Fluorescence spectroscopic measurement of the amino acid tryptophan on the hair fibre surface was extended to include in situ fibre irradiation and a novel cyclical wash off, reapplication protocol. When applied to the investigation of a new damage prevention active, it was shown that the active was preferentially degraded in a sacrificial manner and that the underlying fibre surface was maintained in good condition. In addition, tensile strength measurements were performed to assess the mechanical properties of the treated and untreated fibres following UV and sunlight exposure and the results demonstrate the damage prevention effectiveness of the active.