Abstract:
A color wheel for use in projector systems comprises a disc shaped hub, rotatable around a central axis and color filter segments arranged radially to said central axis and affixed to said hub. This color ring is arranged concentrically with respect to said axis and said segments&#39; contour comprising an outer arc, an inner arc and two radial parts. Each of the radial parts comprises a recess arranged such that during assembly of said segments pairs of neighboring recesses form a hole which again engages in a corresponding peg on the hub in a form fitting manner.

Description:
FIELD OF THE INVENTION  
       [0001]     This disclosure relates to a rotatable color wheel for use in projector systems to enable color sequential illumination of an image panel  
       BACKGROUND OF THE INVENTION  
       [0002]     Most of the image projection apparatus for producing colored images on a screen either only comprise one image panel: In order to form a colored image the panel needs to be sequentially illuminated with light beams of different colors. If the sequence is displayed fast enough the human eye is not able to timely resolve the resulting color image sequence. As a result the impression of a colored image develops. One method to realize the required sequential illumination is to place dichroic filter segments secured on a rotatable hub in the path of a light beam. Since the rotation of such a color wheel needs to be quite fast, strong centrifugal forces act on the dichroic filters and the means for securing them to the rotatable hub need to provide sufficient stability.  
         [0003]     In most cases it is sufficient to use an adhesive only to attach the color filter segments to the hub. U.S. Pat. No. 5,868,482 (Edlinger) discloses such a color wheel comprising a disc shaped carrier and color filter segments attached to the periphery of the disc shaped carrier using an epoxy adhesive. Unfortunately the disclosure tells nothing about the alignment of these color filter segments to the disc shaped carrier. Alignment is crucial because due to the fast rotation the wheel needs to be well balanced and if the segments are not well aligned to the disc shaped carrier additional means for balancing are required. Each step of the assembly comprising applying adhesive, aligning the segments, curing the adhesive and balancing the wheel takes time and hardly allows for standardized or automated procedure.  
         [0004]     In addition in some cases it seems to be not sufficient to secure the segments by adhesive only. Sometimes in cases for which, based on force calculations, the “adhesive only” assembly seems to be not sufficient additional mechanical securing means are required. In such cases according to prior art, holes in the filter segments and screws are used to additionally secure the segments to the disc shaped carriers. Unfortunately it is a complicated procedure and the procedure to drill holes into glass and secure the segments with screws results in decreased durability of the segments.  
         [0005]     In U.S. Pat. No. 6,011,662 a color wheel is provided that has changeable filter elements. Here a hub has specific filter holding structures, including a spring element along the periphery of the hub. Each color element is mounted on a platform component. The platform component is shaped with concave edge elements that engage corresponding pegs on the hub and a downwardly extending nub that engages with a corresponding cavity in the hub. The platform component has a beveled insertion edge on the top side. The hub has a plurality of pegs and nub cavities. The color element is inserted into the hub and pressed down. The concave edge elements engage the pegs. The nub is inserted into the nub cavity. The color element is held in position on the hub by those structures as long as the bottom surface of the color element is pressed down against the hub. The spring element extends just past the beveled edge on the platform component of the color element. The color element can be lifted from the hub by first lifting the spring element.  
         [0006]     The U.S. Pat. No. 6,011,662 is intended to provide a color wheel with easily changeable, user-selected dichroic filters. As can be seen from the description above the system is rather complicated: Filter segments need to be secured to platforms, which comprise springs etc. Therefore this is a rather expensive solution and one skilled in the art would, in his intention to realize a simple and cheap assembly, avoid this way.  
       SUMMARY OF THE INVENTION  
       [0007]     It is the goal of the present invention to, at least partially, provide a solution for the above mentioned problems with prior art. Special focus is given to provide an assembly method, which results in self alignment of the segments and a single step freezing and therefore securing of the color filter segments to a hub. The resulting color wheel, assembled with this method, requires if any only minor balancing effort, is lightweight and cheap.  
