Calcium acts as a universal second messenger in a variety of cells. Numerous functions of all types of cells are regulated by Ca2+, thus calcium measurement is critical for various biological investigations. 

Among visible light-excitable calcium indicators, Fluo-8®, Fluo-4, Fluo-3, Rhod-2 and Rhod-4™ are most commonly used.  The long-wavelength Rhod-4™, Cal-590™ and Cal-630™ are valuable alternative Ca²⁺ indicators to the green fluorescent Fluo-8®, Fluo-4 and Fluo-3 for experiments in cells and tissues that have high levels of autofluorescence. 

Ratiometric, or dual-wavelength, ion indicators are a subcategory of fluorescent dyes utilized for their ability to quantitatively measure intracellular ion concentrations. Compared to single-wavelength indicators, dual-wavelength indicators possess unique spectral properties which are triggered in response to binding its target ion. Among ratiometric calcium indicators, Fura-2 and Indo-1 are the most widely used.