Latent Fingerprint Image: How to Reproduce an Image of a Latent Print to a Specific Size

Introduction

Most law enforcement agencies either have AFIS or have access to AFIS. Most AFIS systems will accept latent fingerprint images at 1:1 or 5:1 ratios. Creating 1:1 or 5:1 images of a latent lift with a copy camera (e.g., the MP-4 Polaroid) is easily accomplished. Another effective way to acquire a 1:1 image of a lift is through the use of a flatbed scanner. However, occasionally the AFIS operator may encounter a photograph or an image that was not captured at a 1:1 ratio (film or digital), and the image needs to be resized for AFIS entry. The following report describes methods to capture 1:1 photographs with a copy camera and also describes a simple way to resize images that were not taken at a 1:1 ratio.

Copy Camera

To establish a 1:1 ratio on any copy camera, adjust the bellows so that the optical center of the lens is twice the focal length of the lens from the film plane (e.g., 135 mm x 2 = 270 mm). On the commonly used MP-4 camera, this equates to setting the bellows at approximately the 22.1 mark when using the 135 mm lens. The distance from the lens to the surface should also be twice the focal length of the lens. The proper distance is easily established by opening the lens aperture at its largest opening and adjusting the camera position up or down until the image comes into focus. Using a ruler, a test photograph can verify the accuracy of the setting (Figure 1). Minor bellows adjustments should be made as needed to adjust the magnification (extend the bellows to increase magnification). Once the correct position on your MP-4 has been determined, the image should always be accurate when using that lens bellows extension setting. The thickness or height of the latent on the item does not matter. The camera height is adjusted to accommodate the variation in the height of a latent on the evidence.

Scanning Photographs

The original photograph must include a scale in the photo. Align or rotate the photograph so that the scale will be scanned in either a vertical or horizontal orientation. Set the scan resolution as recommended in your AFIS system specifications. (It is necessary to scan photographs at a higher resolution if the photograph is printed at less than a 1:1 ration.) Complete the scan and save the image.

Most consumer imaging software programs are capable of the functions needed to resize an image to the proper scale. Some minor variations in the names of the commands and functions may occur, but the functions should be similar. Click on Open File, find the image you just saved, and open it. (I suggest that you have at least 1" visible on the scale in your image.)

Select the Crop tool from the tool bar. Click and drag corner to corner to make a box. Move the box so that one of its sides lines up with the 1" marker you have chosen to be the edge of your image (Figure 2). Now you can enlarge or reduce the size of the box by clicking the other side of the box and dragging until you have sized the width (or height) of the box to 1" (or whatever size you will need to include all of the desired latent print). You can be more precise if you zoom in. If you are satisfied that the size of the box is accurate, click inside the box and drag the box so that the box is centered over the latent fingerprint (Figure 3). Your image will now be 1" wide.

Figure 1
Scale next to image of a scale that was captured by the MP-4 Polaroid camera
on Type 51 Polaroid film with the aid of two blue and two white photo bulbs;
shutter speed 30 and the aperture set at f/11.

Figure 2
The area to be cropped has been changed to include,
top to bottom, twenty-four (24) of the marks (1 1/2").

Figure 3
Click within the box and drag the box over the latent fingerprint.

Figure 4
Adjust the width (or height) to establish the appropriate dimensions.
Be sure to check the constrain proportions and uncheck the resample box.

Select Image>Image Size and a box will appear. Make sure the Constrain Proportions box is checked and the Resample Image box is unchecked. Click in the Document Size: Width box (or the Height Box, if height of image was used) and change the number to the corresponding measurement (1") that you have chosen for the dimension of your scale in the image (Figure 4). In a second step, use the Image>Image Size function to adjust the resolution. Check (turn on) the Resample Image function and then set the resolution to 500 pixels/inch (or whatever your system requires). With the image now scaled, you can use the print preview function to determine the printing scale (e.g., 100% being lifesize and 500% being a 5:1 ratio). To provide 5:1 ratio images, resize the image with the corresponding measurement to five times the scale used (e.g., 5" in lieu of 1").

Digital Photos of Latent Fingerprints

To minimize the degree of scaling of images captured with a digital camera, I researched distance combined with lens size and printed format. I determined that taking a photo using a 60 mm macro lens at a distance of approximately 10 7/16" from the surface of the lens to the latent print, and then printing it at wallet size, would give me a 1:1 ratio printed image. This works with my camera. You may need to test and adjust the measurements to fit your camera.

The same principal applies to achieve a 5:1 ratio printed image. The distance from the lens surface to the latent print is approximately 4 11/16", and you print the image using the 4" x 6" format. A scale should still be implemented when using this method.

Although most digital cameras will provide the needed image resolution for the 4" x 6" 5:1 photographic print, a camera with at least 1125 x 1625 pixel resolution is needed for the 500 ppi in the wallet-size print.

Poli-View

The Poli-View forensic imaging system (Rolfin Company) captures latent fingerprint images on surfaces where latent prints are difficult to develop. In most cases, the latent will illuminate or fluoresce when the item is processed with a chemical and the light source is directed at the chemically treated item (e.g., DFO on paper or Rhodamine on a superglue-fumed nonporous item). There are other uses for this system.

It is possible to save your captured image on a 3" floppy disk and then directly enter that image into AFIS. This is achieved by capturing the image with a scale placed next to the latent print. The size of the image is adjusted to a 1:1 ratio by zooming in and out with the lens until the scale in your image aligns or matches the size of the actual 1:1 ratio image that was saved in your .bmp file. I will be glad to send a copy of the 1:1 ratio .bmp image on a 3" floppy to those of you who have a Poli-V system and would like to try this approach. The benefit is that there is no loss of quality (as may be experienced when the image is printed and then scanned by your AFIS).

Conclusion

A 1:1 or 5:1 ratio (in the final print) can be achieved with any camera that has a close-up lens, as long as there is a scale in the photo. This can be accomplished in the traditional darkroom or by using suitable imaging software. I prefer a digital camera in lieu of film because of the convenience when the latent fingerprint image is already in the computer, thus avoiding the scanning of photographs.

For further information, please contact:


Bill Lackey
Crime Scene Investigative Unit
Plano Police Department
909 14th StreetPlano, TX 75074
billl@plano.gov

*From the Journal of Forensic Identification Vol. 55, No. 3, May/June 2005
The Official Publication of the International Association for Identification
"Reproduction of the Journal of Forensic Identification, in whole or in part, for noncommercial, educational use is permitted provided proper citation of the source is noted. Reproduction for any other use is prohibited without prior written permission. Requests for permission may be addressed to the editor (of the Journal of Forensic Identification -- jfieditor@theiai.org)."

Article posted: December 9, 2008