Jessica N. McCutcheon

Multimode Imaging in the Thermal Infrared for Chemical Contrast Enhancement. Part 3: Visualizing Blood on Fabrics

Infrared thermal imaging using lock-in and molecular factor computing methods for the detection of blood on a dark, acrylic fabric is shown. Contrast differences between the clean fabric and the fabric stained with blood diluted as low as 1:100 are reported. We have also demonstrated that this method can be used to discriminate between a bloodstain and four common interfering agents (bleach, rust, cherry soda, and coffee) to other blood detection methods. These results indicate that this system could be useful for crime scene investigations by focusing nondestructive attention on areas more likely to be suitable for further analysis.

A Study of Electric Field Standing Waves on Reflection Microspectroscopy of Polystyrene Particles

We have been investigating the mid-infrared (MIR) reflection spectrum of microparticles on mirrored substrates. Gold-coated porous alumina filters were used as a substrate to layer the particles and provide consistent reflection spectra. Polystyrene spheres with measured diameters of 0.42 μm were studied using Fourier transform infrared (FT-IR) reflection microspectroscopy, and spectra are shown for coverages in the range 0.5–6 monolayers (ML). Results show that absorption has a nonlinear, stairstep-like dependence on particle coverage and a wavelength dependence that can be explained by electric field standing waves (EFSW) caused by the mirrored substrate. The same effect is found to cause progressive weakening of the observed spectra as a function of increasing wavelength in sub-monolayer coverage measurements. Scattering effects in the spectra are consistent with surface scattering at the antinodes of the EFSW. These observations provide explanations for differences seen between optical properties of particles calculated using the specular-reflection method versus those calculated using traditional aerosol methods. A simple multilayer method for estimating particle absorption coefficients is demonstrated that compares well with values reported using ellipsometry for bulk polystyrene. Another simple method based on sub-monolayer coverage spectra provides spectra suitable for classification analysis but is only semi-quantitative at determining absorption coefficients.

Coating effects on mid-infrared reflection spectra of fabrics.

Polymer films of varying thicknesses were deposited onto cotton and polyester fabric samples by dip-coating from solution. Scanning electron microscopy (SEM) images of the coated fabric samples were used to evaluate the quality of the polymer coating. The samples were analyzed by infrared diffuse reflection spectroscopy to determine the relationship between film thickness and the effect of the coating on the spectroscopy of the two fabrics. Effects observed in four limiting cases are examined: (Case I) weak coating absorption on a fabric with weak absorption at the same frequency; (Case II) strong coating absorption in a spectral region of weak fabric absorption; (Case III) weak coating absorption in a spectral region of strong fabric absorption; and (Case IV) strong coating absorption in a spectral region of strong fabric absorption. In the first case, effects were dominated by reduced scattering as the coating is added. In the second case, the strong coating absorption that was observed at low coverages plateaus at higher coverage due to depth of penetration effects. In the third and fourth cases, reduced Fresnel diffuse reflection is measured as the coating is added, consistent with the reduction of scattering observed in the first case.