Customized Polymer / Glass Diffractive Waveguide Stacks for Augmented / Mixed Reality Applications

Abstract

To provide diffractive waveguide stacks.SOLUTION: A diffractive waveguide stack includes first, second, and third diffractive waveguides for guiding light in first, second, and third visible wavelength ranges, respectively. The first diffractive waveguide includes a first material having a first refractive index at a selected wavelength and a first target refractive index at a midpoint of the first visible wavelength range. The second diffractive waveguide includes a second material having a second refractive index at the selected wavelength and a second target refractive index at a midpoint of the second visible wavelength range. The third diffractive waveguide includes a third material having a third refractive index at the selected wavelength and a third target refractive index at a midpoint of the third visible wavelength range. A difference between any two of the first target refractive index, the second target refractive index, and the third target refractive index is less than 0.005 at the selected wavelength.SELECTED DRAWING: None

Description

[Technical field]

[0001] (CROSS-REFERENCE TO RELATED APPLICATIONS) This application claims the benefit of U.S. Patent Application No. 62 / 865,808, filed June 24, 2019, which is incorporated by reference in its entirety.

[0002] (Technical field) The present invention relates to customized polymer / glass diffractive waveguide stacks with improved performance for augmented / mixed reality applications. [Background technology]

[0003] (background) Augmented reality / mixed reality devices typically utilize a single type of glass or polymer material for all layers (e.g., red (R), green (G), and blue (B) layers). The overall performance of the device can be determined by the performance of the RGB layers. Some important performance indicators, such as modulation transfer function (MTF), efficiency, field of view (FOV), contrast, and eyepiece uniformity, depend on the optical properties of the individual layers. These optical properties include refractive index, yellowness index, haze, light transmittance, surface roughness, and the like. For a given material, optical properties such as refractive index and light transmittance are functions of wavelength. See, for example, Figures 1A and 1B, which show a typical dispersion curve (refractive index vs. wavelength) for a polymer waveguide, and Figure 2, which shows a typical extinction curve (light transmittance vs. wavelength) for a polymer waveguide material. Other properties, such as haze and yellowness index, are not significantly wavelength dependent. This particular relationship of waveguide material and incident wavelength can pose challenges related to optical compatibility when the refractive index and corresponding Yellowness Index value are above a certain threshold value. Summary of the Invention [Means for solving the problem]

[0004] (summary) In a first general aspect, a diffractive waveguide stack includes a first diffractive waveguide for guiding light within a first visible wavelength range and a second diffractive waveguide for guiding light within a second visible wavelength range. The first diffractive waveguide includes a first material and has a first yellowness index and a first refractive index at a selected wavelength, and the second diffractive waveguide includes a second material and has a second yellowness index and a second refractive index at a selected wavelength. Wavelengths within the first visible wavelength range exceed wavelengths within the second visible wavelength range, the first refractive index exceeds the second refractive index at the selected wavelength, and the first yellowness index exceeds the second yellowness index at the selected wavelength.

[0005] Implementations of the first general aspect may include one or more of the following features.

[0006] The diffractive waveguide stack of the first general aspect may include a third diffractive waveguide for guiding light within a third visible wavelength range. The third diffractive waveguide includes a third material and has a third yellowness index and a third refractive index at a selected wavelength. The wavelengths within the second visible wavelength range exceed the wavelengths within the third visible wavelength range, and the second yellowness index exceeds the third yellowness index at the selected wavelength. The second refractive index may exceed the third refractive index at the selected wavelength. The first visible wavelength range includes red light, the second visible wavelength range includes green light, and the third visible wavelength range includes blue light. At the selected wavelength, the first yellowness index is less than about 1.2, the second yellowness index is less than about 0.8, and the third yellowness index is less than about 0.4.

[0007] In some cases, the first material includes a first polymer and the second material includes a second polymer. The first polymer and the second polymer may be different. In some cases, the first material includes a first copolymer having a first monomer and a second monomer and the second material includes a second copolymer having a first monomer and a second monomer. The ratio of the first monomer to the second monomer in the first copolymer may be different from the ratio of the first monomer to the second monomer in the second copolymer. The first material may include a first additive and the second material may include a second additive. The first additive and the second additive can be the same and the ratio of the first additive to the first polymer may be different from the ratio of the second additive to the second polymer.

