Patent Publication Number: US-6710941-B2

Title: Fresnel lens for projection screens

Description:
RELATED APPLICATIONS 
     This is a continuation of parent application Ser. No. 09/229,198, filed on Jan. 13, 1999, now issued as U.S. Pat. No. 6,407,859. 
    
    
     BACKGROUND 
     The present invention is directed generally to a Fresnel lens for use with projection screens, and particularly to a Fresnel lens that reduces the effect of ghost images. 
     Fresnel lenses are often used in projection screens for collimating light received from the illumination source. The Fresnel lens is typically used to increase the gain of the screen at the screen edge, so that a viewer does not notice a lack of brightness uniformity across the screen. 
     However, a Fresnel lens typically generates a ghost image, which is the result of internal reflections within the lens and the substrate to which the lens may be attached. The ghost image may be perceived by the viewer, with the effect that the image quality is reduced and the viewer may be distracted. Consequently, the screen manufacturer has to compromise between brightness uniformity and image quality. 
     Therefore, there is a need for a Fresnel collimating lens, for use with a projection screen, that reduces, or avoids, the production of ghost images. The Fresnel collimating lens should also maintain the capability of effectively collimating light to provide more uniform brightness across the screen. 
     SUMMARY OF THE INVENTION 
     Generally, the present invention relates to a screen having a Fresnel lens laminated to another layer for support. 
     In one embodiment of the invention, a screen includes a Fresnel lens having an output surface, and a dispersing screen supportingly attached on a first side to the output surface of the Fresnel lens. 
     In another embodiment of the invention, a screen includes a Fresnel lens having an output surface, where at least a portion of the output surface includes a Fresnel structure. A first optical layer has a first surface supportingly attached to the output Fresnel structured surface of the Fresnel lens. 
     In another embodiment of the invention, a first layer has a first surface, and a redirecting means for redirecting light passing through the screen, has a Fresnel structured output surface. Attaching means on at least one of the first layer and the redirecting means supportingly attaches the output surface of the redirecting means to the first surface of the first layer. 
     In another embodiment of the invention, a layer of transparent material has an input surface and a Fresnel-structured output surface having ridges formed between functional slopes and riser slopes, at least some of the ridges being truncated with flat portions essentially parallel to the input surface. 
     In another embodiment of the invention, a layer of transparent material has an output surface with a Fresnel-structured portion proximate an edge thereof and a substantially unstructured center portion. 
     In another embodiment of the invention, a first layer has a first surface, and a Fresnel lens having a Fresnel-structured output surface. The Fresnel-structured output surface includes functional slopes and riser slopes, and at least a portion of one functional slope and a portion of a riser slope are embedded in the first surface of the first layer. 
     The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: 
     FIG. 1 illustrates the illumination of a screen without the use of collimating optics; 
     FIG. 2 is a graph showing gain at the center and edge of the screen of FIG. 1 as a function of viewing angle; 
     FIG. 3 illustrates the illumination of a screen using a Fresnel lens for collimation; 
     FIG. 4 illustrates the creation of ghost images in a Fresnel lens; 
     FIGS. 5A-5F illustrate different embodiments of embedded Fresnel lenses according to the present invention; 
     FIG. 6 illustrates the occurrence of an inactive region in a Fresnel lens; 
     FIGS. 7A-7C illustrate flat-top Fresnel lenses according to embodiments of the present invention; 
     FIG. 8 is a graph showing gain at the center and edge of a screen as a function of viewing angle for diffusing screens with and without a Fresnel lens. 
     FIG. 9 illustrates a Fresnel pattern over an entire screen area; 
     FIG. 10 illustrates a partial Fresnel pattern over a portion of a screen, according to the present invention; 
     FIGS. 11A,  11 B and  12  illustrate cross-sections through different embodiments of partial Fresnel screens according to the present invention; 
     FIG. 13 illustrates a viewing apparatus according to the present invention; 
     FIGS. 14A and 14B illustrate applications using screens with Fresnel lenses having nonuniform focal profiles; 
     FIGS. 15A-15D illustrate different embodiments of Fresnel lenses having grooves to avoid air entrapment during lamination and lens manufacturing; and 
     FIG. 16 illustrates a first-surface Fresnel lens according to an embodiment of the present invention. 
    
    
     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     The present invention is applicable to Fresnel lenses, and is believed to be particularly suited to Fresnel lenses for use with rear projection screens and monitors One of the advantages of the invention is that the appearance of ghost images is reduced, if not prevented altogether. Therefore, the invention may be used to improve the uniformity of the brightness perceived across the screen while retaining the quality of the image. 
     Among the many factors important in the design of rear-projection screens and monitors are i) efficient light use, ii) a high resolution and iii) a small form factor. A high efficiency is desirable so that the power of the light source may be reduced, thus reducing problems with disposal of waste heat, and reducing energy costs. There is a trend towards increasingly higher resolution, for example in high definition television (HDTV), to provide the viewer with a sharper, clearer image. Also, it is generally desired to reduce the form factor, such as volume, footprint or weight, so that the monitor takes up as little space in the user&#39;s environment as possible. The implementation of a large screen size, under the restriction of a small form factor leads to the use of wide-angle optical systems. Wide angle optical geometries place higher requirements on the optical components of the screen than exist with narrow angle systems. The present invention addresses this need for wide-angle components, while permitting efficient light use and high resolution operation. The present invention also permits the maintenance and/or improvement in screen resolution, brightness and brightness uniformity from screen center to screen edge. 
