Optically Transparent MAPbX3 Perovskite Ceramic Microstructures: Pores. Three of the four scattering sources described in the first section-optical anisotropy between grains, impurities/secondary phases, and surface roughness-have been minimized using our methodology. Thus, the only remaining, but undoubtedly the most significant, scattering source is residual porosity. Light scattering from pores is linked to refractive index differences between air (n=1) and the polycrystalline medium (n≈1.9 and n≈2.0 between ~800 nm and ~2000 nm for MAPbCl3 and MAPbBr3, respectively). As will be demonstrated by the modelling below, the magnitude/efficiency of pore scattering is intertwined between pore diameter (dp), pore size distribution, and porosity (P, the volume fraction of pores). While pores can never be entirely eliminated the ultimate goal is to minimize both porosity and pore size. It should be noted that accurate experimental quantification of porosity and pore size remains extremely challenging. Porosity is ultimately linked to the ceramic density such that P=1-. Relative densities for the perovskite ceramics-estimated from Archimedes' method (see methods in SI and Table for more details)-range from ~0.973 and ~0.996, leaving residual porosities from P~0.027 to ~0.004. Unfortunately, accurate quantification of P values ≲0.01 via Archimedes' method is not reliable using common laboratory analytical balances and changes in P as low as ~0.0001 can significantly affect transmission. Nonetheless, Archimedes' measurements clearly demonstrate our perovskite ceramics are highly dense. We therefore further investigate pores using X-ray nanotomography (NanoCT) to map three-dimensional cross-sections of the ceramic with a nominal spatial resolution down to ~50 nm and a voxel size of 16 nm. Here, contrast between X-ray absorption (or lack thereof) by perovskite grains and pores permits determination of critical pore attributes (average pore size dp, sizedistribution, and rough morphology). Figure depicts NanoCT data for an 8×6×2 m region of the l=0.989 mm MAPbBr3 ceramic. Figure first illustrates an overlay of the perovskite grains (red shaded regions) and pores (blue shade region), whereas Figure removes all grains and displays only pores. Video S1 shows a three-dimensional, rotating view of this data. Figure shows the entire region analyzed by NanoCT and SEM images of the ceramic fragment. NanoCT methods can be found in the SI.