WO2025224543A1

WO2025224543A1 - Heads-up display with dynamic temperature-induced mirror regions

Info

Publication number: WO2025224543A1
Application number: PCT/IB2025/053476
Authority: WO
Inventors: Stephan J. Pankratz; Steven H. Kong
Original assignee: 3M Innovative Properties Co
Current assignee: 3M Innovative Properties Co
Priority date: 2024-04-22
Filing date: 2025-04-02
Publication date: 2025-10-30
Anticipated expiration: 2026-10-22

Abstract

A heads-up display includes an optical stack having a reflective polarizer between an optical module and a display. The optical stack switches between first and second operating states and emits white first and white second output images in response to an absence and a presence of an external incident light normally incident on an optical module side of the optical stack and would in an absence of the optical module and the reflective polarizer damage the display. In the presence of the external incident light, the optical stack protects the display from the damage by switching from the first to the second operating state. The switching results in a portion of the external incident light to be reflected. The first and second output images have respective average intensities Ioa and Iob and respective standard deviations Soa and Sob, such that the ratio of Sob/Iob is not greater than about 3.5%.

Description

HEADS-UP DISPLAY WITH DYNAMIC TEMPERATURE-INDUCED

MIRROR REGIONS

Summary

In some aspects of the present description, a heads-up display is provided, the heads-up display including an integral optical stack having a reflective polarizer disposed between an optical module and a display. The optical stack is configured to switch between first operating state and a second operating state. The optical stack is configured to emit corresponding substantially white first and substantially white second output images in response to, respectively, an absence and a presence of an external incident light that has a substantially uniform intensity, is substantially normally incident on an optical module side of the optical stack, and would, in an absence of the optical module and the reflective polarizer, damage the display. In the presence of the external incident light, the optical stack protects the display from the damage by switching from the first operating state to the second operating state. The switching results in a sufficient portion of the external incident light to be reflected by the reflective polarizer. The first and second output images have respective average intensities, loa and lob, and respective standard deviations, Soa and Sob, such that Sob/Iob is not greater than about 3.5%.

In some aspects of the present description, a heads-up display is provided, the heads-up display including a display, an optical module, and a reflective polarizer disposed between, and bonded to, the optical module and the display. The display is configured to form and emit an emitted image, the headsup display configured to display a virtual image of the emitted image to an occupant of a vehicle. The reflective polarizer is configured to transmit at least 40% of the emitted image having a first polarization state and reflect at least 40% of the emitted image having an orthogonal second polarization state. The optical module is configured to transmit at least 40% of the emitted image transmitted by the reflective polarizer. When an incident external light having the second polarization state and a substantially uniform incident intensity is substantially normally incident on the optical module opposite the display, and would, if directly incident on the display with a substantially uniform higher, but not a lower, incident intensity generate sufficient heat in the display so as to at least reversibly diminish a desired optical characteristic of the emitted image, then as the incident intensity is increased from the lower to the higher incident intensity, more heat is generated in the display, and at least a fraction of the generated heat is thermally conducted from the display to the optical module resulting in a decrease in an optical transmission of the incident external light by a combination of the optical module and the reflective polarizer.

In some aspects of the present description, a heads-up display is provided, the head-up display including an optical stack having a reflective polarizer disposed between an optical module and a pixelated display. For a substantially normally incident light having a visible wavelength in a visible wavelength range from about 420 nm to about 680 nm, the reflective polarizer transmits at least 60% of the incident light having a first polarization state and reflects at least 60% of the incident light having an orthogonal second polarization state, and the optical module transmits at least 40% of the incident light for each of the first and second polarization states. The display is configured to emit pixelated emitted first and second images in response to, respectively, an absence and a presence of an external incident light that is substantially normally incident on an optical module side of the optical stack and would irreversibly damage the display if directly incident on the display. The heads-up display is configured to display pixelated first and second virtual images of the respective emitted pixelated emitted first and second images for viewing by an occupant of a vehicle. For each pixel in at most Xl% and X2% of the pixels in the respective pixelated first and second virtual images, a magnitude of a maximum difference between an intensity of the pixel and intensities of pixels adjacent to the pixel is greater than by at least 5%, and wherein X2 - XI is no greater than about 65.

Brief Description of the Drawings

FIG. 1 is a schematic side view of a heads-up display, in accordance with an embodiment of the present description;

FIG. 2 is an illustration of a vehicle and occupants associated with a heads-up display, in accordance with an embodiment of the present description;

FIGS. 3A-3B are cutaway side views of a heads-up display with an optical module, in accordance with an embodiment of the present description;

FIG. 4 is a cutaway, side view of a multilayer optical film, in accordance with an embodiment of the present description;

FIG. 5 is a cutaway, side view of an optical assembly including an optical module, in accordance with an embodiment of the present description;

FIG. 6 shows examples of output images of alternate versions of a heads-up display, in accordance with an embodiment of the present description;

FIG. 7 is a chart showing optical characteristics of images created by a heads-up display, in accordance with an embodiment of the present description;

FIG. 8 is a chart showing optical performance of pixels in a heads-up display, in accordance with an embodiment of the present description;

FIGS. 9A-9B illustrate additional information related to pixels in a heads-up display, in accordance with an embodiment of the present description;

FIG. 10 illustrates how the addition of a thermal management layer can improve performance in a heads-up display, in accordance with an embodiment of the present description;

FIG. 11 is a chart showing optical improvements in a heads-up display through the use of a thermal management layer, in accordance with an embodiment of the present description; and FIG. 12 is provided to illustrate the set-up used to create and test the examples of embodiments of the heads-up displays of the present description.

Detailed Description

In the following description, reference is made to the accompanying drawings that form a part hereof and in which various embodiments are shown by way of illustration. The drawings are not necessarily to scale. It is to be understood that other embodiments are contemplated and may be made without departing from the scope or spirit of the present description. The following detailed description, therefore, is not to be taken in a limiting sense.

One concern for commercial head up displays (HUDs) is thermal protection of the imager, especially from incoming sunlight. At certain angles of incidence, sunlight may be focused directly onto the imager, which may be, for example, an LCD panel. In an augmented reality (AR) HUD system, which may have a higher magnification, this focused solar energy density may increase exponentially such that, during such a "solar loading event", the LCD in an AR HUD system may locally experience 10 times or even 50 times as much concentrated solar power compared to a standard HUD system. This issue has led some manufacturers and HUD makers to conclude that LCDs cannot be used as the imager for HUD systems with virtual image distances greater than about 3 to 5 meters, leading them to use alternate imaging systems which are often more expensive and more completed than an LCD.

Given this situation, new approaches and solutions must be found if LCDs are to remain a viable option as an imager in AR HUD systems. What is needed in the art is a dynamic approach to protecting an LCD imager from extreme and damaging solar power, allowing it to continue to be used in future AR HUD systems.

