O
`Ulllted States Patent
`[19]
`[11] Patent Number:
`5,777,796
`
`Burstyn
`[45] Date of Patent:
`Jul. 7, 1998
`
`USOOS777796A
`
`Illllllllllllflllllllllllllllllllllllllllllllllllllllllllllllllllllll
`
`[54] PENTAPRISM COMBINER/SPLIT'I‘ER
`
`Inventor: Herschel Burstyn. Platnsboro. NJ.
`[75]
`[73] Assignee: Delta America Ltd.. Fremont. Calif.
`
`[2” Appl. No.: 686,381
`[22]
`Filed:
`Jul. 25, 1996
`0023 27/14; GOZB 5/04
`[51]
`Int. Cl.‘
`359/634; 359/636; 359/834
`[52] US. Cl.
`[58] Field of Search ..................................... 359/634. 6361
`359/331. 834
`
`[56]
`
`1,319,292
`1.320.625
`2,202,257
`2,267,948
`3,905,684
`4,268,119
`4,784,469
`4,890,899
`4,974,178
`5,097,323
`5,179,658
`5,224,085
`5.404.437
`5.448551
`5,465,177
`5,619,284
`
`,
`References Cl‘ed
`U.S. PATENT DOCUMENTS
`
`
`
`10/1919 Kunz ....................................... 359/636
`..... 359/636
`11/1919 Kuuz ......
`. 359/636
`5/1940 Klaver
`12/1941 Rantsch ......
`359/636
`9/1975 Cook et a].
`. 359/834
`5/1981 Hartman]: ............ 359/834
`
`11/1988 Tsukada et al.
`..... 359/831
`1/1990 Aoki eta],
`.
`..... 359/831
`11/1990 lzeki et a1.
`.
`364/523
`358/60
`3/1992 Sato et a1.
`..
`
`..... 395/164
`1/1993 Izawa ct a1,
`
`6/1993 Shinkai eta]. .
`.. 369/4433
`
`4/1995 Nguyen ..............
`395/152
`
`9/1995 Miyagawa et al.
`..... 369/271
`..... 359/636
`11/1995 Bar-bier et a].
`
`4/1997 Magocs ......
`348/157
`
`
`
`5,664,141
`5,671,202
`
`9/1997 Yamarnuro
`9/1997 Brownstein et a1.
`
`.
`
`
`711/111
`..... 369/58
`
`Primary Examiner—Georgia Y. Epps
`.
`A-W-Wm’ EXOWMr—Pvclyn A Lester
`Attorney, Agent, or hirm—Skjewen. Morrtll, MacPherson.
`Franklin & Frtcl; DaVid W. Herd
`[57]
`ABSTRACT
`Céguiaififii’fieffitfifmfii ‘Lflgflgfi 1°31;3i“;
`systems. is described. The invention utilizes a unique pen-
`taprism core element, partially bounded by five faces. two
`each on either side of a plane of bilateral symmetry and one
`opposite. bisected by the plane of bilateral symmetry.'I\vo of
`the faces of the pentaprism are coated with dichroic films.
`Functioning as a light combiner. the pentaprism uses these
`dichroic surfaces and its geometry to interact with incoming
`Uri—color
`light beams which. after first passing through
`appropriately modulated light valves. are selectively trans-
`mitted and reflected such that all beams exit. axially aligned.
`from one side of the pentaprism. Functioning as a light
`splitter. the pentaprism uses the dichroic surfaces and its
`geometry to split a white light beam into its component
`primary colors and pass these colors out of the pentaprism
`Turning mirrors are used to direct the tri‘color beams into or
`out of the pentaprism region at the necessary angles for
`proper operation. The turning mirrors are configured to
`increase compactness of the optical engine while maintain—
`ing nearly equal path lengths for all of the tri—color beams.
