Unit 2: Impression Evidences - Subjective Questions

Impression Evidence refers to any object or material that has retained the characteristics of another object through direct physical contact, resulting in a mark, pattern, or negative impression.

Key Points:

• It is created when one object is pressed against another, transferring or imprinting surface details.
• Common examples include footwear impressions, tyre marks, toolmarks, bite marks, and lip prints.

Significance:

• Helps in linking a suspect, tool, or vehicle to a crime scene.
• Can establish the presence of an individual at the scene.
• Provides class characteristics (type, size, brand) and individual characteristics (unique wear patterns, defects).
• Aids in reconstruction of events and sequence of activities.
• Serves as corroborative or associative evidence in court proceedings.

Two-Dimensional (2D) Impressions:

• Formed on a flat surface with negligible depth.
• Created by transfer of material (dust, blood, mud) onto or from a surface.
• Examples: A dusty shoeprint on a tiled floor, a bloody fingerprint on paper.
• Can be positive (material deposited) or negative (material removed).

Three-Dimensional (3D) Impressions:

• Formed in a soft, pliable medium that retains depth and contour.
• Contains length, width, and depth information.
• Examples: A footwear impression in mud, snow, sand, or soft soil; a tyre track in wet earth.
• Typically preserved by casting techniques.

Key Differences:

Locating Impression Evidence:

• Systematic search of the scene using proper lighting techniques.
• Use of oblique (grazing) lighting to reveal faint impressions by casting shadows.
• Employ alternate light sources (ALS) and forensic lighting for latent prints.
• Examine entry/exit points, pathways, and surfaces likely to bear impressions.

Identification Steps:

1. Documentation – Photograph with and without scale before any handling.
2. Classification – Determine whether it is footwear, tyre, tool, bite, or lip print.
3. Type determination – Identify if 2D or 3D, positive or negative.
4. Preservation – Protect from contamination or environmental damage.

Tools Used:

• Flashlights, laser pointers, ALS.
• Electrostatic dust print lifters.
• Photography equipment with scales.

Precautions:

• Avoid contamination or destruction of evidence.
• Maintain chain of custody.
• Wear protective gear to prevent adding one's own impressions.

Casting a 3D Footwear Impression with Dental Stone:

Materials Required:

• Dental stone (die stone) powder
• Water, mixing container, spatula
• Fixative spray (for fragile impressions)
• Frame/border for containment (optional)

Procedure:

1. Photograph the impression with a scale before casting.
2. Prepare the impression – Remove loose debris carefully; apply a fixative/hairspray to stabilize fragile soil.
3. Build a border around the impression if needed to contain the mix.
4. Mix dental stone – Combine approximately 2 parts powder to 1 part water to a pancake-batter consistency.
5. Pour the mixture gently from outside the impression, allowing it to flow in, avoiding damage to fine detail.
6. Add reinforcement (sticks/mesh) if the cast is large.
7. Allow to cure – Let it set for at least 30 minutes (longer in cold/wet conditions).
8. Lift the cast carefully once hardened.
9. Do not clean immediately – Allow to air-dry for 24–48 hours before gently cleaning.
10. Label with case number, date, and initials.

Advantages of Dental Stone: High strength, retains fine detail, and resists crumbling compared to plaster of Paris.

Casting Impressions in Snow:

Snow impressions are fragile and can melt from the heat generated during casting, requiring special methods.

Snow Print Wax Method:

1. Photograph the impression with oblique lighting and a scale.
2. Apply Snow Print Wax (a colored aerosol wax) in several thin layers from a distance.
3. The colored wax highlights detail and provides a protective shell.
4. Allow each layer to set before applying the next.

Casting Over the Wax:

• Once the wax hardens, pour dental stone mixed with cold water (to reduce exothermic heat).
• Ice-cold or slightly warmer-than-freezing water minimizes melting.

Precautions:

• Use cold water for mixing to prevent the exothermic reaction from melting the snow.
• Apply wax gently to avoid displacing fine detail.
• Work quickly but carefully due to weather sensitivity.
• Allow longer curing time in cold conditions.

Significance: Snow Print Wax acts as both a contrast enhancer and a thermal barrier, preserving detail during casting.

Casting a Tyre Impression:

Key Consideration: Tyre impressions are much longer than footwear impressions, often requiring casting of a section representing one full circumference (about 2.1–2.5 meters) to capture repeating tread patterns.

Procedure:

1. Document and photograph the full length with scales at regular intervals.
2. Divide long impressions into sections for casting.
3. Apply fixative to stabilize the soil.
4. Build borders along the length of the impression.
5. Mix larger quantities of dental stone due to greater surface area.
6. Pour and reinforce – Use wire mesh or sticks for structural support given the length.
7. Cure and lift each section carefully.

