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In this work we have obtained terahertz images of hidden signatures that were located on the back side of the test-object’s canvas and covered by covered with a thick layer of brown paint. It was demonstrated that in the tomogram obtained at a frequency of 0.3-0.5 THz, the layers of paint applied on the other side of canvas are clearly distinguishable.

The goal of this study was to detect and inspect the paint layers below the surface independently of any surface features. Using the for THz-TDS imaging system, we obtained contrast images of layers of paint applied to the back side of the canvas. The most difficult task that the researchers have set themselves has not yet been fully resolved. When we try to read the signatures through several layers of paint, background and canvas, we cannot get a clear image of the letters, but we can accurately determine the location of all the signatures.

An overview is presented of recent industrial applications of Terahertz and some of the key challenges and solutions towards further the introduction of Terahertz systems into production environments.

In this paper, we present the latest chemical mapping images of heterogeneous mixtures based on far-infrared reflection spectra obtained using a terahertz pulsed imaging system. The terahertz images and spectral data obtained have a spectral resolution of 1 cm−1 and cover a spectral range of 5–80 cm−1. This broad spectral coverage enables us to map out the spatial distribution of individual chemical components of a sample in a single measurement.

Terahertz pulsed imaging (TPI) was employed to explore its suitability for detecting differences in the film coating thickness and drug layer uniformity of multilayered, sustained-release coated, standard size pellets (approximately 1mm in diameter). Pellets consisting of a sugar starter core and a metoprolol succinate layer were coated with a Kollicoat(®) SR:Kollicoat(®) IR polymer blend for different times giving three groups of pellets (batches I, II and III), each with a different coating thickness according to weight gain. Ten pellets from each batch were mapped individually to evaluate the coating thickness and drug layer thickness between batches, between pellets within each batch, and across individual pellets (uniformity). From the terahertz waveform the terahertz electric field peak strength (TEFPS) was used to define a circular area (approximately 0.13 mm(2)) in the TPI maps, where no signal distortion was found due to pellet curvature in the measurement set-up used. The average coating thicknesses were 46 μm, 71 μm and 114 μm, for batches I, II and III respectively, whilst no drug layer thickness difference between batches was observed. No statistically significant differences in the average coating thickness and drug layer thickness within batches (between pellets) but high thickness variability across individual pellets was observed. These results were confirmed by scanning electron microscopy (SEM). The coating thickness results correlated with the subsequent drug release behaviour. The fastest drug release was obtained from batch I with the lowest coating thickness and the slowest from batch III with the highest coating thickness. In conclusion, TPI is suitable for detailed, non-destructive evaluation of film coating and drug layer thicknesses in multilayered standard size pellets.

Pump and probe nonlinear transmission experiments with femtosecond resolution are used to investigate electron tunnelling between the central CdMnTe well and the outer CdTe wells of a CdTe/CdMnTe double barrier structure. The time dependence of the reduction in oscillator strength resulting from bandfilling at the exciton resonance is used to measure population changes in the outer wells. A tunnelling time of 8 ps is measured when a carrier density of 1 x 10 13 cm -2 is used. Monte Carlo simulations show that the timescale of tunnelling via LO phonon scattering is 10 ps. A pump induced increase in absorption at energies smaller than that of the exciton resonance is observed at early times and assigned to band-gap renormalisation.

The ability of terahertz pulsed imaging (TPI) to be employed as an analytical tool for monitoring a film coating unit operation and to assess the success of a subsequent process scale-up was explored in this study. As part of a process scale-up development, a total of 190 sustained-release tablets were sampled at 10% increments of the amount of polymer applied, from a lab-scale and a pilot-scale coating run. These tablets were subjected to TPI analysis, followed by dissolution testing. Information on tablet film coating layer thickness and variations in coating density were extracted using TPI. It was found that both terahertz parameters were more sensitive and informative to product quality when compared with measuring the amount of polymer applied. For monitoring the film coating unit operation, coating layer thickness showed a strong influence on the dissolution behaviour for both the lab-scale and the pilot-scale batches. An R 2 of 0.89, root mean square error (RMSE) = 0.22 h (MDT range = 3.21–5.48 h) and an R 2 of 0.92, RMSE = 0.23 h (MDT range = 5.43–8.12 h) were derived from the lab-scale and pilot-scale, respectively. The scale-up process led to significant changes in MDT between the lab-scale and pilot-scale. These changes in MDT could be explained by the differences observed in the film coating density on samples with similar amount of polymer applied between the lab and the pilot-scale. Overall, TPI demonstrated potential to be employed as an analytical tool to help refine the coating unit operation and the scale-up procedure.

