Patent Publication Number: US-2023152493-A1

Title: Transparent covering having anti-reflective coatings

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 16/584,648, filed Sep. 26, 2019, which relates to and claims the benefit of U.S. Provisional Application No. 62/748,154, filed Oct. 19, 2018 and entitled “TRANSPARENT COVERING HAVING ANTI-REFLECTIVE COATINGS,” the entire disclosure of which is expressly incorporated herein by reference. 
    
    
     STATEMENT RE: FEDERALLY SPONSORED RESEARCH/DEVELOPMENT 
     Not Applicable 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates generally to transparent coverings for windows, eyewear, or display screens and, more particularly, to transparent coverings having multiple lenses stacked one over the other and adhered together by adhesive. 
     2. Related Art 
     In various contexts, it is advantageous to affix transparent coverings to some substrate. Windows of buildings or vehicles may be covered with transparent window films for tinting (e.g. for privacy), for thermal insulation, to block ultraviolet (UV) radiation, or for decoration. Protective eyewear (e.g. goggles, glasses, and facemasks for off-road vehicle use, medical procedures, etc.) may be covered with a stack of transparent lenses for easy tear-away as the eyewear becomes dirty and obstructs the wearer&#39;s vision. Display screens of mobile phones, personal computers, ATMs and vending terminals, etc. may be covered with protective lenses to prevent damage to the underlying screen or block side viewing (e.g. for privacy and security in public places). When using such coverings, anti-reflective coatings may be implemented in order to reduce unwanted reflections, which may be especially problematic in multi-layer coverings that provide multiple interfaces at which incident light may reflect. However, typical anti-reflective coatings may not adequately reduce reflections over the whole visible spectrum (about 390 to 700 nm). Depending on the design wavelength range of the anti-reflective coating, this could result in a noticeable blue reflection (around 450 nm) or red reflection (around 700 nm) when light is incident on the transparent covering. 
     BRIEF SUMMARY 
     The present disclosure contemplates various systems, methods, and apparatuses, for overcoming the above drawbacks accompanying the related art. One aspect of the embodiments of the present disclosure is a transparent covering affixable to a substrate. The transparent covering includes a stack of two or more lenses, an adhesive layer interposed between each pair of adjacent lenses from among the two or more lenses, a first anti-reflective coating on a first outermost lens of the stack, and a second anti-reflective coating on a second outermost lens of the stack opposite the first outermost lens. The first anti-reflective coating has a first design wavelength range, and the second anti-reflective coating has a second design wavelength range that is different from the first design wavelength range. 
     The first design wavelength range may be centered at around 550 nm and the second design wavelength range may be centered at around 450 nm. 
     The first anti-reflective coating and the second anti-reflective coating may have different thicknesses. The first anti-reflective coating may comprise a film of magnesium fluoride (MgF 2 ) having a thickness of around 100 nm, and the second anti-reflective coating may comprise a film of magnesium fluoride (MgF 2 ) having a thickness of around 82 nm. 
     The transparent covering may exhibit normal-incidence reflectance of under 10% for all wavelengths between 390 nm and 700 nm. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the various embodiments disclosed herein will be better understood with respect to the following description and drawings, in which like numbers refer to like parts throughout, and in which: 
         FIG.  1    is schematic side view of a transparent covering according to an embodiment of the present disclosure; 
         FIG.  2    is a closeup view of the outermost surfaces of the transparent covering shown in  FIG.  1   ; 
         FIG.  3    is a graphical representation of normal-incidence reflectance as a function of wavelength for a transparent covering comprising a 200-gauge polyethylene terephthalate (PET) lens with an anti-reflective (AR) coating; 
         FIG.  4    is a graphical representation of normal-incidence reflectance as a function of wavelength for a transparent covering comprising a stack of three layered 200-gauge PET lenses with AR coatings on the outermost lenses, the AR coatings having the same design wavelength range; 
         FIG.  5    is a graphical representation of normal-incidence reflectance as a function of wavelength for the transparent covering of  FIG.  4    in which a comparison is shown between using AR coatings having a design wavelength range centered at 550 nm and using AR coatings having a design wavelength range centered at 450 nm; and 
         FIG.  6    is a graphical representation of normal-incidence reflectance as a function of wavelength for a transparent covering comprising three layered 200-gauge PET lenses with AR coatings on the outermost lenses, the AR coatings having different design wavelength ranges. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure encompasses various embodiments of a transparent covering having anti-reflective (AR) coatings. The detailed description set forth below in connection with the appended drawings is intended as a description of several currently contemplated embodiments and is not intended to represent the only form in which the disclosed invention may be developed or utilized. The description sets forth the functions and features in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions may be accomplished by different embodiments that are also intended to be encompassed within the scope of the present disclosure. It is further understood that relational terms such as first and second and the like are used solely to distinguish one from another entity without necessarily requiring or implying any actual such relationship in order between such entities. 
