Page 221 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Measurand Modulation

214 Chapter Ten

(a) Weak scatter measurement (b) Weak absorption measurement

1 1-d 1 1-d
d
Modulated Modulated I
source Beam- source p
d dump to sync. detection
I to
p
sync.detection
Measurement
uncertainty
Measurement Result (V m ) d Day 1 DV Day 2 Measurement Result (V m ) 1 1-d DV

Day 2
0
Time Axis Day 1 Time Axis
Figure 10.1 Dark-field detection, for example of low levels of optical scatter (a), is limited by
noise and spurious scatter from the sample cell. Weak absorption measurements (b) are addi-
tionally limited by changes in source intensity and detection scale-factor.

However, the noise band given in Fig. 10.1a only shows the high frequency
noise, that which causes variations during the period of the repeated meas-
urements. In reality, the change DV may be due to other types of errors, for
example baseline drift. If we perform a measurement without source modula-
tion, errors will result from ingress of ambient light to the detector and from
voltage offset drifts in the DC-coupled detection electronics. However, we have
shown that these errors can be minimized using light source modulation and
synchronous detection techniques. The only remaining sources of baseline
drift are voltage offsets in the postdemodulation DC amplifiers, and these too
can usually be kept to insignificant levels which do not limit the LOD defined
by DV.
This leaves the errors of high-frequency noise seen during the measurement.
The LOD can be improved, essentially indefinitely, by increasing the source
power, increasing detection gain, and reducing the detection bandwidth, albeit
at the expense of response time. In this way, very low turbidity levels can be
and are routinely detected; with 100mW of incident light and a 1pW sensitiv-
ity receiver, we have a relative sensitivity of 10 -11 of the transmitted power, or
in practice the limit of the perfection of the beam-dump and suppression of stray
reflections. This is a “dark-field” measurement; when there is no measurand
property, turbidity in this case, there should be no detected light. Note that vari-
ations in source power, detector sensitivity and channel gain do not severely
affect the LOD. They just affect the accuracy with which the LOD is measured.

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