Optical Transparency in Polycrystalline Ceramics. Thick polycrystalline ceramics can be defined as transparent, translucent, or opaque. The term "transparent" should largely be reserved for materials having transmission ≳70% of the theoretical maximum (accounting for reflection losses) in the wavelength range of interest. In hundreds of microns to millimeter thick materials, optical transparency occurs at wavelengths with sufficiently low absorption coefficients (i.e., <1). MAPbBr3 and MAPbCl3 have optical bandgaps (Eg) of ~535 nm and ~405 nm, respectively. At shorter wavelengths (≲Eg), is on the order of ~10 5 -10 6 cm -1 . Furthermore, strong vibrational absorption from the organic CH3NH3 + moieties occurs when ≳2100 nm (see Figure ). At wavelengths between the optical bandgaps and vibrational absorption, a transmission window exists where ≲10 -1 cm -1 . Therefore, we focus our transparency discussion in the wavelength regime from the MAPbBr3 and MAPbCl3 optical bandgaps up to ~2000 nm. However, we show other transparency windows at longer wavelengths exist as well.