Fluorolog-3 v.2.2 (10 Sep 2002)
Introduction to Lifetime Measurements
8-2
Lifetime measurements
The lifetime software is based on acquisition of frequency-domain lifetime measure-
ments. In this technique, the excitation light is sinusoidally modulated. The emission
from the sample is a forced response to the excitation, and therefore is modulated at the
same frequency as the excitation light. The excited state has a finite lifetime. This
means that the modulated emission relative to the excitation is offset by a phase angle,
φ
, and is demodulated. The following figure shows various parameters used within the
frequency domain for lifetime measurements.
Parameters used in frequency-domain measurements.
The figure shown above illustrates a sinusoidal excitation with a circular frequency,
ω
,
an amplitude, AC
x
, and offset by a DC voltage, DC
x
. The sample responds with an
identical circular frequency,
ω
, damped amplitude, AC
f
, and offset by a DC voltage,
DC
f
. In a fluorescence-lifetime measurement, the phase angle,
φ
, and demodulation fac-
tor,
m
, are measured at each frequency and used to calculate the phase lifetime (
τ
p
) and
modulation lifetime (
τ
m
). The above figure shows that the relative amplitude of the
emission (AC
f
/ DC
f
) is smaller than that of the excitation (AC
x
/DC
x
). The demodulation
factor,
m
, is AC
f
/ DC
f
divided by AC
x
/DC
x
.
The equations used to calculate the phase (
τ
p
) and modulation (
τ
m
) of lifetime meas-
urements are:
Excitation
Emission
0 180
360 540
Degrees
φ
1
DC
x
DC
f
AC
x
Intensity
AC
f
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