When investigating a crime scene, many different variables are taken into account such as fingerprints, any other forms of DNA, or even clues left behind at a crime scene. Even looking at blood splatter, the forensic technician can conclude an estimated guess to the weapon used, the height of the person, whether it was foul play or not, and even if it was passive patterns or projected patterns. Blood splatter analysis becomes important when recreating a crime scene (Peschel). Bloodstain pattern analysis comes into play; and that is the interpretation of bloodstains at a crime scene in order to recreate the actions that caused the bloodshed (“A Simplified Guide”).
High-speed photography of blood drops impacting different target surfaces
Introduction The results of the experiment carried out in section 5.1 showed that textured/nonglossy surfaces exhibited the greatest amount of error in the angle of impact calculation compared to smooth/glossy surfaces. At acute angles (20°, 30°, 40°) wood was the most inaccurate surface followed by wallpaper. This may be due to the way in which the bloodstains formed on the different surfaces and this experiment was carried out to allow a slow motion view of the formation bloodstains on various surfaces. The objective of this experiment is to determine whether there is a difference in the way in which bloodstains form on smooth/glossy and textured/non-glossy surfaces and to document these differences.
Method Two mirrors were set up so that the surface upon which the blood drop was being released onto could be viewed in two ways – front on and side on (See Fig. 4.6). A high-speed camera was then set up so that it was perpendicular to the surface (Fig. 5.32). One drop of blood was released from a plastic pipette, which was clamped 40 cm above the target surface, onto the surface, which was set at 30° to the vertical.
The high-speed camera was used to record the blood drop impacting each surface at 1037 frames per second with an exposure time of 960 µs. This process was repeated three times for each surface. The surfaces used in the experiment are given in Table 5.9. The width and length of the resulting bloodstains was measured for each surface. Materials Pipette Surface High-speed Camera
Summary of the different target surfaces. Target Surface Description Cardboard 10 cm long x 5 cm wide A4 chlorine free cardboard (225gsm, Klippan Kaskad Boards) Vinyl10 cm long x 5 cm wide lightly coloured vinyl Wallpaper 10 cm long x 5 cm wide textured, brown and gold wallpaper Wood 10 cm² oak
Results and discussion Still images of a single blood drop impacting each target surface taken from high speed videos are shown in Figures 5.34 – 5.45. Following these is a series of observations made from the videos along with a table showing the average width and length of the resulting bloodstains. The diameter of a blood drop prior to impacting the target surface was 4 mm ± 0.5 mm.
• When the blood drop impacted the cardboard it began to flow over the surface (a). • When the bloodstain was about half of its final size, wave cast-off began to form from both sides (b). • These continued to be pushed out from the bloodstain and eventually merged to form a single wave cast-off (c,d). • This finally landed on the cardboard surface below the bloodstain at a distance about the length of the bloodstain (e). • There was a small pool of blood that sat at the bottom of the bloodstain, above the wave cast-off. A small amount of this blood ran down into the wave castoff, elongating it and the resulting bloodstain appeared to have one ‘tail’ below the elliptical body. • The blood was continuously flowing in a downward direction.
Table 5.10 The width and length measurements and the calculated angle of impact of three bloodstains made on a cardboard surface. The actual impact angle was 30°. Cardboard 1 2 3 Average Width 10 10 9 9.7 Length 22 22 20 21.3 Calculated angle of impact (degrees) 27.0 27.0 26.7 27°
Vinyl
• When the blood drop impacted the vinyl it began to flow over the surface (a). • The blood continuously flowed downwards resulting in cast-off forming from the bottom of the bloodstain (c,d). • The wave cast-off had a circular blood drop at the end of it (d). 105 • After the bloodstain formed, the blood from this drop began to run downwards, elongating the cast-off. • The final bloodstain appeared very dilute and throughout the bloodstain and wave cast-off, there was a ‘channel’ through which the blood had travelled in a downward direction.
Table 5.11 The width and length measurements and the calculated angle of impact of three bloodstains made on a vinyl surface. The actual impact angle was 30°. Vinyl 1 2 3 Average Width 10 10 10 10 Length 22 23 23 22.7 Calculated angle of impact (degrees) 27.0 25.8 25.8 26°
Wallpaper • The blood drop moved over the surface of the wallpaper forming an elliptical shape with a small amount of wave cast-off (c,d). • After the bloodstain had formed, a small blood pool formed just above the wave cast-off (d). • This pool of blood was pushed upwards towards the leading edge of the bloodstain and then moved down toward the wave cast-off. • This occurred a number of times before the blood began to run down the wave cast-off. • After about one minute the bloodstain began to crack and flake off from the wallpaper.
