The invention provides an apparatus including (a) a frame having a boundary plane; (b) a flow chamber supported by the frame, the flow chamber placed a distance from the boundary plane; (c) a radiation source, the radiation source directed away from the flow chamber and away from the exterior side of the boundary plane, and (d) a first reflective surface placed to direct a radiation beam in a path crossing the boundary plane to the flow chamber; (e) one or more reflective surfaces placed to direct a radiation beam from the radiation source to the first reflective surface, the path from the radiation source to the flow chamber being at least 1.5 times the distance from the flow chamber to the boundary plane.