[0008] In some cases, the first material includes a first glass and the second material includes a second glass, and in some cases, the first material includes one of a polymer and a glass and the second material includes the other of a polymer and a glass.

[0009] In a second general aspect, a diffractive waveguide stack includes a first diffractive waveguide for guiding light within a first visible wavelength range, a second diffractive waveguide for guiding light within a second visible wavelength range, and a third diffractive waveguide for guiding light within a third visible wavelength range. The first diffractive waveguide includes a first material and has a first refractive index at a selected wavelength and a first target refractive index at a midpoint of the first visible wavelength range. The second diffractive waveguide includes a second material and has a second refractive index at a selected wavelength and a second target refractive index at a midpoint of the second visible wavelength range. The third diffractive waveguide includes a third material and has a third refractive index at a selected wavelength and a third target refractive index at a midpoint of the third visible wavelength range. The first visible wavelength range corresponds to red light, the second visible wavelength range corresponds to green light, and the third visible wavelength range corresponds to blue light. The difference between any two of the first target refractive index, the second target refractive index, and the third target refractive index is less than 0.005 at the selected wavelength.

[0010] In some implementations of the second general aspect, the selected wavelength is 589 nm.

[0011] In a third general aspect, fabricating a diffractive waveguide for a waveguide stack includes combining a first monomer and a second monomer in a first ratio to provide a first polymerizable material, casting the first polymerizable material into a first diffractive waveguide mold to polymerize the first polymerizable material to provide a first diffractive waveguide for guiding light in a first visible wavelength range, combining the first monomer and the second monomer in a second ratio to provide a second polymerizable material, and casting the second polymerizable material into a second diffractive waveguide mold to polymerize the second polymerizable material to provide a second diffractive waveguide for guiding light in a second visible wavelength range. The first diffractive waveguide has a first yellowness index and a first refractive index at a selected wavelength, and the second diffractive waveguide has a second yellowness index and a second refractive index at a selected wavelength. The wavelengths within the first visible wavelength range exceed the wavelengths within the second visible wavelength range, the first refractive index exceeds the second refractive index at the selected wavelengths, and the first yellowness index exceeds the second yellowness index at the selected wavelengths.

[0012] Implementations of the third general aspect may include one or more of the following features.

[0013] The third general aspect may further include combining the first monomer and the second monomer in a third ratio to provide a third polymerizable material, and casting the third polymerizable material into a third diffractive waveguide mold and polymerizing the third polymerizable material to provide a third diffractive waveguide for guiding light in a third visible wavelength range. The third diffractive waveguide has a third yellowness index and a third refractive index at a selected wavelength. The wavelengths in the second visible wavelength range exceed the wavelengths in the third visible wavelength range, and the second yellowness index exceeds the third yellowness index at the selected wavelength. The second refractive index may exceed the third refractive index at the selected wavelength. In one case, the selected wavelength is 589 nm.

