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. My dissertation consists of two parts. First one presents studies on human sensitivity to the light field structure in empty spaces. Another provides the results 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. 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. Our findings contribute to applied areas, such as computer graphics and architectural lighting design. Moreover, the developed 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.
The full thesis can be downloaded here. Below are the studies constituting the body of my PhD.
The global structure of the visual light field and its relation to the physical light field
Kartashova, T., Sekulovksi, D., de Ridder, H., Pas, S. F., & Pont, S. C. (2016).
Journal of Vision, 16(9).
In this study we developed a novel method of measuring and reconstruction of the visual light field. We superimposed a white spherical probe (Koenderink et al., 2007) over a grid of points (one per trial) on an image of a scene. In each trial observers were asked to set the light on a sphere to make it fit the scene. We found that perceptual data are such that you can actually reconstruct a representation of the visual light field from the data. Physical and visual light fields were reconstructed for a scene with three qualitatively different illuminations. The results suggest that human observers have a robust impression of the light properties changes over scenes. However, this impression is simplified with respect to the physical truth.
The visual light field in paintings of Museum Prinsenhof: comparing settings in empty space and on objects
Kartashova, T., de Ridder, H., te Pas, S. F., Schoemaker, M., & Pont, S. C. (2015).
Proceedings of SPIE 9394, Human Vision and Electronic Imaging XX, 9394, 93941M.
In this study we compared observers’ inferences of light in paintings between two conditions: in empty space and on objects. In one condition, a probe replaced one of the objects in a painting and the task was to set the light on it so that it fitted the scene. In a second condition, the probe was placed next to the cut out object, and observers were asked to fit the illumination on a probe to the light on the object. For four paintings with uniform or diverging light, the settings were highly consistent both between conditions and within paintings. However, for two paintings containing zones of contrasting light, individual differences became prominent. We believe that the most plausible explanation of results is that human observers are blind to complex features of a light field.
Visual light zones
Kartashova, T., de Ridder, H., te Pas, S. F., & Pont, S. C. (2018) The visual light Zones, i- Perception 9, 3 (1-20).
The results of the previous study suggested that the presence of light zones might lead to idiosyncratic differences in observers’ settings. We investigated the perception of illumination differences over depicted spaces in a systematic manner, performing experiments in scenes containing light zones of two different orientations with respect to an observer: in the image plane (differing between the left and right sides of a scene) and in depth (differing between the front and back parts of a scene). 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 differences in depth of a scene (front-back), rather than in the picture plane (left-right).
LightShapes: Perception-Based Visualizations of the Global Light Transport
Kartashova, T., de Ridder, H., te Pas, S. F., & Pont, S.C. (2019) LightShapes: Perception-Based Visualizations of the Global Light Transport, Transactions on applied Perception, 16(1).
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.
A toolbox for volumetric visualization of light properties
Kartashova, T., de Ridder, H., te Pas, S. F., & Pont, S. C. (2019) A toolbox for volumetric visualization of light properties, Lighting Research and Technology.
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.
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Tatiana Kartashova 