Understanding Forensic Digital Imaging

Herbert L. Blitzer,Karen Stein-Ferguson,Jeffrey Huang

• Publisher: Academic Press
• Year: 2010
• ISBN:
  9780080569956
• Topic: Media & Performing Arts
• Subtopic: Digital Media

Understanding Forensic Digital Imaging offers the principles of forensic digital imaging and photography in a manner that is straightforward and easy to digest for the professional and student. It provides information on how to photograph any setting that may have forensic value, details how to follow practices that are acceptable in court, and recommends what variety of hardware and software are most valuable to a practitioner.

In addition to chapters on basic topics such as light and lenses, resolution, and file formats, the book contains forensic-science-specific information on SWGIT and the use of photography in investigations and in court. Of particular note is Chapter 17, Establishing Quality Requirements, which offers information on how to create a good digital image, and is more comprehensive than any other source currently available.

• Covers topics that are of vital importance to the practicing professional
• Serves as an up-to-date reference in the rapidly evolving world of digital imaging
• Uses clear and concise language so that any reader can understand the technology and science behind digital imaging

CHAPTER 1

Introduction to Forensic Use of Digital Imaging

WHY TAKE PICTURES?

Taking pictures is such a normal thing to do that we rarely think about why we are doing it. This is especially true today when cameras are so ubiquitous and easy to use that you can take photos with your cell phone. You don’t have to buy film or have it processed, and you might never print some photos or even show your photos to anyone. So why do it? In one of their most effective advertising campaigns, the Eastman Kodak Company addressed the idea of converting special events into memories, and called those situations “Kodak moments.” The most common reason for taking pictures is to jog our memories at some later time and bring back the feelings of that moment. Humans are very good at using these visual clues to resurrect the whole set of feelings and understandings that the photo preserved. This means that the photographer does not really have to be particularly skilled to get photos that will serve the purpose. The amateur photography industry is predicated on these simple facts:

Photos are very good at bringing to mind whole scenarios from the past

People appreciate reliving certain moments

Photos are easy to take

The cost is very reasonable

This has been the case since the 1880s. Prior to that, in the 1860s, photos were being taken, but the complex nature of the technology at that time limited its use to professional photographers. Photos from the civil war in the United States are still compelling to all who see them, but only Matthew Brady and his colleagues could take pictures back then.

But what about before that time: What were the precursors to photography? Drawings and paintings are the obvious responses. These go back to the Stone Age. Unfortunately they require some skill to produce, and if the individual is not so skilled, an artist has to be hired, so the cost is not right for everyone. Most people can make sketches, though, and in many instances that had to suffice. Some of these were no doubt quite rough indeed. Another approach to preserving memories was with verbal descriptions. These could be told around a campfire and easily embellished over time to suit the purposes of each story teller. Adding melody made it easier to remember the words and captured additional feelings. When writing came into being, the oral history could be rendered as a written history. These were effective, could be extended over long time periods and distances, and although embellishment was possible, it was not quite as easy as with the oral version. Drawings and pictures could be added easily, and decorations could be put on the pages to reinforce the importance of the material. All these memory-jogging techniques continue to this day. One interesting aspect of the memory jogger is that it generally requires that the reader have a memory to jog. That is, he was there at the time of the original event, can envision a reasonable semblance of that situation, or has heard or seen the story so often that he has a mental image of it even though he was never there.

In the world of forensics, some of the factors change. First of all, the memory-jogging mission applies only to the people who were there at the time. For all others, the issue is communication. In this situation, the person who was there at the crime scene, the accident scene, or the disaster scene is trying to convey to others what the scene was like, what was there at the time, where those things were in relation to each other, and what condition the items were in at the time. The simple internal, emotional glow of the memory jogger (assuming a happy event) gives way to a more matter-of-fact communication. The photographer, or someone else who was at the scene, will be asked to confirm that the photo is a fair and accurate representation of what was there at the time. This process is sometimes called visual verification. The people who were there can say, in essence, “I was there and it looked like what you see in the photo.” One could use a sketch in such situations, or the description could be simply verbal (written in a report or transcript) or oral (during testimony). The photo however will contain much more detail. And in most situations, time is of the essence; creating a complete and meticulous written listing of what was there and where it was would be difficult, to say the least. Moreover, it would not convey the ambiance of the situation nearly as well as a photo. Without a photograph, the effect of the lighting will be gone, the comprehension of the level of general orderliness (or confusion) will be lost, and the character of any decoration will vanish. Just imagine a person trying to give an oral description of a tire track impression in sufficient detail so as to allow a determination of whether a confiscated tire made a particular track. The photo conveys the gestalt of the setting, not just a few details.