`
`27 Claims. 2 Drawing Sheets
`
`16
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`XLNX-1004
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`

`

`US. Patent
`
`Jul. 7, 1998
`
`Sheet 1 of 2
`
`5,777,796
`
`
`
`FIG. 2
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`XLNX-1004
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`

`5.777.796
`
`]
`PENTAPRISM COMBINER/SPLITTER
`
`BACKGROUND AND SUMMARY OF THE
`INVENTION
`
`This invention pertains to what can be thought of as a
`dualvoperating—mode optical engine that is capable of func-
`tioning bidirectionally both as an optical combiner and as an
`optical splitter respecting the relationship between sovcalled
`white light and the primary components thereof—red. green
`and blue light. In particular. the invention relates to a unique.
`compact. single-piece pentaprism element which forms the
`core optical component of the engine and which functionally
`is compatible with the high speed of modern projection
`systems without requiring the use of problematic high—index
`glass.
`Conventional color combiners. for example. exist prima-
`rily in one of two forms—the ladder configuration and the
`so~called Phillips prism configuration. The standard ladder
`configuration uses dichroic elements oriented at 45-degrees
`to incoming light beams to achieve color combination. If the
`dichroic elements are implemented as dichroic films coated
`on plates. aberrations such as astigmatism and coma may
`become a problem Also. the typical high angle of incidence
`between such dichroic films and impinging light beams
`decreases both transmission and reflection efficiency. and as
`a result. the combiner becomes incompatible with the bright—
`ness requirement typically specified for a high~speed pro
`jector system. Similarly. a Phillips prism system. if config—
`ured without the use of an appropriate high~index glass. is
`incompatible with high-speed projection systems utilizing
`field lenses to achieve compactness due to total internal
`reflection ("FIR") failure.This can be overcome through the
`use of high-index glass prisms. but such prisms suffer
`several inherent problem. such as a tendency to chip. poor
`transparency in the blue range. and unwanted dispersion
`which characterizes the performance of high-index glass.
`A general object of the present invention. accordingly. is
`to offer a unique. compact. high~efficiency optical engine
`which avoids the kinds of drawbacks and difliculties just
`suggested above. and which olfers all of the advantages that
`are sought in relation to sophisticated high-speed projection
`systems.
`According to a preferred embodiment of the present
`invention. the engine proposed thereby employs. as its core
`element. a unique low-index glass penlaprism. on a pair of
`faces in which are coated dichroic films. combined with
`other optical elements that are arranged in a selected geo-
`metrical configuration which can achieve color separation/
`combination without
`the use of TR. and its attendant
`problems thereto. The body of the pentaprism element. in its
`interior. defines what
`is referred to herein as a light—
`modification region. alternatively called a light-integration
`and light~separation region. and in this region. where light
`beams impinge upon the two faces in the pentaprism which
`are dichroically coated. the angle of incidence on both sides
`of the faces is much lower than the typical 45°. In the
`embodiment specifically described and illustrated herein the
`angle of incidence is more in the nature of about 22W’.
`The novel pentaprism element proposed according to the
`invention is a single-piece element. alternatively called a
`solid prism assembly. and by virtue of that fact. offers the
`important is advantage that it contains no internal optical
`boundaries or discontinuities which require light beam tran-
`sition and the opportunity for potential problems.
`Combined in the overall engine with the dichroicaUy
`coated pentaprism element are plural light valves. three to be
`
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`specific. with one provided for eadi of the three primary
`beam colors of red. blue and green. These light valves sit in
`the appropriate paths for these beams. and are operable. for
`example under computer control. to respond to appropriate
`image—containing data streams that are relevant to each of
`these three colors.
`
`In addition to the several important features and advan—
`tages just mentioned above. the engine proposed by this
`invention. cored as it is by the unique pentaprism mentioned.
`afi‘ords the opportunity for selective construction of an
`engine with diflerent possible configurations. each of which
`promotes compact. efficient. and relatively low-cost con-
`struction.
`
`the use of the core. single—piece pentaprism
`Further.
`effectively reduces to zero the requirement for tedious and
`painstaking optical alignments of the type which attend the
`set-up and use of many prior an systems.
