Chapter 2: Theory of Operation 61
Laser Delay
Sample interrogation takes place within the cuvette flow cell. As described in
Optics System on page 36, fiber optic cables are used to direct laser light through
a series of prisms that focus each laser on the stream at a separate position. This
allows optimal detection of fluorescent signals from each laser with minimal
cross-contamination from the other beams.
In a three-laser system, the red laser intercepts the stream first, followed by the
blue and then the violet laser. Because the laser signals are spatially separated,
there is a slight delay between the detection of each laser’s signal (Figure 2-13).
Figure 2-13 Signal separation over time
The delay factor in BD FACSDiva software is used to realign the signals so they
can be measured and displayed on the same time scale. Note that signals are
aligned with respect to the blue laser, so the red laser signals always have a
negative delay value.
time
cell enters and leaves
the first laser intercept
cell enters and leaves the
second laser intercept
cell enters and leaves
the third laser intercept
red
blue
violet
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