Metrology meets the challenges of immersive reality technologies

Just five years ago, it was doubtful to expect massive 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 remains unclear how this revolution in interactive digital communications 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 visual field (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 world.

XR’s emerging solutions are already making an impact in diverse applications in the entertainment, sports, leisure, enterprise, defense and healthcare sectors. It revitalizes training and professional development programs by allowing virtual access to environments that may be too expensive or too dangerous to access in the real world. It is also transforming industrial manufacturing and design, accelerating design to market with rapid prototyping, through process improvements that allow for increased productivity, efficiency and accuracy.

Technology engines for immersive reality

The XR requires immersive optical digital display (DOID) devices, and market expansion will require these devices to be lightweight, comfortable, and easy to use while providing exceptional visual quality, reliability, and processing power.

Miniaturization of the myriad parts and components used in digital optical display devices is a key and a major driver behind the growth in the use of immersive reality. The focus is on the use of micro-optics and integrated optical assemblies, MEM sensors, and small LED displays, among many other parts and components. The XR systems not only include a projection system to create the illusion of 3D objects in space, but a suite of additional devices, many of them optical, for eye tracking, range, and environmental sensing.

One surprising aspect of immersive reality devices is the extensive use of innovative optical systems with unconventional components, potentially with waveguide structures, holographic surfaces, and metasurfaces to direct light into the eye. Working hand in hand with the growth of immersive reality is an unprecedented demand for accurate and free-form aspheric optics, which presents significant challenges for manufacturers, particularly in terms of quality control and design verification. Hence, standards advance to the required level and support and stimulate innovation.

Free optics and waveguides

Immersive reality devices require both a wide field of view and a large “eyebox” size. Eyebox is the set of eye positions at which there is an acceptable view of the image; It is a key parameter in the optical design of XR optics. The larger the eye box, the better and more realistic the user experience. Simulated reality needs to cover the full range of peripheral vision and eye rotation, which is not possible for practical wearable devices that use conventional engineered optics.

Free-form photonic surfaces, which may have no rotational symmetry or description in terms of conical invariances, 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 require diffraction-limited performance in wide fields of view in off-axis directions. Free optics is often the only way to correct the resulting aberrations and presents many challenges to measurement techniques. Because free-form optics often have no axial or off-axis symmetry, both are challenging to design, measure, and manufacture.

The common design of 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 specific desired path. Planar waveguides are regularly used for repeat imaging in MRI systems, resulting in a significant increase in eye square size. They are often made as a transparent thin film with an increased refractive index on a substrate or sandwiched between two layers of a substrate. Multi-wavelength planar waveguides for DOID systems are complex optical structures, and such free-form optics present many measurement challenges.

Facing the metrology challenge

Measurement tools for AR devices determine surface shape, yaw angles, efficiency, aggregate tolerance, and surface roughness, and assist in development, modeling, performance evaluation, and production quality control.

Offline metrology solutions are often preferred to avoid damage to high-quality surfaces and to provide full-area 3D measurement at high data rates. Full 3D images provide a better working view and characterization of surfaces compared to 2D profiles. These requirements favor visual metrics over contact or haptic solutions where possible.

Interferometry uses the wavelength of light and optical coherence to measure distances and surface shapes. Interferometry is often thought of as a high-precision solution for measuring flat and spherical parts, but it is difficult to adapt to modern free-surface shapes and complex surface structures. Designers of measuring instruments have provided ways to extend the range of high-accuracy interferometry methods specifically for measuring DOID components and systems.

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

Controlling free-form optics requires 3D topographic maps and relational scales to locate fiducials and mounting points. The Zygo Compass interference microscope accommodates the shapes of free surfaces by viewing them in a series of different viewpoints. The final image consists of smaller overlapping images, which are secured together using specialized software based on the fine surface texture correlation between adjacent images. Whole area measurements show the errors lost when using the stylus measurement and can detect errors due to shrinkage and defects in the molding process (see Figure 2).

The challenge for all optical methods is to monitor the quality of planar waveguides, which consist of two or more translucent substrates with 3D coatings to guide light from the waveguide to the eye. The substrates measurement interferometry solution is based on advanced light sources with swept wavelength capability, imparting a unique modulation frequency to each of the reflective surfaces. Instruments such as the Zygo Verifire MST laser interferometer evaluate the flatness, overall thickness contrast, and material homogeneity of waveguide substrates.

Measurements for full waveguide beams require evaluation of the spacing between glass substrates, their flatness after assembly, and the uniformity of the gaps between panels. These dimensional properties have a strong impact on image quality and uniformity in the final DOIDs. To measure completed waveguide sets, an effective solution is coherence-scan interferometry, which separates the surfaces using the contrast of the interference fringes. One example of the system is the Zygo Nexview interference microscope, which is equipped with wide-field objectives (see Figure 3).

The growth in the XR market is spurring unprecedented demand not only for traditional optical scales for surface shapes, tissues and microelectronic devices, but also for micro-optics and waveguides. The challenge is for measurement technologies and technologies to keep pace, especially now that XR technologies and support for DOID devices are transitioning from prototyping to large-scale production.

Thanks and appreciation

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

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