Why can residual pores inside alumina grains prevent light from passing through a lamp envelope?

Residual pores within alumina grains prevent light from passing through a lamp envelope because they cause light scattering.1

In their pure, dense state, many ceramics are wide-band-gap insulators that are optically transparent. However, when a ceramic material contains porosity, the pores act as scattering centers for light. Because the refractive index of the air or gas inside these pores differs from that of the surrounding alumina, light is deflected and scattered as it attempts to pass through the material. This scattering effect renders the otherwise transparent material white and opaque, effectively blocking light transmission.1

To achieve the translucency required for applications like high-pressure sodium-vapour lamps, manufacturers must sinter the alumina to a state of high optical density, often by using additives such as magnesia to eliminate these residual pores.21