Multilayer reflective polarizer with low pass state reflection

Abstract

A multilayer optical film includes a plurality of optical repeat units, each optical repeat unit having first and second microlayers. Each optical repeat unit has an optical thickness being a sum of a first optical thickness of the first microlayer and a second optical thickness of the second microlayer. Each optical repeat unit reflects / transmits light primarily by optical interference. For a substantially normally incident light in a predetermined wavelength range, the plurality of optical repeat units transmits light having a first polarization state and reflects light having a second polarization state. Each optical repeat unit has a corresponding f-ratio, and the plurality of optical repeat units define an f-ratio profile across the multilayer optical film. The f-ratio profile is configured to decrease an amount of light of the first polarization state that is reflected and increase an amount of light of the second polarization state that is transmitted.

Description

[0001] MULTILAYER REFLECTIVE POLARIZER WITH LOW PASS STATE REFLECTION

[0002] Summary

[0003] In some aspects of the present description, a multilayer optical film is provided, the multilayer optical film including a packet of microlayers. The packet includes a plurality of optical repeat units, each optical repeat unit having first and second microlayers. Each optical repeat unit has an optical thickness being a sum of a first optical thickness of the first microlayer and a second optical thickness of the second microlayer. Each optical repeat unit reflects or transmits light in a first wavelength range primarily by optical interference. For a substantially normally incident light in the first wavelength range, the plurality of optical repeat units substantially transmits the light having a first polarization state, and substantially reflects the light having an orthogonal second polarization state. The first microlayer of each optical repeat unit has a first refractive index and the second microlayer of each optical repeat unit has a second refractive index. The first refractive index is greater than the second refractive index. Each optical repeat unit of the plurality of optical repeat units has an f-ratio defined as the ratio of the first optical thickness of the first microlayer to the optical thickness of the optical repeat unit. Together, the plurality of optical repeat units defines an f-ratio profile of the packet of microlayers. The f-ratio profile of the packet of microlayers is non-constant and is configured to decrease an amount of light of the first polarization state that is reflected and increase an amount of light of the second polarization state that is transmitted.

[0004] In some aspects of the present description, a multilayer reflective polarizer is provided, the multilayer reflective polarizer configured such that, for a substantially normally incident light in a predetermined wavelength range extending between about 380 nm and 800 nm, the multilayer reflective polarizer substantially transmits light having a first polarization state and substantially reflects light having an orthogonal second polarization state. The multilayer reflective polarizer includes a plurality of optical repeat units, and each optical repeat unit has first and second microlayers. Each optical repeat unit has an optical thickness being a sum of a first optical thickness of the first microlayer and a second optical thickness of the second microlayer. The first microlayer of each optical repeat unit includes a high-index material having a first refractive index, and the second microlayer of each optical repeat unit includes a low-index material having a second refractive index. The first refractive index is greater than the second refractive index. Each optical repeat unit of the plurality of optical repeat units has an f-ratio defined as the ratio of the first optical thickness of the first microlayer to the optical thickness of the optical repeat unit, and the plurality of optical repeat units define an f-ratio profile of the packet of microlayers. The f-ratio profile includes a local minimum corresponding to a maximum difference in dispersion between the high-index material and the low-index material. Brief Description of the Drawings

[0005] FIG. 1 is a schematic view of an optical system including a multilayer optical film with low pass-state reflection, in accordance with an embodiment of the present description;

[0006] FIG. 2 is an illustration of the multilayer structure of a reflective polarizer with low pass-state reflection, in accordance with an embodiment of the present description;

[0007] FIGS. 3A-3C show additional details for a reflective polarizer with low pass-state reflection, in accordance with an embodiment of the present description;

[0008] FIGS. 4A-4C show optical characteristics associated with a typical reflective polarizer as seen in the prior art;

[0009] FIGS. 5A-5C show optical characteristics associated with a reflective polarizer with low passstate reflection, in accordance with an embodiment of the present description;

[0010] FIGS. 6A-6B show plots of refractive index and dispersion characteristics associated with reflective polarizers which might be mitigated using a reflective polarizer with low pass-state reflection, in accordance with an embodiment of the present description;

[0011] FIGS. 7A-7B show possible f-ratio profiles for a reflective polarizer with low pass-state reflection, in accordance with an embodiment of the present description; and

[0012] FIGS. 8A-8B show additional possible f-ratio profiles for a reflective polarizer with low passstate reflection, in accordance with an embodiment of the present description.

[0013] Detailed Description

[0014] 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.

