Forensic Imaging—An Overview

 

George Reis

Photographs have been used in the courtroom dating back to the nineteenth century. In most cases, the photographs were used to document a crime scene or accident scene so that a judge and jury could better understand the testimonies of the witnesses, and verify the veracity of those testimonies.

As technology improved, the use of photographic evidence escalated and expanded to include analysis in the forensics community — a wide range of analysis opportunities that ranged from visualizing bullet trajectories with lasers to enhancing fingerprint images for comparative analysis.

This article will examine the history of imaging technology in forensics, how imaging is used in both documentation and analysis, and how digital imaging plays a significant role.


Figure 1. The Channel Mixer Adjustment Layer in Adobe Photoshop was used to isolate the cancellation stamp.


Figure 2. This image illustrates a deblurring of a fingerprint image using the Photoshop plug-in Fovea Pro by Reindeer Graphics. This software uses a deconvolution algorithm to achieve the deblurring for either focus blur or motion blur.


Figure 3. Stacking eight images as layers and adjusting the opacity of each layer to achieve frame averaging reduces noise in the image. The technique uses an opacity of 100 percent for the bottom layer, 50 percent for the next layer, 33 percent for the third layer, etc.

 

A Brief History of Forensic Imaging

The first recorded case in which a photograph was used in U.S. court was in 1859 "to prove that a document of title for a land grant was, in fact, a forgery." The first use of an accident scene photograph occurred in 1875. A tintype of an injured person was admitted into evidence in 1879. Photographic comparisons of bullets were used in a 1902 case, and in 1910, photographic speed recorders were used to detect speeding drivers. In addition, photographs of fingerprints were used as early as 1911 (although fingerprints had been used for identification prior to this). Ultraviolet photography was used in court as early as 1934. Color photography first made its way into court in 1943; although the use of color photography was still contested as recently as the early 1960s.

When novel scientific methods are to be used in the presentation of evidence in court, attorneys may challenge the technology by calling for a Frye hearing or a Daubert hearing (depending on the jurisdiction) to determine if the technology meets evidentiary requirements. Frye and Daubert hearings have focused on many areas of scientific evidence, including fingerprint individualization, the use of DNA, and digital imaging. When a Frye or a Daubert hearing is held, a judge determines if the science being challenged meets the criteria set forth by these standards. General acceptance of the technology within its respective field is the criterion for a Frye hearing. For a Daubert hearing, a threshold standard is used rather than the general acceptance criterion in Frye (Hak, 2005).

The first Frye hearing on digital imaging technology was held in 1991 in Virginia and resulted in a decision to allow digitally enhanced fingerprints to be used in a homicide case. Since then, there have been two additional Frye hearings related to digital imaging technology.

Today in forensics we see documentation photography used at a majority of crime scenes and traffic accidents. A wide variety of image analysis techniques is used for this photography, including UV and IR photography of gunshot residue and blood stains; fluorescing dye stains; fingerprint enhancements; QTVR (QuickTime Virtual Reality) panoramic images; video analysis; etc. We even have seen a return of the "photographic speed recorders" in the use of red light cameras. Photography and imaging technologies are an important component of many cases, for both documentation and image analysis.

Documentation of Evidence

Just as forensic photography was first used to document evidence for criminal cases and traffic accidents, that documentation function remains its principal use today. Police officers, detectives, and crime scene investigators use photography to document a crime scene and the evidence at the crime scene.

In the courtroom, images are used to support the testimony of a witness. A witness to a crime, a police officer, a doctor, a pathologist, a criminalist or other expert, even a lay witness, will use photographs to help illustrate their testimonies. Expert witnesses use photographs to illustrate how they reached their conclusions.

The photographs may be used to illustrate the condition of objects, the relationship of an object or objects to others, injuries, lack of injuries, etc. Thus, some standard approaches to documentation photography have emerged and are frequently used.

The first rule is: Do not disturb the evidence or scene until an initial set of photographs has been taken. While this may be the goal, it does not always happen. If, for instance, three police officers, two detectives, and a field sergeant have been through the scene before it is photographed, evidence likely has been disturbed-footprints have more than likely been trampled upon, and the relationships between significant items may have been compromised.

The second rule is: Shoot wide, mid-range, and close-up photographs. The wide shot provides some orientation to the scene and its relationship to the surrounding area. Mid-range photographs will show relationships between objects in the scene, such as blood spatter, the victim, and the weapon. Lastly, close-up photographs will document the location and size of a wound or injury, the blood spatter pattern on a wall, or a fingerprint on a countertop.

