Standoff and non-destructive detection of bloodstains via mid-IR diffuse reflectance spectroscopy

A promising approach to the standoff detection of blood is the use of mid-infrared (IR) diffuse reflectance spectroscopy, by which the presence of the proteinic Amide I and II bands in presumptive bloodstains is verified. Because the molecular vibrational response of the Amide I and II bands lies within the ∼1500-1700 cm^-1 region, samples are illuminated with a spectrally broadband IR laser, and the back-scattered spectrum is then collected and analysed in real time. Unfortunately, absorption by water vapour falls within the same spectral region, leading to spectral distortions at several specific wavenumber positions. These distortions are stronger for bloodstains on porous substrates and also at increased standoff distances, hindering detection. In this work, three spectral processing algorithms are evaluated for application in such scenarios, enabling the extraction of useful information even from highly distorted spectra. Depending on the substrate, a considerable improvement in the maximum standoff distance for passive bloodstains of up to 2.0 m is demonstrated, and standoff measurement results on porous substrates (denim, cotton, kitchen paper) are also presented. The approach discussed here can be extended to any target substance (besides blood) that exhibits molecular fingerprint signatures in the IR water vapour absorptive range. © 2026 The Authors. Published by Elsevier B.V.