Photolytic uncaging of biologically-active molecules within cells is a powerful technique. However, the delivery of uncaging light into the cytosol can vary between cell types, individual cells of the same type, and different loci within an individual cell because of optical differences in absorbance and light-scattering properties of the cytoplasm. Here, we demonstrate a simple technique for monitoring the magnitude of cytosolic ultraviolet delivery during uncaging, which also leaves a quantitative and persistent record of this within the cells. The simple method shown here provides a much needed universal monitor of the delivery of ultraviolet light to molecules within the cytosol, providing a much needed parameter for the correct interpretation of uncaging experiments.