## 1 Introduction  
All over the world, objects of historic and cultural values are exhibited and preserved in museum collections. To protect objects in collections from degradation they are held and exhibited in controlled environments, where ideally all ambient conditions are adjusted to minimize future damage to the heritage materials. This strategy, focused in preventing future deterioration acting on the potential causes of deterioration, is known as preventive conservation. However, the perfect environment is impossible to achieve, and sometimes humidity, pollutants or lighting problems can reach a point where the material an exhibited cultural heritage item is made of starts suffering a quick degradation [1, 2, 3]. Sometimes harmful emissions come from inside. Building materials from furniture, showcases or panels, such as paints, woods or adhesives or cleaners, and even some objects from collections can emit gaseous pollutants. This problem is even more serious if the exhibited artifacts are stored in non-ventilated display [4, 5]. Thus, even in the best conservation-suited environment, additional needs for surveillance of cultural heritage are likely to be required. In order to lighten human efforts on the task of surveillance, automatic monitoring resources are encouraged and proposed. These may take any mean for quantifying the possible decay, like sensing humidity or light or checking the change in color of some material present when it is undergoing a degradation process [6]. One of the most evident signs of deterioration of heritage materials are color alterations. Color is probably the most significant property of cultural heritage objects. Not only in painting, but in sculpture and decorative arts color is carefully chosen, selecting materials, finishes and surface treatments. Aging of materials frequently produces color changes that can be related to degradation in different aspects and degrees of intensity. In some cases as in irreversible alterations of pigments and dyes it represents a major concern for conservators. Therefore, the availability of tools to monitor color change is of interest in cultural heritage conservation. At the same time, color change of certain reference materials can been used for indoor environment evaluation as a preventive conservation tool. Colorimetric measurements are in fact frequently used for detecting degradation in different materials, using colorimeters or spectrophotometers [7; 8]. The use of color change as an indicative of environmental changes has also been suggested [9; 10]. Color monitoring is clearly relevant in the conservation field; it does not require complex equipment or specialized personnel, as a colorimeter is easy to operate. However, it is a time-consuming method, and has some application limitations on very irregular or delicate surfaces, as it requires contact. Based on these premises, our research group has proposed a system for monitoring environmental conditions in museums based on the measurement of color changes by image processing. The objective of this system, besides being a low-cost preventive conservation tool, is to increase social awareness in relation to heritage conservation. For this purpose, the system uses crowdsourced pictures taken by visitors with their cell phones [11] (Figure 1). The usage of crowdsourcing techniques is acknowledged as a useful resource, which has been used for long in monitoring of biodiversity, astronomy, and other fields [12]. More recently has been proposed as a monitorization tool for cultural heritage preservation, using smartphone technologies [13; 14; 15]. Photographic cameras represent the acquired information analogously to the human visual system[16; 17]. But, considering fabrication and physical component differences between models, images representing the same scene by disjoint cameras are likely to appear visually different to the same observer [18].