Optical system metrology is important for best product quality of XR headsets and their components. The test technology used to qualify these components and sub-modules originates from two different fields: display metrology and optical system test: Display-test-derived systems use widefield optics and test low-frequency "macro"-scale parameters like ISO contrast and color and brightness homogeneity. Because of design choices, these systems cannot test "micro"-scale higher-frequency features like contrast of projected text or chromatic aberrations. The typically non-diffraction limited optical designs do not allow reporting correct values for e.g. MTF because of physics limitations. Higher spatial frequencies corresponding to finer details ("micro"-scale) are the domain of optics testing-derived technology, however the design choices typically used here make testing at the "macro"-level difficult.
In this paper, we demonstrate that neither one of the two technologies is sufficient to support the requirements of upcoming headset generations. Instead, we describe a new generation of test equipment that integrates both "micro"- and "macro"-scale test capabilities in one instrument.
Diffractive waveguides for Augmented Reality (AR) are often stacks of 2 or 3 single waveguides, with each individual waveguide designed for 1 or 2 narrow-band wavelength bands. For best image quality, the optical axes of all elements of the waveguide stack must coincide to well below 1 arcmin for all 3 RGB colors, otherwise chromatic aberration effects at edges occur, severely reducing the image quality for the user.
Therefore,1) the optical axis of each waveguide in the stack must be characterized after manufacturing and compatible sets of waveguides must be matched; 2) the stacking process must not introduce tilt errors.
Metrology solutions for both 1) and 2) and the effect of errors on the image quality of the finished waveguide will be presented.
A current challenge for VR headsets producers is to reduce the weight and size of headsets. One very promising approach is the use of pancake lenses that help reducing the weight of the VR lens system but at the same time significantly increase the optical quality. Due to their complexity pancake lenses confront manufacturers with many challenges in testing and fabrication. TRIOPTICS´ expertise in active alignment and optical metrology enable our customers to create better pancake lenses for the challenges of tomorrow.
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