Abstract:
In the manufacture of gradient polarized sunglasses an optical retarder is used in front of a portion of a polarized lens to destroy the linear polarization of incoming light to render that portion of the sunglass lens non-polarized, thus to permit viewing of polarized displays through that non-polarized portion of the polarized sunglass lens.

Description:
FIELD OF THE INVENTION 
     This invention relates to polarized sunglasses and more particularly to the utilization of a depolarizer over a portion of the polarized sunglasses to depolarize a portion of the polarized sunglass lens. 
     BACKGROUND OF THE INVENTION 
     As described in U.S. Pat. No. 7,374,282 issued May 20, 2008 and U.S. Pat. No. 8,172,393 issued May 8, 2012 a system is shown for providing a pair of sunglasses with a polarized region in the upper portion of the eyeglass lens and a non-polarized region at the bottom of the eyeglass lens so as to permit reading of polarized instruments through the non-polarized region. 
     In these patents for providing such a gradient polarized pair of sunglasses it is said that the polarizing material is differentially stretched such that at the top region of the sunglasses there is a maximum stretch to provide maximum polarization, whereas at the bottom portion of the sunglass lenses there is little or no stretching, thereby destroying the polarization characteristic of the lens in the lower region of the sunglasses. 
     It will be appreciated that while such a technique is technically feasible it is somewhat difficult to implement without for instance optical distortion. There is therefore a need to be able to provide an implementation of the gradient polarization sunglasses which is easy to manufacture and which is inexpensive, while at the same time preserving optical quality throughout the extent of the sunglass lenses. 
     By way of background, for polarized displays such as those in mobile phones, and as illustrated in U.S. Patent Publication No. 2013/0063684, a quarter wave retardation film is applied to a front polarizer on the display device. Glare due to sunlight reflected at the outer surface of the display panel can be reduced by viewing the display through polarized sunglasses and placing this quarter wave plate on the LCD display device results in making the display more visible through the polarized sunglasses. 
     The destruction of polarization on the polarized displays of handheld mobile devices and other polarized displays is shown in the following U.S. Patent Application Publications, namely 2012/0229732; 2012/0133859; 2012/0069264; 2011/0205471; and 2005/0237440. 
     All of these systems are utilized to improve the readability of a liquid crystal display in which a polarized film is applied to the display itself. Note that the quarter wave plates or other retarders have not been applied to sunglasses for any purpose much less to be able to read a polarized display in one region of the sunglass while at the same time providing a polarized version of a scene in another portion of the sunglass. 
     There is therefore a need to be able to implement the manufacture of gradient polarized sunglasses without having to differentially stretch polarization material within the lenses. 
     SUMMARY OF INVENTION 
     Rather than stretching polarized material in polarized sunglass lenses, in the subject invention a depolarizer in the form of an optical retarder is positioned in front of the polarized sunglass lenses to alter the state of polarization of the polarized light entering into the sunglasses such that what is viewable from behind the sunglasses by the individual wearing the sunglasses is equivalent to an unpolarized field of view. If for instance the individual wearing the subject sunglasses wishes to view a polarized display then the individual looks through the unpolarized portion of the sunglasses to be able to successfully view the display. When viewing a scene through the upper portion of the polarized sunglasses an individual is able to obtain the benefit of polarized sunglass. In one embodiment the optical retarder is a quarter wave plate. 
     It will be appreciated that while the subject invention is described in terms of the utilization of a quarter wave plate which destroys the linear polarization of incoming light and makes it circularly polarized, any transparent depolarizing material may be used, and which may in fact be an optical retarder with more than 5-6 full wave retardation with normal dispersion of birefringence. 
     In one embodiment of the subject invention a polarized lens has a quarter wave depolarizer adhered to the front surface of the lens. It will be appreciated that in this embodiment there may be a visible line on the sunglasses at the boundary between the top portion of the quarter wave strip and the remainder of the polarized sunglass lens. This is because the refractive index of air is 1.0, whereas the refractive index of the polarized lens is approximately 1.5. This difference in refractive indices can produce a visible line on the sunglass lens which may be objectionable to the wearer of the sunglasses or one who views an individual wearing the sunglasses. 
     While this type of quarter wave depolarizer element stuck on the surface of a polarizing lens does in fact provide for viewing of polarized displays, in a second embodiment to eliminate the boundary line a sandwich structure includes a polarized lens in the middle surrounded by glass covers, with the quarter wave or depolarizing element in between the polarized lens and the outer cover. 
     Adhesive is then utilized in the sandwiched structure in which the adhesive matches the refractive index of both the polarizer and the glass covers. The result is that it is virtually impossible to see the line demarcating the top of the depolarizing strip and the remainder of the polarized sunglass lens due to refractive index matching provided by the adhesive. 