       FIG.  1    is schematic side view of a transparent covering  100  according to an embodiment of the present disclosure. Depending on its particular purpose, the transparent covering  100  may be affixed to a substrate such as a window (for tinting, thermal insulation, blocking ultraviolet (UV) radiation, decoration, etc.) protective eyewear (e.g. for easy tear-away), or a display screen (e.g. for scratch protection, side view blocking, etc.). The transparent covering  100  may include a stack of two or more lenses  110   a ,  110   b  (collectively lenses  110 ), an adhesive layer  120  interposed between each pair of adjacent lenses  110  of the stack, and AR coatings  130   a ,  130   b  on the outermost lenses  110  of the stack. In the example of  FIG.  1   , two lenses  110  are shown. However, a stack of three or more lenses  110  is also contemplated, with the number of lenses  110  depending on the particular application. The transparent covering  100  may be affixed to the substrate by adhesive, for example, in selective areas around the periphery of the transparent covering  100  as described in U.S. Pat. No. 6,536,045, the entire contents of which is expressly incorporated herein by reference. The adhesive used to affix the transparent covering  100  to the substrate may be the same as or different from (e.g. stronger than) that of the adhesive layers  120  interposed between each pair of adjacent lenses  110  of the stack. A stronger adhesive may be used, for example, in a case where individual lenses  110  are to be torn off without removing the entire transparent covering  100  from the substrate. The transparent covering  100  may instead be affixed by other means, for example, using tension posts of a racing helmet as described in U.S. Pat. No. 8,693,102, the entire contents of which is expressly incorporated herein by reference. 
     The lenses  110  may be a clear polyester and may be fabricated from sheets of plastic film sold under the registered trademark Mylar owned by the DuPont Company, such as a type of Mylar made from a clear polymer polyethylene terephalate, commonly referred to as PET. The lenses  110  and adhesive layers  120  may have an index of refraction between 1.40 and 1.52. The thickness of each lens  110  may be between 0.5 mil and 7 mil (1 mil is 0.001″), for example, 2 mil. Even after the adhesive material of the adhesive layers  120  is applied to a 2 mil thickness lens  110 , the thickness of the 2 mil thickness lens  110  may still be 2 mil due to the adhesive layer  120  having only a nominal thickness. The term “wetting” can be used to describe the relationship between the laminated lenses  110 . When viewing through the laminated lenses  110 , it may appear to be one single piece of plastic film. 
     The adhesive layers  120  used to laminate the lenses  110  together may be made of a clear optical low tack material and may comprise a water-based acrylic optically clear adhesive or an oil-based clear adhesive. After the lenses  110  are laminated or otherwise bonded together with the interposed adhesive layers  120 , the thickness of each adhesive layer  120  may be negligible even though the adhesive layers  120  are illustrated as distinct layers in  FIG.  1   . 
