Abstract: An optical assembly comprising a light source, at least one sample vessel and a detector. The detector comprises a plurality of detector locations and the vessel comprises a wall and core of relative shape and dimensions adapted to contain a sample for detection and to define at least two spatially separated transmitted light paths, a first wall path which enters and exits the vessel walls only, spatially separated from a second core path which enters and exits the vessel walls and additionally the vessel core. The spatially separated wall and core paths are coupled to individual detector locations on the detector, a module or clip-on device therefor. Methods for detection and uses of the optical assembly are also disclosed.