[0014] The details of one or more embodiments of the presently disclosed subject matter are set forth in the accompanying drawings and description. Other features, aspects, and advantages of the present subject matter will become apparent from the description, drawings, and claims. The present invention provides, for example, the following: (Item 1) 1. A diffractive waveguide stack comprising: a first diffractive waveguide for guiding light within a first visible wavelength range, the first diffractive waveguide comprising a first material and having a first yellowness index and a first refractive index at a selected wavelength; a second diffractive waveguide for guiding light within a second visible wavelength range, said second diffractive waveguide a second diffractive waveguide including a second material and having a second yellowness index and a second refractive index at the selected wavelength; Equipped with wavelengths in the first visible wavelength range exceed wavelengths in the second visible wavelength range; the first refractive index exceeds the second refractive index at the selected wavelength; The first yellowness index exceeds the second yellowness index at the selected wavelength. (Item 2) a third diffractive waveguide for guiding light within a third visible wavelength range, the third diffractive waveguide comprising a third material and having a third yellowness index and a third refractive index at the selected wavelength; the wavelengths in the second visible wavelength range exceed the wavelengths in the third visible wavelength range; Item 2. The diffractive waveguide of item 1, wherein the second yellowness index exceeds the third yellowness index at the selected wavelength. (Item 3) 3. The diffractive waveguide of claim 2, wherein the second refractive index exceeds the third refractive index at the selected wavelength. (Item 4) 3. The diffractive waveguide of claim 2, wherein the first visible wavelength range includes red light, the second visible wavelength range includes green light, and the third visible wavelength range includes blue light. (Item 5) 3. The diffractive waveguide of claim 2, wherein the selected wavelength is 589 nm, the first yellowness index is less than about 1.2, the second yellowness index is less than about 0.8, and the third yellowness index is less than about 0.4. (Item 6) Item 2. The diffractive waveguide of item 1, wherein the first material comprises a first polymer and the second material comprises a second polymer. (Item 7) 7. The diffractive waveguide of claim 6, wherein the first polymer and the second polymer are different. (Item 8) 7. The diffractive waveguide of claim 6, wherein the first material comprises a first copolymer comprising a first monomer and a second monomer, and the second material comprises a second copolymer comprising the first monomer and the second monomer. (Item 9) 7. The diffractive waveguide of claim 6, wherein a ratio of the first monomer to the second monomer in the first copolymer is different from a ratio of the first monomer to the second monomer in the second copolymer. (Item 10) 7. The diffractive waveguide of claim 6, wherein the first material comprises a first additive and the second material comprises a second additive. (Item 11) Item 11. The diffractive waveguide of item 10, wherein the first additive and the second additive are the same and the ratio of the first additive to the first polymer is different from the ratio of the second additive to the second polymer. (Item 12) Item 2. The diffractive waveguide of item 1, wherein the first material comprises a first glass and the second material comprises a second glass. (Item 13) The first material includes one of a polymer and a glass, and the second material includes a polycarbonate. Item 2. The diffractive waveguide stack of item 1, comprising the other of the polymer and the glass. (Item 14) Item 2. The diffractive waveguide stack of item 1, wherein the selected wavelength is 589 nm. (Item 15) 1. A method of fabricating a diffractive waveguide for a waveguide stack, the method comprising: combining a first monomer and a second monomer in a first ratio to provide a first polymerizable material; casting the first polymerizable material into a first diffractive waveguide mold and polymerizing the first polymerizable material to provide a first diffractive waveguide for guiding light within a first visible wavelength range, the first diffractive waveguide having a first yellowness index and a first refractive index at a selected wavelength; combining the first monomer and the second monomer in a second ratio to provide a second polymerizable material; casting the second polymerizable material into a second diffractive waveguide mold and polymerizing the second polymerizable material to provide a second diffractive waveguide for guiding light within a second visible wavelength range, the second diffractive waveguide having a second yellowness index and a second refractive index at the selected wavelength; Including, wavelengths in the first visible wavelength range exceed wavelengths in the second visible wavelength range; the first refractive index exceeds the second refractive index at the selected wavelength; The method, wherein the first Yellowness Index exceeds the second Yellowness Index at the selected wavelength. (Item 16) combining the first monomer and the second monomer in a third ratio to provide a third polymerizable material; casting the third polymerizable material into a third diffractive waveguide mold and polymerizing the third polymerizable material to provide a third diffractive waveguide for guiding light within a third visible wavelength range, the third diffractive waveguide having a third yellowness index and a third refractive index at the selected wavelength; Further comprising: the wavelengths in the second visible wavelength range exceed the wavelengths in the third visible wavelength range; Item 16. The method of item 15, wherein the second yellowness index exceeds the third yellowness index at the selected wavelength. (Item 17) Item 17. The method of item 16, wherein the second refractive index exceeds the third refractive index at the selected wavelength. (Item 18) 16. The method of claim 15, wherein the selected wavelength is 589 nm. (Item 19) 1. A diffractive waveguide stack comprising: a first diffractive waveguide for guiding light within a first visible wavelength range, the first diffractive waveguide comprising a first material and having a first refractive index at a selected wavelength and a first target refractive index at a midpoint of the first visible wavelength range; a second diffractive waveguide for guiding light within a second visible wavelength range, the second diffractive waveguide comprising a second material and having a second refractive index at the selected wavelength and a second target refractive index at a midpoint of the second visible wavelength range; a third diffractive waveguide for guiding light within a third visible wavelength range, the third diffractive waveguide comprising a third material and having a third refractive index at the selected wavelength and a third target refractive index at a midpoint of the third visible wavelength range; Equipped with the first visible wavelength range includes red light, the second visible wavelength range includes green light, and the third visible wavelength range includes blue light; a difference between any two of the first target refractive index, the second target refractive index, and the third target refractive index at the selected wavelength is less than 0.005 at the selected wavelength. (Item 20) 20. The diffractive waveguide of claim 19, wherein the selected wavelength is 589 nm. [Brief description of the drawings]

[0015] [Figure 1] 1A and 1B show typical dispersion curves for a polymer waveguide.

