WO2007109516A2

WO2007109516A2 - Reducing reflection

Info

Publication number: WO2007109516A2
Application number: PCT/US2007/064121
Authority: WO
Inventors: Barret Lippey
Original assignee: Bose Corp
Current assignee: Bose Corp
Priority date: 2006-03-17
Filing date: 2007-03-16
Publication date: 2007-09-27
Anticipated expiration: 2008-09-17
Also published as: US20070217010A1; AU2007227084A1; WO2007109516A3

Abstract

A plate (400) has a low birefringence and a retardation layer (300) is characterized by a fast optical axis and a slow optical axis. The retardation layer is positioned with its fast optical axis at a rotation angle selected to reduce an s-polarized component of light passing through the retardation layer at a particular angle of incidence.

Description

Reducing reflection

TECHNICAL FIELD

This description relates to reducing reflection.

BACKGROUND

Light from sources of information on the dashboards of automobiles can cast images on the windshield that are superimposed on the drivers or passenger's view through the windshield. Liquid crystal displays (LCDs) and other modern display devices used for information, navigation, and entertainment systems create larger sources of such reflected images than the basic displays of radios and other instruments used, in the past. The increasing angle of windshields in modern, aerodynamic cars can result in more reflections from the dashboard into the driver's field of view.

Light may also reflect from design features on the dashboard and cast images on the windshield that are superimposed on the driver's or passenger's view through the windshield.

SUMMARY In general, in one aspect, a plate has a low birefringence and a retardation layer is characterized by a last optical axis 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 the retardation layer at a particular angle of incidence, Implementations may include one or .more of the fol lowing 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 last optical axes at the same rotation angle. The two or more retardation films have different amounts of retardation. The particular angle of incidence is high as measured from a normal vector of the retardation layer. The particular angle of incidence is low as measured from a normal 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 the windshield. A backlight and a housing are included, and the assembly is adapted to be installed 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 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 following features. The polarization state is substantially s-polirization for the reflection from the vehicle window. The method includes affixing the polarizing layer to the surface.

In general, in one aspect an apparatus includes 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 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 features. The window is a vehicle windshield. An antireflection layer is included. The polarizing layer includes stretched and dyed plastic film. The polarizing layer includes a polarizing coating. Tlie polarizing axis is substantially parallel to a viewing direction through, the window. The surface has a primarily diffuse reflection. 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. The polarizing layer is affixed to the surface.

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 the vehicle window, the polarizing layer being configured to absorb a polarization state of the iight source. Implementations may include one or more of the following features. The polarization state is substantially s-polarization for the reflection from the vehicle window. The method includes affixing the polarizing layer to the surface.

Other ge neral 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.

DESCRIPTION FIG. 1 is a schematic side view of a driver in a ear.

FIG 2 A is a diagram of reflection of light showing polarisation components.

FI G. 2B is a graph of the reflectance of light as a function, of incident angle.

FIG 3. is a schematic perspective view of a retardation film.

FIGs.4 and 5 are schematic side views 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. 10A and 10B are schematic cross-section, views of optical filters.

As shown in FIG 1 , an L CD 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 190 passes

through the panel 102 in multiple directions. Direct, light 104 strikes and travels through

the panel 102 at a low angle relative to a vector 103 normal 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 specific

angles involved, reflected light 1 10 may be visible to the driver 112, causing the 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 not.

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 FIG 2A, light can he characterized as including

perpendicular polarisation components referred to as the s-polarized 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 to by that component,

e.g., "s-polarized light." Consider a plane 209 perpendicular to a reflective surface 202,

such that vectors 204 and 206 represent the direction of travel of incident and reflected

light and both rays of light are contained within the plane 200. The s-polarized

component 204s of the incident light 204 has an electric field vector vibrating

perpendicular to the plans 200 (in and out of the page in FIG 2A), and the p-polarized

component 204p has an electric field vector vibrating in the plane 200. Both components

are perpendicular to the direction of travel of incident light 204.

