The “Window to the Brain” is a transparent cranial implant under development, based on nanocrystalline yttria‐stabilized zirconia (nc‐YSZ) transparent ceramic material. Previous work has demonstrated the feasibility of this material to facilitate brain imaging over time, but the long‐term stability of the material over decades in the body is unknown. In this study, the low‐temperature degradation (LTD) of nc‐YSZ of 3, 6, and 8 mol % yttria is compared before and after accelerated ageing treatments following ISO standards for assessing the ageing resistance of zirconia ceramics. After 100 hr of accelerated ageing (equivalent to many decades of ageing in the body), the samples do not show any signs of phase transformation to monoclinic by X‐ray diffraction and micro‐Raman spectroscopy. Moreover, the mechanical hardness of the samples did not decrease, and changes in optical transmittance from 500 to 1000 nm due to ageing treatments was minimal (below 3% for all samples), and unlikely to be due to phase transformation of surface crystals to monoclinic. These results indicate the nc‐YSZ has excellent ageing resistance and can withstand long‐term implantation conditions without exhibiting LTD.

As transparent polycrystalline materials become more important in optical applications, evaluation of their optical properties across a wide range of wavelengths (or frequencies) is crucial for device design. In-line transmission measurements are often used to assess the suitability of materials for a given optical application. We developed a model that describes reflection, scattering, and absorption losses that commonly affect transmission. The model demonstrates the effects that parameters such as absorption type (Lorentzian or Gaussian), scattering regime (Raleigh–Gans–Debye or Rayleigh), and optical path length have on transmission spectra. We also fit the model onto transmission data from three polycrystalline material systems: ruby, yttria stabilized zirconia, and terbia. Parameters extracted from these fits can be used to describe wavelength dependent transmission with one simple analytical expression. The fit can also be used to decouple absorption from scattering, allowing for the extraction of important properties such as absorption coefficients.

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