Abstract:
An optical device for effectively adjusting the F-number of an elliptical lamp is provided, the elliptical lamp for producing a focused light beam at a given focal point having a given cone angle. The optical device comprises a light interaction portion for optically interacting with the focused light beam when the light interaction portion is in general longitudinal alignment with a light emitting aperture of the elliptical lamp, the light interaction portion for triggering optical adjustment of a cone angle of at least a high cone angle portion of the focused light beam to a smaller cone angle. The optical device further comprises a light egress portion, coupled to the light interaction portion, for enabling exit of the focused light beam from the optical device with an effective cone angle smaller than the given cone angle, after the cone angle of the at least the high cone angle portion of the focused light beam has been adjusted to the smaller cone angle.

Description:
FIELD 
   The specification relates generally to optical systems, and specifically to an optical device for adjusting the F-number of an elliptical lamp. 
   BACKGROUND 
   In a projector, for example a digital projector, there are two main optical paths—illumination and projection paths. The illumination path generally comprises a light source such as an elliptical lamp (e.g. an elliptical Hg lamp), an integrator for generating a more uniform beam of light from the light source (i.e. an integrator rod), and illumination relay optics for conveying light from the integrator to the projection path (including the image generation light modulators, such as a digital multi-mirror device (DMD)). The elliptical lamp generally consists of a light source, such as a burner arc, and an elliptical reflector. 
   However, there is a general problem of mismatch in F-number between the elliptical lamp, the illumination relay optics and the light modulator apparatus. For example, the F-number of commercially available elliptical lamps is generally 0.8 to 1.0, and the F-number of commercially available light modulators is generally about 2.5 (e.g. in a 3-chip projector). Regardless of the F-number chosen for the illumination relay optics, then, light will be lost as it travels from the elliptical lamp to the integrator, and through the illumination relay optics to the light modulator due to the loss in high cone angle light from the low F-number elliptical lamp as it tries to enter the high F-number light modulator. 
   One approach to this problem has been to match the input F-number of the illumination relay optics to the elliptical lamp, and provide the illumination relay optics with a magnification factor of 2.5/0.8=3.125, such that the output F-number matches the F-number of the light modulator. However, such a large magnification factor requires that the cross section of the integrator be very small, and hence lowers the light collection efficiency of the system due to the overfilling of the large focal spot from the elliptical lamp on the integrator. A partial solution to the problem may be to increase the input F-number of the illumination relay optics such that a larger illumination rod can be used. For example, if the input F-number is 1.3 and the output F-number is 2.5, the magnification factor of the illumination relay optics will be only 1.923 instead of 3.125, as above. However, the F-number of the elliptical lamp remains small, light with high incident angle will be lost due to the F-number mismatch at the input face of the integrator, again reducing the overall light collection efficiency of the system. 
   SUMMARY 
   A first broad aspect of an embodiment seeks to provide a reflective iris for adjusting the F-number of an elliptical lamp, the elliptical lamp for producing a focused light beam at a given focal point having a given cone angle. The reflective iris comprises a generally spherical convex mirror portion for retro-reflecting a high cone angle portion of the focused light beam back through the elliptical lamp, when the generally spherical convex mirror portion is in general longitudinal alignment with a light emitting aperture of the elliptical lamp and a center of the generally spherical convex mirror portion is generally aligned with the given focal point, such that the high cone angle portion emerges from the elliptical lamp at a smaller cone angle after retroflection. The reflective iris further comprises an optical aperture through the generally spherical convex mirror portion, disposed around a longitudinal axis of the generally spherical convex mirror portion, for enabling transmission there-through of a lower cone angle portion of the focused light beam and the retro-reflected high cone angle portion, such that an effective cone angle of the elliptical lamp is smaller than the given cone angle. 
   In some embodiments of the first broad aspect, the effective cone angle comprises the lower cone angle. 
   In other embodiments of the first broad aspect, an area of the optical aperture is generally circular. 
   In further embodiments of the first broad aspect, the reflective iris further comprises an adjustable aperture apparatus for adjusting an area of the optical aperture. In some of these embodiments, the adjustable aperture apparatus comprises an iris diaphragm. 
   In yet further embodiments of the first broad aspect, the reflective iris further comprises an ultraviolet filter for preventing ultraviolet light from passing through the optical aperture. 
   In some embodiments of the first broad aspect, the shape of the generally spherical convex mirror portion is generally circular, having a diameter that enables interaction of the generally spherical convex mirror portion with the highest angle light ray of the high cone angle portion of the focused light beam. 
   In other embodiments of the first broad aspect, the reflective iris further comprises a body, the body comprising at least one spherical surface, wherein the generally spherical convex mirror portion resides at the at least one planar surface. 
   In further embodiments of the first broad aspect, the body further comprises a bore through the longitudinal axis, the optical aperture comprising the bore. In some of these embodiments, the body comprises a reflective metal. In other embodiments, the reflective iris, further comprises a reflective film applied to the at least one spherical surface, and the generally spherical convex mirror portion comprises the reflective film. In further embodiments, the body comprises a generally transparent material, the generally spherical convex mirror portion comprising a reflective film applied to a first area of the at least one spherical surface, and the reflecting film surrounding a second area of the at least one spherical surface free of the reflecting film, and the optical aperture comprising the second area. In some of these embodiments, the reflective film compress at least one of a reflective metal film and an optical thin film structure. In further embodiments, the generally transparent material comprises a high temperature glass. In some embodiments, the high temperature glass comprises at least one of Vycor™ and Pyrex™. 
   In some embodiments, the body is at least one of mountable between the elliptical lamp and a lens, and mountable on the elliptical lamp. 
   A second broad aspect of an embodiment seeks to provide an optical device for effectively adjusting the F-number of an elliptical lamp, the elliptical lamp for producing a focused light beam at a given focal point having a given cone angle. The optical device comprises a light interaction portion for optically interacting with the focused light beam when the light interaction portion is in general longitudinal alignment with a light emitting aperture of the elliptical lamp, the light interaction portion for triggering optical adjustment of a cone angle of at least a high cone angle portion of the focused light beam to a smaller cone angle. The optical device further comprises a light egress portion, coupled to the light interaction portion, for enabling exit of the focused light beam from the optical device with an effective cone angle smaller than the given cone angle, after the cone angle of the at least the high cone angle portion of the focused light beam has been adjusted to the smaller cone angle. 
   In some embodiments of the second broad aspect, the optical device further comprises a reflective iris, the light interaction portion comprising a generally spherical convex mirror portion of the reflective iris for retro-reflecting the high cone angle portion of the focused light beam back through the elliptical lamp, when the generally spherical convex mirror portion is in general longitudinal alignment with a light emitting aperture of the elliptical lamp and a center of the generally spherical convex mirror portion is generally aligned with the given focal point, such that the high cone angle portion emerges from the elliptical lamp at the smaller cone angle after retroflection; and the light egress portion comprises an optical aperture of the reflective iris through the generally spherical convex mirror portion, disposed around a longitudinal axis of the generally spherical convex mirror portion, for enabling transmission there-through of a lower cone angle portion of the focused light beam and the retro-reflected high cone angle portion, such that an effective cone angle of the elliptical lamp is smaller than the given cone angle. 
   In other embodiments of the second broad aspect, the optical device further comprises a meniscus lens, the light interaction portion comprising a lamp side surface of the meniscus lens, having a first radius of curvature, and the light egress portion comprising an integrator side surface of the meniscus lens, having a second radius of curvature smaller than the first radius of curvature, such that when the focused light beam enters the lamp side surface and exits the integrator side surface, the focused light beam is converged to a cone angle smaller than the given cone angle. 
   A third broad aspect of an embodiment seeks to provide a light production system comprising: an elliptical lamp for producing a focused light beam, the elliptical lamp having a first F-number; and means for effectively adjusting the first F-number of the elliptical lamp to a second F-number, the means for effectively adjusting the first F-number of the elliptical lamp to a second F-number positioned in front of a light emitting aperture of the elliptical lamp. 
   In some embodiments of the third broad aspect, the means for effectively adjusting the first F-number of the elliptical lamp to a second F-number comprises an optical device in longitudinal alignment with the light emitting aperture, the optical device for adjusting the first F-number of the elliptical lamp and comprising: a light interaction portion for optically interacting with the focused light beam, the light interaction portion for triggering optical adjustment of a cone angle of at least a high cone angle portion of the focused light beam to a smaller cone angle; and a light egress portion, coupled to the light interaction portion, for enabling exit of the focused light beam from the optical device with an effective cone angle smaller than the given cone angle, after the cone angle of the at least the high cone angle portion of the focused light beam has been adjusted to the smaller cone angle, such that an effective F-number of the elliptical lamp comprises the second F-number, the second F-number smaller than the first F-number. 
   In some of these embodiments, the optical device comprises a reflective iris, the light interaction portion comprising a generally spherical convex mirror portion of the reflective iris for retro-reflecting the high cone angle portion of the focused light beam back through the elliptical lamp, when the generally spherical convex mirror portion is in general longitudinal alignment with the light emitting aperture and a center of the generally spherical convex mirror portion is generally aligned with the given focal point, such that the high cone angle portion emerges from the elliptical lamp at the smaller cone angle after retroflection; and the light egress portion comprising an optical aperture of the reflective iris through the generally spherical convex mirror portion, disposed around a longitudinal axis of the generally spherical convex mirror portion, for enabling transmission there-through of a lower cone angle portion of the focused light beam and the retro-reflected high cone angle portion, such that an effective cone angle of the elliptical lamp is smaller than the given cone angle. 
   In other embodiments, the optical device comprises a meniscus lens, wherein the light interaction portion comprises a lamp side surface of a meniscus lens, having a first radius of curvature, and the light egress portion comprises an integrator side surface of the meniscus lens, having a second radius of curvature smaller than the first radius of curvature, such that when the focused light beam enters the lamp side surface and exits the integrator side surface, the focused light beam is converged to a cone angle smaller than the given cone angle. 
   In further embodiments of the third broad aspect, the light production system is a component of a projector, the projector further comprising: an integrator, an entrance of the integrator generally located at, at least one of a center of the means for effectively adjusting the first F-number of the elliptical lamp to a second F-number and a focal point of the means for effectively adjusting the first F-number of the elliptical lamp to a second F-number; an imaging component for accepting light from the integrator and causing the light from the integrator to be formed into an image, the integrator arranged to channel light to the imaging component; and at least one projection component for accepting the image from the imaging component and projecting the image. 

