Unit 5: Development of Impressions - Subjective Questions

Impression evidence refers to any pattern or mark left when one object comes into contact with another, transferring or retaining a two-dimensional or three-dimensional replica of the contacting surface.

Key aspects:

• Two-dimensional (2D) impressions: Formed on hard, flat surfaces (e.g., a shoeprint in dust on tile).
• Three-dimensional (3D) impressions: Formed in soft, pliable materials (e.g., a footprint in mud, tool mark in wood).

Significance:

• Helps link a suspect, tool, or vehicle to a crime scene.
• Can individualize evidence when unique wear patterns or defects are present.
• Assists in reconstruction of events (direction of travel, sequence of movement).
• Provides class characteristics (make, model, size) even when individualization is not possible.

Because impressions are often fragile and transient, their proper documentation and collection is critical to preserving evidentiary value.

Impression evidence can be collected through several methods depending on its type and the surface involved:

1. Photography:

   • The first and most important step for all impressions.
   • Use of a scale (ruler) placed alongside, oblique lighting, and a tripod for perpendicular shots.

2. Lifting:

   • Adhesive/gelatin lifters for dust and residue impressions.
   • Electrostatic lifting for dry-origin dust prints.

3. Casting:

   • Used for 3D impressions (mud, snow, sand) using dental stone or plaster.

4. Powdering and Tape Lifting:

   • For latent 2D impressions on smooth surfaces.

5. Chemical Enhancement:

   • Reagents like amido black, leucocrystal violet for blood impressions.

6. Direct Collection:

   • Removing the entire object bearing the impression when portable.

Best practice: Always photograph before applying any destructive technique, and document location, orientation, and conditions.

Casting is the process of creating a permanent three-dimensional replica of an impression using a hardening material such as dental stone (preferred over plaster of Paris due to superior strength and detail retention).

Process:

1. Preparation: Photograph the impression with a scale. Remove loose debris carefully.
2. Fixation (if needed): Apply a fixative/spray (e.g., hairspray or shellac) to fragile impressions in sand or snow.
3. Mixing: Combine dental stone with water (approx. 2 parts powder : 1 part water by weight) to a pancake-batter consistency.
4. Pouring: Pour the mixture gently from the side or onto a spatula/baffle to avoid disturbing detail. Fill to cover completely.
5. Reinforcement: Optionally embed sticks or mesh for large casts.
6. Curing: Allow 20–30 minutes (or longer) to harden before lifting.
7. Labeling: Inscribe case details, date, and orientation on the back before it fully sets.
8. Cleaning: Clean gently only after 24–48 hours of drying.

Precautions:

• Do not pour material directly with force onto the impression.
• Support snow impressions with Snow Print Wax before casting.
• Ensure proper mix ratio to avoid weak or brittle casts.
• Avoid overfilling that spills and destroys surrounding detail.

Summary: 2D impressions are typically lifted or enhanced, while 3D impressions are cast to preserve depth information.

Electrostatic lifting is a technique used to recover dry-origin dust impressions (e.g., dusty shoeprints) from a variety of surfaces using an electrostatic charge.

Principle:

• Based on electrostatic attraction. A high-voltage charge (typically several thousand volts) is applied to a metallized lifting film.
• The charge attracts dust particles from the impression onto the black side of the film, creating a mirror-image lift.

Procedure:

1. Place the metallized (Mylar) lifting film, black (dust-receiving) side down, over the impression.
2. Connect the device electrodes — one to the top of the film, ground plate to the side.
3. Apply high voltage using the electrostatic dust print lifter (ESDL) unit.
4. Use a roller to press the film gently for good contact as the charge builds.
5. The dust transfers to the film, forming a visible lifted print.
6. Photograph the lift immediately under oblique lighting in a darkened area (charge dissipates over time).

Advantages:

• Non-destructive and can lift from porous and non-porous surfaces.
• Works on carpet, paper, and even skin in some cases.

Limitations:

• Effective mainly on dry dust prints, not wet or greasy ones.
• Lifts are fragile; must be documented quickly.

Light sources are essential tools for detecting, enhancing, and visualizing impression evidence that may be faint or invisible under normal lighting.

1. Oblique (Grazing) Light:

• White light directed at a low angle across the surface.
• Creates shadows that reveal the ridges and valleys of 2D and 3D impressions (e.g., dust prints, indented writing).

2. Ultraviolet (UV) Light:

• Reveals impressions through fluorescence or absorption.
• Useful for biological residues and some latent prints.

