TECHNICAL FIEID
This description relates to reducing reflection.
BACKGROUND
Light .from sources of infornation on the dashboards of autornobiles can cast image.s on the windshield that arc superimposed on the driver's or passen0ger's view through the windshield. Liquid cnsta! display.s (LCDs) and other modem display devices used for infomation. navigation, and entertainment systems create larger sources of such reglected images than the basic displays of radios and other instruments used in the past. The increasing angle of windshieids in modem, aerodynamic cars can result in tnore reflections from the dashboard into the driver 's field of view.
Light may also reflect from design features on the dashboard and cast inuiges on the windshield that are superimposed on the driver's or passenger's view through the windshield,
SUMMARY
In general, in one a.spect, a plate has a low birefringence and a retardation layer is characterized by a last optical a.xis and a slow optical axis. The retardation layer is positioned with its last optical axis at a rotation angle that reduces an s-polarized component of light passing through tlte rctardation layer at a particnlar angle of incidence.
Implementations tnay include one or more of the following features. ,A layer of pressure-sensitive adhesive is included. The pressure-.sensitive adhesive has a low birefringence, A layer of antireflective material is included. The retardation layer includes a retardation film. The retardation layer includes two or more retardation films. The two or more retardation films are positioned with their fast optical axes at different rotation angles. The two or more retardation films are positioned with their fast optical axes at the .same rotadon angle, Tlte two or more retardation films have drfferent amounts of retardation. The particular angle of incidence is high as measnred from a normal vector of the retardation layer. The particular angle of incidence is low as

measured from a nonnal vector of the retardation layer, An LCD panel is included. The apparatus is configured to be installed in an automobile having a windshield, and the retardation layer is positioned to reduce the s-polarized component of light from the LCD) panel passing through the retardation layer and towards tire windshield. A backlight and a housing are included, and the assembly is adapted to be instalied into a dashboard of a vehicle.
In general, in one aspect, a method includes decreasing a reflection of light from a surface, the light reflecting from a vehicle window, by placing a polarizing layer between the surface and ihe vehicle window; the polarizing layer being configured to absorb a polarization state of ihe light source.
Implementation.s may include one or more of tire following features. The polarization state is substantially s-polarization for the reilection from the vehicle window, the method includes affixing the polarizing layer to the surface.
In general, in one a.spect, an apparatus includes an object having a surface that is positioned proximate to a window such that an image of the surface is retleeled from the vvindow in at least some lighting environments, and a polarizing layer positioned between the .surface and the window; the polarizing layer having a polarizing axis that is positioned to reduce visibility of the image reflected from the window.
Implementations may include one or more of the following feature.s. The window is a vehicle wind.shield. An antireflection layer is included. Fhe polarizing layer includes stretched and dyed plastic film. The polarizing layer includes a polarizing coaling. The polarizing axis is substantially parallel to a viewing direction through the window. The surface has a primarily diffuse reilection. The surface has a primarily specular reflection. The object is a design feature on a dashboard. The object is a speaker bezel. The object is a design feature on a rear package shelf, fhe polarizing layer is affixed to the surface.
In general, in one aspect, a method nreludes decreasing a reilection of light from a surface, the light reflecting from a vehicle window, by placing a polarlring layer between the surface and the vehicle window, the polarizing layer being configured to absorb a polarization state of the light source.

