Sub-surface characterisation of latest-generation identification documents using optical coherence tomography

Mid-infrared optical coherence tomography with a stabilized OP-GaP optical parametric oscillator

We demonstrate mid-infrared time-domain optical coherence tomography (OCT) with an orientation-patterned GaP optical parametric oscillator. Instantaneous broadband mid-infrared spectra provide reduced scattering for OCT applications including cultural heritage, quality assurance, and security. B-scan calibrations performed across the wavelength tuning range show depth resolutions of 67 µm at 5.1 µm and 88 µm at 10.5 µm. Volumetric imaging inside a plastic bank card is demonstrated at 5.1 µm, with a 1 Hz A-scan rate that indicates the potential of stable broadband OPO sources to contribute to mid-infrared OCT.

Evaluation of photometric stereo and elastomeric sensor imaging for the non-destructive 3D analysis of questioned documents - A pilot study

Photometric Stereo and Elastomeric Sensor Imaging were assessed for measuring the 3-dimensional (3D) morphology of questioned document samples. Photometric stereo is shown to be a powerful non-contact technique for revealing micron level detail of the samples examined. Elastomeric Sensor Imaging is shown to complement photometric stereo by yielding equivalent results. Additionally, this technique allows quantification of the morphological depth information. The techniques were applied to 2 different types of questioned document sample. Firstly, written signatures were examined. Both techniques were able to reveal characteristic features that could be used to infer stroke direction and ink line application sequence. Secondly toner/ink intersections were examined. Both techniques allowed visualisation of 3D features which were used to infer the sequence of application.

Portable and low-cost hologram verification module using a snapshot-based hyperspectral imaging algorithm

One of the challenges in differentiating a duplicate hologram from an original one is reflectivity. A slight change in lighting condition will completely change the reflection pattern exhibited by a hologram, and consequently, a standardized duplicate hologram detector has not yet been created. In this study, a portable and low-cost snapshot hyperspectral imaging (HSI) algorithm-based housing module for differentiating between original and duplicate holograms was proposed. The module consisted of a Raspberry Pi 4 processor, a Raspberry Pi camera, a display, and a light-emitting diode lighting system with a dimmer. A visible HSI algorithm that could convert an RGB image captured by the Raspberry Pi camera into a hyperspectral image was established. A specific region of interest was selected from the spectral image and mean gray value (MGV) and reflectivity were measured. Results suggested that shorter wavelengths are the most suitable for differentiating holograms when using MGV as the parameter for classification, while longer wavelengths are the most suitable when using reflectivity. The key features of this design include low cost, simplicity, lack of moving parts, and no requirement for an additional decoding key.

Magnetic Field-Modulated Plasmonic Scattering of Hybrid Nanorods for FFT-Weighted OCT Imaging in NIR-II

We report a method for fast Fourier transform (FFT)-weighted optical coherence tomography (OCT) in the second biological tissue transparency window by actively modulating the plasmonic scattering of Fe₃O₄@Au hybrid nanorods using magnetic fields. Instead of tracking the nanoparticles' lateral displacement in conventional magnetomotive OCT imaging, we monitor the nanorod rotation and optical signal changes under an alternating magnetic field in real time. The coherent rotation of the nanorods with the field produces periodic OCT signals, and the FFT is then used to convert the periodic OCT signals in the time domain to a single peak in the frequency domain. This allows automatic screening of nanorod signals from the random biological noises and reconstruction of FFT-weighted images using a computer program based on a time-sequence image set. Compared with conventional magnetomotive OCT, the FFT-weighted imaging technique creates enhanced OCT images with dB-scale contrast over an order of magnitude higher than the original images.

A novel approach for forensic identification of automotive paints using optical coherence tomography and multivariate statistical methods

Automotive paint is one of the most important evidence in solving vehicle-related criminal cases. It contains the critical information about the suspected vehicle, providing essential clues for the investigation. In this study, a novel approach based on optical coherence tomography combined with multivariate statistical methods was proposed to facilitate rapid, accurate and nondestructive identification of different brands of automotive paints. 164 automotive paint samples from 8 different manufacturers were analyzed by a spectral-domain optical coherence tomography system (SD-OCT). Two-dimensional cross-sectional OCT images and three-dimensional OCT reconstruction of vehicle paints of different paints were obtained to show the internal structural differences. Visual discrimination of A-scan data after registration and averaging processing was first used to distinguish different samples. An scanning electron microscope was utilized to obtain the cross-sectional image of the sample to evaluate the effectiveness of OCT technique. Then the original A-scan data, first derivative data and second derivative data of 136 paints with four layers from 7 different manufacturers were collected. Multivariate statistical methods, including principal component analysis (PCA), multi-layer perceptron (MLP), k-nearest neighbor (KNN) algorithm and Bayes discriminant analysis (BDA), were used to analyze different datasets. The results show the hybrid PCA and BDA model based on the first derivative OCT data achieved the best result of 100% accuracy on the testing dataset for identifying automotive paints. It is demonstrated that the OCT technique combined with multivariate statistics could be a promising method for identifying the automotive paints rapidly and accurately.

Production of photochromic nanocomposite film via spray-coating of rare-earth strontium aluminate for anti-counterfeit applications

New photochromic film was developed toward the preparation of anti-counterfeiting documents utilizing inorganic/organic nanocomposite enclosing a photoluminescent inorganic pigment and a polyacrylic binder polymer. To generate a translucent film from pigment/polyacrylic nanocomposite, the phosphorescent strontium aluminum oxide pigment should be well-dispersed in the solution of the polyacrylic-based binder without agglomeration. The photochromic nanocomposite was applied efficiently onto commercial cellulose paper documents utilizing the effective and economical spray-coating technology followed with thermofixation. A homogeneous photochromic film was immobilized onto cellulose paper surface to introduce a transparent film changing to greenish-yellow upon exposure to ultraviolet light as depicted by CIE coloration measurements. The photochromic effect was monitored at lowest pigment concentration (0.25 wt%). The spray-coated paper documents exhibit two absorbance bands at 256 and 358 nm, and two fluorescence peaks at 433 and 511 nm. The morphologies of the spray-coated documents were explored. The spray-coated paper sheets showed a reversible photochromic effect without fatigue under ultraviolet irradiation. The rheology of the produced photochromic composites as well as the mechanical properties and photostability of the spray-coated documents were studied.