HOLOGRAPHIC LENS-BASED LIGHT GUIDING SYSTEM
Patent Information
- Application Number
- ES2025032540U
- Authority / Receiving Office
- ES · ES
- Patent Type
- Utility models
- Current Assignee / Owner
- Filing Date
- 2023-07-03
- Publication Date
- 2026-08-03
- Estimated Expiration
- 2033-07-03
Abstract
Description
HOLOGRAPHIC LENS-BASED LIGHT GUIDING SYSTEM FIELD OF INVENTION The present invention relates to a light guidance system based on holographic lenses. This light guidance system can preferably be applied in augmented reality (AR) viewing devices. BACKGROUND OF THE INVENTION Holography is a technique that uses the principles of light diffraction to record and subsequently reconstruct the three-dimensional (3D) optical information of objects. In this context, so-called holographic optical elements (HOEs) play a key role. These HOEs are fabricated from high-resolution photosensitive materials capable of recording the interference pattern produced by two coherent and spatially overlapping beams (typically laser beams). The first of these beams is usually called the reference beam, and the second the object beam—that is, a beam reflected or scattered by the object whose optical information is to be recorded on the photosensitive material. Thus, an HOE is generated by causing the reference beam and the object beam to interfere within a photosensitive material, recording the interference pattern inside the material at the surface where the reference and object beams intersect.The recorded pattern thus comprises the 3D information of the object and creates a photonic structure capable of diffracting light. Consequently, it is possible to obtain a reconstructed image of the object from the diffracted beam that results from striking the EOH with a reconstruction beam. EOHs can be classified as transmission or reflection EOHs, depending on how the light beams strike the photosensitive material during their generation. Typically, the photosensitive material comprises a first outer surface and a second outer surface opposite the first. In the generation of a transmission or reflection EOH, the reference beam strikes, at least partially, the first outer surface of the photosensitive material. In the generation of a transmission EOH, the object beam also strikes, at least partially, the first outer surface of the photosensitive material. Thus, the generated transmission EOH is configured to diffract a reconstruction beam striking the first outer surface, with the resulting diffracted beam passing through the second outer surface.On the other hand, in the generation of a reflection EOH, the object beam strikes, at least partially, the second outer surface of the photosensitive material. Thus, the generated reflection EOH is configured to diffract a reconstruction beam that strikes the first outer surface of the material, where the resulting diffracted beam passes through that first outer surface. Furthermore, EOHs are used in a multitude of devices, such as augmented reality (AR) vision devices. In recent years, AR vision devices have undergone remarkable development, ranging from the fields of video games and automotive engineering to the generation of new smart devices (for example, smart glasses or pilot helmets) that allow access to real-time information without needing to look away from the screen of an accessory device (for example, a mobile device). The need to develop these types of devices has arisen from two recent technological changes: - the widespread consumption of video content accessible through mobile phone networks or the Internet on mobile devices; - the miniaturization of these mobile devices, whose weights and sizes are essentially determined by the size of their screens. Compared to mobile video devices, AR vision devices offer three main advantages: they are hands-free, offer a high degree of privacy, and feature larger screen sizes. Among the various AR vision devices, those with "see-through" capabilities stand out due to their widespread acceptance. However, combining high-quality image display with clear "see-through" visibility for different applications remains a challenge in see-through devices. In this context, EOHs play an essential role in numerous AR vision devices, where they guide light within a light-guiding system, acting as wave couplers that introduce and extract image-forming light from waveguide substrates. An example of such a light-guiding system is described in US patent 10,877,275 B2, which refers to a system based on three EOHs—specifically, three relief gratings. This system, besides being complex and expensive, has the major disadvantage that relief gratings deteriorate much faster over time than an EOH based on refractive index variations, such as holographic lenses. Other AR devices include light-guiding systems based on holographic lenses. A holographic lens is an optical element that contains the three-dimensional information of an analog lens. An analog lens is understood to be any optical element configured to focus or disperse light through refraction. Thus, the holographic lens is generated by recording the interference pattern between the reference beam and the object beam on a photosensitive material. The object beam originates from an analog lens whose three-dimensional information is to be reconstructed using the recorded interference pattern. In this sense, the interference pattern recorded on the photosensitive material is such that the generated holographic lens is capable of focusing or dispersing light as if it were an analog lens. There are several examples of light guidance systems based on holographic lenses, such as the one disclosed in US patent application 2010 / 0157400 A1. Typically, such light guidance systems comprise at least a first holographic lens, a waveguide substrate, a second holographic lens, and an image projector. The light guided by the light guidance system is generated as a first image by the image projector. This light is first introduced into the waveguide substrate by the first holographic lens, then guided within the waveguide substrate, and finally directed out of the substrate by the second holographic lens. Furthermore, within the waveguide substrate, the light is guided by total internal reflection.Thus, from the light guided outwards from the waveguide substrate by the second holographic lens, the first image of the image projector is reproduced with a second image. However, known light-guiding systems that incorporate holographic lenses, such as the one