  We have developed an all-optical method for measuring the lifetimes of $n S$
and $n D$ Rydberg states and demonstrate its capabilities with measurements on
a dilute cloud of ultracold ${}^{87}$Rb atoms in a cryogenic environment. The
method is based on the time-resolved observation of resonant light absorption
by ground state atoms and selective transfer of Rydberg atoms into the ground
state at varying delay times in order to reconstruct Rydberg decay curves. Our
measurements of the ${}^{87}$Rb $30S_{1/2}$ state indicate an increase of the
lifetime at lowered environment temperatures, as expected due to decreased
black body radiation. For the $38D_{5/2}$ state with an attractive
dipole-dipole interaction, ionization and lifetime reduction due to collisional
effects are observed.
