Figure 1
Photograph of dusty shoe impression on automotive window tinting film.
Abstract: Electrostatic devices use a high-voltage electrical current to charge a surface and attract dust particles left from shoe outsoles and other objects to a metallic film. Placing this film in direct contact with a conductive substrate, such as a metal vehicle, can cause damage to the film and increases the risk of injury to the analyst and damage to property. The use of a nonconductive film barrier allows for the use of the metallic film and the electrostatic dust lifter in obtaining shoe impressions from metal surfaces such as automobiles and countertops.
Electrostatic dust print lifters are useful devices for lifting dusty shoe impressions from a variety of surfaces including carpeting, fabric, wood, and human skin (personal observation) [1, 2]. These devices use a high-voltage power unit to charge a metallic film, which is placed over the suspected impression. The electrostatic charge causes the impression to adhere to the black-colored metallic film, giving the impression the best possible contrast. Documenting shoe impressions on vehicles, however, can be challenging. Automobiles can present curved surfaces with limited surface area, making tripod setup for photography difficult. Photography can also be difficult in daylight or adverse weather conditions. Moving vehicles to an indoor processing garage can also damage impressions during transport. For these reasons and others, the use of an electrostatic dust print lifter is ideal for recovering shoe impressions from automobile surfaces.
Two different electrostatic devices were used to lift impressions in both casework and research. A Kinderprint (model 3C, 120v/60hz) unit with attached cable probes and a Sirchie (model ESP900) unit with enclosed probe connectors both produced excellent and comparable results. Axius window tinting film (#53010 in “Limo Dark” shading) was used as a nonconductive film barrier (Figure 1). The film is rated as 20% VLT (visible light transmission) and is typically sold in auto supply stores. Metallic lifting film can be purchased in sheet or roll form from several forensic supply companies. Legal-size lifting sheets work well for most applications. Both types of film can be further cut to conform to conditions found at the crime scene. Several different electrostatic lifting devices are currently marketed in both hand-held and box configurations.
Visible footwear impressions should be marked near the toe and heel portions with drafting dots or masking tape so the window tinting film can be centered accurately. The window tinting film should be cut large enough to allow for a 2" to 3" border edge once the metallic film is centered over the impression (Figure 2). Once the window tinting film has been centered over the shoe impression, it can be secured on the corners with drafting dots or masking tape. The metallic sheet is then placed on top of the window tinting film and is also secured with drafting dots or masking tape. The electrostatic dust lifting device is then set up as it would be for use on other types of surfaces with one exception: In many instances, the metal substrate can be used as the grounding plane, negating the need to set up a grounding plate or sheet. Some vehicles with nonmetallic panels may still require the use of a grounding plate, depending on the location of the impression. Because of the conductivity of metallic substrates, it is recommended that the device be turned to its lowest setting when beginning the operation. The setting can be increased slowly to achieve the desired attraction between the metallic sheet and the substrate. When using a model similar to the Kinderprint 3C, start with the grounding rod several inches away from the lifting film and slowly move it toward the film until the desired attraction between the lifting film and the substrate is achieved.
Geller and Warrington [3] report that stun guns can be modified to provide results that are similar to those of the types of lifting devices described above. One disadvantage of the stun gun technique is that, without major modification, it is difficult, if not impossible in some cases, to regulate the output level of most stun guns. Although the use of stun gun devices on nonmetallic surfaces is possible, the inability to control the output level may produce undesirable results when lifting from highly conductive substrates. The authors also point out that stun guns are illegal, even for police personnel, in several states and large municipalities. It is recommended that you check your state and local statutes before purchasing a stun gun for use as a lifting device.
Some situations may offer a limited time to develop the impression, or window tinting film may not be obtainable. In these limited situations, the analyst should consider the use of another type of barrier such as black plastic garbage bags (Figure 3). One major disadvantage of this type of plastic barrier is the presence of creases and small surface defects that are usually found on the surfaces of plastic bags. These defects may limit the ability of the film to make adequate contact with the shoe impression and create unacceptable shadows during subsequent photography. If a garbage bag must be used, it is recommended that the crease be placed in the arch area of the impression. When using the rubber roller to smooth the film out, start at the crease and roll out toward the edges. Thicker black plastic sheeting, such as heavy-duty garbage bags and landscaping weed barriers, has yielded poor results.
Electrostatic dust print lifters are an effective tool for obtaining dusty shoe impressions on a variety of surfaces. The electrical nature of these devices, however, has made their use on metal surfaces challenging. Placing metallic lifting sheets directly on metal surfaces can cause damage to the film, the evidence, and personal property. The improper use of these devices can also increase the risk of injury to the analyst. Bodziak [1] recommends the use of a Mylar barrier between the film sheet and the metal surface. This technique can yield excellent results; however, some smaller agencies may have difficulty finding clear Mylar sheets, especially on short notice. Mylar may also run at higher costs than window tinting film, depending on the source supplier and quantity ordered. In addition, the metallic lifting sheet must remain attached to the Mylar sheet for viewing and photography. For cost-conscious agencies, the window tinting film offers some distinct advantages. The film can be found at many retail auto supplies stores and the metallic lifting sheet can be reused on several impressions before being discarded. The application of window tinting film between the metallic lifting sheet and the shoe impression has proven to be effective in obtaining dusty shoe impressions. Although photographic methods may generally yield better results, the location of the shoe impression, lighting and weather conditions, and the camera equipment that is available may all limit the success or practicality of these methods. Electrostatic dust print lifters are simple to operate and may yield excellent results. Electrostatic lifts are also generally regarded as a nondestructive collection method that allows for the use of other processing techniques, if desired. As in every case, the quality of the impression, the surface it is on, and the skills of the analyst are all major factors that determine the success of the collection and preservation of dusty shoe impressions. Practice with this method is highly recommended before attempting it on casework. Findings of additional nonconductive barriers should be reported to augment the possible tools available to the crime scene analyst.
*From the Journal of Forensic Identification Vol. 55, No. 5, September/October 2005
The Official Publication of the International Association for Identification
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