Methods and systems using computer-generating holography

Abstract

A method of using computer-generated holography to determine a corrective lens prescription comprises computing a hologram of an image. An aberration phase map is computed using a set of Zernike pol

Description

The invention relates to a method and system for using computer-generated holography to determine a corrective lens specification, the corrective lens specification to be used to determine a prescription for eyeglasses or contact lenses. BACKGROUND In order to provide an accurate prescription for glasses and contact lenses, an eye care professional (such as an optician or optometrist) must assess a person’s current level of vision in order to determine what level of correction is needed. This clinical procedure to determine the optimal prescription for eyeglasses or contact lenses typically involves carrying out a series of tests which the person must complete. An objective eye test, such as retinoscopy or autorefraction, is typically carried out first, to provide an initial estimate of the prescription. Objective eye tests measure the refractive error of the eye without requiring active feedback from the person. These tests provide a baseline for determining whether someone has myopia, hyperopia, astigmatism, or presbyopia. Objective eye tests are generally followed by subjective refraction testing for fine-tuning. Subjective refraction testing relies on the person's feedback to assess their refractive error. This process involves presenting the person with a series of visual comparisons and asking for their preference. A phoropter, which is a device which mimics a glasses frame and can hold multiple lenses, is positioned in front of the person’s eyes. An eye care professional can use the phoropter to introduce various lenses and settings to fine-tune the prescription. While looking through the phoropter, the person looks at a Snellen chart, or similar eye chart, typically 6 meters (20 feet) away, and the person reads out a sequence of letters of decreasing size off the chart. The eye care professional presents different lens options to the person, via the phoropter, and will ask the person for feedback such as which of two lenses is clearer and whether the letters look more or less sharp. This step is carried out multiple times, in a trial-and-error manner, in order to refine the prescription based on the person’s subjective experience of clarity. If astigmatism is present, the eye care professional can make suitable adjustments based on the person's feedback as to whether the chart appears clearer or blurrier. There are several challenges with subject refraction testing. Firstly, the test requires clear, accurate, and consistent feedback from the person, which may be challenging for children, the elderly, or those with communication barriers. Secondly, some individuals may feel fatigued or unsure when repeatedly answering questions, potentially leading to inconsistent responses. Another issue is that the test relies entirely on subjective perception and so it might not detect issues like suppression in one eye (as in amblyopia). The trial-and-error nature of swapping out lenses during the test may also be inefficient and time consuming. The clinical procedure involves the person travelling to a suitable clinic, having suitable assessment equipment, and a qualified eye care professional must be present to carry out the procedure. Individuals that do not live close to a suitable clinic may therefore be put off from having their eyes checked. It would be advantageous to solve at least some of these problems. SUMMARY OF INVENTION According to an aspect of the present invention there is provided a method of using computer-generated holography to determine a corrective lens specification. The method comprises computing a hologram of an image, computing an aberration phase map using a set of Zernike polynomials, applying the aberration phase map to the hologram to generate a first modified hologram, projecting the first modified hologram, and receiving an indication about an image quality of the first modified hologram, the indication indicating whether the image quality of the first modified hologram corresponds to a first desired image quality. When the indication indicates that the image quality of the first modified hologram corresponds to the first desired image quality, the method may further comprise creating a corrective lens specification based on the set of Zernike polynomials and outputting the corrective lens specification. The use of computer-generated holography (CGH) to determine a corrective lens specification can replace traditional eye test. Zernike polynomials represent specific optical aberrations, allowing the modelling and simulation of each imperfection in the eye using the appropriate polynomial. By combining multiple Zernike polynomials, multiple aberrations can be simulated at once. Zernike polynomials can apply known distortions to an image, and if a person can clearly see the distorted image, it means the distortion applied has counteracted the distortion in the person’s eye. The Zernike polynomials used to create the distortion can then be used to determine a corrective lens specification, which comprises values that form a person’s prescription. CHG in combination with Zernike polynomials, can replicate traditional eye tests, providing an accurate, reliable and certifiable glasses prescription. This method can be implemented using a relatively compact system which can be deployed in rural areas and administered by a skilled technician with a remote doctor to approve prescriptions. Computing the hologram may comprise the steps of generating data representing the image, simulating an interaction of light with the image, calculating a wavefront, and computing the hologram for the image based on the calculated wavefront. The wavefront preferably comprises amplitude and phase information. Computing the hologram may comprise encoding the amplitude and phase information of the wavefront. The data representing the image may be a point cloud or mesh. The data may comprise spatial coordinates, reflectivity, and desired textures. The simulating may be done using light propagation models, for example the Fresnel principle or Fourier transforms. The aberration phase map may represent distortions in the wavefront. The Zernike polynomials may model aberrations in the wavefront, and the Zernike polynomials calculate a specific phase pattern that is applied to the hologram. Projecting the first modified hologram may comprise the steps of displaying the first modified hologram using a spatial light modulator and illuminating the displayed first modified hologram using a coherent light source, wherein illuminating of the first modified hologram reconstructs the image plus a distortion effect. Creating the corrective lens specification preferably comprises converting a coefficient of a Zernike polynomial in the set of Zernike polynomials into a value forming part of the corrective lens specification. The coefficient of a Zernike polynomial represents the magnitude of that polynomial, determining its relative contribution to the aberration phase map. Once the person can clearly see the distorted image, the magnitude of the applied distortion has substantially matched the magnitude of the eye’s imperfection. Therefore, the coefficient can be used to determine an appropriate type of lens correction is needed to improve the person’s vision. In some examples, the method may comprise converting a plurality of coefficients into a plurality of values forming part of the corrective lens specification, wherein each coefficient corresponds to a Zernike polynomial in the set of Zernike polynomials that was used to generate the aberration phase map. In some situations, more than imperfection needs to be corrected using the aberration phase map. These different aberrations each correspond to different Zernike polynomials having different coefficients, and so all of these coefficients are used to create the corrective lens specification. When the indication indicates that the image quality of the first modified hologram does not correspond to the first desired image quality, the method may further comprise adjusting the aberration phase map and applying the adjusted aberration phase map to the hologram to generate a second modified hologram. In this case, the one or more aberrations initially modelled are not cancelled out by the eye’s imperfections and so either the magnitude of the modelled aberration needs to be changed, or further additional aberrations can be included in the aberration phase map. The step of adjusting the aberration phase preferably comprises adjusting a coefficient of at least one Zernike polynomial in the set of Zernike polynomials to form a first set of adjusted Zernike polynomials, computing a first revised aberration phase map using the first set of adjusted Zernike polynomials, and applying the first revised aberration phase map to the hologram to generate the second modified hologram. Adjusting the coefficient of a Zernike polynomial has the effect of adjusting the relative contribution of the aberration being simulated by that polynomial, and so stronger or weaker lenses can be simulated by increasing or decreasing the relative contribution of a distortion. The method may comprise iteratively adjusting the aberration phase map, iteratively applying each adjusted aberration phase map to the hologram to generate a modified hologram, and iteratively projecting each modified hologram until an indication is received that the first desired image quality of the image is achieved. In this way, the method can be repeated until the person indicates that they can clearly see the resulting image. Once the person can clearly see the resulting image, the