   
     BRIEF DESCRIPTIONS OF THE DRAWINGS 
     Embodiments are described with reference to the following figures, in which: 
       FIG. 1  depicts an optical system for focusing light from an elliptical lamp onto an entrance of an integrator, according to the prior art; 
       FIG. 2  depicts a perspective view of a reflective iris in alignment with an elliptical lamp, according to a non-limiting embodiment; 
       FIG. 3  depicts a side view of a reflective iris in alignment with an elliptical lamp, according to a non-limiting embodiment; 
       FIG. 4  depicts a cross-section of a reflective iris in alignment with an elliptical lamp, according to a non-limiting embodiment; 
       FIG. 5  depicts a ray trace diagram of light emitted from an elliptical lamp in alignment with a rectangular aperture, according to a non-limiting embodiment; 
       FIG. 6  depicts a ray trace diagram of light emitted from a reflective iris in alignment with an elliptical lamp and a rectangular aperture, according to a non-limiting embodiment; 
       FIG. 7  depicts a projector, according to the prior art. 
       FIG. 8  depicts a projector, according to a non-limiting embodiment; 
       FIG. 9  depicts a cross-section of a reflective iris having a variable optical aperture, in alignment with an elliptical lamp, according to a non-limiting embodiment; 
       FIG. 10  depicts a cross section of a lens for effectively adjusting the F-number of an elliptical lamp, in alignment with the elliptical lamp, according to a non-limiting embodiment; 
       FIG. 11  depicts a perspective view of a lens for effectively adjusting the F-number of an elliptical lamp, in alignment with an elliptical lamp, according to a non-limiting embodiment; 
       FIG. 12  depicts a side view of a lens for effectively adjusting the F-number of an elliptical lamp, in alignment with an elliptical lamp, according to a non-limiting embodiment; 
       FIG. 13  depicts a ray trace diagram of light emitted from a lens for effectively adjusting the F-number of an elliptical lamp, in alignment with an elliptical lamp, according to a non-limiting embodiment; 
       FIG. 14  depicts light distribution of light from an elliptical lamp shining through a rectangular aperture, as a function of angle, with and without a lens for effectively adjusting the F-number of an elliptical lamp, in alignment with an elliptical lamp, according to a non-limiting embodiment; 
       FIG. 15  depicts cumulative throughput of light from an elliptical lamp shining through a rectangular aperture, as a function of angle, with and without a lens for effectively adjusting the F-number of an elliptical lamp, in alignment with an elliptical lamp, according to a non-limiting embodiment; 
       FIG. 16  depicts a projector, according to the prior art; and 
       FIG. 17  depicts a projector, according to a non-limiting embodiment. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   To gain an understanding of embodiments described hereafter, it is useful to first consider  FIG. 1 , which depicts a system for focusing the light from an elliptical lamp  110  onto an entrance  126  of an integrator  125 , according to the prior art. The elliptical lamp  110  and the integrator  125 , are axially aligned along a longitudinal axis  115  of the elliptical lamp  110 . The elliptical lamp  110  is depicted in cross-section, and is generally symmetrical about the longitudinal axis  115 . The integrator  125  is depicted schematically. As known to one of skill in the art, in a projector, the integrator  125  collects the light which impinges on an entrance  126 , and channels it to another optical component, for example illumination relay optics (not depicted) and ultimately a light modulator (not depicted), while simultaneously scattering the light internally to create a more uniform beam of light. 
   The elliptical lamp  110  comprises an elliptical reflector  112 , having an aperture of diameter D, and a light source  114 . The light source  114  is generally located at a first focal point F 1  of the elliptical reflector  114  on the longitudinal axis  115 . In some embodiments, the elliptical lamp  110  comprises an elliptical Hg lamp, and hence the light source  114  may comprise a burner arc. However, other types of elliptical lamps are within the scope of present embodiments. As known to one of skill in the art, light rays emitted from the from light source  114 , for example lights rays  150   a  and  150   b  (collectively light rays  150  and generically light ray  150 ), that are reflected from the elliptical reflector  112 , are focused at a second focal point F 2  of the elliptical reflector  112 . Hence, the entrance  126  of the integrator  125  is generally located at F 2 , while the light source  114  is modeled as a point source in  FIG. 2 , and subsequent figures, the light source  114  is generally an areal light source and hence overfilling of a large focal spot occurs at the entrance  126  (i.e. an image of the areal light source occurs at the entrance  126 ). 
   A person of skill in the art would understand that the light source  114  is generally emitting light in all directions (with the exception of those parts of the light source that comprise the electrical connecting portions of the light source  114  etc., which block portions of the light source  114 ). A person of skill in the art would further understand that the light ray  150   a  is emerging from the elliptical lamp  110  at a high cone angle, and that the light ray  150   a  generally defines a high angle cone which is emerging from the elliptical lamp  110  generally symmetric about the longitudinal axis  115  (as depicted in  FIGS. 5 and 6 ). Similarly, a person of skill in the art would further understand that the light ray  150   b  is emerging from the elliptical lamp  110  at a low cone angle. 
   The F-number of the elliptical lamp  110  is defined by the ratio of the focal length F 2  to the aperture diameter D, or F 2 /D, and generally defines the cone angle of the highest angle cone emerging from the elliptical lamp  110 , in this example the cone defined by the light ray  150   a . It is this high cone angle light that is particularly difficult to capture by the integrator  125 , the illumination relay optics and/or the light modulator. Indeed, the high cone angle light has a tendency to scatter outside the receiving optics of the illumination relay optics and/or the light modulator, reducing the overall light collection efficiency of the system, especially if there is a mismatch between the F-number of the elliptical lamp  110  and the illumination relay optics (and/or the light modulator), the F-number of the illumination-relay optics being generally larger than the F-number of the elliptical lamp  110 . The overfilling of the large focal spot on the entrance  126  further serves to decrease the light collection efficiency of the system. 
