Apple Invents an HMD with an Optical System with Dispersion Compensation that Delivers Quality AR Imagery
Today the US Patent & Trademark Office published a patent application from Apple that relates to a future Head Mounted Display device that includes dispersion compensation structures and methods for optical reflective devices having holographic optical elements that have the capability of reflecting projected images as well as allowing a user to experience quality augmented reality imagery.
Apple's invention covers one or more improved methods, systems, or devices for performing dispersion compensation.
Holograms may be implemented within optical media as holographic optical elements. A holographic optical element may be substantially achromatic, sustaining a reflective angle independent of the wavelength of incident light.
Light traversing certain dispersion boundaries (e.g., air-to-projection coupling element, air-to-waveguide substrate, air-to-waveguide grating medium, waveguide substrate-to-air, waveguide grating medium-to-projection coupling element, waveguide grating medium-to-coupling element, etc.) of the optical device may exhibit waveform separation across disparate frequencies of the light.
A dispersion relationship between an index of refraction of one medium and an index of refraction of another medium for disparate frequencies may be used in techniques to compensate for chromatic dispersion of light in the optical device.
Dispersion compensation techniques using the dispersion relationship may be applied to determine holograms that compensate for the chromatic dispersion effects of certain dispersion boundaries.
A resulting holographic optical element (HOE) may substantially approximate desired achromaticity associated with use of the holographic optical element in an optical device and/or a particular operating environment (e.g., where projection optics are used, where edge coupling is used, and/or in a fluid medium such as air or water).
Apple further notes that a Head-Mounted Displays typically involve near-eye optics to create "virtual" images. In the past HMDs have dealt with a variety of technical limitations that reduced image quality and increased weight and size.
Past implementations have included conventional optics to reflect, refract or diffract light, however, the designs tend to be bulky. Additionally, conventional mirrors and grating structures have inherent limitations.
For example, a conventional mirror may have a reflective axis that is necessarily coincident with surface normal. The reflective axis of a conventional mirror may lead to suboptimal orientation or performance of the mirror. Also, conventional grating structures may include multiple reflective axes that covary unacceptably with incidence angle and/or wavelength.
Accordingly, a device for reflecting light may include features that reflect light about a reflective axis not constrained to surface normal and whose angle of reflection for a given angle of incidence is constant at multiple wavelengths.
Embodiments of the device may have substantially constant reflective axes (e.g., reflective axes that have reflective axis angles that vary by less than 1.0 degree) across a range of incidence angles for incident light of a given wavelength, and this phenomenon may be observed with incident light at various wavelengths.
In Apple's patent FIG. 1 below we're able to see an illustration of a head mounted display (HMD) #100 in which the principles included herein may be implemented. The HMD may include eyewear or headwear in which a near-eye display (NED) #105 may be affixed in front of a user's eyes.
The NED may include a diffractive element portion disposed within or incorporated with a lens assembly of the HMD. In some examples, the diffractive element portion is a holographic optical element (HOE), which may be comprised of a skew mirror #110.
Apple further notes that the skew mirror is a reflective device which may include a grating medium within which resides volume holograms or other grating structure.
The Skew mirror may include an additional layer such as a glass cover or glass substrate. The additional layer may serve to protect the grating medium from contamination, moisture, oxygen, reactive chemical species, damage, and the like. The additional layer may also be refractive index matched with the grating medium.
Apple's patent FIG. 7 below is a side view of an illustrative input prism and an illustrative skew mirror having gratings with skew axes that vary as a function of grating magnitude; FIG. 12 is a side view of an illustrative input prism having different regions with different dispersion characteristics.
Apple's patent application 20200117003 that was published today by the U.S. Patent Office was filed back in Q4 2019. This report may be updated later today. Considering that this is a patent application, the timing of such a product to market is unknown at this time. For those wishing to dive deeper into the details, click here.
One of the key engineers behind this patent is Vikrant Bhakta, PhD: Senior Display Exploration Engineer. Some of the projects that he's worked on at Apple include Optical Imaging, Projection display, Illumination engineering, Structured light illumination for 3D sensing and super-resolution, AR and VR Head mounted display, Computational imaging, Radiometric and photometric modeling and analysis, Fourier Optics, Imaging and non-imaging optical system design