3. Infrared (IR) Light:

• Penetrates certain surfaces and reveals impressions in blood on dark or patterned backgrounds.

4. Forensic Light Sources (Alternate Light Sources / ALS):

• Tunable to specific wavelengths (e.g., 450 nm, 530 nm) with barrier filters/goggles.
• Cause treated prints (fluorescent powders/dyes) to glow.

5. Laser:

• High-intensity coherent light for detecting faint fluorescent residues.

Best practice: Use light sources before applying chemical or physical enhancement, and always photograph findings under the enhancing light with appropriate filters.

Control standards (also called exemplars or known standards) are reference impressions taken under controlled conditions from a known source (e.g., a suspect's shoe, a specific tool) for comparison against the questioned impression recovered from a crime scene.

Importance:

• Provides a known reference with which the unknown/questioned impression can be scientifically compared.
• Enables identification of class characteristics (size, pattern, make) and individual characteristics (wear, cuts, defects).
• Ensures objectivity and reproducibility in comparison.
• Establishes a baseline that helps confirm or exclude a source.

Requirements of good standards:

• Should replicate the conditions of the questioned impression as closely as possible.
• Must be adequate in quantity and clarity.
• Should include multiple exemplars to capture natural variation.

Without valid control standards, a meaningful and defensible comparison cannot be made.

Generating footwear control standards involves creating reproducible reference impressions from a suspect shoe for comparison with a questioned impression.

Methods:

1. Test Impressions:

   • Inked impressions: The sole is coated with ink or fingerprint powder and pressed onto paper.
   • Powder & lift: Sole is powdered and impression lifted with adhesive.
   • Photographic: Sole photographed directly with a scale.

2. Casting Test Impressions:

   • Make the suspect shoe step into a similar medium (e.g., soft soil) to replicate 3D questioned impressions, then cast.

3. Documentation of the Shoe:

   • Record make, model, size, and photograph the sole with a scale showing all wear patterns and defects.

Best practices:

• Take multiple exemplars to show variation in pressure and gait.
• Reproduce the substrate and conditions of the crime scene where possible.
• Capture complete detail of the entire sole, including edges and any accidental characteristics (cuts, gouges, embedded stones).

These standards allow the examiner to compare class and individual characteristics reliably.

The comparison of control (known) and questioned (unknown) impressions is a systematic examination to determine whether they share a common origin.

Steps in Comparison (ACE-V approach):

1. Analysis: Examine the questioned impression to assess quality, quantity, and characteristics present.
2. Comparison: Place known and questioned side by side; compare corresponding features.
3. Evaluation: Weigh the significance of agreements and discrepancies to reach a conclusion.
4. Verification: An independent examiner reviews the findings.

Characteristics Examined:

• Class Characteristics:

  • Shared by a group — e.g., design/pattern, size, shape, manufacturing details.
  • Narrow down the population but do not individualize.

• Individual Characteristics:

  • Unique to a single source — e.g., cuts, scratches, wear patterns, embedded debris, nicks.
  • Acquired randomly through use; provide the basis for individualization.

Possible Conclusions:

• Identification (Match): Sufficient agreement of individual characteristics.
• Exclusion: Significant unexplained differences.
• Inconclusive: Insufficient detail to decide.

Proper lighting, overlays, and 1:1 scale photographs aid accurate comparison.

Summary: Class characteristics tell us the type or category, while individual characteristics allow us to link an impression to one specific source to the exclusion of all others.

Proper handling at the crime scene is critical because impressions are fragile and easily destroyed.

Procedures:

1. Secure and Protect the Scene:

   • Establish a perimeter; restrict access to prevent accidental destruction.
   • Place markers/cover impressions to protect from weather and foot traffic.

2. Document the Scene:

   • Take overall, mid-range, and close-up photographs.
   • Record location, orientation, and environmental conditions in notes and sketches.

3. Photograph Impressions:

   • Use a scale, oblique lighting, and camera perpendicular to the impression.

4. Collect in Proper Sequence:

   • Recover fragile/transient impressions first (dust, snow).
   • Photograph before applying any physical or chemical technique.

5. Apply Appropriate Collection Method:

   • Casting for 3D; lifting/electrostatic lifting for 2D.

6. Package and Preserve:

   • Protect casts and lifts in rigid, labeled containers.
   • Allow casts to dry before packaging.

7. Maintain Chain of Custody:

   • Label with case number, date, time, collector, and location.
   • Document every transfer.

