Unit 4: Introduction to Impression Evidences - Subjective Questions

Impression evidence refers to any object or material that has retained the characteristics of another object through physical contact, resulting in a transfer or reproduction of surface features.

Significance in forensic investigations:

• Linking evidence: Impressions can link a suspect, tool, vehicle, or object to a crime scene.
• Reconstruction: They help reconstruct the sequence of events at a crime scene.
• Individualization: Some impressions carry individual characteristics that can identify a unique source.
• Class characteristics: Even without individualization, they narrow down the class of the source (e.g., shoe brand, tyre type).
• Corroboration: They corroborate other physical or testimonial evidence.

Examples: Fingerprints, footwear impressions, tyre marks, tool marks, bite marks, lip prints, and ear prints. These evidences are governed by Locard's Exchange Principle, which states that every contact leaves a trace.

Various types of impression evidences may be encountered at a crime scene:

• Fingerprints/Palm prints: Friction ridge impressions left by hands.
• Footwear impressions: Marks left by shoes or bare feet.
• Tyre marks: Impressions of vehicle tyres including tread patterns.
• Tool marks: Marks created by tools on surfaces during forced entry or manipulation.
• Bite marks: Impressions of teeth on skin or food items.
• Lip prints (Cheiloscopy): Impressions of lip grooves.
• Ear prints (Otoscopy): Impressions left by the ear pressed against surfaces.
• Fabric/Textile impressions: Impressions of clothing weave.
• Skid marks, drag marks: Related to vehicle and body movement.

Classification by nature:

• Two-dimensional (2D): Formed on flat surfaces (dust prints, blood prints).
• Three-dimensional (3D): Formed in soft/malleable materials (mud, snow, putty).

Each type provides both class and possibly individual characteristics useful for investigation.

Impressions are classified on the basis of visual appearance into three main categories:

1. Patent (Visible) Impressions:

• Clearly visible to the naked eye without any enhancement.
• Formed when a contaminant (blood, dust, paint, grease) is transferred.
• Example: A bloody shoe print on a tiled floor.

2. Latent (Invisible) Impressions:

• Not readily visible; require enhancement techniques to be seen.
• Formed by natural secretions or fine residues.
• Example: A latent fingerprint requiring powder dusting or chemical development.

3. Plastic (Molded/Three-dimensional) Impressions:

• Formed when an object presses into a soft, pliable surface leaving a 3D negative.
• Example: A shoe impression in mud, snow, wet clay, or putty.

Importance: The type dictates the method of preservation — photography, lifting, or casting.

Cheiloscopy is the branch of forensic science that deals with the study of lip prints for personal identification.

Nature of lip prints:

• The external surface of the lips has numerous grooves and furrows forming a characteristic pattern.
• These patterns are unique to each individual and remain stable throughout life (except due to injury/pathology).
• Formed by transfer of lipstick, saliva, or other substances, or as latent impressions.

Classification (Tsuchihashi/Suzuki classification):

• Type I: Clear-cut vertical grooves running across the lip.
• Type I': Similar grooves but not covering the entire lip.
• Type II: Branched grooves.
• Type III: Intersecting/crossed grooves.
• Type IV: Reticular (net-like) pattern.
• Type V: Undetermined/irregular grooves.

Location of occurrence:

• Drinking glasses, cups, cigarette butts.
• Envelopes, letters, tissue papers.
• Clothing, windows, mirrors.
• Food items.

Lip prints thus serve as valuable individualizing evidence.

Foot and footwear impressions are among the most commonly encountered impression evidences.

Nature:

• Barefoot impressions: Show friction ridge detail of the sole plus the shape/size of the foot, revealing individual characteristics.
• Footwear impressions: Reproduce the outsole design, size, wear patterns, and accidental damage of the shoe.
• Can be 2D (on hard surfaces with dust/blood transfer) or 3D (in soft materials).

Class characteristics: Brand, size, tread design, manufacturing pattern.
Individual characteristics: Wear patterns, cuts, nicks, embedded debris, and randomly acquired features.

Locations of occurrence:

• Soft surfaces: Mud, sand, soil, snow, soft soil near windows/doors.
• Hard surfaces: Floors, tiles, papers, counters (dust or bloody prints).
• Points of entry and exit.
• Along the path of movement of the perpetrator.

Proper documentation involves photography with scale, casting (3D), and lifting (2D).

Gait pattern refers to the characteristic manner or style in which a person walks, and it holds significant forensic value.

Components of gait:

• Gait line: An imaginary line drawn along the direction of travel.
• Step length: Distance between successive footprints of alternate feet.
• Stride length: Distance between successive impressions of the same foot.
• Foot angle: Angle formed by the foot axis relative to the gait line.
• Straddle/width: Lateral distance between the feet.

