  A new approach to simulate fluorescence photons produced in extensive air
showers is described. A Monte Carlo program based on CORSIKA produces the
fluorescence photons for each charged particle in the development of the
shower. This method results in a full three-dimensional simulation of the
particles in a shower and of the fluorescence light generated in the
atmosphere. The photons produced by this program are tracked down to a
telescope and the simulation of a particular detector is applied. The
differences between this method and one-dimensional approaches are
quantitatively determined as a function of the distance between the telescope
and the shower core. As a particular application, the maximum distance at which
the Auger fluorescence telescope might be able to measure the lateral
distribution of a shower is determined as a function of energy.
