GLASS MIRROR SUBSTRATES FOR NON-SPECULAR REFLECTION TECHNICAL FIELD [0001] This disclosure generally relates to the field of glass mirror substrates for reflection to non-specular directions. BACKGROUND [0002] Converting solar energy to heat or electrical energy can be accomplished by concentrating sunlight onto a small area. Concentration employs mirrors, which reflect sunlight in a specular direction directed toward a desired receiver area. A heliostat uses a mirror or an array of mirrors to track the sun and reflect the sunlight onto a tower-mounted receiver. In some systems, arrays of thousands or tens of thousands of heliostats are used. [0003] Heliostats have a challenging set of requirements in that they must have optical surface accuracy of around 1 mrad or smaller (Zhu, G., et al., “Roadmap to Advance Heliostat Technologies for Concentrating Solar-Thermal Power,” NREL, 1-179 (2022)). Heliostats must additionally point toward a particular direction (the mean of the vector pointing along the incident sunlight and the vector pointing from heliostat to the receiver) with sub-milliradian accuracy. They also must survive for decades in harsh outdoor environments that include wind, dust, sandstorms, rain, high and low temperatures, seismic activity, and manual abrasion from washing. [0004] A major challenge in heliostat development is aligning a large and low-cost optical system in the field, either during or after installation. Since heliostats are nominally flat, or have an approximate radius of curvature on the order of hundreds or thousands of meters, measuring them using common optical techniques that rely on specular reflection is challenging. Unless the mirror normal vector points nearly toward the illumination source, such as if the illumination source is near the center of curvature, the distance between the optical detection equipment and the illumination source must be large to capture the specular reflection off the heliostat mirrors. This long distance increases the measurement uncertainty. [0005] For solar energy applications, the desired function of the heliostat is to direct sunlight toward the specular direction. Since the solar flux in that direction is high, it is impractical to locate optical equipment there and difficult to measure optical surface shape. [0006] Common measurement techniques for measuring optical surface shape include deflectometry approaches that illuminate the heliostat with an extended light source. Extended light sources include a monitor, or a screen upon which a pattern is projected and 1 UA23-037 062479.0016PCT1 which subsequently scatters light toward the heliostat, or a laser. Other techniques include illuminating the heliostat with a pulse of light with a special time-intensity relationship and detecting the reflected pulse. The direction of propagation of the reflected light and its intensity, phase, polarization state, spectrum, or time of arrival is then detected and analyzed to provide information about the heliostat optical surfaces. [0007] Several techniques have been used to overcome the limitations of measurement techniques based on specular reflection of the heliostat optical surfaces themselves (Sattler, J.C., et al., "Review of heliostat calibration and tracking control methods," Sol. Energy, 207: 110–132 (2020)). One such method is photogrammetry, which is a process of attaching components that diffusely scatter light to the heliostat mirrors and capturing photographs from a variety of viewpoints to reconstruct the surface. Another method attaches retro- reflecting corner cubes to the mirrors that each reflect illumination in the same direction in which it originated. Yet another method attaches a small planar mirror to the heliostat that is offset by a fixed direction from the mirror surface upon which it is mounted. An additional method attaches a small diffraction grating component to the mirror, and some of the that is diffracted off the grating returns to a desired direction such as back toward the illumination source. All of these methods require attaching a separate component to the heliostat mirrors, which increases measurement uncertainty, increases labor and cost, and decreases robustness to environmental conditions. [0008] Attaching components to heliostat mirrors is undesirable because it increases measurement uncertainty. Heliostat mirrors are usually second-surface mirrors, whereby the reflective coating (aluminum or silver) is protected by a layer of float glass (typically with low iron content, and 1-4 mm thick). Attached components are usually glued to the front surface of the glass, which is not the same as the reflecting surface. Thickness variations in the float glass, adhesive layer, or the attached components themselves result in orientation errors between the attached component and the reflecting surface that is supposed to be measured. [0009] Attaching components to heliostat mirrors increases labor and degrades robustness. Attached components must be placed once the heliostat is installed in the field if there is risk of the component becoming damaged during shipping or installation. Precisely placing the components, and waiting for adhesive to cure, is often time-consuming. Robustness is degraded once the component is attached because of possible thermal expansion coefficient mismatch between the component and the mirror to which it is