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Sensor to monitor UV and Visible light on works of art

Summary: The optoelectronic device for measuring UV and Visible light includes 5 sensors and their devoted electronics. The first 3 sensors are realised by a photodiode with an integrated Red, Green or Blue filter. The last 2 perform the UV measurement using a sensor suitable for UV detection and a filter especially realised to transmit the UV spectrum between 250nm and 400nm. The fifth sensor, mounting UV and Red filter, has been introduced for subtracting the red light transmitted by the UV filter.    The final evolution of the device will be a stand-alone module acquiring and saving measurements of UV and Visible light. Several modules can be placed in each museum room to give lighting history of the site; otherwise they can be placed on every exhibited object to monitor the light illuminating the work of art.    The control of the high-energy radiation (UV and Blue light) is fundamental to estimate the damage suffered by a work of art. The damage does not depend only on the quantity of radiation that can hit the artwork, but also on the radiation type since the damage factor of artworks has a critical dependence on wavelengths.

ILLUMINATION AND PRESERVATION OF WORKS OF ART

When a work of art is illuminated with natural or artificial light it may be damaged by the light. The damage is mainly due to photochemical action or/and radiant heating effect. An example of this damage is presented in the "Incoronazione della Vergine" by Lorenzo Monaco shown in Fig. 1, where the colour of Maria's dress has changed from "malva" dark pink to white. Damage sources act differently on each work of art and affect in a different way every colour and every pigment.

Fig. 1

The picture in Fig. 2, "L'incredulità di San Tommaso" by Cima da Conegliano, is particularly interesting since it contains all the colours known in the XVI century. It is well known that to reduce damages the ultraviolet spectrum must be avoided, but not only the UV spectrum: the continuous monitoring of the radiation impinging on a museum object can provide information on its possible undergone damages. The evaluation of the damage due to exposure to photochemical action involves several aspects: Irradiance, Duration of exposure, Spectral power distribution of incident radiation.

Fig. 2

The current state of research is insufficient to provide a clear picture of relationships between photon energy thresholds, incident spectral power distribution, and rates of photochemical action. Nevertheless the International Council of Museum indicates the values for the maximum illuminance recommended for the different types of museum object, which are summarised in the Table 1.

CONTROL OF THE VISIBLE LIGHT

Fig. 3 - Spectral response for RGB sensors.

The 3 sensors for the Visible spectrum provide Red, Green and Blue light detection to evaluate the RGB balancing.

RGB signals correspond to the spectral curves in Fig. 3, normalised for having unitary area.

Thus the RGB balancing results R=1, G=1, B=1 when the light spectral distribution is uniform.

RGB balancing is useful to estimate the colour rendering of a work of art, but it can also evidence colour variations in museum illumination (detecting change or corruption of the lamps).

DETECTION OF THE UV RADIATION

Fig. 4 - Transmittance of the UV filter.

The UV filter (UG11) has been especially realised to transmit the radiation between 250nm and 400nm but, as Fig. 4 shows, it also transmits some red light.

For the UV measurement we use Sensor4 and Sensor5 (in Fig.s 5 and 6): Sensor4 is a UV detector mounting the UG11 filter; while Sensor5 is the same UV detector with both UG11 filter and Red filter.

Therefore Sensor5 detects the red light transmitted by the UG11 filter. The UV detection is then obtained subtracting this value from Sensor4 detection.

OPTOELECTRONIC DEVICE

Fig. 5

Fig. 5 presents a photo of the device, which is mounted in a black box with dimensions 19 x 16 x 5 cm.

SENSORS AND FILTERS

The main components are the 5 sensors mounted on the board in Fig. 6, which is the upper layer in Fig. 5. The other two layers are the circuit boards shown in Fig. 7.

The sensors, from left to right in Fig. 6, detect Green, Red, Blue light and the last two with UG11 filter detect the UV spectrum. The filter on Sensor4 is the UV filter while Sensor5 has the UV and the Red filter.
	FIG. 6 - board mounting the 5 sensors.

HARDWARE

Current signals arriving from the 5 sensors are converted into voltages by operational amplifiers and then into 16-bit numbers sent to PC through a serial port. Fig. 7 presents the electronic scheme and the photo of the hardware, which is realised on three superposed boards.  

Fig. 7 - The 3 circuit boards.

A prototype of the device has performed some test detections on museum lighting at the Galleria dell'Accademia of Florence. These preliminary measurements are fundamental to adjust the prototype and to regulate the levels of the light detection, especially for the UV measument.       

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