One of the key properties of transparent ceramics is its transmittance. When light passes through a medium, light loss and intensity attenuation will occur due to the absorption, surface reflection, scattering and refraction of the medium. These attenuations depend not only on the basic chemical composition of the material, but also on the microstructure of the material. The factors affecting the transmittance of ceramics will be introduced below. 

1.Porosity of ceramics

   The preparation of transparent ceramics is essentially to eliminate the densification of micro-pore completely in sintering process. The size, number and type of pore in materials will have a significant impact on the transparency of ceramic materials.Small changes in porosity can significantly change the transmittance of materials. For example, studies have shown that transparency decreases by 33% when the closed porosity in ceramics changes from 0.25% to 0.85%. Although this may be the result of a particular situation, to some extent, we can see that the effect of porosity on the transparency of ceramics is a direct and violent manifestation. Other research data show that when the stomatal volume is 3%, the transmittance is 0.01%, and when the stomatal volume is 0.3%, the transmittance is 10%. Therefore, transparent ceramics must increase their density and decrease their porosity, which is usually more than 99.9%. Besides the porosity, the diameter of the pore also has a great influence on the transmittance of ceramics. As shown in the figure below, we can see that the transmittance is the lowest when the diameter of the stomata is equal to the wavelength of the incident light.

2. Grain size

   The grain size of ceramic polycrystals also has a great influence on the transmittance of transparent ceramics. When the incident light wavelength is equal to the grain diameter, the scattering effect of light is the largest and the transmittance is the lowest. Therefore, in order to improve the transmittance of transparent ceramics, the grain size should be controlled outside the wavelength range of incident light.

3. Grain boundary structure

   Grain boundary is one of the important factors that destroy the optical homogeneity of ceramics and cause light scattering and decrease the transmittance of materials. The phase composition of ceramic materials usually consists of two or more phases, which can easily lead to light scattering on the boundary surface. The greater the difference in composition of materials, the greater the difference in refractive index, and the lower the transmittance of the whole ceramics.Therefore,the grain boundary region of transparent ceramics should be thin, light matching is good, and there are no pores, inclusions, dislocations and so on. Ceramic materials with isotropic crystals can achieve linear transmittance similar to that of glass. 

4. Surface finish

   The transmittance of transparent ceramics is also affected by surface roughness. The roughness of ceramic surface is related not only to the fineness of raw materials, but also to the machined finish of ceramic surface. After sintering, the surface of untreated ceramics has a larger roughness, and diffuse reflection will occur when light is incident on the surface, which will lead to light loss. The greater the roughness of the surface, the worse the transmittance. 

   The surface roughness of ceramics is related to the fineness of raw materials. In addition to selecting high fineness raw materials, the surface of ceramics should be ground and polished. The transmittance of alumina transparent ceramics can be greatly improved by grinding and polishing. The transmittance of alumina transparent ceramics after grinding can generally increase from 40%-45% to 50%-60%, and the polishing may reach more than 80%.