Chapter 2: Theory of Operation 59
Electronic Processing
As cells or other particles pass through the focused laser beams, they scatter the
laser light and can emit fluorescence. Because each laser beam is focused on a
small spot and particles move rapidly through the flow cell, the scatter or
fluorescence emission has a very brief duration—only a few microseconds. The
PMTs convert this brief flash of light into an electrical signal called a pulse
(Figure 2-12).
Figure 2-12 Anatomy of a pulse
A pulse begins when a particle enters the laser beam. At this point, both the
beam intensity and signal intensity are low.
The pulse reaches a maximum intensity or height when the particle reaches the
middle of the beam, where the beam and signal intensity are the brightest. The
peak intensity, or height of the pulse, is measured at this point.
As the particle leaves the beam, the pulse trails off.
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