Implementations may include one or more of the iollowing features. The polarization state is substantially s-polarization for the reflection from the vehicle window, The method includes affxing the polarizing layer to the surface.
Other general aspects include other combinations of the aspects and features described above and other aspects and features expressed as methods, apparatus, systems, program products, and in- other ways.
Other features and advantages of the invention will be apparent from the following description and claims.
DESCRlPTION
FIG 1 is a schematic side view of a driver in a car,
FIG. 2 A is a diagram of reflection of light.showing polarization component,s.
FIG 2B is a graph of the reflectance of light as a function of incident angie.
FIG 3, is a schematic perspective view of a retardation film.
FlGs. 4 and 5 are schematic side vievvs of components in a car.
FIG. 6 is a schematic plan view of a driver and passenger in a car.
FIG, 7 is a schematic cross-section. view of an optical filter,
FIG. 8 is a schematic side view of a driver in a car,
FIG 9 is a schematic side view of components; in a car.
FIGS. .lO.A. and lOB are 'schematic cross-section views of optica! filters.
As .shown in FIG,1, an LCD screen consist of a backlight 100 and a light valve panel 102 located in the dashboard 12 of a car 10. Light from the backlight 100 passes through the panel 102 in muitipie directions. Direct light 104 strikes and travels through the panel 102 at a low angle relative to a vector 103 nonnai to the panel 102 and travels directly from the backlight 100 to a driver 112, Such light is referred to as light having a low angle of incidence. Indirect light 106 travels through the panel 102 at a high angle

relative to the normal vector 103 of panel 102 and is reflected by windshield 108. Such light is referred to as light having a high angle of incidence. Depending on the specifie angles involved, reiiected light 110 may be visible to the driver 112, causing (he driver to perceive a reflection of the panel 102 in the windshield 108. In some cases, this reflection is undesirable. Alternatively, in some cases, for example, a heads-up display, the reflected light 110 is intended to be visible to the driver and the direct light 104 is nol.
The amount of indirect light 106 that is reflected by the windshield 108 to produce reflected light 110 depends, among other things, on the polarization of' the indirect light 106, .As shown in FlG. 2A, light can be characterized as including perpendicular polarization components referred to as tiie s-polanzed and p-polarized components. These represent components of an electric field vector oscillating, or vibrating, in the corresponding direction. Light having only one component (that is, the other component has a magnitude of zero) is sometimes referred io by thai component, e.g.. "s-polarized light." Consider a plane 200 perpendicular to a refiective surface 202, such that vectors 204 and 206 represent the direction of travel of incident and retlected light and both rays of light are contained within the plane 200. The s-polaurized component 204s of the incident light 204 has an. electric field vector vibrating perpendicular to the phane 200 (in and ont of the page in FIG 2A), and the p-poiarzed component 204p has an electric field vector vibrating in the plane 200. Both componenis are perpendlcular to the direction of travel of incidenl light 204.
If reflective .surface 202 is a .smooth surface such as glass, the s-polarized component 2()4s of incidenl lighl 204 lends to be ref.ected more than the p-polarized component 204p, which tends to be transmitted more at certain angles rather than

reflocted. "The amount of rcflection for each component depends on the angle of."ineidence θi As shown in FIG 2B, for a low angle of incidence, oniy a small pan of both the s-polarized and the p-polarized components will be reflected, while far a high angle of incidence, nearly all of both compooents is reflected. In between, however, the components behave differently. At a point 252 on the graph, corresponding to an incident angle of about 20º, the reilectance of the s-polarized component (line 2,54) begins to substantially increase, while the reflectance of the p-polarized component (line 256) begins to substantially decrease. The reflectance of the p-polarized coraponenl reaches a mininum (no reflectance) at an angle (θp, before begining, to increa.se. The value of θp depends on the index of refraction of the miterial of the reflective surface. The index of refiaction lor most materials varies slightly with the wavelength of the inddeni light. The vertical a.xis of the graph is based on an index of refraction n, equal to 1,5. For other indices of refraction, the vertical axis of the graph would be different, fhe average reflectance (line 258) of the two components, equivalent to light having equal s-polarized and p-polarized components (or natnral, unpolarized light), remains low until about θp and then begins to increase, such that all three lines approach complete reflectance a.s the angle of incidence approaches 90º.
Returaing to FIG 1 one way to decrease the brightness of reilected light 110 is to make sure thal the indirect light 106 has a small s-polarized componer.t and large p-polarixed component, relative to the windshield 108, so that most of the indirect light 106 incident on the windshield 108 will be transmitted. As .shown in FlG 3. a retarding filn 300} (also known as a polarization rotator) reorients the polarization of light passing through it. Commerciaily available retarding films inchude the OptiGrafixTM retarder