distortions present in the image have been cancelled out by the person’s imperfect eye. Iteratively adjusting the aberration phase map may comprise the steps of applying the first revised aberration phase map to the hologram to generate a second modified hologram, projecting the second modified hologram, receiving an indication that an image quality of the second modified hologram does not correspond to the first desired image quality, adjusting a coefficient of at least one Zernike polynomial in the first set of adjusted Zernike polynomials to form a second set of adjusted Zernike polynomials, computing a second revised aberration phase map using the second set of adjusted Zernike polynomials, applying the second revised aberration phase map to the hologram to generate a third modified hologram, projecting the third modified hologram, and repeating these steps until an indication is received that indicates that an image quality of the projected modified hologram corresponds to the first desired image quality. Each time the hologram is modified using a new aberration phase map, the modified hologram is projected, and a person can view the resulting image and provide an indication about whether they can see the image clearly yet or not. This ensures that the corrective lens specification is tailored to the individual, and the iterative approach helps finely tune the corrective lens specification. The method may further comprise the steps of: after computing the hologram, projecting the hologram, wherein the hologram comprises depth information having a first depth; adjusting the hologram to generate a first adjusted hologram, wherein the first adjusted hologram comprises depth information having a second depth; projecting the first adjusted hologram; and receiving an indication about an image quality of the first adjusted hologram, the indication indicating whether the image quality of the first adjusted hologram corresponds to a second desired image quality. When the indication indicates that the image quality of the first adjusted hologram corresponds to the second desired image quality, the method may further comprise creating the corrective lens specification based at least partly on the second depth. Adjusting the perceived depth of a hologram is another mechanism, alternative or in combination with Zernike polynomials, that can be used to determine what type of lens corrections are needed to counter the effect of the imperfect eye. In particular, adjusting the perceived depth of the hologram can be used to simulate the effects of nearsightedness and farsightedness. In this case, creating the corrective lens specification may comprise the steps of calculating a focal length based on the second depth, and converting the focal length into a value forming part of the corrective lens specification. When the indication indicates that the image quality of the first adjusted hologram does not correspond to the second desired image quality, the method may further comprise the steps of adjusting depth information of the first adjusted hologram from the second depth to a third depth, computing a second adjusted hologram, projecting the second adjusted hologram, and repeating these steps until an indication is received that indicates that an image quality of the projected adjusted hologram corresponds to the second desired image quality. In this way, the perceived image depth can be iteratively adjusted until an indication is received that the second desired image quality of the image is achieved. The method is therefore repeated until the person indicates that they can clearly see the resulting image. Once the person can clearly see the resulting image, the depth information is used to determine what focal length a corrective lens would need to correct the person’s focal range. Receiving an indication regarding the quality of any image being projected may comprise receiving feedback from a person indicating whether the desired image quality has been achieved. In some examples, the feedback may be verbal feedback. In some examples, the feedback may be physical feedback for example nodding or shaking the head. In some examples, receiving an indication may comprise receiving an input via a user interface. Any suitable method for providing an indication regarding the quality of an image may be used. The image may comprise at least one letter. Preferably, the image may comprise a Snellen chart. The image may comprise any image that is suitable for conducting the tests necessary to determine a corrective lens specification for a person. According to another aspect there may be provided a system configured to determine a corrective lens specification using computer generated holography. The system may comprise a computing device configured to compute a hologram of an image; compute an aberration phase map using a set of Zernike polynomials; and apply the aberration phase map to the hologram to generate a first modified hologram. The system may further comprise a holographic unit configured to project the first modified hologram. The computing device may be further configured to receive an indication about an image quality of the first modified hologram, the indication indicating whether the image quality of the first modified hologram corresponds to a first desired image quality. When the indication indicates that the image quality of the first modified hologram corresponds to the first desired image quality, the computing device may be further configured to create a corrective lens specification based on the set of Zernike polynomials and output the corrective lens specification. The holographic unit preferably comprises a first monocular unit and a second monocular unit, each in communication with the computing device. Each of the first a second monocular units may comprise a spatial light modulator configured to receive the first modified hologram from the computing device and display the first modified hologram, a coherent light source configured to illuminate the first modified hologram displayed by the spatial light modulator to reconstruct the image, and an optical system comprising at least one optical component configured to direct the reconstructed image out of the monocular unit via a viewer. The viewer may be configured to allow a person to view the reconstructed image. At least one coherent light source may comprise at least one LED. The coherent light source may comprise a plurality of LEDs. A coherent light source may comprise a laser. Both coherent light sources may be the same. Any suitable coherent light source can be used. At least one spatial light modulator may be a digital-micromirror device. Both spatial light modulators may be digital-micromirror devices. At least one of the first and second monocular units may further comprise a diffraction grating configured to receive light from the coherent light source and redirect the light to the spatial light modulator. Each of the first and second monocular units may further comprise the diffraction grating configured to receive light from the coherent light source and redirect the light to the spatial light modulator. At least one of the optical systems may comprises a liquid-crystal shutter positioned between the spatial light modulator and the viewer. The liquid-crystal shutter may be configured to reduce noise in the reconstructed image. Each of the optical systems may comprises a liquid-crystal shutter positioned between the spatial light modulator and the viewer. In some examples, the viewer may be a binocular viewer. This may provide a comfortable viewing experience by the person viewing the holographic images. The system may further comprise an input device configured to receive input from a person and further configured to send the input to the computing device. The input device may comprise at least one of a touch screen, button, or joystick. Any suitable input device may be used. BRIEF DESCRIPTION OF DRAWINGS The present invention will be described by way of example only with reference to the following drawings in which: Figure 1 is an illustration of an eye; Figure 2 is an illustration of an eye; Figure 3 shows illustrations of an eye; Figure 4 shows illustrations of a myopic eye; Figure 5 is an illustration of a myopic eye corrected using a lens; Figure 6 shows illustrations of a hyperopic eye; Figure 7 is an illustration of a hyperopic eye corrected using a lens; Figure 8 is a schematic diagram of a system for generating and viewing a digital hologram; Figure 9 shows illustrations of an eye viewing a holographic image at different depths; Figure 10 shows illustrations of an eye viewing a holographic image at different depths; Figure 11 shows the first 21 Zernike polynomials; Figure 12 is a system for generating and viewing a digital hologram; Figure 13 is part of a system for generating and viewing a digital hologram; Figure 14 is part of a system for generating and viewing a digital hologram; Figure 15 is part of a system for generating and viewing a digital hologram; and Figure 16 is a flow diagram of a method of generating a digital hologram. DETAILED DESCRIPTION In order to help understand the invention better some general information about how the eye works, and some common eye conditions that can be corrected using glasses or contact lenses, will be described first. The human eye uses a two-lens system to focus light from an object onto the retina at a single depth. Light enters the eye through the cornea (the first lens), which is a curved surface that helps focus the light. The light then passes through the pupil, whose size is controlled by the iris to regulate the amount of light entering the eye. The light continues through the lens (the second lens), which further focuses the light onto the retina at the back of the eye. The retina contains rods and