The optical voltage sensor FlaSh, made from a fusion of a GFP "reporter domain" and a voltage-gated Shaker K(+) channel "detector domain," has been mutagenically tuned in both the GFP reporter and channel detector domains. This has produced sensors with improved folding at 37 degrees C, enabling use in mammalian preparations, and yielded variants with distinct spectra, kinetics, and voltage dependence, thus expanding the types of electrical signals that can be detected. The optical readout of FlaSh has also been expanded from single wavelength fluorescence intensity changes to dual wavelength measurements based on both voltage-dependent spectral shifts and changes in FRET. Different versions of FlaSh can now be chosen to optimize the detection of either action potentials or synaptic potentials, to follow high versus low rates of activity, and to best reflect electrical activity in cell types with distinct voltages of operation.