Alternative operational modes(cont.)Generating and modulating sensors:Generating sensors produce their signal output from energy taken directly from the measurand, e.g. photovoltaic cell (it provides an output voltage related to its irradiation).Modulating sensors use the measurand to alter the flow of energy from an external source in a way that affects the output of the sensor, e.g. photoconductive cell (it reflects the external irradiation in resistance change).