When we watch some criminal investigation TV dramas, when the inspectors need to find blood evidence, they usually spray luminol on the relevant areas and turn off the lights. This has added a certain comic effect to the film and television series, but it is not the best solution for the reality detectives who need to find specific blood evidence under less ideal circumstances. In reality, researchers have been looking for alternative methods to detect extremely low concentrations of blood on fabrics. Recently, they have found the answer in thermal imaging technology.
Blood is invisible in its own infrared spectrum, but spraying steam on the blood stained sample can create a thermal signal. This thermal imaging method can replace luminol’s method in forensic detection and become a new detection scheme. Today, let’s talk about the research of chemical researchers Dr. Michael Myrick and Dr. Stephen Morgan and their team at the University of South Carolina on the use of infrared thermography as an alternative to detecting and recording biological liquid (such as blood at the crime scene) evidence in the field of forensic applications.
Problems of traditional Lumino
Luminol itself is a kind of powder, which is used to test the fabric surface after mixing hydrogen peroxide. If blood is present, 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 substances other than iron, which may lead to wrong judgment.
Dr. Myrick explained that luminol reacts with aromatic amines, copper salts, bleaches and other substances. In addition, it has another problem. It may also have a potential impact on DNA detection: although it does not directly damage DNA, it may affect some genetic markers.
The water absorption / desorption characteristics of blood are similar to those of cotton, so even the whole blood imprint on cotton is ambiguous.
When luminol is sprayed 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. Excessive dilution of the blood will also cause subsequent DNA testing of the sample to become 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 detection. Myrick is particularly interested in detection methods that can be observed for more than a few seconds, are repeatable, and do not destroy samples. He and his team began to study the use of infrared reflection to visualize blood. Although infrared reflection does work, blood stains are always blurred in thermal images.
“Thermal imaging alone is not the best way to visualize chemical controls,” Dr. Myrick admitted. 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 spectrum window. However, in the process of trying to improve the method, the team stumbled upon a better method.
The task of Wayne O’Brien, a graduate student, is to soak a piece of cotton cloth with deuterium oxide from a travel steam iron and measure the reflectivity. O’Brien happened to record the infrared video of steam sprayed on the cotton cloth and made an amazing discovery.
“At the moment when the steam was turned on, he showed me an infrared video in which the 100 times diluted blood was like a lighted bulb. This amazing phenomenon was very difficult to see before, and lit up in the image in an instant,” said Myrick.
In addition, unlike luminol, which immediately fades, they found that the effect of water vapor on blood stained fabrics is sustained. “If you take a piece of cloth and put it into a humid environment with elevated temperature, you can see the blood indefinitely. It will not appear and disappear from time to time. As long as you put it in a humid environment, you can see it forever,” Merrick said
Thermal imager + steam, no hiding of blood mark
Myrick’s team used their findings to study blood fingerprints on three types of fabrics. The “fingerprint” comes from a custom rubber stamp, which is wet and printed on three different types of dyed fabrics. Each piece of fabric is printed with two fingerprints, one of which is diluted by 10 times and the other is not diluted. Then, let the blood seal dry for 24 hours.
When it was necessary to image the blood prints, the researchers exposed the samples to the deionized water vapor of the steam hang ironing machine. For a long time, they steam the cloth every three seconds and pause the recording during each steam spraying interval.
Spraying steam on the sample will directly generate heat. Dr. Myrick likens this process to walking out of a dry air-conditioned room to a damp and hot outdoor. Every garment you wear will immediately absorb water vapor, and the temperature will rise 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 can only maintain a very small amount of moisture and quickly reach equilibrium. Therefore, 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.
Complete blood print on acrylic fabric, left: thermal image during steam exposure to moisture, right: evaporative cooling after exposure, with contrast sufficient 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 set of records, they installed a 50mm lens for the FLIR a6751sc SLS thermal imager to image the whole blood print. FLIR a6751sc provides fast frame rate and 480ns integration speed, enabling researchers to record fast thermal transients. The second set of records used a 13mm lens, which enabled Myrick’s team to observe a single enlarged “fingerprint” ridge pattern. In both cases, the team operated the thermal imager through FLIR’s reasearchir software.
The 10 times diluted blood print on polyester shows the fingerprint ridge pattern and halo caused by the core absorption of blood coagulation.
Myrick’s team found it difficult to image the blood marks on the cotton cloth. This is because the proportion of water in the total weight is as high as 20%. The water absorbed by the cotton cloth is equivalent to that absorbed by the blood itself. In contrast, synthetic fibers such as acrylic and polyester have weak water absorption.
“Cotton is a complex fabric, full of loose fibers,” Myrick added. “And the speed of water absorption is different, and the response of a single fiber is very fast.”
The single thread in the whole blood print forms a sharp contrast with the rest of the cotton cloth
Therefore, 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 float and that in other areas. This comparison is only visible during the 30 ms period during which the floating wire can absorb steam.
“The FLIR a6751sc enables us to make high-speed measurements. In fact, the fiber only lights up in one frame of the hot video,” Myrick explained. After that, most of the cloth has absorbed enough water vapor, thus eliminating the thermal difference between the whole blood and the cotton cloth.
The whole blood print is only blurred 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 yarn) is convex, so the blood coagulation on the warp is more obvious.
The ridge fracture occurs at the position where the acrylic fabric prevents the blood mark from completely contacting the fabric
According to Myrick’s research results, thermal imaging technology is a feasible alternative to luminol method when determining whether there is blood on the fabric. It can even be said that thermal imaging is preferable, because the water vapor used for 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. Scientific research thermal imagers such as FLIR a6751sc have the frame rate and integration speed required to record the rapid temperature rise or cooling of loose cotton fiber, which can be enhanced by the magnifying lens. Myrick and his team will continue to study the application of high-speed imaging on cotton threads with a view to improving this process.
FLIR a6750 series
FLIR a6750 medium wave infrared thermal imager has short exposure time and high window frame rate, making it an ideal choice 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 variety of nondestructive tests. With 327680 (640 × 512) pixel infrared resolution and high sensitivity can generate clear images, which is very suitable for the inspection of precision instruments.
FLIR a6750 series thermal imager can be seamlessly connected with FLIR researchir MAX software to browse, record and process the thermal data obtained by the thermal imager. Another software development kit (SDK) is available.