Polarization optimized head-up display

Abstract

To strongly reduce ghost images.SOLUTION: An optical system comprises a display, a windshield, and an optical film configured to receive an image emitted by an active display region and reflect or transmit the received image toward the windshield. The windshield is configured to receive the image reflected or transmitted by the optical film and reflect at least a portion of the received image toward an eye, such that for at least one first location within a predetermined region of the active display region, the emitted image includes a first emitted image cone emitted from the first location, and the first emitted image cone includes a first emitted central image ray emitted from the first location. The optical film is oriented so as to cause the first emitted central image ray to be substantially polarized in a plane of incidence when incident on the windshield.SELECTED DRAWING: Figure 2A

Description

SUMMARY OF THE INVENTION

[0001] In some aspects of this specification, an optical system includes a display having an active display area with a maximum lateral dimension D and configured to emit an image for viewing by an observer's eye. The active display area includes a display center and a predetermined area including the display center. The predetermined area has a maximum lateral dimension d, and d / D ≤ 0.25. The optical system also includes a vehicle windshield and an optical film configured to receive the image emitted by the active display area and reflect or transmit at least a portion of the received image toward the windshield. The windshield is configured to receive the image reflected or transmitted by the optical film and reflect at least a portion of the received image toward the eye. For at least one first position within the predetermined area of the active display area, the emitted image includes a first emitted image cone emitted from the first position. The first emitted image cone includes a first emitted central image ray emitted from the first position. The optical film is oriented to substantially polarize the first emitted central image ray in the plane of incidence when the first emitted central image ray is incident on the windshield. The optical system includes the optical film.

BRIEF DESCRIPTION OF THE DRAWINGS

[0002]

Figure 1

Figure 2A

Figure 2B

Figure 2C

Figure 2D

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Figure 6A

Figure 6B

Mode for Carrying Out the Invention

[0003] In the following description, reference is made to the accompanying drawings which form a part hereof and in which various embodiments are shown by way of illustration. The drawings are not necessarily to scale. It should be understood that other embodiments may be contemplated and may be practiced without departing from the scope or spirit of the present specification. Accordingly, the following mode for carrying out the invention is not to be construed in a limiting sense.

[0004] In modern head-up display (HUD) systems, for example, a display such as a liquid crystal display (LCD) projects an image onto the vehicle's windshield, and the vehicle operator or passenger can view the image. One common problem seen in HUD systems is the "ghosting" problem when the projected image reflects from both the interface between the front glass and air (the inner surface of the windshield) and the interface between the rear glass and air (the outer surface of the windshield), and the two reflected images "bounce back" to the observer at different angles of incidence, generating a main image and an apparent second "ghost" image offset from the main image, which can reduce the readability of the displayed information. One common way to address this ghosting problem is for the windshield manufacturer to create a wedge shape between the outer and inner surfaces of the windshield. That is, the interface between the outer glass and air is arranged at a slightly different angle from the interface between the inner glass and air, which is intended to align the two reflected images into a single image for at least one ideal viewing angle (e.g., the viewing angle of an average-height driver). Another solution to the ghosting problem is to embed a weak-reflection polarizer between two glasses (i.e., sandwich it between an inner glass sheet and an outer glass sheet) rather than creating a wedge shape in the windshield, and reflect linearly polarized light from the reflection polarizer. One such example of a reflection polarizer is the 3M (trademark) Windshield Combiner Film (3M (trademark) WCF) manufactured by 3M Corporation. When a reflection polarizer is used, the image light rays from the display are linearly polarized (e.g., in the P-polarized state, or P-polarized image light rays) and directed towards the windshield at a constant angle of Brewster's angle or at an angle very close to Brewster's angle. The Brewster's angle is the angle of incidence at which P-polarized light passes through the interface between the inner glass and air without reflection and collides with the reflection polarizer, and at least a portion of the P-polarized light is reflected towards the observer as a single image (i.e., the image is reflected only by the reflection polarizer and not by the interface between the inner or outer glass and air).This technique works well to strongly reduce ghost images from the front and rear glass-to-air interfaces for angles of incidence near Brewster's angle. However, as the width of the HUD image increases, when the image is displayed to the observer at a horizontal skew angle (e.g., due to the inherent curvature of the windshield or the driver's angle to the image), the P-polarized image light rays defined according to the axial incidence play are converted into a mixture of P-polarized and S-polarized light, resulting in higher ghost image reflections.