disclosed in US 2010 / 0157400 A1, present difficulties in correctly focusing the second image, which reproduces the first image generated by the image projector, for the human eye. Furthermore, these light-guiding systems require large and heavy image projectors to avoid negatively impacting the size of the second image. Therefore, there is a need to provide a light-guiding system that correctly focuses the second image reproduced by the image generated by the image projector within that light-guiding system. Likewise, there is also a need to reduce the dimensions and weight of the image projector without negatively impacting the size of the second image it reproduces. BRIEF DESCRIPTION OF THE INVENTION The present invention solves the limitations of the prior art mentioned above, providing an improved light guiding system based on holographic lenses. More specifically, the invention provides a light guiding system comprising at least: - an image projector configured to generate light in the form of a first image, - a first holographic lens comprising a first optical center and having a focal length equal to a first focal value f1, - a second holographic lens comprising a second optical center and having a focal length equal to a second focal value f2, and - a waveguide substrate comprising an inlet portion, a center portion, and an outlet portion. Furthermore, the waveguide substrate comprises at least a first outer face and a second outer face opposite the first outer face. The first holographic lens is arranged on one of the outer faces of the waveguide substrate in its input portion, and the second holographic lens is arranged on one of the outer faces of the waveguide substrate in its output portion. Furthermore, the image projector is configured to direct the light it generates toward the first holographic lens. The first holographic lens is configured to guide the light it receives from the image projector into the input portion of the waveguide substrate. The central portion of the waveguide substrate is configured to guide the light guided by the first holographic lens from the input portion to the output portion. The second holographic lens is configured to receive the light guided to the output portion by the central portion and guide that light outward from the output portion of the waveguide substrate.Likewise, the first holographic lens, the second holographic lens, and the waveguide substrate are configured so that the waveguide substrate guides the light generated by the image projector inside by total reflection, so that, from the light guided by the second holographic lens to the outside of the output portion of the waveguide substrate, the first image generated by the image projector is reproduced with a second image. Advantageously, the light guiding system of the present invention is characterized in that: The distance traveled by light between the first optical center of the first holographic lens and the second optical center of the second holographic lens is substantially equal to the sum of the first focal value f1 and the second focal value f2; the image projector comprises an image plane, wherein said image plane is located at a distance from the first optical center of the first holographic lens substantially equal to the first focal value f1; and The second image, which reproduces the first image generated by the image projector, is located at a distance from the second optical center of the second holographic lens substantially equal to the second focal value f2. In the preceding context, the expression "substantially" is interpreted as equal to or within a variation range of ±0.5%. This variation in one or more of the distances between the elements of the light guidance system results in a blurring of the second image that is imperceptible to the human eye. Thus, the present invention addresses the need to provide a light guidance system that correctly focuses the second image, where said second image is a reproduction of the image generated by the image projector of the light guidance system. The light guidance system according to the present invention shall henceforth be referred to as the "4f system." In a preferred embodiment of the invention, the second focal value f2 of the second holographic lens is greater than the first focal value f1 of the first holographic lens. In the 4f system, the final scaling of the second image is directly proportional, with respect to the first image generated by the image projector, to the ratio between the second focal value f2 of the second holographic lens and the first focal value f1 of the first holographic lens. Therefore, when the second focal value f2 is greater than the first focal value f1, the size of the second image is larger than the size of the first image. This allows for reducing the dimensions and weight of the image projector without negatively affecting the size of the second image reproduced by the first image generated by said image projector. In another preferred embodiment of the light-guiding system, the second holographic lens is configured to guide the light it receives from inside the waveguide substrate to a screen, so that the second image, which reproduces the first image generated by the image projector, is projected onto the screen. Preferably, this screen is at least partially transparent. Alternatively, the second holographic lens is configured to guide the light it receives from inside the waveguide substrate to a user's eye, so that the user sees the second image, which reproduces the first image generated by the image projector. In another preferred embodiment of the invention, the first holographic lens and / or the second holographic lens comprises at least one of the following: transmission holographic lens, reflection holographic lens, converging holographic lens, diverging holographic lens. Furthermore, in another preferred embodiment of the invention, the first holographic lens and / or the second holographic lens comprise at least one outer surface with adhesive properties. Preferably, at least the outer surface of the first holographic lens and / or the second holographic lens in contact with the waveguide substrate has these adhesive properties. This allows the first holographic lens and / or the second holographic lens to be held in place on the waveguide substrate. In another preferred embodiment of the invention, the first holographic lens and / or the second holographic lens comprise a photosensitive material that meets at