Sensor and Measurement Method A turbidity sensor and method of measuring turbidity is provided. The turbidity sensor (100) comprises first and second optical detectors for detecting a respective optical response of each optical signal. The first optical detector (20) may be arranged in direct view of the emitter (10) and the second optical detector (30) may be arranged in indirect view of the emitter (10). The two detectors collect light emitted from the emitter (10) when directed through a fluid sample during two optical tests run in very close succession. Firstly, a control sample is illuminated to determine a calibration factor for the control sample with known turbidity. Then, the calibration factor is used to determine the turbidity of a fluid sample with unknown turbidity. Advantageously, background radiation during the data collection process is ignored because the transient behaviour during each optical test is negligible. The approach is more convenient over known turbidity sensors and measurement methods, particularly in light of the calibration step.