         [0008]     In the solution according to the present invention a color wheel is provided with a hub. The color filter segments are formed out of a single piece body and are shaped with concave edge elements that engage in corresponding pegs provided on the hub or on additional means, which are used to axially secure the segments to the hub. In contrast to prior art solutions no additional platform is required to secure the color wheel segments to the hub. Such additional means can be for example a plastic ring. Securing is performed for example by chemical bond, mechanical snap mechanism ultrasonic welding or other suitable measure 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1   a  Prior art color wheel segment  
         [0010]      FIG. 1   b  Explosion view of prior art color wheel  
         [0011]      FIG. 1   c  Assembled prior art color wheel  
         [0012]      FIG. 2   a  Color wheel segment according to first embodiment of invention  
         [0013]      FIG. 2   b  Explosion view of color wheel assembly according to first embodiment  
         [0014]      FIG. 2   c  Assembled color wheel according to first embodiment of invention  
         [0015]      FIG. 3   a  Color wheel segment according to second embodiment of invention  
         [0016]      FIG. 3   b  Explosion view of color wheel assembly according to second embodiment of invention  
         [0017]      FIG. 3   c  Assembled color wheel according to second embodiment of invention with securing disc  
         [0018]      FIG. 3   d  Assembled color wheel without securing disc  
         [0019]      FIG. 4   a  Color wheel segment according to third embodiment of invention  
         [0020]      FIG. 4   b  Explosion view of color wheel assembly according to third embodiment of invention  
         [0021]      FIG. 4   c  Assembled color wheel according to third embodiment of invention, segments not yet secured to the hub  
         [0022]      FIG. 4   d  Assembled color wheel according to the wheel of  FIG. 4   c , securing performed by heat melting. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0023]     In the following the invention is described with the help of the figures in more detail. Several preferred embodiments are discussed. However in the next section a typical prior art solution will be described first in order to specifically point out the advantages of the present invention over prior art. The sequence of  FIG. 1   a  to  1   c  shows the assembly of the prior art solution.  FIG. 1   a  shows a color wheel segment  3  the contour comprising an outer circular arc  5  an inner circular arc  7  and two linear radial parts  9 , 9 ′. In this example the interpolation of the lines along the two linear radial parts  9 , 9 ′ to their crossing point include an angle φ of 120°. Therefore three of such segments  3 ,  3 ′ and  3 ″, as shown in  FIG. 1   b  form a complete color ring  19 . Each of the segments  3 ,  3 ′ and  3 ″ can comprise a color filter which differs from the others. For example segment  3  could comprise a blue transmission filter defined as transmitting wavelengths from 420 nm to 500 nm and reflecting the rest of the visible light, segment  3 ′ could comprise a red transmission filter, the filter defined as transmitting wavelengths from 600 nm to 680 nm and reflecting the rest of the visible light and the segment  3 ″ could comprise a green transmission filter, the filter defined as transmitting wavelengths from 500 nm to 600 nm and reflecting the rest of the visible light. The number of segments to form the ring can be different from 3 and wheels could be formed comprising segments comprising the identical optical filters. The angle φ may be chosen individually for each segment, as long as the sum of the angles equals 360° to form a ring. However for the purpose of the present invention it is sufficient to describe examples with 3 segments only which does not mean that the invention is limited to 3-segment assemblies.  
         [0024]     Referring further to  FIG. 1   b , which shows the explosion view of a prior art color wheel assembly, the color ring  19  is secured to a hub  11 . In some prior art solutions securing is performed gluing the segments to the hub, using an adhesive. Sometimes, as shown in  FIG. 1   b  the hub  11  comprises a cylindrical hole  13  and a securing disc  15  with a securing cylinder  17 . During assembly the securing cylinder  17  is pressed into the cylindrical hole  13  and as a consequence the securing disc  15  is penning in the segments  3 , 3 ′, 3 ″. In other words the color ring  19  is sandwiched between the hub  11  and the securing disc  15 . In this case the hub  11  as well as the securing disc  15  and the securing cylinder are made of metal in order to guarantee a good mechanical contact and to withstand the radial forces the segments exert on the contact between securing disc and segments or between hub and segments.  FIG. 1   c  finally shows the assembled color wheel  23  according to prior art. Please note that according to the example described, the color wheel segments comprised single piece bodies which were glued to the hub and sandwiched between the hub and a securing disc.  
