When we watch some criminal investigation TV series, when the detector needs to find blood evidence, he will spray luminol on the relevant area and turn off the light. This adds a certain comic effect to the film and TV series, but it is not the best solution for the reality detector who needs to find specific blood evidence in the less ideal situation. In reality, researchers have been looking for alternative methods to detect extremely low concentrations of blood on fabrics, and recently they have found the answer in thermal imaging technology.

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Blood is not visible in its own infrared spectrum, but the thermal signal can be created by spraying water vapor on the sample stained with blood. This thermal imaging method can replace the luminol method in forensic detection and become a new detection scheme. Today, let’s talk about chemical researchers Dr. Michael Myrick and Dr. Stephen Morgan and their team at the University of South Carolina working on the use of infrared thermal imaging as an alternative to detecting and recording evidence from biological fluids, such as blood at a crime scene, in forensic applications.

The problems of traditional Lumino

Luminol itself is a kind of powder, which is used to test the surface of fabric after mixed with hydrogen peroxide. In the presence of blood, iron in hemoglobin catalyzes the reaction between luminol and hydrogen peroxide, releasing electrons as photons visible to blue light. However, luminol can also react with other substances besides iron, which may lead to wrong judgment.

Dr. Myrick explained that luminol reacts with aromatic amines, copper salts, bleach and many other substances. In addition, it also has a problem, it may have a potential impact on DNA testing: although it does not directly destroy DNA, it may affect some genetic markers.

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The water absorption / desorption characteristics of blood are similar to those of cotton, so even the whole blood imprint on cotton is blurred.

When you spray luminol on the bloodstain, it may blur or wash away the bloodstain“ If there is a pattern, such as a fingerprint, and you soak it in a liquid, you may lose it completely, “Dr. Myrick said. This will lose all opportunities to identify fingerprints on the fabric. Over dilution of the bloodstain can also lead to the subsequent DNA testing of the sample becoming a bubble.

Research process of infrared imaging application

Dr. Myrick and his team have been looking for a better way: to visualize blood and other biological fluids for medical testing. Myrick is particularly interested in detection methods that can be observed for more than a few seconds and that are repeatable without destroying the sample. He and his team started working on using infrared reflection to visualize blood. Although infrared reflection does work, blood is always blurred in the thermal image.

“Relying on thermal imaging alone is not the best way to visualize chemical controls,” Dr. Myrick admits. He and his team are looking for ways to improve sensitivity to blood and use steam as a way to create strong absorption bands in the infrared spectral window. However, in trying to improve the method, the team came across a better one.

Wayne O’Brien, a graduate student, was to soak a piece of cotton cloth with deuterium oxide from a travel steam iron and measure reflectivity. O’Brien just recorded the infrared video of steam spraying on the cotton cloth and made an amazing discovery.

“The moment he turned on the steam, in the infrared video he showed me, the 100 fold diluted blood was like a light bulb. This amazing phenomenon was very hard to see before, and suddenly lit up in the image, “Myrick said.

In addition, unlike the instant fading of luminol, they found that the effect of water vapor on bloodstained fabrics was sustained. “If you take a piece of cloth and put it in a humid environment with rising temperature, you can see the blood indefinitely, it won’t appear and disappear from time to time, as long as you put it in a humid environment, you can see it forever,” Myrick said

Thermal imager + water vapor, blood imprint

Myrick’s team used their findings to study blood fingerprints on three types of cloth“ The “fingerprints” come from a custom rubber stamp that is wet and printed on three different types of dyed fabric. Each fabric is printed with two fingerprints, one of which is diluted 10 times, and the other is not diluted. Then, let the blood seal air dry for 24 hours.

When it was necessary to image a blood print, the researchers exposed the sample to deionized water vapor from a steam perm. For a long time, they steamed the cloth every three seconds, pausing the recording between each steam injection.

Spraying steam on a sample directly generates heat, a process Dr. Myrick likens to walking out of a dry air-conditioned room into a hot, humid outdoor environment. Every garment you wear will immediately absorb water vapor, and the temperature rises slightly, which is obvious in the infrared image.

Just as increasing moisture produces heat, removing the steam source causes cooling. However, hydrophobic fabrics such as acrylic or polyester retain only a very small amount of moisture and quickly reach equilibrium. As a result, the bloodstain area will cool more slowly than other areas of the cloth, resulting in temperature difference, which is easy to identify in the infrared image.

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Complete blood print on acrylic fabric, left side: thermal image during steam exposure to moisture, right side: evaporative cooling after exposure, contrast enough to identify 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 group of recordings, they installed a 50mm lens for the flira6751scsls thermal imager to image the whole blood print. Flira6751sc provides fast frame rate and 480ns integration speed, enabling researchers to record fast thermal transients. The second group used a 13mm lens, allowing Myrick’s team to observe a single enlarged “fingerprint” ridge pattern. In two cases, the team operated the thermal imager through FLIR’s reasearchir software.

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The 10 fold diluted blood print on polyester shows the ridge pattern of fingerprint and halo caused by the wicking effect of blood coagulation.

Myrick’s team found it difficult to image blood marks on cotton. This is because the proportion of water in the total weight is as high as 20%, and the water absorbed by the cotton cloth is equivalent to that absorbed by the bloodstain itself. In contrast, acrylic fiber and polyester fiber have weak water absorption.

“Cotton is a complex fabric, full of loose fibers,” Myrick added“ And the speed of water absorption is not the same, single fiber reaction is very fast

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The single thread in the whole blood print is in sharp contrast to the rest of the cotton cloth

As a result, the team was very successful in imaging the enlarged ridge on the cotton cloth. They noticed that there was a clear contrast between the whole blood on the cotton floss and that in other areas. This contrast can only be seen during the 30 ms period when the float can absorb steam.

“Flira6751sc allows us to make high-speed measurements, and in fact, the fiber lights up only one frame in the thermal video,” Myrick explains. After that, most of the cloth has absorbed enough water vapor, thus eliminating the thermal difference between whole blood and cotton.

The whole blood print is only blurred and visible during steam spraying. Like the acrylic sample, there is a texture that prevents the fabric from completely contacting the blood print. However, compared with the weft (horizontal direction), the warp (vertical direction) is convex, so the blood clots on the warp are more obvious.

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The ridge fracture occurs at the position where the acrylic fabric structure prevents the blood print from completely contacting with the fabric

According to Myrick’s research results, thermal imaging technology is a feasible alternative to the luminol method when determining whether the fabric is stained with blood. It can even be said that thermal imaging is preferable, because the water vapor of auxiliary imaging will not further dilute the blood, and there is no possibility of destroying the evidence. Although the use of water vapor will bring some challenges to the imaging of blood stains on cotton cloth, the high-speed and high-resolution infrared thermal imager can provide a flexible solution. Flira6751sc and other scientific research thermal imagers have the frame rate and integration speed needed to record the rapid heating or cooling of loose cotton fiber, which can be enhanced by magnifying lens. Myrick and his team will continue to study the application of high-speed imaging on cotton thread in order to improve the process.

Flira6750 series

Flira6750 medium wave infrared thermal imager has short exposure time and high-speed window frame rate, which makes it an ideal choice for recording fast thermal events and fast moving targets. The cooled InSb thermal imager can freeze the movement of moving objects and accurately measure their temperature, as well as perform a variety of non-destructive tests. With 327680 (640 × 512) pixel infrared resolution and high sensitivity, can generate clear images, is very suitable for the inspection of precision instruments.

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Flira6750 series thermal imager can be seamlessly connected with flirresearch irmax software to browse, record and process the thermal data obtained by the thermal imager. Another software development kit (SDK) is available.

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