Reflectance, illumination, and appearance in color constancy

We studied color constancy using a pair of identical 3-D Color Mondrian displays. We viewed one 3-D Mondrian in nearly uniform illumination, and the other in directional, nonuniform illumination. We used the three dimensional structures to modulate the light falling on the painted surfaces. The 3-D structures in the displays were a matching set of wooden blocks. Across Mondrian displays, each corresponding facet had the same paint on its surface. We used only 6 chromatic, and 5 achromatic paints applied to 104 block facets. The 3-D blocks add shadows and multiple reflections not found in flat Mondrians. Both 3-D Mondrians were viewed simultaneously, side-by-side. We used two techniques to measure correlation of appearance with surface reflectance. First, observers made magnitude estimates of changes in the appearances of identical reflectances. Second, an author painted a watercolor of the 3-D Mondrians. The watercolor's reflectances quantified the changes in appearances. While constancy generalizations about illumination and reflectance hold for flat Mondrians, they do not for 3-D Mondrians. A constant paint does not exhibit perfect color constancy, but rather shows significant shifts in lightness, hue and chroma in response to the structure in the nonuniform illumination. Color appearance depends on the spatial information in both the illumination and the reflectances of objects. The spatial information of the quanta catch from the array of retinal receptors generates sensations that have variable correlation with surface reflectance. Models of appearance in humans need to calculate the departures from perfect constancy measured here. This article provides a dataset of measurements of color appearances for computational models of sensation. © 2014 McCann, Parraman and Rizzi

Daylight Glare Evaluation When the Sun is Within the Field of View Through Window Shades

Shading fabrics have the ability to reduce daylight glare and provide privacy when needed. Recent studies have shown that glare indices such as DGP and its simplified version can be used to predict daylight glare through shades when the sun is not within the field of view. However, there are no comprehensive studies on glare sensation with the sun visible through the fabric – a situation that happens in office buildings – and therefore the applicability of glare indices for such conditions is uncertain. Shades with very low openness factors transmit only a small amount of direct sunlight due to their weave density; nevertheless, existing glare metrics may show intolerable conditions for these cases, while specific studies with human subjects are nearly non-existent. This paper presents an experimental study on daylight glare evaluation for the case of shading fabrics with the sun within the field of view. 41 human subjects (n=41) were tested while performing specific office activities, with 14 shade products of different openness factors and visible transmittance values (direct and total light transmission characteristics). The measured variables and survey results were used to: (i) associate discomfort glare with measured and modeled parameters for these cases (ii) evaluate the robustness of existing glare indices for these cases (iii) examine recently suggested alternate (direct and total vertical illuminance) criteria for glare assessment through fabrics, extract discomfort thresholds and suggest a new related index and (iv) propose corrections in the DGP equation when the sun is visible through the shades, which could be generalized for other systems following a similar approach. Combining illuminance-based metrics and existing glare indices can result in a more realistic glare evaluation covering all cases with and without the sun through shading fabrics. The new results can be inversely used as thresholds for selecting optical properties of shades to ensure glare protection, as well as for the development of glare-based shading controls.

A Human-Centered Approach for the Design of Perimeter Office Spaces Based on Visual Environment Criteria

With perimeter office spaces with large glazing facades being an indisputable trend in modern architecture, human comfort has been in the scope of Building science; the necessity to improve occupants’ satisfaction, along with maintaining sustainability has become apparent, as productivity and even the well-being of occupants are connected with maintaining a pleasant environment in the interior. While thermal comfort has been extensively studied, the satisfaction with the visual environment has still aspects that are either inadequately explained, or even entirely absent from literature. This Thesis investigated most aspects of the visual environment, including visual comfort, lighting energy performance through the utilization of daylight and connection to the outdoors, using experimental studies, simulation studies and human subjects’ based experiments

Spatial–Spectral Evidence of Glare Influence on Hyperspectral Acquisitions

Glare is an unwanted optical phenomenon which affects imaging systems with optics. This paper presents for the first time a set of hyperspectral image (HSI) acquisitions and measurements to verify how glare affects acquired HSI data in standard conditions. We acquired two ColorCheckers (CCs) in three different lighting conditions, with different backgrounds, different exposure times, and different orientations. The reflectance spectra obtained from the imaging system have been compared to pointwise reference measures obtained with contact spectrophotometers. To assess and identify the influence of glare, we present the Glare Effect (GE) index, which compares the contrast of the grayscale patches of the CC in the hyperspectral images with the contrast of the reference spectra of the same patches. We evaluate, in both spatial and spectral domains, the amount of glare affecting every hyperspectral image in each acquisition scenario, clearly evidencing an unwanted light contribution to the reflectance spectra of each point, which increases especially for darker pixels and pixels close to light sources or bright patche