[0005] According to some aspects of the present specification, an optical system based on the optimization of output polarization according to the rake angle and skew angle of the windshield is described. In some embodiments, the optimization can be performed on the display itself. In some embodiments, the optimization can be performed at any suitable point in the optical path from the display to the windshield. For example, the optimization can be performed by controlling the orientation of an optical film (such as a reflective polarizer) in the optical path, or the orientation of an optical reflector (such as a mirror used to fold the optical path), the orientation of a beam splitter in the optical path, or the orientation of a retarder layer (such as a quarter-wave plate or a half-wave plate) in the optical path. These examples are not intended to be limiting.

[0006] In some embodiments, an optical system (e.g., a head-up display, or HUD) includes a display, a vehicle windshield, and an optical film configured to receive an image emitted by the display and reflect or transmit at least a portion of the received image toward the windshield. In some embodiments, the display may be a liquid crystal display (LED), an organic light emitting display (OLED), a digital light processing (DLP) display, or any other suitable picture generating unit (PGU). In some embodiments, the display may have an active display area having a maximum horizontal dimension (i.e., the diagonal of the active display area) D, and the display may be configured to emit an image for viewing by an observer's eye, such as the eye of a vehicle operator. In some embodiments, the active display area may include a display center (i.e., the physical center of the display in both the horizontal and vertical dimensions) and a predetermined area surrounding the display center (i.e., the predetermined area is a subset or a portion of the entire active display area). In some embodiments, the predetermined area may have a maximum horizontal dimension d, and the ratio d / D may be about 0.25 or less (i.e., the predetermined area can be about one-fourth of the entire active display area). In some embodiments, the ratio d / D may be about 0.2 or less, about 0.15 or less, or about 0.1 or less.

[0007] In some embodiments, the optical system may be centered about an optical axis extending from a display center point to the windshield and to the observer's eye. In some embodiments, the optical axis may be defined by a first emitted center image ray.

[0008] In some embodiments, the windshield may be a glass windshield. In some embodiments, the windshield may be curved in either the horizontal direction, the vertical direction, or both. In some embodiments, the windshield may be a scoop angle (i.e., angled from vertical towards the operator). In some embodiments, the windshield may include a first interface surface (e.g., the inner glass / air interface surface) having a corresponding Brewster angle, and the first emitted central image ray is incident at the Brewster angle on the first interface of the windshield.

[0009] In some embodiments, the optical film may be configured to receive an image emitted by an active display area and reflect or transmit at least a portion of the received image towards the windshield. In some embodiments, the windshield may be configured to receive an image reflected or transmitted by the optical film and reflect at least a portion of the received image towards the eye. For at least one first position within a predetermined area of the active display area, the emitted image may include a first emitted image cone emitted from the first position. In some embodiments, the first emitted image cone may include a first emitted central image ray emitted from the first position. In some embodiments, the optical film may be oriented to substantially polarize the first emitted central image ray in the plane of incidence when incident on the windshield. Stated another way, the orientation of the optical film may be adjusted (e.g., rotated) such that at least the central image ray is "adjusted" to have polarization in the plane of incidence when the image impinges on the windshield. In some embodiments, the optical film may be oriented such that at least about 90%, or at least about 95%, of the polarization of the first emitted central image ray is in the plane of incidence when incident on the optical film.

