Chapter 03· 4 min read
Pattern Evidence
Reading as a guest
Sign up free to save your progress, highlight passages, and pick up where you left off.
You'll lose your reading position and notes if you leave without an account.
Pattern evidence is the family of physical comparisons that depend on the geometry, distribution, or microstructure of an impression rather than its chemistry. Bloodstain spatter, glass fracture, tool marks, footwear and tyre impressions, fingerprints, hair, and fibre transfer all belong here. The unifying methodology is the class / sub-class / individual hierarchy.
3.1Bloodstain Pattern Analysis
Bloodstain pattern analysis reconstructs events from the geometry and distribution of blood deposits. The single most-cited measurement is the angle of impact.
Velocity classification by mechanism
| Pattern | Droplet size | Mechanism |
|---|---|---|
| HVIS (High-Velocity Impact Spatter) | < 1 mm (mist) | Gunshot, high-energy impact |
| MVIS (Medium-Velocity Impact Spatter) | 1–4 mm | Blunt force (hammer, fist) |
| Cast-off | 4 mm+ in arc / line | Swung weapon between strikes |
| Drip | 4 mm+, circular | Falling drop from slow source |
| Transfer | Smear / wipe | Contact pattern, no impact |
3.2Glass Fracture Analysis
When a projectile or blow strikes a glass pane, two distinct fracture systems form: radial cracks running outward like spokes, and concentric cracks forming rings around the impact.
The 3R Rule: Radial cracks form Right angles on the Reverse side of the impact. On a radial crack, the broken edge shows ridges (Wallner stress lines) that meet the surface at a right angle on the side furthest from the impact. So examining the right-angle side identifies the side opposite the impact — i.e., which side the bullet or blow came from.
Multi-impact ordering (radial-termination rule): new cracks stop at old cracks because the existing fracture has already released the local stress field. Earlier impact's radials dominate.
Cone of debris (bullet through glass): the cone of glass debris ejected on the exit side points in the direction the bullet was travelling.
3.3Tool-Mark Hierarchy
The hierarchy of tool-mark identification is the framework that prevents over-reaching from class match to identification:
Class characteristics
Designed-in features shared by every example of the tool type. They narrow the source population to "all tools of this type" but do not individualise.
Sub-class characteristics — the trap
Features shared by a subset of tools made consecutively under similar manufacturing conditions. They can mimic individual characteristics under the comparison microscope but are not unique. Failure to recognise sub-class has caused documented false-positive identifications.
Individual characteristics
Random acquired features unique to one specific tool: nicks, accidental impacts, polishing tracks, wear patterns. The probability that two distinct tools share these features is vanishingly small. Individual matches are the basis for AFTE-style identification.
3.4Footwear and Tyre Impressions
| Vehicle class | Wheelbase | Track width |
|---|---|---|
| Hatchback (Maruti Swift) | 2400–2500 mm | 1450–1500 mm |
| Compact sedan (Honda Amaze) | 2450–2550 mm | 1480–1520 mm |
| Mid-size sedan (Honda City) | 2550–2650 mm | 1500–1550 mm |
| Mid-size SUV (Toyota Innova) | 2700–2800 mm | 1500–1580 mm |
| Light commercial truck | > 3000 mm | > 1600 mm |
3.5Hair and Fibre
Hair phase determines DNA strategy. Anagen-phase hair (~85% of scalp hairs) has a strong root with attached follicular tissue → nuclear STR profiling possible (~10⁻¹³ random match probability across 13 CODIS loci). Telogen-phase hair (~10–15%) has a club-shaped root, little tissue → fall back to mitochondrial DNA from the keratinised shaft (HV1 / HV2 hypervariable regions). mtDNA is maternally inherited — identical across maternal-line relatives.
Fibre transfer hierarchy: primary (direct contact) → secondary (one intermediate) → tertiary (two). Each step attenuates the signal. Defence will argue that an apparent primary match is secondary / tertiary if intermediate-contact is plausible.
3.6Fingerprint Development by Substrate
| Substrate | Reagent / technique |
|---|---|
| Smooth dry non-porous (glass, polished metal) | Powder dusting; or cyanoacrylate fuming + dye stain |
| Porous (paper) | DFO (with laser), then ninhydrin |
| Porous wet history | Physical developer (silver-based) |
| Wet non-porous (plastic from water) | Small Particle Reagent (SPR) |
| Adhesive surfaces (sticky tape) | Sticky-side powder OR gentian violet |
| Cartridge cases (fired) | Cyanoacrylate + fluorescent dye + 532 nm laser, or VMD |
| Indented writing on paper | ESDA — non-destructive; before chemical development |
3.7Bite Marks and Lip / Ear Prints — Caution
The historical confidence in bite-mark identification has collapsed under empirical scrutiny. The NAS 2009 report and PCAST 2016 report documented that bite-mark comparison lacks scientific validation for individual identification. 24+ documented US wrongful convictions were overturned by DNA evidence.
Modern practice treats bite-marks as exclusion-only evidence. Lip prints (cheiloscopy) and ear prints (otoscopy) are similarly supportive only.
Bloodstain angle: sin θ = w / l. Tail toward direction of travel. Velocity classes: mist (HVIS gunshot), 1–4 mm (MVIS blunt), arc (cast-off swing). Glass 3R: Right angle on Reverse side. Termination rule: new cracks stop at old. Cone of debris: points where the bullet was going. Tool marks: class (type), sub-class (batch — the trap), individual. Foot stature: foot ≈ 0.15 × stature. Hair DNA: tag → nuclear STR; shaft → mtDNA. Fibre transfer: primary direct, then 1, then 2 intermediates. Bite marks: exclusion-only after NAS / PCAST.
Don't lose your place
Save this chapter and the rest of Forensic Physics.
A free ForensicSpot account remembers which chapters you've read, lets you highlight passages, take notes and resume from any device.