Unit 4: Introduction to Impression Evidences

I. Foundations of Impression Evidence

Impression evidence is any pattern created when one object presses against, strikes, or slides across another surface, transferring the shape and surface detail of the first object onto the second. It is a mainstay of physical evidence because it links a specific object (a shoe, tool, tooth, or tyre) to a scene through comparison of transferred detail.

• Principle of transfer (Locard): every contact leaves a trace; an impression is a negative, mirror-image record of the contacting surface.
• Two-dimensional vs three-dimensional: a 2D impression sits on a surface (dust, blood, ink film); a 3D impression displaces a pliable medium (soil, wax, snow) and preserves depth.
• Comparison basis: identification proceeds by matching class characteristics (shared by a group) and then individual characteristics (unique to one object).
• Fragility: impressions are perishable — they must be photographed with a scale before lifting or casting.
• Reference standard: every questioned impression is compared against a known exemplar taken from the suspected source.

II. Types and Classification — Cataloguing Surface Evidence

A. types of impression evidences found at crime scene

Impression evidence at scenes falls into recurring categories defined by the source object.

• Friction ridge impressions: fingerprints, palm prints, and bare footprints carrying ridge detail.
• Footwear and tyre impressions: patterned marks from shoe soles and vehicle treads in soil, dust, or on floors.
• Tool marks: striations and indentations from screwdrivers, crowbars, and cutters on forced entry points.
• Bite marks: tooth impressions on skin or food.
• Lip and ear prints: contact transfers on glass, doors, and windows.
• Fabric and glove marks: weave or grain patterns left on painted or dusty surfaces.

B. classification of impressions on the basis of visual appearance

Visibility dictates the search and enhancement method used.

1. Patent (visible) impressions: formed in a contaminant that is readily seen, e.g., a bloody shoeprint on tile or a greasy palm print. No development needed — photograph directly.
2. Latent (invisible) impressions: present but not visible until treated, e.g., a dust footprint requiring oblique lighting or an electrostatic lifter.
   • Plastic (moulded) impressions: a subset that is three-dimensional, pressed into a soft medium such as putty, mud, or snow, retaining depth and requiring casting.

III. Lip Prints — Cheiloscopy

Cheiloscopy is the study of the grooves on the labial mucosa, whose pattern is claimed to be individual and stable through life.

lip prints; their nature and location

• Nature: the vermilion border bears furrows (sulci) forming patterns classified by Suzuki and Tsuchihashi into types I (clear vertical), I′ (partial vertical), II (branched), III (intersecting), IV (reticular), and V (undetermined).
• Persistence: the groove pattern reappears after minor injury and remains constant, supporting individualisation.
• Location of occurrence: drinking glasses, cups, cigarette butts, letters, window panes, and clothing.
• Visibility: may be visible (lipstick transfer) or latent (from natural lip secretions), developed with lysochrome dyes or fingerprint powders.

IV. Foot and Footwear Impressions — Podiatry at the Scene

A. foot and footwear impressions; their nature and location of occurrence

Footwear impressions record the outsole design and its accumulated damage, forming one of the most common scene impressions.

• Nature (bare foot): carries friction ridge detail on the sole plus shape, arch, and toe morphology; can individualise like a fingerprint.
• Nature (footwear): reproduces the manufacturer's tread pattern, size, and wear plus randomly acquired cuts, nicks, and embedded stones.
• Location of occurrence: entry and exit paths, soft soil and mud outdoors, dusty or waxed floors, and on kicked doors or victims (stamping marks).
• Recovery: 3D casts with dental stone; 2D lifts electrostatically or with gel lifters, always photographed with a scale.

B. importance of gait pattern

Gait is the individual manner of walking, recorded as a sequence of impressions rather than a single mark.

• Definition: the walking pattern showing stride length, straddle width, foot angle, and heel-to-toe pressure distribution.
• Individualising value: habitual gait (limps, pigeon-toe, out-toeing) persists and can associate a track line with a person even without a clear single print.
• Track line measurements: stride length (heel to heel of the same foot), step length, and foot angle relative to the direction of travel.
• Application: reconstructs number of persons, direction, and whether the subject was walking, running, or carrying a load (deeper, wider prints).

V. Tyre Marks, Skid Marks and Drag Marks — Vehicle-Related Impressions

A. tyre marks, skid marks and drag marks; their nature and location of occurrence

These marks connect a vehicle to a scene and inform reconstruction of its movement.