cones which detect light and colour respectively. These cells convert light into electrical signals that are transmitted to the brain where they are processed into the images we see. The cornea 4, illustrated in Figure 1, is responsible for most of the focusing, however the cornea itself is incapable of changing its focal length. The range of focal depths available to the eye is due to the lens 6, which is capable of changing its focal length via a muscular structure around it. When the muscular structure contracts, the lens 6 thickens providing more focal power. When the muscular structure relaxes, the lens 6 thins providing less focal power. The range of focal depth for the ideal human eye 2 is from about 25 cm to infinity, as shown in Figure 1. The relaxed eye 2 is able to focus on objects at infinity, while the eye 2 in its most contracted state is able to focus on objects as near as 25 cm. It is worth noting that in Figure 1 the state of the lens is shown for a focus at infinity and at 25 cm, however the eye 2 can only focus at one of these depths at a time. When the eye 2 is focused on an object at infinity, the lens is in its most relaxed state, providing as little focal power as possible. This is shown in Figure 2. Figure 2 also shows how light from objects closer than infinity is focused behind the retina 8, thus appearing blurry to the viewer. In general, whenever an object is at a depth different than the current focal depth of the eye 2, the light from that object will be focused before or after the retina and both of these will result in a blurry image. As the eye 2 focuses at different object depths, the muscle around the lens 6 contracts or relaxes as necessary to find the focal power needed to focus light onto the retina 8. This is shown in Figure 3, where the lens 6 thickens to focus light from more nearby objects onto the retina 8. From an optics perspective, this is necessary because closer objects emit light that is more divergent when it reaches the pupil, thus requiring more focal power to direct it onto the retina 8. Light from more distant objects, on the other hand, is less divergent, eventually becoming parallel to the optical axis in the case of optical infinity. The three images in Figure 3 show how the contraction of muscles in the eye 2 thickens the lens 6. A thicker lens 6 has a greater focal power allowing the person to focus on the more divergent rays of nearby objects. During an eye test, one aim is to determine the focal range of the person’s eye. If the eye’s focal range does not reach one of the limits of the ideal focal range, corrective lenses are needed. For example, the ideal human eye has a focal range of ~25 cm to infinity. If an eye’s focal range falls short of either of these limits, correction is needed. Typically, this occurs in one of two ways: the eye cannot focus on distant objects (a condition called myopia), or it cannot focus on nearby objects, such as those at ~25 cm (a condition called hyperopia). Other conditions which affect the ability of the person’s eye to focus on objects correctly are also tested for including astigmatism and presbyopia. At the end of the eye test, the eye care professional will determine a corrective lens specification, based on measurements taken during the eye test, that will allow the person to see clearly and document this specification in a prescription. The corrective lens specification will detail the corrections that are needed to minimize, or exclude, optical aberrations experienced by the person. Optical aberrations include spherical aberrations, astigmatism, and coma, and these can generally be corrected using the appropriately prescribed lens. Standard components of a prescription include SPH (sphere), CYL (cylinder), AXIS, and ADD (addition) values. SPH (sphere) measures overall lens power to correct nearsightedness (negative values) or farsightedness (positive values), indicating how much overall lens curvature is needed to focus light correctly on the retina. CYL (cylinder) measures astigmatism correction and indicates the lens power needed to correct irregular cornea curvature. AXIS specifies the orientation of the astigmatism correction and indicates the angle at which the cylindrical lens correction is applied. ADD (addition) corrections for presbyopia and indicates the additional magnifying power needed for reading or close-up work. Together, the SPH, CYL, AXIS, and ADD values make up the corrective lens specification that will form the person’s prescription. Lenses correct focal range problems by redirecting light from objects outside a person’s focal range to focus within it. Eye care professionals use various charts (e.g. a Snellen chart) and tests to determine the limits of a person’s focal range. Based on these results, they prescribe lenses to shift objects from outside the focal range into viewable positions within it. While the prescribed lenses adjust the person’s focal range to align with the ideal range, they do not extend its overall limits. The lens equation is used to calculate the power of corrective lenses needed to address vision problems like nearsightedness (myopia) or farsightedness (hyperopia). The lens equation is: 111 — — —l— j u v where fis the focal length of the lens creating the image, u is the distance to the objection (which will be infinity for far vision correction), and v is the image distance, corresponding to the eye’s focal point. By convention, u is positive when the object is in front of the lens, v is positive if the image and object are on opposite sides of the lens, v is negative if they are on the same side of the lens, and fis positive for a converging lens and negative for a diverging lens. The power of a lens, P, (measured in diopters) is the reciprocal of its focal length in meters: P = -f As explained earlier, myopia (nearsightedness) occurs when the eye 2 cannot focus on distant objects. In this condition, the relaxed lens 6 has too much focal power, causing light from distant objects to converge at a point in front of the retina 8. This results in blurry images for distant objects because the image is focused too far forward of the retina 8. The upper panel of Figure 4 illustrates this effect for an object at infinity. For nearby objects, the lens’s 6 high focal power compensates for the more divergent light rays, allowing the light to focus correctly on the retina 8. This ensures that the image is placed correctly at the retina 8, as shown in the lower panel of Figure 4. In a myopic eye, light cannot be properly focused at infinity, so the farthest point of clear vision (the far-point) is at a finite distance rather than at infinity. To correct this, a lens 10 is used to shift light from an object at infinity to create an image at the myopic eye’s far-point. Eye care professionals determine the farthest distance v at which the person can see clearly, and prescribe a lens 10 that diverges light rays to bring the focal point back to the retina 8. This correction is achieved using a diverging lens 10 with negative power. The lens 10 creates an image of the distant object at a location closer than the object itself, within the range where the eye 2 can focus. Biconcave lenses 10, like the one shown in Figure 5, add sufficient divergence to the light rays to balance the excessive focal power of the myopic eye 2, allowing distant objects to be seen clearly. This is equivalent to the lens 10 creating an image of the object at a point closer than the object itself and the eye 2 focussing on this closer image. Hyperopia (farsightedness) occurs when the eye 2 cannot focus on nearby objects. In this condition, the relaxed lens 6 has insufficient focal power, causing light from nearby objects to focus at a point behind the retina 8. This results in blurry images for close objects, as depicted in the upper panel of Figure 6. For distant objects, the lens’s 6 weak focal power is compensated by the nearly parallel light rays from the object, allowing the light to focus correctly on the retina 8. As a result, distant objects are seen clearly, as shown in the lower panel of Figure 6. To correct hyperopia, eye care professionals determine the closest distance v at which the person can see clearly and prescribe a converging lens 10 with positive power. This lens 10 shifts the focal point forward to the retina 8 by adding focal power, effectively creating a virtual image farther back than the actual object. The convex lens 10 "pushes" the object’s perceived position away, enabling the eye 2 to focus properly. The above theory calculates the power of the corrective lens required to correct myopia or hyperopia, assuming the lens is positioned directly in front of the eye, as with a contact lens. To convert this corrective power to the corrective power needed for a lens worn in a frame, the following equation is used: c 1 - dvPF where Pc is the power of the contact lens, PF is the power of the equivalent lens in a frame, and dv is the distance between the pupil and the centre of the framed lens. Inverting the equation gives " 1 + dvPc as the necessary correction for a framed lens given the correction for a contact lens. As mentioned above, it would be advantageous to provide an improved eye test procedure that can mitigate, or avoid altogether, at least some of the problems present in subjective refraction tests. It has been found that computer-generated holography (CGH) can be used to determine a person’s corrective lens through replication of traditional clinical eye tests, in order to provide an accurate, reliable, and certifiable lens prescription for the person. Computer-generated holography is the process of creating holograms using computer algorithms rather than tradition optical methods. Instead of capturing holograms physically with optical equipment, a computer calculates and simulates the holographic patterns required to reproduce a desired three-dimensional (3D) image when illuminated. This involves numerical computations and simulating the propagation of diffractive light patterns to present a 3D scene that appears to originate from real objects. At a high level, CGH begins with a target scene - the desired image - and calculates the diffraction pattern necessary to produce that scene using an array of light sources at specific distance. Since the target scene is purely digital, CGH can generate physically impossible 3D scenes as if they were real. In more detail, a hologram is a physical structure that diffracts light to create an image with the illusion of depth, appearing 3D. This effect is achieved by encoding both the amplitude and phase of lightwaves into the hologram. In CGH, computers simulate these light wave interactions to produce the holographic interference pattern digitally. This process begins by defining the 3D object or scene the viewer should perceive then simulating how light would interact with the object to compute its wavefront. This wavefront, describing the light’s amplitude and phase, is mathematically encoded to generate a corresponding 2D holographic interference pattern, forming the digital hologram. The wavefront represents the 3D structure of the object including its shape and how light scatters from its surfaces. CGH calculates the wavefront at a specific plane, such as the plane where the hologram will be displayed. The resulting hologram, encoded with this wavefront information, is then displayed on a digital device capable of modulating light, such as a spatial light modulator or a holographic display. When the hologram is illuminated by a coherent light source (e.g., a laser or LED), the light interacts with the hologram and generates a diffraction pattern. This pattern reconstructs the original wavefront of the object, making the 3D image visible to the viewer as a holographic image. The diffraction pattern emerges from the hologram’s interaction with light and represents how light is redistributed to recreate the object’s original wavefront. It is essential for the encoded wavefront information to accurately reproduce the desired diffraction pattern. As the diffracted light propagates through space, it interferes and combines in such a way that the object’s original wavefront is reconstructed. To the viewer or a detector, this reconstructed wavefront appears as the 3D image of the object. Figure 8 shows an exemplary simplified system 100 for generating and viewing a digital hologram. The system 100 comprises a computing device 102 configured to compute the hologram which is then sent to a modulating device 104 which is able to modulate light and display the digital hologram. Typically, the modulating device will take the form of a spatial light modulator 105 which comprises an array of thousands or millions of tiny light-manipulating elements, such as mirrors or liquid-crystal films. These elements can be individually controlled and adjusted to modulate incoming light, creating the diffraction pattern needed to generate the desired object or scene. A coherent light source 106, such as a laser or LED, illuminates the hologram displayed by the modulating device 104 and the diffracted light reconstructs the desired object or scene, producing a holographic image. Various optics 108, such as lenses and eyepieces, guide the light to a viewer 110, allowing the viewer 110 to view the holographic image. Additional components, such as shutters and / or diffraction gratings can also be included in the system 100 as necessary. The hologram can be used to replicate the optical tests traditionally carried out by an eye care professional and determine a person’s corrective lens needs. As mentioned earlier, eye care professionals traditionally use various charts (e.g., a Snellen chart) and tests (e.g., refraction tests) to assess the limits of a person’s focal range and prescribe corrective lenses as needed to bring objects from outside their focal range into focus. Instead of using a physical test chart typically displayed on a wall at a fixed distance and determining the smallest line of letters a person can read clearly, holographic charts can replace the physical chart. This allows the focal range of a person’s eye to be quickly assessed by adjusting the depth of the holographic chart. In CGH, altering the depth of the hologram is achieved by modifying how the hologram encodes the wavefront of the object. This process involves recalculating the hologram with updated depth parameters for the object's wavefront. In holography, depth is encoded by capturing how light from the object travels to the hologram plane at different distances. The phase of the wavefront varies with the distance of the object points from the hologram plane, with closer points experiencing less phase delay than farther points. When generating a hologram, depth is encoded in the phase of the wavefront, which is calculated based on the object’s 3D structure. By simulating wave propagation (e.g., using Fresnel diffraction or Fourier optics), the hologram captures this depth information. To change the depth of the reconstructed image, the hologram must be recomputed with adjusted propagation distances for the object or parts of the object, effectively changing the phase delays encoded in the hologram. To determine a person's focal range, the holographic chart can be placed at one of the ideal eye’s extrema (e.g., infinity). The depth can then be digitally adjusted by modifying the computer-generated wavefront, virtually moving the holographic chart closer until the person can clearly focus on it. The new depth at which clear focus is achieved corresponds to the position where a corrective lens would create a sharp image. For a target focal extremum u, which is the desired point of focus, virtually adjusting the hologram's depth until the image becomes clear is mathematically equivalent to finding the position v where a corrective lens of focal length f would form a clear image. Thus, the process of adjusting the hologram’s depth to achieve clear focus simulates the effect of a physical lens, without actually using a lens to adjust focus. This approach achieves the same result as altering the lens position or power, allowing for quick and accurate focal range testing without the need for physical lenses or repositioning objects. Figure 9 is a schematic illustration showing how nearsightedness can be corrected using CGH by quickly determining the upper bound of a patent’s vision. In this case the holographic image 12 starts at u = infinity, as shown in the upper panel of Figure 9. The depth of the hologram is then adjusted, effectively moving the holographic image closer to the person, as shown in the middle panel of Figure 9. Figure 10 is a schematic illustration showing how farsightedness can be corrected using CGH by quickly determining the lower bound of a patent’s vision. In this case, the holographic image starts at u = 25 cm, as shown in in the upper panel of Figure 10. The depth of the hologram is then adjusted, effectively moving the holographic image further away from the person, as shown in the middle panel of Figure 10. For both nearsightedness and farsightedness, the depth of the holographic image is adjusted until the person indicates that the image is clear. This final depth corresponds to the image position v from which the focal length of the required corrective lens can be calculated using the lens equation above. The lens power is then calculated as the reciprocal of the focal length. Since adjusting a hologram’s depth is equivalent to finding the image created by a lens with a specified focal length, this allows hologram depth to be converted to the appropriate corrective lens power. We have seen how spherical aberrations, which affect a person’s focal range (i.e. the range of object distances over which the eye can focus clearly) can be determined and quantified using CGH, and how any necessary spherical (SP)H corrections to adjust that focal range can be calculated. However, the method described above does not include how astigmatism is quantified or how the appropriate corrections (both cylindrical (CYL) and AXIS) are determined using CGH. A useful property of optical aberrations is that they primarily affect the phase of the wavefront without significantly altering the amplitude. Zernike polynomials are a set of mathematical functions used to describe wavefront shapes in optical systems, particularly for modelling wavefront aberrations. These polynomials form an orthogonal basis set over a circular domain, making them especially useful for representing wavefront errors in systems with circular apertures, such as lenses. Figure 11 shows the first 21 Zernike polynomials, with different colours representing varying intensities (increasingly positive and negative) across the domain. As you progress through the set, the polynomials become increasingly complex. Each term in the Zernike polynomial expansion corresponds to a specific type of optical aberration, such as defocus, astigmatism, or coma. The polynomials are indexed by radial and angular components, enabling precise decomposition of complex wavefront distortions into simpler, well-defined modes. Zernike polynomials are therefore useful because they correspond to known optical aberrations. Any arbitrary optical aberration can be mathematically represented using a subset of Zernike polynomials. These polynomials can be combined and weighted appropriately to reproduce specific aberrations in an image. As previously discussed, holography preserves and utilizes phase information. In this context, Zernike polynomials can be used to model, analyse, or correct wavefront aberrations, adjusting the quality of reconstructed holographic images. Briefly, a phase mask is calculated as a weighted sum of Zernike polynomials, which is then applied as an offset to the phase of the hologram. Conceptually, this mask introduces known artificial aberrations into the beam, which can help determine the corrective lens needed to view the image. After computing the diffraction pattern required to produce an image in the digital hologram, Zernike polynomials are applied to alter the diffraction pattern in a known way, which in turn adjusts the final holographic image that is reconstructed and viewed. The phase map can be represented using the following summation equation: ¢(^0) = ^cnmZ™(r,0) n,m where $(r, 0) is the phase map as a function of radial coordinate r and angular coordinate 0, cnm is the coefficient (weight) of each Zernike polynomial Z™, and Z™(r, 0) are the Zernike polynomial basis functions. The contribution of each Zernike polynomial (i.e. each aberration) to the overall phase map is determined by its coefficient. Since each Zernike polynomial corresponds to a specific type of optical aberration (e.g. defocus, astigmatism, coma), the coefficients of the Zernike polynomials provide information about the type and degree of optical correction required. In particular, the coefficient of a given Zernike polynomial quantifies the magnitude of that aberration in the wavefront phase map, and so by adjusting the coefficients of the relevant Zernike polynomials, specific aberration effects can be simulated or created. Corrective lenses achieve optical correction by introducing aberrations that “undo” or cancel out the aberrations caused by an imperfect eye. The SPH, CYL, and AXIS properties of the corrective lens specification are directly linked to certain known Zernike polynomials. For example, in Figure 11, the fifth disk (third row, center disk) describes the defocus caused by a lens (SPH), while the fourth and sixth disks describe astigmatism (CYL and AXIS). The phase mask behaves like a corrective lens, by “undoing” the aberrations caused by an imperfect eye. The original holographic image is distorted using Zernike polynomials, until the person provides feedback indicating that they can clearly see the distorted holographic image. When clarity is achieved, the applied phase map has effectively cancelled out the eye’s aberrations. Since the combination and weights of the Zernike polynomials used to construct the phase map are known, as these were input values to create the phase map, and each Zernike polynomial corresponds to a specific aberration, the corrective lens specification (including CYL and AXIS values) can be derived from the phase mask that produced the clear image for the person. The coefficients of the Zernike polynomials that were used to construct the phase map can be converted into values defining a corrective lens specification. For example, a first Zernike polynomial may correspond to defocus, and the coefficient of this Zernike polynomial can be used to derive the spherical lens power. As another example, a second Zernike polynomial may correspond to astigmatism, and the coefficient of this Zernike polynomial can be used to derive the CYL value and I or AXIS value. The coefficients play a central role in defining and constructing the aberration phase map. These values, determined by design decisions to simulate specific aberrations, are known before applying the aberration phase map to the hologram. The Zernike coefficients represent the magnitude of specific aberrations desired in the hologram, and the phase map is constructed as a sum of weighted Zernike polynomials, chosen depending on the aberrations that have been chosen to simulate. Applying this phase map to the hologram introduces the desired aberrations to the holographic image. Once the phase map has been applied to the hologram, the quality of the reconstructed image is evaluated to determine whether a desired image quality has been achieved. In this case the desired image quality actually corresponds to a desired distortion effect which exactly cancels out the aberrations caused by the person’s uncorrected eyes such that the person can see the image clearly. The coefficients can be iteratively updated and refined, adjusting the type and magnitude of the distortion to be applied to the hologram, and the recomputed phase maps can be iteratively applied to the original hologram to adjust the output viewed by the person. This iterative process is carried out until the person indicates that the image output is clear. In summary, the quality of the outputted image is iteratively adjusted until the image quality meets the desired clarity. The Zernike coefficients define the desired aberrations and are integral to constructing the phase map. By iteratively adjusting the coefficients and reapplying the updated phase map, fine-tuned corrections ensure the holographic image achieves the intended clarity. As a reminder, in this context, the intended clarity is achieved when the person indicates that they can clearly see the holographic image. Conceptually, the process of applying different phase masks until the person perceives a clear holographic image can be thought of as mimicking the traditional method of swapping lens in a phoropter. In the case of CGH, the person does not see clearly because of virtual lenses but because the holographic image has been distorted in a way that substantially exactly counteracts the distortions in the person’s vision. The extent of distortion applied to achieve clarity, compared to the original image, provides the data needed to determine the corrective lens specification. This specification is then used to determine the person’s prescription and appropriate lenses. Using CGH, particularly in combination with Zernike polynomials, to determine a person’s corrective lens needs enables multiple refraction adjustments (SPH, CYL, AXIS) to be tested simultaneously, eliminating the need for the traditional trial-and-error “yes-no” approach of swapping physical lenses. CGH for subjective refraction testing allows for real-time adjustments of SPH, CYL, and AXIS without physically changing lenses. This allows different optical corrections to be applied to the same image, and so the same image with different corrections can be presented to the viewer simultaneously. By incorporating Zernike polynomials when calculating the diffraction pattern needed to reconstruct the image, the effects of any refractive lens can be simulated, and multiple corrections can be tested at once. This makes subjective refraction testing much faster compared to the traditional "better or worse" method of lens swapping. Additionally, CGH offers the advantage of independent control over both the image size and depth. In traditional refraction tests, different examination charts are required for different depths because the apparent size of an object changes with distance. For example, 20 / 20 (or 6 / 6) visual acuity indicates that a person can resolve features spanning 5 arcminutes in their field of view. As real objects subtend different angles depending on their distance, it is impossible to test acuity using just one chart, requiring charts of varying sizes for different depths. With CGH, however, independent control of both image size and depth means that a single virtual chart can be positioned at any depth while maintaining the correct angular subtense. This not only saves time but also allows for acuity testing at any chosen distance. Figure 12 shows an exemplary system 200 configured to use computer-generated holography to determine a corrective lens specification which can then be used to determine a person’s corrective lens prescription. Generally, the system 200 comprises a first unit 202 and a second unit 204, which together form part of a binocular display unit 206. The binocular display unit 206 comprises a viewer 208 which allows the person to view the holographic images. The binocular display unit 206 may also be referred to as a holographic unit which is configured to project a holographic image for viewing by a person. The system 200 comprises a computing device 210 including a processor 212 (such as a GPU 212) which can be controlled by a technician. The computing device 210 includes custom software in order to perform CGH and refraction testing. A display screen of the computing device 210 can be used by the technician to provide suitable inputs to the computing device 210, as well as to display information necessary to conduct the testing such as information relating to parameters, values, and coefficients. A person can look through the viewer 208 and view holographic images of test charts which can be adjusted accordingly, for example by the technician adjusting one or more relevant parameters and coefficients, in order to mimic conventional refraction tests. Inputs to the computing device 210 can be provided via the display screen (e.g. using a touchscreen display screen) or through one or more input devices such as buttons or joysticks. For example, the display screen may prompt the technician to begin the test and, upon receiving confirmation to start the test, the computing device 210 will generate an initial holographic image for a person to view. Once the person confirms that they can see some form of holographic image (whether blurry or clear), the technician may instruct the computing device 210 to generate one or more modified holographic images for sequential viewing by the person. After each modified image has been projected, the person provides an indication to the computing device 210 on whether the image is clear or not. In some cases, the indication may be provided to the computing device 210 indirectly, for example via the technician who may use an input device of the computing device or interact with the display screen of the computing device to relay whether or not the person can see the most