Standardized Short-Wave Infrared Hyperspectral Imaging for Non-Destructive Spectral Comparison of Coin Denominations, Design Variants, and Surface-Use Conditions

Abstract

Coins contain multiple types of information, including denomination, design variant, year of issue, and surface condition. This study established a standardized 900–1700 nm short-wave infrared hyperspectral imaging (SWIR-HSI) workflow for controlled and non-destructive spectral comparison of metallic coins. Twenty-five independent New Taiwan dollar (NT$10) coins were used to evaluate imaging position, coin side, and aperture setting. At position 13 on the reverse side, the mean spectral standard deviations were 0.192, 0.142, and 0.151 for F1.8, F2.8, and F4, respectively, and F2.8 was selected for subsequent measurements. Using the same NT$10 coin, support-material and in-plane rotation experiments produced spectral angle mapper (SAM) ranges of 0.232–1.087 and 0.132–0.581, respectively. Under the standardized conditions, the SAM value between one new and one circulated NT$10 coin was 0.729. Pairwise SAM values ranged from 0.260 to 1.680 among eight NT$10 design variants and from 0.458 to 2.449 among six denomination or special-year samples. An initial repeatability evaluation under the current experimental conditions, using three additional NT$10 coins and 24 acquisitions, yielded mean SAM values of 0.2450°, 0.6368°, and 0.3323° for repositioning, inter-day, and inter-operator comparisons, respectively, with an overall median spectral CV of 0.83%. These results quantitatively demonstrate that acquisition conditions affect SWIR spectral measurement and that selected coin samples exhibit measurable spectral differences under controlled conditions. The present study focuses on acquisition standardization and descriptive spectral comparison and does not establish validated coin classification, material identification, or surface-condition assessment.