Assisting forensic investigations - FLIR thermal imaging cameras make bloody fingerprints clearer

When we watch some criminal investigation TV series, when the detector needs to find blood evidence, they usually spray luminol on the relevant area and turn off the lights. This adds a certain comedy effect to the TV series, but it is not the best solution for real detectors who need to find specific blood evidence in less than ideal situations.

In reality, researchers have been looking for alternative methods to detect extremely low concentrations of blood on fabrics, and recently they found the answer in thermal imaging technology.

Blood is not visible in its own infrared spectrum, but spraying water vapor on a blood-stained sample can create a heat signature, a method that can be used to image blood.

The method of replacing luminol in forensic testing has become a new detection solution. Today, let’s talk about chemical researcher Dr. Michael Myrick

and Dr. Stephen Morgan and his team at the University of South Carolina, investigating the use of thermal imaging cameras in forensic applications as an alternative for detecting and documenting evidence from biological fluids, such as blood at crime scenes.

Problems with Traditional Luminol

Luminol itself is a powder that is mixed with hydrogen peroxide and applied to the surface of the fabric for testing. If blood is present, the iron in the hemoglobin will catalyze the reaction between the luminol and hydrogen peroxide, releasing electrons as photons visible to blue light. However, luminol can also react with substances other than iron, which may lead to false judgments.

Dr. Myrick explained that luminol reacts with a variety of substances, including aromatic amines, copper salts, and bleach. In addition, there is a problem with it, which may also have a potential impact on DNA testing: although it does not directly damage DNA, it may affect certain genetic markers.

The water absorption/desorption characteristics of blood are similar to those of cotton, so even a whole blood blot will be blurred on cotton.

Luminol can blur or wash away blood when sprayed on it. "If there's a ridge pattern, like a fingerprint, and you soak it with a liquid, you can lose it completely," says Dr. Myrick. All chances of identifying a fingerprint on fabric are lost. Over-diluting a blood stain can also render subsequent DNA testing of samples useless.

Research process of infrared imaging application

Dr. Myrick and his team were looking for a better way to visualize blood and other biological fluids for medical testing. Myrick was particularly interested in methods that could be observed for more than a few seconds, were repeatable, and did not destroy the sample. He and his team began to investigate the use of infrared reflectance to visualize blood. Although infrared reflectance did work, blood was always blurry in thermal images.

“Thermal imaging alone is not the best way to visualize chemical controls,” Dr. Myrick admitted. He and his team were looking for ways to increase sensitivity to blood and steam as a way to create strong absorption bands in the infrared spectral window. However, while trying to improve the method, the team stumbled upon a better approach.

Graduate student Wayne O'Brien was tasked with saturating a piece of cotton with deuterium oxide from a travel steam iron and measuring the reflectivity. O'Brien happened to record infrared video of the steam being sprayed onto the cotton and made a surprising discovery.

"The moment he turned on the steam, he showed me the infrared video of a 100-fold diluted blood stain that lit up like a light bulb. It was an amazing phenomenon that had been very difficult to see before, and it lit up in the image in an instant," said Myrick.

Furthermore, unlike luminol, which fades immediately, they found that the effect of water vapor on blood-stained fabric is persistent. "If you take a piece of fabric and put it in a humid environment with elevated temperatures, you can see the blood stain indefinitely," Myrick said. "It doesn't appear and disappear. As long as you keep it in a humid environment, you can see it forever."

Thermal imager + water vapor, blood stains are visible

Myrick's team applied their findings to studying blood fingerprints on three types of fabric. The "fingerprints" came from a custom-made rubber stamp, which was wetted and pressed onto three different types of dyed fabric. Two blood fingerprints were applied to each piece of fabric, one diluted 10 times and the other undiluted. The prints were then allowed to air dry for 24 hours.

When it came time to image the bloodstains, the researchers exposed the samples to deionized water vapor from a garment steamer, steaming the cloth every three seconds for a long period of time, pausing the recording between each burst of steam.

