Computer Lab: Examination of Digital Fingerprints

Objective: To examine and identify the major ridge pattern types using digital imaging software and digital files of fingerprint impression evidence.

Background: Latent prints or fingerprints are one of the most common forms of forensic evidence routinely collected at a crime scene. Sometimes this evidence is hidden or latent and needs to be processed or developed with dusting powder to make the ridge patterns visible.  Since every human fingerprint is unique, a suspect’s fingerprint found at a scene of a crime can be used for comparison and for identification.  Often, only a partial print is found at the scene. Forensic scientist will than use computer assisted software to process the print and help with the identification. Sometimes this computer system allows the examiner to search large databases of digital prints for a comparison and hopefully an identification. One such computer systems used nationally by the F.B.I. is called the Automated Fingerprint Identification System or A.F.I.S. for short.

          In the following investigation, you will act as a fingerprint examiner. You will be required to identify a print that was found on the neck of a broken bottle used to knock the victim over the head during a party. The examiners found numerous prints at the scene. A group of 7 people at the scene were asked to have their prints inked for comparison. The inked prints were scanned into a computer as a digital image file. As the fingerprint examiner, you will need to compare the print found at the scene with the inked prints of the suspects and identify the suspect who left their fingerprint on the bottle.

Procedures:

1.)   Open Scion Image Software , it should be located in the student folder on the desktop.

2.)   Maximize the window.  Open the file 08Fingerprints.tif  found in the Largeprints folder in your CDROM.   

GOTO File…Open…D:/ ForensicsIPX/Largeprints/08fingerprints.tif.

3.)   Examine all the prints collected in the investigation.

Question: Whose fingerprints do you think the crime scene most resembles? (answer on the data sheet)

4.)   Close 08fingerprints.tif

5.)   Open the all the images numbered #1 to #7. You will see the crime scene print and all the suspect’s prints. Arrange them in a way that you can view them all at the same time, GOTO Windows… Tile Images.(Ctrl-t)

6.)    Open the files called 10Fingerprinttypes.tif and Info.tif

7.)  You will label each of the suspect’s prints with the ridge type pattern.

 Question: What are the 3 main types of ridge patterns? ( answer on your datasheet)

8.)   For each of the 6 suspects and the crime scene print, classify the fingerprints by ridge pattern type. Use the eyedropper  to pick up the red color from the LUT (this will make your text red. Choose the text tool from the tool window   . Place the cursor on the fingerprint you wish to label. Label each print as a loop, arch or whorl. WARNING: YOU WILL NOT BE ABLE TO REMOVE TEXT ONCE YOU HAVE TYPED IT IN AN HIT RETURN.

Question: What is the ridge pattern type of the print from the crime scene? (answer on the data sheet)

9.)   Show your labeled images ( 7 total) to Ms. Evans. Have her initial your data sheet.

10.)                      Close the windows of the suspects you can eliminate because they do not have the ridge pattern type found at the crime scene.

11.)                      At this point you may think you know who was holding the broken bottle. To help make a positive identification, you need to gather more data from the fingerprints. You will mark and label key ridge features and calculate ridge count.

12.)                      Activate the Info.tif file by clicking on the window.

13.)                      Use the < and > keys to move through the slices of the stack. For example,  would be the 3^rd slice in the stack of 5, 3 of 5 slices.

14.)                      When evidence in presented to the jury, labeled diagrams of fingerprints are used. The key features of a fingerprint that are useful to individualize the evidence include the core, the delta, its ridge count, and some points of comparison.

15.)                      Move to the second slice in the stack . The core is the central point in the fingerprint pattern. The lines in the fingerprint pattern are called the ridges. The delta  is the point closest to the core where the ridges diverge or pull apart. Cores and deltas are unique features of loops and whorl type patterns. Arch patterns do not have cores and deltas.

16.)                    Ridge Count Procedure: Move to the third slice in the stack . The number of ridges between the core and delta is called the ridge count.  Use the histogram tool  to count the number of ridges between the core and the delta on slice # 3. Do this by selecting the histogram tool in the tool window. Drag the cursor from the core to the delta in the image. Release the mouse. The histogram plot window will pop up. Draw what you see in the plot window on your data sheet. Count the number of ridges. Record on your data sheet.

17.)                    Use the ridge count procedure from #16 to count the ridges in the crime scene print. Record on your data sheet.

18.)                    Use the ridge count procedure from #16 to count the ridges for each of your 2 main suspects. Record on your data sheet.

Question: which suspect has a ridge count that is closet to the crime scene print?

19.)        Minutiae Procedure: Move to the 4^th slice  in the info.tif image. Ridges also have unique characteristics that help an examiner individualize the fingerprint further. These characteristics are used for comparison and matching. They are called minutiae. Points were ridge lines split apart in a fingerprint are known as bifurcations . Another type of minutiae is an enclosure  or “eye”. Any of these can be marked and identified as their location and number are unique to the fingerprint.

Typically, between five and ten points of comparison are selected to have a match. Label your crime scene print and your PRIMARY suspect print with 3 points of comparison or minutiae/on each image. Show the labeled image to Ms. Evans. Get her initials  on your data sheet.

20.)        Which suspect is a match to the crime scene print? Answer on the data sheet.