“When we watch some criminal investigation TV dramas, when the examiner needs to find evidence of blood stains, they usually spray luminol to the relevant area and turn off the lights. This adds a certain comedy effect to film and television dramas, but it is not the best solution for reality detectors who need to find specific evidence of blood stains in less than ideal situations. In reality, researchers have been looking for alternative methods to detect very low concentrations of blood on fabrics, and recently they have found the answer in thermal imaging technology.
When we watch some criminal investigation TV dramas, when the examiner needs to find evidence of blood stains, they usually spray luminol to the relevant area and turn off the lights. This adds a certain comedy effect to film and television dramas, but it is not the best solution for reality detectors who need to find specific evidence of blood stains in less than ideal situations. In reality, researchers have been looking for alternative methods to detect very low concentrations of blood on fabrics, and recently they have found the answer in thermal imaging technology.
Blood is invisible in its own infrared spectrum, but spraying water vapor on blood stained samples can create a thermal signal. This thermal imaging method can replace luminol in forensic testing and become a new detection solution. Today, let’s talk about chemistry researchers Dr. Michael Myrick and Dr. Stephen Morgan and their team at the University of South Carolina researching the use of thermal imaging cameras as an alternative to detecting and recording evidence of biological fluids (such as bloodstains at crime scenes) in the field of forensic applications. .
Problems with traditional luminol
Luminol itself is a kind of powder, mixed with hydrogen peroxide and used on the surface of the fabric for testing. If blood is present, the 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 misjudgment.
Dr. Myrick explained that luminol reacts with various substances such as aromatic amines, copper salts, and bleach. In addition, it has a problem. It 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 the water absorption/desorption characteristics of cotton, so even if whole blood is imprinted on cotton, it is blurred.
When spraying luminol on blood stains, it may blur or wash away the blood stains. “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 blood stains can also cause subsequent DNA testing of the sample to become a bubble.
Research process of infrared imaging application
Dr. Myrick and his team are always looking for a better way: to visualize blood stains and other biological fluids for medical testing. Myrick is particularly interested in detection methods that can be observed for more than a few seconds, are repeatable, and do not damage the sample. He and his team began to study the use of infrared reflection to visualize blood stains. Although infrared reflections do work, blood stains are always blurred in thermal images.
“Relying solely on thermal imaging is not the best way to visualize chemical controls,” Dr. Myrick admitted. He and his team are looking for ways to increase 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 graduate student Wayne O’Brien was to soak a piece of cotton cloth with deuterium oxide sprayed from a travel steam jet and measure the reflectance. O’Brien happened to record an infrared video of steam sprayed on cotton cloth and made an amazing discovery.
“At the moment the steam was turned on, in the infrared video he showed me, the blood stains diluted 100 times were like a light bulb. This amazing phenomenon was very difficult to see before, and it was in the middle of the image in an instant. Bright,” Myrick said.
In addition, unlike luminol, which fades immediately, they found that the effect of water vapor on the blood-stained fabric is continuous. Myrick said: “If you take a piece of cloth and put it in a humid environment where the temperature rises, you can see 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.”
Thermal imager + water vapor, the blood mark is invisible
Myrick’s team used their findings to study blood fingerprints on three types of fabrics. The “fingerprints” came from a custom rubber stamp. These “fingerprints” were wetted and printed on three different types of dyed fabrics. Each piece of fabric is printed with two fingerprint blood prints, one of which is 10 times diluted and the other is undiluted. Then, let the blood mark air dry for 24 hours.
When it is necessary to image the bloodstain, the researchers exposed the sample to the deionized water vapor of the garment steamer. For a long period of time, they steamed the cloth every three seconds and paused the recording during each steaming interval.
Spraying water vapor on the sample directly generates heat, and Dr. Myrick likens this process to walking out of a dry air-conditioned room to the hot and humid outdoors. Every piece of clothing you wear will immediately absorb water vapor, and the temperature will rise slightly, which is obvious in the infrared image.
Just like adding moisture to produce heat, removing the steam source will cause cooling. However, hydrophobic fabrics like acrylic or polyester can only hold a very small amount of water and reach equilibrium quickly. Therefore, the blood stain area will cool down more slowly than other areas of the cloth, which creates a 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, the contrast is sufficient to distinguish fingerprint ridge patterns.
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 entire bloodstain. 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, allowing Myrick’s team to observe a single enlarged “fingerprint” ridge pattern. In both cases, the team used FLIR’s ResearchIR software to operate the thermal imager.
The 10-fold diluted blood print on polyester shows fingerprint ridge patterns and halos caused by blood coagulum wicking.
Myrick’s team found it difficult to image blood marks on cotton cloth. This is because the ratio of water to the total weight is as high as 20%, and the water absorbed by the cotton cloth is equivalent to the water absorbed by the blood stain 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 thread absorbs water at different speeds, and the response of a single fiber is extremely fast.”
The single thread in the blood stamp is in Sharp contrast with the rest of the cotton cloth
Therefore, the team was very successful in imaging the enlarged ridges on the cotton cloth. They noticed that there was a clear contrast between the whole blood on the cotton float and the whole blood in other areas. This comparison is only visible during the 30 ms period when the float silk can absorb steam.
“FLIR A6751sc allows us to perform high-speed measurements. In fact, the fiber will only light up in one frame of the thermal video,” Myrick explained. After that, most of the fabric has absorbed enough water vapor, thus eliminating the thermal difference between whole blood and cotton.
The blood mark is only faintly visible during steaming. Like the acrylic sample, there is a weave that prevents the fabric from coming into contact with the blood mark. However, compared with the weft yarn (horizontal direction), the warp yarn (vertical direction yarn) is convex, so the blood coagulum on the warp yarn is more obvious.
The ridge break occurs at the position where the acrylic fabric prevents the blood mark from contacting the fabric completely
According to Myrick’s research results, thermal imaging is a viable alternative to luminol when determining whether there are blood stains on the fabric. It can even be said that thermal imaging is more preferable, because the water vapor that assists in imaging will not further dilute the blood stains, and there is no possibility of ruining the evidence. Although the use of water vapor will bring some challenges to blood stain imaging on cotton cloth, a high-speed, high-resolution thermal imaging camera can provide a workaround. Scientific research thermal imaging cameras such as FLIR A6751sc have the frame rate and integration speed required to record the rapid heating or cooling of loose cotton fibers, which can be enhanced by magnifying the lens. Myrick and his team will continue to study the application of high-speed imaging on cotton threads in order to improve this process.
FLIR A6750 series
The FLIR A6750 mid-wave infrared thermal imaging 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 refrigerated indium antimonide thermal imager can freeze the motion of moving objects and accurately measure their temperature, as well as perform a variety of non-destructive tests. With 327,680 (640×512) pixel infrared resolution and high sensitivity, it can generate clear images, which is very suitable for inspection of precision instruments.
FLIR A6750 series thermal imaging cameras can be seamlessly connected with FLIR ResearchIR Max software,
Browse, record and process the thermal data obtained by the thermal imager. Another software development kit (SDK) is available.