       FIG.  2    is a closeup view of the outermost surfaces of the transparent covering  100  shown in  FIG.  1   . In the upper portion of  FIG.  2   , a first outermost lens  110   a  is shown coated with the first AR coating  130   a . In the example of  FIG.  2   , the first AR coating  130   a  is a thin film AR coating that operates on the principle of destructive interference. A ray of light i (e.g. sunlight) incident on the transparent covering  100  first crosses a first interface  132   a  between the external environment (e.g. air) and the first AR coating  130   a  and thereafter crosses a second interface  134   a  between the first AR coating  130   a  and the first outermost lens  110   a . At each interface  132   a ,  134   a , a portion of the light i is reflected to produce a reflection ray r 1 , r 2 . By appropriately selecting a material and thickness of the first AR coating  130   a , the reflection ray r 2  produced at the interface  134   a  may be 180° out of phase with the reflection ray r 1  produced at the interface  132   a  for a given range of wavelengths referred to as the design wavelength range (which may be centered at a given wavelength referred to as the design wavelength). The resulting reflection rays r 1 , r 2  may thus destructively interfere with each other (i.e. peaks canceling troughs), such that the transparent covering  100  exhibits reduced reflection of light for wavelengths falling within the design wavelength range. 
     The AR coating  130   a  may be a single thin film of magnesium fluoride (MgF 2 ), which is a common material used in single-layer interference AR coatings due to its relatively low index of refraction (n D ≈1.37, where n D  refers to the index of refraction at the Fraunhofer “D” line) suitable for use on many transparent materials. However, any known AR coating materials and structures may be used, including multi-layer interference structures. The thickness of the first AR coating  130   a  may be selected to optimize the reduction in reflection for a desired design wavelength range. For example, in a case where the first AR coating  130   a  is a single-layer interference AR coating, the thickness of the first AR coating  130   a  may be a so-called quarter-wavelength thickness, for example, thickness d 1 =((n air /n coating )λ 1 )/4, where the design wavelength range is centered at with n air  being the index of refraction of the external medium, e.g. 1.00 for air, and n coating  being the index of refraction of the first AR coating  130   a , e.g. 1.37 for MgF 2 . When the light i is incident at 90° to the transparent covering  100 , the additional path length  2   d   1  traveled by the light through the first AR coating  130   a , from the interface  132   a  to the interface  132   b  and back again, causes the reflection ray r 1  to be advanced by half a period (i.e. 180° out of phase) relative to the reflection ray r 2  for the design wavelength λ 1 . This results in destructive interference between r 1  and r 2 , causing reduced reflectance for the design wavelength λ 1 . The effect may be less significant for off-normal incidence due to the angled path traveled by the light within the first AR coating  130   a.    
     In the lower portion of  FIG.  2   , a second outermost lens  110   b  is shown coated with the second AR coating  130   b . The second AR coating  130   b  may similarly be a thin film AR coating that operates on the principle of destructive interference. When the light i reaches the second AR coating  130   b , it first crosses a third interface  134   b  between the second outermost lens  110   b  and the second AR coating  130   b  and thereafter crosses a fourth interface  132   b  between the second AR coating  130   b  and the external environment (e.g. air). At each interface  134   b ,  132   b , a portion of the light i is reflected to produce a reflection ray r 3 , r 4 . Just like in the case of the first AR coating  130   a , by appropriately selecting a material and thickness of the second AR coating  130   b , the reflection ray r 4  produced at the interface  132   b  may be 180° out of phase with the reflection ray r 3  produced at the interface  134   b  for a given design wavelength range. The resulting reflection rays r 3 , r 4  may thus destructively interfere with each other, such that the transparent covering  100  exhibits reduced reflection of light for wavelengths falling within the design wavelength range. 
