  We present an experimental method to measure the transverse spatial quantum
state of an optical field in coordinate space at the single-photon level. The
continuous-variable measurements are made with a photon-counting,
parity-inverting Sagnac interferometer based on all-reflecting optics. The
technique provides a large numerical aperture without distorting the shape of
the wave front, does not introduce astigmatism, and allows for characterization
of fully or partially coherent optical fields at the single-photon level.
Measurements of the transverse spatial Wigner functions for highly attenuated
coherent beams are presented and compared to theoretical predictions.
