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.