Abstract:
Improved optical elements having rises and faces, which may have annular or linear grooves, such as Fresnel spherical or cylindrical lenses, and methods of making them, are disclosed wherein an opaque coating is imparted to the rises by methods including positioning pre-printed opaque elements on a substrate such that when the optical element is pressed, stamped, embossed or molded from the substrate, rises of the completed element comprise said pre-printed opaque elements. A reflective element is made by imparting a reflective coating onto a substrate and then a non-reflective coating to the rises, e.g. by applying non-reflective material to the element and selectively cleaning it to leave a coat of non-reflective material adhering to the rises but not the faces. Photoresist, photographic emulsion, or ink may be used as the coating. Unwanted light may also be minimized by using circularly polarized light.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. § 119(e) from provisional U.S. application No. 60/023,677, filed Aug. 16, 1996. 
    
    
     SUMMARY OF THE INVENTION 
     The improvement to optical elements having rises and faces (ones that are stepped, e.g. a Fresnel lens) disclosed herein employs a light-absorptive element adjacent the vertical rises so that the amount of light passing through or reflected from the rises is minimized. 
     Thus the present invention provides an improved optical element having rise portions and face portions, wherein light transmitting through or reflected from the rise portions is substantially prevented from being seen by an observer viewing an image formed by light passing through the annular lens. More particularly, the rise portions are coated with an opaque light-absorbent material. 
     The improved optical element may be a refractive optical element, in which event the rise portions are substantially opaque and the face portions are substantially transparent. It may also have an anti-reflective coating. It may, for example, be a Fresnel lens or a Fresnel semi-lens or a lenserF lens, as disclosed hereinbelow. 
     The improved optical element may be a reflective element, such as an annular mirror. 
     Various methods of making the improved optical element are part of the present invention. Such methods comprise the steps of producing an optical element having rise portions and face portions, and imparting a light-absorbent coating to the rise portions. The optical element may have a series of annular grooves, or the grooves may be linear. The element may be refractive, in which event an antireflective coating may also be applied, or the element may be reflective. 
     The light-absorbent coating may be imparted to the rises by applying light-absorbing material to the element and selectively cleaning it so as to leave a coat of light-absorbent material adhering to the rise portions but not to the face portions. Adherence may be enhanced by roughening the rise portions in advance, such as by scratching, scoring, or abrading 
     Positive or negative photoresist may be used in the process in various ways. A coating of negative photoresist may be applied to the optical element so as to coat all faces and rises, then illuminating the photoresist so that the rises are not illuminated, so that exposed photoresist is only on the faces. Alternatively, positive resist can be used, and only the rises are illuminated. In either case, the element is then developed to remove the photoresist from the rises, and opaque material that is capable of adhering to the rises is then applied to the element. The photoresist is then removed from the faces by more developing to dissolve away remaining resist together with any of the opaque material coated on it. 
     Alternatively, the photoresist itself may be dark colored or dyed and left on the rises. Negative photoresist may be made to coat the rises selectively by exposing the rises but not the faces to light and then developing the optical element to remove unexposed photoresist. Alternatively, dark positive photoresist may be made to coat the rises selectively by exposing the faces but not the rises to light and then developing the optical element to remove exposed photoresist. 
     Other means for reducing flare and the like with stepped optical elements that are disclosed herein are also within the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is an elevation view of a Fresnel lens. 
     FIG. 1B is a cross-section view of a Fresnel lens taken at line  1 B— 1 B. 
     FIG. 2 is a cross-section view of a “lenserF” lens (LenserF is Fresnel backwards). 
     FIG. 3 is a cross-section view of a plano-convex lenserF lens. 
     FIG. 4 is a cross-section view of a portion of a Fresnel lens schematically illustrating light passing through the rise portion of the lens. 
     FIG. 5 is a cross-section view of a portion of a Fresnel lens which has a scored rise and shows light reflecting and scattering from the scored rise. 
     FIG. 6 is a cross-section view of a portion of a Fresnel lens which has an opaque coating on the rise, reducing scatter and flare. 