least one of the following: said photosensitive material is a phase material or an amplitude material; said photosensitive material is composed, at least partially, of at least one of the following: one or more photopolymers, one or more photoresins, one or more dichromated gelatins; the resolution value of said photosensitive material is equal to or less than 0.5 m; the refractive index of said photosensitive material is equal to or greater than 0.9 times the refractive index of the waveguide substrate, and less than or equal to 1.1 times the refractive index of the waveguide substrate; The thickness of said photosensitive material is between 5 m and 20 m, and more particularly, between 10 m and 15 m. On the other hand, in another preferred embodiment of the invention, the waveguide substrate is transparent. Thus, the waveguide substrate guides light within it by total internal reflection, from the inlet portion to the outlet portion, without the light being absorbed or scattered by it. In another preferred embodiment of the invention, the light directed by the image projector strikes the first holographic lens through its first outer surface, substantially perpendicular to said first outer surface. Furthermore, in another preferred embodiment of the invention, the image projector is one of the following: a projector based on one or more spatial light modulators, or a laser projector. Preferably, said image projector is one of the following: an LCD projector or a DLP projector. In another preferred embodiment of the invention, the image projector is configured to do at least one of the following: generate the first image in high definition and / or ultra-high definition format, emit light of any combination of colors, or emit monochromatic light. Furthermore, in another embodiment of the invention, the image projector has a spectral width of 40 nm or less. Another object of the invention relates to an augmented reality vision device comprising a light guidance system according to any of the embodiments described herein. Preferably, the augmented reality vision device comprises one of the following: smart glasses, a smart helmet, or a vehicle windshield. DESCRIPTION OF THE DRAWINGS To complete the description and aid in a better understanding of the invention, a set of drawings is provided. These drawings form an integral part of the description and illustrate different embodiments of the present invention, which should not be interpreted as restricting the scope of the invention, but rather as examples of how it can be carried out. The accompanying drawings specifically comprise the following figures: Figure 1 shows a schematic representation of a light guiding system according to an embodiment of the present invention. Figure 2 shows a schematic representation of a light guiding system according to another embodiment of the present invention. Numerical references used in the figures: DETAILED DESCRIPTION OF THE INVENTION As mentioned above, the object of the present invention relates to a light guiding system based on holographic lenses. The use of holographic lenses in light guidance systems is well-established. Typically, a light guidance system comprises at least a first holographic lens, a waveguide substrate, a second holographic lens, and an image projector. In this context, the light guided by the light guidance system is the light generated as a first image by the image projector. This light is first introduced into the waveguide substrate by the first holographic lens, then guided through the interior of the waveguide substrate, and finally directed out of the substrate by the second holographic lens. Light guidance systems are typically used in augmented reality (AR) vision devices. Examples, but not limited to, include smart glasses or headsets, where the waveguide substrate is typically a transparent element through which the user views the world. This type of waveguide substrate is generally made of glass, plastic, or other materials known in the prior art. In this context, the light from the light guidance system is guided outward from the waveguide substrate by means of a second holographic lens, so that the user typically sees, through the waveguide substrate, a second image that reproduces the first image generated by the image projector. This second image is superimposed on the real world that the user sees through the waveguide substrate.In this sense, the light has to propagate by total reflection inside the waveguiding substrate, so as not to lose information from the first image during the propagation of the light in the light guiding system. Figure 1 shows a schematic representation of a light guidance system (1) according to an embodiment of the invention. Said light guidance system (1) comprises an image projector (2), a first holographic lens (3), a waveguide substrate (4), and a second holographic lens (5). The image projector (2) is configured to generate light in the form of a first image (6). In particular, in Figure 1, the image projector (2) generates the image (6) of the number three. Non-limiting examples of image projectors (2) are projectors based on one or more spatial light modulators (SLMs). There are various types of projectors based on SLMs. For example, LCD projectors (liquid crystal displays), such as those based on liquid crystal on silicon (LCoS). Another example is DLP projectors (from the English, "Digital Light Processing", that is, digital light processing), such as those based on micromirrors on a DMD (from the English, "Digital Micromirror Device", that is, digital micromirror device).The light source for image projectors (2) based on SLMs can be one or more halogen lamps, LEDs, lasers, among others. Other, non-exhaustive examples of image projectors (2) include laser projectors, where several beams are deflected by one or more mirrors at very high speeds to give the impression of a continuous image. On the other hand, the first holographic lens (3) comprises a first optical center (31) and has a focal length equal to a first focal value (32) f1. Likewise, the second holographic lens (5) comprises another second optical center (51) and has a focal length equal to a second focal value (52) f2. The holographic lenses (3, 5) of Figure 1 are, in particular, transmission holographic lenses. Furthermore, the waveguide substrate (4) comprises an inlet portion (7), a central portion (8), and an outlet portion (9). The waveguide substrate (4) also comprises a first outer face (10) and a second outer face (11) opposite the first outer face (10). The first holographic lens (3) is arranged on the first outer face (10) of the waveguide substrate (4) in its inlet portion (7), while the second holographic lens (5) is arranged on the first outer face (10) of the waveguide substrate (4) in its outlet portion (9). Moreover, the waveguide substrate (4) is preferably composed of one or more materials such as glass, plastic, etc., such that the waveguide substrate (4) is transparent. In this context, the image projector (2) is configured to direct the light it generates toward the first holographic lens (3). This light strikes the first holographic lens (3) on its first outer surface (i.e., the outer surface where the reference beam strikes the photosensitive material during the specific method used to generate the first holographic lens (3)). The first holographic lens (3) is configured to guide the light it receives from the image projector (2) into the entrance portion (7) of the waveguide substrate (4). Likewise, the central portion (8) is configured to guide the light guided by the first holographic lens (3) from the entrance portion (7) to the exit portion (9). The second holographic lens (5) is configured to receive the light guided by the central portion (8) to the exit portion (9).Furthermore, the second holographic lens (5) is configured to guide the light it receives outwards from the output portion (9) of the waveguide substrate (4). Thus, the light guided by the second holographic lens (5) outwards from the waveguide substrate (4) contains information from the first image (6) generated by the image projector (2). Furthermore, the first holographic lens (3), the second holographic lens (5), and the waveguide substrate (4) are configured so that the waveguide substrate (4) guides the light generated by the image projector (2) within it by total internal reflection. In this respect, the light guidance system (1) is embedded in an external medium (e.g., air, water, etc.) with a lower refractive index than that of the waveguide substrate (4), so that total internal reflection occurs within the waveguide substrate (4). Thus, each ray of light guided within the waveguide substrate (4) strikes a point on the interface between the waveguide substrate (4) and the surrounding medium, at an angle of incidence greater than the critical angle for total internal reflection.The critical angle of total internal reflection is equal to the arcsine of the ratio between the refractive index of the medium in which the waveguide substrate (4) is embedded and the refractive index of the waveguide substrate (4). In Figure 1 (and in the subsequent Figure 2) a light beam from a beam propagating by total internal reflection has been represented inside the waveguide substrate (4), to illustrate the above description. As the light generated by the image projector (2) propagates by total internal reflection within the waveguide substrate (4), no information from the first image (6) is lost during propagation. Thus, from the light guided by the second holographic lens (5) to the outside of the waveguide substrate (4), the first image (6) generated by the image projector (2) is correctly reproduced with a second image (12). Furthermore, the first holographic lens (3) in Figure 1 is a converging holographic lens, which facilitates the total internal reflection of all light rays guided by this first holographic lens (3) from the entrance portion (7) to the exit portion (9). In other embodiments, this first holographic lens (3) is diverging. On the other hand, the second holographic lens (5) in Figure 1 is a converging holographic lens. In this respect, the two outermost light rays guided by the second holographic lens (5) are shown in Figure 1 extending outward from the waveguide substrate (4). On the other hand, in the embodiment of Figure 1, the light-guiding system (1) further comprises a screen (13). In particular, this screen (13) is located in front of the first outer face (10) of the waveguide substrate (4), opposite the second holographic lens (5). The second holographic lens (5) is configured to guide the light it receives from inside the waveguide substrate (4) towards the screen (13), so that the second image (12) is projected onto the screen (13). This second image (12) is a reproduction of the first image (6) generated by the image projector (2). In a preferred embodiment, the screen (13) is at least partially transparent. This allows the second image (12) to be seen on the screen (13), while at the same time everything behind the screen (13) is at least partially visible.In non-limiting examples, the screen (13) is the window of a vehicle (car, train, airplane, ship, rocket, among others) of an AR vision device comprising the light guidance system (1) of Figure 1. Thus, advantageously in the light guiding system (1) of Figure 1, the distance traveled (41) by the light between the first optical center (31) of the first holographic lens (3) and the second optical center (51) of the second holographic lens (5) is substantially equal to the sum of the first focal value (32) f1 and the second focal value (52) f2; the image projector (2) comprises an image plane (21), wherein said image plane (21) is located at a distance from the first optical center (31) of the first holographic lens (3) substantially equal to the first focal value (32) f1; and the screen (13) and, consequently, the second image (12) projected thereon are located at a distance from the second optical center (51) of the second holographic lens (5) substantially equal to the second focal value (52) f2. In this context, the expression "substantially" will be interpreted as equal to or within a range of variation of ± 0.5%.This range of variation in one or more of the distances between the elements of the light guiding system (1) implies a blur in the second image (12) that is imperceptible to a human eye. The image plane (21) of the image projector (2) is the plane where the image projector (2) generates the first image (6). Preferably, the distance from the image plane (21) of the image projector (2) to the geometric center of the first holographic lens (3) is measured from the center of the first image (6) formed on that image plane (21). Thus, for example, in an image projector (2) based on a single liquid crystal on silicon (LCoS) display, the image plane (21) is the one containing the LCoS itself within the image projector (2). Furthermore, the center of the first image (2) generally