   Attention is now directed to  FIG. 2 , which depicts an embodiment of an optical device for adjusting the f-number of an elliptical lamp. Specifically,  FIG. 2  depicts a perspective view of a reflective iris  210  for effectively adjusting the F-number of an elliptical lamp, for example the elliptical lamp  110 , according to a non-limiting embodiment. In  FIG. 2 , the elliptical lamp  110  is depicted in a partial cutaway view. The reflective iris  210  comprises a generally spherical convex mirror portion  211  for retro-reflecting a high cone angle portion of a focused light beam back through the elliptical lamp  110 , when the generally spherical convex mirror portion  211  is generally axially aligned with the elliptical lamp  110  along the longitudinal axis  115 , and a center FS of the generally spherical convex mirror portion  211  is generally aligned with the focal point F 2 . The high cone angle portion then emerges from the elliptical lamp at a smaller cone angle after retroflection, as depicted in  FIG. 4  and described in detail below. The reflective iris  210  further comprises an optical aperture  213  through said generally spherical convex mirror portion  211 , disposed around a longitudinal axis of the generally spherical convex mirror portion  211 , for enabling transmission there-through of a lower cone angle portion of the focused light beam and the retro-reflected high cone angle portion, such that an effective cone angle of the elliptical lamp is smaller than the given cone angle. 
   The reflective iris  210  further comprises an inner side  212  opposite the generally spherical convex mirror portion  211 . While the generally spherical convex mirror portion  211  is both generally reflective and generally spherical, the properties of the inner side  212  are generally non-limiting as long as the inner side  212  does not interfere with the reflection of the focused light beam back through the elliptical lamp  110 , and the transmission of the lower cone angle portion of the focused light beam and the retro-reflected high cone angle portion through the optical aperture  213 . Indeed while the reflective iris  210  is generally depicted a shell of a spherical portion in  FIG. 2  and subsequent figures, in other embodiments, the reflective iris  210  may be a solid spherical portion with the optical aperture  213  being a shape suitable for enabling transmission there-through of the lower cone angle portion of the focused light beam and the retro-reflected high cone angle portion. 
   While the optical aperture  213  is depicted as circular, the shape of the optical aperture  213  is not particularly limiting. Indeed, the shape of the optical aperture  213  may depend on the application. For example, if the elliptical lamp  110  and the reflective iris  210  are to be used in a projector with a rectangular integrator, the optical aperture  213  may be rectangular, and of the same aspect ratio as the integrator and/or the light modulator. 
   Moreover, while the reflective iris  210  is also depicted as generally circular, the shape of the reflective iris  210  is generally limited only by the shape of the elliptical lamp  110 . For example, if the elliptical reflector  112  is not generally circular, but has been designed to provide generally elliptical areas that intersect at an angle to form a unified body, the generally spherical convex mirror portion  211  may reflect the shape of the resulting elliptical lamp, being comprised of generally convex mirror sections that intersect at an angle to form a unified generally spherical reflecting surface. 
     FIG. 3  depicts a side view of the elliptical lamp  110  and the reflective iris  210  in general alignment, with the elliptical lamp  110  depicted in cross-section, according to a non-limiting embodiment. 
     FIG. 4  depicts a schematic of the reflective iris  210  and the elliptical lamp  110  in general alignment, with the elliptical lamp  110  depicted in cross-section, according to a non-limiting embodiment, as in  FIG. 1 , with like numbers depicting like elements. The generally spherical convex mirror portion  211  is depicted schematically, while the optical aperture  213  is depicted in cross-section. The optical aperture  213  is further depicted having a diameter of Do, with Do being less than the diameter D of the aperture of the elliptical reflector  112 .  FIG. 4  further depicts the integrator  125  in axial alignment with both the reflective iris  211  and the elliptical lamp  110 . 
     FIG. 4  further depicts the light ray  150   a  impinging on the generally spherical convex mirror portion  211 . Due to the spherical nature of the generally spherical convex mirror portion  211 , and the general axial alignment of the elliptical reflector  112  and the reflective iris  210 , the light ray  150   a  impinges on the generally spherical convex mirror portion  211  generally normally (i.e. generally at a right angle). Hence a reflected light ray  460  travels directly back along the same path as the light ray  150   a , passing generally back through the light source  114  to again reflect from the elliptical reflector  112 . However, as the light ray  460  is now travelling at an angle which is 180° to the light ray  150   a , when it again reflects from the elliptical reflector  112 , the light ray  460  emerges from the elliptical lamp  110  at a smaller cone angle than light ray  150   b . Hence, the light ray  460  passes through the optical aperture  213  and enters the integrator  125  at the smaller cone angle. 
   In contrast, the light ray  150   b  passes through the optical aperture  213  after being reflected from the elliptical reflector  112 . As depicted, the light ray  150   b  comprises the largest angle light ray emitted from the elliptical lamp  110  that is not reflected by the generally spherical convex mirror portion  211 . Light rays which emerge from the elliptical lamp  110  having a cone angle greater than that of the light ray  150   b , are retro-reflected back through the elliptical reflector  112  by the generally spherical convex mirror portion  211 . The overall result is that, when the reflective iris  210  is generally axially aligned with the elliptical lamp  110 , and the center FS of the reflective iris  210  is generally aligned with the second focal point F 2 , the F-number of the elliptical lamp  110  is effectively adjusted from F 2 /D to FS/Do, with the specific F-number being defined by the diameter Do and the center FS of the reflective iris  210 . 
   Hence, the reflective iris  210  may be enabled for effectively adjusting the F-number of the elliptical lamp  110  to a different F-number for better compatibility with the integrator  125 , the illumination relay optics and/or the light modulator of a projector system, increasing the overall light collection efficiency of the system, by choosing a suitable diameter Do and a suitable center FS of the reflective iris  210 . 
   The reflective iris  210  is generally comprised of a suitable material or combination of materials to enable the retroflection as described and is generally heat resistant: when the reflective iris  210  is aligned with the elliptical lamp  110 , the reflective iris  210  is in proximity to the elliptical lamp  110  which can get hot in operation (for example an elliptical Hg lamp). Hence, the reflective iris  210  is comprised of a material, or combination of materials, which can withstand the heat of the elliptical lamp  110 , and further the generally spherical convex mirror portion  211  is comprised of a suitable generally reflective material, or combination of materials for reflecting light emitted from the elliptical lamp  110 . 