In most cases, the role of forensic photography is to document the items of evidence, the relationships between objects, and the overall scene; however, it is not used to record or duplicate the lighting conditions at the time of the event. Since the photographs do not need to duplicate the lighting conditions at the scene, the crime scene investigator or forensic photographer can control the lighting and exposure to provide the best detail of the other elements.

Reproduction of the light conditions present at the time of a traffic accident or other incident is known as photographic reconstruction. This area of photography requires an expertise that is usually provided by civil evidence photographers. Providing precise point of view, measuring light in multiple areas, reproducing conditions photographically, and understanding the human visual system are all part of the specialty of reconstruction photography.

Image Analysis and Digital Imaging

Comparisons of physical objects are an important component of the forensic investigator's work. For example, when a suspect is recorded on video during the commission of a robbery, the investigator may compare a suspect's clothing to video images of that clothing. Also, an investigator may make a comparison of a footwear impression at a crime scene to a suspect's shoes, or comparison of a latent fingerprint on a piece of evidence to the victim's prints.

When forensic images are used in comparative analysis, there is frequently a need to enhance detail in those image(s) so that as much information as possible is considered in the analysis. This detail enhancement is generally accomplished with digital techniques, using a forensically valid workflow.

Because every municipal, county, and state law enforcement agency in the United States is autonomous, there are no protocols or standards with which all agencies are required to comply. There are, however, Federal and State Rules of Evidence, case law, and recommended guidelines published by various public and private entities.

The Rules of Evidence place the integrity of the image on the testimony of the witness. That is, they don't require specific file encryption, image security, matching hash values (values calculated from digital data that serves to distinguish them from other data), file formats, etc., for digital imaging. Further, they do not require any specific format, filing method, resolution, or retrieval methods for film. In essence, the Rules of Evidence only require that the witness be able to testify that the image is a fair representation of the subject. If opposing counsel has reason to believe otherwise, he or she can personally analyze the images and offer arguments to support his or her opinion.

Images associated with case law fall into two categories-those that are challenged and those images that are unchallenged and subsequently allowed into evidence. In court, if a challenge to an image is made based on the technology used, or based on chain-of-custody issues, the court may hold a separate hearing to determine if this evidence meets court requirements. Since 1991, three challenges have been made to digital imaging technology. In each challenge, the images were allowed as evidence.

These challenges all have been related to image analysis, rather than to scene or evidence documentation. As mentioned above, image analysis frequently involves image enhancement as a first step. And, while there are no required protocols to follow, most recommendations state that a valid forensic workflow should provide a secure method for archiving images; provide enhancement and analysis that changes only the quality, not the content of the images; include notes or an audit trail for any significant changes to an image; use processes that are repeatable with predictable results; and provide training for all personnel working with images.

A variety of applications are used in image analysis and enhancement. Adobe Photoshopô is a key application and used by many forensic image analysts. Avid and Ocean Systems, among other companies, provide systems for video analysis. Plug-in filters such as FoveaPro and Optipix from Reindeer Graphics offer additional analysis capabilities within Photoshop.

Some of the key techniques used in image analysis include color adjustment, use of channels to isolate detail in specific color ranges (Figure 1), deconvolution to correct for focus or motion blur (Figure 2), histogram adjustments to separate tonal ranges frame, averaging to reduce image noise when multiple images are available (Figure 3), and Fast Fourier Transform to eliminate repetitive patterns and reverse projection for calibration and subject measurement (the Fourier Transform separates the image into its frequency components. This allows one to do many processes to the image, and the one most relevant in forensics is identifying repeating patterns and filter them out of the image).

Conclusion

The field of imaging forensics has come a long way since its beginnings in the mid-nineteenth century. Yet, its primary application, the use of images to illustrate the testimony of a witness, is still the cornerstone of photography in a forensic workflow. The introduction of image analysis, color photography, and digital imaging have provided new tools, and broadened imaging capabilities; the basics, however, remain the same.

About the Author

George Reis is President of Imaging Forensics, providing consulting, training and litigation support in forensic image analysis. He was with the Newport Beach Police Department for 15 years as a forensic photographer, latent print examiner and crime scene investigator. He has provided training and consulting services to law enforcement agencies throughout the United States, including the Secret Service, U.S. Army Crime Lab, and state, county, and local police departments. Visit his website at www.imagingforensics.com.

Reference

Hak, Jonathan W., 2005. Forensic Video Analysis and the Law. Presentation at the 2005 LEVA Training Session.

Bibliography

Miller, L. 1977. Police Photography. Anderson Publishing Co. Cincinnati, Ohio.

Copyright 2005, The Journal of Biocommunication, All Rights Reserved

Table of Contents for VOLUME 31, NUMBER 3