     FIG. 7 is a plan view of an annular lens of the present invention which is coated with photoresist and is partially covered with a light shield. 
     FIG. 8 is a cross-sectional elevation view of the annular lens of FIG. 7, taken along the line  8 — 8 . 
     FIG. 9 is a cross-section view of a sheet of plastic lens material having opaque pre-printed rings thereon, preparatory to being formed into an annular lens of the present invention. 
     FIG. 10 is a cross-section view of a step in the formation of an annular lens of the present invention by a press or mold, forming a lens with opaque rises from the sheet of plastic material shown in FIG.  9 . 
     FIG. 11 is a cross-section view of a portion of an annular lens, and a wave plate, schematically illustrating the passage of beams of polarized light therethrough. 
     FIG. 12 is a cross-section view of a portion of an annular lens coated with anti-reflective (AR) coating, schematically illustrating the effect of AR coating on a beam of light. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In general, a Fresnel lens is an optical element resembling a plano-convex or plano-concave lens that is cut into narrow rings and flattened out. Fresnel lenses can be large glass structures as in lighthouses, floodlights or traffic signals, or thin molded plastic plates with fine steps. 
     Fresnel lenses are commonly made from plastic, thus allowing them to be mass-produced inexpensively and quickly from a metal master. The metal master is typically made by rotating a copper or other material blank with a computer-manipulated cutting tool of diamond or other hard material. The master is then used to cast, emboss, compression mold or injection mold plastic replicas. 
     Conventional Fresnel lenses are formed with a series of annular rings and are therefore relatively thin, and relatively light. In contrast, conventional plano-convex and plano-concave lenses are thick, heavy, and expensive. 
     Fresnel lenses have poor resolving power when compared to conventional lenses because of the shape of the surface of each annulus. Each annulus of a Fresnel lens, when viewed in cross-section, has a vertical surface (a rise) and a lens-function surface (a face). Together the faces therefore merely approximate the desired curvature of the lens. 
     Annular lenses, such as Fresnel lenses, also suffer from flare or scatter of light as some of the light passes through and reflects off of the rise instead of through the lens face. To improve the optical performance of annular lenses, the amount of such light scattered by the rises should be reduced to a minimum. 
     Earlier workers have disclosed scratching or abrading the rises of Fresnel lenses. However, such methods do not provide a satisfactory solution to the problem of scattered light because the amount of scattered light is not reduced; rather it is merely scattered and flared in a different way. 
     The “lenserF” is another type of annular lens. The lenserF provides a lens which has as one side a series of spaced ring-shaped planar, or annular, faces, which together approximate the planar surface of a normal plano-convex or plano-concave lens. The annular faces are separated by rises. The concentric annular faces are reminiscent of a Fresnel lens, but with the planar face being stepped, rather than the curved face as in a Fresnel lens. Thus the name: “LenserF” is “Fresnel” spelled backwards. 
     A lenserF is thin, but, unlike a Fresnel, it is not flat. It retains the curved shape and depth of the plano-convex lens or plano-concave lens, and provides much better resolving power than a corresponding Fresnel lens that approximates the same curve. The effect of such a curvature is to create a cavity in which additional lenses (lenserF or others) can be placed. Compounding or cascading of lenses can take place in a much smaller volume, allowing lenses to be closer together than with conventional lenses (allowing greater magnification, chromatic aberration correction, lower F number, etc.). Compound plastic lenses are capable of being made to minimize chromatic aberration. 
     An improvement to annular lenses, including Fresnel and lenserF lenses, disclosed herein comprises means for decreasing the amount of light scattered from the rises of the lens. 
     A number of techniques are disclosed herein which are optionally used alone, or in combination with scratching or abrading the rises, to reduce the light reflecting from the rises. These include: (1) coating the rise with opaque material; (2) applying an anti-reflective (AR) coating to portions of the annuli of the lens; (3) pressing the lens from a blank having pre-printed opaque rings so that the pre-printed rings cover the rises of the lens; (4) using a transparent polar-analyzer in conjunction with polarized light; (5) using an opaque annular ring mask in conjunction with the lens; and (6) using photographic exposures of rings onto photosensitized rises and employing hydrophilic and hydrophobic coatings, copper over-coated with nickel with acid etched rings in the nickel, greasy ink, and water. 