`These and other objects and advantages that are attained
`by the invention will become more clearly understood and
`apparent from a consideration of the accompanying drawv
`ings when viewed in light of the text material which now
`follows.
`
`DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a simplified isometric view illustrating a pre—
`ferred embodiment of the system in which. at the upper and
`left sides of this view. certain components present in the
`system are shown in dashed lines to afiord a clearer view
`than might otherwise exist of what has been referred to
`above as the central optical pentaprism element
`in the
`system.
`FIG. 2 is a view. on a slightly smaller scale than FIG. 1.
`taken generally from the lower left side of FIG. 1. with all
`of the main structural components in the system/engine
`shown in solid lines. FIG. 2 illustrates the system/engine of
`the invention being employed as a combiner ofred. blue and
`green light beams.
`FIG. 3. which is drawn on substantially the same scale as
`FIG. 2. pictures the system/engine being employed as a
`splitter of white light into red. blue and green components.
`FIG. 4 is a block/schematic diagram illustrating a projec—
`tor which utilizes two engines constructed in accordance
`with the present invention——with one functioning as a light
`splitter and the other functioning as a light combiner.
`FIG. 5 is a very simplified and schematic isometric view
`illustrating generally a configuration of the engine/system
`performing as a light combiner. wherein incoming red. blue
`and green light beams approach the engine from the same
`direction along parallel axes (which might be thought of as
`being contained generally in one plane). with emergent.
`combined white light exiting orthogonally in the same plane.
`
`55
`
`DETAILED DESCRIPTION OF PREFERRED
`EMBODIMENT OF THE INVENTION
`
`Turning now to the drawings. and referring initially to
`FIGS. 1. 2 and 3 collectively. indicated generally at 10 and
`10‘ are two versions (reverse—directionally related) of an
`optical system. also referred to herein as an optical engine.
`each of which is constructed in accordance with a preferred
`embodiment of the present invention. In quick summary.
`engines 10. 10' include. as what is referred to herein as a
`central element. a unique low-index glass pentaprisrn 12. on
`faces 12a and 12b in which are provided dichroic coatings
`140. 1412.
`respectively. These coatings are alternatively
`called dichroic surfaces and dichroic films. and as can be
`
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`5.777.796
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`3
`seen they are disposed at acute angles relative to one other.
`The dichroic surfaces function in a manner that will be
`described shortly.
`Engine 10 further includes first. second. and third con-
`ventionally available light valves 20. 18. 16. respectfully.
`alternatively called optical elements. light-modulation struc-
`tures and pixelating light-valve structure. No such light
`valves are present in engine 10'. Included in both engines 10.
`10' are three conventional
`turning minus. 22. 24. 26.
`alternatively called reflective surfaces and reflective
`coatings. whose functions also will shortly be described.
`Engines 10. 10' further include and define a first optical
`path segment [50 extending between faces 120 and 12b and
`normal
`to a plane of bilateral symmetry (that will he
`described shortly below) and second and third optical path
`segments 15!: 15c. respectively. disposed at oblique angles
`to that same plane of bilateral symmetry. Segment 15b
`extends as shown between face 12a and the plane of bilateral
`symmetry and segment 15c extends between face 12b and
`the plane of bilateral symmetry. The geometrical composite
`of optical path segments lSa. 15b. 15c takes the form of a
`right triangle.
`
`5
`
`10
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`15
`
`Focusing attention on FIG. 2 among these three figures.
`indicated by dash-dot line 28 is what has been referred to
`herein above as a plane of bilateral symmetry which char—
`acterizes a geometric feature of pentaprism 12. As can be
`seen in FIG. 2. on one side of this plane are a first and second
`face 124:. 12c. respectively. and on the other side are a third
`and fourth face 12b 12d. respectively. A fifth face 12: in the
`pentaprism is bisected by normally intersecting plane 28.
`Anotha way of looking at the central pentaprism of the
`invention is to envision an optical plane through which the
`red. blue. and green light beams transmit. best thought of as
`the plane of FIG. 2. Then. as can be envisioned from FIG.
