Voltage sensitive dyes technique: What is the underlying measure?
Since these are fluorescent dyes, the most likely method of use is with some sort of fluorescent (likely confocal) microscope. The various types of dyes have different hydrophobic tails, so they target different types of membranes (plasma membrane, mitochondrial membranes, etc.). An excitatory light source (typically a laser, with a single wavelength) is applied to the sample, and the $F_0$ or baseline fluorescence intensity at time zero is measured. When an event happens, such as the contraction of a muscle, the $\Delta F$ or change in fluorescence intensity is measured. This difference is then divided by the baseline, and called the fractional fluorescence (${\Delta F}/{F_0}$). It is plotted like so:



Bu G et al. Biophys J. 2009 Mar 18;96(6):2532-46.