Dimensional Review of Scales for Forensic Photography

Area of Science:

• Forensic Science
• Metrology
• Image Analysis

Background:

• Photographic scales are crucial for accurate documentation in forensic science.
• The American Board of Forensic Document Examiners (ABFO) No. 2 scale is a widely used standard reference.
• Assessing the long-term geometric accuracy of these scales is essential for reliable evidence analysis.

Purpose of the Study:

• To evaluate the dimensional accuracy and stability of ABFO No. 2 scales from multiple vendors.
• To investigate changes in scale features, including graduations, circle dimensions, center placement, and leg perpendicularity, over a four-year period.

Main Methods:

• Four vendors' ABFO No. 2 scales were selected for analysis.
• Measurements of key features were performed using a vision-based coordinate measurement system.
• Scales were re-measured after four years to assess temporal stability.

Main Results:

• All scales met specified tolerances for centimeter graduations.
• Scale length and center-to-center measurements showed acceptable stability over four years.
• A significant change was observed in the perpendicularity of the scale legs over time.

Conclusions:

• ABFO No. 2 scales exhibit good dimensional stability for length and circle measurements.
• The perpendicularity of scale legs may degrade over time, impacting geometric accuracy.
• Regular quality control checks using certified calibration standards are recommended for forensic scales.