Came across this excellent video from Matthew Steiner on Wired and went ahead and excerpted a few key images for anyone writing a forensic scene. (The blood in the video is all lamb’s blood!) The whole video is worth a watch, but here’re some highlights.
Edge Characteristics -Â Â Blood on different surfaces leaves different edge patterns.
Satellite Stains - Stains that come off of parent stains. The drop on the bottom is the ‘parent stain’ in the image below. In this example, the blood is being forced out.
Also occurs when blood is being dripped in to blood (image below). These drips patterns are particularly common in stabbings. In some cases, these can even be followed.
Transfer Patterns - A bloody surface coming in contact with another surface.
The image below is of a shoeprint covered in blood. The pattern gets lighter and lighter as the subject moves away.
The example below is from blood that was applied to jeans and then bumped up against a surface. Easy to misinterpret as a palm/handprint at first glance, but the lines are too straight.
Below is a blood-stained hand being wiped across a surface. The effect of movement on blood produces ‘feathering’, similar to a paintbrush. Useful for interpreting movement.Â
Pooling (below) shows that the blood has been accumulating for some time, meaning that the source likely wasn’t moved. Usually dries a lot slower, and will produce clotting.
Serum Separation - The plasma of the blood is visible, more commonly found in blood that has pooled.
Force being applied to an open wound produces a distinct splatter pattern (below). Directly opposite where the force was applied, the blood will strike the surface at 90 degrees, and become more elliptical at other angles. Most common with bludgeoning.
Calculating the angle of impact can help show where a body was positioned at the moment of impact. This is calculated by measuring the length and width of the stain (usually using a digital calipers), dividing those numbers, and then multiplying them by the arcsin. Always measure in millimeters (or smaller).
More trigonometry if you want to find the area of origin. The 22cm is the distance between the stain and the area of convergence, and the 33.25 degrees was the previously-calculated angle of impact (above). x will then tell you how high the area of origin is (i.e.: how far the source was from the ground).
Wipe & wipe patterns. The feathering indicates the direction of movement.
Cast-off pattern. There is blood on something (e.g.: subject’s hand, a knife) and it is dripping as it is being moved through space. Blood is flung off in distinct curvilinear patterns. Found on walls, floors, ceilings.
Void. A continuous spatter pattern that is being interrupted/blocked by something. Could indicate something like a bag, wallet, cell phone. Can indicate movement of something from the crime scene.
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