not restricted to light integrators. This invention could be useful for any optical assembly comprising a number of substrate walls to be assembled where a fast preassembly is required and a second reinforcement fixation is needed.  
      What has been described above includes exemplary implementations of the present invention. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the present invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the present invention are possible. Accordingly, the present invention is intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims.