Fura2 is a ratiometric fluorescence probe for the measurement of calcium. Its derivative Fura-2-acetoxymethyl ester (Fura2-AM) is membrane permable and can thus be used to measure intracellular free calcium concentration (Grynkiewicz et al., 1985). For this purpose, cells are incubated with Fura2-AM, which crosses the cell membrane by diffusion and is cleaved into free Fura2 and acetoxymethyl groups by cellular esterases. Intracellular free calcium is measured by exciting the dye at 340 nm and 380 nm, which are the excitation optima of calcium-bound and free Fura2, respectively, and emission detection above 500 nm. Through the ratiometric detection unequal distribution of the dye within the cell and other potential disturbances are largely cancelled out, making this a widely used and relatively reliable tool for calcium measurements.
Reference: Grynkiewicz G, Poenie M, Tsien RY. (1985) A new generation of Ca2+ indicators with greatly improved fluorescence properties. J Biol Chem 260(6): 3440-50. 
MitoPedia methods: Fluorometry