`1. this plane would be in'bctween. and parallel to. spaced—
`apan end surfaces 13a and 13b. with faces 120. 12b. 12c.
`12d. and 12c being transverse to the optical plane. If the
`optical engine were being used as a combiner. for example.
`light would enter through three of the faces. be combined
`within the pentaprism. and exit a fourth face. thereby leaving
`a fifth face optically inoperative.
`Continuing with attention focused principally on FIG. 2.
`let us now consider how engine 10 performs as a light
`combiner. Through any suitable means and organization
`which is not relevant to the core of the present invention.
`green. blue and red incoming light beams are presented. and
`these are shown by dashed lines 30. 32. 34. respectively. As
`pictured in FIG. 2. these three beams are generally coplanar
`and lie within the plane of FIG. 2. Green beam 30. labeled
`G. enters from the left side of FIG. 2. passes (when
`permitted) through light valve 16. is reflected 90° by mirror
`24. is reflected again 90“ by mirror 22 into a path which
`parallels its incoming path. passes through face 12b and
`coating 14b (at a low angle of incidence-about 22V2°). and
`then passes directly. and straight through. the pentaprlsm as
`illustrated In connection with this remark about passing
`straight
`through the pentaprism. one will recognize that.
`with the engine of the invention being used as a combiner.
`what emerges from the pentaprism is a combined beam.
`axially aligned. that includes elements drawn from all three
`of the incoming beams. It is noted that the green. blue and
`red beams traverse an isosceles uiangular path within the
`interior region of the pentaprism. The length of a shorter side
`of this triangle is of length X. as indicated in FIG. 2. and
`represents the length of the traverse on the isosceles triangle
`of the green light beam.
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`Further with respect to the path followed by the beam of
`green light. measured from where the beam emerges from
`what can be thought of as the exit face of valve 16 (the right
`side of thereof is. FIG. 2) to the point along that path where
`it exits the pentaprism at face 120 this overall length is called
`the green-me path length. An interesting feature of the
`present invention is that the path lengths. so defined. for all
`three beams. including each of the other two beams (blue
`and red). are substantially identical.
`The beam of blue light 32. labeled B. enters angularly
`from the upper side of FIG. 2. passes (as permitted) through
`light valve 18. is reflected 90“ by mirror 26. passes through
`coating 14a and prism face 12a (at substantially the same
`low angle of incidence mentioned above). is reflected and
`tumed (at the same low angle of incidence) by dichroic
`coating 14b. and emerges from the side 12c of the pcntap—
`rism as a portion of the combined output beam. and along the
`same final portion of the path followed by the green beam.
`The beam of blue light may be seen to traverse two sides of
`the triangle described above. over a length thereon of
`approximately 2.4x. Reinforcing the notion that path
`lengths are equal.
`the overall blue-beam path length is
`measured from the point at which the beam emerges from
`the downstream side of valve 18 (the underside thereof in
`FIG. 2) to the point where the beam emerges from face 12c
`in FIG. 2.
`
`Completing a description of the “combining" perfor-
`mance now being described in relation to FIG. 2. red beam
`34. labeled R. enters from the bottom side of the figure.
`passes (as permitted) through light valve 20 to enter the
`underside of prism 12 in FIG. 2. is reflected and turned (at
`substantially the same low angle of incidence mentioned
`repeatedly above) by dichroic coating 14a. is reflected and
`turned again. and at the same low angle of incidence. by
`dichroic coating 14b. and then passes through prism face 12c
`to exit along the same final portions of the paths followed by
`the previously described green and blue beams -- thus to
`form the final. combined. constituent in the output combined
`beam The beam of red light may be seen to traverse all three
`sides of the triangle described above. over a length thereon
`of appoximately 3.4x. The overall path length followed by
`the red beam is defined as that length between the point at
`which the beam exits or emerges from the downstream side
`of valve 20 (its upper side in FIG. 2) to the point where it
`exits face 12c of prism 12.
