Advances in Fingerprint Technology.pdf

laser on
image intens.
off
gate
delay
background
fluorescence
on fingerprint
phosphoresc.
off off
off
on on
gate width
Figure 6.2 Time-gating scheme for time-resolved imaging to suppress background
fluorescence.
synchronously with the laser. One then has the scheme depicted in Figure 6.2,
in which the gate width denotes the time the imaging device is on and the
gate delay represents the time interval between laser cut-off and imaging
device turn-on. The delay is needed to ensure that the background fluorescence
has decayed by the time the imaging device becomes active. The imaging
device turns off before onset of the next laser illumination cycle. The
imaging device is typically a gateable CCD camera, namely a CCD camera
equipped with a proximity-focused microchannel plate image intensifier. 9,13
Figure 6.3 shows a typical block diagram of the pertinent imaging apparatus.
The system depicted in the figure utilizes a mechanical light chopper, namely
a rotating wheel with holes in it. This is appropriate when the fingerprint
luminescence has a lifetime of millisecond order. The system in Figure 6.3
calls for a laser of relatively high power, such as the Ar-laser typically
employed in current routine fingerprint work. One can operate with much
smaller lasers in systems in which fingerprints are scanned by a focused laser
beam that is deflected by a pair of rotating mirrors, 14 in much the same way
as the electron beam rastering in the acquisition of television images. Such
systems tend to be slow, but have the virtue, apart from the small laser, of
requiring only a cheap detector, such as a photomultiplier tube, instead of
the expensive gateable CCD camera. For shorter lifetimes, which are beyond
the capability of mechanical devices, an electro-optic light modulator or
similar device would replace the light chopper. Now, however, optical alignment
and electrical biasing of the modulator make for difficulties as well as