Abstract: A mechanism for positionally restraining a microplate is disclosed. The mechanism is defined by a base having at least one surface with a receptacle for the insertion of a microplate into the base. Supports and/or positioning structures on a surface of the base have point contacts to restrain movement of the microplate in a stable position for repeatable optical detection measurements. The supports and/or positioning structures permit insertion of the microplate for an initial measurement, removal of the microplate for analytical manipulation, and re-insertion of the microplate to an exact position so as to allow analysis of precise comparative measurements by an optical reader.