  Imaging atmospheric Cherenkov telescopes are continuously exposed to varying
weather conditions that have short and long-term effects on their response to
Cherenkov light from extensive air showers. This work presents the
implementation of a throughput calibration method for the VERITAS telescopes
taking into account changes in the optical response and detector performance
over time. Different methods to measure the total throughput of the instrument,
which depend on mirror reflectivites and PMT camera gain and efficiency, are
discussed as well as the effect of its evolution on energy thresholds,
effective collection areas, and energy reconstruction. The application of this
calibration in the VERITAS data analysis chain is discussed, including the
validation using Monte Carlo simulations and observations of the Crab Nebula.