Sims available from Grafix Plastics, Cleveland, OH. A retarding film has two optical
axes, a.fast axis! f and a slow axisS (in FIG, 3, f and S are orthogonal to each other in the plane of the retarding film 300, rotated 45º from the edges of the film). The rotation of the film deseribes rotation of the film abont a normal vector through its center, avid is measured by the angle between the last axis and some extenal reference, such as the edge of the film. Depending on the orientation of the film the speed of light passing through the film will be different in the two direetions. 'The speed of incident light
vibrating along the fast axis f will be faster than the light vibrating along the slow axis s, AS a result, the polarization of linearly polarized light can be rotated.
For example, in FIG3, incident light 302 has a relatively iarger component 302s, and a relatively smaller component 302f, resulting in a net polarization 302n, Retarding fiim 300 effectively rotates the polarization of incident light 302 so thai exiting light 302' has a relatively smaller coinponent 302s' and a relatively iarger component 302f, resulting in a net polarization 302n' at a substantially diffcrent angle than the original net polarization 302n,
The effect of the retarding film depends on its orientation relative io the polarization of the incoming light, its thickness T, and the angle of incidence θ. The amount by which each component is shortened or lengthened deponds on how much of the retarding film material the light passes through. Light passing through the film at incident angles other than peipendicuiar passes through a greater araoujit of rnaieriai. increasing its effect. In the case of a dashboard-mounted LCD panel 102, the light irom the LCD has a known polarization and passes throngh the panel 102 and strikes the
windshield 108 at known angles. As shown in FIG 4, a thickness of retarding film 300

can be selacted and the film positioned between the panel 102 and the windshield 108 such that the indirect light 106 will have a relatively larger p-polarized component 106p and smaller s-polarized component 106s and thereby minirnize its reflection by the windshield 108g.
In the example of FIG 4, the retarding film) 300 is laminated onto a low-birefringenee plats 400 to form, a t'dier 402. l^iretringence h the property of a material where there ait-. different indices of refraction depending on the direction of the light passing through the material, .Retardation films have high birefringence, A low-birefringence plate is one in which the index of retraction of light is nearly the same for ail directions and i.s therefore the same for both orthogonal components of polarization, and Is used in this example to reduce any effect the plate 400 may have on the polarization beyond the effect of the retarding, film 300. Attaching the retarding film 300 to the plate 400 assues that the retarding film 300 is positioned at the proper incident angles relative to the light 106 from the LCD backlight 100 and LCD panel 102 and at the proper rotatiatial angle relative to the horizotnal and vertical axes of the LCD panel File birefrigence plate 400 also protects the retarding film 300 from damage, separating it from the environment. Commercially available low-birefringence plates include the Clarex® brand nnule by Nitto lushi Kogyo, Tokyo, japan.
With this arrangement, a retarding film connfigured to assure that light passing through at a high angle is p-polarized relative to the wind.shield i 08 can have the beneiicial side effect of increasing the brightness of the LCD when directly viewed by a driver vveanng polarized sunglasses. Polarized sunglasscs are typically designed to block s-polarized light (since sunlight reflected off a horizontal surface, such as the ground

water, will be s-polarized relative to that surface). Since light from small and medium size LCD screens is tpically polarized at a 45 degree angle relative to horizontal, half of the energy of such light is blocked by polarized sunglasses, decreasing its apparent brightness, A retarding film configured to rotate the light to have a large p-polarized component relative to the windshield 108 can also be arranged to rotate the direct light 104 to have a large p-polarized component relative to the driver's .sunglasses.
in some exarnplcs, as shown in FIG 5, the filter 402, including the retarding film 300, is placed between the panel 102 and the windshield 108, but not in the driver's field of view, I'his can allow the .filter 402 and the retarding tilm 300 to be reduced in size, as only a small aperture is necessary to intercept all of the light 106 shioing from the LCD

00 to the windshield 108.