According to some aspects of the present description, a heads-up display may include an integral optical stack having a reflective polarizer disposed between an optical module and a display. In some embodiments, the optical stack may be configured to switch between first and second operating states and emit corresponding substantially white first and substantially white second output images in response to, respectively, an absence and a presence of an external incident light (e.g., incoming sunlight) that has a substantially uniform intensity, leb, is substantially normally incident on an optical module side of the optical stack, and would, in an absence of the optical module and the reflective polarizer, damage the display.

In some embodiments, in the presence of the external incident light, the optical stack may protect the display from the damage by switching from the first operating state to the second operating state. In some embodiments, the switching may result in a sufficient portion of the external incident light to be reflected by the reflective polarizer, preventing damage to the display.

In some embodiments, the first and second output images may have respective average intensities, loa and lob, and respective standard deviations, Soa and Sob, such that the ratio of Sob/Iob is not greater than about 3.5%, or not greater than about 3%, or not greater than about 2.5%, or not greater than about 2%, or not greater than about 1.5%, or not greater than about 1%, or not greater than about 0.75%, or not greater than about 0.5%.

In some embodiments, the square root of ((Sob/Iob)2 - (Soa/Ioa)2) may not be greater than about 3.5%, or about 3%, or about 2.5%, or about 2%, or about 1.5%, or about 1%, or about 0.75%, or about 0.5%. In some embodiments, the ratio lob/Ioa may be greater than or equal to about 0.1, or about 0.2, or about 0.3, or about 0.4, or about 0.5, or about 0.6, or about 0.7, or about 0.8, or about 0.9. In some embodiments, (Sob/Iob)/(Soa/Ioa) may be less than or equal to about 10 or about 9, or about 8, or about 7, or about 6, or about 5, or about 4, or about 3, or about 2, or about 1.

It should be noted that the analyses described herein related to the measurement of nonuniformities and values such as intensity (e.g., loa, lob) and standard deviation (Soa, Sob) were achieved by applying a high-pass filter to the image data to eliminate the effects of the overall “bowl” shape of the dimmed spot in the measured output images. Examples of these output images are provided and described elsewhere herein.

That is, before analyzing a non-uniformity of an output image, the underlying radial gradient in the region of interest needs to be removed. One method for accomplishing this is to apply a spatial high pass Fourier filter with an appropriate cutoff frequency to the image. For our examples, a Gaussian edge with spatial cutoff frequency of 50 kdp'¹ was used. The unit kdp is defined as kilo-display pixels.

In some embodiments, the external incident light may have an intensity of no more than about 20mW per square millimeter. In some embodiments, in the absence of the external incident light, the integral optical stack may be substantially at room temperature.

In some embodiments of the heads-up display, for a substantially normally incident light having a visible (i.e., human-visible) wavelength in a visible wavelength range from about 420 nm to about 680 nm, the reflective polarizer transmits at least 60%, or at least 70%, or at least 80%, or at least 90%, or at least 95% of the incident light having a first polarization state (e.g., a p-polarization state) and reflects at least 60%, or at least 70%, or at least 80%, or at least 90%, or at least 95% of the incident light having an orthogonal second polarization state (e.g., a s-polarization type).

In some embodiments, wherein the external incident light is substantially of the second polarization state. This may be because polarization state of the external incident light may have been modified by an element disposed in the potential pathway of the external light before it enters the HUD system, or once it enters the HUD. For example, the HUD may further include a polarizing element disposed in the optical path of the external incident light before the external incident light reaches the optical module, the polarizing element transmitting the external incident light of the second polarization type and one of reflects and absorbs the external incident light of the first polarization type.

In some embodiments, the optical module may include any module that can activate or change optical characteristics locally in any region of the LCD having dangerously elevated temperatures. In some embodiments, the optical module may include a passive (no switching electronics or electrodes), single-pixel liquid crystal cell onto which, in some embodiments, is laminated a separate reflective polarizer. In some embodiments, the optical module may be attached to the front surface of an imaging display of the heads-up display, such that the reflective polarizer is facing the imaging display.

In order for the optical module to optimally function, it may be desirable to modify the polarization of the incoming external light. In some embodiments, for example, the heads-up display may include a polarizing element (e.g., an absorbing or reflective polarizer) to pre-polarize the incident external light (e.g., the sunlight entering the heads-up display). This polarizing element may be, for example, a polarizer that is part of heads-up display, such as the dust cover (part of the dash providing a cover for the heads-up display), a mirror (such as a cold mirror within the heads-up display), or another element within the optical path of the heads-up display.

When the imaging display is heated locally due to intense solar loading, the heat is transferred to the attached optical module (e.g., through heat conduction) and when the optical module locally reaches the liquid crystal clearing temperature the optical module becomes a partial mirror to the polarized incident sunlight. The clearing temperature of the liquid crystal material in the optical module may be chosen to be lower than the temperature at which the imaging display will experience temporary failure or permanent damage. In some embodiments, the mirror function is not binary (i.e., the mirror is not “on” or “off’), but rather forms an intermediate state that may be essentially uniform or may be a mixed state at a micro-domain level to provide a partial mirror with the minimum degree of reflectivity needed at each location to maintain a dynamic, steady-state equilibrium temperature. This partial mirror simultaneously protects the imaging display from failure or damage and also allows a fraction of the emitted image light to pass through and be visible to an occupant of the vehicle. In some embodiments, the stmcture of the optical module may be a twisted nematic structure. Under normal temperature operating conditions (e.g., at room temperature), the optical module may act as a uniform polarization rotator for the output polarized light, which is uniformly reflected or transmitted by an external polarizing element in the headsup display system (e.g., a mirror, or mirror film).

According to some aspects of the present description, a heads-up display may include a display configured to form and emit an emitted image, an optical module, and a reflective polarizer. In some embodiments, the heads-up display may be configured to display a virtual image of the emitted image to an occupant (e.g., a driver or passenger) of a vehicle.

In some embodiments, the reflective polarizer may be disposed between, and bonded to, the optical module and the display. In some embodiments, the reflective polarizer may be configured to transmit at least 40%, or at least 50%, or at least 60%, or at least 70%, or at least 80%, or at least 90%, or at least 95%, of the emitted image having a first polarization state (e.g., a p-polarization type, or polarized to an x-axis of the reflective polarizer) and reflect at least 40%, or at least 50%, or at least 60%, or at least 70%, or at least 80%, or at least 90%, or at least 95%, of the emitted image having an orthogonal second polarization state (e.g., an s-polarization type, or polarized to an y-axis of the reflective polarizer). In some embodiments, the optical module may be configured to transmit at least 40%, or at least 50%, or at least 60%, or at least 70%, or at least 80%, or at least 90%, or at least 95% of the emitted image transmitted by the reflective polarizer.

In some embodiments, when an incident external light (e.g., sunlight entering the system) having the second polarization state and a substantially uniform incident intensity is substantially normally incident on the optical module opposite the display, and would, if directly incident on the display with a substantially uniform higher, but not a lower, incident intensity, generate sufficient heat in the display so as to at least reversibly diminish a desired optical characteristic of the emitted image, then as the incident intensity is increased from the lower to the higher incident intensity, more heat is generated in the display, and at least a fraction of the generated heat is thermally conducted from the display to the optical module resulting in a decrease in an optical transmission of the incident external light by a combination of the optical module and the reflective polarizer.

