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Advances in Fingerprint Technology.pdf

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Ar-laser<br />

image monitor<br />

lens<br />

light chopper<br />

computer<br />

pr<strong>in</strong>ter<br />

Figure 6.3 Block diagram of time-gated imag<strong>in</strong>g apparatus.<br />

<strong>in</strong>creased expense. For time-resolved imag<strong>in</strong>g <strong>in</strong> such situations, a different<br />

approach is often taken. Time-gated imag<strong>in</strong>g typically perta<strong>in</strong>s to the doma<strong>in</strong><br />

of lifetimes longer than roughly 1 µs.<br />

Basics of Phase-Resolved Imag<strong>in</strong>g<br />

gateable<br />

CCD<br />

camera<br />

filter<br />

liquid<br />

light<br />

guide<br />

sample<br />

Note that a f<strong>in</strong>gerpr<strong>in</strong>t detection system based on the phase-resolved concept<br />

has yet to be developed. However, this is just a matter of time, given that<br />

phase-resolved imag<strong>in</strong>g systems have been <strong>in</strong> operation for some time <strong>in</strong><br />

applications such as cell microscopy. If one modulates the <strong>in</strong>tensity of the<br />

illum<strong>in</strong>at<strong>in</strong>g laser light s<strong>in</strong>usoidally <strong>in</strong>stead of on-off as <strong>in</strong> Figure 6.2, the<br />

thus excited lum<strong>in</strong>escence is then also s<strong>in</strong>usoidal <strong>in</strong> <strong>in</strong>tensity but is delayed<br />

with respect to the excitation by a phase that is related to the lum<strong>in</strong>escence<br />

lifetime. There is also a related lum<strong>in</strong>escence demodulation. The situation is<br />

shown <strong>in</strong> Figure 6.4, with normalized excitation and emission. φ is the phase<br />

and m the demodulation. The effect of the angular modulation frequency ω,<br />

namely 2πf (where f is the modulation frequency), on the phase and demodulation<br />

is depicted <strong>in</strong> Figure 6.5. Multiple lum<strong>in</strong>escence lifetimes <strong>in</strong> phaseresolved<br />

spectroscopy and imag<strong>in</strong>g can be dist<strong>in</strong>guished by vary<strong>in</strong>g the modulation<br />

frequency. Modulation frequencies of hundreds of megaHertz are<br />

readily obta<strong>in</strong>able. Thus, lifetimes of nanosecond order become accessible.<br />

It is only necessary that the analyte lum<strong>in</strong>escence have a lifetime significantly<br />

different from that of the background for phase-resolved imag<strong>in</strong>g to suppress<br />

the background. However, the f<strong>in</strong>gerpr<strong>in</strong>t lum<strong>in</strong>escence lifetime will generally<br />

be longer <strong>in</strong> practise than that of the background because shorten<strong>in</strong>g of<br />

lum<strong>in</strong>escence lifetime is generally attended by decrease <strong>in</strong> lum<strong>in</strong>escence<br />

quantum yield (i.e., decrease <strong>in</strong> lum<strong>in</strong>escence <strong>in</strong>tensity). The desired f<strong>in</strong>gerpr<strong>in</strong>t<br />

lum<strong>in</strong>escence lifetimes typically range from about 10 ns to about 1 µs<br />

(as compared to background fluorescence lifetimes of roughly 1 ns). A basic

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