Advances in Fingerprint Technology.pdf

fingerprints has been instrumental in the solution of many major cases (e.g.,
Polly Klaas, Nightstalker). To the best of this author’s knowledge, laser detection
holds the record for the oldest fingerprint developed and identified in
a criminal case over 40 years old. 10 In the author’s laboratory, ridge detail of
fingerprints dating back to the American Civil War has been developed by
laser. The photoluminescence approach has found use also in other criminalistic
applications, 9 such as document examination, fiber analysis, locating
seminal fluid, and various instances of trace evidence detection. 11 The current
state of the methodology has been dealt with extensively in journals as well
as books, including the first edition of Advances in Fingerprint Technology. 12
Thus, the focus in this second edition is on upcoming methodologies
designed to permit detection of latent fingerprints on articles that display
very intense background luminescence which masks the fingerprint luminescence
obtained with the current procedures. This is a pervasive major problem
that still plagues the photoluminescence detection of fingerprints and,
indeed, all photoluminescence-based analytical techniques.
The Essence of Time-Resolved Imaging
Whether in analytical chemistry, biotechnology, or criminalistics, the detection
of a weak photoluminescence in the presence of a strong background
fluorescence is carried out following the same basic principles. They make
use of the difference between the luminescence lifetimes of the background
and analyte emissions. We begin by inquiring into the origin of this lifetime
difference.
Luminescence Lifetimes 9
The emission of light by substances can have a number of causes, among
them heat (incandescence) and chemical reaction (chemiluminescence), as
in the luminol reaction with blood. For our purposes, the most important
origin of luminescence, however, is the prior absorption of light (excitation)
to give rise to photoluminescence. On the basis of luminescence lifetime (i.e.,
the decay time of the luminescence intensity once the excitation is cut off)
two categories of photoluminescence were distinguished long before the
quantum-mechanical underpinnings were understood; namely fluorescence
of short lifetime and phosphorescence of long lifetime. The slowness concept
distinguishing the two light emissions has evolved over time (toward shorter
lifetimes of phosphorescence). The distinction between fluorescence and
phosphorescence is placed on a less arbitrary footing when one examines the
quantum-mechanical origin of the photoluminescence of typical organic
molecules — which we are mostly concerned with in fingerprint work. For