In some such embodiments, if the incident external light having the second polarization state and the substantially uniform incident intensity is directly incident on the display with the substantially uniform higher incident intensity, then the incident external light generates sufficient heat in the display so as to irreversibly diminish the desired optical characteristic of the emitted image (e.g., irreversibly damage one or more of the display, polarizer, display electronics, or other element of the HUD system).

In some embodiments, the heads-up display may further include a polarizing element (e.g., a film designed to alter or selectively transmit/reflect a polarization type of the light transmitted or reflected by the polarizing element) disposed in the optical path of the incident external light before the incident external light reaches the optical module. In some such embodiments, the polarizing element may transmit incident external light of the second polarization type and one of reflects and absorbs the incident external light of the first polarization type.

In some embodiments of the heads-up display, in the absence of the incident external light, the heads-up display may be substantially at room temperature (i.e., the presence of the incident external light causes elements of the heads-up display to rise significantly above room temperature).

According to some aspects of the present description, a heads-up display (HUD) may include an optical stack. In some embodiments, the optical stack may include a reflective polarizer disposed between an optical module and a pixelated display (e.g., a liquid crystal display), such that for a substantially normally incident light having a visible wavelength in a visible wavelength range from about 420 nm to about 680 nm, the reflective polarizer may transmit at least 60%, or at least 70%, or at least 80%, or at least 90%, or at least 95% of the incident light having a first polarization state (e.g., p-pol, or aligned to an x-axis) and may reflect at least 60%, or at least 70%, or at least 80%, or at least 90%, or at least 95%, of the incident light having an orthogonal second polarization state (e.g., s-pol, or aligned to a y-axis). In some embodiments, the optical module may transmit at least 40%, or at least 50%, or at least 60%, or at least 70%, or at least 80%, or at least 90% of the incident light for each of the first and second polarization states. In some embodiments, the display may be configured to emit pixelated emitted first and second images in response to, respectively, an absence and a presence of an external incident light that is substantially normally incident on an optical module side of the optical stack, and which would cause reversible loss of the image or irreversibly damage the display if directly incident on the display. In some such embodiments, the heads-up display may be configured to display pixelated first and second virtual images of the respective emitted pixelated emitted first and second images for viewing by an occupant of a vehicle.

In some embodiments, for each pixel in at most Xl% and X2% of the pixels in the respective pixelated first and second virtual images, a magnitude of a maximum difference between an intensity of the pixel and intensities of pixels adjacent to the pixel may be greater than by at least 5%, and wherein (X2 - XI) may be no greater than about 65, or about 60, or about 50, or about 40, or about 35, or about 30, or about 25, or about 20, or about 15, or about 10, or about 5, or about 2, or about 1.

In some embodiments of the heads-up display, for each pixel in at most Xl% and X2% of the pixels in the respective pixelated first and second virtual images, a magnitude of a maximum difference between an intensity of the pixel and intensities of pixels adjacent to the pixel may be greater than by at least 6%, and wherein (X2 - XI) may be no greater than about 55, or about 50, or about 45, or about 40, or about 30, or about 20, or about 10, or about 5, or about 2, or about 1. In some embodiments, a magnitude of a maximum difference between an intensity of the pixel and intensities of pixels adjacent to the pixel may be greater than by at least 8%, and wherein (X2 - XI) may be no greater than about 35, or about 30, or about 25, or about 20, or about 10, or about 5, or about 2, or about 1. In some embodiments, a magnitude of a maximum difference between an intensity of the pixel and intensities of pixels adjacent to the pixel may be greater than by at least 10%, and wherein (X2 - XI) may be no greater than about 20, or about 15, or about 10, or about 5, or about 2, or about 1. In some embodiments, a magnitude of a maximum difference between an intensity of the pixel and intensities of pixels adjacent to the pixel may be greater than by at least 12%, and wherein (X2 - XI) may be no greater than about 15, or about 10, or about 8, or about 5, or about 2, or about 1. In some embodiments, a magnitude of a maximum difference between an intensity of the pixel and intensities of pixels adjacent to the pixel may be greater than by at least 14%, and wherein (X2 - XI) may be no greater than about 10, or about 9, or about 8, or about 5, or about 2, or about 1.

In some embodiments of the heads-up display, irreversibly damaging the display may include damage to one or more of the display polarizer, color filters in the display, electronics in the display, and pixel elements in the display.

In some embodiments, no two adjacent layers in the optical stack define an air gap therebetween. In some embodiments, the optical stack may be an integral optical stack. In some embodiments, a first bonding layer may bond the reflective polarizer to the optical module, and a second bonding layer may bond the reflective polarizer to the display. In some embodiments, the reflective polarizer may include a plurality of polymeric layers numbering at least 10, or at least 20, or at least 50, or at least 75, or at least 100, or at least 150, or at least 200, or at least 250, or at least 300, in total. In some such embodiments, each of the polymeric layers may have an average thickness of less than about 500 nm, or less than about 400 nm, or less than about 300 nm, or less than about 200 nm. In some embodiments, the plurality of polymeric layers may include a plurality of alternating polymeric first and polymeric second layers, wherein the polymeric first layers have a different composition than the polymeric second layers. In some such embodiments, the reflective polarizer may further include at least one skin layer disposed on the plurality of polymeric layers and may have an average thickness of greater than about 500 nm, or greater than about 750 nm, or greater than about 1000 nm, or greater than about 1500 nm, or greater than about 2000 nm. This multilayer embodiment of a reflective polarizer is not meant to be limiting. In other embodiments, any appropriate type of reflective polarizer may be included.

In some embodiments of the heads-up display, the optical module may include at least one liquid crystal material. In some such embodiments, the optical module may further include an optically transparent front cover, and the liquid crystal material may be disposed between the front cover and the reflective polarizer. In some embodiments, the liquid crystal material may include one or more of a liquid crystal molecule, a nematic liquid crystal material, a chiral liquid crystal material, a guest host liquid crystal material, a polymer dispersed liquid crystal (PDLC) material, and a cholesteric liquid crystal material. In some embodiments, the optical module may be configured to modify a polarization state of at least a portion of an incident polarized light in transmission.

In some embodiments, for the substantially normally incident light having the visible wavelength and the second polarization state, and for a first operating temperature, the optical module may transmit at least 40%, or at least 50%, or at least 60%, or at least 70%, or at least 80%, or at least 90% of the polarized incident light, and may rotate the polarization state of at least a portion of the polarized incident light from the second polarization state to the first polarization state, so that at least 60%, or at least 70%, or at least 80%, or at least 90% of the transmitted light has the first polarization state. In some embodiments, wherein the first operating temperature may be a room temperature (e.g., an ambient temperature of the environment the heads-up is in when operating without external incident light, such as sunlight).

