Since the development of ratiometric fluorescent calcium indicators like Fura-2 and lndo-1 in the mid-eighties, calcium imaging has become a major approach to studying life cell physiology. The ability to calibrate the spectral properties of the dye as a function of the calcium concentration led to measurements of nearly absolute intracellular calcium concentrations in a large variety of preparations. However, the use of ratiometric calcium indicators in confocal calcium imaging is limited, since the dyes are excited by UV light while the lasers of most confocal microscopes provide visible light only. In addition, high energy UV light may cause rapid dye bleaching and photodamage. Thus it appears advantageous to reduce the illumination of the preparation to the minimum possible minimum. The most frequently used dual-excitation dye Fura-2 requires rapidly alternating excitation at 340 ran and 380 nm, respectively, to permit ratiometric calcium measurements with a maximal efficacy.