[0016] [Diagram 2] FIG. 2 shows a typical extinction curve for a waveguide material.

[0017] [Diagram 3] FIG. 3 shows a plot of efficiency versus yellowness index for a red / green / blue (RGB) waveguide.

[0018] [Figure 4] 4A and 4B show RGB configurations based on three different base materials.

[0019] [Diagram 5] 5A and 5B show RGB configurations based on the same base material with different refractive indices.

[0020] [Figure 6-1] 6A-6D show an RGB configuration based on a combination of glass and polymer waveguides with different refractive indices. [Figure 6-2] 6A-6D show an RGB configuration based on a combination of glass and polymer waveguides with different refractive indices.

[0021] [Figure 7] FIG. 7 depicts a multi-head system for processing RGB layers based on a two-part liquid resin.

DETAILED DESCRIPTION

[0022] (Detailed description) The present disclosure relates to the use of optically tuned materials for various color layers (e.g., RGB) in an augmented reality (AR) / mixed reality (MR) diffractive waveguide-based eyepiece lens to optimize the overall optical performance of the eyepiece lens. The material for each color waveguide can be prepared for optimal optical properties (refractive index, yellowness index, transmittance) according to the operating wavelength. Various implementations of glass and polymer-based waveguides configured to achieve optimal optical properties are described.

[0023] For a given waveguide material at RGB wavelengths, differences in refractive index (dispersion curve) typically result in different fields of view (FOV) for each layer and can limit the overall FOV of the waveguide stack. In addition, materials with higher refractive indices (e.g., glass as well as polymers) tend to exhibit higher yellowness indices (b * ) related to the light transmittance of the waveguide and the overall efficiency of the eyepiece lens. When the yellowness index (b * lim ) exceeds a certain value, material absorption limits the overall efficiency of the eyepiece lens, at least in part, due to absorption by most of the waveguide. The threshold of b * lim depends on the spectrum, i.e., b * lim is different for different colors (R, G, B, C,...) in the order of B < G < R. That is, the red layer can tolerate a higher value of b * lim compared to the green and blue layers. FIG. 3 shows a schematic of the threshold b * lim values for various colors based on the reduction in eye box efficiency as the waveguide approaches b * , and plots 300, 302, and 304 correspond to red, green, and blue, respectively. Therefore, instead of using the same waveguide material for all three colors, the refractive index and the corresponding b* An appropriate combination of values ​​may be selected for the individual R, G, and B layers.

[0024] b * To demonstrate the dependence of eyepiece efficiency on wavelength with * The threshold value of is 1.71 at the refractive index of 589, and the starting b * The wavelength-dependent efficiency was obtained by fabricating all three waveguides in LUMIPLUS LPB-1102 polymer (Mitsubishi Gas Chemical), where the wavelength-dependent efficiency is 0.3. The waveguides were then exposed to an additional UV dose to induce a higher yellowness in the waveguides, resulting in a color-by-color efficiency of b * was measured as a function of b * The threshold value is then the b on the plot where the efficiency starts to drop significantly. * By localizing the position, the efficiency per color vs. b * Extracted from the plot.

[0025] There are various implementations to achieve a suitable combination of refractive index and yellowness index for the R, G, and B layers separately. The first implementation employs three different waveguide materials, each with a different base material composition. The second implementation employs the same base material for each waveguide and adjusts the chemical composition or synthesis conditions to modify the optical properties. The third implementation combines glass and polymer waveguides. For higher operating wavelengths (e.g., red, 625 nm), the red wavelength has a higher b * A higher refractive index and a higher b * For lower operating wavelengths (e.g., blue, 455 nm), materials with a higher refractive index at 455 nm and a lower b * A lower refractive index at the wavelength of choice (e.g., 589 nm) and a lower b *Materials with

[0026] Higher b * Materials with a refractive index of 1.71 and higher are suitable for the red and green waveguide layers. Thus, a material with a refractive index of 1.71 may not be an ideal choice for the red and green waveguide materials because the refractive index at the green (530 nm) and red (625 nm) wavelengths is lower than 1.75. As depicted in FIG. <n2<n3およびBb * <Gb * <Rb * Wherein Bb * , Gb * , and Rb * are the b * Represents.