     Consider a rear projection optical system  100  as shown in FIG. 1, having a light source  102  that illuminates a rear projection screen  104 . The light source  102  is located at a distance d from the screen  104  and the half-angle cone of light emitted by the source  102  is given by θ. The distance from the center of the screen  104  to the edge is given by t. A viewer&#39;s eye is typically centrally located at position  106  at a distance r from the screen  104 . The angle formed between a normal to the screen  104  at the screen edge and the viewer&#39;s eye is given by the angle α. 
     At the edge of the screen  104 , the light is incident on the rear surface  108  at an angle equal to θ, where θ is measured relative to a normal to the screen. Therefore, the forward direction of light traveling through the screen  104  continues through the screen at an angle θ. The viewer perceives only that portion of light from the edge of the screen  104  that has been scattered through an angle equal to θ+α. Accordingly, the viewer perceives that the brightness at the edge of the screen is reduced. 
     This is illustrated further in FIG. 2 which shows the measured values of gain for a screen with a viewing angle of 64°. In other words, a collimated beam of light incident on the back surface of the screen is scattered into a cone having an angle of 64° as measured by the points where the intensity falls to half of the maximum. The measured gain curves are for illuminating the screen at angles of θ=0° (continuous line,  202 ), and θ=20 20   (dashed line,  204 ). The viewing angle, α, is the angle of the ray of light relative to the normal to the screen. 
     At the central position  106 , the viewer views the center of the screen  104  at a viewing angle of zero, which is normal incidence. At the same position  106 , the viewer views the edge of the screen at an angle α. Consider first the case where the light incident on the screen  104  is not redirected between the light source and the screen, for example by a collimating Fresnel lens. In this case, the light detected by the viewer from the center of the screen  104  was incident on the screen at an angle θ=0°, and so we use the upper curve  202 . Since the viewer is looking directly at the center position of the screen, the viewing angle α=0°. Therefore, the gain for light at the center of the screen is 0.97, point A. 
     The light detected by the viewer from the edge of the screen was incident on the screen at an angle θ=20°, and so we use the lower curve  204 . The viewer sees light from the edge of the screen at a viewing angle of α=−30°, i.e. light that has been scattered through an angle of 50° (20°+30°). Therefore, the gain for light at the edge of the screen  104  is 0.51, point B. Therefore, the screen brightness perceived by the viewer is 47% less at the screen edge (gain=0.51) than at the screen center (gain=0.97). This large drop in brightness across the screen  104  is undesirable and may be very noticeable to a viewer. 
     In one approach to substantially increase the uniformity of brightness perceived across a screen, a Fresnel lens  302  may be used to redirect the light from the light source  102  prior to incidence on the rear surface  108  of the screen  104 , as illustrated in FIG.  3 . In the illustrated case, the light incident on the screen  104  is collimated. The light propagates through the edge of the screen  104  in a direction parallel to the light passing through the center of the screen  104 . Therefore, the light perceived by the viewer at position  106  from the edge of the screen does not need to be scattered through an angle of θ+α, but only an angle α. Since, in this case, all light is incident on the screen  104  at an angle of θ=0°, we use only the upper curve  202 . Again, the light reaching the viewer from the center of the screen (α=0°) has a gain of 0.97. However, in this case, the gain of the light reaching the viewer from the edge of the screen, α=30°, is 0.78, point C. 
     Therefore, the perceived drop in brightness from the center to the edge of the screen  104  is only about 20% when the light reaching the screen  104  is collimated by the Fresnel tens. This figure may be further improved if the Fresnel lens is configured to bend the light at the edge of the screen towards the viewer. 
     The use of a Fresnel lens, however, introduces additional difficulties. For example, the Fresnel lens is normally supported either around its edge or on its input surface, since there is typically an air gap between the Fresnel lens output surface and the following optical component. Where the Fresnel lens is edge-mounted, the lens is made relatively thick to so that there is some degree of self-support, otherwise the lens may move or droop into contact with other components, and change the optical characteristics of the system. Where the Fresnel lens is supported on its input surface, the lens is typically attached to a transparent sheet, such as a sheet of glass. In both of these approaches, the Fresnel lens, or Fresnel lens/support combination, is relatively thick. 
     A Fresnel lens typically includes at least one structured surface, each portion of the structured surface lens having a functional surface that is angled with respect to the lens in order to re-direct light passing through that particular portion. Adjacent functional surfaces are typically connected by riser slopes. The functional surfaces and riser slopes typically present a grooved structure when viewed in cross-section. The pattern of functional slopes and riser slopes is generally referred to herein as a Fresnel structure. 
     One problem associated with Fresnel lenses is the generation of ghost images, which is discussed with reference in FIG.  4 . An incoming ray  402  enters a Fresnel lens  400  as internal ray  406  and then exits the lens  400  on an angled face  404 , also referred to as a functional slope. The ray  418  that propagates through the functional slope  404  is redirected to form the primary image. However, a portion of the internal ray  406  is reflected by the functional slope  404  as reflected ray  408 . A portion of the reflected ray  408  may be reflected off the entrance face  410  of the Fresnel lens is reflected ray  412 . The reflected ray  412  is incident upon another facet such as a riser slope  414  of the lens  400  and passes out as emerging ray  416 . The emerging ray  416  forms a secondary image. The ghost image formed by the emerging ray  416  may be perceived by the viewer and detracts from the quality of the primary image presented to the viewer, and effectively reduces the resolution of the image presented to the viewer. Therefore, it is desirable to reduce the effect of the ghost image. 
