Advances in Fingerprint Technology.pdf

here. The optical or photoreproductive methods include simple photocopying
methods, standard film photographic methods, and digital image capture
methods.
Optical Methods
Photocopying Methods
Photocopying methods are usually used to intensify a weak print. The effect
is caused in part by the high contrast their images are meant to have, but
also by the fact that the spectral sensitivity of their “cameras” is different
from that of the human eye. In the latter case, if the spectral sensitivity reaches
out into the near-infrared region, the contrast of certain background colors
weakens while that of the Ag-PD print does not. (“Ag-PD” is also used as an
adjective to mean silver physically developed.)
Film Photographic Methods
The versatility available to both film and digital photographic methods
includes the choice of illuminating and viewing filters. In fact, use of laser
illumination or illumination from an alternate light source is often used in
photographing evidence. With film photography, one also has a choice of
films of different sensitivities and of chemical development methods. Thus,
photographic enhancement techniques involve a judicious choice of illuminating
filters, viewing filters, film sensitivities, and processing methods to
obtain the optimum contrast from a latent print image.
Digital Imaging Methods
As with film photography, digital photography also uses diverse illuminating
and viewing filters. Similar to the human eye, photographic films, CCD
cameras, optical scanners, and digital cameras have their own spectral sensitivities
and this plays a critical role in the enhancement scheme. However,
the greatest advantage of capturing a digital image is what can be done with
the image afterward. Image processing software that is designed to enhance
image contrast includes Image-Pro (from Media Cybernetics) and Photoshop
(from Adobe). Combining this enhancement capability with the choices of
filtered illuminating/viewing conditions provides a powerful approach to
optimize contrast. For example, the image of an Ag-PD print on a colored
background viewed at two separate wavelengths can be manipulated to
remove the background. This follows because the silver image has the same
level of gray over a large spectral range but not the colored background. Also,
well-defined patterned backgrounds can be removed by Fourier transform
methods. Images can be taken before and after development and compared
to remove background completely.