[0010] In some embodiments, the optical film may be a reflective polarizer. In some embodiments, for substantially perpendicular incident light and for at least one visible wavelength within the visible wavelength range from about 420 nm to about 670 nm, the reflective polarizer reflects about 20% to about 40% of the incident light having a first polarization state (e.g., P-polarized light) and transmits at least 60% of the incident light having a second orthogonal polarization state (e.g., linearly S-polarized type light, or S-polarized light). In some embodiments, the reflective polarizer may be a multilayer optical film including a total number of more than about 30, or a total number of more than about 50, or a total number of more than about 100 layers. In some embodiments, the plurality of layers of the reflective polarizer includes a plurality of alternating first and second layers. In some embodiments, each of the first and second layers may have an average thickness of less than about 500 nm. In some embodiments, the difference in refractive index between the first and second layers along the first polarization state may be greater than about 0.05. In some embodiments, for substantially perpendicular incident light and for at least one infrared wavelength within the wavelength range from about 700 nm to about 1500 nm, the reflective polarizer may reflect at least 40% of the incident light for at least one of the first and second polarization states. In other words, in some embodiments, the reflective polarizer can function to reflect heat from sunlight and prevent the heat from entering the HUD system.

[0011] In some embodiments, the optical film may be an absorption polarizer. In some embodiments, for substantially perpendicular incident light, the absorption polarizer may transmit at least 60% of the incident light polarized along a first direction and absorb about 60% of the incident light polarized along a second orthogonal direction, and when the first emitted central image ray is incident on the absorption polarizer, it is polarized along a third direction, and the first and third directions form an inclination angle greater than about 5 degrees.

[0012] In some embodiments, the optical film may be an optical reflector (e.g., an optical mirror). In some embodiments, the optical reflector may be configured to receive an image emitted by an active display region and reflect at least a portion of the received image toward the windshield. In some embodiments, the windshield may be configured to receive the image reflected by the optical reflector and reflect at least a portion of the received image toward the observer's eye. In some embodiments, the optical reflector may be oriented to substantially polarize the first emitted central image ray in the plane of incidence when it is incident on the windshield. In some embodiments, for substantially perpendicular incident light and for at least one visible wavelength within the visible wavelength range from about 420 nm to about 670 nm, the optical reflector may reflect at least 60% of the incident light polarized along each of a first direction and a second direction orthogonal to each other (e.g., linearly P-polarized light or linearly S-polarized light). In some embodiments, for substantially perpendicular incident light and for at least one infrared wavelength within the wavelength range from about 700 nm to about 1500 nm, the optical reflector may transmit at least 40% of the incident light polarized along at least one of the first direction and the second direction orthogonal to each other. In some embodiments, the optical reflector may be a multilayer optical film including a plurality of alternating first layers and second layers, each of the first layer and the second layer having an average thickness of less than about 500 nm.

[0013] In some embodiments, the optical system may further include a retarder layer (e.g., a quarter-wave plate or a half-wave plate) disposed between the optical film and the windshield. In some embodiments, the optical film may be an optical reflector configured to receive an image emitted by an active display region and reflect at least a portion of the received image toward the windshield. In some embodiments, the windshield may be configured to receive the reflected image after the image reflected by the optical reflector passes through the retarder layer and reflect at least a portion of the received image toward the eye. In some embodiments, the retarder layer is oriented to substantially polarize the first emitted central image ray in the plane of incidence when incident on the windshield.

[0014] Referring now to the drawings, FIG. 1 is a top schematic view of a head-up display as seen in the prior art. As shown in FIG. 1, a typical head-up display 200 projects an image onto a windshield 40, which an observer 30 views as a reflected image 22. Most windshields available today are sometimes curved in both the horizontal and vertical directions (i.e., top to bottom as viewed by observer 30), as shown in the top view of FIG. 1. For example, as shown in FIG. 1, the image projected onto the windshield 40 by the HUD 200 strikes the windshield at a horizontal skew angle of about 10 degrees, which is not uncommon in modern vehicles and can sometimes be even greater than 10 degrees. The position of the HUD 200 may vary depending on the type of vehicle and may be located at a point farther from the observer 30 than shown in FIG. 1 (imparting an even greater skew angle to the reflected image 22), which is also important to note. When the image is projected such that it is primarily P-polarized light with respect to the axial plane of incidence (i.e., with respect to the optical axis), the light shifts to a mixture of P-polarized and S-polarized light due to being projected onto the surface at a significant horizontal skew angle. This increased S-polarization component may increase ghosting as seen by the observer 30.