If reflective surface 202 is a smooth surface such as glass, the s-polarized

component 204s of incident light 204 tends to be reflected more than the p-polarized

component 204p, which tends to be transmitted more at certain angles rather than reflected. The amount of reflection for each component depends on the angle of incidence

θ_i, As shown in FIG 2B, for a low angle of incidence, only a small part of both the s~

polarized and the p-polarized components will be reflected, while for a high angle of

incidence, nearly all of both components is reflected. In between, however, the

components behave differently. At a point 252 on the graph, corresponding to an incident

angle of about 20°, the reflectance of the s-polarized component (line 254) begins to

substantially increase, while the reflectance of the p-polarized component (line 256)

begins to substantially decrease. The reflectance of the p-polarized component reaches a

minimum (no reflectance) at an angle θ_n before beginning to increase. The value of θ_!,

depends on the index of refraction of the material of the reflective surface. The index of

refraction for most materials varies slightly with the wavelength of the incident light The

vertical axis of the graph is based on an index of refraction n_i equal to 1.5. For other

indices of refraction, the vertical axis of the graph would be different. The average

reflectance (line 258) of the two components, equivalent to light having equal s-polarized

and p-polarized components (or natural, uopolarized tight), remains low until about Θ_p

and then begins to increase, such that all three lines approach complete reflectance as the

angle of incidence approaches 90°,

Returning to FfG. 1. one way to decrease the brightness of reflected light 110 is to

make sure that the indirect light 106 has a small s-polarized component and large

p-polarized 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

film 300 (also known as a polarization rotator) reorients the polarization of light passing

through it. Commercially available retarding films include the OptiGrafix^{T M} retarder films available from Grafix Plastics, Cleveland, OH. A retarding film has two optical

axes, a fast axis f and a slow axis s (in FIG. 3. f and s are orthogonal to each other, in

the plane of the retarding film 300, rotated 45° from the edges of the film). The rotation

of the film describes rotation of the film about a normal vector through its center, and is

measured by the angle between the last axis and some external 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 directions. 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 FKI 3, incident, light 302 has a relatively larger component 302s,

and a relatively smaller component 302f, resulting in a net polarization 302n Retarding

film 300 effectively rotates the polarization of incident light. 302 so that exiting light 302'

has a relatively smaller component 302s' and a relatively larger component 302f ,

resulting in a net polarisation 302n' at a substantially different angle than the original net

polarisation 302n,

The effect of the retarding film depends on its orientation relative to 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 depends on how much of

the retarding film material the light passes through. Light passing through the film at

incident angles other than perpendicular passes through a greater amount of material,

increasing its effect In the case of a dashboard-mounted LCD panel 102, the light from

the LCD has a known polarization and passes through the panel 302 and strikes the

windshield 1 08 at known angles. As shown in FIG 4, a thickness of retarding film 300 can be selected 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 minimize its reflection by the

windshield 108.

In the example of FlG, 4, the retarding film 300 is laminated onto a low-

birefringence plate 400 to form a filter 402. Birefringence is the property of a material

where there are 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

all directions and is 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 fllni 300

to the plate 400 assures 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 rotational angle relative to the horizontal and vertical axes of the LCD panel.

The birefringence plate 400 also protects the retarding film 300 from damage, separating

it from the environment. Commercially available low- birefringence plates include the

Clarex® brand made by Nitto Jushi Kogyo, Tokyo, Japan.

With this arrangement, a retarding film configured to assure that light passing

through at a high angle is p-polarized relative to the windshield 108 can have the

beneficial side effect of increasing the brightness of the LCD when directly viewed by a

driver wearing polarized sunglasses. Polarized sunglasses are typically designed to block

s-polarized light (since sunlight reflected off a horizontal surface, such as the ground or water, will be s-polarized relative to that surface). Since light from small and medium

size LCD screens is typically 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 Io 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 examples, as shown in FlG. 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. This can allow the filter 402 and the retarding film 300 to be reduced in size, as

only a small aperture is necessary to intercept all of the light 106 shining from the LCD

100 to the windshield 108.

In some examples, it is desirable to reduce the reflection of the LCD screen for

both the driver and the passenger, who may view the reflection in the windshield at

different compound angles φ_d and φ_p, especially if the LCD screen is angled towards the

driver, as shown in FIG 6. Some indirect light 106d strikes the windshield 108 and is

reflected to the driver at one angle φ_d, while other indirect light 106p strikes the

windshield 108 at a different angle φ_p. An angle of polarization that reduces the intensity

of the reflected light 110d seen by the driver 112d might increase the intensity of the

reflected light 1 10p viewed by the passenger 112p. In such a case, the retarding film 300

may be configured to achieve a polarization that reduces the reflection for both driver and

passenger, though typically not to as great an extent as 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 mid the position of their seat, 1 he retarding film, may be configured to optimize the reduction in reflection for the greatest range of

seating positions.

