The use of imaging techniques has severely catalyzed the gain in knowledge about biological systems. In photodynamic therapy (PDT) imaging in general is used in applications ranging from research to clinical use.¹  The focus is often on imaging of the photosensitizer (PS) fluorescence, which correlates with the localization of the PS. However, imaging of the main mediator of PDT, singlet oxygen, is of great interest as such measurements could enlighten the modes of photodynamic action. It might eventually help to optimize therapeutic parameters, such as light dose, by monitoring singlet oxygen quenching efficiencies and oxygen saturation in situ.^1,2  The possibility of direct imaging of singlet oxygen in PDT relevant systems will be a leap in methodology.