   In some non-limiting embodiments, the reflective iris  210  may comprise a suitable metal of a suitable shape, with the generally spherical convex mirror portion  211  being generally reflective of light emitted from the elliptical lamp  110 . For example, the reflective iris  210  may comprises aluminum, with the generally spherical convex mirror portion  211  being polished, treated and/or coated to reflect light emitted from the elliptical lamp  110 . In these embodiments, the optical aperture  213  may comprise an opening in the metal. 
   In another non-limiting embodiment, the reflective iris  210  may comprise a substrate material of a suitable shape, while the generally spherical convex mirror portion  211  may comprise a coating on the substrate material. In a non-limiting example, the substrate material may comprise a suitable transparent material, for example a high temperature glass (e.g., Vycor™, Pyrex™, N-BK7, fused silica and the like), of a suitable shape, and the generally spherical convex mirror portion  211  may comprise a suitable generally reflective coating on the glass, such as a thin film metal or a dielectric coating. Further, in some embodiments, if the glass is itself a generally spherical portion, the coating may be on the outside of the glass or on the inside of the glass (i.e. deposited on the inner side  212 ). In some of these embodiments, the optical aperture  213  may comprise an opening in the substrate material. In embodiments where the reflective iris  210  is comprised of a suitable transparent material and the generally spherical convex mirror portion  211  comprises a suitable generally reflective coating, the optical aperture  213  may comprise an opening in the generally reflective coating (i.e. an area of the reflective iris  210  that was not coated with the generally reflective coating). In these embodiments, the suitably transparent material may further comprise an optical filter for filtering unwanted light, for example UV light and/or infrared light. The optical filter may comprise an optical coating on the suitable transparent material, on any suitable side or area. Alternatively, the suitable transparent material may comprise inherent light filtering properties (e.g. a glass which absorbs UV light). 
   The outer dimensions of the reflective iris  210  are generally configured so that the reflective iris  210  retro-reflects light rays emitted from the elliptical lamp  110  that have the highest angle cone, for example the light ray  150   a . Further, the outer dimensions of the reflective iris  210  are generally configured so as to not interfere with the impingement of the light that is transmitted through the optical aperture  213  on the integrator  125 . 
   It will be recalled that the reflective iris  210  may be enabled for effectively adjusting the F-number of the elliptical lamp  110  to a specific F-number for better compatibility with the integrator  125 , the illumination relay optics and/or the light modulator of a projector, to increase the overall light collection efficiency of the system. Moreover, the F-number of the elliptical lamp  110  can be effectively and freely adjusted by choosing a suitable Do of the optical aperture  213  for each application, and a suitable center FS. In addition, since the cone angle of the focused light beam that enters the integrator  125  is narrower (i.e. due to the larger F-number) with the reflective iris  210  in alignment (i.e. in  FIG. 4  vs.  FIG. 1 ), the contrast ratio of a projector using the reflective iris  210  will improve due to reduced light overlapping between an on-state and off-state light path from the light modulator (e.g. a Digital Micromirror Device or DMD). 
   In a non-limiting example, the F-number of the elliptical lamp  110  may be adjusted to match the input F-number of the illumination relay optics. In particular non-limiting embodiment, the input F-number is 1.3 and the F-number of the elliptical lamp  110  is 0.8. Hence, the reflective iris  210  may be configured to effectively adjust the F-number of the elliptical lamp  110  to 1.3 by choosing a suitable Do and a suitable center FS. By doing this, the light throughput increases resulting in a higher brightness of the projector. As well, the use of the reflective iris  210  improves the use of an input F-number for the illumination relay optics that is intermediate the elliptical lamp  110  and the light modulator, as the light collection efficiency at the integrator  125  is increased. 
   In order to demonstrate the performance of the reflective iris  110 , two non-limiting models were created.  FIG. 5  depicts a ray diagram of a model of the system depicted in  FIG. 1 , with the elliptical lamp  110  in alignment with a rectangular aperture  510  representing the entrance  126  of integrator  125 , but without the reflective iris  210 .  FIG. 6  depicts a ray diagram of a model of the system depicted in  FIG. 4 , similar to that of  FIG. 5  but with the reflective iris  210 . In each figure, the light source  114  of  FIGS. 1 and 4  is modeled as an areal light source rather than as a point light source. 
   In each model, the F-number of the elliptical lamp  110  is 0.8, while the rectangular aperture  510  has dimensions of 6.8×5.85 mm with a collection F-number of 1.3, and is located at the second focus F 2  of the elliptical lamp  10 . In  FIG. 6 , the diameter Do of the optical aperture  213  is 24 mm. 
     FIG. 5  further depicts a focused cone of light  520  as it emerges from the elliptical lamp  110 , and a cone of light  530  that emerges from the rectangular aperture  510 , as the focused cone of light  520  impinges on the rectangular aperture  510 . In contrast,  FIG. 6  also depicts the focused cone of light  520  as it emerges from the elliptical lamp  110 , but  FIG. 6  further depicts that with the reflective iris  210  in alignment, a high cone angle portion of the focused cone of light  520  is retro-reflected back through the elliptical lamp  110  and through the reflective iris  210 . The result is that a focused cone of light  620  that emerges from the reflective iris  210  (in combination with the elliptical lamp  110 ) has a smaller cone angle than the focused cone of light  520  that emerges from the elliptical lamp  110 . As the focused cone of light  620  impinges on the rectangular aperture  510 , a cone of light  630  that emerges from the rectangular aperture  510 , has a smaller cone angle than the cone of light  530  that emerges from the rectangular aperture  510  in the system of  FIG. 5 . 
   Table 1 further records the gain in light collection efficiency between the system depicted in  FIG. 6  and the system depicted in  FIG. 5  using ray-tracing illumination software such as TracePro from Lambda Research Corporation, 25 Porter Rd, Littleton, Mass. 01460-1434, USA. Light emitted from the elliptical lamp  110  was modeled as 21928 lumens. Light emitted through the rectangular aperture  510  without the reflective iris  110  in alignment (as in  FIG. 5 ) was then determined to be 12937 lumens, while light emitted through the rectangular aperture  510  with the reflective iris  210  in alignment (as in  FIG. 6 ) was determined to be 13957 lumens. In other words, with the reflective iris  210  in alignment, as in  FIG. 6 , an increase in light collection efficiency of 8% was achieved. 
   
