CL is often described as a dark-field technique:
the absence
of a strong
background light level, as
in absorption spectroscopy,
reduces the background
signal,
improving detection limits.
Small
changes in a small signal are more easily observed
than small changes
in the presence of a large signal.
As a principle,
it is possible to measure CL in a
fluorometer by turning off the excitation source.
What is actually needed,
is a photomultiplier tube,
sufficiently sensitive in the spectral region of interest.