     There are two major approaches to reducing the effects of ghosting. One is to reduce the spatial separation between the primary and the ghost images in the plane of the screen to the point where there is no detectable separation, and the other is to reduce the amount of light in the ghost image. 
     The displacement of the ghost image relative to the primary image is dependent on a) the thickness of the Fresnel lens and b) the distance between the Fresnel lens and the screen and c) the angle of the functional surface. Reducing a) the Fresnel thickness and b) the separation between the screen and the Fresnel, or both, results in a reduction in the displacement of the ghost relative to the primary image. Also, reducing the angle of the functional surface results in a reduction of the separation between the ghost image primary image. The angle of the functional surface depends in part on the distance from the center of the lens and the difference in refractive index between the Fresnel lens material and the material into which the primary ray  418  travels. 
     When the spatial separation between the ghost and primary images in the plane of the screen is reduced to the point where the ghost image illuminates the same pixel as the primary image, then the viewer is unable to detect a ghost image. In such a case, the resolution of the image on the screen is unaffected by the Fresnel lens, while the advantages of increased brightness uniformity and light use efficiency are maintained. For example, in the case of high definition television (HDTV) having 1024 pixels across the screen, if the ghost image is separated from the primary image by less than approximately 0.098% of the screen width, then no transverse ghost image is visible. 
     In some situations, the Fresnel lens may be supported on a transparent sheet, such as a sheet of glass, that is attached to the input face of the Fresnel lens. Where this is the case, the reflected ray  412  that leads to the production of the ghost image may predominantly arise from reflection off a face of the transparent sheet, rather than the input surface of the Fresnel lens: the reflection at the interface between the Fresnel lens and the transparent sheet may be small due to index matching. In such a case, the separation between the ghost and primary images is not only dependent on the thickness of the Fresnel lens, but also the thickness of the transparent sheet. 
     Earlier approaches to reducing the brightness of the ghost image include depositing an absorptive coating on the riser slopes  414  of the Fresnel structure. This is difficult to do without the absorptive coating spreading on to an adjacent functional face. Another approach is to make the riser slopes  414  highly scattering, so that the light  416  exiting through the riser slopes  414  is highly scattered, thus reducing the brightness of the ghost image. Again, it is difficult to make the riser slopes  414  highly scattering without adversely affecting the functional surface  404 . In addition, this does not eliminate the ghost image  416 , but scatters it, resulting in a reduction in resolution. 
     The input face  410  of the Fresnel lens  400  may be treated, for example with a matte finish, to reduce the amount of light specularly reflected into the reflected ray  412 . This approach may, however, reduce the amount of light entering the Fresnel lens, or may scatter light, thus affecting resolution. The input face  410  and/or the functional face  404  may also be treated with an anti-reflection coating to reduce the amount of light reflected into ray  412 . This approach has limited utility, however, since anti-reflection coatings have a limited bandwidth and effective cone angle, outside of which the reflection is not significantly reduced. Therefore, since the anti-reflection coating on the input face  410  and/or functional surface  404  has to operate over a wide range of wavelengths and incident angles, the anti-reflection coating is not a very satisfactory approach to reducing the brightness of the ghost. 
     It is important to note that a mild matte finish or an antireflection coating may also be provided on the input face  410  of the Fresnel lens  400  in order to reduce ghost images that arise from specular reflections from the input face  410  interacting with the optical system used with the screen, for example turning mirrors. 
     Another approach to reducing the brightness of the ghost image is to provide some optical interaction within Fresnel lens itself, by loading the Fresnel lens with optically interacting particles  422 , to reduce the ghost image. Examination of FIG. 4 shows that light in the primary image  418  travels only a short path through the Fresnel lens  400 , whereas the light  416  in the ghost image travels a much longer distance within the Fresnel lens. If the optically interacting particles  422  disposed within the lens  400  are scattering particles, then the light in the reflected beams  408  and  412  has a high probability of being scattered, for example as scattered ray  420 , before emerging as the ghost image  418 . On the other hand, the light  418  in the primary image has a smaller probability of being scattered. Furthermore, since the ghost image  416  has a relatively long path length within the lens  400 , there is typically a greater separation between the ghost image and the light scattered from reflected beams  408  and  412 . Therefore, the ghost image  416  may be made less noticeable because of significant scattering, while there is only a small reduction in resolution in the primary image  418 . The degree of scattering within the Fresnel lens is selected to reduce the ghost image while maintaining the primary image. 
     In another approach, the optically interacting particles  422  disposed within the Fresnel lens  400  may be absorbing particles, in which case the light  416  in the ghost image has a high probability of being absorbed due to its long path length within the Fresnel lens  400 . On the other hand, the light  418  in the primary image has a smaller probability of being absorbed. Therefore, the ghost image  416  may be made less noticeable because a significant fraction of its light has been absorbed, with only a small reduction in brightness of the primary image  418 , and no affect on the resolution. The degree of absorption within the Fresnel lens  400  is selected to reduce the ghost image  416  while maintaining the primary image  418 . It will be appreciated that the optically interacting particles  422  may include a mixture of absorbing and scattering particles. 
