Apparatus and methods are described for determining a property of a biological sample. A sample carrier (22) includes one or more sample chambers (40) that define at least a first region (42) and a second region (44), the height of the one or more sample chambers varying between the first and second regions in a predefined manner. A computer processor (28) receives data relating to a first optical measurement that is performed upon a portion of the sample that is disposed within the first region, receives data relating to a second optical measurement that is performed upon a portion of the sample that is disposed within the second region, and determines the property of the sample by using a relationship between the first optical measurement, the second optical measurement, and the predefined variation in height between the regions. Other applications are also described.