Small borosilicate glass microspheres have fewer spherical imperfections and inclusions than other glass microspheres. They also have better mechanical and thermal stability. This series has been calibrated with NIST traceable methods of optical microscopy. They are packaged as dry powders in 1 gram bottles. They have a density of 2.50-2.55g/cm 3 and an index of refraction of 1.56 @ 589nm (23°C). Brands: Thermo Scientific