attached. Furthermore, the component or the adhesive may not be sufficiently robust to withstand the decades of harsh UA23-037 062479.0016PCT1 environmental conditions in which a heliostat must operate. Failure of these components requires further labor to replace them. Finally, cleaning heliostat mirrors is accomplished with water and mechanical and chemical means. A component that is attached to the heliostat mirrors is at risk of being damaged during cleaning, or the cleaning process must accommodate the attached components. [0010] Therefore, there exists a need for a system that better measures optical surface shapes of heliostats. SUMMARY [0011] It is to be understood that this summary is not an extensive overview of the disclosure. This summary is exemplary and not restrictive and it is intended to neither identify key or critical elements of the disclosure nor delineate the scope thereof. The sole purpose of this summary is to explain and exemplify certain concepts of the disclosure as an introduction to the following complete and extensive detailed description. [0012] The present disclosure relates to an embedded, small-diffraction grating. The grating can be embedded in the glass substrate of a second-surface mirror. The grating can be one-dimensional, two-dimensional, or three-dimensional. The grating provides periodic or aperiodic phase modulation. The grating can additionally provide periodic or aperiodic amplitude modulation. Periodic or aperiodic phase modulation and periodic or aperiodic amplitude modulation can be provided together or separately. [0013] The present disclosure relates to light from a metrology system propagating through a grating before and after reflection. Such propagation allows light to be directed to non- specular directions to be measured by an optical system. [0014] The present disclosure additionally relates to multiple gratings written in a single layer or multiple layers in the glass to provide additional diffraction orders or more complex control of the reflected wave fronts. Such gratings can enable metrology of large specular surfaces with compact optical setups. As a non-limiting example, point source illumination and reflection into a camera are enabled. Such gratings can also be used to re-direct an amount of sunlight to a direction offset from a specular direction. This allows measurement of the mirror orientation while directing most sunlight to a receiver for thermal energy generation. [0015] The present disclosure relates to gratings written with ultrafast laser pulses. Laser pulses may have pulse duration in the femtosecond to picosecond range. Laser pulses impart index of refraction variation, scattering, or both. This provides phase modulation, amplitude modulation, or a combination thereof. UA23-037 062479.0016PCT1 BRIEF DESCRIPTION OF THE DRAWINGS [0016] The features and components of the following figures are illustrated to emphasize the general principles of the present disclosure. Corresponding features and components throughout the figures can be designated by matching reference characters for the sake of consistency and clarity. [0017] FIGS. 1A-1B depict a grating (FIG. 1A) and a differential interference contrast micrograph (FIG 1B) according to the present disclosure. [0018] FIGS. 2A-2B depict sunlight reflected off a mirror and grating (FIG. 2A) and ray trace simulations (FIG.2B) according to the present disclosure. [0019] FIG.3 depicts a simulated spectrum of sunlight according to the present disclosure. [0020] FIG.4 illustrates an example mirror in accordance with the present disclosure. [0021] FIG.5 illustrates an example system and dispersion pattern in accordance with the present disclosure. [0022] FIG.6 illustrates an example system and dispersion pattern in accordance with the present disclosure. [0023] FIG.7 illustrates an example system and dispersion pattern in accordance with the present disclosure. [0024] FIG.8 illustrates an example system and dispersion pattern in accordance with the present disclosure. DETAILED DESCRIPTION [0025] The present disclosure can be understood more readily by reference to the following detailed description, examples, drawings, and claims, and their previous and following description. However, before the present compositions, systems, and/or methods are disclosed and described, it is to be understood that this disclosure is not limited to the specific devices, systems, and/or methods disclosed unless otherwise specified, as such can, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting. I. Definitions [0026] It should be appreciated that this disclosure is not limited to the systems and methods described herein. It is also to be understood that the terminology used herein is for the purpose of describing certain embodiments only, and is not intended to be limiting, since the scope of the present disclosure will be limited only by the appended claims. [0027] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this UA23-037 062479.0016PCT1 disclosure belongs. Although any systems, methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention. All publications mentioned are incorporated herein by reference in their entirety. [0028] The use of the terms "a," "an," "the," and similar referents in the context of describing the presently claimed invention (especially in the context of the claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. [0029] Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. [0030] Use of the term "about" is intended to describe values either above or below the stated value in a range of approx. +/- 10%; in other embodiments the values may range in value either above or below the stated value in a range of approx. +/- 5%; in other embodiments the values may range in value either above or below the stated value in a range of approx. +/- 2%; in other embodiments the values may range in value either above or below the stated value in a range of approx. +/- 1%. The preceding ranges are intended to be made clear by context, and no further limitation is implied. