## 1 Introduction
This is the main concern regarding the crowdsourced information, therefore a homogenisation via color calibration is needed to  
Figure 1: Visual outline of the conceived cultural heritage conservation system by color monitoring via crowdsourcing. perform an effective monitoring of the pieces. This requires the presence of a color chart when acquiring the content to be calibrated. The mentioned approaches use mobile images individually adjusted using X-Rite charts as color reference. However, the X-Rite color chart has been designed provide bright colors for outdoor photography, and some of the tiles have too saturated colors [13], and its materials have not been tested to be compatible with heritage objects. Our monitoring system pretends a further step on these first approaches, designing and testing an optimized color chart, an accurate calibration method and developing an efficient color change detection system. The previous stages of this project, the conception of the color chart, the verification of the suitability of the materials for the monitoring system, and an adaptive calibration algorithm for image processing have already been presented [11; 19]. The color chart is mounted on an inert substrate together with several metal coupons used as sensitive materials. This reference color chart has been specifically designed to cover all the range of the sRGB color space, which is shared by most phone cameras in usage. It is placed in the display case or exhibition space to be monitored in this project. Alongside the chart, metal coupons are placed. Possible inadequate environmental conditions that may affect the heritage artifacts will induce corrosion in the coupons. This corrosion process might take the form of a tarnishing, more or less uniform colored corrosion layers, or colored spots on the surface of the metal. These colorimetric cues are the relevant information to be extracted from the crowdsourced pictures, as warnings of unsuitable environmental conditions for the conservation of heritage assets. The calibration process is one of the most critical parts of the monitoring system. Assuming most crowdsourced pictures are taken with different cameras, a quality calibration for every image is needed for homogenizing color information. According to this, the color chart needs to be consistent and stable enough to be used as a reliable reference for the color calibration algorithm. So, the materials employed to build the display and chart and the coupons need to be carefully selected so they do not undergo possible strong chromatic deviations from the initial state; and they should not produce alterations in the metal coupons and, more important, in the heritage artifacts [11]. It is utmost important that the reference color chart remains unaffected so the quality of the picture calibration does not lower when deployed for a prolonged time. In addition, if by any reason the color references change, it is imperative to check how this change can affect the outcomes of the calibration process. Thus, the aim of this paper is to evaluate robustness of the crowdsourced color evaluation system against environmental degradation of the reference display. The criterion adopted is mathematical robustness to discoloration. Since the main agent causing color modification in organic materials (including boards, papers and inks) is light, variations in the color chart on different materials due to exposure to UV radiation have been evaluated. For this purpose, periodic measurements of the exposed references have been carried out. These measures are processed in different mathematical ways to evaluate the robustness of environmentally-induced color degradation in them. From this information, it is evaluated how possible light induced color variations may affect the accuracy of the calibration method. Finally, conclusions are drawn and the selection of the most suitable material to build the crowdsourcing-based monitoring display are proposed.