In some embodiments, the optical module may be configured to modify, in transmission, a polarization state of at least a portion of an incident polarized light (e.g., sunlight having first passed through a polarization element to become substantially polarized in having the second polarization type) having the visible wavelength as a function of a temperature of the optical module. In some embodiments, the optical module may be intentionally thermally coupled to the display so that heat generated in the display is intentionally thermally conducted to the optical module changing an operating temperature of the optical module. Stated another way, a change in temperature in the display may be thermally conducted into the optical module. In some embodiments, for the external incident light having the second polarization state and the optical module operating at a first operating temperature, the optical module transmits at least 70%, or at least 80%, or at least 90% of the polarized external incident light as a first transmitted light and rotates the polarization state of the polarized incident light from the second polarization state to the first polarization state so that at least 70%, or at least 80%, or at least 90% of the first transmitted light has the first polarization state. In some such embodiments, the reflective polarizer may transmit at least 70%, or at least 80%, or at least 90% of the at least 70% of the first transmitted light as a second transmitted light having the first polarization state, such that the second transmitted light heats the display.

In some embodiments, the generated heat may be thermally conducted to the optical module, changing an operating temperature of the optical module from the first operating temperature to a higher second operating temperature (e.g., above room temperature). In some such embodiments, at the second operating temperature, the optical module may transmit at least 70%, or at least 80%, or at least 90% of the polarized external incident light as a third transmitted light and may modify the polarization state of the polarized incident light so that a first portion of the third transmitted light has the first polarization state and a second portion of the third transmitted light has the second polarization state. In some such embodiments, the reflective polarizer may substantially reflect the second portion of the third transmitted light and may substantially transmit the first portion of the third transmitted light as a fourth transmitted light having sufficiently low intensity so as to not damage the display.

In some such embodiments, the first operating temperature may be a room temperature. Amd the second operating temperature may be at least 10 degrees, or at least 20 degrees, or at least 30 degrees, or at least 40 degrees, or at least 50 degrees, or at least 60 degrees higher than the room temperature.

In some embodiments, the pixelated first and second virtual images may have respective average intensities Iva and Ivb and respective standard deviations Sva and Svb, such that the ratio Svb/Ivb is not greater than about 3.5%, or about 3%, or about 2.5%, or about 2%, or about 1.5%, or about 1%, or about 0.75%, or about 0.5%. In some embodiments, wherein the square root of ((Svb/Ivb)² - (Sva/Iva)²) may not be greater than about 3.5%, or about 3%, or about 2.5%, or about 2%, or about 1.5%, or about 1%, or about 0.75%, or about 0.5%. In some embodiments, the ratio Ivb/Iva may be greater than or equal to about 0.1, or about 0.2, or about 0.3, or about 0.4, or about 0.5, or about 0.6, or about 0.7, or about 0.8, or about 0.9. In some embodiments, the ratio (Svb/Ivb)/(Sva/Iva) may be less than or equal to about 10, or about 9, or about 8, or about 7, or about 6, or about 5, or about 4, or about 3, or about 2, or about 1.

Turning now to the figures, FIG. 1 is a schematic side view of an embodiment of a heads-up display according to the present description. A heads-up display 300 includes an integral optical stack 200 which includes a reflective polarizer 30 disposed between an optical module 20 and a display 10. In some embodiments, the optical module may be, for example, a liquid crystal cell, which may have a first operating state and a second operating state based on the amount of external light incident on the headsup display, especially light that follows the optical path of the heads-up display and causes the display to heat up. Additional information on these operating states are discussed elsewhere herein. In normal operation, the display 10 may be configured to form and emit an emitted image which is emitted as image rays 14 , and the heads-up display 300 may be configured to display a virtual image 13 of the emitted image to an occupant of a vehicle (see, e.g., occupants 320, 325 of vehicle 310 illustrated in FIG. 2). In some embodiments, embodiments, virtual image 13 includes a plurality of pixels 15. The virtual image 13 may be a first virtual image 13a when the heads-up display is operating in the first operating state, and may be a second virtual image 13b when the heads-up display is operating in the second operating state. Additional details on the operating states is discussed elsewhere herein.

In some embodiments, the heads-up display may have a folded optical path (i.e., the path followed by emitted image rays 14. Emitted image rays 14 may be emitted from optical stack 200 having a first polarization state (indicated by the double-headed arrow along the path of emitted image rays 14. The emitted image rays 14 may, in some embodiments, be redirected by a first mirror 70 and a second mirror 75 before being reflected by windshield 80 and redirecting the image rays 14 into the eye 315 of an occupant of the vehicle, creating the virtual image 13 at a point in front of windshield 80.

As previously discussed, FIG. 2 is an illustration of a vehicle 310 and occupants 320 (e.g., a driver) and 325 (e.g., a passenger) associated with heads-up display 300 of FIG. 1.

FIGS. 3 A and 3B are cutaway side views of an embodiment of a heads-up display with an optical module, according to the present description. Turning first to FIG. 3 A, in some embodiments, heads-up display 300 includes an optical stack 200 having a reflective polarizer 30 disposed between an optical module 20 and a pixelated display 10.

In some embodiments, the optical stack may be configured to switch between first and second operating states and emit corresponding substantially white first 1 la and substantially white second 1 lb output images in response to, respectively, an absence and a presence of an external incident light 40b (e.g., external sunlight entering the heads-up display) that has a substantially uniform intensity, leb, is substantially normally incident on an optical module side 201 of optical stack 200, and would, in an absence of the optical module 20 and the reflective polarizer 30, temporarily disable or irreversibly damage the display 10.

In some embodiments, in the presence of external incident light 40b, optical stack 200 may protect display 10 from damage by switching from the first operating state to the second operating state. In some embodiments, this switching may result in a sufficient portion 42 of the external incident light 40b to be reflected 43 by reflective polarizer 30, with a remainder of light 41 being transmitted toward display 10.

In some embodiments, the first output image 1 la (emitted in the absence of external incident light 40b and in the first operating state) and the second output image 1 lb (emitted in the presence of external incident light 40b and in the second operating state) may have respective average intensities, loa and lob, and respective standard deviations, Soa and Sob, such that the ratio Sob/Iob may not be greater than about 3.5%, or about 3%, or about 2.5%, or about 2%, or about 1.5%, or about 1%, or about 0.75%, or about 0.5%. FIG. 3B, provided for illustration purposes, shows various light rays and their interactions with some of the elements of FIG. 3 A. FIGS. 3 A and 3B may be examined together for the following discussion.

In some embodiments, for a substantially normally incident light 31, 32 having a visible wavelength in a visible wavelength range from about 420 nm to about 680 nm, optical module 20 may transmit at least 40%, or at least 50%, or at least 60%, or at least 70%, or at least 80%, or at least 90% of the incident light for each of the first and second polarization states (e.g., when optical module 20 is operating in the first operating state).