[0015] With the rapid development of Virtual Reality (VR) displays, there is a growing need for smaller, brighter, and more efficient VR headset displays. A key component in some VR designs is a reflective polarizer based on a multilayer optical film (MOF). To enable better image quality and viewing experience, the reflective polarizer should have a high contrast ratio and an extremely low pass state reflection. However, MOF-based reflective polarizers inevitably have residual pass-state reflection due to the index mismatch between the two materials along the pass axis. For example, a MOF-based reflective polarizer may include a plurality of optical repeat units, where each optical repeat unit (ORU) includes a first microlayer of a first material (e.g., a high-index material) and a second microlayer of a second material (e.g., a low-index material). For some wavelengths of light (e.g., above 500 nm), the pass-state dispersion of the high-index material and the low-index material may match well, but for other wavelengths (e.g., below 500 nm), there may be a significant mismatch between the refractive indexes of the two materials. This mismatch can cause residual reflection, which in turn lead to ghost images and other undesired artifacts in VR displays. It is desirable to minimize the pass state reflection without major sacrifice in block state reflectivity.

[0016] It should be noted that, although the example described above has an index mismatch in the lower wavelengths (e.g., blue wavelengths), in other examples and embodiments, the index mismatch can occur substantially anywhere in the human-visible wavelength range (e.g., in the green wavelengths, or in the red wavelengths).

[0017] According to some aspects of the present description, a multilayer optical film includes a new design that preferentially lowers the pass state reflection but with a smaller reduction in block state reflectivity by intentionally modifying the f-ratio profile of the multilayer optical film. The design targets the spectral region where the most refractive index mismatch happens (e.g., in the lower blue wavelengths) and reduces the pass state reflection using a localized f-ratio.

[0018] Because the high index material has a higher refractive index in the block state, the f-ratio along the block state is higher than that of the pass state. Therefore, if the local f-ratio is intentionally adjusted to be below a mean value for the entire f-ratio profile of the film, the reflective power of the pass state will be lower relative to the block state in that area. In this way, the pass state reflectivity can be preferentially reduced in exchange for a small increase in block state transmission.

[0019] According to some aspects of the present description, a multilayer optical film includes a packet of microlayers. In some embodiments, the packet may include a plurality of optical repeat units, wherein each optical repeat unit includes first and second microlayers. In some embodiments, the first and second microlayers may be of different materials, a first material and a second material, respectively. In some embodiments, the first material and the second material may have different indices of refraction. In addition, in some embodiments, the first material may have a higher index of refraction than the second material.

[0020] In some embodiments, the first microlayer may have a first refractive index in a first direction (e.g., x-direction of the first microlayer) of the multilayer optical film and a second refractive index in a second, orthogonal direction (e.g., y-direction) of the multilayer optical film. In some such embodiments, the first refractive index is different than the second refractive index.

[0021] In some embodiments, each optical repeat unit may have an optical thickness which is a sum of a first optical thickness of the first microlayer and a second optical thickness of the second microlayer. In some embodiments, each optical repeat unit may reflect and / or transmit light in a first wavelength range (e.g., a human visible wavelength range, extending from about 380 nm to about 800 nm) primarily by optical interference. In some embodiments, a total number of the optical repeat units less than about 1000, or less than about 900, or less than about 800, or less than about 700, or less than about 600, or less than about 500.

[0022] In some embodiments, for a substantially normally incident light in the first wavelength range, the plurality of optical repeat units may substantially transmit (e.g., transmit at least about 95%, or about 90%, or about 85%, or about 80%, or about 75%, or about 70%) the light having a first polarization state, and may substantially reflect (e.g., reflect at least about 95%, or about 90%, or about 85%, or about 80%, or about 75%, or about 70%) the light having an orthogonal second polarization state. For example, the first polarization state may be an s-type polarization, and the orthogonal second polarization state may be a p-type polarization, or the first polarization state may be an p-type polarization, and the orthogonal second polarization state may be a s-type polarization.

[0023] In some embodiments, the first microlayer of each optical repeat unit may have a first refractive index and the second microlayer of each optical repeat unit may have a second refractive index. In some embodiments, the first refractive index may be greater than the second refractive index (e.g., the first microlayer may be of a high-index material, and the second microlayer may be of a low- index material). In some embodiments, each optical repeat unit of the plurality of optical repeat units may have an f-ratio defined as the ratio of the first optical thickness of the first microlayer to the optical thickness of the optical repeat unit. In some embodiments, the plurality of optical repeat units may define an f-ratio profile of the packet of microlayers (i.e., a plot of ORU f-ratios across the plurality of ORUs). In some such embodiments, the f-ratio profile of the packet of microlayers may be configured to decrease an amount of light of the first polarization state that is reflected and increase an amount of light of the second polarization state that is transmitted. In some embodiments, an amount of light of the first polarization state that is reflected (i.e., an unwanted reflection of light in at least some wavelengths of light in the pass state) may be minimized for a given increase in the amount of light of the second polarization state that is transmitted (i.e., unwanted transmission of light in at least some wavelengths of light in the block state). In some embodiments, the f-ratio profile may be nonconstant (i.e., may vary across the plurality of ORU layers).

