Page 96 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Interlude: Alternative Circuits and Detection Techniques

Interlude: Alternative Circuits and Detection Techniques 89

of efficient materials will also distort pulse shapes. To quantify the UV signal
free from these effects, it may be necessary to use a narrow-band UV interfer-
ence filter in front of the detector.
Alternatively, you might need to suppress the excitation wavelength and
detect only the fluorescence. This is the requirement for fluorescent detection
used in chemical analysis. It is the basis of many of the most sensitive assays
possible. The big benefit, as discussed in more detail in Chap. 10, is that fluo-
rescent detection provides a dark-field measurement. Unlike the case of trace-
level detection of chemicals by transmission spectroscopy, where almost all the
light passes straight through the sample and we must detect tiny reductions
in transmitted intensity, in fluorescence detection when no sample is present,
we should have no detected light. This allows the detection gain to be greatly
increased to improve the limit of detection (LOD). The key to good perform-
ance in fluorescence measurement is perfect separation of the excitation and
converted wavelengths (Fig. 4.9). Unless a very well-defined wavelength is used
for excitation, spurious longer emission wavelengths may be confused with the
fluorescent signal. Similarly, at the detector the excitation wavelengths must be
very well suppressed. This usually demands two sets of filters, one to clean up
the source and one to limit the detection bandwidth. Multilayer interference
and holographic filters are commonly used, although liquid and absorbing glass
filters can be useful in some cases, as interference filters have additional pass-
bands at longer wavelengths. In addition to requiring excellent filtering,
because the fluorescent processes emit isotropically, it is also desirable to collect

Scattered light contains
excitation line,
Excitation spurious emission,
line filter and fluorescent signal

Source Detection filter
spectrum response
Excitation filter
response Fluorescence

Spurious
emission
Wavelength
Wavelength
Excitation Detector: Maximize collection solid
line angle of emitted light
Figure 4.9 Sensitive detection of fluorescence requires well-defined wavelengths of excitation and
detection. This generally requires both source filters (to remove fluorescence wavelengths) and
detection filters (to remove the source light wavelengths).

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