Abstract: A method for performing measurements includes providing relative movement between a receptacle and a cover over a measurement interval comprising multiple non-overlapping time periods. A distance between the receptacle and the cover changes along a first axis over at least a portion of each time period. An apparatus for performing measurements includes a receptacle comprising a substrate and a plurality of wells within the substrate, and a cover. The cover includes: (1) a device configured to engage with the receptacle, (2) a control subsystem configured to provide relative movement between the receptacle and the cover, and (3) a measurement subsystem.