Improving the Measurement of the Diameter of Si Spheres

For a potential new definition of the kilogram using silicon spheres to determine Avogadro´s constant, extremely low measurement uncertainties are required for all influence quantities, e.g., lattice constant, isotopic composition, mass, and volume. For the determination of the volume of the silicon sphere, an unprecedented accuracy for dimensional measurements is required. To reduce the measurement uncertainty, improvements in the PTB interferometer for measuring the spheres were made. Investigations on the wavefront aberration of the objectives and on the bulk temperature of the sphere allow a reduction in the measurement uncertainty to 3.4 10^-8