In some examples, it is desirable to reduce the reflecfin of the LCD screen for both the driver and the passenger, who may view the retlection in the windshield at different compound angles φd and φp especially if the LC.D screen isangled towards the driver, as shown in FIG 6. Some indirect light 106d strikes the will dshield 108 and is retlected to the driver at one iuigle φd while other indirect light 106p strikes the windshield 108 at a different angle (p„. An angle of polarization that reduces the intensity of the reflected light 1 lOd seen by the driver 1 ]2d might increase the intensity of the reflected light 110p -viewed by the passenger 11.2p. In such a case, the retarding film 300 may be configiired to achieve a polarization tliat redxtces the reileciion ti>r both, driver and passenger, though typically not to as great an extent a,s could be achieved if it were optimized for only one seating position. Similarly, the actual position of the driver and passenger will vary with the height of each and the position of their .seat. the retarding

film may be conflgured to optimize the redaction in reflection for the greatest range of seating positions.
Retarding film are generaliy commercially available in a finite set of retardation values. As shown in FIG: 7, multiplc layers of retarding film may be combined to achieve the retardation values needed to produce the desired adjustment to polarization. Layers of retiirding film 300a and 300b are adhered to each other, to the low-birefringence plate 400, and to ari anderlying substrfite film 7()6 with a pressure-sensitive adhesive (PSA) 704 having a low birefringenee, such as the optical adhesives available from Adhesives Research, Glen Rock, ?A. Ami-reflective coatings 702 and 708 are deposited or adhered to the top and bottom of filter 402 to help prevent rctlecfions from the top and bottom surfaces. The iissembled filter 402 is separated from the LCD panel 102 by an air gap 710. Different layers of retardation films may be positioned with their fast axes at different rotatior.a! angles to achieve a desired effect. The speeillc rotaiiotia! angles chosen will depend on the angle of the windshield 108 relative to the LCD pane! 102. the posilioTis of the driver and ptussenger, and the polarization aiigle of the light generated by the LCD panel ! 02. In) one case, it was found that a film with 165 nm of retiu'daiion at a wavelength of 560 nm and a film with 300 nm of retardadon at a wavelength of 560 nni with their last axes rotated 13 degrees counterclockwise from the vertical axis of the LCD produced the minimum amount of reflection from the windshieid of a lest vehicle for botli the driver and passenger positiotts. hi anotlier case, two layers of 250 nm retardation film were each mtated at 90 degrees relative to each other with the back layer rotated 14 degrees counterclockwise from the screen horizontai and the front layer rotated 14 deerees counterclockwise from the screen veriical.

Another implementation concerns the retlection of light from the dashboard or objects on the dashboard. The reflection from an object on the dashboard may rdlect from a window and form an image that is a distraction io drivers or passengers in the veliicle. By adding a polarizing layer between the object and the window, the polarization of the reiiection irom the ot'yect is converted frotn natural polarization to p-polarization, I'Ms reduces the reflection from the window and solves the problem of the distracting image. The object may reffect light primarily in ail directions (diffuse reflection), or it may reflect light primarily in one direction with the angle of incidence equal to the angle of reflection (specular reflection).
in the example of FIG 8, vehicle 800 has viewer 802, dashboard 804, surface 806. window' 813, pespendicular 818, and incident angle 820. the boid arrows show light rays that form images. Outside light 808 passes through window 812, diffusely retlecis from surface 806, and forms directly reflected light 816. Outside light 808 also diffuseiy reflects from surface 806, forms indirectly reflected light 810 which impinges on window 812 at incident angle 820 (measured from perpendicular 818), specularly reflects from window 812 and forms window reflected light 814. Both directiy reflected light 816 and window reflected light 814 form images visible to viewer 802. Viewer 802 may be a driver or a passenger in vehicle 800. Window reflected ligth814 forms an image of the reflected light from surface 806 in the window. The image of surface 806 is undesirable because it is superimposed on and interferes with the viewer image of the surrounding environment.
in the example of FIG 9, oui-side light 908 passes through window 904, passes
through polarizing layer 902, impinges on surface 900, specularly reflects from surface

