Systems and methods for reading machine-readable marks on racks and receptacles

A sample processing or assay instrument (100) includes a moveable support (112). The moveable support (112) defines a first pocket (140A) configured to receive a first object (136) having a first machine-readable mark (147). The moveable support (112) defines a second pocket (140B) configured to receive a second object (136) having a second machine-readable mark (147). The moveable support (112) also includes a first fiducial machine-readable mark (15OA) and a second fiducial machine-readable mark (150B). The instrument also includes an image capture device (128) that captures a first image including the first fiducial machine-readable mark (150A) and the first machine-readable mark (147) of the first object (136). The image capture device (128) captures a second image that includes the second fiducial machine-readable mark (150B) and the second machine-readable mark (147) of the second object (136). The instrument (100) also includes a processor configured to associate information decoded from the first and second machine-readable marks (150A, 150B) with first and second locations on the moveable support (112).