Differences from Footwear Casting:

Additional Value: Tyre impressions also yield track width and wheelbase measurements useful for vehicle identification.

Class Characteristics:

• Features shared by all footwear of a particular type or manufacture.
• Determined during the manufacturing process.
• Examples: Tread pattern design, shoe size, brand logo, general shape.
• Help narrow down to a category or model of shoe.

Individual Characteristics:

• Unique features acquired through use, damage, or wear over time.
• Randomly acquired and specific to a single shoe.
• Examples: Cuts, nicks, scratches, embedded stones, wear patterns, holes.
• Allow positive identification of a specific shoe.

Comparison Table:

Conclusion: Individual characteristics carry greater evidential weight as they can uniquely link a specific shoe to a scene.

Cheiloscopy is the study of lip prints. Suzuki and Tsuchihashi (1970) proposed a widely used classification of the grooves/wrinkles on the labial mucosa.

Classification (Types I–V):

• Type I: Clear-cut vertical grooves running across the entire lip.
• Type I′ (I prime): Vertical grooves that do not cover the entire lip (partial length).
• Type II: Branched (bifurcated) grooves forming a 'Y' shape.
• Type III: Intersecting (crossed) grooves forming an 'X' pattern.
• Type IV: Reticular (net-like) pattern.
• Type V: Grooves that cannot be classified into the above categories (irregular/undetermined).

Significance:

• Lip prints are considered unique to each individual, even in identical twins.
• They are permanent and can aid in personal identification.
• Useful in cases involving contact with cups, glasses, cigarettes, and cosmetics.

Note: Lip prints remain relatively stable throughout life, changing only due to trauma or pathology.

Lip Print (Cheiloscopy) Analysis Procedure:

1. Locating Lip Prints:

• Search cups, glasses, bottles, cigarette butts, tissues, envelopes, and clothing.
• Lip prints may be visible (from lipstick) or latent (invisible).

2. Development Techniques:

• Visible prints: Photographed directly with scale.
• Latent prints: Developed using fingerprint powders (aluminum, magnetic), lysochrome dyes (Sudan black, oil red O) which react with lipids, or fluorescent reagents.

3. Lifting:

• Use transparent adhesive tape or lifting film to transfer the developed print.
• Transfer to a contrasting backing card.

4. Recording Exemplars:

• Apply lipstick to suspect's lips, blot on bond paper or cellophane to obtain reference prints.

5. Analysis and Comparison:

• Divide the lip print into quadrants/compartments for systematic study.
• Classify grooves using the Suzuki-Tsuchihashi system (Types I–V).
• Compare pattern type, location, and unique features with the exemplar.

6. Documentation:

• Photograph, chart, and record all findings maintaining chain of custody.

Applications: Personal identification, sex determination, and linking suspects to a scene.

Bite Mark Analysis is a branch of forensic odontology dealing with the examination, documentation, and comparison of bite mark injuries on skin or objects to identify the biter.

A bite mark is a patterned injury or impression left by teeth, typically consisting of two opposing (upper and lower) arches.

Role in Forensic Odontology:

• Suspect identification – Comparing the bite mark to a suspect's dentition.
• Victim identification – In cases of assault, abuse, or homicide.
• Evidence in violent crimes – Common in sexual assault, child abuse, and homicide cases.
• Establishing association between a biter and the bitten object/person.

Sources of Bite Marks:

• Human skin (victims or suspects), foodstuffs (cheese, apples, chocolate), and other objects.

Characteristics Analyzed:

• Arch shape and size, tooth alignment, spacing, missing teeth, rotations, and unique dental features.

Limitations: Skin is a poor impression medium (elastic, subject to distortion and swelling), so bite mark evidence is considered corroborative and increasingly scrutinized for reliability.

Bite Mark Evidence Procedure:

1. Collection from Victim:

• Photograph the bite mark using the ABFO No. 2 scale (right-angle reference scale) with and without scale.
• Take photographs at 90° angle to avoid distortion.
• Swab the area for salivary DNA before other procedures.
• Take impressions of the bite mark using dental impression material if 3D detail exists.

2. Collection from Suspect (with legal authorization):

• Record dental impressions and study casts of upper and lower arches.
• Photograph the dentition.
• Obtain bite registration (wax bite).

3. Comparison Methods:

• Direct comparison – Overlay of suspect's cast onto life-size photographs.
• Transparent overlay technique – Trace biting edges onto acetate and superimpose on the bite mark.
• Computer-generated overlays for accuracy.
• Compare arch shape, tooth position, size, spacing, and unique features.

4. Analysis Parameters:

• Number and size of teeth marks, inter-canine distance, missing/rotated teeth, and individual peculiarities.