`Having thus described how engine 10 performs as an
`optical combiner. let us turn attention now to FIG. 3 which
`shows the other. reverse mode of operation where engine 10
`functions as a light splitter. When engine 10' is employed in
`this mode. no light valves are required. and accordingly.
`FIG. 3 omits a showing of these valves. It does. however.
`contain the turning mirrors previously described. Those
`skilled in the art will recognize that no fresh. elaborate
`discussion is required to explain what really is occurring in
`the splitting activity pictured in FIG. 3. The white-light
`beam to be split enters as indicated by arrowhead 36 at face
`12c of prism 12. experiences internal low~angle of incidence
`reflection from coating 14b (with respect to the red and blue
`components) while 15 passing the green component through
`and out of the system via turning mirrors 22. 2A. The blue
`component passes through coating 14a to exit the system
`after striking turning mirror 26. and the red component exits
`downwardly in FIG. 3 through the prism after low-angle of
`incidence reflection from coating 14a. The exiting. separated
`light beams are alternatively called substantially monochro-
`matic light beams.
`With attention now directed to FIG. 4. and as was
`mentioned above in the description of the drawings. here
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`5.777.796
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`5
`
`there is pictured in block/schematic form an overall projec-
`tor which utilizes an engine 10' which performs as a splitter
`and an engine 10 which performs as a combiner. Progressing
`through this figure from the left to the right sides therein.
`what is here pictured in block form is a white light source 38
`optically coupled to an engine 10' which is labeled “SPLIT-
`TER“ and which is organized in accordance with what is
`pictured in FIG. 3. another engine 10 which is labeled
`“COMBINER” and which is constructed in accordance with
`the organization pictured in FIG. 2. and a conventional
`optical stack 40 which includes atypical projection lens that
`sends a projected image to the outside world.
`Interposed engines 10'. 10. and indicated by a block 42.
`are three light valves. such as previously mentioned valves.
`16. 18. 20. whose operations are under the conventional
`control of block 44 which functions as a light modulation
`controller. which might typically be a computer. micropro—
`cessor or other suitable control element.
`
`The simplicity of such a projection organization afforded
`by utilization of engines 10. 10' will be apparent.
`FIG. 5 in the drawings illustrates in highly schematized
`way how an engine 10". constructed in accordance with the
`present invention. might be employed with suitable turning
`mirrors to allow for red. blue and green input beams to enter
`the engine along parallel axes generally directed upwardly in
`a common plane. and be combined to emerge generally
`orthogonally within the same plane as indicated at 46.
`Engine [0" is like engine 10 except for the necessary and
`different organization of is turning mirrors.
`Accordingly. a preferred embodiment of the present
`invention has been described in organirations allowing for
`both modes of operation. splitting and combining. and the
`obvious simplicity and elegance of the high efficiency
`pentaprism are made evident by the description and draw-
`ings. Low-index glass or even plastic in the prism offers all
`of the advantages discussed earlier. and the construction and
`geometry of the pentaprism lead to low angles of incidence
`for transmission and reflection. which translates into high
`optical efficiency. Internal optical activity that might cause
`problems were there to be internal optical discontinuities in
`the prism are avoided. inasmuch as the prism is a single-
`piece unit. That is not to say. however. that the penraprism
`can’t successfully be implemented as a system of dichroic
`films mounted on transmissive plates arranged in empty
`space. Since the angles of incidence and reflection are low
`(about 22%“) it is possible to construct a combiner/splitter
`which has relatively low polarization sensitivity (i.e.. the
`band edges of the dichroic layers don‘t shift when there is a
`change made from S polarization to P polarization). Thus.
`the proposed structure can be used with a polarizing cube in
`conjunction. if desired. with reflective light valves.
`While several organizations of the engine have been
`disclosed with turning mirrors specifically illustrated in
`certain locations. it will be apparent to those skilled in the art
`that other specific turning-mirror organizations. such as the
`one alluded to for engine 10". can be used to take advantage
`of the engines capabilities and to afford opportunities for the
`construction of systems having different footprints and con—
`figurations. For example. the organization shown in FIG. 5.
