The ultimate goal of in vitro measurements of sunscreen protection is replacement of invasive tests on human volunteer subjects. This requires validation of methods against a measurable in vivo benchmark, such as SPF. SPF is measured in the laboratory using artificial light sources that do not completely simulate sunlight. Once a method for in vitro measurements is validated, sunscreen protection against measured solar spectra may be achieved. Critical aspects of in vitro measurements of sun protection include the ability of substrates to emulate human skin, performance of instrumentation and use of correct algorithms to account for changes in sunscreen absorbance during irradiation. We have developed a dose-response model that accounts for changes in absorbance and facilitates evaluation of broad spectrum protection, as well as SPF, and provides an index of sunscreen photostability.