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How to define the response speed of an opto-electric sensor?

Q:How to define the response speed of an opto-electric sensor?


      A:  The response speed of an opto-electric sensor represents the response speed of the sensor to light, which is usually characterized by response time. The response time of a sensor usually refers to the time required for output signal to rise from zero to 63.2% of the peak value (namely 1-1/e), or for the signal to decline from the peak to 36.8% of the peak value (namely 1/e) in response to pulsed light. Alternatively, the rise time can also be defined as the time required for the output signal to rise from 10% to 90% of its full value, while the fall time can be defined as the time required for the output signal to drop from 90% to 10% of its full value. Usually the response speed of an opto-electric sensor is limited by two factors: (1) resistance-capacitance delay; (2) the transition time of photon-generated carriers between electrodes. Besides, for devices with parasitic photoconduction, the capture and release of photon-generated carriers by defects would seriously affect the response speed of the sensor.