Generating and modulating sensors: 生成和調製感測器: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).