[0024] Stated another way, while a multilayer optical film in the prior art may use a constant f-ratio profile (e.g., a constant f-ratio of about 0.5 across the plurality of ORUs) in order to maximize the amount of reflection in the block state of the film, it may be advantageous, as described herein, to instead allow for a small increase in block state transmission to minimize the about of light that is reflected in the pass state. It is often this unwanted pass state reflection that can cause ghosting of images on a display. It is thus taught herein that these two characteristics can be balanced for an overall increase in display quality by intentionally adjusting and shaping the f-ratio profile to provide optimal display quality.

[0025] In some embodiments, the f-ratio profile of the packet of microlayers may be configured such that the lowest f-ratios within the f-ratio profile substantially correspond to locations within the first wavelength range corresponding to a maximum difference in dispersion between the first microlayer and the second microlayer. In some such embodiments, the lowest f-ratios may occur in a wavelength range extending from about 380 nm to about 495 nm (e.g., blue wavelengths), or in a wavelength range extending from about 600 nm to about 800 nm (e.g., red wavelengths), or in a wavelength range extending from about 495 nm to about 600 nm (e.g., green wavelengths). The optimizing effects of the multilayer optical films described herein may be achieved with a variety of different f-ratio profiles. For example, in some embodiments, the f-ratio profile include a plot with a substantially positive slope, such that the f-ratios of ORUs increasing from lowest wavelengths to highest wavelengths. The plots may be substantially linear or may be curved, as described herein. In some embodiments, the f-ratio profile may include a plot with a substantially negative slope, decreasing from lowest wavelengths to highest wavelengths. In still other embodiments, the f-ratio profile may define a U-shaped or V-shaped plot, have a substantially negative slope from a first end of the first wavelength range (e.g., in the blue wavelengths) to a local minimum of the f-ratio profile (e.g., in the green wavelengths), followed by a substantially positive slope from the local minimum to a second end of the first wavelength range (e.g., in the red wavelengths).

[0026] In some embodiments, a deviation of a local f-ratio at a given wavelength from a mean value of the f-ratio profile may be substantially proportional to the difference in dispersion between the first microlayer and the second microlayer at that wavelength. Stated another way, the shape of the f-ratio profile may be dictated by or may correspond to the differences in dispersion of the refractive indices of the high-index materials and the low-index materials (with lower f-ratios typically corresponding to the greatest amounts of dispersion difference). In some such embodiments, the deviation of the local f-ratio from the mean value may be greater than 0.05, or greater than about 0.06, or greater than about 0.07, or greater than about 0.08, or greater than about 0.09, or greater than about 0.1, or greater than about 0.15, or greater than about 0.2.

[0027] According to some aspects of the present description, an optical system includes a display configured to form and emit an image, the optical system configured to display a virtual image of the emitted image for viewing by an eye disposed at or proximate an exit pupil of the optical system, and an optical lens assembly. In some embodiments, the optical lens assembly may include at least a first optical lens disposed in an optical cavity defined between a partial reflector and any of the multilayer optical films described herein.

[0028] In some embodiments, image light emitted from each location on the image formed by the display may pass through the partial reflector, through the at least first optical lens, be reflected at least once by the multilayer optical film, and exit the optical lens assembly and is incident on the exit pupil as a substantially collimated image light.

[0029] According to some aspects of the present description, a multilayer reflective polarizer may be configured such that, for a substantially normally incident light in a predetermined wavelength range extending between about 380 nm and 800 nm (i.e., a human-visible wavelength range), the multilayer reflective polarizer substantially transmits light having a first polarization state, and substantially reflects light having an orthogonal second polarization state.

[0030] In some embodiments, the multilayer reflective polarizer may include a plurality of optical repeat units, wherein each optical repeat unit includes at least first and second microlayers. In some embodiments, each optical repeat unit has an optical thickness which is a sum of a first optical thickness of the first microlayer and a second optical thickness of the second microlayer.

[0031] In some embodiments, the first microlayer of each optical repeat unit may include a high- index material having a first refractive index, and the second microlayer of each optical repeat unit may have a low-index material having a second refractive index. In some embodiments, the first refractive index greater than the second refractive index.

[0032] In should be noted that, for the purposes of this document, the term “high-index material” shall be a material which has a refractive index which is high relative to the low-index material, and the term “low -index material” shall be a material which has a refractive index which is low relative to the high-index material. For example, the high-index material (also called a high index optics, or HIO, material) may be PEN (poly(ethylene naphthalate) and may have a refractive index, n, of approximately 1.75, and the low -index material (or LIO material) may be PMMA (poly(methyl methacrylate)) and have a refractive index, n, of approximately 1.49.

[0033] The materials discussed above are examples only and any appropriate materials may be substituted within the scope of the films discussed herein. Materials used in the first and second microlayers may also be blended with other materials as required to alter the refractive indices of those layers to fit any application. It should also be noted that at least one of the microlayers may be birefringent, meaning that the refractive index of that microlayer may be different in different directions. For example, the first microlayer may have a first refractive index in a first direction (e.g., x-direction) of the multilayer optical film and a second refractive index in a second, orthogonal direction (e.g., y-direction).

