Metrology meets the challenges of immersive reality technologies

Just five years ago, it would have been questionable to anticipate exponential growth in immersive reality technologies. Today, professionals in many industries are discovering that interacting with information-rich 3D displays is much better than using a 2D display. While it is still unclear how this revolution in interactive digital communication will develop, there is no doubt that it is coming.

The definition of immersive or extended reality (XR) systems includes virtual reality (VR) for a fully artificial field of view (see Figure 1), augmented reality (AR) for adding digital information to normal vision, and mixed reality (MR) for the complete fusion of the real and digital worlds.

Emerging XR solutions are already making an impact in diverse applications across the entertainment, sports and leisure, enterprise deployments, defense and healthcare industries. It drives professional training and development programs by providing virtual access to environments that would be too expensive or dangerous in the real world. It is also transforming industrial manufacturing and design, accelerating design to market with rapid prototyping, to process improvements that enable greater productivity, efficiency and accuracy.

Technology drivers for immersive reality

XR requires digital optical immersive display (DOID) hardware, and to expand the market for these devices they need to be lightweight, comfortable and easy to use, while also delivering exceptional visual quality, reliability and processing power.

Miniaturization of the myriad of parts and components used in digital optical display devices is fundamental and a major driver of the growth in the use of immersive reality. The focus is on the use of smaller and smaller micro-optics and integrated optical assemblies, MEMs sensors and micro-LED displays, among many other integral parts and components. XR systems include not only the projection system for creating the illusion of 3D objects in space, but also a host of additional devices, many of which are optical, for eye tracking, distance measurement and environmental detection.

A striking aspect of immersive reality devices is the extensive use of innovative optical systems with non-traditional components, possibly with waveguide structures and holographic and meta-surfaces to direct light to the eye. Going hand in hand with the growth of immersive reality is an unprecedented demand for micro-aspherical and free-form optics, which present significant challenges for manufacturers, especially for quality control and design verification. This is where metrology comes to the fore, supporting and stimulating innovation.

Free form optics and waveguides

Immersive reality devices require both a wide field of view and a large “eyebox” volume. An eyebox is the set of eye positions for which an acceptable view of the image exists; it is an important parameter in the optical design of XR optics. The larger the eyebox volume, the better and more realistic the user experience. Simulated reality must cover the full range of peripheral vision and eye rotations, and this is not feasible for practical wearable devices using conventional geometric optics.

Free-form optical surface shapes, which may lack rotational symmetry or description in terms of conical constants, have evolved from an intriguing optical design concept to a practical necessity for applications ranging from aerospace and defense to consumer electronics. For immersive reality, which combines exceptionally high optical performance with the ergonomic limitations of wearable interactive technologies, designs often call for diffraction-limited performance at large fields of view in off-axis orientations. Freeform optics are often the only way to correct the resulting aberrations and pose several challenges for measurement technologies. Because free-form optics often lack axial or off-axis symmetry, they are both challenging to design, measure and manufacture.

A common design for XR optics includes planar waveguide structures, which are important for image transmission and coupling. An optical waveguide is a structure that constrains a light wave to travel along a certain desired path. Planar waveguides are regularly used to replicate the imaging pupil in MR optical systems, resulting in greatly increased eyebox volume. They are often manufactured in the form of a thin transparent film with increased refractive index on a substrate or embedded between two substrate layers. Multi-wavelength planar waveguides for DOID systems are complex optical structures, and like free-form optics, present numerous metrological challenges.

Meeting the metrological challenge

Metrology tools for immersive reality devices quantify surface shape, diffraction angles and efficiency, mounting tolerances and surface roughness, and aid in development and prototyping, performance evaluation and production quality control.

Non-contact metrology solutions are often preferred to prevent damage to high-value surfaces and to provide full-area 3D measurements at high data rates. Full 3D images provide better process insight and surface characterization compared to 2D profiles. These requirements favor optical metrology over contact or tactile solutions whenever possible.

Interferometry uses the wavelength of light and optical coherence to measure distances and surface shapes. Interferometry is often considered a high-precision solution for measuring flat and spherical parts, but is difficult to adapt to modern free-form surface shapes and complex surface structures. Measurement tool designers have introduced ways to extend the range of high-precision interferometric methods specifically for measuring DOID components and systems.

Free-form optics without spherical symmetry is a good example of how interferometry has evolved to meet the challenge. Accurate metrology, not only for surface shape but also for surface orientation with respect to reference points, plays a key role in providing essential quality control and feedback during the manufacturing process. One solution for free-form interferometric measurements is computer-generated holograms (CGHs), which when used in conjunction with a laser Fizeau interferometer such as the Zygo Verifire create wavefronts to compensate for the high surface slopes and often asymmetrical shape of aspherical and free form optics.

Process control of free-form optics requires 3D topographic maps and relational metrology to locate reference points and mounting points. The Zygo Compass interference microscope accommodates free shapes of surfaces by viewing them from different perspectives. The final image is formed from overlapping smaller images, which are locked together using specialized software based on the correlation of fine-scale surface texture between adjacent images. Full-surface measurements reveal errors that are missed when using tactile stylus metrology and can detect errors due to shrinkage and imperfections in the molding process (see Fig. 2).

A challenge for all optical methods is the quality control of planar waveguides, which consist of two or more semi-transparent substrates with holographic coatings to direct light out of the waveguide and into the eye. An interferometric solution for measuring the substrates is based on advanced light sources with swept wavelength power, which give each of the reflective surfaces a unique modulation frequency. Instruments such as the Zygo Verifire MST laser Fizeau interferometer evaluate the flatness, overall thickness variation and material homogeneity of waveguide substrates.

Metrology for complete waveguide stacks requires evaluation of the distance between glass substrates, their flatness after mounting and uniformity of gaps between the plates. These dimensional features have a strong influence on image quality and uniformity in final DOIDs. To measure the completed waveguide assemblies, an effective solution is coherence scanning interferometry, which separates surfaces using interference fringe contrast. An example system is the Zygo Nexview interference microscope, equipped with wide-angle objectives (see Fig. 3).

Growth in the XR market is driving unprecedented demand not only for traditional optical metrology for surface shapes, textures and miniature microelectronics, but also for micro-freeform optics and waveguides. The challenge is for metrology techniques and technologies to keep pace, especially as XR technologies and supporting DOID hardware move from prototypes to large-scale production.

THANK YOU

Many thanks to Chris Young, MicroPR&M, for discussions and contributions to this article.

Compass, Nexview, Verifire and Verifire MST are trademarks of Zygo Corporation.

Leave a Comment