     The second AR coating  130   b  may have a structure and function equivalent to that of the first AR coating  130   a  but with a different design wavelength range (e.g. a design wavelength range centered at a different design wavelength λ 2 ≠λ 1 ), as will be described in more detail below. For example, the second AR coating  130   b  may similarly be a single-layer interference AR coating whose thickness may be a so-called quarter-wavelength thickness, for example, thickness d 2 =((n air /n coating )λ 2 )/4, where the design wavelength range is centered at λ 2 , with n air  being the index of refraction of the external medium, e.g. 1.00 for air, and n coating  being the index of refraction of the second AR coating  130   b , e.g. 1.37 for MgF 2 . In this way, the design wavelength range of the second AR coating  130   b  may be adjusted (relative to that of the first AR coating  130   a ) by changing the thickness of the second AR coating  130   b , without needing to use a different AR coating material or structural configuration. For example, in a case where the AR coatings  130   a  and  130   b  are single-layer interference AR coatings made of MgF 2  (n D ≈1.37), respective design wavelength ranges centered at 550 nm and 450 nm may be achieved using respective thicknesses d 1  and d 2  of around 100 nm and around 82 nm as shown below: 
     
       
         
           
             
               
                 
                   
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     In the above examples represented by Expressions 1 and 2, the two AR coatings  130   a  and  130   b  are single-layer interference AR coatings made of MgF 2  (n D ≈1.37). However, it is contemplated that the materials and structures and even the principles of operation of the first and second AR coatings  130   a ,  130   b  may differ, as long as the first and second AR coatings  130   a  and  130   b  have different design wavelength ranges. 
     It should be noted that the above description is somewhat simplified for ease of explanation. For example, the reflection rays r 1  and r 2  may experience an additional 180° phase shift that is not experienced by the reflection rays r 3  and r 4 , due to the interfaces  132   a  and  134   a  being interfaces going from low to high index of refraction relative to the incoming light i. However, since both the reflection ray r 1  and the reflection ray r 2  experience the same additional phase shift, the additional phase shift does not affect the destructive interference between the reflection rays r 1  and r 2 . 
       FIG.  3    is a graphical representation of normal-incidence reflectance as a function of wavelength for a transparent covering comprising a 200-gauge PET lens with an AR coating. Normal-incidence transmission as a function of wavelength is also shown. In the example of  FIG.  3   , the AR coating has a design wavelength range centered at around 550 nm (i.e. green light). The transparent covering of  FIG.  3    exhibits normal-incidence reflectance of under 10% for all wavelengths between 500 nm and 700 nm. Because the reflectance is higher for wavelengths shorter than 500 nm, rising to over 20% while still within the range of human vision (which extends down to around 390 nm), the transparent covering of  FIG.  3    produces a perceivable blue or violet reflection. 
       FIG.  4    is a graphical representation of normal-incidence reflectance as a function of wavelength for a transparent covering comprising a stack of three layered 200-gauge PET lenses with AR coatings on the outermost lenses. Normal-incidence transmission as a function of wavelength is also shown. The transparent covering of  FIG.  4    may have the structure of the transparent covering  100  shown in  FIGS.  1  and  2    with a third layer  110  between the layers  110   a ,  110   b , except that, in the example of  FIG.  4   , the AR coatings have the same design wavelength range as each other (unlike the AR coatings  130   a ,  130   b  of  FIG.  1   ). As in the example of  FIG.  3   , the design wavelength range of the AR coatings of  FIG.  4    is centered at around 550 nm (i.e. green light). In this case, however, due to internal reflections between the three PET lenses, the reflectance is somewhat worse in the low wavelength end, rising to over 30% while still within the range of human vision (which extends down to around 390 nm). Significant blue or violet reflections may be observed despite the use of two AR coatings. 
       FIG.  5    is the same as  FIG.  4    except that  FIG.  5    further depicts an additional curve shown as a dashed line. The dashed line represents normal-incidence reflectance as a function of wavelength for the same transparent covering, but with AR coatings having a design wavelength range centered at 450 nm used in place of the AR coatings having a design wavelength range centered at 550 nm. As can be seen, by using AR coatings having a design wavelength centered at 450 nm, the entire reflectance curve may be shifted to the left, thus improving the reflectance for low wavelengths. As shown, reflectance is under 10% all the way down to around 390 nm before rising for lower wavelengths outside the range of human vision. While this may greatly reduce or eliminate the perceivable blue or violet reflection, it comes at the expense of increasing reflectance at higher wavelengths (e.g. reflectance over 15% at around 700 nm), thus introducing a red reflection that was not perceivable using the AR coatings of  FIG.  4   . The choice between AR coatings centered at around 550 nm and AR coatings centered at around 450 nm thus represents a tradeoff between unwanted reflections of different colors. 
