Ratiometric

Ratiometric imaging uses a ratio of images to determine ion concentration, voltage or pH levels in a cell of a cell network.  One common example is the Fura-2 fluorophore, which absorbs light preferentially at 380nm in high intracellular concentrations of calcium and absorbs light preferentially at 340nm in low concentrations of calcium. By exciting the sample at these two wavelengths and taking a ratio of the image, a semi-quantitative determination of calcium levels is possible.  Other ratiometric fluorophores, like INDO-1,  undergo a change to their emission wavelength under differing ion concentrations.