     Specific substances appropriate to these purposes are known to those of skill in chemical technology. This information may be ascertained from customary reference works in this field, including for example the  Kirk - Othmer Encyclopedia of Chemical Technology, Beilstein&#39;s Handbuch , and  Chemical Abstracts.    
     These methods will be described below in greater detail using as an example of annular lens, However, it is to be understood that they are similarly applicable to other optical elements having rises. 
     (1) One method employs coating the rises of the lens with an opaque absorptive material. Generally, an opaque material such as ink is made to adhere to the rise, while leaving other portions of the lens free of opaque material. The rise portion of the lens is preferably scratched, abraded or scored during manufacture to promote better adherence of opaque material to the rise portion. 
     (1a) The rise is desirably scratched, abraded or scored during manufacture to facilitate adherence. Opaque material such as ink may thereafter be applied to an annular lens or other optical element by dipping, spraying or other conventional means, then subjecting it to a brief wash, with or without wiping by rubber or other material with appropriate grooves. Such a technique results in a coat of ink adhering to the scratched rise and leaves the smooth lens faces uncoated. 
     (1b) In another embodiment of the invention, the application of opaque material to selected areas of an annular lens is optionally achieved by the use of a photoresist. The photoresist is then used to protect the coated regions of the lens from the opaque material. This embodiment of the invention is optionally implemented by applying a coating of positive photoresist to the entire lens. Positive photoresist generally softens or depolymerizes upon exposure to light. A light shield with an appropriate opening (e.g. wedge-shaped) is then placed over the lens, so that when the lens is exposed to light from the proper angle, the faces are not exposed to light. The lens is rotated to provide successive exposure of the lens rises around the entire 360 degrees. By this process the rises, but not the faces, are exposed to light. A resist developer is then used to rinse away the photoresist from the rises, while leaving the unexposed photoresist on the face portions. The entire lens can then be coated with opaque material that is capable of adhering to the lens rises but that is prevented from adhering to the lens faces because of the presence of the photoresist thereon. The photoresist is then removed from the lens faces by further rinsing with developer to dissolve away the unexposed resist together with the opaque material coating it. This process leaves rises coated with opaque material and clear faces. 
     (1c) Yet another embodiment of the invention that uses a negative photoresist is as follows. Negative photoresist generally hardens or polymerizes upon exposure to light. As in the method described in section (1b) above, the lens is dipped or sprayed in photoresist and then exposed to a properly masked light. In this method, however the light exposure is arranged to strike the resist only on the faces. A developer will rinse away the unexposed negative photoresist from the rises. Coating of the lens with opaque material, and removal of remaining resist and opaque material from the faces is then performed as disclosed above. 
     (1d) Yet another embodiment of the invention involves using a photoresist that is black or other dark color, dyed black or other dark color, or is otherwise opaque once the process is finished. With proper exposure and development, e.g. as described above, the rises are left coated with the black resist, while the faces are left uncoated. 
     (1e) In another embodiment of the invention, pre-printed opaque rings or striations are positioned on material such as flat plastic such that when the lens or other optical element is pressed, stamped, embossed or molded, the rise portions of the completed lens will be made up of the opaque pre-printed rings. 
     (1f) In yet another embodiment, a transparent element having pre-printed thereon opaque lines or circles is positioned with respect to the rises of an existing stepped optical element such that the opaque markings intercept a substantial portion of the light which passes through, or which reflects off of, the rises. Optionally the opaque markings may be on the reverse side of the substrate from which the optical element is constructed. 
     (1g) In another embodiment, emulsion coated on both faces and rises of the lens are scanned by laser or exposed to an annular pattern to become dark on rises while being clear on faces after standard photographic development. 
     (1h) In another embodiment, grooves are formed in the lens surface when the lens is made. Then ink is put on and “doctor bladed” off, leaving ink in the grooves. Drying of the ink can be done by air, UV, heat, etc. 