         [0025]     The sequence for  FIGS. 2   a  to  2   c  shows segments, and assembly of a color wheel according to a first preferred embodiment of the present invention.  FIG. 2   a  shows a color wheel segment  103  with a contour that comprises an outer circular arc  105  and an inner circular arc  107  and two radial parts  109 ,  109 ′. Each of the radial parts  109 ,  109 ′ comprises at least one triangular shaped recess  110 ,  112 . When three of such color wheel segments  3 , 3 ′, 3 ″ are put together to form a complete color ring  119 , the pairs of neighboring recesses  110 ,  112 ′ of neighboring segments form a rectangular hole. The segments are placed on a hub in such a way that the recesses engage in corresponding pegs  150 ,  150 ′ and  150 ″ in a form fitting manner. With these pegs engaging in the recesses of the segments the segments are well aligned to the hub and no additional adjustment procedure is required. In the example as described here the height of the pegs  150 ,  150 ′  150 ″ is less than the thickness of the glass segments  103 ,  103 ′,  103 ″. Glass segments could be, for example 1.2 mm thick and pegs could be 1 mm in height. In the same way as already described for prior art solutions the hub  111  comprises a cylindrical whole  113  and the color ring  119  is sandwiched between a securing disc  115  and the hub  111 , whereby a securing cylinder  117  foreseen at the securing disc  115  engages and is pressed into the cylindrical hole  113 . In order to secure the disc to the hub an anchor principle can be used as option or in addition to pressing a securing cylinder  117  into a cylindrical hole  113 . Such an anchor principle requires an anchor  153  foreseen at the securing cylinder  117  and an additional recess  157  foreseen within the cylindrical hole  113  of the hub  111 . One of the advantages with using pegs  150 ,  150 ′ and  150 ″ which engage form fitting into the recesses  110 ,  110 ′  110 ″ and  112 ,  112 ′,  112 ″ is that the color wheel segments are by this automatically aligned to the hub. However, another, major advantage of the use of pegs and recesses is that once the segments are axially fixed to the hub by means of for example the securing disc  115 , the rather strong radial forces, when rotating the wheel as required will act mainly upon the pegs  150 ,  150 ′ and  150 ″. In other words the means for withstanding the strong radial forces are mechanical means in form of pegs foreseen on the hub  111  and these means are well separated from the means for axially securing the segments to the hub. This is why for example the hub  111  and/or the securing disc  115  together with the securing cylinder  117  and, if any, the anchor  153  could be formed out of plastic which is cheap to produce. In addition this allows an assembly without the use of adhesive.  
         [0026]     It is clear, that the pegs to engage into the recesses of the segments could be as an option, foreseen at the bottom side of the securing disc  115 . As already mentioned in this example the height of the pegs needs to be less than the thickness of the segments in order be able to effectively press the securing disc to the segments.  
         [0027]     This is very different with the second embodiment of the present invention as shown in the sequence of  FIG. 3   a  to  3   d . In order to give different examples of recesses, the segment as shown in  FIG. 203  comprises concave recesses located at the radial parts  209 ,  209 ′ of the its contour. Note that in this embodiment of the present invention the contour only comprises an outer circular arc, no inner circular arc is required, the inner part could be of any shape as long as the shape does not affect the stability or the optically usable zone. Three of such segments  203 ,  203 ′,  203 ″ are then put together as shown in  FIG. 3   b  to form a color ring  219  with the recesses forming approximately oval shaped holes. This assembling can be done on the hub  211  and the segment will be well aligned because of the pegs  250 ,  250 ′ and  250 ″. Special with this embodiment is that the pegs  250 ,  250 ′ and  250 ″ foreseen on the hub  211  themselves form oval cylinders with heads to act as anchors. This time the height of the oval parts of the pegs correspond to the thickness of the segments. If now the segments  203 ,  203 ′, and  203 ″ are placed on the surface of the hub  211  the pegs together with their heads act as anchors and sufficiently secure the segments to the hub, without using an additional securing disc. Such an assembled wheel is shown in  FIG. 3   d . In the  FIG. 3   b  shown are cleaved pegs  250 ,  250 ′ and  250 ″. However, the one skilled in the art knows different variations of this anchor principle to be applied here. It is for example well possible that rather than cleaving the pegs, the heads of the pegs are deformable in order to allow the assembly. As another option an additional securing disc  215 , comprising oval holes  216 ,  216 ′ and  216 ″ but this time not comprising a securing cylinder can be used as shown in  FIG. 3   b  and  FIG. 3   c .  FIG. 3   c  shows the finally assembled color wheel  223  comprising such a securing disc  215 . If the securing disc  215  is used the height of the pegs  250 ,  250 ′ and  250 ″ needs to be adjusted.  