coincides with the geometric center of the LCoS. In the context above, in the present invention the image distance of the first holographic lens (3) is approximately or equal to infinity, while the image distance of the second holographic lens (5) is substantially equal to the second focal value (52) f2. In a lens with focal length f, the image distance is understood as the distance at which said lens forms an image (real or virtual) of an object from the light that reaches it from the same, when the object is located at a given distance with respect to said lens. Thus, as described above, the light guidance system (1) of the present invention solves the need to provide a light guidance system (1) that correctly focuses the second image (12) that reproduces the first image (6) generated by the image projector (2) included in said light guidance system (1). The light guidance system (1) according to the present invention is referred to as system 4f. Figure 2 shows a schematic representation of a light-guiding system (1) according to another embodiment of the invention. Thus, said light-guiding system (1) is a 4f system that also comprises an image projector (2), a first holographic transmission lens (3), a waveguide substrate (4), and a second holographic transmission lens (5). As in Figure 1, the light guiding system (1) of Figure 2 guides the light generated by the image projector (2) so that, after being guided first by the first holographic lens (3) into the waveguide substrate (4), and subsequently by total reflection inside the waveguide substrate (4), this light is guided to the outside of the waveguide substrate (4) by means of the second holographic lens (5). However, unlike the embodiment in Figure 1, the second holographic transmission lens (5) is a diverging holographic lens, and the light-guiding system (1) lacks a screen (13). In fact, in the light-guiding system (1) of Figure 2, the light generated by the image projector (2) is guided toward a user's eye (14), rather than toward a screen (13). Thus, the second holographic lens (5) is configured to receive the light guided to the output portion (9) by the central portion (8), and to guide this received light toward the eye of a user (14) located outside the waveguide substrate (4). The two outermost rays of light guided by the second holographic lens (5) toward the user's eye (14) are shown in Figure 2. This user sees the second image (12), which reproduces the first image (6) generated by the image projector (2).Specifically, in Figure 2, the second image (12) is a virtual image that the user sees through the waveguide substrate (4), located outside of it and in front of its second outer face (11). In non-limiting examples, the light guidance system (1) in Figure 2 is contained in AR smart glasses or a headset, where the user of said glasses or headset sees, through the waveguide substrate (4), the outside world and the second image (12) superimposed on it. Again, the light-guiding system (1) of Figure 4 is advantageously a 4f system. Thus, the distance traveled (41) by the light between the first optical center (31) of the first holographic lens (3) and the second optical center (51) of the second holographic lens (5) is substantially equal to the sum of the first focal value (32) f1 of the first holographic lens (3) and the second focal value (52) f2 of the second holographic lens (5), the image plane (21) of the image projector (2) is located at a distance from the first optical center (31) of the first holographic lens (3) substantially equal to the first focal value (32) f1, and, thanks to the crystalline lens of a user's eye (14) acting as a converging lens, the user sees the second image (12) at a distance from the second optical center (51) of the second holographic lens (5) substantially equal to the second focal value (52) f2.Again, the expression "substantially" will be interpreted in this context as equal to or within a range of variation of ± 0.5%. In this way, the user's eye (23) perfectly focuses on the second image (12) that reproduces the first image (6) generated by the image projector (2), and the need to provide a light-guiding system (1) that correctly focuses the second image (12) is resolved. In alternative embodiments of the 4f system of Figure 2, the second holographic lens (5) that guides the light to the user's eye (14) is a converging holographic lens. Thus, for the user to see the second virtual image (12), the user's near point must be located in the space between the waveguide substrate (4) and the user's eye (14). As is known, a user's near point is the point at the closest distance to the user's eye (14) where the user can clearly see a target with perfect accommodation. For example, for a person under 40 years of age, this distance is typically 10 cm. Under these conditions, the user sees the second image (12) at a distance substantially equal to the second focal value (52) f2, outside the waveguide substrate (4) and in front of its first outer face (10).Again, the expression "substantially" will be interpreted here as equal to or within a range of variation of ± 0.5%. On the other hand, in the light-guiding systems (1) of the present invention, the final scaling of the second image (12) is directly proportional, with respect to the first image (6), to the ratio between the second focal value (52) f2 of the second holographic lens (5) and the first focal value (32) f1 of the first holographic lens (3). Thus, in the embodiments of Figures 3 and 4, the second focal value (52) f2 of the second holographic lens (5) is smaller than the first focal value (32) f1 of the first holographic lens (3), so that the size of the second image (12) is smaller than the size of the first image (6). As described above, in preferred embodiments of the invention, the second focal value (52) f2 of the second holographic lens (5) is advantageously larger than the first focal value (32) f1 of the first holographic lens (3), such that the size of the second image (12) is larger than the size of the first image (15). This allows for reducing the dimensions and weight of the image projector (2) without negatively affecting the size of the second image (12) that reproduces the first image (6) generated by said image projector (2). Other embodiments of the light-guiding system (1) according to the present invention can be deduced by someone skilled in the art from the contents of this document. In non-limiting examples, the light-guiding systems (1) comprise two holographic