types of photodetectors can be substituted but optimal results result from detectors that are small in size as compared to the waveguide 80 the multimode optical fibers 72 74 and 76 can be longer or shorter or even eliminated depending on the precise configuration and the type of photodetector turning now to fig3 where the sample cell stream 130 is shown the sample cell stream 130 consists of a flow chamber 132 a sheath fluid 134 cells 136 and a core sample fluid 137 in which the cells are suspended during analysis cells travel through the low chamber 132 at a rate of approximately 250 cells s the sheath fluid 134 provides a laminar flow for the core fluid 137 the diameter of cells 136 in the sample fluid 137 is gradually reduced by hydrodynamic focussing downstream from the injection point to provide a single file of cells as shown in fig3 the sample cell stream 130 flows in a downward direction although an upward direction could also be utilized the single cells in sample fluid 137 pass through the laser light beam 20 at interrogation point 138 once the cells have been exposed to laser 20 they continue on to a waste tank not shown the miniature pfcm is ideally used in the analysis of sperm chromatin structure assay scsa scsa data has been correlated with fertility and evaluations regarding sperm quality can be easily and comprehensively derived from scsa tests evenson d p et al 1997 apoptotic human ejaculated sperm contribute to infertility evenson d et al 1995 flow cytometry of sperm chromatin structure as related to toxicology and fertility seventh international spermatology symposium cairns australia october proceedings individual semen samples.