Importance:

• Individual identification: Gait characteristics are relatively unique to individuals.
• Determining physical condition: Reveals limps, injuries, or disabilities.
• Estimating characteristics: Approximate height, sex, and physical build.
• Reconstruction: Indicates whether the person was walking, running, or carrying a load.
• Number of persons: Helps determine how many individuals were present.
• Direction of movement: Reveals entry and exit paths.

A series of footprints (a gait pattern) is more valuable than a single impression as it reveals behavioural and locomotive traits.

These three types of marks differ in their formation mechanism and forensic value:

1. Tyre Marks (Tyre Impressions):

• Nature: Reproduction of the tyre tread pattern when a vehicle rolls over a surface.
• Location: Soft soil, mud, snow, or as dust/blood prints on hard surfaces.
• Value: Reveal tyre brand, tread design, size, and wear (class and individual characteristics).

2. Skid Marks:

• Nature: Produced when tyres slide/lock during sudden braking without rolling; the tread pattern is usually absent or smeared.
• Location: On road surfaces at accident scenes.
• Value: Help estimate vehicle speed, direction of travel, and braking behaviour.

3. Drag Marks:

• Nature: Marks left when a body, object, or heavy item is dragged across a surface.
• Location: Floors, soil, carpets, from crime scene to disposal site.
• Value: Indicate movement of a body/object, direction, and possible number of persons involved.

Summary: Tyre marks reproduce tread; skid marks arise from braking/sliding; drag marks arise from dragging objects/bodies.

Skid marks are produced when a vehicle's tyres lock and slide during hard braking. They are valuable for reconstructing accidents and estimating pre-braking speed.

Principle:
The kinetic energy of the moving vehicle is dissipated as friction between the tyres and the road surface over the skid distance.

Speed estimation formula:

Where:

• = initial speed of the vehicle (m/s)
• = coefficient of friction between tyre and road
• = acceleration due to gravity ()
• = length of the skid mark (m)

A commonly used practical form:

Where = speed (mph), = skid distance (feet), = drag factor.

Factors affecting accuracy:

• Road surface condition (wet/dry).
• Type and condition of tyres.
• Braking efficiency of the vehicle.
• Slope/gradient of the road.

Thus, skid mark analysis provides a scientific basis for reconstructing accident dynamics.

Bite marks are impressions produced by teeth and are studied under Forensic Odontology.

Nature:

• Represent the arrangement, size, and shape of an individual's teeth (dentition).
• May show individual characteristics such as spacing, rotation, missing/broken teeth.
• Typically appear as circular or oval bruises/abrasions with two opposing arches.

Types of bite marks:

• Hemorrhage: Small bleeding spot.
• Abrasion: Undamaging mark on skin.
• Contusion: Bruise.
• Laceration: Puncture or tearing of skin.
• Incision: Neat puncture of skin.
• Avulsion: Removal of skin.
• Based on aggression: offensive/defensive bite marks.

Location of occurrence:

• On victims: Skin (breasts, arms, shoulders), commonly in cases of assault, sexual offences, and child abuse.
• On suspects: Defensive bites by the victim.
• On objects/foodstuffs: Cheese, chocolate, apples, chewing gum left at the scene.

Bite mark analysis involves comparison of the mark with dental casts of a suspect for identification.

Ear prints are impressions left by the external ear (auricle) when pressed against a surface, studied under Otology/Earology.

Nature:

• The morphology of the ear — including the helix, antihelix, tragus, antitragus, lobe, and concha — is unique to each individual.
• Ear structure is considered stable after a certain age and does not change significantly.
• Ear prints are usually latent, formed by transfer of sebum/sweat, requiring enhancement to visualize.

Location of occurrence:

• Doors and windows: When a burglar presses the ear against them to listen for occupants before breaking in.
• Glass panes, walls, and other smooth surfaces.

Evidential value:

• Provides class characteristics (general shape) and possible individual characteristics (unique contours).
• Useful in burglary and breaking-in cases.
• Requires expert comparison with known ear impressions of suspects.

Limitation: Reliability is debated due to variability from pressure and angle of contact, so it is often used as corroborative rather than conclusive evidence.

Tyre impressions can originate from either the original tyre or a re-treaded (replaced) tyre, and distinguishing between them is important in vehicle identification.

Original Tyre Impressions:

• Produced by tyres in their factory-manufactured condition.
• Tread design and dimensions exactly match manufacturer specifications.
• Sidewall markings (size, brand, DOT code) correspond to the tread pattern.
• Wear characteristics develop uniformly according to use.

Replaced / Re-treaded Tyre Impressions:

• Produced by tyres whose worn tread has been replaced with a new tread (recapping/re-treading).
• The tread pattern may not match the original tyre casing or sidewall markings.
• Often show a seam or joint line where the new tread was applied.
• May display irregularities, buffing marks, or mismatched patterns.
• Common in commercial/heavy vehicles for economy.

