The reason why fluorescence is more sensitive than UV-Vis absorption is that they are measured in different ways. Absorbance is measured as the difference in intensity between light passing through the reference and the sample, whereas fluorescence is measured directly without any reference beam. In other words, fluorescence is measured over a dark background, while absorbance is measured over a bright background. It is relatively easy to detect low levels of light, but difficult to identify small differences in intensity. Therefore, fluorescence is more sensitive than UV-Vis absorption spectroscopy.