The colour of light for urban spaces

Light in urban spaces was first introduced to prevent crimes against both people and properties and, while there is no study that confirms crimes' reduction in lit spaces, it is undeniable that people feel safer in lit urban spaces. Over the years it became evident that light can also improve the liveability of cities and enhance their characteristics, therefore in the last years a growing attention was paid to urban lighting design taking into account the environmental impact and energy saving requirements too. The will to use the lighting design project to improve the liveability of historical centres it’s clear: the city during the night is no more conceived like a sum of lit architectures but, thanks to a good lighting design project, it could complete the image that the city has during the day with perspectives, views and glimpses correctly illuminated, giving a global sense of the environment while making it safer. To properly highlight building facades and monuments, an accurate study of materials and chromaticity needs to be conducted and in this paper a methodology to do so is proposed. This method was developed after a research on the evaluation of chromatic changes of objects lit by sources with different SPDs and it is also based on the concept of visual saliency. Visual saliency is the perceptive quality that makes an object discernible and capable of instantly catching the eyes and mind attention; our brain automatically creates "saliency maps" of everything we see and using an algorithm it is possible to graphically recreate these maps to understand what attracts the eyes' attention in order to enhance it. Luminance is one of the factor that adds or takes saliency to an object: by increasing or decreasing luminance values it is possible to attract or repel the eyes' attention. Today there is a tendency to recommend a specific luminance ratio between an object and its background, using a higher luminance contrast to achieve a higher visibility level and this is typical of objects with a uniform background. However, in urban situations objects have complex backgrounds and it is important to remember that an object's saliency also depends on its background. A single salient target draws the attention irrespective of the number of the other not salient elements on the background, but when these elements increase in luminance, the main target loses visibility and saliency. It is also really important to consider that there may also be different levels of saliency. The methodology was then applied to a case study in the historical centre of Naples.