In some embodiments, reflective polarizer 30 may transmit at least 60%, or at least 70%, or at least 80%, or at least 90%, or at least 95% of the incident light 31 having a first polarization state 3 Ip (e.g., transmits p-polarized light) and may reflect at least 60%, or at least 70%, or at least 80%, or at least 90%, or at least 95% of the incident light 31 having an orthogonal second polarization state 31s (e.g., reflects s-polarized light). The transmitted light 3 Ip then passes into the display as normally incident light 41 with the first polarization type (e.g., p-pol) and an intensity Io (see FIG. 3B).

In some embodiments, display 10 may be configured to emit pixelated emitted first 12a and second 12b images in response to, respectively, an absence and a presence of external incident light 40b that is substantially normally incident on an optical module side of the optical stack, and would temporarily disable or irreversibly damage the display if directly incident on the display.

In some embodiments, heads-up display 300 may be configured to display pixelated first 13a and second 13b virtual images of the respective emitted pixelated emitted first 12a and second 12b images for viewing by an occupant 320, 325 of a vehicle 310 (see also, e.g., FIG. 2).

Emitted first 12a and second 12b images travel as corresponding substantially white first 1 la and substantially white second 1 lb output images in response to, respectively, an absence and a presence of an external incident light 40b. First 1 la and second 1 lb output images may have respective average intensities loa and lob and respective standard deviations Soa and Sob, such that the ratio Sob/Iob is not greater than about 3.5%, or about 3%, or about 2.5%, or about 2%, or about 1.5%, or about 1%, or about 0.75%, or about 0.5%.

In some embodiments, the external incident light may have varying intensity which may effect the operating state and performance of the heads-up display. For example, when an incident external light 40a, 40b which have the second polarization state (e.g., have been pre-polarized to have the second polarization state) and has a substantially uniform incident intensity, respectively, of lea and leb, and is substantially normally incident on the optical module 20 opposite display 10, and would, if directly incident on the display with a substantially uniform higher leb, but not a lower lea, incident intensity generate sufficient heat in display 10 so as to at least reversibly diminish a desired optical characteristic of the emitted image 12a, 12b, then as the incident intensity is increased from the lower lea to the higher leb incident intensity, more heat is generated in display 10, and at least a fraction of the generated heat is thermally conducted from display 10 to optical module 20, resulting in a decrease in an optical transmission of the incident external light by a combination of the optical module 20 and the reflective polarizer 30.

Emitted images 12a and 12b are perceived by the viewer 320, 325 as pixelated first 13a and second 13b virtual images, which may have respective average intensities Iva and Ivb and respective standard deviations Sva and Svb, such that Svb/Ivb is not greater than about 3.5%, or about 3%, or about 2.5%, or about 2%, or about 1.5%, or about 1%, or about 0.75%, or about 0.5%. In some embodiments, the square root of ((Svb/Ivb)² - (Sva/Iva)²) may not be not greater than about 3.5% (or about 3%, or about 2.5%, or about 2%, or about 1.5%, or about 1%, or about 0.75%, or about 0.5%. In some embodiments, the ratio Ivb/Iva may be greater than or equal to about 0.1, or about 0.2, or about 0.3, or about 0.4, or about 0.5, or about 0.6, or about 0.7, or about 0.8, or about 0.9. In some embodiments, the ratio (Svb/Ivb)/( Sva/Iva) may be less than or equal to about 10, or about 9, or about 8, or about 7, or about 6, or about 5, or about 4, or about 3, or about 2, or about 1.

In some embodiments, external incident light 40a, 40b may be pre-polarized to a first polarization type by an element disposed in the optical path of the heads-up display. For example, the pre-polarization may be done by an element such as the dust cover (part of the dash providing a cover for the heads-up display), a mirror (such as a cold mirror within the heads-up display, for example cold mirror 70 shown in FIG. 3 A), or another element within the optical path of the heads-up display. The mirror 70 shown in FIG. 3 A may, for example, transmit the first polarization type therethrough, and substantially reflect the second polarization type toward optical module 200.

FIG. 4 is a cutaway, side view of a multilayer optical film, such as one embodiment of a reflective polarizer 30, according to the present description. In some embodiments, reflective polarizer 30 may include a plurality of polymeric layers 33, 34 numbering at least 10, or at least 20, or at least 50, or at least 75, or at least 100, or at least 150, or at least 200, or at least 250, or at least 300 in total. In some embodiments, each of the polymeric layers may have an average thickness of less than about 500 nm, or about 400 nm, or about 300 nm, or about 200 nm. In some embodiments, the plurality of polymeric layers 33, 34 may include a plurality of alternating polymeric first layers 33 and polymeric second layers 34. In some such embodiments, the polymeric first layers 33 having a different composition than the polymeric second layers 34. In some such embodiments, polymeric first layers 33 and polymeric second layers 34 may have, for example, different indices of refraction. In some embodiments, the reflective polarizer 30 may further include at least one skin layer 35 disposed on the plurality of polymeric layers 33, 34 and may have an average thickness of greater than about 500 nm, or about 750 nm, or about 1000 nm, or about 1500 nm, or about 2000 nm.

FIG. 5 is a cutaway, focused side view of an embodiment of an optical stack 200 including an optical module 20, according to the present description. In some embodiments, optical stack 200 may include a reflective polarizer 30 disposed between an optical module 20 and the display 10. In some embodiments, a first bonding layer 50 may bond reflective polarizer 30 to optical module 20. In some embodiments, a second bonding layer 51 may bond reflective polarizer 30 to display 10. In some embodiments, the first bonding layer 50 and second bonding layer 51 may include optically clear adhesives.

In some embodiments, optical module 20 may include at least one liquid crystal material 21. In some embodiments, the liquid crystal material 21 may include one or more of a liquid crystal molecule, a nematic liquid crystal material, a chiral liquid crystal material, a guest host liquid crystal material, a polymer dispersed liquid crystal (PDLC) material, and a cholesteric liquid crystal material. In some embodiments, optical module 20 may further include an optically transparent front cover 22, wherein the liquid crystal material 21 is disposed between the front cover 22 and the reflective polarizer 30.

In some embodiments, display 10 may include additional layers that contribute to the performance of display 10. For example, other layers of display 10 may include light sources, sensors, color filters, adhesive layers, protective covers, heatsink layers, or any other appropriate functional layers. In some embodiments, display 10 may include display electronics 18 which drive the display 10 to emit images. It should be noted that, although the display electronics 18 are depicted here as a separate functional block, they may, in some embodiments, be integrated into display 10. Other layers of display 10, as listed above or as yet unidentified but known in the art, may be integrated into display 10. Stated another way, FIG. 5 is intended to show the major functional blocks of the optical stack 200, and each functional block may be assumed to include elements necessary for that block’s operation, or for the improvement or modification of the block’s operation.

