We introduce a way to simultaneously measure the light density, light vector and diffuseness of the light field using a cubic illumination meter based on the spherical harmonics representation of the light field. This approach was applied to six light probe images of natural scenes and four real scenes built in our laboratory, and the results were compared to those obtained using Cuttle’s method. We also demonstrated a way to simultaneously and intuitively visualise the global structure of the light distribution using light tubes and colour coding for the light density, light flow and diffuseness variations through the space. Together with Mury’s work, we have a complete way to describe, measure and visualise the local and global low-order properties of light distributions in three-dimensional spaces.@en

The light density, direction and diffuseness are important indicators of the spatial and form-giving character of light. Mury presented a method to describe, measure and visualise the light field’s structure in terms of light density and direction variations in three-dimensional spaces. We extend this work with a theoretical and empirical review of four diffuseness metrics leading to a novel metric proposal DXia. In particular, the relationships between these diffuseness metrics were studied using a model named ‘probe in a sphere’. Diffuseness metric DXia re-frames the diffuseness metric of Cuttle in an integral description of the light field. It fulfils all diffuseness criteria and has the advantage that it can be used in a global, integrated description of the light flow and diffuseness throughout three-dimensional spaces.@en

The lighting and furnishing of an interior space (i.e., the reflectance of its materials, the geometries of the furnishings, and their arrangement) determine the appearance of this space. Conversely, human observers infer lighting properties from the space's appearance. We conducted two psychophysical experiments to investigate how the perception of the light direction is influenced by a scene's objects and their layout using real scenes. In the first experiment, we confirmed that the shape of the objects in the scene and the scene layout influence the perceived light direction. In the second experiment, we systematically investigated how specific shape properties influenced the estimation of the light direction. The results showed that increasing the number of visible faces of an object, ultimately using globally spherical shapes in the scene, supported the veridicality of the estimated light direction. Furthermore, symmetric arrangements in the scene improved the estimation of the tilt direction. Thus, human perception of light should integrally consider materials, scene content, and layout.@en

Humans are able to estimate light field properties in a scene in that they have expectations of the objects' appearance inside it. Previously, we probed such expectations in a real scene by asking whether a "probe object" fitted a real scene with regard to its lighting. But how well are observers able to interactively adjust the light properties on a "probe object" to its surrounding real scene? Image ambiguities can result in perceptual interactions between light properties. Such interactions formed a major problem for the "readability" of the illumination direction and diffuseness on a matte smooth spherical probe. We found that light direction and diffuseness judgments using a rough sphere as probe were slightly more accurate than when using a smooth sphere, due to the three-dimensional (3D) texture.We here extended the previous work by testing independent and simultaneous (i.e., the light field properties separated one by one or blended together) adjustments of light intensity, direction, and diffuseness using a rough probe. Independently inferred light intensities were close to the veridical values, and the simultaneously inferred light intensity interacted somewhat with the light direction and diffuseness. The independently inferred light directions showed no statistical difference with the simultaneously inferred directions. The light diffuseness inferences correlated with but contracted around medium veridical values. In summary, observers were able to adjust the basic light properties through both independent and simultaneous adjustments. The light intensity, direction, and diffuseness are well "readable" from our rough probe. Our method allows "tuning the light" (adjustment of its spatial distribution) in interfaces for lighting design or perception research.@en

Light is a kind of medium that makes objects visible without being visible itself in empty space. The combination of spatial and spectral characteristics of the light source, material reflectance and shape of an object determine its appearance, and as such the appearance of an object is the main cue for perception of its lighting. In the current study we tested whether types of shapes and scene layout influence lighting perception. In real scenes composed of several objects we tested observers' perception of lighting direction with a real light probe – using a novel experimental setup to optically mix the probe object into a real scene by a semi-transparent mirror. Participants (N=15) had to adjust the illumination direction on the probe such that it fitted the scene. We tested nine lighting directions for four different scenes, existing of five objects: a cylinder, star shape, and cross shape plus combinations of bowling pin(s) and pentagon shape(s). When using one bowling pin and one pentagon shape, the result showed that the light direction estimation was systematically contracted near the pentagon body, but not near the bowling pin. When horizontally mirroring this scene, also the light direction estimation was mirrored. Replacing the bowling pin with another pentagon body resulted in systematic contractions of the light direction slants near both pentagon bodies. Replacing the pentagon bodies with two bowling pins finally resulted in close to veridical light direction settings. The globally convex smoothly curved bowling pins, in comparison with the facetted pentagon shapes, improved observers' setting of light direction. Mirror arrangements of objects in the scene improved the estimation of the tilt direction. These very systematic effects suggest that human perception of the global "light flow" in a scene might be systematically deformed depending on scene layout and content.@en

Human observers can perceive intensity and direction differences of the illumination on objects and in scenes. They also have a sense for the light diffuseness. Reviewing studies into light diffuseness perception and practical lighting guidelines we encountered the problem that there is no agreement on how to describe and measure the light diffuseness, complicating comparisons. We found a large variety of metrics relating to visual effects of light diffuseness, including contrast, shape expressing, material expressing, and atmosphere effects. Moreover, many metrics appeared to be application-, context- or even object-specific. We compared four approaches and propose a normalized metric for light diffuseness, ranging from 0, meaning fully collimated light (a beam with zero spread), to 1, meaning fully diffuse or Ganzfeld illumination. We developed a measurement method for real scenes using cubic illuminance metering. We tested metric and method using simulations, measurements on Debevec luminance maps using a cubic and tetrahedron shaped meter, and measurements in real scenes using the cubic meter. We also tested the influence of scene properties (lighting, geometry and furnishing) and variations within scenes. We compared optical against psychophysical data from our own and other studies, and against practical lighting guidelines. We found that the cubic meter method and metric give robust measurements of light diffuseness. Measurements in real scenes fell in a wide range of 0.1 – 0.9. We found extremely strong effects of furnishing and geometry. Such material-lighting interactions in scenes / architectural spaces are not well-understood and form a challenge in practical lighting design. Most practical guidelines note a broadband range centered slightly above medium diffuseness or hemispherical diffuse light (overcast sky). The psychophysical data contract to narrow bands, depending on the type of scene (varying per experiment), suggesting a template representation of light diffuseness that depends on the overall appearance of a scene.@en