Forensics: How digital microscopy helps to gather evidence from hair, fibers, paint, explosives and insects

Hair and fiber evidence obtained from crime scenes [10, 11] are often imaged and analyzed with a microscope [10-13]. The Leica DVM6 digital microscope can provide high quality images of hair or fiber samples under investigation quickly and easily. Images of human and animal hair and fibers in a textile sample taken with the Leica DVM6 are shown.

The human hair is composed of a shaft with three major features: medulla (central core), cortex (middle layer), and cuticle (outer layer) [11, 12]. The hair’s root grows from the follicle which is part of the scalp [11, 12].

Frequently, the medulla and cuticle are analyzed. In human hairs, the medulla is either continuous, interrupted, fragmented, or absent and the cuticle has an imbricate (flattened scale) structure [11, 12]. If a hair strand with its root intact is found to contain tissue from the follicle, it can be used for DNA analysis [11, 13]. Leica DVM6 images of various human hairs, recorded using the extended depth of field (EDOF) or multi-focus mode, are seen here in Figures 1–6.

Fig. 1: Treated hair strand showing an absent medulla.

Fig. 2: Dark brown hair strand with a continuous medulla.

Fig. 3: Gray hair strand with interrupted or fragmented medulla.

Fig. 4: Cuticle of a treated hair strand with coating.

Fig. 5: Hair root with follicular tissue.

Fig. 6: 3D image of the hair root (on the left).

The medulla of animal hairs often occupies a much larger volume of the shaft, known as the medulla index [11, 12], compared to human hairs. The cuticle of animal hairs can be coronal (crown-like scales), spinous (petal-like scales), or imbricate as noted above for humans [11, 12]. Leica DVM6 images (multi-focus mode) of an otter and sheep hair are shown in figures 7 and 8 below.

Fig. 7: Otter hair strand with large, continuous medulla.

Fig. 8: Sheep hair strand cuticle, the same as an untreated wool fiber.

Fibers are the smallest components of textiles. They can be natural from plants, insects, or animals, such as cotton, silk or wool, or synthetic from polymers, such as nylon or polyester [11, 14]. Textile fibers found at a crime scene can be transferred from the surroundings or victim onto the perpetrator during physical contact [11, 14]. Forensic analysis of such fibers can find unique characteristics or traits which help to identify and match them. Below in Figures 9–12 are some Leica DVM6 images of a textile sample composed of nylon. Figure 13 shows a line profile measurement made in the multi-focus image of Figure 12.

Fig. 9: Mosaic overview of a textile sample containing many nylon fibers.

Fig. 10: Zoom-in on a smaller area of a textile sample: individual nylon fibers resolved and reflection of ring light seen from glue binding sample to substrate.

Fig. 11: Higher magnification images of individual nylon fibers to better characterize them.

Fig. 12: 3D image (multi-focus mode) of a nylon textile sample with profile measurement (red line) and false color z-scale.

Fig. 13: Contour from line profile measurement in 3D image just above.

Property damage found at crime or accident scenes is often analyzed in forensic investigations [15, 16], e.g., painted metallic surfaces of automobiles, buildings, etc. There can be subtle, yet unique differences in the paint coatings of automobiles, especially in relation to the manufacturer and model. These differences make it possible to match paint evidence with a specific automobile or type of automobile [15, 16]. Paint chips found at a crime or accident scene can be imaged and its multiple layers analyzed with microscopy to identify some of its unique characteristics. Leica DVM6 images of a paint chip are shown in Figures 14–17 below.

Fig. 14: Blue metallic paint chip: 2D topview.

Fig. 15: Blue metallic paint chip: 3D side view of image.

Fig. 16: Cross section of the same paint chip above: 2D topview; multiple layers in the chip are visible.

Fig. 17: Cross section of the same paint chip above: 3D side view of image; multiple layers in the chip are visible.

Forensic investigations can also involve the study of residues from explosions which have occurred at crime or accident scenes [17, 18]. Typically, these residues are composed of small particles created by fragmentation of the explosive device itself or nearby objects [19, 20]. Swabbing areas around the center of the explosion is a way to collect these residue particles [17, 18]. The explosive residue swabs are often imaged and analyzed with microscopy, among other techniques. The images in Figures 18 and 19 below of swab samples were taken with a Leica DVM6.

Fig. 18: Multi-focus image of an aluminium particle present in a swab of an explosive residue.

Fig. 19: Another multi-focus image of an aluminium particle present in a swab of an explosive residue.

Forensic entomology is the study of insects and arthropods found at crime scenes, usually in relation to death investigations [21–23]. The insects most often found on and around a corpse are flies and beetles. The presence of a particular species of fly or beetle usually correlates with the time since death [21–
23].

As is also the case in general entomology, samples of insects and arthropods collected during forensic investigations can be identified from their anatomy via microscopy imaging. Leica DVM6 images of a green blow or bottle fly, a species commonly found on a corpse shortly after death [21–23], are shown in Figures 20 and 21.

Fig. 20: Multi-focus image of a green blow fly at higher magnification with ring light illumination: portion of the abdomen (with hair) and wing shown.

Fig. 21: Same part of the blow fly seen from the left, except imaged with coaxial illumination.

Summary table showing which features of the Leica DVM6 are useful for certain forensic analyses [9]. An X means the feature is “useful” for the particular analysis.

	Magnification			Illumination			Observation perspective		3D imaging			2D imaging
	Low objective	Middle objective	High objective	Integrated ring / Coaxial light	Spotlight (LED5000 SLI)	Transmitted light (BLI)	Tilting	Rotating	Extended Depth of Field (infinite focus)			X-Y stitching (scan)
Leica DVM6 S									Z image builder			X-Y image builder
Leica DVM6 A										Montage	3D models		Mosaic
Hair and fibers		X	X	X	X	X			X	X	X
Paint chips	X	X	X	X	X		X	X	X	X	X
Explosives	X	X	X	X	X		X	X	X	X
Particles	X	X	X	X	X		X	X	X	X
Soil	X	X	X	X	X		X	X	X	X
Glass	X	X	X	X	X	X	X	X	X	X
Entomology	X	X		X					X	X	X	X	X

The results in this report show the use of the Leica DVM6 digital microscope for forensic analysis of evidence from hair, fibers, paint layers, explosive residues, and insects (entomology). A fast, efficient workflow for analysis of forensic evidence is accomplished due to the practical features of the Leica DVM6, e.g., a convenient way to change magnification rapidly over the full range, one-handed tilting and sample rotation, intuitive software for microscope operation and data analysis, and versatile illumination. An efficient workflow leads to efficient data acquisition and analysis for forensic investigations.

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