     In order to avoid the above tradeoff and eliminate reflections over a broader range of wavelengths, the transparent covering  100  shown in  FIGS.  1  and  2    makes use of two different AR coatings  130   a ,  130   b  having different design wavelength ranges. For example, the two AR coatings compared in  FIG.  5    may be combined in a single transparent covering  100 , with one AR coating on a first outermost lens  110   a  of the stack (e.g. the top lens  110   a  in  FIGS.  1  and  2   ) and the other AR coating on a second outermost lens  110   b  of the stack (e.g. the bottom lens  110   b  in  FIGS.  1  and  2   ). The transparent covering  100  may thus have a first AR coating  130   a  with a first design wavelength range centered at around 550 nm and a second AR coating  130   b  with a second design wavelength range centered at around 450 nm. In this way, reflections can be prevented both for low wavelengths below 500 nm and for high wavelengths above 600 nm. 
       FIG.  6    illustrates the resulting reflectance as a function of wavelength. The same transparent covering comprising three layered 200-gauge PET lenses with AR coatings on the outermost lenses is used, but with the AR coatings having design wavelength ranges centered at around 550 nm and 450 nm, respectively. As can be seen, the transparent covering of  FIG.  6    exhibits normal-incidence reflectance of under 10% for all wavelengths between 390 nm and 700 nm. 
     The design wavelength ranges of the AR coatings  130   a ,  130   b  need not be centered at 550 nm and 450 nm but may be centered at any appropriate design wavelengths for the particular application. For example, if red reflection is not a problem but ultraviolet reflection is, the design wavelength ranges may be further shifted to lower wavelengths, e.g. centered at 450 nm and 300 nm, respectively. Non-overlapping design wavelength ranges are also envisioned, such as where it is desired to reduce reflections of red and blue/violet light but to allow reflections of green light, which may be achieved, for example, by using design wavelength ranges centered at 750 nm and 250 nm, respectively. By combining the effects of the two AR coatings  130   a ,  130   b  having different design wavelength ranges in this way, reflections over a broad range of wavelengths may be eliminated using relatively inexpensive AR coatings such as single-layer interference coatings made of MgF 2 . 
     In the above examples, the external environment of the transparent covering  100  is assumed to be air having an index of refraction of around 1.00. However, it is also contemplated that the external environment may not be air. For example, in the case of a transparent covering  100  for a window of an underwater building or vehicle, the external environment may be water having a higher index of refraction. In some instances, the external environment may even be vacuum having a lower index of refraction than air. The above selection of AR coatings  130   a ,  130   b  can be made accordingly, with n air  referring generally to the index of refraction of the external medium. 
     In the above examples, the transparent covering  100  is described as being affixed to some substrate. However, it is also contemplated that the transparent covering  100  itself may be used without an underlying substrate, for example, affixed at its periphery to a surrounding wall or garment, such as is described in relation to  FIG.  6 C  of U.S. Patent Application Pub. No. 2018/0029337, the entire contents of which is expressly incorporated herein by reference. 
     Throughout this disclosure, the word “transparent” is used broadly to encompass any materials that can be seen through. The word “transparent” is not intended to exclude translucent, hazy, frosted, colored, or tinted materials. 
     The AR coatings  130   a ,  130   b  described throughout this disclosure may be applied according to known methods such as spin coating, dip coating, or vacuum deposition. 
     The above description is given by way of example, and not limitation. Given the above disclosure, one skilled in the art could devise variations that are within the scope and spirit of the invention disclosed herein. Further, the various features of the embodiments disclosed herein can be used alone, or in varying combinations with each other and are not intended to be limited to the specific combination described herein. Thus, the scope of the claims is not to be limited by the illustrated embodiments.