Golden Rule: Always photograph first, then collect, and never rush destructive techniques.

Casting impressions in snow presents unique challenges because heat from setting materials can melt and destroy the detail.

Techniques:

1. Photography First:

   • Photograph with a scale and oblique lighting before any treatment.

2. Snow Print Wax (Highlighting):

   • A colored aerosol wax (e.g., Snow Print Wax) is sprayed in light layers onto the impression.
   • It highlights detail and forms a protective, insulating shell that reinforces the impression.

3. Cooling the Casting Material:

   • Dental stone is mixed with cold water to minimize heat generation from the exothermic setting reaction.

4. Careful Pouring:

   • Pour gently onto a baffle/spatula to avoid disturbing the delicate detail.

5. Extended Setting Time:

   • Allow longer curing because low temperatures slow the setting.

Precautions:

• Apply wax in thin, multiple coats to avoid melting.
• Avoid direct sunlight and body heat over the impression.
• Sulfur casting is an alternative but is hazardous and less common.

These steps preserve the fragile snow impression for laboratory comparison.

Photography is considered the most important and first step because it captures the impression in its original, undisturbed state before any potentially destructive collection method is applied. It creates a permanent record that is admissible and reproducible.

Reasons for Importance:

• Impressions are fragile and may be destroyed during lifting or casting.
• Provides a 1:1 record usable for direct comparison and overlays.
• Documents location, orientation, and condition.
• Serves as backup evidence if physical collection fails.

Correct Photographic Technique (Examination-Quality Photography):

1. Scale: Place a ruler/scale in the same plane as the impression.
2. Perpendicular alignment: Camera lens must be exactly parallel to the impression to avoid distortion.
3. Tripod: Use a tripod for stability and precise framing.
4. Oblique lighting: Direct light at a low angle from multiple directions to reveal 3D detail through shadows.
5. Fill the frame: Capture the impression as large as possible without cropping detail.
6. Multiple exposures: Vary lighting angles and use bracketing.
7. Include identifiers: Case number, date, and scale in the frame.

A well-executed photograph can itself serve as the comparison specimen even if physical recovery is not possible.

Summary: Casting preserves volumetric detail of 3D impressions, while lifting recovers surface residue of 2D impressions. The choice depends on the dimensionality and substrate of the impression.

Tire track impressions may be 2D (on hard surfaces) or 3D (in soil, mud, snow) and require methods matching their type.

Collection Methods:

1. Photography:

   • Photograph the entire circumference (approx. 6–7 ft of continuous track to capture one full rotation).
   • Use scales, oblique lighting, and take overlapping perpendicular shots along the track.

2. Casting (3D tracks):

   • Use dental stone in overlapping sections because tire tracks are long.
   • Reinforce large casts and label each section with sequence/orientation.

3. Lifting (2D tracks):

   • Gel or adhesive lifters for dust tracks; electrostatic lifting for dry residue.

4. Chemical/enhancement:

   • For tracks in blood or other residues.

Important Considerations:

• Determine track width, wheelbase, and turning diameter for vehicle class identification.
• Look for individual characteristics: cuts, nicks, embedded stones, wear patterns.
• Document the direction of travel.

A sufficient length must be captured to include repeating tread design and unique wear features.

Blood impressions such as bloody shoeprints may be faint or invisible and require enhancement to visualize ridge and tread detail.

Chemical Enhancement Reagents:

1. Amido Black:

   • A protein stain that binds to proteins in blood, producing a blue-black color.

2. Leucocrystal Violet (LCV):

   • Reacts with the heme (peroxidase activity) in blood, staining it violet.

3. Luminol:

   • Produces chemiluminescence (glow) with blood; excellent for detecting cleaned/diluted bloodstains in darkness.

4. Fluorescein:

   • Fluoresces under an alternate light source when reacting with blood.

5. Diaminobenzidine (DAB):

   • Stains blood dark brown.

Physical/Optical Methods:

• Infrared (IR) photography: Reveals blood on dark or patterned backgrounds.
• Alternate Light Sources with filters: Enhance contrast.

Procedure Notes:

• Photograph first in normal and IR light.
• Presumptive testing and DNA sampling should be considered before/along with enhancement.
• Apply reagents carefully; some are destructive to DNA — collect samples first.

These techniques dramatically improve the visibility of pattern detail for comparison.

Oblique lighting (also called grazing light) is the technique of directing light across a surface at a low, shallow angle rather than straight on.