[0034] In some embodiments, each optical repeat unit of the plurality of optical repeat units may have an f-ratio defined as the ratio of the first optical thickness of the first microlayer (the high-index material microlayer) to the optical thickness of the optical repeat unit, such that the plurality of optical repeat units defines an f-ratio profile of the packet of microlayers. In some embodiments, the f-ratio profile may include a local minimum (corresponding to the optical repeat units having the lowest f- ratios) corresponding to a maximum difference in dispersion between the high-index material and the low-index material. Each optical repeat unit of the plurality of optical repeat units reflects and / or transmits light in the predetermined wavelength range primarily by optical interference. In some embodiments, a total number of the optical repeat units less than about 1000, or less than about 900, or less than about 800, or less than about 700, or less than about 600, or less than about 500.

[0035] In some embodiments, the maximum difference in dispersion between the high-index material and the low-index material (i.e., the largest mismatch between the first microlayer and the second microlayer) may occur in a wavelength range extending from about 380 nm to about 495 nm. In some other embodiments, the maximum difference in dispersion may occur in a wavelength range extending from about 600 nm to about 800 nm. In still other embodiments, the maximum difference in dispersion may occur in a wavelength range extending from about 495 nm to about 600 nm. In some embodiments, the f-ratio profile may define a plot with a substantially positive slope, increasing from lowest wavelengths (e.g., blue wavelengths) to highest wavelengths (e.g., red wavelengths). In some other embodiments, the f-ratio profile may define a plot with a substantially negative slope, decreasing from lowest wavelengths to highest wavelengths. In yet some other embodiments, the f-ratio profile may define a U-shaped or a V-shaped plot having a substantially negative slope from a first end of the first wavelength range to a local minimum of the f-ratio profile, followed by a substantially positive slope from the local minimum to a second end of the first wavelength range, the second end higher than the first end.

[0036] In some embodiments, a deviation of a local f-ratio at a given wavelength from a mean value of the f-ratio profile may be substantially proportional to the difference in dispersion between the first microlayer and the second microlayer at that wavelength. For example, in some embodiments, the deviation of the local f-ratio is greater than 0.05, or greater than about 0.06, or greater than about 0.07, or greater than about 0.08, or greater than about 0.09, or greater than about 0.1, or greater than about 0.15, or greater than about 0.2.

[0037] According to some aspects of the present description, an optical system includes a display configured to form and emit an image, the optical system configured to display a virtual image of the emitted image for viewing by an eye disposed at or proximate an exit pupil of the optical system, and an optical lens assembly. In some embodiments, the optical lens assembly may include at least a first optical lens disposed in an optical cavity defined between a partial reflector and any of the multilayer reflective polarizers described herein.

[0038] In some embodiments, image light emitted from each location on the image formed by the display passes through the partial reflector, through the at least first optical lens, is reflected at least once by the multilayer reflective polarizer, and exits the optical lens assembly and is incident on the exit pupil as a substantially collimated image light.

[0039] Turning now to the figures, FIG. 1 is a schematic view of an embodiments of an optical system including a multilayer optical film with low pass-state reflection, according to the present description. Optical system 300 includes a display 10 configured to form and emit an image 11. In some embodiments, optical system 300 may be configured to display a virtual image 12 of emitted image 11 for viewing by an eye 13 disposed at or proximate an opening 22 of an exit pupil 20 of optical system 300. In some embodiments, optical system 300 may be centered on optical axis 301, and at least a portion 302 of optical axis 301 may be a folded optical axis (i.e., light is reflected back along or proximate to optical axis 301 before being redirected toward exit pupil 20).

[0040] In some embodiments, optical system 300 further includes optical lens assembly 100. Optical lens assembly 100 may include at least a first optical lens 35 disposed in an optical cavity 36 defined between a partial reflector 37 and a multilayer reflective polarizer 34. In some embodiments, image light 50 emitted from each location 51 on the image 11 formed by display 10 passes through partial reflector 37, through at least first optical lens 35, is reflected at least once by the multilayer reflective polarizer 30, and exits optical lens assembly 100 and is incident on the exit pupil 20 as a substantially collimated image light 52.

[0041] In some embodiments, optical lens assembly 100 may further include at least a second optical lens 34. In some such embodiments, at least second optical lens 34 may be disposed outside optical cavity 36 proximate reflective polarizer 30 and away from partial reflector 37. In other such embodiments, at least second optical lens 34 may be disposed inside optical cavity 36 proximate reflective polarizer 30 and facing partial reflector 37 (not shown in FIG. 1). In some embodiments, optical system 300 may further include an absorbing polarizer 60 disposed outside optical lens assembly 100, proximate reflective polarizer 30 and away from partial reflector 37.