Retarding films are generally commercially available in a finite set of retardation

values, As shown in FIG. 7, multiple layers of retarding film may be combined to achieve

the retardation values needed to produce the desired adjustment to polarization. Layers of

retarding film 300a and 300b are adhered to each other, to the low-birefringence _.plate

400, and to an underlying substrate film 706 with a pressure-sensitive adhesive (PSA)

704 having a low birefringence, such as the optical adhesives available from Adhesives

Research, Glen Rock. PA. Anti -reflective coatings 702 and 708 are deposited or adhered

to the top and bottom of filter 402 to help prevent reflections from the top and bottom

surfaces. The assembled filter 402 is separated from the LCD panel 302 by an air gap

710. Different layers of retardation films may be positioned with their last axes at

different rotational angles to achieve a desired effect. The specific rotational angles

chosen will depend on the angle of the windshield 108 relative to the LCD panel 102, the

positions of the driver and passenger, and the polarization angle of the light generated by

the LCD panel 102. In one case, H was found that a film with 165 nπi of retardation at a

wavelength of 560 mil and a film with 300 nm of retardation at a wavelength of 560 nm

with their fast axes rotated 13 degrees coαπterclockwi se from the vertical axis of the I -CD

produced the minimum amount of reflection from the windshield of a iest vehicle for

both the driver and passenger positions. In another case, two layers of 250 nm retardation

π\m were each rotated at 90 degrees relative to each other with the back layer rotated 14

degrees counterclockwise from the screen horizontal and the front layer rotated \ 4

degress counterclockwise from the screen vertical. Another implementation concerns the reflection of light from the dashboard or

objects on the dashboard. The reflection from an object on the dashboard may reflect.

from a window and form an image that is a distraction to drivers or passengers in the

vehicle. By adding a polarizing layer between the object and the window, the

polarization of the reflection from the object is converted from natural polarization to p-

polarization. This reduces the reflection from the window and solves the problem of the

distracting image. The object may reflect light primarily in all 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 RG. 8, vehicle 800 has viewer 802. dashboard 804, surface 806,

window 812, perpendicular 818, and incident angle 820. The bold arrows show light rays

that form images. Outside light 808 passes through window 812, diffusely reflects from

surface 806, and forms directly reflected light 816. Outside light 808 also diffusely

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 directly 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 light 814 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's image of the surrounding

environment.

In the example of FIG 9, outside light 908 passes through window 904, passes

through polarizing layer 902, impinges on surface 900, specularly reflects from surface 900, forms light 914, and forms an image visible to viewer 906. Another ray of outside

light 910 (propagating in a different direction than, the direction of outside light 908)

passes through window 904, passes through polarizing layer 902, specularly reflects from

surface 900, forms light 912, impinges on window 904 at incident angle 924 (measured

from perpendicular 922), and specularly reflects from window 904 to form weak light

916 (as explained later) that does not form an image visible to viewer 906.

In FIG. 9, strong light rays are shown with bold lines and weak light rays are

shown with dotted lines. There are two polarization states shown: p-polarization Is

shown by a short arrow perpendicular to the direction of light propagation, S-

polarization is shown by a dot at the base of the p-polarization arrow signifying

polarization in a direction perpendicular to the plane of the page. These polarization

slates are defined with respect, to the reflection plane for window 904. The reilection

plane is defined at the plane that includes both the incident light ray and the reflected

light ray. In this example, the reflection plane for window 904 is the same as the

reflection plane for surface 900. Outside light 908 and outside light 910 have natural

polarization which is signified by having both a p-polarization arrow and an s-

polarization dot

In general, a polarizing layer has a polarization axis which is defined to be the

axis in the same plane as the polarizing layer which is parallel to the direction of the

polarizing layer that passes p-polarized light. In FIG. 9, polarizing layer 902 is rotated so

that the polarization axis its in the same plane as the p-polarization arrow. With this

orientation, polarizing layer 902 will transmit p~polarization and absorb s-polarization. In other words, the polarizing axis of polarizing layer 902 is substantialIy parallel to the

viewing direction through the vehicle window.

Outside light 908 with natural polarization passes through polarizing layer 902,

and becomes p-polarized light 914. With polarizing layer 902, the image of surface 900

is about 50% weaker in brightness than what it would be without polarizing layer 902.

Outside light 910 with natural polarization passes through polarizing layer 902,

and becomes p-polarized light 912. As shown in FIG 2B, the reflection of p-polarized

light 916 from window 904 is at an intensity less than or equal to approximately 4% for

incident angles less than roughly 70 degrees. Incident angles near 60 degrees are typical

for the dashboard and from windshield geometries of many automobiles, The weak light

916 forms an image that is much reduced in brightness relative to the image that forms

without polarizing layer 902.