[0027] 4A and 4B depict RGB waveguide stacks having RGB layers with different base materials for at least two of the layers. FIG. 4A depicts an RGB eyepiece stack 400 in which a red waveguide 402 is made of a first glass having a first refractive index, and green and blue waveguides 404 and 406 are made of a second glass having second and third refractive indices, respectively, where the second and third refractive indices are different. In one example, the red waveguide has a refractive index of 1.8 at 625 nm and a * The green waveguide is made of glass having a refractive index of 1.8 at 530 nm and a b * The blue waveguide is made of glass having a refractive index of 1.8 at 455 nm and a b *<0.35. FIG. 4B depicts an RGB eyepiece stack 410 in which a red waveguide 412 is made of a first polymer having a first refractive index, a green waveguide 414 is made of a second polymer having a second refractive index, and a blue waveguide 416 is made of a third polymer having a third refractive index, where the first, second, and third polymers are different and the first, second, and third refractive indices are different. For the implementation depicted in FIG. 4B, optical polymers such as thiolene, polycarbonate, CR-39, PMMA, MR167, MR174, and other suitable polymers can be utilized to form a suitable combination of multi-color waveguide stacks for AR / MR applications.

[0028] 5A and 5B depict waveguide stacks having waveguides with different operating wavelengths (e.g., R, G, B, etc.) made of the same base glass or polymer material. A high refractive index component can be combined with the base material, curing conditions can be selected, or both, to achieve a desired combination of refractive index and yellowness index for each color. FIG. 5A depicts an RGB waveguide stack 500 having the same base glass material for the red waveguide 502, green waveguide 504, and blue waveguide 506. FIG. 5B depicts an RGB waveguide stack 510 having the same base polymer material for the red waveguide 512, green waveguide 514, and blue waveguide 516. Each layer in the waveguide stacks of FIGS. 5A and 5B has a concentration of a high refractive index component selected to achieve a suitable refractive index and yellowness index for each layer.

[0029] 6A-6D depict waveguide stacks fabricated using a combination of glass and polymer waveguides with various refractive indices. FIG. 6A depicts an RGB eyepiece stack 600 having a red polymer waveguide 602 with a refractive index n1, a green glass waveguide 604 with a refractive index n2, and a blue glass waveguide 606 with a refractive index n3. FIG. 6B depicts an RGB eyepiece stack 610 having a red polymer waveguide 612 with a refractive index n1, a green polymer waveguide 614 with a refractive index n2, and a blue glass waveguide 616 with a refractive index n3. FIG. 6C depicts an RGB eyepiece stack 620 having a red polymer waveguide 622 with a refractive index n1, a green glass waveguide 624 with a refractive index n2, and a blue glass waveguide 626 with a refractive index n3. FIG. 6D depicts an RGB eyepiece lens stack 630 having a red polymer waveguide 632 with a refractive index n1, a green polymer waveguide 634 with a refractive index n2, and a blue glass waveguide 636 with a refractive index n3.

[0030] The customized RGB polymer waveguides described herein can be fabricated using a multi-head system 700, as shown in Figure 7, using a two-part high refractive index polymer resin, where base resins 702 and 704 are used for all three waveguides. The desired refractive index and yellowness index can be achieved by selectively controlling the relative amounts of base resins 702 and 704 that are dispensed and combined. This approach allows for rapid and efficient high volume processing.

[0031] In one embodiment, as depicted in FIG. 7, base resins 702 and 704 are provided to a red layer mixer 706 in a 1:1 ratio. The resulting mixture is provided to a red layer casting head 708 and used to form a red waveguide for an RGB eyepiece stack. Base resins 702 and 704 are provided to a green layer mixer 710 in a 1:0.8 ratio. The resulting mixture is provided to a green layer casting head 712 and used to form a green waveguide for an RGB eyepiece stack. Base resin 704 is provided to a blue layer mixer 714 without base resin 702. Base resin 704 from blue layer mixer 714 is provided to a blue layer casting head 716 and used to form a blue waveguide for an RGB eyepiece stack. Different ratios of base resins 702 and 704 can be used in other embodiments.

[0032] Although the present disclosure contains many specific embodiment details, these should not be interpreted as limitations on the scope of the present subject matter or the scope of what may be claimed, but rather as descriptions of features that may be specific to a particular embodiment. Certain features described in the context of separate embodiments in this disclosure can also be implemented in combination in a single embodiment. Conversely, various features described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Also, although the features described above may be described as acting in a combination and may even be initially claimed as such, one or more features from the claimed combination may, in some cases, be deleted from the combination, and the claimed combination may be directed to a subcombination or a variation of the subcombination.

