  Analytical expressions are derived for the distribution rates of spatial
coincidences in the counting of photons produced by spontaneous parametric down
conversion (SPDC). Gaussian profiles are assumed for the wave function of the
idler and signal light created in type-I SPDC. The distribution rates describe
ellipses on the detection planes that are oriented at different angles
according to the photon coincidences in either horizontal-horizontal,
vertical-vertical, horizontal-vertical or vertical-horizontal position
variables. The predictions are in agreement with the experimental data obtained
with a type-I BBO crystal that is illuminated by a 100 mW violet pump laser as
well as with the results obtained from the geometry defined by the
phase-matching conditions.