     In one embodiment of the present invention, the thickness of the Fresnel lens is reduced, with a resultant reduction in the separation between the ghost image and the primary image. In the present invention, the thin Fresnel lens may be supported by being attached to an optical layer on the output side of the Fresnel lens. Supporting the Fresnel lens on the optical layer on the output side of the lens also reduces the separation between the lens and the optical layer, further reducing the separation between the ghost and primary images. 
     One particular approach to supporting a thin Fresnel lens is illustrated in FIGS. 5A-5C, which show different embodiments of an “embedded” Fresnel lens, where at least a portion of the Fresnel lens is embedded in the screen. 
     Considering first the embodiment illustrated in FIG. 5A, the screen  500  is formed from a support layer  504  and the Fresnel lens  502 . The Fresnel structure  506  of the Fresnel lens  502  is embedded completely in the support layer  504 . The support layer  504  may be a diffusing screen film. However, there is no requirement that the support layer  504  be a diffusing film, and there is no intention to limit the invention to such. The support layer may be another suitable type of dispersing screen, such as a lenticular screen, a beaded screen, a surface diffusing screen, a holographic diffusing, or a micro-structured diffusing screen. This list is not intended to be exhaustive. 
     An example of a support layer including a beaded screen, for example as is described in U.S. patent application Ser. No. 09/192,118, and incorporated herein by reference, is illustrated in FIG.  5 D. The screen  560  includes a Fresnel lens  562  embedded in a first transparent layer  564  having a lower index of refraction than the Fresnel lens  562 . A layer of refracting beads  566  is disposed between the first transparent layer  564  and a second transparent layer  568 . The beads  566  are embedded in a layer of absorbing material  570  that prevents light from passing through the interstices between the beads. The upper surfaces of the beads  566  receive light  572  from the Fresnel lens  562 . The light  572  is focused by the beads  566 , with the result that the light  572  diverges after passing through the screen  560 . 
     An example of a screen  580  having a surface diffuser screen is illustrated in FIG.  5 E. The screen  580  includes a Fresnel lens  582  embedded in a first transparent layer  584  having a lower index of refraction than the Fresnel lens  582 . The transparent layer  584  is laminated to a surface diffuser  586  having a refractive index different from that of the transparent layer  584 . Light passes through the Fresnel lens  582 , where it is re-directed, through the first transparent layer  584  and then through the surface diffuser  586 : the light is typically scattered when passing through the diffusing surface  588  of the surface diffuser  586 . In the example illustrated, the transparent layer is laminated to the diffusing surface  588  of the surface diffuser  586 . The diffusing surface  588  may be, for example, a holographic diffusing surface as shown, or may be a random or microstructured surface. The diffusing surface  588  may also be on the output surface of the surface diffuser  586 , rather than the input surface. Furthermore, the surface diffuser may also be loaded with scattering particles to provide additional bulk diffusion. 
     The redirecting effect of a functional surface of the Fresnel structure depends on the difference in refractive index between the material of the Fresnel lens and the material into which the redirected rays propagate. In a conventional Fresnel lens with the Fresnel structure in air, there is a large refractive index difference because the redirected rays pass into air from the Fresnel lens. In the case of an embedded Fresnel, the difference in refractive index is reduced, since the support layer  504  has a refractive index higher than that of air. It is generally advantageous to increase the difference between refractive indices of the Fresnel lens  502  and the support layer  504 . Materials that may be used for the Fresnel lens  502  include polycarbonate, polystyrene, epoxy acrylates and modified acrylates, or other suitable materials, such as a resin loaded with fine, high index inorganic particles. Materials that may be used for the support layer  504  include fluoropolymers and acrylics, such as polyvinyl fluoride, cellulose acetate, cellulose tri-acetate or cellulose acetate butyrate. The design of the Fresnel structure  506  is based on the refractive index difference between the lens  502  and the support layer  504 , so that light rays  508  and  510  entering the Fresnel lens  502  from an illumination source positioned at a design distance from the Fresnel lens  502  source, emerge from the Fresnel lens  502  into the support layer  504  in parallel directions. 
     The Fresnel lens need only redirect light towards the viewer to have a beneficial effect, and need not collimate the light. Nevertheless, in certain situations, collimation may be preferred in order to maximize overall screen performance. Therefore, in the description of the invention, the use of the term redirecting should be understood to include redirecting light through the Fresnel lens so as to be more advantageous to the viewer. This covers redirecting light so that the beam of light diverges from the Fresnel lens; parallelizing, or collimating, the light so that the transmitted beam essentially neither diverges not converges, and redirecting the light beam so as to converge at some point beyond the Fresnel lens. This range of possibilities may be regarded as bending the light so that it emerges from the Fresnel lens at one, or more, angles selected within a continuum of angles ranging from very little redirection, in which case the light diverges from the lens, to a significant amount of redirection, in which case the light converged from the lens. 