[0033] Particular embodiments of the present subject matter have been described. Other embodiments, modifications, and permutations of the described embodiments, as will be apparent to those skilled in the art, are within the scope of the following claims. Although operations are depicted in the figures or in the claims in a particular order, this should not be understood as requiring such operations to be performed in the particular order or sequential order shown, or that all of the illustrated operations be performed (some operations may be considered optional) to achieve desirable results.

[0034] As such, the exemplary embodiments described above do not define or limit the disclosure. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of the disclosure.

Claims

1. 1. A diffractive waveguide stack comprising: a first diffractive waveguide for guiding light within a first visible wavelength range, the first diffractive waveguide comprising a first material and having a first yellowness index, a first refractive index at a selected wavelength, and a first target refractive index at a midpoint of the first visible wavelength range; a second diffractive waveguide for guiding light within a second visible wavelength range, the second diffractive waveguide comprising a second material and having a second yellowness index, a second refractive index at the selected wavelength, and a second target refractive index at a midpoint of the second visible wavelength range; a third diffractive waveguide for guiding light within a third visible wavelength range, the third diffractive waveguide comprising a third material and having a third yellowness index, a third refractive index at the selected wavelength, and a third target refractive index at a midpoint of the third visible wavelength range; the first visible wavelength range includes red light, the second visible wavelength range includes green light, and the third visible wavelength range includes blue light; the first yellowness index exceeds the second yellowness index, the second yellowness index exceeds the third yellowness index; A diffractive waveguide stack, wherein a difference between any two of the first target refractive index, the second target refractive index, and the third target refractive index is less than 0.

005.

2. The diffractive waveguide stack of claim 1 , wherein the selected wavelength is 589 nm.

3. The diffractive waveguide stack of claim 1 , wherein the first material is different from the second material and the third material, and the second material is different from the third material.

4. A diffractive waveguide stack as described in claim 1, wherein the first yellowness index is less than 1.2, the second yellowness index is less than 0.8, and the third yellowness index is less than 0.

4.

5. 2. The diffractive waveguide stack of claim 1, wherein the first material comprises a first copolymer comprising a first monomer and a second monomer, and the second material comprises a second copolymer comprising the first monomer and the second monomer, and a ratio of the first monomer to the second monomer in the first copolymer is different from a ratio of the first monomer to the second monomer in the second copolymer.

6. The diffractive waveguide stack of claim 1 , wherein the first material, the second material, and the third material each independently comprise a polymer or a glass.

7. the first material comprises a first glass, the second material comprises a second glass, and the third material comprises a third glass, and the first glass, the second glass, and the third glass are different; or 10. The diffractive waveguide stack of claim 1 , wherein the first material comprises a first polymer, the second material comprises a second polymer, and the third material comprises a third polymer, and the first polymer, the second polymer, and the third polymer are different.

8. The diffractive waveguide stack of claim 1 , wherein a field of view of the first diffractive waveguide is different from a field of view of the second diffractive waveguide.

9. The diffractive waveguide stack of claim 1 , wherein the first material comprises a first polymer and the second material comprises a second polymer.

10. The diffractive waveguide stack of claim 9 , wherein the first polymer and the second polymer are different.

11. 10. The diffractive waveguide stack of claim 9, wherein the first material comprises a first copolymer comprising a first monomer and a second monomer, and the second material comprises a second copolymer comprising the first monomer and the second monomer.

12. 12. The diffractive waveguide stack of claim 11, wherein a ratio of the first monomer to the second monomer in the first copolymer is different from a ratio of the first monomer to the second monomer in the second copolymer.

13. The diffractive waveguide stack of claim 9 , wherein the first material comprises a first additive and the second material comprises a second additive.

14. 14. The diffractive waveguide stack of claim 13, wherein the first additive and the second additive are the same and a ratio of the first additive to the first polymer is different from a ratio of the second additive to the second polymer.

15. The diffractive waveguide stack of claim 1, wherein wavelengths within the third visible wavelength range are shorter than wavelengths within the second visible wavelength range, and wavelengths within the second visible wavelength range are shorter than wavelengths within the first visible wavelength range.

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