LUMAT LB 9507 - 1.3 The Principle of Measurement

Chapterl 1: System Description LB 9507
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1.3 The Principle of Measurement
The emitted light is measured by a selected high-sensitivity, low-
noise photomultiplier. Its spectral sensitivity covers a range
between 390 and 620 nm. All well-established applications in bio-
and chemiluminescence emit in this wavelength range.
The photomultiplier operates as an ultra-fast photon counter. The
photoelectrons released from the photocathode by the light
quanta are multiplied via a dynode chain and trigger a fast pulse
with a rise time of a few nanoseconds at the anode. These pulses
are then amplified by a very fast amplifier. A threshold
discriminator suppresses the low-energy pulses caused by the
noise of the photomultiplier. The single pulses are counted
digitally, their total number being directly proportional to the
emitted quantity of light.
However, the ”relative light units” (= RLU) are used as unit of
measurement for the raw data, and not the number of pulses. The
number of RLUs is found by dividing the directly counted pulses
by ten; moreover, the raw data are multiplied by the RLU-factor
(see Instrument Parameters) which allows compensation of the
inevitable individual fluctuations of the cathode sensitivity of
various photomultipliers.
The kinetics of many chemiluminescence reactions is so fast that
typical counting times are in the range of 1-5 s per measurement.
Thus, sample throughputs of 600 samples per hour are possible.
Bioluminescent samples usually require longer measurement
times.
The individual measurement time can be found in method
publications or in your kit description.