A latent image scanner 10 comprising a three-color laser-diode interferometer 11 having either thickness gauge optical geometry 12 or profiler optical geometry 12a is focused on a spot on a surface of a reference optic 14 having a latent image residue thereon. The interferometry thickness gauge 10 or profiler 10a measures and records the height and location of the spot. The scanner 10 is then focused on a new spot by moving an associated X-Y mechanical translator 13. A complete scan of the surface of the reference optic 14 results in a three dimensional data array correlating height profile to X-Y coordinates of the surface of the reference optic 14. The difference between the absolute height profile and the known height of the reference optic 14 represents the latent image.

FIELD OF THE INVENTION 
The present invention relates to latent image scanning and more 
particularly to the application of a 3-color laser-diode interferometry to 
detect contact latent images which may not be visible to the naked eye or 
to a microscope. 
DESCRIPTION OF THE PRIOR ART 
There are many instances where latent image detection, for example, 
fingerprint, footprint, or the like, is a useful method for personal 
identification. Such methods for identification are frequently used by 
local law enforcement agencies, security agencies and hospitals. Latent 
image detection methods typically compare a detected latent image to a 
reference latent image to determine whether the detected print matches the 
reference. If a match occurs then additional evidence of positive 
identification has been discovered. 
Heretofore, forensic scientists, law enforcement agencies and the like have 
typically "dusted" and or "lifted" the latent images from a surface to 
expose the latent prints, photographed the resulting exposed latent 
images, and compared the results with known references either visually or 
by other means, such as digital graphical techniques. Unfortunately, such 
prior art techniques are destructive of the latent image on the surface of 
an object, thus, requiring that such techniques be carried out carefully 
because only one opportunity to detect them exists. 
No prior art method of detecting latent images is known to have employed 
three-color laser-diode interferometry to detect latent images. The 
three-color laser-diode interferometry of the present invention can detect 
latent images that may not be visible to the naked eye or to a microscope 
without actually contacting the latent image or destroying the sample. 
Three color interferometers may be operated as thickness gauges or as 
profilers by using different optical geometry. Three color interferometry 
and the operation of a three-color interferometry profilers are discussed 
in P. DeGroot, "Three-Color Laser-Diode Interferometer," Applied Optics 
236, 3612-3636 (1991). 
SUMMARY OF THE INVENTION 
The present invention contemplates a non-contact, non-destructive latent 
image scanner to provide a means for detecting contact latent images, such 
as fingerprints resulting from the contact of a finger with a surface. The 
scanner of the present invention can detect latent images that may not be 
visible to the naked eye or even to a microscope. The present invention 
allows the creation of a digital image of the print without destruction 
thereof. The invention achieves non-destructive scanning by using 
three-color laser-diode interferometry methods to detect the change in the 
surface profile, or optical thickness, of a reference optic caused by the 
presence of a latent image. The three-color laser-diode interferometers of 
the present invention can have thickness gauge geometry or profiler 
geometry. 
Thus, as briefly discussed above, the present invention may be used to 
detect fingerprints. When a finger contacts a surface, natural oils in the 
skin are deposited onto the surface. In this way, a polished flat surface 
contacted by a finger leaves a latent print. The latent print changes both 
the surface height and/or the local index of refraction thereof and 
provides a surface that can be analyzed by the three-color laser-diode 
interferometry thickness gauge or profiler. The latent image scanner of 
the present invention can automatically scan over the area of the latent 
image and record the changes in optical thickness or local index of 
refraction as a function of surface position. The natural oils deposited 
on the surface when scanned by the thickness gauge or profiler are 
translated into digital peaks and valleys corresponding to the whorls of 
the latent image oil residue. Thus, the resulting scan data of a completed 
scan is in the form of a digital array of optical thickness or surface 
profile data, and is readily stored in a disk file for future processing. 
A primary objective of the present invention is to provide a latent image 
scanner. 
Another objective of the present invention is to provide a non-contact and 
a non-destructive latent image scanner. 
Another objective of the present invention is to provide a non-destructive, 
non-contact latent image scanner that can translate a surface having a 
latent image into a digital array of optical thicknesses for given surface 
coordinates which can readily be stored on a disk. 
Other objects and advantages will become apparent to those skilled in the 
art from the following detailed description read in conjunction with the 
attached drawings and claims appended hereto.

DETAILED DESCRIPTION OF THE PRESENT INVENTION 
FIG. 1 shows a latent image scanner employing a three-color laser-diode 
interferometer with thickness gauge optical geometry. FIG. 2 shows an 
alternative embodiment of the latent image scanner of the present 
invention employing a three-color interferometry profiler. Referring to 
FIGS. 1 and 2, a non-contact, non-destructive latent image scanner 10 
comprises a standard three-color laser-diode interferometer 11 having 
thickness gauge geometry 12 or profiler optical geometry 12a, a movable 
X-Y mechanical translator 13, and an associated reference optic 
specifically substrate 14. Interferometers 11 having thickness gauge 
geometry 12 or profiler geometry 12a are used to detect the change in 
surface profile or optical thickness of an object. The thickness gauge 
geometry shown in FIG. 1 transmits both reference 15a and object 15b beams 
to the "substrate" 14, such that the reference beam is focused on the 
substrate surface 21a nearest the interferometer and the object beam on 
the surface 21b farthest from the interferometer. The primary advantage of 
this device is that the measurements are independent of stage motions in 
the direction of the measurement, e.g., vertical runout of the translation 
stage in FIG. 1, since such motions are common to both reference and 
object beams. The primary constraint of this device is that the reference 
surface 21a of the "substrate" needs to be polished and transmissive, 
e.g., a polished optical flat or equivalent, to permit the object beam to 
impinge on the measurement surface 21b. However, the measurement surface 
21b need not be polished. 