reflecting lenses (3, 5) instead of two holographic transmitting lenses (3, 5), wherein said holographic lenses (3, 5) are arranged on the second outer face (11) of the waveguiding substrate (4) instead of on the first outer face (10) thereof. In these examples, when the image projector (2) directs the light it generates toward the first holographic lens (3), typically said light strikes the first holographic lens (3) on its first outer surface, after passing through the interior of the waveguiding substrate (4) via its entrance portion (7).Thus, the first holographic lens (3) receives the light generated by the image projector (2) and reintroduces it into the interior of the waveguide substrate (13) at its entrance portion (7). Likewise, the second holographic lens (3) receives the light guided to the exit portion (9) by the central portion (8), and guides this received light outwards from the exit portion (9), so that, before exiting the interior of the waveguide substrate (4), it passes through the interior of said waveguide substrate (4) at its exit portion (9). In other non-limiting embodiments of the invention, the light guidance system (1) comprises holographic lenses (3, 5), wherein one is a transmitting holographic lens and the other is a reflecting holographic lens. Likewise, in other embodiments of the invention, the first holographic lens (3) is disposed on an outer face (10, 11) of the waveguide substrate (4) distinct from the outer face (10, 11) on which the second holographic lens (5) is disposed, and the image projector (2) and the screen (13) or a user's eye (14) are located on opposite sides of the waveguide substrate (4). Furthermore, as is known, a holographic lens is typically generated by recording the interference pattern between two coherent light beams on a photosensitive material. Thus, each holographic lens (3, 5) of the light guidance system (1) comprises a photosensitive material on which an interference pattern has been previously recorded. In preferred embodiments of the invention, the photosensitive material of the first holographic lens (3) and / or the second holographic lens (5) is a phase material. In other embodiments, this photosensitive material is an amplitude material. A photosensitive material is phase-sensitive when, upon incident light, its refractive index or thickness changes, such that when a wave passes through it, the photosensitive material modifies the phase of the wave. A photosensitive material is called amplitude-sensitive when, upon illumination, its light absorption changes, so that when illuminated after the holographic lens has been generated, it modifies the amplitude of the wave, that is, its intensity. A phase-sensitive material can guide up to 100% of the incident light, while an amplitude-sensitive material guides less than 33% of the incident light. In another preferred embodiment of the invention, the photosensitive material of the first holographic lens (3) and / or the second holographic lens (5) is composed, at least partially, of at least one of the following: one or more photopolymers, one or more photoresins, or one or more dichromated gelatins. Preferably, the photosensitive material is composed of one or more photopolymers. Advantageously, the generation of holographic lenses with photopolymers requires minimal pre-processing and post-processing treatments compared to other photosensitive materials, such as photoresins or dichromated gelatins. Furthermore, in another preferred embodiment of the invention, the resolution of the photosensitive material of the first holographic lens (3) and / or the second holographic lens (5) is equal to or less than 0.5 m. Preferably, when the first holographic lens (3) and / or the second holographic lens (5) are reflection holographic lenses, said photosensitive material has a resolution of equal to or less than 0.2 m. In the generation of holographic lenses, a resolution of 0.5 m corresponds to being able to record an interference fringe density of approximately two thousand fringes per millimeter of photosensitive material. On the other hand, a resolution of 0.2 m corresponds to being able to record an interference fringe density of approximately five thousand fringes per millimeter of photosensitive material. In this context, the fringes are the maxima and minima of the interference pattern recorded on the photosensitive material. In another preferred embodiment of the invention, the refractive index of the photosensitive material of the first holographic lens (3) and / or the second holographic lens (5) is equal to or greater than 0.9 times the refractive index of the waveguide substrate (4), and less than or equal to 1.1 times the refractive index of the waveguide substrate (4). When the deviation of the refractive index of the photosensitive material of the holographic lens (3, 5) from the refractive index of the waveguide substrate (4) is no greater than ±10%, said holographic lens (3, 5) is more effective at guiding light into / outside the waveguide substrate (4). In this context, the refractive index of the photosensitive material of each holographic lens (3, 5) is the index it exhibits when light is incident upon it.For example, as is known, if the photosensitive material is a phase material, its refractive index is sometimes different when light is incident on it than when it is not, whereas if the photosensitive material is an amplitude material, the index is the same whether or not light is incident on it. Furthermore, the refractive index of the photosensitive material of the first holographic lens (3) and / or second holographic lens (5) is preferably greater than the refractive index of the medium in which the light-guiding system (1) is typically embedded. On the other hand, the thickness of the photosensitive material of the first holographic lens (3) and / or the second holographic lens (5) is preferably between 5 and 20 µm, and more specifically, between 10 and 15 µm. In this context, the thickness of the photosensitive material of each holographic lens (3, 5) is the thickness it has when light is incident upon it. For example, as is known, if the photosensitive material is a phase material, its thickness is sometimes different when light is incident upon it compared to when it is not. This difference can be a reduction in thickness of less than 5%. On the other hand, if the photosensitive material (2) is a amplitude material, the thickness is the same whether or not light is incident upon it. With the above thickness