• Tyre marks: rolling impressions reproducing tread design, width, and circumference; found in soil, snow, or as dust/mud transfers on roads and victims.
• Skid marks: friction abrasions on the road surface from locked or braking wheels, indicating speed and braking; located on carriageways at collision approaches.
• Drag marks: trails left when a body or object is pulled across a surface; found from a point of assault to a disposal site.
• Reconstruction use: skid length feeds speed estimation; track width and wheelbase from turning marks identify vehicle class.

B. original and replaced tyre impressions

Distinguishing factory-fitted from later-fitted tyres refines vehicle identification.

1. Original (OE) tyres: those specified and fitted by the manufacturer; all four often share brand, pattern, and size, giving a consistent set.
2. Replaced tyres: aftermarket fittings that may mismatch the others in brand, pattern, or wear, producing differing left/right or front/rear impressions that narrow identification.

C. tread nomenclature and indicator of wear and tear

Standard terms describe the tread so patterns can be compared and wear assessed.

• Tread elements: ribs (circumferential raised bands), grooves (circumferential channels), lugs/blocks (raised tread sections), sipes (fine slits), and shoulder (outer edge).
• Noise treatment: varied block pitch reduces hum and creates recognisable sequences.
• Wear indicators: moulded tread wear bars appearing when tread depth reaches ~1.6 mm; uneven wear (feathering, cupping, one-sided) signals misalignment or under-inflation and adds individualising detail.
• Value: wear depth and damage convert a class match into an individual association.

VI. Bite Marks — Forensic Odontology

Bite marks are impressions of the biting edges of teeth, studied to link a suspect's dentition to a mark.

bite marks; nature, types and location of occurrence

• Nature: typically two opposing arches (upper and lower) showing arch shape, tooth size, spacing, rotations, and missing teeth.
• Types:
  • Class I – hemorrhagic: small bleeding points, minimal pattern.
  • Class II – abrasive/contused: clear tooth marks with abrasion.
  • Class III – lacerated: tissue torn, indicating force.
  • Also distinguished as animal vs human and aggressive vs amorous (self vs assailant).
• Location of occurrence: on victims' skin (breasts, arms, shoulders in assaults), on offenders (defensive bites by victims), and on foods (cheese, fruit, chocolate) or chewing gum.
• Recording: photograph with an ABFO No. 2 scale; compare against dental casts of the suspect.

VII. Ear Prints — Otoscopy

Ear prints are contact transfers of the outer ear, used where a person presses an ear against a surface to listen.

ear prints; nature and location of occurrence

• Nature: an impression of the helix, antihelix, tragus, antitragus, and lobe; the anthropometric shape is considered highly individual.
• Location of occurrence: doors and window panes at burglary points where the intruder listened before entry.
• Recovery: developed like latent prints with powders, then lifted; compared with exemplar ear prints taken at controlled pressure.
• Caution: pressure and angle distort the print, so class-level use dominates over strict individualisation.

VIII. Class and Individual Characteristics — The Basis of Comparison

This section defines the two-tier logic that governs all impression identification.

class and individual characteristics of various impression evidences

1. Class characteristics: features shared by all items of a group, narrowing but not individualising the source.
   • Footwear: manufacturer's tread design, size, mould.
   • Tyres: pattern, width, tread design.
   • Tools: blade width, type (cutting vs prying).
   • Bite/ear: general arch shape or ear form.
2. Individual characteristics: randomly acquired features unique to one object, enabling positive identification.
   • Footwear: cuts, gouges, embedded gravel, wear pattern.
   • Tyres: cuts, uneven wear, repairs.
   • Tools: microscopic striations and nicks on the working edge.
   • Teeth: fractures, rotations, restorations.
   • Reasoning: class characteristics establish the possible source group; individual characteristics confirm the single source when sufficient in number and clarity.

IX. Tool Marks — Mechanical Impressions

Tool marks are impressions or scratches produced when a tool contacts a softer surface, common in forced-entry cases.

tool marks; their types, nature and location of occurrence

• Types:
  • Impressed (compression) marks: a tool pressed into a softer surface leaving a negative shape, e.g., a hammer or pliers indentation.
  • Striated (sliding/friction) marks: parallel scratches from a tool dragged across a surface, e.g., a screwdriver prying, cut wire ends.
  • Combination marks: both compression and slide, as when a jaw grips and twists.
• Nature: reproduces the tool's contour (class) plus microscopic edge imperfections that transfer as fine striae (individual), examined by comparison microscopy.
• Location of occurrence: door frames, window latches, locks and hasps, safes, cut chains and wires, and padlocks in burglaries.
• Recovery: photograph and preferably submit the whole object; where impossible, cast with silicone-based material rather than cleaning the mark.