Forensic significance:

• Identifying a re-tread helps narrow down the vehicle.
• Mismatches between tread pattern and casing markings are strong indicators of re-treading.
• Re-tread seams can act as individual characteristics.

Tread nomenclature refers to the standardized terminology used to describe the various features of a tyre's tread pattern.

Key tread elements:

• Tread: The outermost rubber part that contacts the road.
• Ribs: Continuous raised sections running circumferentially.
• Lugs / Blocks: Raised rubber segments that provide traction.
• Grooves: Channels between ribs/blocks; circumferential (around) or lateral (across).
• Sipes: Fine slits within tread blocks for grip on wet surfaces.
• Shoulder: The edge region where the tread meets the sidewall.
• Voids: Empty spaces between tread elements.

Tyre sidewall markings:

• Manufacturer name, tyre size (e.g., 195/65 R15), load index, speed rating, and DOT code.

Forensic identification value:

• The tread design provides class characteristics — identifying brand, model, and type.
• The arrangement of ribs, blocks, and grooves in an impression can be compared with known databases.
• Noise treatment/pitch sequence variations aid comparison.
• Combined with wear and damage, tread analysis helps narrow down and eventually individualize the source tyre.

Wear and tear features on tyres develop through usage and provide valuable individual characteristics for forensic comparison.

Indicators of wear and tear:

• Tread depth reduction: Uniform wear reduces tread pattern depth over time.
• Uneven wear patterns:
  • Cupping/scalloping — due to worn suspension.
  • Feathering — due to misalignment.
  • Centre wear — over-inflation.
  • Edge/shoulder wear — under-inflation.
• Wear indicators (Tread Wear Indicators - TWI): Raised bars in grooves indicating minimum legal tread depth.
• Cuts, nicks, and gouges: From road hazards.
• Embedded objects: Stones, glass, nails.
• Punctures and repairs.

Importance as individual characteristics:

• While tread pattern is a class characteristic, wear features are randomly acquired, making them potentially unique.
• The exact combination and position of cuts, nicks, and wear on a specific tyre can individualize it to a particular vehicle.
• Enables matching a crime scene impression to a specific suspect tyre, not just a type.

Thus, wear and tear elevates tyre impression evidence from class-level to individual-level identification.

Impression evidences carry two levels of characteristics that determine their evidential power.

Class Characteristics:

• Features common to a group or category of objects produced during manufacturing.
• Help narrow down the source but cannot identify a unique object.
• Examples:
  • Shoe: brand, size, outsole design.
  • Tyre: tread pattern, size, model.
  • Tool: type (screwdriver, crowbar), width.

Individual Characteristics:

• Features that are unique to a single object, acquired randomly during use, damage, or manufacture.
• Allow individualization — matching to one specific source.
• Examples:
  • Shoe: cuts, nicks, wear patterns, embedded stones.
  • Tyre: specific gouges, punctures, re-tread seams.
  • Tool: striations, nicks on the working edge.

Comparison table:

Conclusion: A match in class characteristics is necessary but not sufficient; individual characteristics provide conclusive identification.

Tool marks are impressions, cuts, scratches, or abrasions produced when a tool comes into contact with a surface. They are commonly found in burglary and forced-entry cases.

Types of tool marks:

1. Impression / Compression Marks:

• Produced when a tool is pressed against a softer surface, leaving a negative impression.
• Example: A hammer struck on a metal surface, pliers gripping a wire.

2. Striated / Sliding (Abrasion) Marks:

• Produced when a tool slides across a surface, leaving parallel lines (striations).
• Example: A screwdriver prying open a window frame, a knife scraping.

3. Combination Marks:

• Both compression and sliding action occur together.
• Example: Cutting tools like bolt cutters and pliers that both grip and slide.

4. Cutting Marks:

• Produced by cutting tools separating a material.
• Example: Bolt cutters on a chain, saw marks.

Nature:

• The harder tool imprints its characteristics onto the softer surface.
• Contains both class (tool type, width) and individual (striations, nicks) characteristics.

Tool marks are compared using a comparison microscope with test marks made from the suspect tool.

Location of occurrence of tool marks:

• Points of entry/exit: Doors, windows, frames, locks, latches.
• Safes and cash boxes: Marks from prying, drilling, or cutting.
• Padlocks and chains: Cut marks from bolt cutters/saws.
• Vehicles: Marks on ignition, doors during theft.
• Wooden, metal, plastic surfaces that have been pried, cut, or struck.

Methods of collection and preservation:

1. Photography:

• Photograph the mark with a scale and oblique lighting to reveal depth and striations.