WHAT IS CLAIMED IS;
1. An apparatus comprising
a plate liaving low birefringence, and
a retardation layer characterized by a fast optical axis and a slow optical axis,
and in which the retardation layer is positioned with its fast optical axis at a rotation angle that reduces an s-polarized component of light passing dirough the retardation iayer at a particuiar angle of incidence.
2. The apparatus of claim \ also comprising a layer of pressure-sensidve adhesive.
3. The apparatus of claim 2 in which the pressure-sensitive adhesive has a low birefringenec.
4. The apparatus of claim I also comprising a layer of antireflective material.
5. The apparatus of claim 1 in which the retai'dation layer comprises a retardation film,
6. The apparatus of claim 1 in which the retardation layer comprises two or more retardation films.
7. The apparatus of claim 6 in which the two or more retardation films are positioned with their fast optical axes at different rotation angies.
8. 'fhe apparjitus of claim 6 in which the two or more retardation films are positioned with their fast optical axes at the same rotation angle.
9. The apparatus of claim 6 in which the two or more retardation films have different amounts of retardation.
10. The apparatus ofclaim 1 in which the particular angle of incidence is high as measured from a normal vector of the retardation layer.
11. The apparatus of claim 1 in which the particular angle of incidence is low as measured from a normal vector of the retardalion layer.
12. The apparatus of claim I also comprising an LCD panel
13. The apparatus of' claim 12 in which
the apparatiis is configured to be inslailed in an automobile having a wiadshieid, and
the retardation layer is positioned to reduce the s-polarized eomponent of tight from the LCD panes passing through the retardation layer and towards the wiadshieid.
14. A method comprising
decreasing refections from a light source by placing a film between the light source and a reileetive surface, the film being configured to rotate a polarization of light from the light source.
15. The method of claim 14 in which the film. is configured to rotate a polarization of light from the light source by decreasing a magnitude of a polarization, component of the light titat is perpendicular to a plane deiined by the angle of ineidence of the light on the reflective surface.
16. The method of claim 14 in which the light source- comprises an LCD panel.
17. The method, of claim 14 in wlijch the reflective surface comprises a windshield.
18. An apparatus comprising

an object having a surface that is positioned proximate to a window such that an image of the surface is reflected from the window in at least aorae lighting environments, and
a polarizing layer positioned between the surface and the window, the polarizing layer having a polarizing axis that is positioned to reduce visibility of the image reflected from thc window.
19. 'The apparatus of claim 18 in which the window is a vehicle windshield.
20. The apparatus of claim 18 also comprising an antireflection layer.
21. The apparatus of claim 18 in which thc polarizing layer comprises stretched and dyed plastic film.
22. The apparatus of claim 18 in which the polarizing layer comprises a polarizit.\g coating.
23. The apparatus of claim 18 in which the polarizing axis is substantially parallei to a viewing direction through the window.
24. The apparatus of claim 18 in which the surface has a primarlly diffuse reliection.
25. The apparatus of claim 18 in vvhich the surface has a primarily specular reliection.
26. The apparatus of claim 18 in which the object is a design feature on a dashboard,
27. ['he apparatus of claim 18 in which the object is a speaker bezel
28. The apparatus of claim 18 in which the object is a design feature on a rear package shelf
29. The apparatus of claim 18 in which the polarizing layer is affixed to the surfece.
30. A method comprising
decreasing .a reflection of light from a surface, the light retlecting from a vehicle window, by placing a polarizing layer between the surface and the veiiicle window, the polarizing layer being configured to absorb a polarization state of the light source.
31. The method of claim 30 in which the polarization stale is substanfially s-polarization tor the reflection from the vehicle window.

.5i.

The method of claim 30 further comprising affixing the polarizing layer to the

surface.