5. Documentation and Reporting:

• Maintain chain of custody; conclusions expressed with appropriate certainty given known distortion issues.

Limitation: Skin distortion and swelling reduce reliability, so results are treated cautiously.

Toolmark: A toolmark is any impression, cut, gouge, scratch, or abrasion produced when a tool comes into contact with a softer surface, transferring the tool's surface characteristics.

Classification of Toolmarks:

1. Impression (Compression) Marks:

• Produced when a tool is pressed against a softer surface.
• The tool's shape is impressed as a negative image.
• Example: Hammer strike, pliers grip marks.

2. Striated (Abrasion/Friction) Marks:

• Produced when a tool slides across a surface with pressure.
• Creates parallel scratches/striations reflecting the tool's edge irregularities.
• Example: Screwdriver prying, knife scraping, wire cutter drag marks.

3. Combination Marks:

• Contain both impression and striated features.
• Example: A tool that first grips then slides, such as pliers pulling wire.

Characteristics Analyzed:

• Class characteristics: Tool type, width, general shape.
• Individual characteristics: Unique nicks, wear, and manufacturing imperfections on the tool's working surface.

Common Tools: Screwdrivers, pliers, crowbars, hammers, bolt cutters, knives.

Toolmark Impression Procedure:

1. Documentation:

• Photograph the toolmark with a scale using oblique lighting to enhance detail.
• Note location, orientation, and surrounding context.

2. Collection:

• Preferred method: Collect the entire object bearing the toolmark whenever possible.
• If not feasible, make a cast using silicone-based casting material (e.g., Mikrosil, Duplicast).
• Never fit the suspect tool directly into the mark (risk of altering/contaminating evidence).

3. Casting Toolmarks:

• Apply silicone casting material to the mark, allow to set, and lift a detailed replica.

4. Preservation:

• Protect toolmark surfaces from damage; package rigidly.
• Package the suspect tool separately to preserve any transferred trace evidence (paint, metal).

5. Analysis and Comparison:

• Prepare test marks using the suspect tool on a similar soft medium (lead, wax) at various angles.
• Use a comparison microscope to align and compare striations/impressions side by side.
• Match class characteristics first, then individual characteristics.

6. Trace Evidence:

• Examine for paint, metal filings, or material transfer between tool and surface.

Conclusion: A match of individual striation patterns can positively link a specific tool to a toolmark.

Comparison of Casting Materials:

Dental Stone Advantages:

• Produces harder, more accurate casts.
• Requires no reinforcement for most impressions.
• Better preserves individual characteristics.
• Can be cleaned without damaging the cast.

Conclusion: Dental stone is preferred in modern forensic casting due to superior strength, minimal distortion, and excellent detail reproduction, while Plaster of Paris is now largely obsolete for footwear/tyre casting.

Reconstruction Using Footwear Impressions:

1. Determination of Height (Stature):

• There is an approximate correlation between foot length and body height.
• A commonly cited estimate: Foot length is roughly 15% of body height, so:
  
• Regression formulae specific to population groups provide better estimates.

2. Determination of Gait Pattern:

• A series of footprints (gait line/walking pattern) reveals movement characteristics:
  • Stride length – distance between successive prints of the same foot.
  • Step length – distance between right and left foot.
  • Foot angle – angle of each foot relative to the direction of travel.
  • Straddle/width – lateral distance between feet.
• Abnormalities (limping, dragging) indicate injury or physical peculiarity.

3. Determination of Direction of Travel:

• The orientation of the toe indicates direction of movement.
• Deeper heel impressions at contact and toe push-off marks show forward motion.
• Slippage or kick-back debris indicates running vs. walking.

Significance: Combined analysis assists in crime scene reconstruction and offender profiling.

Electrostatic Dust Print Lifting (ESDL):

This technique recovers dry, dusty two-dimensional impressions (footwear/tyre) from surfaces such as floors, paper, and clothing.

Principle:

• Uses a high-voltage electrostatic charge to attract charged dust particles from an impression onto a metalized lifting film.

Equipment:

• Electrostatic dust lifter (high-voltage unit), grounding plate, and a black metalized lifting film (Mylar-type).

Procedure:

1. Locate the dust impression using oblique lighting.
2. Place the black (conductive) side of the film down over the impression.
3. Connect the ground plate beside the film and the charging probe to the film.
4. Apply high voltage – the film becomes charged and presses against the surface.
5. Dust particles are attracted and transferred to the film's underside.
6. Disconnect power, carefully lift the film.
7. Photograph immediately using oblique lighting (the lift is fragile).

Advantages:

• Highly sensitive for faint, dry-origin dust prints.
• Non-destructive to the original surface.