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., "such as") provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non- claimed element as essential to the practice of the invention. [0031] As used herein, “specular reflection” refers to reflection wherein a light wave encounters a surface at a first angle known as the angle of incidence and leaves the surface at the same angle of incidence. For a smooth surface, an angle of incidence can be found between a normal to the smooth surface and the light wave approaching the surface. [0032] As used herein, “non-specular reflection” refers to reflection other than specular reflection. Examples of non-specular reflection include but are not limited to diffuse reflection, which may occur when light encounters a non-smooth surface. [0033] As used herein, “metrology” refers to the science of measurement, embracing both experimental and theoretical determinations at any level of uncertainty in any field of science and technology. UA23-037 062479.0016PCT1 [0034] As used herein, a “second-surface mirror” refers to a mirror wherein a protective surface abuts a second coating that reflects light. As a non-limiting example, a protective surface of glass can abut a reflective coating of aluminum or silver. II. Structures Embedded within Glass Mirror Substrates [0035] The present disclosure relates to large, optomechanical structures. Such structures include but are not limited to heliostats, second-surface mirrors, and the like. Heliostats are used for solar energy concentration and require sub-milliradian precision surface metrology. Heliostats and systems thereof include surfaces of large sizes and long focal lengths. [0036] The present disclosure relates to compact metrology systems for accurately measuring heliostat surfaces. Such systems allow for locating optical detection equipment in a direction separated from the specular reflection direction of a heliostat surface. The solar flux is high in this direction. Optical detection equipment can be so located by re- directing light to a non-specular direction. This allows for measuring optical surface shape, as well as the pointing direction. The ability to detect light in a non-specular direction from the illumination source enables these systems to be more compact. Increased compactness allows lower measurement uncertainty. [0037] The present disclosure relates to gratings. Such gratings are capable of being directly integrated or embedded into glass substrates. Glass substrates include but are not limited to second-surface mirrors. Gratings direct light differently based on its wavelength and can be designed to modify the spectrum of reflected light. [0038] The present disclosure relates to diffraction gratings. As a non-limiting example, diffraction gratings include one or more periodic arrangements of lines. Such arrangement periodically modifies the phase, intensity, or a combination thereof of an incident electromagnetic wave. Electromagnetic wave includes light waves. [0039] The present disclosure relates to phase gratings. Phase gratings can be used to achieve minimal loss of light. Phase gratings disclosed herein do not scatter or absorb light. [0040] The present disclosure additionally relates to non-periodic gratings. Non-periodic gratings are useful when the desired reflected wave front is not the same as the incident wave front. As a non-limiting example, non-periodic gratings can shape a spherical incident wavefront to a planar wavefront. [0041] The present disclosure relates to systems of multiple grating layers. Multiple grating layers may include layers each with different patterns or layers with identical patterns. Layers with different patterns provide efficiency of diffraction that is high in some UA23-037 062479.0016PCT1 directions and low in others. Layers with different patterns also allow complex, two- dimensional diffraction patterns. Multiple identical grating layers allows for altering the optical energy diffracted into different orders. [0042] The present disclosure relates to a method of creating variations in the index of refraction in glass mirror substrates using ultrafast lasers. Ultrafast lasers, with pulse durations in the range of about 100 femtoseconds up to about 20 picoseconds, are used for processing materials. As non-limiting examples, such lasers are used for processing transparent materials, including, but not limited to glass, that normally do not absorb light in the visible or near-infrared region of the electromagnetic spectrum. [0043] The present disclosure relates to ultrafast laser pulses capable of being focused to a small area. Such focusing provides high intensity in a small volume. This high intensity results in non-linear