Photographing blood under layers of soot with infrared cameras

Credit: Image by Arshad Sutar from Pexels

Fire destroys buildings and leaves only ash and rumble in its wake. Arsonists believe that their fiery M.O. protects them from the threat of biological evidence. However, researchers in Australia reveal buried bloodstains from dense, sooty remains. 

Fire; a force of nature that destroys everything in its path and leaves nothing but traces behind. Fictional shows like CSI and real-life arsonists demonstrate that fire is an extremely effective method to destroy physical evidence that could link the perpetrator back to the offense. But does it truly destroy all evidence? Researchers recently demonstrated that by modifying an infrared camera, they could uncover bloodstains buried beneath an arson’s sooty remains. 

Blood is one of the strongest indicators of a violent crime and an incredibly important type of biological evidence. Unfortunately, in the event of an arson, when fire is deliberately set through a combination of oxygen, fuel, and an ignition source, blood is often believed to be unusable as physical evidence. Fire’s extreme heat, upwards of 1000° C / 1832° F, degrades DNA and incinerates physical evidence such as murder weapons and electronic devices. 

In addition, fire instigated with chemical accelerants like gasoline leaves behind a thick soot, interfering with common blood detection methods such as luminol and visible light photography. However, the perpetrator may unknowingly leave behind blood and other biological fluids under the assumption that the fire would destroy, not mask, them. This motivated researchers at the Western University in Australia to modify a digital single-lens reflex camera (SLR) to discern bloodstains in soot by detecting differences in the material’s density using reflected infrared photography

Reflected infrared photography takes advantage of blood’s absorptive properties in the infrared part of the electromagnetic spectrum. As blood dries, sped up by the fire’s heat, it turns from a ruby red to a deep brown, and absorbs energy at wavelengths greater than 820 nm. The infrared camera captures this absorption event by gathering the other reflected wavelengths that are not absorbed by the blood to the camera, resulting in a high-contrast infrared photograph (Fig 1). This works well for photographing things under soot, which although is dense enough to prevent the full transmission of infrared light, hinders transmission of visible light, captured in normal photography, twice as much as infrared light.

Fig 1: Two examples of infrared (IR) photography, one colored and one black and white. IR photography provides greater contrast than traditional visible light photography because of how objects reflect or transmit the IR light. Sources: David on Flickr and Carol Highsmith.

To test their new device, the authors spread commercially obtained horse blood inside a wooden crib and set it ablaze to recreate an arson scene (Fig 2). They even extinguished the flames using heavy water spray to mimic firefighter response and gathered information on how different arson properties, such as exposure to oxygen and type of chemical accelerant impacted soot deposit. They compared their infrared photography technique with standard visible light photography already employed at crime scenes. The standard method reached its limit of visualization when the soot reached about 550 nanometers or 1/50 of a mil. To visualize the density, a single layer of paint is 1 mil and a sheet of aluminum foil is 3/5 of a mil. In comparison, they modified their reflected infrared method and could detect bloodstains in soot up to 900 nanometers deep, or 1/33 of a mil, a 66% improvement.

Fig 2: The scientists set a wooden crib, a box-like structure on fire, to recreate an arson scene (shown above); or something to that effect. Source: Image by Pexels from Pixabay 

The authors modified a typical single-lens camera with infrared capabilities to reveal latent bloodstains buried under thick soot. They further optimized their imaging capabilities by employing a painted background, increasing their sensitivity, and changing the spectral output of the lighting sources of the images. Normal photography failed to reveal latent bloodstains under a thin soot layer, limiting the standard camera’s utility at a soot-covered crime scene. To obtain widespread usage, more institutions (commercial or otherwise) should validate this new camera method and begin usage in the field. Bastide et al. showed that with the right technology, CSI can reveal more evidence at arsons scenes and prove the past, especially a violent past, never remains buried forever.

TitleDetection of Latent Bloodstains at Fire Scenes using Reflected Infrared Photography
AuthorsBelinda Bastide, Glenn Porter, and Adrian Renshaw
JournalForensic Science International
Year2019
Linkhttps://www.sciencedirect.com/science/article/pii/S0379073818306601

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