This disclosure describes techniques for deflection estimation for a protective cover in a wearable device such as an augmented reality (AR) headset. Per techniques of this disclosure, the birefringent property of a coverglass under stress is utilized to estimate coverglass deformation/deflection. The coverglass includes two polarization layers, applied to the outer and inner sides of the coverglass, that act as orthogonal polarizers. An image sensor is placed adjacent to the inner surface of the coverglass, and is optionally attached to the coverglass using an adhesive with a refractive index matched to that of the coverglass material. Polarized light emerging from the outer polarization layer propagates through the coverglass and is split into an ordinary ray and an extraordinary ray due to the different effective refractive indices of the birefringent material. The two rays pass through the inner polarization later which is effective to polarize incident light along a plane orthogonal to the first polarization layer. The segregated (isolated) emergent ray is detected by the image sensor. Coverglass deflection is determined based on measured optical properties of the segregated ray by use of prior calibration and/or network training.
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Anonymous, "Deflection Estimation Using Birefringence", Technical Disclosure Commons, (August 17, 2020)