     It should also be appreciated that the focal length of the Fresnel structure need not be constant across the width of the lens. For example, the focal length of the Fresnel lens reduce from a high value at the center of the lens to a low value the edge of the lens. In such a configuration, the light in the center portion may be barely affected by the lens, while the light at the edge of the lens is redirected through a large angle. Moreover, the profile of the focal length, i.e. the value of focal length compared with position across the lens, need not be symmetrical, but may be asymmetrical so as to direct light towards one edge of the screen. This may be useful where, for example, the screen  1400  is mounted close to a wall  1402  and the viewer  1404  is positioned away from the wall  1402 , as shown in FIG.  14 A. This may also be useful where the screen  1410  is positioned at the edge of an array  1412  of screens, and it is desired to direct the light towards the viewer  1414  who is positioned centrally relative to the array  1412  of screens. 
     Several different methods may be used for making the screen  500 . The Fresnel lens  502  may be formed using one or more of several different methods, including, but not limited to, embossing, extrusion, casting and curing, compressive molding and injection molding. After the Fresnel lens  502  has been formed, the support layer  504  may be formed by one of a number of coating techniques. For example, polymeric material for the support layer  504  may be poured on to the Fresnel structure, and the material knife-coated thereover to fill in the grooves of the Fresnel structure. The polymeric material may then be processed, for example cured, dried, or cooled, to create a permanent support layer  504 . Without limiting the invention, the polymeric material may be UV curable, solvent-based, solventless, dryable, or thermoplastic. Other coating techniques that may be used include rolling, dipping, die coating, spinning, and spray coating. 
     It will be appreciated that a complementary process may be followed, where the support layer  504  is formed first, having the complement of the Fresnel structure on one surface. In such a case, the support layer  504  may be formed by a process such as embossing, extrusion, casting and curing, compressive molding and injection molding. The Fresnel layer may then be formed on top of the support layer  504  using a coating technique, for example as described in the previous paragraph. The complementary surface on the support layer  504  acts as a mold to form the Fresnel lens. 
     In another approach to an embedded Fresnel screen, illustrated in FIG. 5B, the screen  520  includes a Fresnel lens  522  attached to a central layer  526 . The support layer  524  is attached to the other side of the central layer  526 . In this embodiment, the central layer  526  is formed from a material having a lower refractive index than that of the Fresnel lens  522 . The central layer may be, for example an adhesive layer, such as a pressure sensitive adhesive, iso-octal acrylate-acrylic acid copolymer or thermoplastic hot melt adhesive. The three layer structure may be assembled by lamination, thermoforming, compression molding, or ultrasonic or RF welding. 
     It will be appreciated that the design of the Fresnel lens structure  506  and  522  in the respective embodiments takes into account the change in refractive index for light propagating from the Fresnel lens  502  or  522  into the adjacent layer  504  and  526 , respectively. Therefore, the design of the Fresnel lens need not be identical to a design for a Fresnel lens operating in air. However, the Fresnel groove structure  506  and  528  is designed to substantially redirect light propagating through the Fresnel lens  502  and  522  in a preferred direction, which may include collimation. 
     In another approach to an embedded Fresnel, illustrated in FIG. 5C, the screen  540  includes a Fresnel lens  542  attached to a central layer  546 . The support layer  544  is attached to the other side of the central layer  546 . The Fresnel structure  548  of the Fresnel lens is partially embedded in the central layer  546 , leaving air gaps  550  between the functional slopes  552  of the Fresnel structure and the central layer  546 . Such a screen may be formed, for example, by coating the support layer  544  with a thin layer of adhesive to form the central layer  546 , and then pressing the Fresnel lens  542  through the adhesive central layer  546 . The screen  540  may then be processed to fix the central layer  546 , for example by UV curing, heating, cooling, drying and the like. An advantage of this embodiment is that a thin Fresnel lens is provided with support, while still maintaining a large refractive difference between the lens material and air. The support layer  544  may be a diffusing layer. 
     Another approach to an embedded Fresnel is illustrated in FIG. 5F. A screen  1560  includes a Fresnel lens  1562  attached to a central layer  1566 . The support layer  1564  is attached to the other side of the central layer  1566 . The output surface  1568  of the Fresnel lens  1562  has a Fresnel structure  1570  that includes rising slopes  1572  and functional slopes  1574 . The peak  1576  between adjacent functional and rising slopes  1572  and  1574  of the Fresnel structure  1570  need not have the simple triangular cross-section as illustrated in FIG. 5C, where part of the functional slope is embedded in the central layer  1546 . In this case, the peak  1576  has an embedded portion  1578  having different cross-sectional shape. The case illustrated has embedded portions that are square or rectangular in cross-section. Other cross-sections may be used. An advantage provided by the embedded portion  1578  is that the peak  1576  may suffer less damage when being pushed through the central layer  1546 , than the structure illustrated in FIG.  5 C. 
     The embedded portions  1578  may correspond to the inactive, or unused, portions of the Fresnel structure  1570 , which are larger closer to the edge of the lens  1562 . In one approach, the embedded portions  1578  may be larger closer to the edge than the center of the lens  1562 , with smaller embedded portions  1580  closer to the center of the lens  1562 . Furthermore, the center portion of the lens  1562  may be provided with the embedded portions  1580 , even though corresponding functional slopes have no inactive portions. The embedded portions  1580  at the center of the lens  1562  need not significantly affect the re-directing capabilities of the lens  1562 , since light propagating through the center portion of the lens  1562  typically requires less re-direction than light propagating through the edge of the lens  1562 . 
