A team at the University of Calgary in Alberta, Canada, has used two-photon excitation to improve the specificity of photodynamic therapy. The tight focus of two-photon excitation confines the energy to a volume of about 1 µm3, which reduces unwanted photodamage in the surrounding tissues. The researchers compared how one- and two-photon activation affected photosensitizers in solution. In a session at Photonics West, they reported that protoporphyrin IX and verteporfin reacted similarly to both techniques. They will compare the methods in vitro to determine how environmental factors may influence the process.