**5. Conclusions**

In our research, the spectral change detection limit was determined on the basis of measurement repeatability over a time span of a few weeks. However, for long-term monitoring applications, long-term repeatability should be established. Future studies will have to look into practical calibration and validation procedures that are stable over long periods of time. In conclusion, imaging spectroscopic instruments can be valuable tools for monitoring the condition of archival documents and the present case study should be useful to establish the suitability of other instruments and similar case studies.

**Author Contributions:** Conceptualization, R.P.; methodology, R.P., M.E.K., M.S.; software, M.E.K.; formal analysis, R.P., M.E.K.; investigation, R.P.; resources, T.A.G.S. and G.d.B.; data curation, R.P. and M.E.K.; writing—original draft preparation, R.P.; writing—review and editing, M.S., M.E.K., R.M.G.; visualization, R.P., M.E.K.; supervision, M.S.; project administration, R.P.; funding acquisition, R.P., T.A.G.S., G.d.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding, but it was made possible thanks to the resources made available by the Nationaal Archief to Roberto Padoan for building the light-aging chamber and preparing and carrying out the measurements in 2014 during his work as books and paper conservator.

**Conflicts of Interest:** The authors declare no conflict of interest.