The profiler geometry shown in FIG. 2 transmits only one beam, the object 
beam 15b, to the substrate on the X-Y translation stage. The reference 
path is contained within the profiler head assembly 23, thus each measured 
profile includes the motions of the stage, e.g., vertical runout. The 
primary advantage of such a device is the ability to scan opaque objects, 
that is the substrate need not be transmissive. Discussion of the 
operation of three-color interferometers can be found in P. DeGroot, 
"Three-Color Laser-Diode Interferometer," Applied Optics 36, 3612-3636 
(1991). 
The latent image scanner of the present invention is ideally suited for 
non-destructive and non-contact fingerprint scanning. When a finger 
contacts a surface of an object, the skin of the finger deposits natural 
oils onto the surface. The oil residue left on the object varies in height 
according to a mirror image of a pattern of whorls of the actual 
"fingerprint" of the finger. Therefore, the height profile along any 
direction of the plane of the surface where the fingerprint lies varies in 
height. 
If a fingerprint is deposited on the surface of a reference optic 14, the 
natural oil deposits from the skin change the height profile of the 
surface of the reference optic 14 in the same manner as the surface of any 
other object with a fingerprint thereon. An image of the fingerprint can 
be stored in the memory 16 of a computer 17 by scanning the surface of the 
reference optic 14 with the latent image scanner 10 and recording the 
height profile of the reference optic 14 with the fingerprint thereon. 
Thus, to scan the reference optic 14 for a fingerprint, the reference 
optic 14 is placed on the high precision X-Y mechanical translator and is 
scanned to derive a reference profile for the optic 14. The scan is 
commenced by focusing the scanner 10 onto a starting spot on the surface 
of the optic 14. The height or thickness of the spot is measured by a trio 
of detectors 18 connected to an input/output communications interface 20. 
The height measurement data is downloaded to computer 17 through the 
interface 20 and a communications link 22 where it is recorded in computer 
memory 16. Because the recorded measurement is stored in a digital form in 
memory 16 it can be readily imported into a database for storage, recall, 
display, digital comparison to other reference latent images in a database 
or any other useful digital data manipulations. After measurement and 
recordation, the X-Y mechanical translator 13 is then moved incrementally 
to a new spot on the surface in a predetermined manner where a new 
measurement and recordation is performed. Thus, by repetitive measurement, 
recordation and movement of the X-Y mechanical translator 13 so that the 
scanner 10 scans over the entire surface area, a profile of the reference 
optic 14 can be derived. 
Once the reference profile is derived, a fingerprint profile can be 
obtained by commencing another scan of the optic 14 with a fingerprint 
thereon. The process is begun by placing a fingerprint on the reference 
optic 14. The scanner 10 is then run through the same scan routine as was 
performed when deriving the reference profile. 
A complete fingerprint scan results in a three dimensional array of scan 
data relating height profile to the position on the X-Y mechanical 
translator 13. FIG. 3 shows a three dimensional graphic representation of 
an absolute fingerprint profile derived from the three-dimensional scan 
data array produced by the scanner 10. The resulting scan profile is an 
absolute measurement of the surface height of the fingerprint on the 
reference optic 14. The difference between the fingerprint reference 
profile and the known height profile of the reference optic 14 for a given 
X and Y coordinate yields the particular characteristics of the 
fingerprint at a given spot on the surface. The difference between the 
absolute height profile and the known profile of the reference optic 14 
for all the coordinates represents the fingerprint. The scan technique 
described above is not restricted to using an X-Y stage, a properly 
controlled beam deflector system scanning the output beam across the 
sample containing a latent image thereon could be used as an alternative 
means for scanning the reference optic 14. 
Although, the invention has been described by scanning the reference optic 
14 first to derive a reference profile, this scan is not necessary to 
practice the present invention. This reference scan will, however, allow a 
derivation of qualitative data by the user of the present invention. 
Whereas the invention has been described for measuring a latent image, such 
as a fingerprint, on the top surface of the reference optic 14, the 
invention will perform equally well if the latent image is located on the 
bottom surface of the reference optic 14. 
Because the scan never contacts the latent image, the scan is 
non-destructive and the latent image can be scanned repeatedly if desired. 
The latent image scanner can be made transportable and compact, thus 
providing equipment for producing rapid identification for an 
investigative team at an investigation site. 
Thus, what has been described is a latent image scanner that can scan a 
surface for residue of a latent image left thereon without contact and 
destruction of the residue. The latent image scanner provides a means for 
precision digital analysis of a latent image residue.