         [0028]     In a third embodiment of the present invention, as shown in the figure sequence  4   a  to  4   d  semicircular recesses  310  and  312  are realized within the radial parts of the contour of the segment  303 . In  FIG. 4   b  three of such segments are put together and a hub  311  with pegs  350 ,  350 ′,  350 ″ is provided which, as shown in  FIG. 4   c  engage into the recesses of the segments which form circular holes. The height of the 350, 350′,  350 ″ is well above the thickness of the segments. Axial securing can then be performed for example just by melting the upper parts of the pegs, as shown in the readily assembled wheel in  FIG. 4   d.    
         [0029]     According to another aspect of the present invention the pegs engaging into the recesses could be used to facilitate assembly procedures. Especially, but not only, in cases where more than 3 segments are used to form the color ring it is quite important to place the correct neighboring segments next to each other. Incorrect placement of the segments results in low yield. In order to prevent these incorrect placements it is possible to give the recesses and the corresponding pegs on the hub or on the securing disc different specific geometrical forms. Only the correct neighboring segments may then be placed and assembled. Shown in  FIG. 5  is an embodiment, where the securing disc  415  comprises pegs in form of prisms, cylinders, cubes and different geometrical shape. (No references are shown for the pegs in order not to overload the figure) and the hub  411  comprises the corresponding recesses to let the pegs engage into the recesses. Shown in addition is another type of mechanical fixture. Some of the pegs of the securing disc comprise screws  480 ,  480 ′,  480 ″, to secure the securing disc to the hub.  
       REFERENCE NUMERALS  
       [0000]    
       
           3  color wheel segment  
           3 ′ color wheel segment  
           3 ″ color wheel segment  
           5  outer circular arc  
           7  inner circular arc  
           9  linear radial part  
           9 ′ linear radial part  
           11  hub  
           13  cylindrical hole  
           15  securing disc  
           19  color ring  
           23  color wheel  
           103  color wheel segment  
           103 ′ color wheel segment  
           103 ″ color wheel segment  
           105  outer circular arc  
           107  inner circular arc  
           109  linear radial part  
           109 ′ linear radial part  
           110  recess  
           110 ′ recess  
           110 ″ recess  
           111  hub  
           112  recess  
           112 ′ recess  
           112 ″ recess  
           113  cylindrical hole  
           115  securing disc  
           117  securing cylinder  
           119  color ring  
           123  color wheel  
           150  peg  
           150 ′ peg  
           150 ′ peg  
           153  anchor  
           157  recess  
           203  color wheel segment  
           203 ′ color wheel segment  
           203 ″ color wheel segment  
           210  recess  
           212  recess  
           209  radial part of contour  
           209 ′ radial part of contour  
           205  outer circular arc  
           211  hub  
           215  securing disc  
           216  oval hole  
           216 ′ oval hole  
           216 ″ oval hole  
           219  color ring  
           250  peg  
           250 ′ peg  
           250 ″ peg  
           261  head of peg  
           261 ′ head of peg  
           261 ″ head of peg  
           223  color wheel  
           303  color wheel segment  
           303 ′ color wheel segment  
           303 ″ color wheel segment  
           310  recess  
           312  recess  
           311  hub  
           319  color ring  
           350  peg  
           350 ′ peg  
           350 ″ peg  
           323  color wheel  
           403   i  color wheel segment  
           403   ii  color wheel segment  
           403   iii  color wheel segment  
           403   iv  color wheel segment  
           403   v  color wheel segment  
           403   vi  color wheel segment  
          hub  
           411  securing disc  
           415  color ring  
           419  screw  
           480  screw  
           480 ′ screw  
           480 ″ screw