The present invention relates generally to the field of biochemical laboratory instrumentation for different applications of measuring properties of samples on e.g. microtitration plates and corresponding sample supports. The object of the invention is achieved by providing an optical measurement instrument for photoluminescence, chemiluminescence and/or AlphaScreen measurements wherein different optical modules are used for alternative measurements. The excitation pulses for the alternative measurements are guided via two different routes to optical modules, the routes reaching the module in different angles. This way it is possible to use alternative radiation sources without optical switches and without changing the optical system. The object of the invention is further achieved by providing an additional lens in the optical module when a thermo plate is used. This way it is possible to achieve a correct optical focus in different measurement modes not depending on the use of the thermo plate.