CRIME-LITE APPLICATIONS //
Forensic Applications of Infrared Light
Blood on fabric as seen by the naked eye (left) vs. under IR illumination (right)
Infrared wavelengths of light are those wavelengths that are too long for our eyes to see, above 700nm. Historically, this meant that we couldn’t view any of the information that could be gained from IR examinations. However, the use of an IR sensitive camera allows us to use these wavelengths to visualise many important evidence types.
Fig.1 The spectrum of light.
Light ranges from Ultraviolet (100-400nm) through to visible light (400-700nm) and finally through to IR light (700-1000nm)
There is always more evidence at a crime scene than we can visibly see, and this is why we use visible examination techniques like oblique light or fluorescence examination which can be very effective. In order to maximise the evidence that we can detect and prevent missing important evidence, we also need to search beyond the visible and consider techniques like reflected infrared examinations.
Fig.2 the principle of reflected IR examinations.
IR light is directed at a surface and different parts of the surface will absorb and reflect differing amounts of IR. It is this difference in absorption and reflection which creates contrast on the substrate and allows us to see different evidence types which we cannot see using standard visible light examination techniques
For IR examinations, there are 3 key requirements:
– An Infrared light source
– An IR pass filter
– An IR sensitive camera
All of these are necessary to successfully visualise the differences in IR absorption and reflection, which can allow us to reduce the interference from dark or patterned backgrounds and clearly see the evidence on the surface. This effect is not always possible to achieve, not all inks and dyes reflect IR light, but exploiting the differences in absorption and reflection between our substrate and evidence can provide meaningful information that we otherwise wouldn’t have access to.
The Crime-lite Auto is a highly portable, full spectrum camera which allows for the quick screening and searching of a crime scene. It has been designed to help an examiner quickly and effectively search for and detect evidence across the spectrum from ultraviolet through to infrared. The product is built around a 20.1 MP camera with high intensity LEDs which have the capability to detect a range of different evidence types.
The DCS5 is a fingerprint photography workstation, which consists of a high-resolution DSLR camera, specialist lighting modules and specifically designed enhancement software to allow the user to photograph any fingerprint that has been developed on virtually any surface. One of our lighting modules for the DCS-5 is the fls with over 300 wavelengths of IR light to allow the user to fine-tune the illumination to enhance the reflection in the background.
Crime-lite® ML PRO
The ML-Pro is our full-spectrum laboratory-based screening tool. It has been specifically designed for the biology lab – incorporating full-spectrum illumination and automated filtration with integrated reporting software. The ML-Pro is equipped with IR LEDs and an appropriate filter to allow examiners to carry out IR examinations, quickly and efficiently in the laboratory.
Relevant Evidence Types:
On light coloured substrates, blood is very easy to see. It appears as a dark red-brown stain and can easily be distinguished from the background on these substrate types. Dark and patterned backgrounds are the substrate types on which it is challenging to see blood as it can be hard to see the staining, and even more so to visualise any fine patterns – for example from blood spatter.
This is a shirt made from black material, and therefore it is challenging to find the areas of bloodstaining on this substrate using white light alone. When IR light and an appropriate filter are used, it is much easier to view the areas of blood staining – the dyes in the black material reflect the IR light and lighten, whereas the blood strongly absorbs the IR light and darkens. This allows us to not only locate areas for further testing but could also provide us with the valuable information of the pattern created on the surface when the blood was deposited.
ON BLACK FABRIC
Fig.3 An area of blood staining on a black surface photographed on the Crime-lite Auto illuminated with a white light and viewed with a visible pass filter (left) and when illuminated with near infrared light and viewed with a 780LP filter (right).
ON PATTERNED FABRIC
Fig.4 An area of blood staining on a patterned surface photographed on the ML-Pro using white light and a visible pass filter (left) and IR light with a 780nm long pass filter.
This highly patterned sample shows another application of IR light. In this case, all of the dyes used on this surface are reflecting IR light and all of them are lightening, which allows us to view the absorption of the blood on the surface. It’s possible to see the smaller area of staining in white light, but the larger stain in this case is almost completely obscured by the dark purple surface. Without the use of IR light it is challenging to see the blood on this surface at all.
Paints can be formulated in many different ways with a range of different pigment types. If these pigments reflect infrared light, it can be possible to see beneath paint to view evidence which may have been painted over in an attempt to conceal it.
In this example an area of the wall has been plastered over and repainted to match the rest of the wall. This is quite hard to see when using white illumination, but when IR light is used, the difference in absorption and reflection of the two areas is plain to see and allows this area to be more closely inspected as the contrast is alerting the examiner to an unexpected difference on the wall.
UNDER PLASTER & PAINT
Fig.5 An area on a wall which has been plastered over and repainted to match the rest of the wall photographed on the CL-Auto using white light and a visible pass filter and an IR light with a 780nm filter.
Gunshot residue is a very challenging evidence type to search for, as the possibilities of the composition are extremely varied. The composition of GSR can be dictated by not just the ammunition and the gun, but ammunition that has previously fired. One of the methods that has been reported has being highly successful for the visualization of gunshot residue is the use of IR light, as so many fabric types reflect IR light, but many components of GSR may absorb IR and darken.
Fig.6 A bullet hole surrounded by GSR photographed on the Crime-lite auto in white light with a visible pass filter (left) and with IR light and a 780nm filter
In this case, the GSR is visible in the singed area immediately surrounding the bullet hole, as well as the speckled pattern over the surrounding area. This is not possible to see using white light as the deposited pattern is so fine. Using IR light makes it possible to record how the pattern looks visibly prior to carrying out any further testing.
Fingerprints can be deposited and developed on a huge range of substrates, which is why the photography of fingerprints can be such a complex and challenging subject. Ultimately, our aim within fingerprint photography is to capture high quality images of the prints, allowing us to see as many features within the fingerprint as possible. Often, what is challenging is interference which arises from the background, IR reflection is one technique which may work to reduce this interference and improve the images that can be captured of a fingerprint.
Our fingertips and hands come into contact with a range of different surfaces every day and often these surfaces may have printing which eventually may interfere with treated fingerprints. In the case of this example, the fingerprint has been treated with black powder – a treatment which absorbs IR light – so when illuminated in the IR part of the spectrum, this part of the fingerprint still appears as dark. Whereas the dark text which interferes with the fingerprint initially in the white light, virtually disappears completely, as it reflects the infrared.
Fig.7 Black powder fingerprints on a printed surface photographed on the DCS-5 with white light and a visible pass filter (left) and IR light (right)
Fig.8 A fingerprint treated with black powder photographed on the DCS-5 using white light (left), 767nm light (middle) and 831nm light (right)
These images demonstrate the affect that can be achieved when fine control over the wavelength of IR illumination is possible. Initially, under white light there is printing and a pattern which interfere with the fingerprint. When 767nm light is used to illuminate the sample, most of the printing reflects the IR light, and in fact all of the interfering printing reflects the IR but when the illumination is changed to 831nm, almost total background removal is possible and the print can be seen as dark ridges on a plain background, making the identification of a print like this much easier – without the interference of a background pattern. This control is achieved by using the fls to select a single wavelength of IR light to use as the illumination for the DCS-5.