[0015] FIG. 2A is a cross-sectional view of a polarization-optimized head-up display system according to an embodiment of the present specification. In some embodiments, the polarization-optimized head-up display (HUD) 300 includes a display 10 such as an LED, an OLED, or another PGU. The display 10 is configured to emit an image 20 along the optical axis 60 and is projected onto a windshield 40 having a first interface 44 (e.g., the interface between air and glass inside the windshield 40). The emitted image 20 includes an emission light cone 23 emitted from a first position 14 on the display, and the emission light cone 23 includes an emission center image ray 24. In some embodiments, the emitted image 20 passes through an optical film 50, and at least a portion 21 of the image is transmitted through it and projected onto the windshield 40, and at least a portion 21 of the image is then reflected from the windshield as a reflected image 22 and travels to the observer 30's eye. In some embodiments, the center image ray 24 may coincide with the optical axis 60.

[0016] In some embodiments, the windshield 40 may include an inner reflective polarizer layer 140 (e.g., a polymer film reflective polarizer sandwiched between an inner glass layer and an outer glass layer). In some embodiments, this may be a weak reflective polarizer that reflects 20% - 40% of the incident light having a first polarization state and transmits at least 60% of the incident light having a second orthogonal polarization state. The reflective polarizer 140 is discussed in more detail in the discussion of FIG. 3 elsewhere in this specification.

[0017] Figure 2B is a front view of the display 10 and the optical film 50, showing further details. When Figures 2A and 2B are considered together, the following discussion can be best considered. A predetermined area 13 of the active display area 11 of the display 10 includes the display center 12 (the physical center of the display in both the horizontal and vertical dimensions) and a first position 14 from which the light cone 23 is emitted. In some embodiments, the first position 14 and the display center 12 may be arranged together (i.e., they may be the same point). The emitted light cone 23 includes at least a first emitted central image ray 24, which is emitted by the display 10, travels through the optical film 50, is reflected from the windshield 40, and continues to the observer 30's eye. In some embodiments, the first emitted central image ray 24 may coincide with the optical axis 60 or may be substantially parallel to the optical axis 60. Referring to Figure 2B, the predetermined area 13 can have a maximum horizontal dimension d, the active display area 11 can have a maximum horizontal dimension D, and the ratio of d / D is about 0.25, or about 0.2, or about 1.5, or about 1.0. In other words, the predetermined area 13 represents the central portion of the active display area 11.

[0018] In some embodiments, the optical film 50 may be oriented (e.g., rotated) such that the first emitted central image ray 24 is substantially polarized in the plane of incidence when it impinges on the windshield 40. In other words, the optical film 50 may be rotated or otherwise oriented from its "conventional" orientation such that the polarization of the first emitted central image ray 24 is substantially polarized at the point of incidence with the windshield 40, which, as described above, can be a substantial skew angle with respect to the observer 30 on the curved windshield 40. In this way, the system can compensate for any ghosting that may be visible to the observer 30 due to the skew angle of the windshield.

[0019] Note that the optical film 50 is shown in FIG. 2B as having a rectangular shape that is physically rotated with respect to the display 10. However, the shape of the physical optical film 50 is arbitrary, and the orientation of the polarization feature portions within the optical film 50 is important. Also, as seen in other embodiments described herein, the optimization of the polarization of the image light ray 24 may be performed at other positions along the optical path of the image light ray 24.

[0020] FIGS. 2C and 2D show an embodiment in which the optical film 50 is an absorption polarizer 70. FIGS. 2C and 2D can be considered together for the best discussion of the following. When the optical film 50 is an absorption polarizer 70, for vertical incident light 71 (which may include the central image light ray 24), the absorption polarizer 70 can transmit at least 60% of the incident light 71 polarized along a first direction (polarization direction 72 as shown in FIG. 2C) and absorb at least 60% of the incident light 71 polarized along a second, orthogonal direction (polarization direction 73). In some embodiments, the absorption polarizer 70 may be oriented such that the light passing through the absorption polarizer 70 can be polarized along a new third direction 74. In some embodiments, the first direction 72 and the third direction 74 form an inclination angle α greater than about 5 degrees therebetween. In other words, referring to FIG. 2D, the central image light ray 24 is incident on the absorption polarizer 70 in the polarization direction 72 as shown as 24(p72), and at least a portion of the image light ray 24 exits the absorption polarizer 70 in the polarization direction 74 shown as 24(p74). In other words, the polarization of the central image light ray 24 is rotated by the optical film 50 and is substantially polarized within the plane of incidence on the distorted windshield.

