  A system for time-correlated single photon counting applications based on
CMOS single photon avalanche diodes is presented. An instrument response
function of 79 ps allows a fluorescence decay of high-affinity Ca2+ indicator
Oregon Green BAPTA-1 to be analyzed with unprecedented temporal resolution in
the two-photon excitation regime. A triple exponential decay model is shown to
best fit the fluorescence dynamics of OGB-1.