Table 5.12 The width and length measurements and the calculated angle of impact of three bloodstains made on a wallpaper surface. The actual impact angle was 30°. Wallpaper 1 2 3 Average Width 9 10 10 9.7 Length 23 23 2323 Calculated angle of impact (degrees)23.0 25.8 25.8 25°
Wood • The blood drop travelled over the grainy wood surface and as the wave castoff began to form, it lifted off the surface of the wood leaving a space without blood directly above the wave cast-off. • The bloodstains that formed on the wood surface had areas that had no blood because of the grains on the surface.
Table 5.13 The width and length measurements and the calculated angle of impact of three bloodstains made on a wood surface. The actual impact angle was 30°.
Wood 1 2 3 Average Width 11 9 9 9.7 Length28 27 29 28 Calculated angle of impact (degrees) 23.1 19.5 18 20°
The blood formed differently on all of the surfaces examined in this experiment. On surfaces including the wood and wallpaper, the blood ‘skimmed’ over the grains and the bloodstain that formed on the wood had voids where the blood had not touched the surface. The blood glided smoothly over the vinyl and cardboard resulting in a bloodstain with more defined perimeters compared to those that formed on wood and wallpaper.
The widths of the resulting bloodstains were very similar for each different surface (10° ±1°). The length however was very different for each different surface. The resulting bloodstain was longer for wood and wallpaper compared to that of cardboard and vinyl. This may be due to the fact that on these more textured surfaces the blood moved over the grains in the surface leaving voids where blood had not touched the surface.
Because the blood drops consisted of the same volume of blood, it may have been that because the blood ‘skimmed’ the surface it could travel further down the surface as opposed to when it formed on a surface such as cardboard where it covered the entire surface. Elongation of blood drop may account for the underestimation of the angle of impact at acute angles that was found in Section 5.2. The time taken for a bloodstain to form on cardboard and wallpaper was different by about 24 ms, the bloodstain forming on cardboard much more quickly than on wallpaper (Table 5.14). The bloodstain on wood however only took 39.7 ms to form, which was similar to the time taken for a bloodstain to form on vinyl.
It could be seen in the high-speed videos that the blood glided over the cardboard surface without any blood moving back up the bloodstain (all the blood moved in a downwards direction forming a long wave cast-off). This was however not the case for wallpaper where the blood could be seen moving up and down the bloodstain before finally forming the bloodstain. This may have been due to the amount of friction the blood encountered when moving along the surface – there would not have been much friction force in the case of the cardboard, however on the wallpaper, because it is a textured surface, there would have been a greater friction force.
Given this finding, it would have been expected that the time taken for a bloodstain to form on the wood surface would have taken approximately the same time as the time taken for a bloodstain to form on wallpaper. This was however not the case, with the bloodstain forming in only 39.6 ms on wood. It was seen in the high-speed video of a bloodstain forming on wood that the blood travelled continuously in a downward direction meaning no extra time was spent with blood moving up and down the bloodstain. The bloodstains that took the shortest amount of time to form had the longest wave castoff. Therefore in general perhaps the longer that the wave cast-off is, the less time that was taken to form a bloodstain.
Table 5.14 The time taken for bloodstains to form on different surfaces Surface Time at which blood touched surface (ms) Time at which bloodstain had formed (ms) Time to form bloodstain (ms)
Cardboard 8.7 36.6 27.9 Wood 16.4 56.0 39.6 Vinyl 11.5 52.0 40.5 Wallpaper 6.7 58.8 52.1
Conclusion The high-speed photography used in this study showed that blood drops behave differently on different surfaces. An impact spatter pattern can be found on a number of different surfaces at a crime scene. Regardless of the surface type, if it is necessary, a region of origin determination may be carried out. The experiments carried out in this chapter have shown that the surface type can affect the accuracy of the angle of impact calculation, which may consequently affect the accuracy of the region of origin determination.
Surfaces including wood and wallpaper produced less accurate angle of impact results compared to surfaces including cardboard, tile glass and vinyl. This study has shown that the way in which bloodstains form on a surface is unique for that particular surface. It is therefore important for forensic analysts to be aware that the surface upon which an impact spatter pattern is can affect the accuracy of the angle of impact calculation. While the exact amount of uncertainty in the result may not be able to be quantified, it should at least be recognized that the surface type might affect the overall region of origin result.
