Reading the Scene
When blood contacts a surface, it leaves stains whose shape, size, and distribution encode information about the event that produced them. Bloodstain pattern analysis (BPA) uses physics and geometry to reconstruct impact angles, velocities, and the spatial origin of bloodshed. First formalized by Eduard Piotrowski in 1895 and refined by Victor Balthazard and Herbert MacDonell, BPA remains a key forensic discipline.
Stain Geometry
A blood drop is nearly spherical in flight due to surface tension. When it strikes a surface at an angle, the resulting stain is an ellipse whose eccentricity reveals the impact angle: sin(α) = width/length. At 90° the stain is circular; at 10° it becomes a narrow ellipse 5.7 times longer than wide. The pointed end (tail) indicates the direction of travel, allowing analysts to trace trajectories back to their origin.
Velocity and Classification
The energy of impact determines stain size and the extent of satellite spatters. Low-velocity events (dripping blood) produce large, round stains with smooth edges. Medium-velocity impacts (punches, bludgeoning) create smaller stains with some spines. High-velocity events (gunshots) atomize blood into a mist of sub-millimeter droplets. This classification helps investigators distinguish between mechanisms of injury even before other evidence is examined.
Area of Origin
The most powerful application of BPA is reconstructing the three-dimensional point where blood originated. By measuring the directionality and impact angle of multiple stains, analysts trace convergence lines to find the area of origin on the floor plane, then use the tangent of each impact angle to estimate the height. Modern software tools replace physical strings with computational reconstruction, improving accuracy and documentation.