Strengths, Limitations, and Recommendations for Instrumental Color Measurement in Forensic Soil Characterization

Abstract

Color determination of soil evidence is often done by visual comparison to soil color charts. A handheld spectrophotometer was tested with representative materials for its suitability for forensic soil characterization. Instrumental colorimetry provides accurate colorimetry with ~10‐fold better precision than a soil color chart. The minimum sample size for accurate color determination was between 0.02 and 0.04 mg of fine soil for the specific instrument tested. Reporting colors in the L*a*b* space permits quantification of ΔE00, a measure of perceptible color difference, could enable objective quantification of small color differences and thresholds for forensic soil comparisons. A ΔE00 greater than ~ 3.5 to 6 likely indicates disparate soil sources in a forensic comparison, in the absence of confounding factors like sample alteration. Despite the superior precision of instrumental colorimetry, this approach is inappropriate for samples which are mottled at an inseparable scale, attached to a substrate, or too small for instrumental measurement.

Forensic Analysis of Bloodstain Color

“…There is a wide range of spectroscopic examination methods in forensics that define or compare the color of trace materials to obtain information about their origin and to determine whether they were altered at the time of the crime. Examples include forensically motivated examinations of soil [3], vehicle paint [4,5], textile fibers [6], postmortem interval [7], drugs [8], and others.…”