Limitation: Works best on dry impressions; ineffective on wet or greasy prints. Lifts are fragile and must be documented promptly.

Vehicle Identification from Tyre Impressions:

1. Tread Pattern Analysis:

• The tread design helps identify the tyre brand and model (class characteristic).
• Wear patterns and damage (cuts, embedded stones) provide individual characteristics.

2. Track Width (Track/Gauge):

• The distance between the centers of the left and right tyres on the same axle.
• Measured from corresponding points of left and right tyre impressions.
• Front and rear track widths may differ.

3. Wheelbase:

• The distance between the front and rear axles.
• Measured from the front tyre impression to the rear tyre impression on the same side.
• Best measured where the vehicle turns or stops, exposing both axle positions.

4. Turning Diameter:

• Analysis of curved tracks reveals the vehicle's turning radius.

Vehicle Class Determination:

• Combining track width, wheelbase, number of axles, and tread type helps narrow down the make and model of the vehicle.

Additional Features:

• Number of tyres/axles indicates vehicle type (car, truck, motorcycle).
• Tread depth and wear can indicate tyre age/condition.

Conclusion: Systematic measurement of these parameters allows investigators to classify and potentially identify the suspect vehicle.

Limitations and Challenges of Bite Mark Analysis:

1. Distortion of Skin:

• Skin is elastic and non-rigid, causing significant distortion.
• Body position, movement, and location affect the mark's shape.

2. Time-Dependent Changes:

• Swelling, bruising, and healing alter the appearance of bite marks over time.
• Postmortem changes further distort marks on deceased victims.

3. Lack of Uniqueness Validation:

• The assumption that human dentition is unique and reliably transferred to skin is scientifically contested.
• Studies show low reliability and reproducibility among examiners.

4. Subjectivity:

• Comparisons often rely on examiner interpretation, introducing bias.
• Lack of standardized, validated methodology.

5. Wrongful Convictions:

• Several cases have been overturned where bite mark testimony proved erroneous.
• Scientific bodies (e.g., NAS 2009, PCAST 2016) have criticized its validity.

6. Poor Substrate Quality:

• Foods and skin often provide incomplete or unclear impressions.

Current Status:

• Increasingly treated as corroborative rather than conclusive evidence.
• DNA from saliva at the bite site is considered far more reliable.

Conclusion: While historically used, bite mark analysis faces serious scientific scrutiny and should be applied with caution and appropriate qualification.

Importance of Photography and Documentation:

1. Preservation of Perishable Evidence:

• Impressions in snow, mud, dust, or blood may deteriorate rapidly; photography preserves them permanently.

2. Primary Record for Comparison:

• For 2D impressions, a high-quality photograph may be the only recoverable evidence.
• Serves as a 1:1 (life-size) record for laboratory comparison.

Photographic Best Practices:

• Take photographs with and without a scale.
• Position the camera perpendicular (90°) to the impression to avoid distortion.
• Use a tripod for stability.
• Employ oblique/side lighting to reveal three-dimensional detail and texture.
• Include a scale (ruler) on the same plane as the impression.
• Photograph the overall scene, then mid-range, then close-up (evidence establishing shots).

3. Legal and Evidentiary Value:

• Maintains chain of custody.
• Supports courtroom presentation and expert testimony.

4. Documentation Elements:

• Notes on location, dimensions, orientation, lighting conditions, date, time, and photographer.
• Sketches and measurements to fix position within the scene.

Conclusion: Meticulous photography and documentation are critical, as impression evidence is often fragile and non-reproducible, and errors can render evidence inadmissible.

Comparison Microscope in Toolmark Analysis:

Definition: A comparison microscope consists of two microscopes connected by an optical bridge, allowing the examiner to view two specimens side by side in a single split field of view.

Role in Toolmark/Impression Analysis:

1. Side-by-Side Comparison:

• Enables simultaneous viewing of the evidence toolmark and the test mark made by the suspect tool.
• The optical bridge merges both images so striations can be aligned.

2. Striation Matching:

• Striated toolmarks contain parallel lines from tool edge irregularities.
• The examiner aligns striation patterns to determine if they originate from the same tool.

3. Individual Characteristic Identification:

• Reveals microscopic nicks, ridges, and wear unique to a specific tool.

Procedure:

1. Prepare test marks from the suspect tool on soft material (lead, wax) at various angles.
2. Mount evidence mark and test mark on respective stages.
3. Adjust magnification and lighting for both sides equally.
4. Rotate and align specimens until striations correspond across the dividing line.

Applications:

• Toolmarks, firearm bullet/cartridge comparisons, and striated impression evidence.

Conclusion: The comparison microscope is the cornerstone instrument for establishing that two marks share a common origin through matching striation patterns.