     (2) Another embodiment of the invention involves applying an antireflective coating to virtually all surfaces of the lens to reduce reflection of any light from it. This embodiment of the invention effects its result in a manner similar to the way in which an AR coating on a television tube eliminates the reflections from room lights. In this embodiment of the invention, an AR coating applied to the Fresnel lens also coats the rises, thus reducing the reflection of light that impinges on or reflects from the rises. 
     Yet a further embodiment of the invention includes an AR coating on virtually all surfaces of an annular lens and an opaque coating on the rises. This embodiment combines a reduction of light reflected from the faces and an absorption of light that would otherwise pass outwardly through the rises, to further reduce light passing through or reflecting from the rise portions. 
     (3) In another embodiment of the invention, scattering from the rises of the lens is reduced by using an image source that produces circularly polarized light. Such an image source is optionally an LCD in conjunction with an appropriate polarizer, or it is optionally another source of light that has been circularly polarized. In this embodiment, only light which passes through the lens face directly without being reflected from the rise will be transmitted by a circular analyzer. 
     Annular Lens Embodiments 
     FIG. 1A shows a plan view of a Fresnel lens  35 - 300  of the prior art. FIG. 1B is a cross-sectional view of a Fresnel lens  35 - 300 , taken at line  1 B— 1 B. Shown is a rise portion  35 - 310  and a face portion  35 - 315 . 
     LenserF lenses are shown in FIGS. 2 and 3. FIG. 2 shows a cross section of a plano-concave lenserF  36 - 300 , which has rise portion  36 - 310  and a face portion  36 - 315 . FIG. 3 shows a cross section of a plano-convex lenserF  37 - 300 . 
     In FIG. 4 are shown details of a Fresnel lens  38 - 300  in cross-sectional view. Light ray  38 - 100  is shown. Rise portion  38 - 310  and face portion  38 - 315  are depicted. 
     Light ray  38 - 100  passes through rise portion  38 - 310 . Light ray  38 - 100  then reflects from face portion  38 - 315 , contributing to scatter or flare. 
     FIG. 5 shows details of a Fresnel lens  39 - 300  in cross-sectional view. Light rays  39 - 101 ,  39 - 102 , and  39 - 103  are shown. Also shown is scored rise  39 - 310 . FIG. 5 schematically illustrates additional scatter or flare arising from light ray  39 - 101  being reflected from a scratched rise  39 - 310  of a prior art Fresnel lens. Light ray  39 - 102  is shown to produce additional scatter or flare as it passes through the scored rise  39 - 310  and is refracted. Light ray  39 - 102  is shown to contribute additional scatter and flare. Light ray  39 - 103  contributes to scatter and flare. 
     FIG. 6 shows light rays  40 - 100 ,  40 - 101 ,  40 - 102 , and  40 - 103 . FIG. 6 also shows details of a Fresnel lens  40 - 300  in cross-sectional view. Also shown are each opaque rise portion  40 - 320 . Light rays  40 - 100 ,  40 - 101 ,  40 - 102 , and  40 - 103  are each shown to be absorbed upon hitting the rise portion  40 - 320 . 
     In accordance with the present invention, an opaque material such as ink or other coating material is made to adhere to the rise, while leaving other portions of the lens free of opaque material. The rise of the lens is preferably scratched, abraded or scored during manufacture to promote better adherence of material to the rise. 
     Opaque material is optionally applied to a lens by dipping or spraying the lens with a coating such as ink, then subjecting the lens to a brief wash. Such a technique results in a coat of ink adhering to the scratched rise and leaves each smooth lens face uncoated. 
     FIG. 7 shows another way to provide such opacity using photoresist on a Fresnel lens  41 - 300 . A light shield  41 - 328  with a wedge-shaped opening  41 - 329  is shown. 
     FIG. 8 is a cross sectional view of FIG. 7 taken along line  8 . Light rays  41 - 100  and  41 - 101  are shown. Unexposed photoresist  41 - 322  is shown. Exposed photoresist  41 - 324  is shown. Also shown is light shield  41 - 328 . 
