  A unified scheme for quantum measurement processes is formulated on the basis
of Micro-Macro duality as a mathematical expression of the general idea of
quantum-classical correspondence. In this formulation, we can naturally
accommodate the amplification processes necessary for magnifying quantum state
changes at the microscopic end of the probe system into the macroscopically
visible motion of the measuring pointer. Its essence is exemplified and
examined in the concrete model of the Stern-Gerlach experiment for spin
measurement, where the Helgason duality controlling the Radon transform is seen
to play essential roles.
