Spatial resolution as a trade-space for low-light imaging of sensitive cultural heritage documents

Abstract

Abstract A 14th-century manuscript was imaged at high image quality with an HSI detector, using a low-illumination dose of 57 lx h, significantly less than one day’s worth of museum display (≈ 400 lx h). Initial work digitally downsampled spatial resolution from 333 pixels per inch (ppi) to 161 and 80 ppi, and increased the signal-to-noise ratio (SNR) by a factor of 2.00 and 2.79. Despite downsampling, pan-sharpened images were up to 1.5 times spatially sharper than the reference HSI image. Relatively high spectral accuracy was maintained, with spectral angle mapper (SAM) measurements of 0.0527–0.0963. The process was then repeated with physical adjustments to imaging height replacing the digital downsampling process, decreasing resolution from 403 ppi to 111 ppi, increasing SNR from 27.1 to 95.4, and sharpness by 60% using pan-sharpening, while maintaining SAM values of 0.110. An overall image quality metric is proposed, composed of fundamental spatial, spectral, and SNR metrics. This metric highlights efficient lighting and processing methods for increased image quality while limiting total illumination dose on the target, and can be tailored based on one’s imaging constraints.