Facilitating forensic investigations-FLIR thermal imaging camera makes "blood fingerprints" clearer

When we watch some criminal investigation TV dramas, when the examiner needs to find evidence of blood stains, they usually spray luminol on the relevant area and turn off the lights

Blood is invisible in its own infrared spectrum, but spraying water vapor on blood stained samples can create a thermal signal.

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

Dr.

The water absorption/desorption characteristics of blood are similar to the water absorption/desorption characteristics of cotton, so even if the whole blood is imprinted on the cotton, it is blurred

When spraying luminol on blood stains, it may blur or wash away the blood stains

Research process of infrared imaging application

Dr.

"Relying solely on thermal imaging is not the best way to visualize chemical controls," Dr.

The task of graduate student Wayne O'Brien was to soak a piece of cotton cloth with deuterium oxide sprayed from a travel steam iron and measure the reflectance

"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

In addition, unlike luminol, which fades immediately, they found that the effect of water vapor on the blood-stained fabric is continuous

Thermal imager + water vapour, there is no way to hide the blood mark

Myrick's team used their findings to study blood fingerprints on three types of fabrics

When it is necessary to image the bloodstain, the researchers exposed the sample to the deionized water vapor of the garment steamer

Spraying water vapor on the sample directly generates heat.

Just like adding moisture to produce heat, removing the steam source will cause cooling

　　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 mark 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 imaging camera 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 uses a 13mm lens, allowing Myrick's team to observe a single enlarged "fingerprint" ridge pattern
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In both cases, the team used FLIR's ResearchIR software to operate the thermal imager
.

　　The 10-fold diluted blood mark on polyester shows fingerprint ridge patterns and halos caused by blood coagulation 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
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In contrast, synthetic fibers such as acrylic and polyester are relatively weak in 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
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They noticed that there was a clear contrast between the whole blood on the cotton float and the whole blood in other areas
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This comparison is only visible during the 30 ms period when the float silk can absorb steam
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　　"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 fabrics have 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 full 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 cloth tissue prevents the blood mark from contacting the fabric completely

　　According to the results of Myrick's research, thermal imaging is a viable alternative to the luminol method 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
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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
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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 a magnifying lens
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Myrick and his team will continue to study the application of high-speed imaging on cotton threads 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
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This refrigerated indium antimonide thermal imaging camera can freeze the motion of moving objects and accurately measure their temperature, as well as perform a variety of non-destructive tests
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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
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　　The 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
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Another software development kit (SDK) is available
.

　　About FLIR Systems, Inc.

　　Founded in 1978, FLIR Systems, Inc.
is a world-leading industrial technology company focusing on the development of smart sensing solutions for defense, industrial and commercial applications
.
The vision of FLIR Systems is to become the "sixth sense in the world" and to create technology to help professionals make smarter decisions, save lives and improve lives
.