     
       
             
           
             
             
             
             
           
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Total from Lamp = 21928 lm 
             
           
        
         
             
                 
               With Iris 
                 
                 
             
             
                 
               (lm) 
               Without Iris (lm) 
               Improvement 
             
             
                 
                 
             
           
        
         
             
               Light through 
               13957 
               12937 
               8% 
             
             
               Rectangular Aperture 
             
             
               Light Collection Efficiency 
               (63.7%) 
               (59.0%) 
             
             
                 
             
           
        
       
     
   
     FIG. 7  depicts a schematic of a light collection system of a projector comprising two elliptical lamps  710 , similar to the elliptical lamp  110 , focused on two entrances of an integrator rod  725  which performs substantially the same function in substantially the same way as the integrator rod  125 . The integrator  725  channels light from each of the elliptical lamps  710  perpendicular to the light output path of each of the elliptical lamps  710  to illumination relay optics  750 , which subsequently magnifies and channels the light to a light modulator  755 . In contrast,  FIG. 8  shows how reflective irises  810 , similar to the reflective iris  210 , can be incorporated into the system of  FIG. 7  to improve the light collection efficiency of the projector. 
   Attention is now directed to  FIG. 9  is substantially similar to  FIG. 4 , with like elements depicted with like numbers, however the light rays  150  have been omitted for simplicity.  FIG. 9  depicts another non-limiting embodiment of the reflective iris  210 , in which the reflective iris  210  further comprises an apparatus  820  for varying the diameter of the optical aperture  213 . Hence, in this embodiment, the optical aperture  213  has a variable diameter Do′. In some embodiments the apparatus  820  resides within the optical aperture  213  (as depicted). In other embodiments, the apparatus  820  may be mounted on the lamp side of the reflective iris  210 , while in yet other embodiments, the apparatus  820  may be mounted on the integrator side of the reflective iris  210 . In yet other embodiments, the apparatus  820  may be a separate element from the reflective iris  210  and be mounted either between the reflective iris  210  and the integrator  125 , or between the reflective iris  210  and the elliptical lamp  110 . 
   In some embodiments, the apparatus  820  is a generally spherical portion (as depicted), with a radius and center that is generally similar to the radius and center FS, respectively, of the reflective iris  210 . In some of these embodiments, an elliptical lamp side surface  825  is generally reflective and retro-reflects light back towards the elliptical lamp  110  in a manner similar to the generally spherical convex mirror portion  211 . 
   In other embodiments, the apparatus  820  may be generally planar. 
   In some embodiments, the apparatus  820  may also generally comprise a device for a user of the system of  FIG. 8  to adjust the variable diameter Do′. In some embodiments, a lamp-facing surface of the apparatus  820  is reflective. In some non-limiting embodiments, the apparatus  820  comprises an iris diaphragm. In some of these embodiments, the iris diaphragm is a generally spherical portion. 
   By varying the variable diameter Do′ of the optical aperture  213 , the F-number of the system of  FIG. 8 , may be varied according to F=F 2 /Do′. Hence, a smaller Do′ aperture will lead to a larger F-number. This has the effect of tightening the cone angle of the light impinging on the entrance  126 , which results in a better contrast ratio for the optical components towards which the integrator  125  channels the light toward, such as a light modulator (e.g. a DMD). 
   In one non-limiting example, the systems of  FIGS. 2 ,  3 ,  4 ,  6  and  9  comprise light production systems for an optical projector. In some of these embodiments, the optical projector comprises an analog optical projector, while in other embodiments, the optical projector comprises a digital optical projector, for example a digital optical projector as manufactured by Christie Digital Systems Canada, Inc., 809 Wellington St. N., Kitchener, Ontario, Canada N2G 4Y7. 
   In some embodiments the reflective iris  210  may be adapted for mounting between the elliptical lamp  110  and the integrator  125 . In other embodiments, the reflective iris  210  may be adapted for mounting to the elliptical lamp  110 , for example by gluing the reflective iris  640  to the aperture of the elliptical lamp  110  In some of these embodiments, a suitable spacer may be provided to protect the reflective iris  210  from the heat of the elliptical lamp  110 , and to ensure a suitable optical path of the light rays  150 . 
   Turning now to  FIG. 10 , an alternative embodiment of an optical device for adjusting the F-number of an elliptical lamp is depicted.  FIG. 10  depicts the elliptical lamp  110  and the integrator of  FIG. 1  in schematic, along with light rays  150 , with like elements depicted with like numbers.  FIG. 10  further depicts an F-Number Lens (FNL)  1010  recovering the loss of a high-angle portion of the focused light beam emerging from the elliptical lamp  110 . In essence, when the FNL  1010  is axially aligned with the elliptical lamp  1010 , with an elliptical lamp side surface  1011  facing the elliptical lamp  110 , the FNL  110  refracts, diverges and focuses the focused light beam emerging from the elliptical lamp  110  onto the entrance  126  of the integrator. Hence, the F-number of the elliptical lamp  110  can effectively be adjusted (e.g., from 0.8 to 1.3, as above) to match the input F-number of an illumination relay system in a projector. By doing this, the light collection efficiency will be increased which will result in a higher brightness of the projector. 