     In yet another embodiment of the subject invention a polarized lens may be provided an orientable clear plastic sheet in which the upper portion of the sheet has a vertical machine direction and in which the lower portion of the clear plastic sheet is stretched at 45 degrees with respect to the machine direction of the upper portion of the sheet. This stretching is easily accomplished and provides and optical thickness approximating a quarter wave retarding element. This element is then laminated to the polarized lens so that the orientation direction of the element is at 45 degrees to the polarization axis of the polarized lens. 
     Regardless of the manner in which polarization is destroyed at the front portion of the polarized sunglass lenses, linearly polarized light entering the sunglasses at least in a bottom portion of the sunglasses has its polarization destroyed. This provides the ability to view a polarized display by looking downwardly through the unpolarized portion of the sunglass lens, thereby to be able to view polarized displays with a gradient polarized pair of sunglasses. 
     By way of definition as used herein the term depolarized is used to mean an alteration of the state of polarization which is equivalent to depolarization. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of the subject invention will be better understood in connection with the Detailed Description, in conjunction with the Drawings, of which: 
         FIG. 1  is an exploded view of the manufacture of a gradient pair of sunglasses in which a depolarizer quarter wave plate is adhered to the front surface of the polarized lenses; 
         FIG. 2  is a diagrammatic illustration of the effect of the adhering of the depolarizer to the front portion of the sunglass lenses of  FIG. 1  showing that there is a depolarized portion of the sunglasses for the viewing of polarized displays; 
         FIG. 3A  is a diagrammatic illustration of one embodiment of a method of manufacture of a gradient polarized sunglass lens illustrating the adhering of a depolarizer in the form of a quarter wave plate to the front surface of a polarized sunglass lens; 
         FIG. 3B  is a diagrammatic illustration of a sandwich structure for minimizing the visible boundary line between a depolarizer and the remainder of the polarized lens illustrating a polarizer sandwiched between two covers, with adhesive used to laminate the sandwich to get matching the refractive index of both the polarized lens and the covers; 
         FIG. 3C  is a diagrammatic illustration of the utilization of an unlaminated clear plastic sheet over the front surface of a polarized lens in which the bottom portion of the unlaminated clear plastic sheet is stretched in a direction 45 degrees to the machine direction of the plastic sheet, thereby to provide an optical thickness in the stretched portion that approximates a quarter wave retarding element; 
         FIG. 4A  is a diagrammatic illustration of the view of polarized instruments on a vessel when viewed by polarized sunglasses, illustrating that the displays are black due to cross polarization between the polarization of the polarized displays and the polarization of the sunglasses; 
         FIG. 4B  is a diagrammatic illustration of the polarized displays of  FIG. 4  when viewed with gradient polarized sunglasses illustrating that indicia on the displays is visible when viewed through the unpolarized portion of the gradient polarized sunglasses, whereas a distant object is made more visible by virtue of the fact of viewing the object through the polarized portion of the gradient sunglasses; 
         FIG. 5  is a diagrammatic illustration of utilizing an optical retarder in front of a linearly polarized film such that at least a portion of the linearly polarized light improving on the sunglasses through the optical retarder is viewable from the back side of the polarized lens, thus to be able to view a polarized display through the portion of the sunglass lens on which the optical retarder is located; and 
         FIG. 6  is a diagrammatic illustration of the conversion of linearly polarized light into circularly polarized light utilizing a quarter wave retarder to allow viewing of a polarized display through this portion of the lens regardless of the relative orientation of the plane of polarization of the impinging light and a linearly polarized lens. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1 , a fully polarized pair of sunglasses  10  is provided with fully polarized lenses  12  and  14 . Were a polarized display to be viewed by such a pair of sunglasses extinguishing due to cross polarization would occur making the polarized display seem black. 
     In order to avoid this effect, in one embodiment a pair of depolarizers  16  and  18  is applied to the front surfaces of polarized elements  12  and  14  for converting the state of the polarization of linearly polarized light so that what reaches the interface between the depolarizer and the polarized elements is circularly polarized and thus allowed to go through the linearly polarized lens rather than being extinguished due to cross linear polarization. 
     As illustrated in  FIG. 2 , such a pair of gradient polarized sunglasses includes regions  26  and  28  which are fully polarized, whereas due to the utilization of the depolarizing elements in front of the polarized lenses there are nonpolarized sections  22  and  24  such that in effect the lenses are nonpolarized at the regions  30  and  32 . 
     It has been found that by sticking on quarter wave retarders onto the front of polarized lenses an individual wearing such polarized sunglasses may in fact view a polarized display by looking downwardly through regions  30  and  32  towards a polarized display. 
     Referring now to  FIG. 3A , in one embodiment a cross section of the lenses of  FIG. 2  is shown in which there is a linearly polarized layer  40  to which is laminated a depolarizer  42  which in one embodiment is a quarter wave plate. It will be appreciated that there is an air interface between the top boundary  44  of depolarizer  42  and polarized layer  40 . The result is that there will be a noticeable line either when viewing the pair of sunglasses worn by an individual or plate by an individual looking outwardly from the polarized sunglasses. The reason for this is that the refractive index of the polarized layer  40  is on the order of 1.5, whereas air refractive index of air is 1.0. This difference in refractive index at boundary  44  causes boundary  44  to be visible and is in some senses undesirable, although sunglasses manufactured in this manner will in fact provide the unpolarized view of a polarized display while providing a polarized view of a scene. 