recently generated and projected image clearly, based on verbal or physical cues (e.g. head nodding / shaking, thumbs up / thumbs down). In other cases, the indication may be provided to the computing device 210 directly, for example the person may use an input device of the computing device 210 or interact with the display screen of the computing device 210 to relay whether the most recently generated image is clear. In some examples, the indication provided to the computing device 210 may be inferred based on a person’s response or reaction to viewing the generated image. For example, the technician may observe the person, and the technician may provide the indication to the computing device 210 based on the person’s behaviour. In other examples, the indication provided to the computing device 210 may be generated from electrical signals related to the person’s brain activity upon viewing the image. If the computing device 210 receives an indication that the person cannot see the image clearly, the testing continues by progressively modifying the holographic image using Zernike polynomials, as described above. Each newly generated image is then projected for the person to view. During this process, the technician may be required to confirm each step of the modification. In some cases, the technician may be asked to specify the next test to be conducted, for example by confirming which parameters or values should be adjusted or used as inputs for the next stage of the test. In other cases, the software may run automatically to generate the new images, determining which parameters or values should be adjusted without technician intervention but based on the indication received regarding image quality of the previous image. However, even if the software determines the next image automatically, the next step will only begin once the computing device 210 receives an indication regarding the quality of the previous image. The indication could comprise a binary indication regarding the image quality. For example, the computing device may ask “Is the image clear” and the indication received may be yes / no. The indication could comprise a relative indication. For example, the computing device may ask “How is the image quality compared to the previous image” and the indication received may be better / worse or more / less blurry. Once the person confirms that they can clearly see the image, the computing device 210 records the parameters and values used to generate the image that was clearly visible. These parameters and values are then used to create the corrective lens specification which is output by the computing device 210. In some cases, the output may be displayed on the screen, printed, or saved as a data file that can be used straight away or sent. Additionally, instead of outputting the corrective lens specification, the computing device 210 may convert it into a suitable prescription, which is then outputted to the display screen, printed, or saved as a data file that can be sent or stored. Each monocular unit 202, 204 is in communication with the computing device 210, and configured to receive holographic data from the processor 212 which is used to reconstruct the holographic images to be viewed by a person. Each monocular unit 202, 204 is also in communication with the viewer 208 allowing the person to view the holographic images reconstructed by the monocular units 202, 204. As can be seen in Figure 12, each monocular unit 202, 204 comprises its own optical engine 216, 218, power supply, control unit 220, 222, illumination source, and internal optics. The computing device 210 controls synchronization between the two optical engines 216, 218. Figure 13 is a more detailed view of one monocular unit 202. For the avoidance of doubt, both monocular units 202, 204 are the same and so only one will be described in detail. The GPU 212, which generates the holographic image, and any phase map to be applied to the hologram and sends this data to a modulating device 224 in the optical engine 216. In the example shown in Figure 13, the modulating device 224 takes the form of a digital micromirror device (DMD) which comprises an array of 5 micro-electromechanical system (MEMS) mirrors to spatially modulate light. The DMD can spatially control the characteristics of light across its surface, through the binary tilting of each of the microscopic mirrors in the array, which allows for precise light modulation. Illumination optics 226 within the optical engine 216 focus and direct light from a coherent light source 228 in the control unit 220 to the DM D 10 224 to reconstruct the holographic image. In this example, the coherent light source 228 comprises a plurality of RGB LEDs which provide colour information for the holographic image. Drivers 230, such as LED drivers in the control unit 220, control the pulse width and current of the LEDs to ensure the holographic image is illuminated as intended. A liquid crystal (LC) shutter 232 further 15 modulates the light, by selectively blocks or transmits light from the DMD 224, to enhance reconstruction of the holographic image which can be viewed by a person through the viewer 208. A driver 234, such as an LC driver, controls the LC shutter 232 in the optical engine 216. It is noted that while Figure 13 shows an LC shutter 232, this component is optional. 20 Figure 14 shows the optical path of light travelling through the optical engine 216. The incoming coloured light from the LEDs 228 is collimated by a first lens L1 and then passed to a diffraction grating 226a which splits the collimated RGB light into its individual colour components. This light is redirected onto the DMD 224 by a mirror 226b. The first lens L1, diffraction grating 226a, and mirror 226b may all be 25 components forming part of the illumination optics 226. In some arrangements, the diffraction grating may be a Thorlabs (RTM) GR25-1205 grating, having 1200 grooves / mm, which may help to balance any smearing which may otherwise occur in the image as a result of the broadband nature of the RGB light sources being used with the DMD. 30 Light reflected from the DMD 224 is focussed onto the LC shutter 232 via second and third lenses L2, L3 which work together to recombine the spatially separated RGB colour beams from the diffraction grating into a single, combined RGB beam. In some examples, second and third lenses L2, L3 may together have an equivalent focal length of 46.7 mm. The LC shutter 232 may help to enhance image quality by noise-reduction windowing in the Fourier plane of the L2-L3 lens pair. A further pair of lenses L4, L5, positioned after the LC shutter 232 act as a relay to efficiently transfer the image from the LC shutter 232 to the viewer 208 without distorting or losing information about the reconstructed holographic image. The lenses L4, L5 also allow the viewer 208 to be positioned a sufficient distance away from the LC shutter 232 such that the person can look through the viewer 208 at a comfortable distance. Figure 15 shows the binocular display unit 206 which houses the two monocular units 202, 204. The system 200 is able to produce eye-testing images (such as the Snellen, Tumbling E, and Jaegar charts) at suitable optical distances (e.g ranging from 6 m to 40 cm) and which are free of speckle noise, penumbra, and other visual irregularities that could interfere with a person's ability to resolve the charts. The system 200 is able to replicate the tests necessary to provide an accurate, reliable, and certifiable prescription for eyeglasses or contact lenses. As mentioned above, the computing device 210 forming part of the system 200 includes custom software designed to perform CGH and refraction testing. The software can simply be run by a technician, trained on how to use the system 200, without the need for a skilled eye care professional (such as an optician or optometrist) to be present to conduct the testing. This enables the system 200 to be deployed in rural or remote areas, where a trained technician can perform the tests, optionally supervised by a remote doctor, with the remote doctor approving any prescriptions generated from the test results. The custom software comprises detailed step-by-step instructions for performing refraction testing using the system 200, so the technician only needs to following any prompts and instructions provided to them by the computing device 210 to carry out the eye test. The system 200 is compact overall, requiring only the computing device 210 and the binocular display unit 206 for successful operation. This system 200, is therefore much more compact that a traditional refraction testing set-up, making it significantly more portable, allowing the system 200 to be deployed and used in a large variety of locations here traditional setups might not be feasible. Figure 16 shows an exemplary flow diagram of a method of using computer-generated holography to determine a corrective lens specification. The method begins at step S300 by computing a hologram of an image by the computing device 210. The computing device 210 then computes an aberration phase map using a set of Zernike polynomials at step S302. At step S304, the computing device 210 applies the aberration phase map to the hologram to generate a modified hologram, which may be referred to as a first modified hologram. The first modified hologram is projected, by the binocular display unit 206, at step S306. At this point, the person can view the reconstructed holographic image, based on the first modified hologram. The computing device 210 receives, at step S308, an indication about the image quality of the first modified hologram, wherein the indication indicates whether the image quality of the first modified hologram corresponds to a first desired image quality. As has been discussed previously, in this context