A dual-wavelength ion indicator, also known as a ratiometric indicator is a specific type of fluorescent dye that’s used to quantitatively measure intracellular calcium ion (Ca2+ ) concentrations.

Calcium ions are among the most widespread second messengers used in signal transduction pathways in eukaryotic cells. Ca2+ concentrations are regulated by the parathyroid hormone but they cannot be visualized directly in living cells. Fluorescent dual-wavelength ion indicators help to visualize and accurately measure intracellular calcium concentrations.

Fura-2 is the most widely used dual-wavelength ion indicator.