**1. Introduction**

Imaging spectroscopy (IS) is a well-established analytical method in heritage science [1,2]. Based on the literature review done in 2014 [3] and a more recent one in 2020, it was possible to identify a deficiency about standards and experimental studies for the quantification of the impact and monitoring sensitivity of IS on heritage materials. With the intent of partly filling this knowledge gap we have been using the "SEPIA" hyperspectral imager of the *Nationaal Archief* (National Archives of The Netherlands) as a case study instrument. The described method can be transferred to validating other IS instruments and applications.

Assessing the impact of the environment and potential changes on cultural heritage during exhibitions is part of the professional standards of conservators [4] and their code

**Citation:** Padoan, R.; Klein, M.E.; Groves, R.M.; de Bruin, G.; Steemers, T.A.G.; Strliˇc, M. Quantitative Assessment of Impact and Sensitivity of Imaging Spectroscopy for Monitoring of Ageing of Archival Documents. *Heritage* **2021**, *4*, 105–124. https://doi.org/10.3390/ heritage4010006

Received: 18 December 2020 Accepted: 7 January 2021 Published: 12 January 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

of ethics [5]. The environmental parameters for long-term preservation of historical documents in storage are recommended to be 16 ◦C to 20 ◦C, 35 to 50% RH, and complete darkness [6–8]. When a historical document is not in storage but exhibited or otherwise exposed to light, it could undergo unwanted degradation. In conservation assessment, qualitative methods such as visual examination or photographic documentation are usually employed. More advanced and objective methods for monitoring changes are point-based measurements done with non-destructive methods such as single beam spectrometry or with micro-invasive chemical analysis by micro-sampling the monitored object. Both methods are effective in giving information about the analysed area, but they preclude the assessment of the overall condition of those artefacts that are very inhomogeneous. Moreover, the use of micro-invasive tests is in most of the cases not allowed and it cannot be repeated multiple times on the same spot. IS represents for this reason the most promising method for monitoring the optical properties of heritage materials in a non-invasive way and in the long term [9–11]. The spatial and spectral characteristics of IS instruments can in fact allow for the identification of entire areas on the monitored document where changes occur rather than single spots.

There is a clear distinction between using IS as an analytical tool and as a monitoring instrument. Monitoring involves technical challenges such as the alignment and comparison of images taken of the same area at different times, repositioning of the camera and illumination and choice of calibration standards. A project on the digitization of paintings using multispectral imaging at the National Gallery of London in 1988 was one of the first works in the field of heritage science that addressed these issues [12]. Following to that, the VASARI project was the first attempt to standardize monitoring of paintings [13,14]. This project was then followed by the MARC and the CRISATEL projects [15–17]. In book and paper conservation, only a few research projects have so far addressed the use of IS for monitoring purposes [18–24].

From a conservation point of view, ideally repeated measurements should not damage the monitored objects at all. It is of course essential that the monitored condition indicators are not changed more significantly by the monitoring instrument itself than by the exhibition conditions. The cumulative light dose depends on the measurement frequency, monitoring period (typically years), and the construction of the IS instrument itself. The IS instrument is deemed to be suitable for the intended monitoring applications, if any cumulative colour changes induced by the measurement itself remain well below the threshold of visual perception for even the most light-sensitive material. Note that different criteria and thresholds for the acceptable impact of the instrument may be chosen, depending on the range of materials, the measurement schedule, and the intended monitoring period.

Ideally, the IS instrument has to enable the detection of changes before such changes become visibly detectable so to prevent the occurrence of damages rather than documenting them. The repeatability of measurements, which determines the sensitivity limit for detecting changes in the object, depends on the construction of the instrument and the measurement method, spectral calibration, and methods and software algorithms applied to the comparison of measurements of the same object taken at different times.