the image quality of the first modified hologram is a perceived image quality because it is based on the person’s opinion as to whether or not they can see the reconstructed first modified holographic image clearly or not. Similarly, the first desired image quality is when the person viewing the reconstructed holographic image can see the image clearly, and so the first desired image quality is achieved when the person indicates that the reconstructed, projected holographic image is clear. If the received indication indicates that the image quality of the first modified hologram corresponds to the first desired image quality, at step S310, the computing device 210 then creates a corrective lens specification based on the set of Zernike polynomials. It should be understood that creating a corrective lens specification based on the set of Zernike polynomials does not necessarily mean that all the Zernike polynomials in the set are used to create the corrective lens specification. Instead, at least one Zernike polynomial of the set of Zernike polynomials must be used to create the corrective lens specification. For example, at step S312 at least one Zernike polynomial that was used to compute the aberration phase map, and which corresponds to at least one aberration of interest, is identified in the set of Zernike polynomials and the coefficient of said at least one identified Zernike polynomial is converted, at step S314, by the computing device 210 into a value forming part of the corrective lens specification. In some cases, the computing device 210 converts a plurality of coefficients into a plurality of values forming part of the corrective lens specification, wherein each coefficient corresponds to a Zernike polynomial in the set of Zernike polynomials that was used to generate the aberration phase map. The corrective lens specification is output at step S316. The corrective lens specification is then used by an eye care professional to determine an appropriate prescription for eyeglasses or contact lenses. As we have seen above, the eye care professional can be remote from the system 200 and so the eye care professional can receive the corrective lens specification from the computing device 210, for example the corrective lens specification can be sent to a remote computing device accessible by the eye care professional. If the received indication indicates that the image quality of the first modified hologram does not correspond to the first desired image quality, at step S318, the computing device 210 adjusts the aberration phase map at step S320 in order to continue the testing. The aberration phase map is adjusted at step S320 by adjusting a coefficient of at least one Zernike polynomial in the set of Zernike polynomials to form a set of adjusted Zernike polynomials (which may be referred to as a first set of adjusted Zernike polynomials). A revised aberration phase may (which may be referred to as a first revised aberration phase map) is the computed by the computing device at step S322 using the set of adjusted Zernike polynomials. As we have seen, adjusting a coefficient of any given Zernike polynomial adjusts the magnitude, and therefore relative contribution, of the aberration modelled by said Zernike polynomial and so the aberration phase map can be revised to increase or decrease the contribution of different aberrations as necessary. At step S324 the computing device 210 applies the revised aberration phase map to the original hologram to generate another modified hologram (which may be referred to as the second modified hologram). The second modified hologram is then projected by the binocular display unit 206, at step S326, and the person views the reconstructed holographic image, based on the second modified hologram. The computing device 210 receives, at step S308, another indication about the image quality of the second modified hologram, this indication again indicating whether the image quality of the second modified hologram corresponds to a first desired image quality. Method steps S320 to S326 are repeated until the computing device 210 receives an indication that indicates that the image quality of the most recently projected modified hologram corresponds to the first desired image quality. At least one of the Zernike polynomials and their coefficients are then identified based on the last aberration phase map that was used to construct the latest modified hologram, and it is these coefficient values which are used to create the corrective lens specification. The method therefore iteratively adjusts the aberration phase map, iteratively applying each adjusted aberration phase map to the hologram to generate a modified hologram, and iteratively projects each modified hologram until an indication is received that the first desired image quality of the image is achieved. The above-described method steps are used to determine at least the CYL and AXIS values that form part of the corrective lens specification. Looking again at Figure 16, at step S328, after computing the hologram the binocular display unit 206 projects the hologram and the person view the original reconstructed hologram. This hologram comprises depth information having a first depth, and so the viewer perceives the holographic image at a first distance. The computing device 210 then adjusts the hologram at step S330 to generate an adjusted hologram (which may be referred to as a first adjusted hologram), the adjusted hologram comprising depth information having a second depth. The adjusted hologram is projected by the binocular display unit 206, and the person perceives the holographic image resulting from the adjusted hologram to be at a second distance. The computing device 210, at step S332, then receives an indication about an image quality of the adjusted hologram, the indication indicating whether the image quality of the first adjusted hologram corresponds to a second desired image quality. Again, the second desired image quality (or any nth desired image quality) is equivalent to the first desired image quality: a desired image quality is when the image being viewed is clearly to the person viewing the image, and so a desired image quality is achieved when the person indicates that the reconstructed, projected holographic image is clear. If the received indication indicates that the image quality of the first adjusted hologram corresponds to the second desired image quality, at step S334, the computing device 210 creates a corrective lens specification based on the second depth. This is done by calculating, by the computing device 210, a focal length based on the second depth at step S336 and then converting, by the computing device 210, the focal length into a value forming part of the corrective lens specification at step S338. The corrective lens specification is again output at step S316. It should be noted that in some cases the corrective lens specifications created at steps 314 and 338 can be combined and output as one corrective lens specification and in other case the corrective lens specifications created at steps 314 and 338 can be output separately. Regardless, the one or more corrective lens specifications output by the computing device 210 are used by the eye care professional to determine one prescription. Thus, all the values that form the one or more corrective lens specifications are used to determine the final prescription. If the received indication indicates that the image quality of the first adjusted hologram does not correspond to the second desired image quality at step S340, the computing device 210 adjusts the depth information of the first adjusted hologram from the second depth to a third depth at step S342 and then computes another adjusted hologram (which may be referred to as a second adjusted hologram) at step S344. The second adjusted hologram is projected by the binocular display unit 206, at step S346, and the person views the reconstructed holographic image, based on the second adjusted hologram. As a result of adjusting the depth information, the person perceives the holographic image resulting from the adjusted hologram to be at a third distance. The computing device 210 receives, at step S332, another indication about the image quality of the second adjusted hologram, this indication again indicating whether the image quality of the second adjusted hologram corresponds to the second desired image quality. Method steps S342 to S346 are repeated until the computing device 210 receives an indication that indicates that the image quality of the most recently projected adjusted hologram corresponds to the second desired image quality. The depth information of the latest adjusted hologram is used to calculate the focal length and create the corrective lens specification. The method therefore iteratively adjusts the depth information of the hologram and iteratively projects each adjusted hologram until an indication is received that the second desired image quality of the image is achieved. These method steps are used to determine at least the SPH values that form part of the corrective lens specification. Although the method starting at step S302 and the method starting at step S328 have been described separately and shown as separate branches on the flow diagram in Figure 16, it will be appreciated that the computing device 210 may carry out these method steps substantially simultaneously. The computing device 210 is able to make multiple adjustments and modifications to the hologram in one step, and so multiple different aberrations and visual effects can be models at the same time. It should also be noted that in some cases, the SPH value may be calculated using Zernike polynomials and the method starting at step S302. In this case, the method starting at step S328 may not necessarily be carried out.

