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A new LED lighting system for Leonardo's The Last Supper

In 2015 iGuzzini donated a new system for lighting Leonardo Da Vinci's The Last Supper. The system features latest generation luminaires that highlight the splendid colours and details of this world famous masterpiece. In its role as technical sponsor, in the years to come, iGuzzini has also agreed to monitor the rapid technological developments taking place in the lighting sector and identify the best ways to conserve and illuminate the painting. This will be done in compliance with  its philosophy of supporting and protecting Italy's cultural heritage that has characterised all the initiatives in the company's program: Light is Back. This particular project dates back to 2014, when the Milan Heritage Office decided that The Last Supper needed a new lighting system. Naturally, this included the entire refectory of the Church of Santa Maria delle Grazie in Milan where the masterpiece is located, and which in 1980 was declared a UNESCO World Heritage Site. The refectory is a long, rectangular room with two famous paintings on opposite walls: Leonardo's The Last Supper on the North wall and Donato Montorfano's Crucifixion on the South wall. From a technical point of view, The Last Supper, painted for the Duke of Milan, Ludovico "il Moro" Sforza, between 1494 and 1497, is an amazing experiment. Leonardo wanted to apply the light effects that Flemish painters were beginning to import into Italy, to mural painting. To achieve this he needed the same kind of surface and colours used when painting wooden panels. So he prepared the surface of the wall with a primer that we still do not know the exact contents of, and which was the primary cause of the damage that began to inflict the painting from the beginning of the sixteenth century. Milan has a particularly damp climate that is unfortunately extremely detrimental to this kind of technique and even in those very early years, the painting had already deteriorated so badly that Cardinal Federico Borromeo of the Ambrosiana Academy had a large copy made to conserve its memory as he was afraid it would be lost altogether. Over the centuries numerous other attempts were made to “restore” the painting up until its definitive restoration that began in the mid 1970s and ended in 1999, the year in which the decision was made to install the lighting and air conditioning systems that are still in place. During this time the Galileo Ferraris Electrotechnical Institute photometry laboratory compiled the specifications for the lighting system. The position of the luminaires was limited on account of distribution requirements and they were therefore concealed behind the crush barriers that run parallel to the murals. Special fluorescent lamp luminaires were used that created a marked asymmetrical light beam to guarantee light uniformity across the vertical surface. The total colour temperature produced was about 4050°K. The specifications for the new lighting system defined by the Heritage Office focused on the need to limit lighting to the surface of the paintings, not illuminate the side walls of the refectory or the section of the vault above the paintings, restore the work's chromatic qualities, reduce the heat created by the luminaires and lower energy consumption. The design procedure consisted of two separate phases involving laboratory testing the luminaires and then defining the solution on site. The first phase was conducted in the ISCR light research laboratory with the aim of assessing the optimal spectral distributions. The new system features Palco spotlights fitted with Chip On Board LEDs installed on a DALI track including an iGuzzini control unit that can individually regulate the luminous flux of each spotlight. This creates a superior level of light uniformity on the painting while remaining within the lighting levels produced on the painting set by the conservation specifications defined and tested by the Higher Institute for Conservation and Restoration. The LED spectral quality and relative colour temperature were chosen following a visual assessment of the tests on the painted surface made by a commission of experts including the Head of the Milan Architectural and Landscapes Heritage Office, the Director of the Last Supper Site, the painting's Chief Restorer and the Director of the Lombardy Regional Museum Group, who together judged that the optimal colour balance produced by the spotlights is an LED colour temperature of 3384 K. On the basis of this decision a number of assessments were also made regarding conservation. Tests were made and all the measurements taken indicate that the consistent reduction of dissipated heat in the room achieved by the new lighting system has a positive effect on the stability of the exhibition environment. The many improvements introduced have also had a positive effect not only on The Last Supper, but also on Montorfano's Crucifixion that has been lit using 3 optical units for each of the two Cestello luminaires mounted on the side walls with crossover aiming that creates uniform light distribution over the whole painting. The careful regulation of the quantity and orientation of the luminous flux means that the cracks and irregularities in the wall are hardly visible. Other zones that have been lit include the public area in the refectory and the visitor paths. Cestello standard lamps have been installed in the room with low voltage AR111 halogen lamps with a power level of 38 watts for each optical unit that have now been replaced with 20 watt optical units fitted with multichip LEDs. For the visitor paths in the cloisters, on the other hand, the existing design solution, featuring ceiling-mounted Cestello luminaires, was kept, but the 30 x 50W halogen lamps were replaced with the same number of 11 watt LED optical units that reduce consumption by 78%. The positive results of the new system has enabled the site to extend its visiting hours. Taking into account other environmental factors (such as dust and pollution), after an initial monitoring period, the estimated increase in the number of visitors will allow 45,000 more people a year to experience Leonardo's masterpiece.

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    Archaeology, Arts and Landscapes Heritage Office for the City of Milan

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