Applicability of the terahertz pulsed spectroscopy (TPS) and the terahertz pulsed imaging (TPI) to detect tulobuterol (TBR) crystal in transdermal tape has been investigated. The TBR crystal was identified by comparison between spectrum of tapes and TBR reference standard. The TBR crystal in patch matrix was successfully detected by the TPS and TPI. These results indicate that the TPS and TPI are applicable to identify an active substance in patch matrix for quality control.

Terahertz pulsed imaging is now used for routine non-destructive testing of pharmaceutical products. The technique provides valuable information about a tablet's coating uniformity and can provide additional information about its core. Terahertz pulsed spectroscopy is a powerful technique in the analysis of polymorphic forms as terahertz region of the electromagnetic spectrum probes intermolecular interactions in the crystalline form.

Fourier-transform infrared transmission spectroscopy has been used for the determination of glucose concentration in whole blood samples from 28 patients. A 4-vector partial least-squares calibration model, using the spectral range 950-1200 cm(-1), yielded a standard-error-of-prediction of 0.59 mM for an independent test set. For blood samples from a single patient, we found that the glucose concentration was proportional to the difference between the values of the second derivative spectrum at 1082 cm(-1) and 1093 cm(-1). This indicates that spectroscopy at these two specific wavenumbers alone could be used to determine the glucose concentration in blood plasma samples from a single patient, with a prediction error of 0.95 mM.

Many time-resolved spectroscopy techniques require two broadly and independently tunable, synchronised light pulses. This requirement can be met by optical parametric oscillators (OPOs) [1] and travelling wave optical parametric amplifiers (OPAs) [2, 3, 4, 5]. The latter capable of providing higher pulse energies. Our aim was to develop a multikilohertz repetition rate OPA system capable of providing femtosecond/picosecond, synchronised and independently tunable pump and probe pulses in the visible part of the spectrum for time-resolved absorption and resonance Raman spectroscopies.

Spectral distortions are commonly observed in terahertz spectra of granulated materials. These spurious structures in the spectroscopy are caused by scattering due to the refractive index mismatch between the particles and their surrounding medium. We find that the scattering contribution is random across sample positions and could be eliminated by summing and averaging multiple measurements over a sample area. We present experimental results of both absorbing and nonabsorbing particles in the size range 50–250μm and also give an empirical expres-sion to describe the effect of grain size on the scattering-induced extinction as a function of frequency.

Studies in terahertz (THz) imaging have revealed a difference between diseased and healthy tissue in the THz regime. Since water has strong absorptions at THz frequencies, a likely contributing factor to the contrast mechanism is variation in water content. We have previously modelled the propagation of a THz waveform through water and normal skin in the time domain. In these preliminary results, we extract the complex dielectric coefficients from THz spectroscopy measurements of diseased tissue and predict the waveform differences which have been seen in THz images of skin cancer.

The polymorphic modifications of carbamazepine show distinct spectral features in the far infrared between 2 and 130 cm-1. Using time-resolved terahertz pulsed spectroscopy the enantiotropic solid state phase transition between two polymorphic forms was recorded. The terahertz spectra revealed that the solid state conversion follows a two step mechanism: form III sublimates and then condenses in a second step as form I. The sublimation and condensation steps follow different kinetics.

Various aspects of tablet coating quality were analysed with terahertz pulsed imaging (TPI). Coating layer thickness variability and reproducibility, coating distribution and uniformity were determined and validated with optical microscopy imaging. Results indicated that TPI has the potential to be employed as a process analytical tool to determine tablet coating quality due to its non-destructive nature.