Underlying Optical Principle:

• When light strikes a surface at a low angle, the raised ridges of an impression cast shadows into the adjacent recessed valleys.
• This exaggerated contrast between light and shadow makes otherwise invisible or faint 3D texture visible to the eye and camera.
• Mathematically, the shadow length increases as the angle of incidence () from the surface decreases — the lower the angle, the longer and more pronounced the shadow, enhancing surface relief.

Significance:

• Essential for visualizing dust prints, indented impressions, and 3D marks.
• Reveals shallow surface detail not seen under direct (flat) lighting.
• Non-destructive and applied before any chemical/physical treatment.

Practical Application:

• Move the light source around the impression (0°–360°) to find the optimal angle and direction.
• Photograph from multiple lighting directions to capture all detail.

Thus, oblique lighting exploits shadow formation to dramatically improve the detection and documentation of textured impression evidence.

When impression evidence coexists with other evidence types, a logical sequence of processing prevents accidental destruction.

General Sequence:

1. Scene Assessment & Documentation:

   • Secure scene, take overall photographs, notes, and sketches before entry.

2. Protect Fragile Impressions:

   • Cover and flag transient impressions (dust prints, snow tracks) immediately.

3. Photograph All Impressions:

   • Examination-quality photography with scale and oblique lighting.

4. Collect Fragile/Transient Evidence First:

   • Recover evidence most likely to be lost — e.g., dust prints (electrostatic lift), snow casts.

5. Recover Trace Evidence Consideration:

   • Collect any trace (fibers, DNA on blood impressions) before destructive enhancement.

6. Casting of 3D Impressions:

   • After photographing, cast footwear/tire marks.

7. Chemical/Physical Enhancement:

   • Apply enhancement reagents last, as many are destructive.

8. Packaging & Chain of Custody:

   • Label, package rigidly, and document every step.

Guiding Principles:

• Least destructive to most destructive.
• Most fragile/transient collected first.
• Always photograph before altering.

This order maximizes evidentiary yield and preserves integrity across all evidence types.

After comparing a questioned impression with a control standard, the examiner evaluates the agreement and disagreement of features to reach one of several conclusions.

Possible Conclusions:

1. Identification (Positive Association / Match):

   • Basis: Agreement in class characteristics AND sufficient corresponding individual characteristics (unique wear, cuts, defects) with no unexplained differences.
   • Meaning: The impression was made by the specific known source to the exclusion of others.

2. Probably / Could Have Made (Association):

   • Basis: Agreement in class characteristics and some individual detail, but insufficient for certainty.
   • Meaning: The source cannot be excluded; supports but does not confirm identity.

3. Inconclusive:

   • Basis: Insufficient detail, poor quality, or limited quantity of the questioned impression.
   • Meaning: No definite determination can be made.

4. Exclusion (Elimination):

   • Basis: Significant unexplained differences in class or individual characteristics.
   • Meaning: The known source did not make the questioned impression.

Key Principles:

• Class agreement alone cannot individualize.
• Individual characteristics are decisive for identification.
• Conclusions must be verified by a second examiner (ACE-V).

The examiner must clearly document the reasoning behind whichever conclusion is reached.

The Electrostatic Dust Print Lifter (ESDL) is used to recover dry dust impressions through electrostatic attraction.

Equipment & Components:

1. High-Voltage Power Unit:

   • Generates several kilovolts (kV) of static charge; usually battery-operated and portable.

2. Metallized Lifting Film (Mylar):

   • A two-sided film: black (dust-receiving) side faces down, aluminized/conductive side up.

3. Charging Probe/Electrode:

   • Delivers charge to the top (conductive) surface of the film.

4. Ground Plate:

   • Completes the circuit; placed near the film to establish the electrostatic field.

5. Roller:

   • Ensures good contact between film and surface.

Working Principle:

• Applying high voltage charges the film, creating an electrostatic field that attracts dust particles from the impression onto the black side, forming a mirror-image lift.

Practical Limitations:

• Works best on dry dust impressions; ineffective on wet, greasy, or heavily embedded residues.
• Lift is fragile and temporary — charge dissipates, so it must be photographed immediately under oblique light.
• Requires a relatively flat surface for good contact.
• High voltage safety hazard — must avoid conductive/wet surfaces and metal.
• The lifted image is reversed (mirror image), which must be accounted for during comparison.

Despite limitations, it is highly effective and non-destructive for recovering latent dust prints.