Directionality is defined as the balance between the diffuse and directional componentsof lighting. It is an indicator about the spatial distribution of light flow onto an element or into a space. The directional character of light has a significant impact on the appearance of a luminous environment, and the visual performance and comfort within that environment. Poor directionality may produce harsh shadows on the task, cause veiling reflections towards the viewing angle, or create a dull environment. Suitable directionality may model 3D surfaces, reveal the surface textures or details of a task, and create an aesthetically pleasing environment. There are a number of illuminance based performance indicators that are used for quantifying the directionality of light. This study is based on luminance values, rather than illuminance. Lighting patterns on any object are determined by three factors: the directionality of light, geometric and surface properties of the object, and the viewing angle. A luminance based metric incorporates all the three factors and thoroughly describes the scene as perceived by the human visual system.
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