absorption processes such as multi-photon absorption. Absorbed electromagnetic energy results in structural changes to many materials including but not limited to glass. As a non-limiting example of such structural change, a change in the index of refraction of the material is achievable. Ultrafast lasers enable modifying the index of refraction of transparent materials underneath a material surface. Such modification is achievable with lateral resolution on the order of about 1 micrometer up to about 50 micrometers. Such modification is achievable with depth resolution on the order of about 1 micrometer up to about 100 micrometers. Beyond the region of high intensity (the focal region), the intensity decreases as 1/z ² , where z is the distance from the observation plane to the focal region. The intensity decreases to a level where it does not damage other materials such as an aluminum or silver layer used for reflection. [0044] Ultrafast lasers can write periodic or aperiodic patterns. Such lasers can additionally write three-dimensional or two-dimensional patterns. Periodic or aperiodic patterns in three-dimensions or two-dimensions, including any combinations, thereof, controllably reflect light off mirrors in non-specular directions. Non-specular reflection is particularly valuable for measuring characteristics of heliostats (via metrology) for solar energy applications. Non-specular reflection is additionally useful for measuring any large optical surface composed of second-surface mirrors. [0045] This approach of writing index variations in the glass mirror substrate to form a grating inherently protects the micrometer-scale features from environmental effects. No adhesives or attached components are required to provide the same benefits of other approaches. The reflective surface of the original, unmodified mirror is used for reflection. Any uncertainty in the direction of the diffracted light is dictated by the error in the pitch UA23-037 062479.0016PCT1 of the grating. Uncertainty may also be dictated by the location of the index of refraction modulations imparted by an ultrafast laser. [0046] Gratings may be efficiently written in a factory using mature laser processing techniques. This is partly due to the glass surface of the mirror remaining smooth after writing. As non-limiting examples, laser processing techniques include but are not limited to high-speed galvo or polygon scanners. Factory processing increases the number of gratings that may be included on a heliostat. An increased number of gratings increases the number of points that are measurable from non-specular directions. [0047] FIG. 1A displays an exemplary grating according to the disclosure. The grating is written in a 1 cm x 1 cm portion of the upper left corner of the second-surface float glass mirror. A differential interference contrast micrograph (FIG.1B) shows variations in index of refraction wherever lines are written using, for example, an ultrafast laser. Intensity variations in the micrograph correspond to changes in index of refraction in the glass displayed in FIG.1A. The ultrafast laser used for this prototype may have a wavelength of 1030 nm, a pulse duration of 350 fs, a pulse energy of 550 nJ, at a repetition rate of 450 kHz and a translation speed of 45 mm/s. The laser was focused by an objective lens with 0.4 numerical aperture, to a depth of 200 micrometers below the surface. It is understood that other parameters may be used to tune the system for desirable results. As an example, laser pulse duration may be 100fs to 20ps; wavelength may be 500nm to 1550nm; repetition rate may be 450khz to 80Mhz; numerical aperture may be 0.05 to .6. Parameters may be set to minimize any damage to the coating or to prevent any cracking or voiding. Desirable outcomes may include a smooth index modulation with continuous grating lines. Desirable outcomes may include configuring the mirror for absorption only in the focal volume to create index modulation. [0048] FIG.2A shows an exemplary grating reflecting sunlight. The reflection is captured using a camera. The camera records images with an f-stop value of f/22 and an exposure time of 1/200 seconds. Other camera settings may be used. In the main image (left), the camera is located below the main reflected beam from the mirror. Each image on the right of FIG. 2A was taken at a camera location 25mm lower than the image above it. The spectrum of reflected light changes as the camera position changes. This is represented in FIG. 2B where ray trace simulations of the reflected and diffracted sunlight to the region sensed by the camera are shown at various positions. The colors in FIG. 2B do not correspond to the colors seen by the camera. The colors are merely used to distinguish the location that a selection of wavelengths intersects the camera pupil. UA23-037 062479.0016PCT1 [0049] As shown in FIGS. 2A-2B, as the mirror orientation changes, the spectrum of observed reflected sunlight light will change. This provides a means of determining the orientation of the mirror surface using a grating. [0050] FIG. 3 shows the simulated spectrum based on ray tracing solar rays of various wavelengths to the camera according to the disclosure. The simulated spectrum corresponds to sunlight reflected off the grating area and captured by a camera. Each curve represents the spectrum seen by the camera at a particular