`and under a circumstance where the entering red. blue and
`green beams enter engine 10"upwardly and substantially
`vertically. a system obviously is constructablc which can be
`thought of as having a high vertical aspect ratio and a
`relatively tiny footprint. Also. an alternative embodiment
`might include the use of solid prism assemblies with exter-
`nal reflective coatings instead of turning mirrors.
`
`6
`Thus. while a preferred embodiment and certain sug~
`gested modifications of the invention have been illustrated
`and described herein. other variations and modifications that
`come to the minds of those skilled in the art are appreciated
`and will clearly come within the scope of the claims cov-
`ering the present invention.
`It is claimed and desired to secure by Letters Patent:
`1. An optical device for processing light beams. the device
`comprising:
`a first optical interface. wherein the first optical interface
`is configured to transmit light beams of a first color and
`to reflect light beams not of the first color;
`a first light beam structure having a first end and a second
`end. the second end of the first light beam structure
`being operatively coupled to the first optical interface.
`wherein the first light beam structure is configured to
`guide a substantially monochrome light beam of the
`first color in a path intersecting the first optical
`interface. wherein the substantially monochrome light
`beam of the first color is transmitted through the first
`optical interface;
`a second optical
`interface. wherein the second optical
`interface is configured to transmit
`light beam of a
`second color and to reflect light beams of not of the
`second color;
`a second light beam structure having a first end and a
`second end. the second end of the second light beam
`structure being operatively coupled to the second optii
`cal interface. wherein the second light beam structure is
`configured to guide a substantially monochrome light
`beam of the second color in a path intersecting the
`second optical
`interface. wherein the substantially
`monochrome light beam of the second color is trans-
`mitted through the second optical interface; and
`a third optical interface. wha'ein the third optical interface
`is configured to transmit light beams of a third color
`and to reflect light beams not of the third color.
`wherein the first. second and third optical interfaces are
`disposed to define three faces of a pentaprism so that a
`first optical path between a fourth face of the pentap—
`rism and the first end of the first light beam structure
`has a number of reflections equal
`to a number of
`reflections of a second optical path between the fourth
`face of the pentaprism and the first end of the second
`light beam structure.
`2. The optical device of claim 1 wherein the first optical
`path has two reflections.
`3. The optical device of claim 1 wherein the first. second
`and third optical interfaces are disposed so that a third
`optical path between the third optical
`interface and the
`forn1h face of the pentaprism has a number of reflections
`substantially equal to the number of reflections in the first
`optical path.
`4. The optical device of claim 3 wherein the first. second
`and third optical paths have a substantially equal number of
`transmissions through dichroic layers.
`5. The optical device of claim 1 wherein the first light
`beam structure comprises a pair of substantially parallel
`reflective surfaces.
`6. The optical device of claim 5 wherein the pair of
`substantially parallel reflective surfaces are disposed so that
`a light beam of the first color traveling along the first optical
`path is reflected twice within the first light beam structure
`and transmitted through the first optical interface.
`7. The optical device of claim 6 wherein the first optical
`path has a segment without reflection within the pentaprism
`between the first optical interface and the fourth face of the
`pentaprism.
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`5.777.796
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`8. The optical device of claim 1 wherein the second light
`beam structure comprises a reflective surface disposed so
`that a light beam of the second color traveling along the
`second optical path is reflected once within the second light
`beam structure and transmitted through the second optical
`interface.
`9. The optical device of claim 8 wherein the second
`optical path has a first segment within the pentap'ism
`between the second optical interface and the first optical
`interface and has a second segment within the pentap'ism
`substantially equivalent to the segment within the pentap—
`rism of the first optical path.
`10. The optical device of claim 8 wherein the second
`optical path includes a reflection within the pentaprism off of
`the first optical interface.
`11. The optical device of claim 1 wherein the third optical
`path includes two reflections within the pentaprism
`12. The optical device of claim 11 wherein the third
`optical path includes a reflection of? of the first optical
`interface and a reflection oflr of the second optical interface.
