In computer graphics, illuminating a scene is a complex task, typically consisting of cycles of adjusting and rendering the scene to see the effects. We propose a technique for visualization of light as a tensor field via extracting its properties (i.e., intensity, direction, diffuseness) from (virtual) radiance measurements and showing these properties as a grid of shapes over a volume of a scene. Presented in the viewport, our visualizations give an understanding of the illumination conditions in the measured volume for both the local values and the global variations of light properties. Additionally, they allow quick inferences of the resulting visual appearance of (objects in) scenes without the need to render them. In our evaluation, observers performed at least as well using visualizations as using renderings when they were comparing illumination between parts of a scene and inferring the final appearance of objects in the measured volume. Therefore, the proposed visualizations are expected to help lighting artists by providing perceptually relevant information about the structure of the light field and flow in a scene.@en

It is impossible to see the light in an empty space, we can only observe light as emission from a light source or reflections from objects. Yet, human observers can estimate the illumination in empty parts of an observed scene, based on the appearance of surrounding objects. This dissertation presents studies on human sensitivity to the light field structure in empty spaces and description of the development of a light visualization tool that implements our knowledge of light fields, light design and perception. In our perceptual studies, we reconstructed and compared physical and visual light fields. Physical measurements of the illuminance were made in real and modelled scenes with Cuttle's cubic measurement approach. The measurement device was a cube (a simulated one for modelled scenes) with small sensors on each side. The device was positioned over a grid of points in scenes creating regular measurements. For each position six measurements were translated to light properties (intensity, vector direction, diffuseness) with Cuttle's formulas. Then the resulting data was interpolated in order to obtain a full light field. In psychophysical experiments we used a probe proposed by Koenderink et al., a white matte sphere on which the illumination could be controlled by an observer. The task was to make the probe visually fit to a scene or an object. Placing the probe over grids of positions we obtained user data that was proven to be robust enough to reconstruct the global visual light field. We demonstrated that observers' data is robust enough to reconstruct the global structure of the visual light field. We also found that the visual light field is simplified with respect to the physical truth. In particular, it does not reflect subtle variations of the physical light field. In studies on scenes with complex light field structures (i.e. light zones, neighboring light fields with contrasting differences in one or more light properties), we found that observers are quite sensitive to the difference in light properties between the light zones. However, they showed idiosyncratic behavior especially for light zones with diferences in depth of a scene (front-back), rather than in the picture plane (left-right). The second goal of this thesis was to develop a tool that incorporates our knowledge in measurement and perception of light in its visualization. Modern light visualizations often focus on surfaces or show light in a sophisticated manner understandable only for experts. We augment existing approaches with our tool that visualizes light in 3D volumes and in a perceptually-relevant manner. The measurement approach was the same as the physical measurements used for the perceptual studies above. The measurement cubes could be implemented physically, for real, or virtually, for modelled scenes. Resulting measurements were translated into light properties - mean illuminance, vector direction and diffuseness of light - and represented via variation of shapes' proportions. We tested our visualizations performance compared to image renderings and found that the visualizations led to at least as good task performance as renderings. Moreover, we developed a web-based tool, which can be used for visualizing of cubic measurements by anyone and described applications of this tool for architectural lighting design. Our findings expand knowledge on the structure of the visual light field and help to understand it better. This can contribute to applied areas, such as computer graphics and architectural lighting design. Moreover, our visualization tool can immediately be used by lighting artists or architectural light designers for increasing their work efficiency by providing quick and quantitative representation of the light conditions. @en

In this article, we studied perception of a particular case of light fields that is characterized by a difference in its consistent structure between parts of a scene. In architectural lighting design, such a consistent structure in a part of a light field is called a light zone. First, we explored whether human observers are sensitive to light zones, that is, zones determined primarily by light flow differences, for a natural-looking scene. We found that observers were able to distinguish the light conditions between the zones. The results suggested an effect of light zones’ orientation. Therefore, in Experiment 2, we systematically examined how the orientation of light zones (left-right or front-back) with respect to a viewer influences light inferences in symmetric scenes. We found that observers are quite sensitive to the difference in the light flow of the light zones. In addition, we found that participants showed idiosyncratic behavior, especially for front-back-oriented light zones. Our findings show that observers are sensitive to differences in light field structure between two parts of a scene, which we call visual light zones.@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

Human observers are sensitive to light properties such as the intensity, direction and diffuseness. We study how well they are able to estimate the light field (the structure of the net flux transport) in rooms, by empirical measurement and comparison of the physical and visual light fields. We made regularized measurements of the physical light field in a room under three light conditions with a custom-made cubic illuminance meter based on six Konica Minolta T-10MA sensors. Next, we photographed the room and conducted a psychophysical experiment, in which the observer’s task was to change the lighting (direction, intensity and ambient components) of a “probe” in order to make it appear like it belonged to the pictured scene. The probe, a white Lambertian sphere on a black monopod (defining the location of the probe in the scene), was superimposed on predetermined locations in the image. As a separate measure of the visual light field, the participants indicated the perceived (“subjective”) light source position on the extended scene pictures and described it in words. For each light condition we made reconstructions of light fields consisting of light vectors in a grid: physical from measurements, visual from observers’ settings on a probe, and simplified models of light fields with all vectors pointing to the subjective light source positions. Participants’ interpretations varied remarkably in position and number of the subjective light sources. Many of them were far from veridical. We made pairwise comparisons between light field reconstructions. For the most prominent case, with a visible light source, the average angular difference between visual and physical light vectors was 57 degrees, whereas between visual and simplified model for subjective light sources it was 29 degrees. The results suggest that, overall, the visual light field’s structure is simplified with respect to the physical one.@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