As discussed elsewhere herein, optical module 20 may be configured to modify a polarization state of at least a portion of an incident polarized light in transmission. For example, for the substantially normally incident light having a human-visible wavelength and the second polarization state, and for a first operating temperature, the optical module may transmit at least 40%, or at least 50%, or at least 60%, or at least 70%, or at least 80%, or at least 90%, of the polarized incident light and may rotate the polarization state of at least a portion of the polarized incident light from the second polarization state to the first polarization state, so that at least 60%, or at least 70%, or at least 80%, or at least 90% of the transmitted light has the first polarization state.

In some embodiments, optical module 20 may be configured to modify, in transmission, a polarization state of at least a portion of an incident polarized light having the visible wavelength as a function of a temperature of the optical module. In some embodiments, optical module 20 may be intentionally thermally coupled to display 10 so that heat generated in display 10 is intentionally thermally conducted to optical module 20 changing an operating temperature of the optical module.

In some embodiments of the heads-up display, wherein for an external incident light having the second polarization state and for an optical module 20 operating at a first operating temperature, optical module 20 may transmit at least 70%, or at least 80%, or at least 90% of the polarized external incident light as a first transmitted light and may rotate the polarization state of the polarized incident light from the second polarization state to the first polarization state so that at least 70% or at least 80%, or at least 90% of the first transmitted light has the first polarization state, reflective polarizer 30 may transmit at least 70%, or at least 80%, or at least 90% of the at least 70% of the first transmitted light as a second transmitted light having the first polarization state, such that the second transmitted light heats display 10. This generated heat may then be thermally conducted to optical module 20, changing an operating temperature of the optical module from the first operating temperature to a higher second operating temperature. In some embodiments, at the higher second operating temperature, optical module 20 may transmit at least 70%, or at least 80%, or at least 90% of the polarized external incident light as a third transmitted light and may modify the polarization state of the polarized incident light so that a first portion of the third transmitted light has the first polarization state and a second portion of the third transmitted light has the second polarization state. In some embodiments, reflective polarizer 30 may substantially reflect the second portion of the third transmitted light and may substantially transmit the first portion of the third transmitted light as a fourth transmitted light having sufficiently low intensity so as to not damage the display.

In some embodiments, the first operating temperature may be a room temperature. In some embodiments, the second operating temperature is at least 10, or at least 20, or at least 30, or at least 40, or at least 50, or at least 60 , or at least 70, or at least 80 or at least 90 degrees (e.g., degrees Celsius) higher than the room temperature.

FIG. 6 shows examples of output images of alternate versions of an embodiment of a heads-up display, according to the present description. The test setup shown in FIG. 12 was used to test various versions of optical modules, and the images shown here in FIG. 6 represent actual output images produced in the test runs.

A series of liquid crystal cells with different liquid crystal materials were created to test different optical modules in a test setup simulating a real HUD system with incident solar radiation. Each column of FIG. 6 corresponds to a different optical stack in which different optical modules having different liquid crystal materials were tested. These different optical module compositions are labeled OM1, OM2, OM3, OM4, and OM5 in the images of FIG. 6.

The rows in FIG. 6 are labeled with power levels P1-P4, which represent the percentages of the maximum brightness of a test device (a spotlight) used to provide external incident light for the various test cases. In the pictures shown in FIG. 6, Pl (the percentage for the top row of output images) was 50%. Similarly, P2 was 60%, P3 was 70%, and P4 was 80%.

The images of FIG. 6 represent portions of white test images which were produced in the different tests. In the absence of an external light (i.e., if the input power level of the light source were set to zero percent), each test image would be a substantially uniform white square. However, as can be seen in the test images, the heads-up display of the present description produces various levels of a dark region based on the power of the external incident light and the makeup of the liquid crystal material used in the optical module. For example, looking at image OMla, we see a slightly darker spot near the center, representing a reduced optical output based on the Pl power level (i.e., 50% of full power for the input light source) and the liquid crystal material composition of OM1, OM2a, OM3a, OM4a, and OM5a show spots with various levels of darkening based on the different liquid crystal materials used, while all are at the Pl power level. While all the optical module-containing stacks show increasing darkening of the dark region in the output image as the input power increases, some show that the uniformity of this darkening varies between sample modules. For example, OM5d, in the lower right-hand comer of FIG. 6, shows more luminance non-uniformity in its dark region than the other optical module-containing stacks, and such “grainy” non-uniformity is detrimental to the image quality.

The darkness of the spots in the images of FIG. 6 represents the level of protection the optical module is providing to the display and other elements of the heads-up display. A darker spot corresponds directly to the degree of disordering in the liquid crystal material as it approaches or passes its bulk clearing temperature, as described elsewhere herein, and has begun to transition to a mirror, reflecting a portion of the polarized incident sunlight (which also means that a portion of the intended white image will be reflected by the optical module, leading to a darker spot on the output image). That is, the optical module begins switching to the “second operating state” in order to protect the display and other elements of the heads-up display. Lower levels of input sunlight will allow for a higher partial transmission of the output image, such that the heads-up display is still substantially operable in the region effected by the external incident sunlight.

FIG. 7 is a chart showing optical characteristics of images created by various embodiments of a heads-up display, according to the present description. The five test compositions OM1, OM2, OM3, OM4, and OM5 discussed elsewhere herein (see, e.g., FIG. 6 and the corresponding description) were measured to have average intensity Iv and standards deviations Sv as shown and plotted here in FIG. 7. As described elsewhere herein (see, e.g., FIGS. 2-3 and the corresponding descriptions), emitted images 12a and 12b may be perceived by the viewer 320, 325 as pixelated first 13a and second 13b virtual images, which may have respective average intensities Iva and Ivb and respective standard deviations Sva and Svb. The ratios of Sva/Iva and Svb/Ivb are plotted in FIG. 7. FIG. 7 also shows the ratio of standard deviation to intensity for the emitted image themselves, which is very low (typically about 0), indicating a substantially uniform emitted image from the display.

As shown here, the ratio Svb/Ivb is typically not greater than about 3.5%, or about 3%, or about 2.5%, or about 2%, or about 1.5%, or about 1%, or about 0.75%, or about 0.5%, depending on the composition chosen for the optical module. This data also applied to the quadratic version of the comparison, for the square root of ((Svb/Ivb)² - (Sva/Iva)²), which can be used to ensure the values for pixelated first 13a and second 13b virtual images (representing the absence and presence of external incident light) are both considered to eliminate any other sources of image corruption.

FIG. 8 is a chart showing optical performance of pixels in various embodiments of a heads-up display, and FIGS. 9A-9B provide an illustration and backup data for the chart of FIG. 8. It may be helpful to examine all three figures together for the following discussion. Starting with FIG. 9A, and with reference to heads-up display 300 of FIG. 3, the heads-up display is configured to display pixelated first 13a and second 13b virtual images of the respective emitted pixelated emitted (i.e., emitted from the display directly) first 12a and second 12b images for viewing by occupant 320, 325 of a vehicle 310.

