  A key assumption in the reconstruction of extensive air showers using the air
fluorescence technique is the hypothesis that fluorescence is proportional to
energy deposition at all depths in the shower. This ansatz, along with the
supposition that particle distribution and energy loss can be well modeled by
modern shower simulation software, must be thoroughly verified in order to
validate the air fluorescence technique. We report here the results of the
first direct measurement of air fluorescence yield as a function of shower
depth, as performed in the thick-target phase of the FLASH (FLuorescence in Air
from SHowers) experimental program at the SLAC Final-Focus Test Beam facility.
We compare observed fluorescence light yields as a function of shower depth to
concurrently measured charged particle yields, to the predictions of the EGS
and GEANT software packages, and to empirical energy-loss models. We also
examine the extent to which the relative yield versus shower depth is
independent of wavelength within the fluorescence spectrum.