[0021] As discussed with respect to FIG. 2A, the windshield 40 may include a reflective polarizer 140 embedded therein. FIG. 3 is a cross-sectional view of one embodiment of such a reflective polarizer 140. In some embodiments, the reflective polarizer 140 may be configured as a multilayer optical film (MOF). In some embodiments, the reflective polarizer 140 includes a plurality of alternating first polymer layers 142 and second polymer layers 143. In some embodiments, the combined number of alternating first polymer layers 142 and second polymer layers 143 can be from 30 to 700. In some embodiments, the first polymer layer 142 and the second polymer layer 143 may each have an average thickness of less than about 500 nm.

[0022] In some embodiments, the first polymer layer 142 may be substantially isotropic (i.e., exhibits substantially the same refractive index when measured in different directions). In some embodiments, the second polymer layer 143 may be substantially birefringent (i.e., exhibits two different refractive indices when measured in two different orthogonal directions), having in-plane refractive indices nx and ny along orthogonal directions, and the difference between nx and ny may be greater than about 0.05, or greater than about 0.03, or greater than about 0.01. In some embodiments, an additional layer 141 (e.g., an adhesive layer, another substrate layer) may be present on the outer surface of the reflective polarizer 140.

[0023] In some embodiments, the reflective polarizer 140 may be relatively weak, and when the incident light 145 impinges on the film, for at least one visible wavelength within the visible wavelength range from about 420 nm to about 670 nm, the reflective polarizer reflects about 20% to about 40% of the incident light 145 having a first polarization state (e.g., P-polarized light) and transmits at least 60% of the incident light having a second orthogonal polarization state (e.g., S-polarized light).

[0024] In some embodiments, for substantially perpendicular incident light 145 and for at least one infrared wavelength within the infrared wavelength range from about 700 nm to about 1500 nm, the reflective polarizer may reflect at least 40% of the incident light for at least one of the first polarization state and the second polarization state. In some embodiments, this can help reduce unwanted heat transfer (to the HUD) caused by sunlight striking the outer surface of the windshield.

[0025] Figures 4 and 5 are cross-sectional views of alternative embodiments of a polarization-optimized head-up display system according to this specification. Many of the elements of Figures 4 and 5, as well as those of Figure 2A, are common. Elements of Figures 2A, 4, and 5 that share reference numerals with the same numbers have the same functions, and additional explanations beyond the foregoing description may not be provided.

[0026] In Figure 4, the polarization-optimized head-up display system 400 has a "folded" optical path (when defined by the optical axis 60), and the image 20' is emitted from the active display region 11 of the display 10 and is first reflected from an optical reflector 90 (e.g., a mirror, a reflective film, etc.), and then at least a portion of the image 21' is redirected towards the windshield 40. In some embodiments, a retarder layer 100 (e.g., a quarter-wave plate or a half-wave plate) may be disposed on the optical reflector 90. In some embodiments, one or both of the optical reflector 90 and the retarder layer 100 may be oriented to substantially polarize the first emitted central image ray 24 in the plane of incidence when it is incident on the windshield 40. After the image 21' is reflected from the windshield 40, at least a portion of the image 22' is redirected towards the observer 30 as a perceived image. In some embodiments, the optical reflector 90 may be in a substantially flat form, a curved (e.g., convex or concave to apply force) form, or a free form. In some embodiments, the optical reflector 90 may be a polarizing reflector (e.g., a polarizing reflective multilayer optical film) capable of rotating polarization without a separate retarder layer 100.