FIG. 9A represents a schematic illustration of pixelated virtual images 13a/13b, including reference pixels 15a- 15i. The data in FIG. 9B and plotted in FIG. 8 can be described as follows: for each pixel 15a in at most Xl% (of virtual image 13a, in the absence of external incident light) and X2% (of virtual image 13b, in the presence of external incident light) of the pixels in the respective pixelated first 13a and second 13b virtual images, a magnitude of a maximum difference between an intensity of the pixel 15a and intensities of pixels 15b-l 5i adjacent to the pixel is greater than by at least 5%, then (X2 - XI) may be no greater than about 65 (e.g., in FIG. 9B, the entry for “>5%” for OM5b is about 63.76).

When the magnitude of the maximum difference between an intensity of the pixel 15a and intensities of pixels 15b-15i adjacent to the pixel is greater than by at least 6%, then (X2 - XI) may be no greater than about 55 (e.g., in FIG. 9B, the entry for “>6%” for OM5b is about 51.52).

When the magnitude of the maximum difference between an intensity of the pixel 15a and intensities of pixels 15b-15i adjacent to the pixel is greater than by at least 8%, then (X2 - XI) may be no greater than about 35 (e.g., in FIG. 9B, the entry for “>8%” for OM5b is about 31.60).

When the magnitude of the maximum difference between an intensity of the pixel 15a and intensities of pixels 15b-15i adjacent to the pixel is greater than by at least 10%, then (X2 - XI) may be no greater than about 20 (e.g., in FIG. 9B, the entry for “>10%” for OM5b is about 18.52).

When the magnitude of the maximum difference between an intensity of the pixel 15a and intensities of pixels 15b-15i adjacent to the pixel is greater than by at least 12%, then (X2 - XI) may be no greater than about 15 (e.g., in FIG. 9B, the entry for “>12%” for OM5b is about 10.79).

When the magnitude of the maximum difference between an intensity of the pixel 15a and intensities of pixels 15b-15i adjacent to the pixel is greater than by at least 14%, then (X2 - XI) may be no greater than about 10 (e.g., in FIG. 9B, the entry for “>14%” for OM5b is about 6.36).

FIG. 8 is a plot of the data captured in FIG. 9B for pixels as illustrated in FIG. 9A.

FIG. 10 illustrates how the addition of a thermal management layer can improve performance in an embodiment of a heads-up display, according to the present description. As shown in FIG. 10, any of the embodiments of the heads-up display as described herein may be combined with a secondary thermal management component to further enhance its performance while protecting the LCD from overheating. In one embodiment, a transparent component 24 such as glass may be attached to, or disposed adjacent to, the backside of display 10 and act as a thermal mass and thermal conductor to absorb and conduct heat energy 1020b away from display 10.

This function slows down the temperature rise of display 10 when it is under solar irradiation 44 and in some cases may also lower the maximum equilibrium temperature of display 10. When such a transparent, backside heat sink 24 component (e.g., a thermal management layer) is utilized in combination with a frontside optical module 20 as described elsewhere herein, it enables optical module 20 to achieve its nominal equilibrium temperature at a lower level of reflectivity, and correspondingly a lower degree of local dimming of the output heads-up display image. This enables the protective heat rejection function to be accomplished while maintaining a higher image quality.

For example, incoming focused and pre-polarized external light 44 may entered optical stack 200, initially passing through optical module 20, reflective polarizer 30, and entering into display 10. This external light 44 (e.g., incident, incoming sunlight) may cause a “hot region” 1000 to form within optical stack 200. When this occurs, heat generated within display 10 and other elements of optical stack 200 may be transmitted to optical module 20, which, as described elsewhere herein, changes from a first operating state to a second operating state, which, in combination with the reflective polarizer 30, can create a partial mirror 1010. The dynamic, temperature-induced partial mirror 1010 works to reflect some of the incoming external light 44 as reflected rays 1020a, protecting display 10 and other components of optical stack 200, but also reducing the luminance of images output by display 10 when the images are viewed by a viewer. By providing a back side heat sink component 24, some of the heat from hot region 1000 may be conducted away from display 10 into heat sink component 24 where it is distributed throughout heat sink component 24 and may be at least partially radiated 1020b from a back side (a side away from display 10) of the heat sink component 24. As the heat affecting display 10 is thereby reduced, the amount of dimming of the output images is similarly reduced (partial mirror 1010 effects the polarization state of the light being transmitted through the optical module 20 less), while the display 10 and other components of the optical stack are still protected from excess heat.

In another embodiment, a second reflective polarizer 35 may be laminated to the transparent heat sink component 24 that is attached to, or disposed adjacent to, the backside of display 10. Second reflective polarizer 35 may, in some embodiments, function to reject light from a backlight (e.g., light emitting diodes providing light from a far-right side of FIG. 10, not shown) that has a polarization state that would otherwise be absorbed by the back display polarizer. In this embodiment, the backside component (including backside heat sink 24 and second reflective polarizer 35) functions both as a conductive heat sink for display 10 and a polarized reflector for the incident light from any backlight present. In combination with the optical module 20 and reflective polarizer as described herein, this may provide an even higher thermal mitigation performance to protect display 10 and any related components, as described elsewhere herein.

FIG. 11 is a chart showing optical improvements in a heads-up display employing a thermal management layer such as thermal management layer 24 for FIG. 10. The four plot lines on the chart are, as labeled in the legend, plots of luminance in nits of the output images for two different power levels of incident external light (e.g., sunlight) comparing the effects on luminance for those levels for an optical stack with and without a back side heat sink. In both cases, the results show a significant increase in luminance when the heat sink component is present (e.g., increase 1100a for the case of lower power [about 11.5 kW/m²] of the incoming external light, and increase 1100b for the case of the higher power [about 24 kW/m²] of the incoming external light). FIG. 12 describes a test setup as used in the testing of sample compositions 0M1-0M5 and is described in additional detail in the following Examples section.

Examples

A series of liquid crystal (LC) cells with different LC materials were created and tested in a test setup simulating a real HUD system with incident solar radiation (see FIG. 12). 3M Reflective Polarizer Mirror (RPM) film 430 was laminated to one surface of each LC cell 420 to create the optical modules 420. These optical modules 420 were then individually connected thermally and optically to the front polarizer of an imager LCD 410 with a thin layer of mineral oil, after which the LCD+optical module stack was inserted into the test setup. An output HUD image 440 was generated by driving an LED backlight projector 415 while the imager LCD 410 was electronically driven to display a white or an HMI image. The output HUD image 440 was recorded with a Radiant Imaging IM-29 Imaging Colorimeter 405a after reflecting off a glass laminate 480 simulating a windshield. Simultaneously, simulated solar energy 495 from a 4000W Lancelot spot lamp 490 was incident on the HUD setup, which after reflecting off a spherical mirror 475 and then a polarizing cold mirror 470, consisting of 3M Cold Mirror Film laminated to flat glass, was incident onto a portion of the optical module-fitted imager LCD 420/430/410. A white HUD image was output to enable the activated optical module region to be clearly visible and the degree of dimming caused by the optical module response to be easily recorded. This optical module response was recorded for all the optical modules at a range of incident spot lamp energies between 50% and 100% output power.