`13. The optical device of claim ll wherein the third
`optical path includes:
`a first segment between the third optical interface and the
`second optical interface:
`a second segment between the second optical interface
`and the first optical interface; and
`a third segment between the first optical interface and the
`founh face of the pentaprism
`14. An optical device for processing light beams.
`device comprising:
`first optical means for transmitting light beams of a first
`color and to reflect light beams not of the first color;
`first reflecting means for directing a substantially mono—
`chrome light beam of the first color in a path intersect-
`ing the first optical means. wherein the substantially
`monochrome light beam of the first color is transmitted
`through the first optical means;
`second optical means for transmitting light beams of a
`second color and to reflect light beams of not of the
`second color;
`
`the
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`second reflecting means for directing a substantially
`monochrome light beam of the second color in a path
`intersecting the second optical means. wherein the
`substantially monochrome light beam of the second
`color is transmitted through the second optical means;
`and
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`45
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`third optical means for transmitting light beams of a third
`color and to reflect light beams not of the third color.
`wherein the first. second and third optical means are
`disposed to define three faces of a pcntaprism so that a
`first optical path between a fourth face of the pentap—
`risrn and a first end of the first reflecting means has a
`number of reflections equal to a number of reflections
`of a second optical path between the fourth face of the
`pentaprism and a first end of the second reflecting
`means.
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`15. The optical device of claim 14 wherein the first optical
`path has two reflections.
`16. The optical device of claim 14 wherein the first.
`second and third optical means are disposed so that a third
`optical path between the third optical means and the fourth
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`8
`face of the pentaprism has a number of reflections substan—
`tially equal to the number of reflections in the first optical
`path.
`17. The optical device of claim 16 wherein the first.
`second and third optical paths have a substantially equal
`number of transmissions through dichroic layers.
`18. The optical device of claim 14 wherein the first
`reflecting means comprises a pair of substantially parallel
`reflective sm'faces.
`
`19. The optical device of claim 18 wherein the pair of
`substantially parallel reflective surfaces are disposed so that
`a light beam of the first color traveling along the first optical
`path is reflected twice within the first reflecting means and
`transmitted through the first optical means.
`20. The optical device of claim 19 wherein the first optical
`path has a segment without reflection within the pentaprism
`between the first optical means and the fotnth face of the
`pentapn'sm.
`21. The optical device of claim 14 wherein the second
`reflecting means comprises a reflective surface disposed so
`that a light beam of the second color traveling along the
`second optical path is reflected once within the second
`reflecting means and transmitted through the second optical
`means.
`
`22. The optical device of claim 21 wherein the second
`optical path has a first segment within the pentaprism
`between the second optical means and the first optical means
`and has a second segment within the pentaprism substan-
`tially equivalent to the segment within the pentaprism of the
`first optical path,
`23. The optical device of claim 21 wherein the second
`optical path includes a reflection within the pentaprism off of
`the first optical means.
`24. The optical device of claim 14 wherein the third
`optical path includes two reflections within the pentaprism.
`25. The optical device of claim 24 wherein the third
`optical path includes a reflection off of the first optical means
`and a reflection off of the second optical means.
`26. The optical device of claim 24 wherein the third
`optical path includes:
`a first segment between the third optical means and the
`second optical means;
`a second segment between the second optical means and
`the first optical means: and
`a third segment between the first optical means and the
`fourth face of the pentaprism.
`27. A method of combining light beams.
`comprising:
`directing a light beam of a first color to a first dichroic
`interface along a first optical path;
`directing a light beam of a second color to a second
`dichroic interface along a second optical path; and
`directing a light beam of a third color to a third dichroic
`interface along a third optical path.
`wherein the first. second and third optical paths have
`substantially equal numbers of reflections and substan—
`tially equal numbers of transmissions through dichroic
`interfaces and wherein the first. second and third dich-
`roic interfaces form three faces of a pentaprism.
`ii
`it
`3'
`It!
`It
`
`the method
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`XLNX—1004
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`