     In an embodiment of the lens improvement, the application of opaque material to each rise of the lens is optionally achieved by the use of a photoresist. This embodiment of the invention is optionally implemented by applying a coating of negative photoresist to the entire lens  41 - 300 . A light shield  41 - 328  with a wedge-shaped opening  41 - 329  is then placed over the lens  41 - 300 , so that when the lens is exposed to light as from light ray  41 - 100  from the proper angle, each rise  41 - 310  is not exposed to light. The lens  41 - 300  is rotated to provide successive exposure of each lens face  41 - 315  around the entire 360 degrees. By this process each face  41 - 315 , but not any rise  41 - 310 , is exposed to light. 
     A resist developer is then used to rinse away the photoresist from each rise  41 - 310 , while leaving the exposed photoresist on each face  41 - 315 . The entire lens  41 - 300  can then be coated with opaque material that is capable of adhering to the lens  41 - 300  but that is prevented from adhering to the lens faces because of the presence of the photoresist thereon. The photoresist is then removed from each face  41 - 315  by further rinsing with developer to dissolve away the exposed resist  41 - 324  together with the opaque material coating it. This process leaves each rise  41 - 310  coated with opaque material and every face  41 - 315  clear. 
     Yet another embodiment of the invention that uses a positive photoresist is as follows. As in the method described above, the lens  41 - 300  is dipped or sprayed in photoresist and then exposed to a properly masked light. In this method, however, the light exposure is arranged to strike the resist only on each rise  41 - 310 . A developer will rinse away the exposed positive photoresist from each rise  41 - 310 . Coating of the lens  41 - 300  with opaque material is performed as disclosed above. The resist is developed away each face  41 - 315 . 
     Yet another embodiment of the invention involves using a photoresist that is dyed dark. With proper exposure and development, each rise  41 - 310  is left coated with the black resist, while each face  41 - 315  is left uncoated. 
     Referring now to FIGS. 9 and 10, in another embodiment of the invention there are opaque pre-printed rings  43 - 340  positioned on lens material  43 - 342 . The opaque pre-printed rings  43 - 340  are positioned such that when a lens  44 - 300  is pressed, stamped, embossed or molded, each rise  44 - 310  of the completed lens  44 - 300  will be made up of the opaque pre-printed rings  43 - 340 . 
     FIG. 11 shows an embodiment involving circularly polarized light. Light ray  45 - 100 , light ray  45 - 101 , and light ray reference point  45 - 102  are shown. Also shown are the Fresnel lens  45 - 300 , each rise portion  45 - 310 , each face portion  45 - 315 , and a circular polarizer  45 - 350 . 
     Scattering from each rise  45 - 310  of the lens  45 - 300  is reduced by providing an image source that produces circularly polarized light. Such light is optionally provided by the output from an LCD or is optionally other light that has been circularly polarized. In this embodiment, a circular polarizer  45 - 350  is provided so that light ray  45 - 100  which passes through the face will be properly transmitted by circular polarizer  45 - 350 . However, when light ray  45 - 101  reflects from the rise  45 - 310  at light reference point  45 - 102 , its polarization will change and it will not pass through the circular polarizer  45 - 350 . 
     Referring now to FIG. 12, another embodiment of the invention involves applying an anti-reflective (AR) coating to the lens  46 - 300  to reduce reflection of any light from any face  46 - 315  and any rise  46 - 310 . This embodiment of the invention effects its result in a manner similar to the way in which an AR coating on a television tube eliminates the reflections from room lights. In this embodiment of the invention an AR coating applied to the Fresnel lens also coats each rise  46 - 310 , thus reducing the reflection of light  46 - 100  that passes through the lens  46 - 300 . 
     Yet a further embodiment of the invention uses an AR coating together with an opaque coating  46 - 355 . This embodiment combines a reduction of light reflected from any face  46 - 315  and an absorption of light to further reduce light passing through or reflecting from any rise  46 - 310 . 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects as illustrative and not restrictive. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope. The inventor intends that all patentable subject matter disclosed herein eventually be the subject of patent claims, regardless of whether presented upon the initial filing of the application represented by this document or in a subsequent filing.