Fig. 2From: Highly sensitive terahertz non‐destructive testing technology for stone relics deterioration prediction using SVM-based machine learning modelsThe optics performance of the flaked stone of hollowing samples in THz waveband. a The time-domain signals of the reference and the flaked stone with thickness are 1.5 mm and 2 mm; b–d are the THz frequency spectra, transmission spectra, and refractive index spectra of the two flaked stones, respectivelyBack to article page