2. Collection of the original object:

• Preferred method — the entire object bearing the mark (or a cut-out section) is submitted to the lab.

3. Casting:

• When the object cannot be removed, a cast is made using materials like silicone rubber (e.g., Mikrosil).

4. Documentation:

• Note the location, direction, dimensions, and orientation of the mark.

Precautions:

• Never fit the suspect tool directly into the mark (may cause contamination/alteration).
• Protect the mark from damage during transport.

Proper preservation ensures reliable comparison microscopy with the suspect tool.

Impressions are broadly categorized as 2D or 3D based on the surface on which they form.

Two-Dimensional (2D) Impressions:

• Formation: Formed on hard, flat surfaces by the transfer or removal of material (e.g., dust, blood, grease).
• Have only length and width, minimal depth.
• Examples: Bloody shoe prints on tiles, dust prints on paper, latent fingerprints.
• Preservation:
  • Photography with scale.
  • Lifting using adhesive lifters, gelatin lifters, or electrostatic dust lifters.
  • Chemical/powder enhancement.

Three-Dimensional (3D) Impressions:

• Formation: Formed when an object presses into a soft, malleable surface.
• Have length, width, and depth.
• Examples: Shoe/tyre impressions in mud, snow, sand, or putty.
• Preservation:
  • Photography with scale and oblique lighting.
  • Casting with materials like dental stone (plaster of Paris) for soil, or sulphur/snow-print wax for snow.

Comparison table:

The correct classification determines the appropriate documentation and recovery technique.

Locard's Exchange Principle, proposed by Dr. Edmond Locard, states that "every contact leaves a trace" — whenever two objects come into contact, there is a mutual exchange of materials or features between them.

Application to impression evidence:

• When a shoe contacts the ground, it leaves an impression (transfer of tread pattern) and simultaneously picks up soil/debris.
• A tool contacting a surface imprints its features and may retain paint/material from the surface.
• A tyre rolling over mud transfers its tread while collecting soil in its grooves.
• Teeth biting into skin leave a mark and may retain saliva/skin traces.

Significance:

• Provides the theoretical foundation for the existence and collection of all impression evidence.
• Justifies searching both the crime scene (for impressions) and the suspect's belongings (for transferred traces).
• Emphasizes two-way transfer, aiding reconstruction and linkage.

Example: In a burglary, a crowbar leaves tool marks on a window frame (impression) while paint from the frame adheres to the crowbar — both pieces of evidence connect the tool to the scene.

Thus, Locard's principle underpins the collection, analysis, and interpretation of impression evidence.

Footwear (and barefoot) impressions provide substantial information beyond mere identification of the shoe.

Estimation of physical characteristics:

• Height/Stature: Foot length has a proportional relationship to height (approx. foot length is ~15% of stature); regression formulae are used.
• Sex: Foot size, shoe type/size, and gait can suggest the probable sex.
• Body weight: Depth and clarity of a 3D impression may indicate weight and load carried.
• Age: Broadly estimated from foot size and gait characteristics.

Event reconstruction:

• Direction of movement: Orientation of impressions reveals entry/exit and path.
• Mode of locomotion: Walking, running, or jumping (deduced from spacing, depth, and toe-digging).
• Number of persons: Multiple distinct patterns indicate multiple individuals.
• Carrying a load: Deeper impressions or altered gait suggest a heavy burden.
• Behaviour: Hesitation, standing, or turning can be inferred.

Gait analysis: Sequential impressions (gait line, step/stride length, foot angle) enhance individual identification.

Conclusion: Footwear impressions serve dual purposes — identification of the footwear and reconstruction of the suspect's actions and characteristics at the scene.

Tyre marks (tyre impressions) are reproductions of a tyre's tread produced when a vehicle passes over a surface.

Nature:

• Can be 2D (dust, mud, blood transfer on hard surfaces) or 3D (impressions in soft soil, mud, snow).
• Reproduce the tread design, dimensions, and any acquired defects.
• May include track width (wheelbase) information when multiple wheels leave marks.

Location of occurrence:

• Soft ground near crime scenes — mud, soil, sand, snow.
• Roadways and driveways (as dust or oil transfer).
• Grass, gardens, and unpaved areas.
• Entry and exit routes used by a vehicle.

Class characteristics:

• Tread design pattern (ribs, blocks, grooves).
• Tyre size and width.
• Brand and model of the tyre.
• Track width and wheelbase of the vehicle.

Individual characteristics:

• Cuts, nicks, and gouges in the tread.
• Wear patterns unique to the specific tyre.
• Embedded objects (stones, glass).
• Re-tread seams and repairs.

Significance: Class characteristics identify the tyre type and vehicle category, while individual characteristics can link the impression to a specific tyre and vehicle, enabling conclusive identification.