Terms such as “about” will be understood in the context in which they are used and described in the present description by one of ordinary skill in the art. If the use of “about” as applied to quantities expressing feature sizes, amounts, and physical properties is not otherwise clear to one of ordinary skill in the art in the context in which it is used and described in the present description, “about” will be understood to mean within 10 percent of the specified value. A quantity given as about a specified value can be precisely the specified value. For example, if it is not otherwise clear to one of ordinary skill in the art in the context in which it is used and described in the present description, a quantity having a value of about 1 , means that the quantity has a value between 0.9 and 1.1, and that the value could be 1.

Terms such as “substantially” will be understood in the context in which they are used and described in the present description by one of ordinary skill in the art. If the use of “substantially equal” is not otherwise clear to one of ordinary skill in the art in the context in which it is used and described in the present description, “substantially equal” will mean about equal where about is as described above. If the use of “substantially parallel” is not otherwise clear to one of ordinary skill in the art in the context in which it is used and described in the present description, “substantially parallel” will mean within 30 degrees of parallel. Directions or surfaces described as substantially parallel to one another 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 otherwise clear to one of ordinary skill in the art in the context in which it is used and described in the present description, “substantially aligned” will mean aligned to within 20% of a width of the objects being aligned. Objects described as substantially aligned may, in some embodiments, be aligned to within 10% or to within 5% of a width of the objects being aligned. All references, patents, and patent applications referenced in the foregoing are hereby incorporated herein by reference in their entirety in a consistent manner. In the event of inconsistencies or contradictions between portions of the incorporated references and this application, the information in the preceding description shall control.

Descriptions for elements in figures should be understood to apply equally to corresponding elements in other figures, unless indicated otherwise. Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations can be substituted for the specific embodiments shown and described 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. Therefore, it is intended that this disclosure be limited only by the claims and the equivalents thereof.

Claims

What is claimed:

1. A heads-up display comprising an integral optical stack comprising a reflective polarizer disposed between an optical module and a display, the optical stack configured to switch between first and second operating states and emit corresponding substantially white first and substantially white second output images in response to, respectively, an absence and a presence of an external incident light that has a substantially uniform intensity, is substantially normally incident on an optical module side of the optical stack, and would in an absence of the optical module and the reflective polarizer damage the display, in the presence of the external incident light, the optical stack protecting the display from the damage by switching from the first operating state to the second operating state, the switching resulting in a sufficient portion of the external incident light to be reflected by the reflective polarizer; the first and second output images having respective average intensities loa and lob and respective standard deviations Soa and Sob, such that Sob/Iob is not greater than about 3.5%.

2. The heads-up display of claim 1, wherein the square root of ((Sob/Iob)² - (Soa/Ioa)²) is not greater than about 3.5%.

3. The heads-up display of claim 1 , wherein lob/Ioa > 0.1.

4. The heads-up display of claim 1, wherein (Sob/Iob)/(Soa/Ioa) < 10.

5. The heads-up display of claim 1, wherein the external incident light has an intensity of no more than about 20mW per square millimeter.

6. The heads-up display of claim 1, wherein, in the absence of the external incident light, the integral optical stack is substantially at room temperature.

7. The heads-up display of claim 1, wherein, for a substantially normally incident light having a visible wavelength in a visible wavelength range from about 420 nm to about 680 nm, the reflective polarizer transmits at least 60% of the incident light having a first polarization state (p-pol, x-axis) and reflects at least 60% of the incident light having an orthogonal second polarization state.

8. The heads-up display of claim 7, wherein the external incident light is of the second polarization state.

9. The heads-up display of claim 1, further comprising a polarizing element disposed in the optical path of the external incident light before the external incident light reaches the optical module, the polarizing element transmitting the external incident light of the second polarization type and one of reflects and absorbs the external incident light of the first polarization type.

10. A heads-up display comprising: a display configured to form and emit an emitted image, the heads-up display configured to display a virtual image of the emitted image to an occupant of a vehicle; an optical module; and a reflective polarizer disposed between, and bonded to, the optical module and the display, the reflective polarizer configured to transmit at least 40% of the emitted image having a first polarization state and reflect at least 40% of the emitted image having an orthogonal second polarization state, the optical module configured to transmit at least 40% of the emitted image transmitted by the reflective polarizer, such that when an incident external light having the second polarization state and a substantially uniform incident intensity is substantially normally incident on the optical module opposite the display, and would, if directly incident on the display with a substantially uniform higher, but not a lower, incident intensity generate sufficient heat in the display so as to at least reversibly diminish a desired optical characteristic of the emitted image, then as the incident intensity is increased from the lower to the higher incident intensity, more heat is generated in the display, and at least a fraction of the generated heat is thermally conducted from the display to the optical module resulting in a decrease in an optical transmission of the incident external light by a combination of the optical module and the reflective polarizer.

11. The heads-up display of claim 10, wherein if the incident external light having the second polarization state and the substantially uniform incident intensity is directly incident on the display with the substantially uniform higher incident intensity, then the incident external light generates sufficient heat in the display so as to irreversibly diminish the desired optical characteristic of the emitted image.

12. The heads-up display of claim 10, further comprising a polarizing element disposed in the optical path of the incident external light before the incident external light reaches the optical module, the polarizing element transmitting the incident external light of the second polarization type and one of reflects and absorbs the incident external light of the first polarization type.

13. The heads-up display of claim 10, wherein, in the absence of the incident external light, the headsup display is substantially at room temperature.

14. A heads-up display comprising an optical stack comprising a reflective polarizer disposed between an optical module and a pixelated display, such that for a substantially normally incident light having a visible wavelength in a visible wavelength range from about 420 nm to about 680 nm: the reflective polarizer transmits at least 60% of the incident light having a first polarization state and reflects at least 60% of the incident light having an orthogonal second polarization state; and the optical module transmits at least 40% of the incident light for each of the first and second polarization states; the display configured to emit pixelated emitted first and second images in response to, respectively, an absence and a presence of an external incident light that is substantially normally incident on an optical module side of the optical stack, and would irreversibly damage the display if directly incident on the display, the heads-up display configured to display pixelated first and second virtual images of the respective emitted pixelated emitted first and second images for viewing by an occupant of a vehicle, wherein, for each pixel in at most Xl% and X2% of the pixels in the respective pixelated first and second virtual images, a magnitude of a maximum difference between an intensity of the pixel and intensities of pixels adjacent to the pixel is greater than by at least 5%, and wherein (X2 - XI) is no greater than about 65.

15. The heads-up display of claim 14, wherein, for each pixel in at most Xl% and X2% of the pixels in the respective pixelated first and second virtual images, a magnitude of a maximum difference between an intensity of the pixel and intensities of pixels adjacent to the pixel is greater than by at least 6%, and wherein (X2 - XI) is no greater than about 55.