A photo can convey a comprehensive impression of an environment, and since much will depend upon doing this fairly and accurately, the photographer and subsequent image preparer must do their work with more skill than the average amateur to avoid the bias of the freelance storyteller. The photos must be exposed properly to give the viewer a clear impression of what the scene was like at the time. They must show both the relationships among objects as well as detail in key areas. This is usually accomplished by taking establishment shots from some distance away, medium shots to juxtapose selected items accurately, and close-ups to show important details. Finally, it is important to avoid bias.

Freelance photographers are often out to tell a story as opposed to presenting a balanced set of facts. As a result they will carefully compose photos to do just that. For example, if the story involves enforced separations, they will look for some fencing and then position a subject in front of that fence to help the storyline even if the fence in the photo has nothing to do with the separations. If they are seeking to express slovenliness, they may take photos in a workshop or laundry room at some inopportune time. In general, they have a preplanned story to tell and are looking for ways to convey that message. In forensic assignments, the story is probably not known at the time the photos are taken, and in fact, the photos should be able to play an important part in determining what the true story is. But it must be a fair and accurate story. Then, later, they can be used to help tell that story to a jury or judge.

In the typical forensic photography assignment, the timeline is an important issue. The first representatives of authority on the scene are normally patrol officers. They ascertain the nature of the situation, care for any injured people, and at the same time, protect the area from contamination and change. The technicians, including the photographer(s), will be next on the scene. They have limited time to document the setting as it was found, and to collect samples and items that could be useful in understanding what happened. As they do their work, the scene will start to undergo change, and as they complete their assignment, the rate of change will accelerate. There is no going back. They must get it right the first time. While they are working the crime scene, other investigators are starting to question witnesses. The story will begin to unfold. And later, after a lot of detective work, the story of the situation will start to become clear. This means that the photographer(s) had to do their work without knowing the story their work eventually would help to tell. In most jurisdictions, all the photos taken by the police or crime lab may have to be given to the defense team. So any attempts to bias the story using photos taken before the whole story is known could lead to extremely embarrassing outcomes and the release of a potentially dangerous defendant. Fairness is required.

The most common purpose for photos is to revive memories, the second is to communicate, and the third is to provide a base for measurements. If the purpose for the photos is to recall memories, no special care is required in taking the photos. If the purpose is to tell a story, a sequence of photos will be needed, and it must be possible for viewers of the images to make the connections among the various shots. If the images will be used for making measurements, great care must be taken to ensure that the intended measurements will be valid. The particulars will vary with the anticipated analytical purposes. In many instances, special analytical tools are used to extract information from photographs. Some tools extract dimensions or colors that are attributable to the item that was photographed. More recently, sets of photos have been used to create three-dimensional renditions of objects. In these situations, great care must be taken to ensure that when the photo(s) was taken close attention was paid to the intended measurement process that would follow. A significant amount of image processing, sometimes using complex tools in complicated combinations, might be used to prepare the image prior to measurement. Some of those processing tools might introduce distortions that could make the measurements difficult or inaccurate if not properly applied. In a number of image measurement situations, the image that actually is measured may not be visually verifiable. This arises when the object is not visible to the human eye, and therefore, no one actually could have seen the result prior to processing.

In these situations, the person who analyzed the image has to be able to show that the end result was properly and scientifically extracted from an original photo and that the original photo was a properly and scientifically constructed representation of the original scene or object.

The subsequent chapters of this book explain the basics of the science supporting the most frequently used tools and techniques in forensic photography. The objective is to make the analyst aware of the principles upon which the tools are based, the limitations associated with those tools, and to some degree, why the tools and techniques are designed the way they are. The chapters at the end of the book describe the applicable law and thereby provide guidance to the analyst as needed as he prepares to deliver testimony regarding the work done and the conclusions drawn.

PHOTOGRAPHY AS A SURROGATE

As indicated, photography serves as a surrogate for actually being at the scene. This is generally taken for granted, but in fact a lot of careful design work was required to make the equipment and software suitable for the task. The photographic system employed must capture the optical information from a scene; in most cases this is the visual information. This is the information that a person at the scene would be able to glean visually.¹ The photographic system must then process that information and render it in such a way that a person looking at the image will recognize what he or she is viewing. That is, they can look beyond the photograph and ...