     It will be appreciated that the partially embedded Fresnel lenses  542  and  1562  illustrated in FIGS. 5C and 5F may be also be formed from Fresnel lenses partially embedded directly into the respective support layers  544  and  1564 , rather than being partially embedded into a central layer. 
     The embedded Fresnel approach permits the Fresnel lens to be very thin, for example below 0.010 inches thick, thus substantially reducing the separation between the primary image and the ghost image formed by the Fresnel lens. 
     Another approach for mounting a Fresnel lens to a support layer is described with reference to FIGS. 6 and 7. This approach is termed the “flat-top” Fresnel. The basis of the flat-top Fresnel approach is described with reference to FIG. 6, which illustrates a portion of a Fresnel lens  600  and two rays  602  and  604  incident on the input face  606  of the lens  600 . Each ray  602  and  604  is refracted upon entering the lens  600  to produce internal rays  602 A and  604 A, respectively. The internal rays  602 A and  604 A are incident on the functional surface face  608 , and are refracted upon passing therethrough, to produce redirected rays  602 B and  604 B. 
     A portion  608 A of the functional surface  608  remains optically unused since it lies in the shadow of the adjacent riser slope  610 . Accordingly, that triangular portion of material labeled ABC incorporating the inactive portion  608 A is not used to redirect light passing through the Fresnel lens  600 . This triangular portion of material ABC may be removed from the Fresnel lens  600  to produce a flat surface  612  along the line AB. The surface  612  may be used for attaching the Fresnel lens  600  to a supporting film. 
     A screen  700  incorporating a flat-top Fresnel lens is illustrated in FIG.  7 A. The screen  700  is formed from a Fresnel lens  702  that is contacted to a support layer  704 . The support layer  704  may be, for example, a diffusing screen. The Fresnel lens  702  has a Fresnel structure  706  with truncated tips  708  and  709  that have respective flat surfaces  710  and  712  for attaching to the support layer  704 . The width of the flat surface  712  of the outer tips  708  is typically wider than the width of the flat area  710  of the inner tips  708 , because the angle at which light is incident on the Fresnel lens is greater towards the lens edge, thus creating a larger “shadow” region at the edge that may be removed to produce the flat contacting surfaces. 
     The flat-top Fresnel screen  700  is advantageous in that it provides a substantial flat area for attaching the Fresnel lens  702  to the support layer  704 , thus providing support to a thin Fresnel lens. The flat-top design also maintains an air gap between the active portions of the Fresnel structure  706  and the support screen  704 , permitting the lens designer to rely on a large refractive index difference when designing the Fresnel structure. A larger refractive index difference permits the reduction in the angle of the functional portions of the Fresnel structure  706 , thus increasing the manufacturability of the Fresnel lens. In addition, from a manufacturing viewpoint, there is a low probability of adhesive migrating into the grooves of the Fresnel lens when assembling the screen, and the use of truncated tips reduces the opportunity for damaging the tips of the Fresnel lens. Furthermore, the separation between the ghost image and the primary image may be substantially reduced, if not removed, because the Fresnel lens is in close proximity to the support layer and the Fresnel lens may be made to be thin, for example down to 0.010 inches or less. 
     Another advantage provided by the flat-top Fresnel lens  702  is that, even if not in direct contact with the second layer  704   a , the truncated tips  708  and  709  permit the Fresnel structure to approach more to the second layer  704   a , for example as illustrated in FIG.  7 B. The close proximity between the lens  702  and the second layer  704   a , for example a dispersing screen such as a diffuser or the like, reduces the separation between the ghost image and the primary image. In such a case, the lens  702  may be held taut in a perimeter frame. 
     Another example of a screen that uses a flat-top Fresnel lens is illustrated in FIG.  7 C. Here, the screen  750  includes a flat-top Fresnel lens  752  attached to a beaded screen  754  having an upper transparent layer  756 , a layer of beads  758 , and a lower transparent layer  760 . An opaque layer  762  prevents light from passing through the interstices between the beads  758 , and may also prevent reflection of ambient light from the front of the screen  750 . As is discussed in U.S. patent application Ser. No. 09/192,118, the gain of the screen may be adjusted by varying the difference in refractive index between the beads  758  and the upper transparent layer  756 . An advantage of using a flat-top Fresnel lens  752  with the beaded screen  754  is that light passing through the Fresnel lens  752  passes into the air gaps  764  between the Fresnel lens  752  and the upper transparent layer  756 . Therefore, the refractive index of the upper transparent layer  756  may be adjusted to produce a desired screen gain without affecting the re-directing, or collimating, effect of the Fresnel lens  752 . 
     The improvement in uniformity of brightness across a screen that may be gained from using a flat-top Fresnel screen is illustrated in FIG. 8, which shows gain as a function of a viewing angle. Curves  802  and  804  correspond to the gain curves for a diffusing screen alone, and were previously presented in FIG.  2 . The upper curve  802  illustrates the gain for light incident on the screen at an angle of θ=0°, while the lower curve  804  illustrates the gain for light incident on the at the edge of the screen. For normal-incidence viewing at the center of the screen, the gain at the center (at a viewing angle of 0°) is 0.97, point D. The gain at the edge of the screen (at a viewing angle of −30°) is 0.51, point E. 
