Terahertz reflection spectroscopy of palm leaf manuscripts with spatially contextualized analysis

Reflection-mode THz-TDS of spatially heterogeneous palm-leaf manuscripts reflects contributions from both material response and measurement geometry. We present a workflow using five duration-labeled specimens (0–160 d; one specimen per label), each measured at left, center, and right positions. Between-specimen comparisons are descriptive, and one ancient specimen serves as an external descriptive case. Mean reflectivity and integrated RAI showed modest position variability, whereas phase slope showed pronounced position variability and was sensitive to measurement context; spectral-shape deviation remained reference-specific. Based on normalized mirror-reference amplitude and position variability, 0.5–1.2 THz was retained as a system- and dataset-dependent working range. Signal-quality and sensitivity checks led to exclusion of local-band descriptors that were sensitive to interpolation or limited by weak reference amplitude. The workflow establishes a measurement-validity basis for spatially contextualized THz analysis and provides a foundation for subsequent material validation and non-invasive monitoring studies in heterogeneous documentary heritage.