The objective of this chapter is to familiarize scientists working in photodermatology with the basic concepts related to light measurement and light source characterization that are necessary for reproducible scientific tests. Hopefully this chapter will encourage the reader to look differently at the optical source being used and how correctly to express the exposure dose. In performing any phototoxicity study, it is imperative that the spectral characteristics of the optical source be known. For a specific photobiological action spectrum, different light sources delivering the same optical power can produce completely different dermatological effects if the sources have differing spectra (Figure 1). Indeed, a photodermatologist will choose a specific ultraviolet (UV) source to match best a given biological action spectrum (if known) in order to achieve the greatest efficiency in delivering a photobiologically significant dose (Diffie, 1982). Different applications require different light sources, and a variety of measurement techniques may be in order when attempting to conduct different types of studies.