[0042] FIG. 2 is a schematic, side view showing the structure of an embodiment of a multilayer optical film (such as multilayer optical film / reflective polarizer 30 shown in FIG. 1). In some embodiments, multilayer optical film may have a multilayer structure and include a plurality of polymeric layers 31, 32 as shown in FIG. 2. In some embodiments, an average thickness of each of polymeric layers 31, 32 may be less than about 500 nm, or less than about 450 nm, or less than about 400 nm, or less than about 350 nm, or less than about 300 nm, or less than about 250 nm, or less than about 200 nm. In some embodiments, multilayer optical film 30 may further include at least one skin layer 33 having 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 1250 nm, or greater than about 1500 nm.

[0043] In some embodiments, the indices of refraction of alternating polymeric layers 31, 32 may be different from each other. In some embodiments, the indices of refraction of at least one of the alternating polymeric layers 31, 32 may vary across a profile of layers in the z-direction, as shown by the coordinate system in FIG. 7. In some embodiments, a profile plotting various physical and optical characteristics of alternating polymeric layers 31, 32 such as index of refraction, thickness, etc. may vary (e.g., may show a changing gradient of one or more of the characteristics, may vary randomly, continuously, non-continuously, etc.). In this way, optical transmission characteristics of the substrate layer may be modified to meet a specific requirement. For example, in some embodiments, for a substantially normally incident light and a visible wavelength range extending from about 420 nm to about 680 nm, the plurality of polymeric layers 31, 32 may have an average reflectance of greater than about 50%, or greater than about 60%, or greater than about 70%, or greater than about 80%, or greater than about 90%, or greater than about 95% when the incident light is polarized along a first inplane direction (e.g., along the x-axis of the film) and an average transmittance of greater than about 60%, or greater than about 70%, or greater than about 80%, or greater than about 90%, or greater than about 95% when the incident light is polarized along an orthogonal second in-plane direction (e.g., the y-axis).

[0044] FIGS. 3A-3C show additional details for an embodiment of a reflective polarizer with low pass-state reflection, according to the present description. For example, the embodiment of multilayer optical film 30 shown in FIG. 3 A, the plurality of microlayers 31, 32 are arranged in optical repeat units, or ORUs. It should be noted that FIG. 3A is an exploded view, showing gaps between some microlayers to distinguish boundaries between ORUs, but these gaps do not exist in the final film.

[0045] Turning to FIGS. 3B and 3C, each ORU may include a first microlayer 31 and a second microlayer 32. An optical thickness Topt, of each ORU, therefore, can be defined as a sum of the optical thickness Thio of first microlayer 31 (the high index optics layer) and the optical thickness of second microlayer 32 Tuo(the low index optics layer).

[0046] Although optical thickness is shown in FIG. 3B and 3C as a physical thickness of the layer, and while optical thickness can be related to the physical thickness of the microlayer, optical thickness (OT) shall be defined by the following:

[0047] OT = -log(T) where T is the amount of transmission of light through the medium and is between 0 (for 0% optical transmission of the light) and 1 (for 100% optical transmission of the light). For example, for 100% transmission, T would be equal to 1 and the optical thickness would be calculated to be 0.

[0048] Each ORU in multilayer optical film 30 also defines an f-ratio. For the purposes of this document, f-ratio shall be defined as the ratio of the first optical thickness Thio of first microlayer 31 (the “high-index layer”) to the optical thickness of the entire optical repeat unit (such as ORUx or ORUy in FIGS. 3B and 3C, respectively). Looking at FIG. 3B, ORUx has an f-ratio approximately equal to 0.5 because the optical thicknesses of the first 31 and second 32 microlayers are approximately equal. That is, Thio is approximately 0.5 times Toptfor ORUx. For ORUy shown in FIG. 3C, Thio is approximately a third of Topt, so the f-ratio of ORUy is approximately 0.3.

[0049] A total number of the optical repeat units for a given multilayer optical film 30 may be less than about 1000, or less than about 900, or less than about 800, or less than about 700, or less than about 600, or less than about 500. If you plot the f-ratio of all of the ORUs on a chart, the resulting plot is considered an f-ratio profile of the total packet of microlayers for the multilayer optical film 30.

[0050] For example, FIG. 4A shows an f-ratio profile 70 typical to an optical film of the prior art. In this example, the f-ratio of each ORU is plotted against the ORU number, from a first ORU (ORU 0, on the left of the plot) to a last ORU (which, in the example shown in FIG. 4A, appears to be about ORU 700). This example from the prior art uses a substantially constant f-ratio of 0.5 across all of the ORUs, which is typical for existing reflective polarizers in the art.

[0051] However, this conventional, constant f-ratio of 0.5 can cause unwanted reflection in the pass state for the reflective polarizer, which can produce ghost images on the display. As shown in FIG. 4B, the plot 72 of percent reflection in the pass state shows unwanted reflection in the blue wavelength range in area 75 of the plot. Even a small amount of reflection of light in the pass state can lead to ghost images. As can be seen in FIG. 4C, which shows percent transmission of light in the block state, this constant f-ratio of 0.5 is used in prior art systems because it tends to create an ideal plot 73 which shows substantially zero transmission of light in the human-visible wavelength range (between about 400 nm and about 800 nm) in the block state.

