INTRODUCTION

The Video Spectral Comparator (VSC) is one of the most important tools used in questioned document examination (QDE). It is a nondestructive system designed to study, capture, and analyse documents under a wide range of lighting conditions. By using the VSC, forensic experts can detect alterations, compare different inks, check security features, and prepare reliable visual records for investigations and court presentations.

Initially introduced by Foster + Freeman, the VSC has evolved into different generations with improved optics, imaging, and analytical capabilities. It combines ultraviolet (UV), visible, and infrared (IR) illumination with high-quality cameras, magnification lenses, and specialised software to reveal details that are otherwise invisible to the naked eye. Today, VSC instruments are standard in forensic laboratories, border control, banking security, and cultural heritage institutions.

Principle

The working principle of the VSC is based on how light interacts with materials such as inks and paper. It uses”

1. Reflection and absorption: Each ink reflects and absorbs light differently at specific wavelengths, allowing hidden differences to be seen
2. Fluorescence: Certain inks or paper fibers glow under UV light, helping reveal tampering or security features.
3. Transmission: When light passes through a document, structures like watermarks or erasures become visible
4. Infrared luminescence: Some inks absorb IR light and re-emit visible light, which is useful in identifying alterations
5. Spectral analysis: By scanning across different wavelengths, unique optical “signatures” of inks or paper can be recorded for comparison.

This multi-spectral imaging approach allows the VSC to expose additions, erasures, or overwriting that cannot be detected under normal light.

INSTRUMENTATION & WORKING

Figure 1. VSC

COMPONENTS:

1. Illumination Sources

PARTS:

• Visible light (white light source)
• Ultraviolet (UV) light sources (short-wave and long-wave)
• Infrared (IR) light sources
• Oblique and transmitted light systems

USES: Provide a wide range of electromagnetic radiation to reveal hidden or invisible document features.

FUNCTION:

• UV light reveals fluorescence in inks, fibers, and security threads
• IR light penetrates surface inks, helping distinguish between chemically similar inks
• Oblique lighting exposes indentations, erasures, or surface alterations
• Transmitted light shows watermarks and embedded security features

WORKING: The light source illuminates the document, and the reflected, transmitted, or fluoresced light is captured by optical systems for analysis.

2. OPTICAL SYSTEM (LENSES & FILTERS)

PARTS:

• High-resolution zoom lens
• Bandpass filters (UV, visible, IR range)
• Polarizers

USES: Focuses and filters light to isolate specific wavelengths for clearer document examination

FUNCTION:

• Filters allow only selected wavelengths to pass, enabling comparison of inks and papers.
• Lenses provide magnification to view micro-texts, fine security printing, and alterations

WORKING: The optical system controls how light interacts with the sample, magnifies it, and enhances contrast under selected wavelengths.

3. IMAGING & CAMERA SYSTEM

PARTS:

• High-resolution CCD/CMOS camera
• Digital image processor

USES: Captures images of documents under varied illumination for observation and record-keeping.

FUNCTION:

• Provides real-time visualization of the document under different spectral conditions.
• Enhances image clarity, allowing forensic experts to detect hidden details.

WORKING: The camera collects the reflected/transmitted/fluorescent light after interaction with the document and sends it to the display system.

4. SPECTRAL ANALYSIS UNIT

PARTS: 

• Infrared luminescence system
• Absorption and fluorescence detectors

USES: Enables scientific differentiation of inks, dyes, and paper

FUNCTION:

• Detects ink variations through absorption and luminescence in IR/UV ranges.
• Differentiates between inks of different formulations that appear identical under normal light

WORKING: The unit records how the material responds to specific wavelengths and displays variations invisible to the naked eye.

5. DOCUMENT STAGE (EXAMINATION PLATFORM)

PARTS:

• Glass-based stage with adjustable height
• Rotating and movable platform

USES: Holds documents in position during analysis.

FUNCTION:

• Provides flexibility for illumination (top, bottom, side).
• Provides flexibility for illumination (top, bottom, side).

WORKING: The stage enables consistent placement of documents while different light sources are applied for examination

6. COMPUTER & SOFTWARE INTERFACE

PARTS:

• Dedicated forensic analysis software
• Data storage system
• Display monitor

USES: Provides control, analysis, and documentation.

FUNCTIONS:

• Controls light sources, filters, and camera settings
• Processes and enhances images.
• Stores, compares, and prints examination results

WORKING: The examiner uses the software to switch between spectral modes, capture images, and generate reports.

7. DETECTORS

PARTS:

• Magnetic ink detection unit
• Infrared and UV detectors
• Microprint verification tools

USES: Detects hidden security features in banknotes, passports, and secure documents

FUNCTION:

• Detects hidden security features in banknotes, passports, and secure documents
• Reveals microtext, holograms, and latent images.

WORKING: By activating specialized detectors, the system highlights features that are otherwise invisible under natural light.

WORKING PROCESS:

1. Place the document on the stage.
2. Capture initial images under normal white light.
3. Switch through multiple light sources (UV, visible, IR) to reveal hidden details
4. Record high-resolution images at each wavelength.
5. Compare different areas using overlays, false-color images, or subtraction methods.
6. Use the spectrometer (if available) to collect spectral data for ink discrimination
7. Prepare documentation with annotated images and results for case files.

OUTPUTS & RESULTS

The VSC produces a range of results, including

• Images under UV, visible, and IR illumination
• False-colour composites that highlight differences between inks or papers.
• Fluorescence images showing security features or erased text.
• Transmission images revealing watermarks or paper structures.
• Spectral graphs for detailed ink comparisons
• Case reports containing annotated images and metadata for court use.

KEY FEATURES OF MODERN VSC SYSTEM

1. Multispectral light sources covering UV, visible, and IR
2. High-resolution, sensitive imaging systems
3. Automated imaging workflows
4. Advanced software tools for image enhancement and comparison
5. Compatibility with optional spectrometers
6. Ability to handle both small and large-format documents

APPLICATIONS

• Detecting alterations, erasures, and overwriting in documents
• Differentiating inks to check if entries were made with the same or different pens
• Determining the sequence of strokes in intersecting lines
• Authenticating passports, ID cards, and banknotes by analyzing built-in security features
• Revealing watermarks and fibers in paper
• Examining forgeries in contracts, wills, and certificates.
• Studying old manuscripts and artworks in cultural heritage research

ADVANTAGES

• Completely non-destructive.
• Covers a broad spectral range (UV to IR)
• Produces high-quality, court-admissible documentation
• Quick and reliable analysis
• Suitable for many types of documents

LIMITATIONS

• Expensive to purchase and maintain
• Calibration: Require regular calibration to maintain accuracy.
• Maintenance: The sensitive components, such as light sources and filters, may require frequent maintenance.
• Training Needed: Often requires specialized training, which can be time-consuming and costly.