This report documents the optical characteristics of a number of photodynamic therapy (PDT) light sources of varied types, measured and indexed relative to estimated effectiveness for activation of the PDT chromaphore protoporphyrin IX (PpIX). PDT sources in use at several clinics, including intense pulsed light (IPL) sources, lasers, and continuous wave (CW) light sources, were spectroradiometrically measured and indexed relative to their overlap to an absorption spectrum of PpIX. The sources were highly disparate, varying in power from irradiance in the mW/cm(2) range for the CW sources up to ∼30 J/cm(2) per flash for the IPL sources. Our PpIX Index ranged by a factor of nearly 100 (0.008-0.630) in estimated PpIX PDT effectiveness following the distinct spectral characteristics of the light sources surveyed. Application of this PpIX Index, tempered with an understanding of the biology of the lesion being treated and effective spectrum of the light source reaching the lesion requiring therapy, provides a rational algorithm to approximate equivalent light doses prior to clinical protocols to establish equivalent patient outcomes employing alternative PDT light sources.