position and corresponds to the camera positions shown in FIG.2A. The shading of the lines in FIG.3 does not correspond to any color observed by the camera, and is merely used to distinguish the lines from one another. As the camera position changes, the edge of the spectrum changes, which is detectable by a camera with a color sensor. [0051] FIG. 4 illustrates a second-surface mirror comprising a glass substrate 402 and a reflective film 404 disposed on a second-surface (back surface) of the substrate 402 according to the disclosure. As described herein, one or more grating layers 406, 408, 410 may be formed in the substrate to provide a modification of the index of refraction. Various layers or patterns may be provided and may be tuned to redirect light, for example in a non- specular direction. [0052] FIG. 5 illustrates a system comprising a mirror in accordance with the present disclosure and showing the main reflection, +1 ^st order dispersed reflection and -1 ^st order dispersed reflection. As such, one or more cameras may be configured to capture one or more of the +1 ^st order dispersed reflection and -1 ^st order dispersed reflection. [0053] As illustrated in FIG.6, the camera and/or the dispersion pattern may be configured to span the camera. In some instances, depending on one or more of the sun, grating, and target positions, the dispersed light misses the camera, as shown in FIG. 7. As such, including multiple grating orientations, either in multiple layers, or in multiple grating patches, spreads the dispersed light laterally, as shown in FIG.8. Various grating patterns may be used to effect a desired dispersion pattern. [0054] The present disclosure comprises at least the following aspects: [0055] Aspect 1: An optical device comprising: a second-surface mirror comprising a substrate and a reflective surface; and a diffraction grating formed in a portion of the substrate of the second-surface mirror, the diffraction grating configured to controllably reflect light off the second-surface mirror in a non-specular direction. [0056] Aspect 2: The optical device of claim 1, wherein the substrate comprises glass. UA23-037 062479.0016PCT1 [0057] Aspect 3: The optical device of any one of claims 1-2, wherein the diffraction grating comprises a periodic pattern. [0058] Aspect 4: The optical device of any one of claims 1-3, wherein the diffraction grating comprises a non-periodic pattern. [0059] Aspect 5: The optical device of any one of claims 1-4, wherein the diffraction grating comprises a one-dimensional pattern. [0060] Aspect 6: The optical device of any one of claims 1-5, wherein the diffraction grating comprises a two-dimensional pattern. [0061] Aspect 7: The optical device of any one of claims 1-6, wherein the diffraction grating comprises a three-dimensional pattern. [0062] Aspect 8: The optical device of any one of claims 1-7, wherein the diffraction grating comprises a plurality of grating layers. [0063] Aspect 9: The optical device of claim 8, wherein each of the plurality of grating layers is the same. [0064] Aspect 10: The optical device of claim 8, wherein each of the plurality of grating layers is the different. [0065] Aspect 11: The optical device of any one of claims 1-10, wherein the diffraction grating is formed in an approximately one cm x one cm portion of the second-surface mirror. [0066] Aspect 12: A system comprising: an optical device, wherein the optical device comprises: a second-surface mirror comprising a substrate and a reflective surface; and a diffraction grating formed in a portion of the substrate of the second-surface mirror from a plurality of grating layers, the diffraction grating configured to controllably reflect light off the second-surface mirror in a non-specular direction. [0067] Aspect 13: The system of claim 12, further comprising one or more cameras configured to capture at least a portion of the reflected light off the mirror. [0068] Aspect 14: The system of any one of claims 12-13, wherein the substrate comprises glass. [0069] Aspect 15: The system of one of claims 12-14, wherein the diffraction grating comprises a periodic pattern. [0070] Aspect 16: The system of one of claims 12-15, wherein the diffraction grating comprises a non-periodic pattern. [0071] Aspect 17: The system of one of claims 12-16, wherein each of the plurality of grating layers is the same. UA23-037 062479.0016PCT1 [0072] Aspect 18: The system of one of claims 12-17, wherein each of the plurality of grating layers is the different. [0073] Aspect 19: A method of making an optical device comprising: providing a second- surface mirror comprising a substrate and a reflective surface; and forming a diffraction grating in a portion of the substrate of the second-surface mirror, the diffraction grating configured to controllably reflect light off the second-surface mirror in a non-specular direction. [0074] Aspect 20: The method of claim 19, further comprising using an ultrafast laser to write at least a portion of the diffraction grating in the substrate. [0075] The present disclosure relates to methods of use of gratings described herein. Such methods include redirecting light to non-specular directions. Such directions can be chosen for desired purposes. Gratings disclosed herein are capable of being used for measurement of any second-surface mirror in any non-specular direction. 11 UA23-037 062479.0016PCT1