   The FNL  1010  generally comprises a meniscus or concave comprising a lamp side surface  1011  having a radius of curvature R 1 , and an integrator side surface  1012  having a radius of curvature R 2 . In the depicted embodiment, R 2  is less than R 1 , and hence the FNL  1010  further comprises corners  1013  to connect the lamp side surface  1011  and the integrator side surface  1012 . However, present embodiments are not particularly limited by the corners  1013  and the lamp side surface  1011  and the integrator side surface  1012  may be connected by any suitable structure. Moreoever the FNL  1010  has a thickness TL. 
   Further, a reference point on the FNL  1010  is located at a position DL relative to the aperture of the elliptical lamp  110 . In some embodiments the reference point on the FNL  1010  is located at the center of the FNL  1010  (as depicted), however the reference point may be located at any suitable point on the FNL  1010 , for example on the lamp side surface  1011  or the integrator side surface  1012 . 
   The FNL  1010  may comprise any suitable optical material or combination of materials. In general the FNL  1010  should be enabled to tolerate the heat generated from the elliptical lamp  110 . Non-limiting examples of suitable optical materials include but are not limited to fused silica, N-BK7, Vycor™, and Pyrex™. In some embodiments, for higher light transmission, N-BK7 may be used as long as the design of the system allows the N-BK7 to tolerate the heat generated from the elliptical lamp  110 . In some embodiments, the surface of the lens side surface  1011  may be coated with a UV coating to block transmission of UV light from through the FNL  1010 . This obviates the need for a separate UV filter in the system. In some embodiments, the surface of the integrator side surface  1012  and/or the surface of the lamp side surface  1011 , can be coated with multi-layer anti-reflection coating to increase transmission through the FNL  1010 . 
   Indeed, given the F-number of the elliptical lamp  110 , and the desired effective F-number of the elliptical lamp, the relationships between the behavior of the system of  FIG. 10  and parameters such as DL, TL, R 1 , R 2 , may be determined using optical design software such as ZEMAX® (from ZEMAX Development Corporation, 3001 112th Avenue NE, Suite 202, Bellevue, Wash. 98004-8017 USA), CODE V® (from Optical Research Associates, 3280 East Foothill Boulevard, Suite 300 Pasadena, Calif. 91107-3103), OSLO® (from Lambda Research Corporation, 25 Porter Rd, Littleton, Mass. 01460-1434 USA), and the like. Using such optical design software, DL, R 1  and R 2  and TL of the FNL  1010  may be determined, using as inputs the F-number of the elliptical lamp  110  and the desired effective F-number of the elliptical lamp  110  with the FNL  1010  in alignment with the elliptical lamp  110 , as well as the distance between the entrance  126  and the elliptical lamp  110 . Further, limits can be placed on some or all of the parameters. DL, for example, may be limited to a minimum distance that the FNL  1010  should be from the elliptical lamp  110  to prevent heat damage. Further DL, TL, R 1  and R 2  may be limited to reflect space considerations in the system. For example, there may be a preferred maximum distance between the entrance  126  and the FNL  1010  and or a preferred maximum distance between the entrance  126  and the elliptical lamp  110 . With such inputs, the optical software may freely design the system depicted in  FIG. 10 . The FNL  1010  can then be manufactured as required. 
   In some embodiments, R 1  of the lamp side surface  1011  is generally chosen so that the focused beam of light that emerges from the elliptical lamp  110  impinges on the lamp side surface  1011  at a normal or near normal angle, as depicted, such that the refraction of the focused light beam generally occurs at the integrator side surface  1012 . 
   Further examination of  FIG. 10  shows that, in the depicted embodiment, the entrance  126  is not located at the second focal point F 2  of the elliptical lamp  110 . Rather, the entrance is located at the focal point of the FNL  1010 , which is depicted as a distance FL from the elliptical lamp  110 . From the point of view of the entrance  126 , the distance FL is the effective focal length of the elliptical lamp  110 . Note that in  FIG. 10 , the second focal length F 2  is represented as the distance F 2  from the elliptical lamp  110  along the longitudinal axis  115 . 
   Furthermore,  FIG. 10  depicts the light ray  150   a  and the light ray  150   b  from  FIG. 1 . As in  FIG. 1 , the light ray  150   b  represents a low cone angle light ray while the light ray  150   a  represents the highest cone angle light ray that emerges from the elliptical lamp  110 . In this embodiment, however, each light ray is refracted, diverged and focused by the FNL  110 . Indeed,  FIG. 10  depicts two paths for each light ray  150 , the path of the light ray  150  in the absence of the FNL  1010  (broken line) and the path of the light ray  150   a  in the presence of the FNL  1010  (solid line). For example, in the absence of the FNL  1010 , each light ray  150  would be focused onto the second focal point F 2  (at the intersection of broken lines, as depicted). In the presence of the FNL  1010 , a refracted portion of each light ray  150  is focused onto the entrance  126 , at the distance FL from the elliptical lamp  110 . The light ray  150   a  comprises a refracted portion  150 ′ a , and the light ray  150   b  a refracted portion  150 ′ b.    
   The refracted portion  150 ′ a  represents the highest cone angle light ray emerging from the FNL  1010 , as the light ray  150   a  represent the highest cone angle light ray impinging on the lamp side surface  1011  of the FNL  1010 . Furthermore, it is understood that the refracted portion  150 ′ a  generally defines a high angle cone which is emerging from the FNL  1010  and which is generally symmetric about the longitudinal axis  115  (as depicted in  FIG. 13 ). However, if a path  1050  of the refracted portion  150 ′ a  is directly traced back towards the elliptical lamp  110 , the intersection of the path  1050  and the aperture of the elliptical lamp  110  defines a distance D′/2 from the longitudinal axis  115 . Again turning to the point of view of the entrance  126  the cone defined by the refracted portion  150 ′ a  effectively appears to emerge from the elliptical lamp  110 , but the aperture of the elliptical lamp  110  effectively appears to have a diameter D′, rather than D. Hence, the effective F-number of the elliptical lamp  110 /FNL  110  system is FL/D′. 
