Natural materials cover a wide range of surface scattering properties, which can be described by the bidirectional reflectance distribution function (BRDF). The lobes of the BRDF may be described by their primary scattering direction (resulting in salient features of the appearance modes at different locations) and generic BRDFs typically consist of several of such lobes. Thus, similarly to describing BRDFs as linear superpositions of scattering lobes, we can describe the appearance of objects consisting of any materials as linear superpositions of objects of different canonical materials. We approach this problem via optically mixing of objects that represent canonical reflectance modes, allowing systematical and gradual variations of materials to test perceptual qualities of generic materials. We performed two experiments with a shiny, matte and velvety green bird, representing the forward, diffuse and surface scattering modes. The mixing was done with combinations of two materials in a real setup, using a semi-transparent mirror, and with combinations of three materials on a screen, using superpositions of photographs. In both experiments we tested how observers rated glossiness, softness, warmth and heaviness for systematically varied weights of the materials combinations. Finally we conducted a survey about how observers experienced the optical mixtures. We found systematical, gradual variations of the ratings as a function of the weightings, for the real stimuli consisting of dual mixes and for the images consisting of triple mixes. The data patterns for the real stimuli and for the images were consistent with each other for glossiness, softness and warmth ratings, but showed a global scaling and slight local modulation for heaviness. Observers thought we were varying material, lighting and color and they found the birds to look real. These results show that we can use optical mixtures as a basis for a material probe to investigate perception of generic, real materials.@en

In this paper, we introduce a toolbox for the perceptually based visualization of light in a volume, focusing on the visual effects of illumination. First, our visualizations extend the conventional methods from a two-dimensional representation on surfaces to the whole volume of a scene. Second, we extend the conventional methods from showing only light intensity to visualizing three light properties (mean illuminance, primary direction and diffuseness). To make our methods generally available and easily accessible, we provide a web-based tool, to which everybody can upload data, measured by a cubic or simple illuminance meter or even a smartphone-app, and generate a variety of three-dimensional visualizations of the light field. The importance of considering the light field in its full complexity (and thus as a three-dimensional vector field instead of its two-dimensional sections) is widely acknowledged. Our toolbox allows easy access to sophisticated methods for analysing the spatial distribution of light and its primary qualities as well as how they vary throughout space. It is our hope that our results raise interest in ‘third stage’ approaches to lighting research and design, and the toolbox offers a practical solution to this complex problem.@en

Human observers have been demonstrated to be sensitive to the local (physical) light field, or more precisely, to the primary direction, intensity, and diffuseness of the light at a point in a space. In the present study we focused on the question of whether it is possible to reconstruct the global visual light field, based on observers’ inferences of the local light properties. Observers adjusted the illumination on a probe in order to visually fit it in three diversely lit scenes. For each scene they made 36 settings on a regular grid. The global structure of the first order properties of the light field could then indeed be reconstructed by interpolation of light vectors coefficients representing the local settings. We demonstrate that the resulting visual light fields (individual and averaged) can be visualized and we show how they can be compared to physical measurements in the same scenes. Our findings suggest that human observers have a robust impression of the light field that is simplified with respect to the physical light field. In particular, the subtle spatial variations of the physical light fields are largely neglected and the visual light fields were more similar to simple diverging fields than to the actual physical light fields.@en