  The optical transmission matrix (TM) characterizes the transmission
properties of a sample. We show a novel experimental procedure for measuring
the TM of light waves in a slab geometry based on sampling the light field on a
hexagonal lattice at the Rayleigh criterion. Our method enables the efficient
measurement of a large fraction of the complete TM without oversampling while
minimizing sampling crosstalk and the associated distortion of the statistics
of the matrix elements. The procedure and analysis described here is
demonstrated on a clear sample which serves as an important reference for other
systems and geometries such as dense scattering media.