   Hence, the FNL  1010  may be enabled for effectively adjusting the F-number of the elliptical lamp  110  from F 2 /D to FL/D′ for better compatibility with the integrator  125 , the illumination relay optics and/or the light modulator in a projector system, increasing the overall light collection efficiency of the system. Moreover, the F-number of the elliptical lamp  110  can be effectively and freely adjusted by choosing a suitable FNL, similar to the FNL  1010 , for each application. In addition, since the cone angle of the focused light beam that enters the integrator  125  is narrower (i.e. due to the larger F-number) with the FNL  1010  in alignment (i.e. in  FIG. 4  vs.  FIG. 1 ), the contrast ratio of a projector using the FNL  1010  will improve due to reduced light overlapping between an on-state and off-state light path from the light modulator (e.g. a Digital Micromirror Device or DMD). 
   In a non-limiting example, the F-number of the elliptical lamp  110  may be adjusted to match the input F-number of the illumination relay optics. In particular non-limiting embodiment, the input F-number is 1.3 and the F-number of the elliptical lamp  110  is 0.8. Hence, the FNL  1010  may be configured to effectively adjust the F-number of the elliptical lamp  110  to 1.3 by choosing a suitable R 1 , R 2  and a suitable FL. By doing this, the light throughput increases resulting in a higher brightness of the projector. As well, the use of the FNL  1010  improves the use of an input F-number for the illumination relay optics that is intermediate the elliptical lamp  110  and the light modulator, as the light collection efficiency at the integrator  125  is increased. 
   Attention is now directed to  FIG. 11 , which depicts a perspective view of the FNL  1010  and the elliptical lamp in general alignment, according to a non-limiting embodiment. In  FIG. 11 , the elliptical lamp  110  is depicted in a partial cutaway view. 
     FIG. 12  depicts a perspective side view of the FNL  1010  and the elliptical lamp  110  in general alignment, with the elliptical lamp  110  depicted in cross-section, according to a non-limiting embodiment. 
   In order to demonstrate the performance of the FNL  1010 , two non-limiting models are created. The first model is similar to the model depicted in  FIG. 5 , with the elliptical lamp  110  in alignment with the rectangular aperture  510  representing the entrance  126  of integrator  125 , but without the FNL  1010 . However in this model, a UV filter is placed in front of the elliptical lamp  110  in order to reject UV from the elliptical lamp  110 .  FIG. 13  depicts a perspective view of a ray diagram of the elliptical lamp  110  in alignment with the FNL  1010 . The second model is similar to  FIG. 13 , with the rectangular aperture  510  at the focal point. Compared to the first model, the FNL  1010  replaces the UV filter in each model, the light source  114  (e.g., as in  FIGS. 1 and 10 ) is modeled as an areal light source rather than as a point light source. 
   In each model, the F-number of the elliptical lamp  110  is 0.8, while the rectangular aperture  510  has dimensions of 6.8×5.85 mm with a collection F-number of 1.3. In the each model, the rectangular aperture  510  is located at the appropriate focal position, and is representative of the entrance  126  of the integrator  125 . 
     FIG. 13  further depicts the focused cone of light  520  as it emerges from the elliptical lamp  110 , and a focused cone of light  1320  that emerges from the FNL  1010  (in combination with the elliptical lamp  110 ). The focused cone of light  1320  has a smaller cone angle than the focused cone of light  520  that emerges from the elliptical lamp  110 . 
   In one non-limiting example, the systems of  FIGS. 10-13  comprise light production systems for an optical projector. In some of these embodiments, the optical projector comprises an analog optical projector, while in other embodiments, the optical projector comprises a digital optical projector, for example a digital optical projector as manufactured by Christie Digital Systems Canada, Inc., 809 Wellington St. N., Kitchener, Ontario, Canada N2G 4Y7. 
     FIG. 14  depicts the light distributions as a function of angle through the rectangular aperture  510  for both models. With the FNL  1010  in alignment with the elliptical lamp  110 , the light distribution shifts to a lower angle as compared to the elliptical lamp  110  alone. Hence, means more light will be collected in an illumination relay system with an input F number of 1.3.  FIG. 15  depicts the cumulative throughput of both models as a function of angle (i.e. an integration of the curves of  FIG. 14 ). With the FNL  1010  in alignment with the elliptical lamp  110 , the effective half angle of the elliptical lamp  110  is adjusted to approximately 210 from a half angle of grater than 30° without the FNL  1010 . Hence the cone angle of the light emerging from the elliptical lamp  110  is adjusted from a higher cone angle (&gt;30°) to a lower cone angle (˜21°), demonstrating that the F-number of the elliptical lamp  110  has been adjusted from a lower F-number (0.8) to a higher F-number (1.3). 
   Table 2 further records the gain in light collection efficiency between the models (i.e. without the FNL  1010  in alignment with the elliptical lamp  110  and with the FNL  1010  in alignment with the elliptical lamp  110 ) using ray-tracing illumination software such as TracePro from Lambda Research Corporation, 25 Porter Rd, Littleton, Mass. 01460-1434, USA. Light emitted from the elliptical lamp  110  was modeled as 21251 lumens. Light emitted through the rectangular aperture  510  without the FNL  1010  in alignment was then determined to be 12460 lumens, while light emitted through the rectangular aperture  510  with the FNL  1010  in alignment was determined to be 15140 lumens. In other words, with the FNL  1010  in alignment, as in  FIGS. 10-13 , an increase in light collection efficiency of 21.5% was achieved. 
   