Claims

1. A method of using computer-generated holography to determine a corrective lens specification, the method comprising:computing a hologram of an image;computing an aberration phase map using a set of Zernike polynomials;applying the aberration phase map to the hologram to generate a first modified hologram;projecting the first modified hologram;receiving an indication about an image quality of the first modified hologram, the indication indicating whether the image quality of the first modified hologram corresponds to a first desired image quality;when the indication indicates that the image quality of the first modified hologram corresponds to the first desired image quality;creating a corrective lens specification based on the set of Zernike polynomials;outputting the corrective lens specification.

2. The method of claim 1, wherein creating the corrective lens specification comprises:converting a coefficient of a Zernike polynomial in the set of Zernike polynomials into a value forming part of the corrective lens specification.

3. The method of claim 1 or 2, wherein the creating comprises:converting a plurality of coefficients into a plurality of values forming part of the corrective lens specification, wherein each coefficient corresponds to a Zernike polynomial in the set of Zernike polynomials that was used to generate the aberration phase map.

4. The method of any preceding claim, wherein when the indication indicates that the image quality of the first modified hologram does not correspond to the first desired image quality, the method further comprises;adjusting the aberration phase map and applying the adjusted aberration phase map to the hologram to generate a second modified hologram.

5. The method of claim 4, wherein adjusting the aberration phase map comprises:adjusting a coefficient of at least one Zernike polynomial in the set of Zernike polynomials to form a first set of adjusted Zernike polynomials;computing a first revised aberration phase map using the first set of adjusted Zernike polynomials; andapplying the first revised aberration phase map to the hologram to generate the second modified hologram.

6. The method of claim 4 or claim 5, wherein the method comprises iteratively adjusting the aberration phase map, iteratively applying each adjusted aberration phase map to the hologram to generate a modified hologram, and iteratively projecting each modified hologram until an indication is received that the first desired image quality of the image is achieved.

7. The method of claim 6, wherein iteratively adjusting comprises:applying the first revised aberration phase map to the hologram to generate a second modified hologram;projecting the second modified hologram;receiving an indication that an image quality of the second modified hologram does not correspond to the first desired image quality;adjusting a coefficient of at least one Zernike polynomial in the first set of adjusted Zernike polynomials to form a second set of adjusted Zernike polynomials;computing a second revised aberration phase map using the second set of adjusted Zernike polynomials;applying the second revised aberration phase map to the hologram to generate a third modified hologram;projecting the third modified hologram;repeating these steps until an indication is received that indicates that an image quality of the projected modified hologram corresponds to the first desired image quality.

8. The method of any preceding claim, wherein the method further comprises:after computing the hologram, projecting the hologram, wherein the hologram comprises depth information having a first depth;adjusting the hologram to generate a first adjusted hologram, wherein the first adjusted hologram comprises depth information having a second depth;projecting the first adjusted hologram;receiving an indication about an image quality of the first adjusted hologram, the indication indicating whether the image quality of the first adjusted hologram corresponds to a second desired image quality;when the indication indicates that the image quality of the first adjusted hologram corresponds to the second desired image quality;creating the corrective lens specification based at least partly on the second depth.

9. The method of claim 8 wherein creating the corrective lens specification comprises:calculating a focal length based on the second depth; andconverting the focal length into a value forming part of the corrective lens specification.

10. The method of claim 8 or 9, wherein when the indication indicates that theimage quality of the first adjusted hologram does not correspond to the second desired image quality, the method further comprises;adjusting depth information of the first adjusted hologram from the second depth to a third depth;computing a second adjusted hologram;projecting the second adjusted hologram;repeating these steps until an indication is received that indicates that an image quality of the projected adjusted hologram corresponds to the second desired image quality.

11. The method of any preceding claim, wherein receiving an indication comprises receiving feedback from a person indicating whether the first desired image quality has been achieved.

12. The method of claim 11, wherein receiving an indication comprises receiving an input via a user interface.

13. The method of any preceding claim, wherein the image comprises at least one letter.

14. The method of any preceding claim, wherein the image comprises a Snellen chart.

15. The method of any preceding claim, wherein computing the hologram comprises:generating data representing the image;simulating an interaction of light with the image and calculating a wavefront; andcomputing the hologram for the image based on the calculated wavefront.

16. The method of claim 15, wherein the wavefront comprises amplitude and phase information and computing the hologram comprises encoding the amplitude and phase information of the wavefront.

17. A system configured to determine a corrective lens specification using computer generated holography, the system comprising:a computing device configured to:compute a hologram of an image;compute an aberration phase map using a set of Zernike polynomials;apply the aberration phase map to the hologram to generate a first modified hologram;a holographic unit configured to project the first modified hologram;the computing device further configured to receive an indication about an image quality of the first modified hologram, the indication indicating whether the image quality of the first modified hologram corresponds to a first desired image quality;when the indication indicates that the image quality of the first modified hologram corresponds to the first desired image quality, the computing device is further configured to;create a corrective lens specification based on the set of Zernike polynomials; andoutput the corrective lens specification.

18. The system of claim 17, wherein the holographic unit comprises:a first monocular unit and a second monocular unit, each in communication with the computing device, wherein each of the first a second monocular units comprises:a spatial light modulator configured to receive the first modified hologram from the computing device and display the first modified hologram;a coherent light source configured to illuminate the first modified hologram displayed by the spatial light modulator to reconstruct the image;an optical system comprising at least one optical component configured to direct the reconstructed image out of the monocular unit via a viewer;the viewer configured to allow a person to view the reconstructed image.

19. The system of claim 18, wherein each coherent light source comprises at least one LED.

20. The system of claim 18 or 19, wherein each spatial light modulator comprises a digital-micromirror device.

21. The system of any of claims 18 to 20, wherein each ofthe first and second monocular units further comprises a diffraction grating configured to receive light from the coherent light source and redirect the light to the spatial light modulator.

22. The system of any of claims 18 to 21, wherein each optical system comprises a liquid-crystal shutter positioned between the spatial light modulator and the viewer.

23. The system of any of claims 18 to 22, wherein the viewer is a binocular viewer.

24. The system of any of claims 17 to 23, further comprising an input device configured to receive input from a person and further configured to send the input to the computing device.

25. The system of claim 24, wherein the input device comprises at least one of a touch screen, button, or joystick.

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