## 3 Materials and Methods
### Layer 1: Measurements  
The aim of the first layer is to get measurements of environmental entities in the room where the wall painting is placed. The system is realized by some sensors directly connected to an ARDUINO electronic card (also called sensor cards in the remainder of the article). The choice of this card, compared to other possibilities [16], was due to its characteristics which have made it a standard for low frequency control applications. ARDUINO is an open-source electronics prototyping platform based on flexible easy-to-use hardware and software [27]. An ARDUINO board consists of an Atmel 8-bit AVR microcontroller with complementary components to facilitate programming and incorporation into other circuits. An important aspect of the ARDUINO is the standard way that  
Figure 1: Block Scheme of the measurements system: the layered architecture. connectors are exposed, allowing the CPU board to be connected to a variety of interchangeable add-on modules known as shields. The ARDUINO board exposes most of the microcontroller's I/O pins for use by other circuits. It provides 20 digital I/O pins, six of which are analog inputs connected to a 10 bits ADC. Several plug-in application shields are also commercially available. The shield has been designed by us and it is connected to a temperature sensor and a humidity one. The first is MCP9700 with an accuracy of \(\pm 4\)\(\mathrm{\SIUnitSymbolCelsius}\) over the 0-70 \(\mathrm{\SIUnitSymbolCelsius}\) temperature range [35]. The second is a HONEYWELL model HIH-5031-001, with an accuracy of 3% over the 0%\(-\)100% humidity range [36]. The need to limit the power consumed obliged us to choose electronics with reduced consumption. The ARDUINO is particularly suited for low energy applications because it is provided with a sleep mode with very low power consumption that is extremely useful when the card is not acquiring data. Other devices, such as data acquisition electronic cards based on Microchip microcontroller [37], Raspberry-Pi [38] or Beaglebone [39] could be used. Table 1 lists some principal characteristics of these devices. As Table 1 shows, the choice of which device to use to realize the sensor cards depends both on the personal experience of the users in the programming and in the design of electronic cards and on the type of project one wants to develop. If you want to do a software project, the Raspberry-Pi and the Beaglebone are surely the best choice. Their audio, video and internet capabilities make them the winners in this aspect. There is no need to attach external components, so there is no real need to learn electronics and they have the possibility of driving both a GPRS modem and a WiMAX one. If you want to do a hardware project, the ARDUINO and the Microchip microcontroller family are the best choice. In fact, although the Beaglebone has analog inputs to acquire analog signals, its power consumption is higher than that of the others making it less competitive for extreme applications such as ours, while the Raspberry-Pi has no native analog inputs. The cost is the element that makes ARDUINO the best choice, in fact, although the basic cost promotes the microcontrollers, the need to