The diffuseness of illumination on an object plays a key role in how we identify the material of that object. Quantifying its impact is needed to determine the most favorable lighting conditions to recognize a material. With such an aim, the authors have conducted psycho-visual experiments, performed with real materials and a specific device designed to illuminate the objects under various controlled diffuseness and for a fixed illuminance.

Several materials (polyresin, fur, wood, stainless-steel) were presented to observers. The first step of the experiments consisted in memorizing the appearance of the objects by viewing and touching them in a daily environment. The objects were then illuminated under different diffuseness and the observers evaluated the fidelity and the ideality of their appearance. In practice, they chose the best lighting condition that faithfully reproduces the object appearance, and the one that ideally reproduces the characteristics of the material.

The main result is that moderate diffuseness values are the most suitable. One hypothesis is that these values correspond the most to diffuseness in daily life. Rather expectedly, glossy materials are more sensitive to illumination diffuseness than matte materials. These results may seem intuitive in hindsight. Nevertheless, as for any sensory metrology study, the interest is precisely to formalize and quantify observations that are, by nature, subjective.

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