  Phase contrast imaging is used to observe Bose-Einstein condensates (BECs) at
finite temperature in situ. The imaging technique is used to accurately derive
the absolute phase shift of a probe laser beam due to both the condensate and
the thermal cloud. The accuracy of the method is enhanced by using the
periodicity of the intensity signal as a function of the accumulated phase. The
measured density profiles can be described using a two relevant parameter fit,
in which only the chemical potential and the temperature are to be determined.
This allows us to directly compare the measured density profiles to different
mean-field models in which the interaction between the condensed and thermal
atoms is taken into account to various degrees.
