Hidden Evidence
Written by Jessica Zarate, M.S. and Jodi Lynn Barta, PhD   


Blood-based impression evidence is often difficult to recover and visualize on certain substrates, such as human skin, carpet, and clothing. These difficulties arise for various reasons: substrate color, background pattern, texture, levelness, porosity, and impression age. The lack of contrast between the impression and dark-colored substrates poses difficulties in the detection of some impressions, even when using an alternate light source (ALS). Furthermore, many background patterns and substrate textures can mimic ridge details found in bloody impression evidence, obscuring the quality of the impression when visualized on the substrate.

Even more cumbersome are the large or immovable objects encountered at crime scenes. This evidence often requires chemical enhancement methods that can be laborious and time intensive, as well as unnecessarily expose the practitioner to toxic chemicals. It is best practice to conduct chemical enhancement in the controlled setting of a laboratory, yet this requires the disassembly of large items to be transported to the lab, which is not always possible. In the case of immovable objects, chemical enhancement must be conducted at the scene, and photographs of the impression are the only recovered evidence.

Another challenge is the enhancement of bloody impressions on human skin, either living or deceased. In the case of impression evidence from human skin, living subjects must be protected from exposure to toxic chemicals often used for enhancement due to the porosity of human skin. In addition to the toxicity concerns, chemical enhancement of bloody impression evidence on other porous substrates may often result in complete staining of the substrate background, leading to potential alteration or destruction of impression evidence. As a result of these difficulties, bloody impression evidence on these problematic substrates may be overlooked or not optimally enhanced.

Currently, the most effective method for recovering non-blood latent impression evidence is through the use of dusting powder as a detection method followed by lifting with various tapes, gel lifters, or casting compounds. These methods are highly effective for the simple reason that they remove the impression from the substrate and in turn eliminate substrate variables, such as color, background pattern, and texture that compete with the visualization of impression details. They also allow for the recovery of lifted impressions from large, non-level, or immovable objects. The recovery of the impression from these substrates is less cumbersome and timely than disassembly, and also eliminates transportation and storage concerns that arise with the large items.

One product that allows the recovery of bloody impressions from different substrates for analysis is a lifting strip from Zar-Pro. The Zar-Pro Blood Lifting Strips employ the same methodology that is so effective for non-blood latent impressions, but are designed specifically to bond with proteinaceous impressions, such as blood. Bloody impressions can be recovered from any substrate regardless of color, background pattern, texture, porosity, or non-level surface. The Zar-Pro membrane is highly sensitive to proteinaceous materials, lifting the bloody impression from various substrates onto the white background of the lifting strip to provide necessary contrast for analysis without competing variables from the substrates. Lifted impressions are subject to metal-enhanced fluorescence, which further optimizes the contrast of the impression to a bright fluorescent color on a darkened background when visualized with an ALS. This novel method is particularly useful for aged or diluted bloody impressions because of its ability to optimize contrast for visualization.

To demonstrate the effectiveness of the Zar-Pro lifters, a series of impressions were deposited on problematic substrates, such as human skin, carpet, cotton, and denim. Photographs of the impressions were taken pre- and post-lift, as well as after fluorogenic enhancement. In some cases, the impression details were not readily visible on the substrate prior to lifting; only the presence of blood was observed under both normal (Figures 1.1 and 4.1) and alternate lighting (Figures 2.1 and 3.1). The Zar-Pro lifters were cut to fit the size of the various substrates and then activated with the Zar-Pro activator and placed firmly onto the bloodied substrates. The lifters were then removed from the substrates to reveal the lifted impressions now permanently affixed onto the lift membrane for visualization (Figures 1.2, 2.2, 3.2, and 4.2). In most cases, it was not until the impressions were lifted from the substrates that the quality of the impression details was evident. Once dried, the lifted impressions were enhanced with a Polilight Flare Plus using an orange or yellow barrier filter at 505 nm (Figures 1.3, 2.3, and 3.3). The fluorogenic capability of the lifters allowed for the visualization of even the faintest of bloody impression evidence. In addition, the lifted impressions are preserved long-term on the membrane with consistent fluorogenic capabilities.

The Zar-Pro lifters can optimize the recovery of bloody impression evidence. It is important that practitioners in the field of forensics are not overlooking hidden impression evidence, even though the impression may not be readily visible to the examiner. Any time blood is visible, whether it be on human skin, or other porous or textured substrates, consider the possibility that impression evidence may be hidden and just waiting to be recovered.

Figures 1.1, 1.2, 1.3

Figure 1.1—Four-finger bloody impression on human skin (neck area).

Figure 1.2—Lifted bloody impression from human skin.

Figure 1.3—Lifted bloody impression from human skin visualized with an ALS.


Figures 2.1, 2.2, 2.3

Figure 2.1—Bloody palm impression on cotton t-shirt (dark-green color) visualized with an ALS.

Figure 2.2—Lifted bloody palm impression from cotton t-shirt.

Figure 2.3—Lifted bloody palm impression from cotton t-shirt visualized with an ALS.

Figures 3.1, 3.2, 3.3

Figure 3.1—Bloody footwear impression on carpet (dark-blue color) visualized with an ALS.

Figure 3.2—Lifted bloody footwear impression from carpet.

Figure 3.3—Lifted bloody foowear impression from carpet visualized with an ALS.

Figures 4.1, 4.2

Figure 4.1—Bloody thumb print on denim (indigo-washed blue jeans).

Figure 4.2—Lifted bloody thumb print from denim.

About the Authors

This e-mail address is being protected from spam bots, you need JavaScript enabled to view it , M.S. is research scientist and instructor at Madonna University, and is the inventor of the Zar-Pro Fluorescent Blood Lifting Strips. She is a former Michigan certified police officer experienced in evidence collection and analysis, who recognized a need for improvement in methods for the collection and enhancement of blood-based evidence. She continues her research into ways to improve evidence collection and techniques for use in the field.

Jodi Lynn Barta, Ph.D. is the Director of the Forensic Science Program and Research Facility at Madonna University and is a National Institute of Justice funded researcher engaged in research to improve methods for use by forensic professionals.

Note: Photographs were taken and laboratory assistance was provided by Jessika Williams, Madonna University student and Military Police Officer in the U.S. Army Reserves.

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