[0027] In FIG. 5, the polarization-optimized head-up display system 500 adds further bending to the optical path represented by the optical axis 60. The display 10 emits an image 20' from the active display region 11. The image 20' travels towards the optical beam splitter 80. In some embodiments, the optical beam splitter 80 may be a reflective polarizer that substantially reflects incident light in a first polarization state (e.g., linearly S-polarized light) and substantially transmits incident light in a second, orthogonal polarization state (e.g., linearly P-polarized light). In one embodiment, the central image ray 24 may be emitted as light having an S-polarization state, reflected by the optical beam splitter 80, and redirected towards the optical reflector 90. The retarder layer 100 disposed on the optical reflector may then convert the S-polarized light to P-polarized light when at least a portion of the image 21' is redirected towards the optical beam splitter 80 and then passes through. The image 21' is reflected from the windshield 40, and at least a portion of the image 22' is redirected towards the observer 30. The orientation and rotation angle of the beam splitter 80 may be used to set the output polarization as seen in the plane of incidence. In the embodiments of FIGS. 4 and 5, any of the elements including the optical beam splitter 80, the optical reflector 90, and the retarder layer 100 may be oriented such that the image (including the image ray 24) is substantially polarized at the point of incidence on the distorted windshield 40.

[0028] Finally, FIGS. 6A and 6B provide graphical plots illustrating the effects of a polarization-optimized head-up display according to this specification. FIG. 6A provides a plot of the ratio of the contrast of the ghost image of the output of a P-polarized head-up display when the polarization optimization adjustment is not performed (i.e., in the case of the embodiment of FIG. 2A, the optical film 50 is not rotated with respect to the display 10). The x-axis of FIG. 6A plots the horizontal skew angle of the windshield with respect to the axial plane (the angle from the axial plane at the point of incidence of the image). The y-axis plots the vertical angle of incidence. The dashed rectangular box represents the "eyebox" or active viewing area (approximately 5°×12°) of the displayed image as seen by the vehicle operator. The shaded contour in FIG. 6A indicates that the contrast ratio of the ghost image increases significantly towards the right side of the "eyebox", exceeding 2 percent and approaching 3 percent.

[0029] FIG. 6B shows the improved performance of the same system achieved when the output polarization of the HUD is rotated by approximately 5 degrees. The entire "eyebox" in FIG. 6B shows a ghost contrast ratio of less than approximately 1%, and the ghost image perceived by the operator is significantly reduced.

[0030] Terms such as "about" will be understood by those skilled in the art in the context in which they are used and described herein. When the use of "about" applied to quantities representing the size, amount, and physical characteristics of a feature is not apparent to those skilled in the art in the context in which it is used and described herein, "about" will be understood to mean within 10 percent of a particular value. An amount given as about a particular value can be exactly the particular value. For example, when it is not apparent to those skilled in the art in the context in which it is used and described herein, an amount having a value of about 1 means that the amount has a value between 0.9 and 1.1, and that the value can be 1.

[0031] Terms such as "substantially" will be understood by those skilled in the art in the context in which they are used and described herein. If the use of "substantially equal" is not clear to those skilled in the art in the context in which it is used and described in the description of the present invention, "substantially equal" means approximately equal when "about" is as described above. If the use of "substantially parallel" is not clear to those skilled in the art in the context in which it is used and described herein, "substantially parallel" means within 30 degrees of parallel. Directions or surfaces described as being substantially parallel to each other may, in some embodiments, be within 20 degrees, or within 10 degrees, of parallel, or may be parallel or nominally parallel. If the use of "substantially aligned" is not clear to those skilled in the art in the context in which it is used and described in the description of the present invention, "substantially aligned" means being aligned within 20% of the width of the object being aligned. Objects described as being substantially aligned may, in some embodiments, be aligned within 10% or within 5% of the width of the object being aligned.

[0032] All of the references, patents, or patent applications referred to above are hereby incorporated by reference in their entirety and consistently herein. If there are any inconsistencies or contradictions between the incorporated reference portions and the present application, the information in the foregoing description shall prevail.

[0033] It should be understood that the descriptions of the elements in the figures apply equally to the corresponding elements in other figures, unless otherwise indicated. Although specific embodiments have been illustrated and described herein, it will be understood by those skilled in the art that the specific embodiments shown and described may be replaced by various alternative embodiments and / or equivalent embodiments without departing from the scope of the present disclosure. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Accordingly, it is intended that the present disclosure be limited only by the claims and their equivalents.