[0052] As discussed elsewhere herein, the unwanted reflection in the pass state shown in FIG. 4B corresponds to the amount of mismatch in the refractive indices of the first microlayer and the second microlayer and the lower wavelengths. In the example of FIGS. 4A-4C, this mismatch is greatest in the blue wavelengths at the lower end of the human visible spectrum, and so the corresponding unwanted reflection (and the ghost imaging resulting from it) occurs at these wavelengths.

[0053] FIGS. 5A-5C show optical characteristics associated with an embodiment of a reflective polarizer with low pass-state reflection, according to the present description. Looking first at FIG. 5A, embodiments of the present description are configured to have an f-ratio profile which exhibits lower f-ratios in the ORUs corresponding to the maximum mismatch in the refractive indices between microlayers. As with the examples shown in FIGS. 4A-4C, the embodiments of FIGS. 5A-5C are configured for an optical film where the maximum mismatch in refractive indices occurs in the lower, or blue, wavelengths. By configuring the f-ratio profile 80 to be non-constant, such that the lowest f- ratios occur in the ORUs corresponding to the maximum mismatch in refractive indices. In this case, ORU 0 (first, left- most ORU in the plot) has an f-ratio of approximately 0.1, and the f-ratios increase linearly to a maximum f-ratio of 0.5 on the right-most side of the plot.

[0054] Comparing plot 82 in region 85 of FIG. 5B to plot 72 in region 75 of FIG. 4B, it is evident that the f-ratio profile of FIG. 5 A significantly reduces the amount of unwanted reflection in the blue wavelengths (corresponding to a similar reduction in ghost imaging). This significant reduction in reflection in the pass state of FIG. 5B is balanced with a slight increase in transmission shown on plot 83 and region 86 of FIG. 5C. This slight increase in block state transmission will not cause significant problems with display image quality, and the decrease in pass state reflection reduces or eliminates problems with ghost imaging.

[0055] FIGS. 6A-6B show plots of refractive index and dispersion characteristics associated with reflective polarizers which might be mitigated using a reflective polarizer with low pass-state reflection, as discussed in the present description. Each of the plots shown in FIGS. 4A-5C are based on a model using the graph shown in FIG. 6A, in which the largest mismatch in refractive indices between the high-index material 91 (of first microlayer 31) and the low-index material 92 (of second microlayer 32) occurs in region 90 of FIG. 6A. Region 90 corresponds to blue wavelengths of about 380 nm to about 450 nm.

[0056] In other embodiments, such as that shown in FIG. 6B, the greatest mismatch of refractive indices between high-index material 91 and low -index material 92 may occur in the higher wavelengths, such as those shown in region 90b, or may exist in multiple regions, such as regions 90a and 90b shown in FIG. 6B. In the embodiments where the index mismatch (or the greatest difference in dispersion occurs), other types of f-ratio profiles may provide the best optical performance. Examples of alternative f-ratio profiles are illustrated in FIGS. 7A-7B and 8A-8B.

[0057] The example f-ratio profile shown in FIG. 5A showed a non-constant linear, positive slope (i.e., increasing f-ratio magnitude toward the higher wavelengths of light). As shown in FIG. 7A, similar performance improvements may be seen with plot 80a, showing a non-linear (curved) f-ratio profile. FIG. 7A shows a curved f-ratio profile with a positive slow, increasing from about 0.1 at ORU 0 to about 0.45 around ORU 600. This f-ratio profile still relies on the greatest difference in dispersion (or index mismatch) in the lower, blue wavelengths of light, such as that shown in FIG. 6A.

[0058] When the index mismatch occurs in other wavelengths, such as the plot shown in FIG. 6B, the f-ratio profile may have a negative slope, such as that shown by plot 80b of FIG 7B. This means that the lowest f-ratios will occur in the higher ORU numbers corresponding to the index mismatch seen in the higher (red) wavelengths of FIG. 6B.

[0059] Other types of dispersion / index mismatch may occur than those shown in FIGS. 6 A and 6B. For example, in a situation where the greatest amount of index mismatch occurs in the green wavelengths, the f-ratio profile may be similar to that of plot 80c of FIG. 8A, which shows a beginning negative slope to a low point, followed by a positive slope to the highest ORU numbers. In this case, the concept is to match the location of the lowest f-ratios to the ORUs associated with the greatest differences in dispersion. Finally, FIG. 8B shows a curved (non-linear) f-ratio profile with a negative slope, which may be used when the index mismatch occurs in the higher (red) wavelengths.

[0060] Examples

[0061] Modeling Procedure

[0062] The 4x4 matrix formalism is used to model the transmission and reflection spectra of a multilayer optical film involving birefringent materials. The original formulae were developed by Berreman1and Yeh2. A computer program is developed internally to implement the algorithm. Input parameters for this optical model include individual layer refractive index tensor, layer thicknesses, and number of layers. Transmission and reflection spectra shown below are predicted by this optical model.

