A device for detecting whether a sample well (352) contains a sample is presented. The sample well contains one or more fluorescent substances needed for an optical analysis of the sample. The device includes a controller (312) configured to compute, on the basis of a luminescence, e.g. fluorescence, emission signal measured from the sample well, an indicator value indicative of a decay time of the measured fluorescence emission signal. The controller is configured to compare the indicator value to a reference value and to set, in accordance with the comparison, a detection result to express that the sample well contains the sample or that the sample well does not contain the sample.