ranges, the first holographic lens (3) and / or the second holographic lens (5) are typically volume EOHs.A volume EOH is one that, when a beam of light falls on it, does not divide the incident beam into more than two beams. Furthermore, preferably the arrangement of the first holographic lens (3) and the second holographic lens (5) is such that there is no air between said holographic lenses (3, 5) and the waveguide substrate (4). In preferred embodiments of the invention, the photosensitive material of the holographic lenses (3, 5) is such that the outer surfaces of said holographic lenses (3, 5) in contact with the waveguide substrate (4) have adhesive properties. Other ways of arranging the holographic lenses (3, 5) on the outer faces (10, 11) of the waveguide substrate (4) are known to those skilled in the art. For example, a transparent glue or silicone with a refractive index substantially equal to that of the waveguide substrate (4) may be used. In this context, the expression "substantially" shall be interpreted as equal to or within a range of ±2%. Furthermore, in preferred embodiments of the invention, the image projector (2) has a spectral width less than or equal to 40 nm. Generally, 40 nm is the spectral width of image projectors (2) comprising LEDs. In another preferred embodiment of the invention, the image projector (2) has sufficient resolution to preferably generate the first image (6) in high definition (HD) and / or ultra high definition (UHD) format. In this context, UHD format refers to 4K, 8K, and / or 16K resolutions. In another preferred embodiment of the invention, the image projector (2) is configured to emit light of any combination of colors from the color palette (e.g., an RGB projector). In other embodiments, the image projector (2) is configured to emit monochromatic light, that is, to emit light of a single color. On the other hand, in one embodiment of the invention, the light directed by the image projector (2) strikes the first holographic lens (3) at its first outer surface, substantially perpendicular to it. In this respect, the expression "substantially" will depend on the thickness of said first holographic lens (3). As is known to those skilled in the art, the angular selectivity of a holographic lens depends on its thickness, with the greater the thickness, the more restrictive the angular selectivity becomes. Thus, for example, when the first holographic lens (3) has a thickness of 20 µm, and a light ray strikes a point on its first outer surface substantially perpendicularly, the expression "substantially" is interpreted as meaning that the direction of said ray is parallel to the normal to said first outer surface, with a range of variation from said direction of ± 5°.When, in another example, the thickness is 5 m, the expression "substantially" is interpreted as meaning that the direction of the ray is parallel to the normal to the first outer surface, with a range of variation of ±10° with respect to that direction. In the context above, the normal to the first outer surface of a holographic lens is typically taken as the direction that defines the axis around which the interference pattern recorded inside the photosensitive material of the holographic lens is centered. Thus, the normal to the first outer surface of a holographic lens is fixed during its generation. Generally, the normal to the first outer surface of the holographic lens is the normal to the point located at the center of that first outer surface, such that the interference pattern of the holographic lens is centered with respect to the first outer surface.In other examples, the interference pattern is not centered with respect to the first outer surface of the holographic lens, so the normal direction to the first outer surface is the normal direction of another point on that first outer surface. In Figures 1 and 2, a straight line represents a ray of light directed by the image projector (2) that strikes the first holographic lens (3) at its first outer surface, perpendicular to it, as described above. However, the thickness of the waveguide substrate (4) of the light guidance system (1) is not particularly relevant. This thickness only determines the number of internal reflections that occur within the waveguide substrate (4) when light propagates by total internal reflection. Advantageously, this allows the light guidance system (1) to be easily adapted to the specific dimensions of AR vision devices such as smart glasses, smart helmets, etc. Furthermore, the waveguide substrate (4) is not limited to a waveguide substrate (4) with flat, parallel outer faces (10, 11) as shown in Figures 1 and 2. In other examples, the opposite outer faces (10, 11) of the waveguide substrate (4) are curved (e.g., a convex or concave waveguide substrate (4)). The conditions necessary for a light ray to propagate by total internal reflection within a waveguide substrate (4) of any geometric shape are known. For example, consider a waveguide substrate (4) with curved outer faces (19, 20) made of glass with a refractive index of 1.5, and an external medium in which the waveguide substrate (4) is embedded, made of air with a refractive index of 1.In such a case, a light ray propagates inside said waveguide substrate (4) by total reflection, when between a first reflection at a first point on an outer face (10, 11) inside the waveguide substrate (4) and the immediately subsequent reflection at a second point on the opposite outer face (10, 11), the angle formed by the normal direction at the first point and the normal direction at the second point is preferably equal to or less than about 20º. Furthermore, the curvature of the outer faces (10, 11) of the waveguide substrate (4) does not influence the values of the distances between the image projector (2), the holographic lenses (3, 5), and the second image (12) that define the 4f system. In this sense, the optical path of a light ray propagating inside the waveguide substrate (4) by total internal reflection, from when it is introduced by the first holographic lens (3) until it is extracted by the second holographic lens (5), is substantially equal to the ratio between the sum of the first focal value (32) f1 and the second focal value (52) f2, and the refractive index of the waveguide substrate (4), regardless of the curvature of the outer faces (10, 11) of the waveguide substrate (4). Again, the expression "substantially" is interpreted here as equal to or within a range of variation of ± 0.5%.