Spraying water vapor onto the sample directly generates heat, a process Dr. Myrick likens to walking out of a dry, air-conditioned room into the hot, humid outdoors. Each piece of clothing you wear immediately absorbs the water vapor, warming slightly, and this warming is apparent in the infrared image. Just as adding moisture generates heat, removing the vapor source results in cooling. However, hydrophobic fabrics like acrylic or polyester can only hold very small amounts of moisture and reach equilibrium quickly. Therefore, the bloodstained area will cool more slowly than the rest of the fabric, creating a temperature difference that is easily visible in the infrared image.

An intact blood print on acrylic fabric, left: thermal image during steam exposure to moisture, right: evaporative cooling after exposure, with enough contrast to discern the fingerprint ridge pattern.

Complete blood print on polyester fabric, left: thermal image during steam exposure to moisture, right: evaporative cooling after exposure.

In the first set of recordings, they mounted a 50mm lens on a FLIR A6751sc SLS thermal imager to image the entire blood print. The FLIRA6751sc provides a fast frame rate and 480ns integration speed, allowing the researchers to record fast thermal transients. The second set of recordings used a 13mm lens, allowing Myrick's team to observe a single magnified "fingerprint" ridge pattern. In both cases, the team operated the thermal imager through FLIR's ReasearchIR software.

A 10-fold diluted blood print on polyester shows the fingerprint ridge pattern and halos caused by wicking of blood clots.

Myrick's team found it difficult to image blood stains on cotton. That's because cotton, which makes up 20 percent of its weight in water, absorbs as much water as the blood itself. In contrast, synthetic fibers such as acrylic and polyester are less absorbent.

“Cotton is a complex fabric, full of loose fibers,” Myrick added. “And the threads absorb water at different rates, and the individual fibers react differently.

Very fast.

The single thread within the full blood print contrasts sharply with the rest of the cotton fabric

The team was therefore able to image the magnified ridges on the cotton cloth with great success. They noticed a clear contrast between the whole blood on the cotton float and the whole blood in other areas. This contrast was only visible during the 30 milliseconds when the float was able to absorb the steam.

“The FLIR A6751sc enables us to make measurements at such high speed that the fiber actually lights up in only one frame of the thermal video,” explains Myrick.

By then, most of the fabric had absorbed enough water vapor to eliminate the thermal difference between the whole blood and the cotton.

The full blood stain was only faintly visible during the steaming period, and like the acrylic sample, there was a weave pattern that prevented the fabric from coming into full contact with the stain. However, the warp yarns (vertical yarns) were raised compared to the weft yarns (horizontal yarns), so the blood clots were more visible on the warp yarns.

The ridge breaks occur where the acrylic fabric prevents the bloodstain from making full contact with the fabric.

According to Myrick's research, thermal imaging is a viable alternative to the luminol method when determining whether blood is present on fabric. It can even be argued that thermal imaging is preferable because the water vapor used to aid imaging does not further dilute the blood and has no potential to destroy the evidence. While the use of water vapor presents some challenges in imaging blood on cotton, high-speed, high-resolution infrared cameras offer a workaround.

Research thermal imaging cameras such as the FLIR A6751sc have the frame rates and integration speeds needed to record the rapid heating or cooling of loose cotton fibers, which can be enhanced with a magnifying lens. Myrick and his team will continue to investigate the application of high-speed imaging on cotton threads in hopes of improving this process.

FLIR A6750 Series

The FLIR A6750 mid-wave infrared camera has a short exposure time and a high-speed window frame rate, making it ideal for recording fast thermal events and fast-moving targets. This cooled InSb thermal imager can freeze the motion of moving objects and accurately measure their temperature, as well as perform a wide range of non-destructive tests. With an infrared resolution of 327,680 (640×512) pixels and high sensitivity, it can produce clear images and is ideal for inspection of precision instruments.

The FLIR A6750 series thermal imagers can be seamlessly connected to the FLIR ResearchIR Max software to browse, record and process the thermal data acquired by the thermal imager. A software development kit (SDK) is also available.