In the example of FIG. 10A, polarizing layer 930 covers surface 940. Polarizing

layer 930 includes protective layers 950 and stretched, dyed plastic layer 960. Protective

layers 950 may be made of cellulose triacetate. Stretched, dyed plastic layer 960 may be

made of polyvinyl acetate. Optional anti reflection coating 970 may be added to reduce

the reflection from the top of layer 930.

In the example of FlG. 10B, polarizing layer 990 covers surface 940. Polarizing

layer 990 is a polarizing coating. The polarizing coating may be a liquid crystal material

applied with a wet roller process. Optional antireflection coating 970 may be added to

reduce the reflection from the top of layer 990. In FIGS. 10A and 10B, the polarizing layer forms an optical filler that transmits

one state of polarization and absorbs another state of polarization. Polarizing layer 930

or 990 may be attached to surface 940 by laminating to surface 940. Alternatively,

polarizing layer 930 or 990 may be attached to surface 940 by pressure sensitive

adhesive, UV-cure adhesive, or other attachment methods. Polarising layer 930 or 990

may be placed in front of surface 940 without making an attachment. Surface 940 may

be the dashboard itself, or may be a design feature on the dashboard. Design features are

surfaces that have distinctive colorings or textures that highlight specific areas for

ornamentation purposes such as a speaker bezel or a logo.

The vehicle may be an automobile, airplane, ship, or other vehicle that has a

window. Any transparent or translucent surface may be considered a window. Windows

may include windshields, and sunroofs. The windows may be located at the front, rear,

sides, or other areas of the vehicle. In the case of an automobile, the front window is

generally located close to and above the dashboard and the rear window is generally

located close to and above the rear package shelf The design feature may be on the

dashboard, on the rear package shelf, or in another area of the vehicle.

In some implementations, the s-polarized component of light passing through the

retardation layer is reduced for purposes other than reducing window reflections in a

vehicle.

Other implementations are within the scope of the claims. For example, the retarding film may be included in the LCD screen as part of the manufacturing process. A display based on liquid crystal on silicon (LCOS) or other technology could he used.

Claims

WHAT IS CLAIMED IS:

1. An apparatus comprising

a plate having 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 uptieaj axis at a rotation angle that reduces an s-polarized component of lighi passing through the retardation saver at a particular angjε of incidence.

2, The apparatus of claim I also comprising a layer of pressure-sensitive adhesive.

3. The apparatus of claim 2 in which the pressure-sensitive adhesive has a low birefringence.

4. The apparatus of claim 1 also comprising a layer of antireflective material.

5. The apparatus of claim 1 in which the retardation 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 angles.

8. The apparatus of claim 6 in which the two or more retardation films are positioned with their last optical axes at the same rotation angle.

9. The apparatus of claim 6 in which the two or more retardation turns have different amounts of retardation.

10. The apparatus of claim 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 retardation layer.

12. The apparatus of claim 1 also comprising an LCD panel

13. The apparatus of claim 12 in which

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 panes passing through the retardation layer and towards the windshield.

14. A method comprising

decreasing reflections from a light source by placing a film between the light source and a reflective 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 that is perpendicular to a plane defined by the angle of Incidence 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 which 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 some lighting env ironments. and

a polarizing layer positioned between the surface and the window, the polarising layer having a polarizing axis that is positioned to reduce visibility of the image reflected from the window.

19. T he apparatus of claim 18 in which the window is a vehicle windshield

20. The apparatus of cIaim 18 also comprising an antireflection layer.

21. The apparatus of claim 18 in which the polarizing layer comprises stretched and dyed plastic film.

The apparatus of claim 18 in which the polarizing layer comprises a polarising coating.

23 The apparatus of claim 18 in which the polarizing axis is substantially parallel to a viewing direction through the window.

24 The apparatus of claim 38 in which the surface has a primarily diffuse reflection

25. The apparatus of claim 18 in which the surface has a primarily specular reflection

26. The apparatus of claim 38 in which the object b a design feature on a dashboard.

27 T he apparatus of claim 18 in which the object is a speaker bezel.

28. T he apparatus of claim 38 in which the object is a design feature on a rear package shelf .

29 The apparatus of claim 38 in which the polarizing layer is affixed to she surface.

30. A method comprising decreasing a reflection of light from a surface, the light reflecting from a vehicle window , by placing a polarizing layer between the surface and the vehicle 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 state in substantially s- polari zation for the reflection from the vehicle window .

31 The method of claim 50 further comprising affixing the polarizing layer to the surface