Conspicuity of urban objects
Introducing a model of saliency level based on visual complexity and luminance contrast with implication in urban lighting
In recent decades, one of the main objectives of lighting masterplans has been to highlight important urban objects and landmarks at night to create a more pleasant ambiance. This process of lighting design, as with other types of visual design, needs to consider the relationship between the object and its context and how the background lighting could influence the design of the lighting applied on the urban objects. Different studies have shown that salience is strongly influenced by visual context; which means the same target may be salient or not, dependent on how it is embedded in the scene.
Two variables have been investigated - the density of background patterns of light, and their proximity to the target. Traditionally, increasing luminance has been the main tool used to improve the saliency of a particular object. For this reason interaction of these factors with luminance contrast also has been studied, this suggesting a model of saliency level based on luminance contrast and complexity of background lighting. This is a novel approach - most studies about the factors which influence saliency of urban objects consider only luminance contrast and colour contrast of objects against their simple background and the complexity of background has been almost ignored. This study applies the previous studies in psychophysics into urban lighting.
This PowerPoint presentation (.ppt, 1.4MB) describes the methodology used in tests.
The graph below shows the results of the tests carried out to examine the effect of background density. It can be seen that higher background densities reduce the saliency of an object. Note that a target of luminance contrast 3 with a low density of background lighting has a similar saliency to a target of luminance contrast 5 but with a medium or high level of background density: reducing background density allows the luminance of an object to be reduced whilst maintaining the same level of saliency.
Davoudian N and Fotios S, The impact of spatial characteristics of background lighting on the visual saliency of urban objects, Lux Europa, Istanbul 2009, 1121-1126.
Davoudian N and Fotios SA, Background patterns of light and their impact on the saliency of illuminated objects in urban spaces, Light Pollution & Urban Lighting 2007, Istanbul, Turkey, September 2007.
Davoudian N and Fotios SA, Alternatives to brightness for achieving night-time conspicuity, Urban Nightscape 2006, annual meeting of CIE divisions 4 & 5, Athens, 21-23 September 2006, pp. 62-63.