     
       
             
           
             
             
             
             
           
             
             
             
             
           
         
             
               TABLE 2 
             
           
           
             
                 
             
             
               Total from Lamp = 21251 lm 
             
           
        
         
             
                 
                 
               Without 
                 
             
             
                 
               With FNL (lm) 
               FNL (lm) 
               Improvement 
             
             
                 
                 
             
           
        
         
             
               Light through 
               15140 
               12460 
               21.5% 
             
             
               Rectangular Aperture 
             
             
               Light Collection Efficiency 
               (71.2%) 
               (58.6%) 
             
             
                 
             
           
        
       
     
   
   In addition, to the higher light collection efficiency, the contrast ratio of the projector can be enhanced.  FIG. 14 , further shows that the peak of the light emitted from the elliptical lamp  110  shifts from 15° to 13° when the FNL  1010  is in alignment. Hence, more light is now at lower cone angles and minimizes the amount of light overlapping between the on-state and off-state light inside the projector. 
     FIG. 16  depicts a schematic of a light collection system of a projector comprising two elliptical lamps  1610 , similar to the elliptical lamp  110 , focused on two entrances of an integrator rod  1625  which performs substantially the same function in substantially the same way as the integrator rod  125 . The integrator  1625  channels light from each of the elliptical lamps  1610  perpendicular to the light output path of each of the elliptical lamps  1610  to illumination relay optics  1650 , which subsequently magnifies and channels the light to a light modulator  1655 . In contrast,  FIG. 17  shows how two FNLs  1710 , similar to the FNL  1010   210 , can be incorporated into the system of  FIG. 16  to improve the light collection efficiency of the projector. In modeling each system, it was found that the dual-lamp projector of  FIG. 16  can only achieve 7739 lm. In contrast, when the FNLs  1710  are used to effectively adjust the F-number of the elliptical lamps  1610  to 1.3, as in  FIG. 17 , the total screen throughput now becomes 9482 lm, 22.5% brighter than before. 
   In one non-limiting example, the systems of  FIGS. 10-13  comprise a light production system for an optical projector. In some of these embodiments, the optical projector comprises an analog optical projector, while in other embodiments, the optical projector comprises a digital optical projector, for example a digital optical projector as manufactured by Christie Digital Systems Canada, Inc., 809 Wellington St. N., Kitchener, Ontario, Canada N2G 4Y7. 
   In some embodiments the FNL  1010  may be adapted for mounting between the elliptical lamp  110  and the integrator  125 . In other embodiments, the FNL  1010  may be adapted for mounting to the elliptical lamp  110 , for example by gluing the FNL  1010  to the aperture of the elliptical lamp  110  In some of these embodiments, a suitable spacer may be provided to protect the FNL  1010  from the heat of the elliptical lamp  110 , and to ensure a suitable optical path of the light rays  150 . 
   Persons skilled in the art will appreciate that there are yet more alternative implementations and modifications possible for implementing the embodiments, and that the above implementations and examples are only illustrations of one or more embodiments. The scope, therefore, is only to be limited by the claims appended hereto.