     In order to eliminate the visible boundary line, in one embodiment shown in  FIG. 3B  a polarized layer  50  is sandwiched between two transparent covers  52 , in one embodiment glass covers, with the retarder element  54  being positioned between polarized layer  50  and the front cover here shown at  52 ′. 
     This sandwich structure is adhered together utilizing an adhesive  56  which is disposed between rear cover  52 ″ and polarized layer  50  and between front cover  52 ′ and polarized layer  50 . The adhesive is chosen so that the refractive index of the adhesive matches the refractive index of both the polarizer and the covers, thereby to minimize any difference in refractive index between retarder  54  and other elements of the sandwiched structure. 
     As to the types of adhesives that are useable in the subject application, one can use thermal epoxy optical adhesives or a UV-cured optical adhesive. One thermal epoxy is available as EPO-TEK 301 from Epoxy Technology, Inc. of Billerica, Ma. 
     Referring now to  FIG. 3C , in another embodiment a polarized layer  60  is provided with an unlaminated clear plastic sheet  62  which due to its manufacture has a vertical viewable direction as illustrated by line  64 . Here it can be seen that a boundary  66  is formed between the upper portion of clear plastic sheet  62  and a lower portion  68  which is in fact stretched at a 45 degree angle with respect to the machine direction of the clear plastic sheet. By stretching the clear plastic sheet below boundary  66  so as to form a stretched portion  68  the optical thickness associated with the stretched portion has an optical thickness equal to Δnt=λ/4 to provide optical retardation. The result is that the boundary  66  is virtually invisible using a simple manufacturing to provide depolarizing in front of a polarized lens. 
     As illustrated in  FIG. 4A , which polarized sunglasses  10  are used to view polarized displays  70  on board a vessel  72  due to the polarization of these displays they appear black. While an object in a scene, here a buoy  74 , can be viewed through the polarized sunglasses  10  without difficulty, the polarized displays cannot be viewed by such a pair of polarized sunglasses. 
     However, referring to  FIG. 4B , when a gradient polarized pair of sunglasses  20  is utilized, polarized displays  70  have indicia clearly visible through the bottom portions  30  and  32  of the subject gradient polarized sunglasses due to the conversion of the state of the polarization due to the optical retarder. Here it can be seen that buoy  74  viewed through polarized portions  26  and  28  of the gradient polarizer is viewed as would normally be viewed using standard polarized sunglasses. 
     Referring to  FIG. 5 , in the theory of operation of the subject invention a split field linear/circular polarizing filter scenario is shown in which one has a linear polarizing film  80  and an optical retarder  82  adhered to the front surface  84  of the linear polarized film. In normal operation, linearly polarized light  86  is extinguished by the polarizing film such that little or no light is transmitted as illustrated by grey arrow  88  at the back side of the linearly polarized film. 
     However, by providing an optical retarder to convert the linear polarization of the incident linearly polarized light  86 , polarization is converted such that in effect the light is circularly polarized. When this circularly polarized light impinges upon linearly polarizing film  80  a portion of the polarized light here shown by arrow  90  exits the rear surface of the linearly polarized film such that if a polarized display is being viewed through the optical retarder its indicia will nonetheless be visible although at about half the light amplitude of the light impinging on the optical retarder. 
     Referring to  FIG. 6 , how this is achieved is shown by linearly polarized wave forms  90  and  92 . Wave form  90  impinges upon a polarizing element  94  which is linearly polarized as illustrated by vector  96  such that when the direction of vector  96  is orthogonal to the direction  98  of the impinging polarized light, the light is extinguished as illustrated by dotted line  100 . 
     On the other hand when linearly polarized light  92  impinges on a quarter wave element  102  the linearly polarized light as illustrated by  104  is converted as illustrated at  106  into a circularly polarized pattern. This pattern is equivalent to two orthogonal components with a relative phase lag of one quarter of a wave such that when light from the retarder impinges upon a polarizer  108  having a vertical polarization direction  110 , the vertical component  120  passes through the polarizer having a polarization direction  110  and the horizontal component  112  is extinguished as illustrated by dotted line  114  because it is crossed with direction  110 . 
     However, for the light that passes through the retarder and that impinges upon polarizer  108  that has a vector  120  parallel to vector  110  this light passes through polarizer  108  as illustrated at  116 . 
     While the present invention has been described in connection with the preferred embodiments of the various figures, it is to be understood that other similar embodiments may be used or modifications or additions may be made to the described embodiment for performing the same function of the present invention without deviating therefrom. Therefore, the present invention should not be limited to any single embodiment, but rather construed in breadth and scope in accordance with the recitation of the appended claims.