     The brightness is significantly more uniform across the screen that includes the flattop Fresnel lens, curves  806  and  808 . The gain measured for the center of the screen is represented by the solid curve  806  and the gain measured for the edge of the screen is represented by the dashed curve  808 . The solid curve  806  closely tracks the ideal curve for a Fresnel-collimated screen  802 , except for a small loss in gain that is caused by reflective loss introduced by the flat-top Fresnel lens. The gain seen by a viewer at the center of the screen having the flat-top Fresnel lens (viewing angle=0°) is 0.91, point F, while the gain at the edge of the screen (at a viewing angle of −30°) is 0.70, point G. Therefore, the drop in brightness from the center to the edge of the screen with the flat-top Fresnel lens is approximately 23%. The drop in brightness from center to edge for the ideal screen, curve  802 , is about 20%, very close to the value of 23% for the screen with the flat-top Fresnel. Therefore, the flat-top Fresnel screen is effective at collimating the light from the light source and making the screen brightness uniform. The flat-top Fresnel lens has no additional support other than the diffusing screen, and may be made sufficiently thin that no ghost images are apparent to the viewer. 
     It should be noted that the Fresnel lens  542 , illustrated in the embedded Fresnel embodiment of FIG. 5C, may be embedded into the central layer  546  to a depth where the optically inactive portions of the Fresnel structure are embedded while the optically active portions  550  of the functional slopes  552  are exposed to the air gaps. Such a design permits the Fresnel lens  542  to operate with a large refractive index difference between the Fresnel lens and the air, while maintaining the supporting function of the support layer, and also while holding the thin Fresnel lens close to the support layer to reduce the appearance of ghost images. 
     Another approach to supporting a Fresnel lens on a support layer is illustrated with respect to FIGS. 9-12. This approach is termed a “partial Fresnel”. 
     Screens having a Fresnel lens typically have the Fresnel structure covering the entire screen. This is illustrated in FIG. 9, where the screen  900  is formed from a Fresnel lens  902  and a support layer  904 . The Fresnel structure  906  on the Fresnel lens  902  covers the entire surface of the screen  900 . 
     One method of fabricating a screen  900  is to apply the Fresnel lens  902  to the support layer  904 . However, certain difficulties may arise in applying a Fresnel lens to a support layer  904 . For example, the radially grooved structure  906  of the Fresnel lens  902  may lead to air entrapment when laminating one film to another, resulting in cosmetic defects apparent to the viewer. Therefore, it may be advantageous to avoid having the Fresnel structure where it is not needed. In particular, from the discussion above with regard to FIG. 8, it is apparent that redirection is required mostly at the side edges, especially across the width of the screen, and at the four corners of the screen to increase perceived brightness in these areas, while little redirection, if any, is required at the center of the screen. 
     Therefore, a screen may include a Fresnel lens that has a Fresnel structure only at those portions of the screen where redirection is required, e.g. at the edges and corners. Such an approach is illustrated in FIG. 10, which shows a screen  1000  with a Fresnel pattern  1002  at the edges and corners of the screen  1000 . The central portion  1004  of the screen does not have any Fresnel structure, since there is less need for redirection of light in the central portion  1004  of the screen  1000 . An advantage of this embodiment is that the number of defects arising during manufacture, e.g. due to air entrapment, may be reduced, thus enhancing the yield of acceptable screens. Additionally, since air facets are eliminated in the central portion and the angle of incidence is low, thus reducing reflection losses, the gain of the central portion of the Fresnel lens may be increased. 
     One particular approach to implementing a partial Fresnel lens is illustrated in FIG. 11A, which shows a cross-section of a screen that uses a Fresnel film  1102  attached to a support layer  1104 . The Fresnel film  1102  has a central portion  1106  that lacks a Fresnel structure and is, in this case, essentially flat. The edges of the Fresnel film  1102  are provided with a Fresnel grooved structure  1108  to redirect the light passing through the edges  1114  of the screen  1100 . 
     Here, the term attached is used to describe any method by which the Fresnel lens is joined to the support layer, such as adhesion, with or without an adhesive layer, lamination, heat lamination, fusion, or ultrasonic or RF bonding or welding, or the like. 
     In this particular embodiment, the Fresnel lens  1102  is also a flat-top lens where the tips of the Fresnel structure have flat surfaces  1110  for contacting to the support layer  1104 . 
     In another embodiment  1120 , illustrated in FIG. 11B, the Fresnel lens  1122  includes an output face  1124  having a flat central portion  1126 . The Fresnel structure  1128  at the edges of the lens  1122  has tips  1130  formed by the functional and riser slopes. The height of the flat central portion  1126  is selected to be approximately the same height as the tips  1130  so that the tips  1130  are close to contacting, or are in contact with, the surface  1132  of the support layer  1104 . 
     Another approach to a partial Fresnel structure, that also includes an embedded structure, is illustrated in FIG.  12 . The screen  1200  has a Fresnel lens  1202  embedded in a support layer  1204 . The Fresnel lens  1202  has a central region  1206  that is free of a Fresnel structure and is attached to the support layer  1204 . There is a Fresnel structure  1208  at the edges of the Fresnel lens  1202  that is embedded in the support layer  1204 . The refractive index of the support layer  1204  is less than the refractive index of the Fresnel film  1202 . The Fresnel structure  1208  is designed to redirect light entering the support layer  1204  from the Fresnel film  1202 . 
