Improved robustness by concurrent rapid and non-destructive terahertz sensing of pharmaceutical tablet thickness and porosity

Conventional porosity measurements of pharmaceutical tablets using terahertz time-domain spectroscopy (THz-TDS) in transmission geometry require supplementary thickness measurements - typically using an optical laser gauge - to enable accurate porosity determination. However, discrepancies in beam profiles between the laser and THz systems, as well as the need for precise mechanical positioning of the tablet for the two measurements, introduce potential systematic errors and complicate the measurement setup. This study presents a robust and rapid alternative: a transflection-based THz-TDS method that simultaneously determines both tablet thickness and porosity from a single terahertz waveform. By utilising a state-of-the-art, high-power THz-TDS system, we obtained sufficiently strong reflection features to extract both parameters without additional instrumentation. This approach overcomes the limitations of conventional transmission setups, offering a non-destructive, calibration-free solution for in-line porosity monitoring in pharmaceutical tablet manufacturing.