Indo-1 (potassium salt) is a ratiometric fluorescent calcium indicator. It is ideal for analyses using flow cytometry, as it uses a single excitation source, typically 349-364 nm light from an argon-ion laser. The emission maximum shifts from 475-485 nm without calcium to 400-410 nm when Indo-1 (potassium salt) binds calcium. Indo-1 (potassium salt) is prone to photobleaching, which limits its usefulness in methods involving microscopy.