     It will be appreciated that other combinations of approaches may be used for manufacturing a screen with a supported, thin Fresnel lens. For example, a flat-top Fresnel lens may be embedded in a support layer, either having a full Fresnel pattern or a partial Fresnel pattern. Furthermore, a partial Fresnel lens may be partially embedded in a support layer, or a central layer. These are only examples of other approaches, and are not intended to limit the invention. 
     Another approach to reducing the problem of trapping air bubbles when the Fresnel lens is laminated to a screen is illustrated in FIGS. 15A-15D. In this approach, a Fresnel lens  1500  has a full Fresnel structure  1502 . Air-relief grooves  1504  cut across the Fresnel structure  1502  permit the passage of air out of one valley of the Fresnel structure into another valley when the lens is being laminated to its supporting layer or when the Fresnel lens is being manufactured. The air-relief grooves  1504  may be cut in different patterns across the Fresnel structure  1502 . FIG. 15A illustrates a partial radial pattern of air-relief grooves  1504 , such as may be used when the lamination process proceeds in a direction substantially parallel to the arrow. FIG. 15B illustrates a full radial pattern of air relief grooves  1504 . 
     A partial linear pattern of air relief grooves  1504  is illustrated in FIG. 15C, and a full linear pattern of grooves  1504  is illustrated in FIG.  15 D. Such linear patterns may be used when the lamination or Fresnel lens manufacturing process proceeds in a direction illustrated by the respective arrows, i.e. substantially parallel to the grooves  1504 . 
     The air-relief grooves  1504  need not be straight, and may take on other shapes so long as they provide relief for air to flow from one valley in the Fresnel structure to another valley. 
     Another embodiment of a Fresnel-screen that reduces the effect of ghost images is illustrated in FIG.  16 . The screen  1600  includes a front surface Fresnel lens  1602  attached, for example by lamination, to a diffusing layer  1604 . The diffusing layer  1604  may be attached to a substrate layer  1606  to provide support. An advantage of this screen  1600  is that the “land” between the Fresnel surface  1608  and the diffusing layer  1604 , in other words the thickness of Fresnel lens  1602  between the surface of the diffusing layer  1604  and the bottom  1610  of the groove between the riser slope  1612  and the functional slope  1614 , may be very small. This reduces the separation of the ghost image from the primary image. 
     A projection system that employs a screen with a thin Fresnel lens is illustrated in FIG.  13 . The projection system  1300  includes a light projector  1302  that includes a light source  1304  (l.s.) that generates a beam of light  1305 . The beam of light  1305  may propagate through beam handling optics  1306  before illuminating a reflective polarizer  1308 , for example a reflective polarizing sheet as described in PCT publication WO 96/19347. Light of a certain polarization is reflected by the polarizing beam splitter  1308  to a LCD array  1310 . The LCD array reflects the light back towards the polarizing beam splitter  1308 . The LCD array  1310  spatially modulates the light beam  1309  incident thereon by rotating the polarization through approximately 90°. Therefore, those portions of the light beam  1311  reflected by the LCD array  1310  whose polarization is rotated by the array  1310  are transmitted by the polarizer  1308  as beam  1313 . The beam  1313  may pass through transmission optics  1312  before illuminating the screen  1314 . The transmission optics  1312  may include, for example, projection lenses and/or a polarizer for cleaning up the image on the screen  1314 . The screen  1314  includes a Fresnel lens  1316  followed by a diffusing screen  1318 . As discussed above, the diffusing screen may be a bulk diffuser, a surface diffuser, a beaded screen, or the like. The screen  1314  may be any one of the embodiments described above, or a combination thereof, in which a thin Fresnel lens  1316  is supported on a diffusing screen  1318 . Light from the screen  1314  is detected by the viewer at position  1320 . 
     FIG. 13 does not show the lateral extent of any of the light beams, but indicates a central ray in each light beam. The lateral extent of the beams is determined, at least in part, by the particular beam handling optics  1306  and transmission optics  1312  employed in the projection system  1300 . The projection system  1300  may include one or more folding mirrors to reduce the depth of the system. When the form factor of the system is made smaller for a given size of screen, the divergence of light along the light path between the light source and the screen increases. This typically increases the need for a redirecting lens, such as a Fresnel lens, at the screen to maintain brightness uniformity and efficient light use. 
     It will be appreciated that the projection system need not be configured exactly as shown. For example, transmission optics may be positioned between the polarizer  1308  and the LCD array  1310  in addition to, or instead of, the transmission optics  1312  between the polarizer  1308  and the screen  1314 . In addition, the projection system may be configured using a transmissive LCD display, rather than a reflective LCD display. 
     It will be appreciated that the embodiments presented above have been used for illustrative purposes, and that certain features of the illustrated embodiments may be changed without affecting the present invention. For example, the Fresnel lens need not have a circular Fresnel pattern, as illustrated, but may have a Fresnel pattern which is a linear Fresnel pattern, for redirecting light along one axis, or may also be a two-dimensional Fresnel pattern, other than circular, for redirecting light along two axes. 
     As noted above, the present invention is applicable to display systems incorporating a Fresnel lens. It is believed to be particularly useful in reducing the effect of ghost images in back projection displays and screens. The use of the Fresnel lens of the present invention permits reduction in the form factor of the screen and high light use efficiency, while reducing ghost images to permit high resolution operation. Accordingly, the present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the present specification. The claims are intended to cover such modifications and devices.