[0063] Baseline Example

[0064] For the baseline example, the performance of the baseline reflective polarizer with a constant f-ratio = 0.5 is shown in FIG. 4A. This represents the conventionallA wave stack design in prior arts. This constant f-ratio contributes to a pass-state reflection see FIG. 4B which increases around 400 nm due to the index mismatch between the high index (fHO) and low index (LIO) layers along the pass axis (y). In the baseline example, the mismatch increases towards the shorter wavelength as shown in FIG. 6A and hence the reflection increases.

[0065] Einear F-ratio Compensation

[0066] To compensate for the index mismatch as shown in FIG. 6 A, an f-ratio profile, such as that shown in FIG. 5A can be used, showing an increasing f-ratio through the layer stack (see f-ratio definition described elsewhere herein). As discussed earlier in the application, an f-ratio lower than 0.5 preferentially suppresses pass-state reflection with minimal decreases in block-state reflection. By creating a f-ratio profile which is non-constant, the f-ratio of the layer pairs which reflect the lower wavelengths towards 400 nm can be preferentially lowered. One such example is a linear f-ratio profile of FIG. 6A. With this design, the pass-state reflection can be suppressed, as shown in FIG. 5B (compared to the amount of pass state reflection shown in FIG. 4B with the constant f-ratio profile).

[0067] Curved F-ratio Compensation

[0068] Leveraging the same principle, the f-ratio profile does not have to be linear to compensate for the index mismatch as shown in FIG. 6A. For example, FIG. 7A shows a curved f-ratio profile to suppress passstate reflection towards 400 nm. This example illustrates the design freedom in terms of the f-ratio profile to compensate for different index mismatch shapes.

[0069] Negative Slope F-ratio Compensation

[0070] In this example, we illustrate the possibility of compensating for a different index mismatch as shown in FIG. 6B. Instead of the increasing mismatch towards 400 nm as in FIG. 6A, the index difference increases towards 780 nm. Using the same design principle, we specifically lower the f-ratio for layers that reflect wavelengths towards 780 nm as shown in FIG. 7B. The pass-state reflection remains substantially suppressed even though the index mismatch moves to a new spectral location. References for Examples

[0071] 1. Berreman, D. W. Optics in Stratified and Anisotropic Media: 4x4-Matrix Formulation. J Opt Soc Am 62, 502 (1972).

[0072] 2. Yeh, P. Electromagnetic Propagation in Birefringent Layered Media. J Opt SocAm 69, 742-756 (1979).

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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 multilayer optical film comprising: a packet of microlayers, the packet comprising a plurality of optical repeat units, each optical repeat unit comprising first and second microlayers, each optical repeat unit having an optical thickness being a sum of a first optical thickness of the first microlayer and a second optical thickness of the second microlayer, each optical repeat unit reflecting or transmitting light in a first wavelength range primarily by optical interference, such that for a substantially normally incident light in the first wavelength range, the plurality of optical repeat units substantially transmits the light having a first polarization state, and substantially reflects the light having an orthogonal second polarization state; wherein the first microlayer of each optical repeat unit has a first refractive index and the second microlayer of each optical repeat unit has a second refractive index, the first refractive index greater than the second refractive index, and each optical repeat unit of the plurality of optical repeat units has an f-ratio defined as the ratio of the first optical thickness of the first microlayer to the optical thickness of the optical repeat unit, the plurality of optical repeat units defining an f-ratio profile of the packet of microlayers, such that the f-ratio profile of the packet of microlayers is non-constant and is configured to decrease an amount of light of the first polarization state that is reflected and increase an amount of light of the second polarization state that is transmitted.

2. The multilayer optical film of claim 1, wherein the first wavelength range extends from about 380 nm to about 800 nm.

3. The multilayer optical film of claim 1, wherein the f-ratio profile of the packet of microlayers is configured such that the lowest f-ratios within the f-ratio profile substantially correspond to locations within the first wavelength range corresponding to a maximum difference in dispersion between the first microlayer and the second microlayer.

4. The multilayer optical film of claim 3, wherein the lowest f-ratios occur in a wavelength range extending from about 380 nm to about 495 nm.

5. The multilayer optical film of claim 3, wherein the lowest f-ratios occur in a wavelength range extending from about 600 nm to about 800 nm.

6. The multilayer optical film of claim 3, wherein the lowest f-ratios occur in a wavelength range extending from about 495 nm to about 600 nm.

7. The multilayer optical film of claim 1, wherein an amount of light of the first polarization state that is reflected is minimized for a given increase in the amount of light of the second polarization state that is transmitted.

8. The multilayer optical film of claim 1, wherein the first microlayer has a first refractive index in a first direction of the multilayer optical film and a second refractive index in a second, orthogonal direction of the multilayer optical film, the first refractive index different than the second refractive index.

9. The multilayer optical film of claim 1, wherein the f-ratio profile comprises a plot with a substantially positive slope, increasing from lowest wavelengths to highest wavelengths.

