SUBSTRATE, SUBSTRATE PRODUCTION METHOD, ND FILTER, AND OPTICAL CHARACTERISTIC MEASUREMENT DEVICE

摘要

Disclosed is a substrate (10) for an ND filter (16), made from a spinel sintered material which is a material wherein breaks caused by thermal stress during use are unlikely to occur. It is desirable that the substrate (10) has a Young's modulus of 150-350 GPa, that the composition of the spinel sintered material is MgO·nAl2O3 (1.05?n?1.30), and that the Si content is 20 ppm or less.