Claims

1. 1. An optical system comprising: a display having an active display area having a maximum lateral dimension D and configured to emit an image for viewing by an eye of an observer, the active display area including a display center and a predetermined area that includes the display center, the predetermined area having a maximum lateral dimension d, where d / D≦0.25; Vehicle windshields, an optical film configured to receive the image emitted by the active display area and to reflect or transmit at least a portion of the received image toward the windshield, the windshield being configured to receive the image reflected or transmitted by the optical film and to reflect at least a portion of the received image toward the eye, and for at least one first location within the predetermined region of the active display area, the emitted image includes a first emanated image cone emanating from the first location, the first emanated image cone including a first emanated central image light ray emanating from the first location, the optical film being oriented to polarize the first emanated central image light ray substantially in a plane of incidence when incident on the windshield; An optical system comprising:

2. The optical system of claim 1 , substantially centered on an optical axis extending from the display center to the eye.

3. The optical system of claim 2 , wherein the optical axis includes the first emerging central image ray.

4. The optical system of claim 1 , wherein the display comprises a liquid crystal display, an organic light emitting diode display, or a digital light processing display.

5. 2. The optical system of claim 1, wherein d / D≦0.

2.

6. 2. The optical system of claim 1, wherein the windshield includes a first boundary surface and a corresponding first Brewster angle, the first emanating central image ray being incident on the first boundary surface of the windshield at the first Brewster angle.

7. The optical system of claim 1 , wherein the windshield comprises a reflective polarizer embedded therein.

8. 8. The optical system of claim 7, wherein for substantially normally incident light and for at least one visible wavelength in a visible wavelength range spanning from about 420 nm to about 670 nm, the reflective polarizer reflects from about 20% to about 40% of the incident light having a first polarization state and transmits at least 60% of the incident light having an orthogonal second polarization state.

9. 9. The optical system of claim 8, wherein for the substantially normally incident light and for at least one infrared wavelength in an infrared wavelength range spanning from about 700 nm to about 1500 nm, the reflective polarizer reflects at least 40% of the incident light for at least one of the first polarization state and the second polarization state.

10. 9. The optical system of claim 8, wherein the reflective polarizer comprises a plurality of layers including a plurality of alternating first and second layers totaling greater than about 30, each of the first and second layers having an average thickness less than about 500 nm, and a difference in refractive index between the first and second layers along the first polarization state is greater than about 0.

05.

11. 2. The optical system of claim 1 , wherein the optical film comprises an absorbing polarizer that, for substantially normally incident light, transmits at least 60% of the incident light polarized along a first direction and absorbs approximately 60% of the incident light polarized along an orthogonal second direction, and the first emerging central image ray is polarized along a third direction when incident on the absorbing polarizer, and the first direction and the third direction form an inclination angle greater than approximately 5 degrees.

12. 2. The optical system of claim 1, wherein the optical film comprises an optical reflector configured to receive the image emitted by the active display area and reflect at least a portion of the received image toward the windshield, the windshield configured to receive the image reflected by the optical reflector and reflect at least a portion of the received image toward the eye, and the optical reflector is oriented to polarize the first emitted central image light ray substantially in the plane of incidence when incident on the windshield.

13. 13. The optical system of claim 12, wherein for substantially normally incident light and for at least one visible wavelength in a visible wavelength range spanning from about 420 nm to about 670 nm, the optical reflector reflects at least 60% of the incident light polarized along each of mutually orthogonal first and second directions.

14. 13. The optical system of claim 12, wherein for the substantially normally incident light and for at least one infrared wavelength in an infrared wavelength range spanning from about 700 nm to about 1500 nm, the optical reflector transmits at least 40% of the incident light polarized along at least one of a first direction and a second direction that are orthogonal to each other.

15. 2. The optical system of claim 1, further comprising a retarder layer disposed between the optical film and the windshield, the optical film comprising an optical reflector configured to receive the image emitted by the active display area and reflect at least a portion of the received image toward the windshield, the windshield configured to receive the image reflected by the optical reflector after the image passes through the retarder layer and reflect at least a portion of the received image toward the eye, the retarder layer oriented to polarize the first emitted central image light ray substantially in the plane of incidence when incident on the windshield.

Images