10. The multilayer optical film of claim 1, wherein the f-ratio profile comprises a plot with a substantially negative slope, decreasing from lowest wavelengths to highest wavelengths.

11. The multilayer optical film of claim 1, wherein the f-ratio profile comprises a substantially negative slope from a first end of the first wavelength range to a local minimum of the f-ratio profile, followed by a substantially positive slope from the local minimum to a second end of the first wavelength range, the second end higher than the first end.

12. The multilayer optical film of claim 3, wherein a deviation of a local f-ratio at a given wavelength from a mean of the f-ratio profile is substantially proportional to the difference in dispersion between the first microlayer and the second microlayer at that wavelength.

13. The multilayer optical film of claim 12, wherein the deviation of the local f-ratio is greater than 0.05.

14. An optical system, comprising: a display configured to form and emit an image, the optical system configured to display a virtual image of the emitted image for viewing by an eye disposed at or proximate an exit pupil of the optical system; and an optical lens assembly comprising at least a first optical lens disposed in an optical cavity defined between a partial reflector and the multilayer optical film of claim 1; wherein image light emitted from each location on the image formed by the display passes through the partial reflector, through the at least first optical lens, is reflected at least once by themultilayer optical film, and exits the optical lens assembly and is incident on the exit pupil as a substantially collimated image light.

15. A multilayer reflective polarizer, configured such that, for a substantially normally incident light in a predetermined wavelength range extending between about 380 nm and 800 nm, the multilayer reflective polarizer substantially transmits the light having a first polarization state, and substantially reflects the light having an orthogonal second polarization state; the multilayer reflective polarizer comprising a plurality of optical repeat units, each optical repeat unit comprising first and second microlayers, each optical repeat unit having an optical thickness being a sum of a first optical thickness of the first microlayer and a second optical thickness of the second microlayer, wherein the first microlayer of each optical repeat unit comprises a high-index material having a first refractive index, and the second microlayer of each optical repeat unit comprises a low-index material having a second refractive index, the first refractive index greater than the second refractive index, and each optical repeat unit of the plurality of optical repeat units has an f-ratio defined as the ratio of the first optical thickness of the first microlayer to the optical thickness of the optical repeat unit, the plurality of optical repeat units defining an f-ratio profile of the packet of microlayers, wherein the f-ratio profile comprises a local minimum corresponding to a maximum difference in dispersion between the high-index material and the low-index material.

16. The multilayer reflective polarizer of claim 15, wherein each optical repeat unit of the plurality of optical repeat units reflects or transmits light in the predetermined wavelength range primarily by optical interference.

17. The multilayer reflective polarizer of claim 15, wherein a total number of the optical repeat units less than about 1000.

18. The multilayer reflective polarizer of claim 15, wherein the maximum difference in dispersion between the high-index material and the low-index material occurs in a wavelength range extending from about 380 nm to about 495 nm.

19. The multilayer reflective polarizer of claim 15, wherein the maximum difference in dispersion between the high-index material and the low-index material occurs in a wavelength range extending from about 600 nm to about 800 nm.

20. The multilayer reflective polarizer of claim 15, wherein the maximum difference in dispersion between the high-index material and the low-index material occurs in a wavelength range extending from about 495 nm to about 600 nm.

21. The multilayer reflective polarizer of claim 15 , wherein the first microlayer has a first refractive index in a first direction of the multilayer optical film and a second refractive index in a second, orthogonal direction of the multilayer optical film, the first refractive index different than the second refractive index.

22. The multilayer reflective polarizer of claim 15, wherein the f-ratio profile comprises a plot with a substantially positive slope, increasing from lowest wavelengths to highest wavelengths.

23. The multilayer reflective polarizer of claim 15, wherein the f-ratio profile comprises a plot with a substantially negative slope, decreasing from lowest wavelengths to highest wavelengths.

24. The multilayer reflective polarizer of claim 15, wherein the f-ratio profile comprises a substantially negative slope from a first end of the first wavelength range to a local minimum of the f- ratio profile, followed by a substantially positive slope from the local minimum to a second end of the first wavelength range, the second end higher than the first end.

25. The multilayer reflective polarizer of claim 15, wherein a deviation of a local f-ratio at a given wavelength from a mean of the f-ratio profile is substantially proportional to the difference in dispersion between the first microlayer and the second microlayer at that wavelength.

26. The multilayer reflective polarizer of claim 25, wherein the deviation of the local f-ratio is greater than 0.05.

27. An optical system, comprising: a display configured to form and emit an image, the optical system configured to display a virtual image of the emitted image for viewing by an eye disposed at or proximate an exit pupil of the optical system; and an optical lens assembly comprising at least a first optical lens disposed in an optical cavity defined between a partial reflector and the multilayer reflective polarizer of claim 15; wherein image light emitted from each location on the image formed by the display passes through the partial reflector, through the at least first optical lens, is reflected at least once by the multilayer reflective polarizer, and exits the optical lens assembly and is incident on the exit pupil as a substantially collimated image light.

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