Claims
1. A light guidance system (1) comprising at least an image projector (2) configured for generating light in the form of a first image (6), and further comprising a first holographic lens (3) comprising a first optical center (31) and having a focal length equal to a first focal value (32) f1, a second holographic lens (5) comprising a second optical center (51) and having a focal length equal to a second focal value (52) f2, and a waveguide substrate (4) comprising an inlet portion (7), a center portion (8), and an outlet portion (9), wherein the waveguide substrate (4) comprises at least a first outer face (10) and a second outer face (11) opposite said first outer face (10); the first holographic lens (3) is disposed on one of the outer faces (10,11) of the waveguide substrate (4) in its inlet portion (7); the second holographic lens (5) is disposed on one of the outer faces (10, 11) of the waveguide substrate (4) in its outlet portion (9); the image projector (2) is configured to direct the light it generates towards the first holographic lens (3); the first holographic lens (3) is configured to guide the light it receives from the image projector (2) into the inlet portion (7) of the waveguide substrate (4); the central portion (8) of the waveguide substrate (4) is configured to guide the light guided by the first holographic lens (3) from the inlet portion (7) to the outlet portion (9); the second holographic lens (5) is configured to receive the light guided to the outlet portion (9) by the central portion (8),and guide the light it receives to the outside of the output portion (9) of the waveguide substrate (4); and the first holographic lens (3), the second holographic lens (5), and the waveguide substrate (4) are configured so that the waveguide substrate (4) guides the light generated by the image projector (2) inside it by total internal reflection, so that, from the light guided by the second holographic lens (5) to the outside of the output portion (9) of the waveguide substrate (4),The first image (6) generated by the image projector (2) is reproduced with a second image (12); the light guiding system (1) being characterized in that the distance traveled (41) by the light between the first optical center (31) of the first holographic lens (3) and the second optical center (51) of the second holographic lens (5) is substantially equal to the sum of the first focal value (32) f1 and the second focal value (52) f2; the image projector (2) comprises an image plane (21), wherein said image plane (21) is located at a distance from the first optical center (31) of the first holographic lens (3) substantially equal to the first focal value (32) f1; and the second image (12) that reproduces the first image (6) generated by the image projector (2) is located at a distance from the second optical center (51) of the second holographic lens (5) substantially equal to the second focal value (52) f2.
2. Light guiding system (1) according to claim 1,wherein the second focal value (52) f2 of the second holographic lens (5) is greater than the first focal value (32) f1 of the first holographic lens (3).
3. Light guidance system (1) according to any of claims 1-2, wherein the second holographic lens (5) is configured to guide the light it receives from inside the waveguide substrate (4) towards a screen (13), wherein the screen (13) is at least partially transparent.
4. Light guidance system (1) according to any of claims 1-3, wherein the first holographic lens (3) and / or the second holographic lens (5) comprises at least one of the following: a transmitting holographic lens, a reflecting holographic lens, a converging holographic lens, or a diverging holographic lens.
5. Light guidance system (1) according to any of claims 1-4,wherein the first holographic lens (3) and / or the second holographic lens (5) comprise at least one outer surface with adhesive properties.
6. Light guidance system (1) according to any of claims 1-5, wherein the first holographic lens (3) and / or the second holographic lens (5) comprise a photosensitive material that meets at least one of the following: said photosensitive material is a phase material or an amplitude material; said photosensitive material is composed, at least partially, of at least one of the following: one or more photopolymers, one or more photoresins, one or more dichromated gelatins; the resolution value of said photosensitive material is equal to or less than 0.5 μm; the refractive index of said photosensitive material is equal to or greater than 0.9 times the refractive index of the waveguide substrate (4), and less than or equal to 1.1 times the refractive index of the waveguide substrate (4); the thickness of said photosensitive material is between 5 μm and 20 μm, and more particularly, between 10 μm and 15 μm.
7. Light guidance system (1) according to any of claims 1-6, wherein the waveguide substrate (4) is transparent.
8. Light guidance system (1) according to any of claims 1-7, wherein the image projector (2) is one of the following: a projector based on one or more spatial light modulators, a laser projector.
9. Light guidance system (1) according to claim 8, wherein the image projector (2) is one of the following: an LCD projector, a DLP projector.
10. Light guidance system (1) according to any of claims 1-9,wherein the image projector (2) has a spectral width less than or equal to 40 nm.
11. Augmented reality vision device comprising a light guidance system (1) according to any of claims 1-10.
12. Augmented reality vision device according to claim 11, wherein the augmented reality vision device comprises one of the following: smart glasses, a smart helmet, a vehicle windshield.