Methods for assessing the effects of spatial luminance patterns on perceived qualities of concert lighting

This paper presents experimental approaches for evaluating concert lighting from the viewpoints of audience members and performers in Cambridge King's College Chapel. We develop image zoning and abstraction techniques to quantify and interpret photometric data acquired under four different electric lighting conditions. Assessed by 78 participants, these lighting scenarios are compared across six different viewing positions using a set of structured questionnaires. Ordered logistic regression modelling shows that the ratios and functions describing uniformity, brightness and light patterns are common explanatory variables for predicting perceived visual clarity, visual uniformity, brightness and spatial intimacy. Uniformity-related attributes are observed to be among the strongest variables for all these perceived qualities, except for visual clarity, which is better explained by acuity-related measures. These experimental results confirm the applicability of our approaches, highlighting the importance of combining multiple methods and integrating complex architectural situations into the process of understanding luminous appearance. Cambridge Trus

IMPACT OF NEW LIGHTING TECHNOLOGIES ON OFFICE ERGONOMICS

The goal of this study was to find the impact of cutting-edge light-emitting diodes (LED) lighting technologies on the office ergonomics in modern offices. An experiment was conducted in a windowless office at the University of Kansas. This experiment used four test conditions with two levels of light sources (LED and fluorescent lamps) and two levels of lighting control (with/without lighting control). A total of 30 subjects with an average age of 21.6 and eyesight of 20/20 and 20/16 participated in this experiment. Subjects performed typing tasks and color matching tasks under each one of four test conditions. Subjective evaluation of lighting quality and task satisfaction were collected using a questionnaire. High Dynamic Range (HDR) photography was used as a new approach of light measurement in offices. The results revealed that LED lighting had more consistent color rendering performance than fluorescent lighting, and also led to better typing task satisfactions. LED lighting could reduce the lighting power consumption (by up to 65.5% in this study) without any negative impact on office ergonomics. Introduction of individual lighting control could lead to better satisfaction toward the lighting quality and higher savings in lighting power consumption

Which tone-mapping operator is the best? A comparative study of perceptual quality

Altres ajuts: CERCA Programme/Generalitat de CatalunyaPublicat sota la llicència Open Access Publishing Agreement, específica d'Optica Publishing Group https://opg.optica.org/submit/review/pdf/CopyrightTransferOpenAccessAgreement-2022-06-27.pdfTone-mapping operators (TMOs) are designed to generate perceptually similar low-dynamic-range images from high-dynamic-range ones. We studied the performance of 15 TMOs in two psychophysical experiments where observers compared the digitally generated tone-mapped images to their corresponding physical scenes. All experiments were performed in a controlled environment, and the setups were designed to emphasize different image properties: in the first experiment we evaluated the local relationships among intensity levels, and in the second one we evaluated global visual appearance among physical scenes and tone-mapped images, which were presented side by side. We ranked the TMOs according to how well they reproduced the results obtained in the physical scene. Our results show that ranking position clearly depends on the adopted evaluation criteria, which implies that, in general, these tone-mapping algorithms consider either local or global image attributes but rarely both. Regarding the question of which TMO is the best, KimKautz ["Consistent tone reproduction," in Proceedings of Computer Graphics and Imaging (2008)] and Krawczyk ["Lightness perception in tone reproduction for high dynamic range images," in Proceedings of Eurographics (2005), p. 3] obtained the better results across the different experiments. We conclude that more thorough and standardized evaluation criteria are needed to study all the characteristics of TMOs, as there is ample room for improvement in future developments

Contemplation of tone mapping operators in high dynamic range imaging

The technique of tone mapping has found widespread popularity in the modern era owing to its applications in the digital world. There are a considerable number of tone mapping techniques that have been developed so far. One method may be better than the other in some cases which is determined by the requirement of the user. In this paper, some of the techniques for tone mapping/tone reproduction of high dynamic range images have been contemplated. The classification of tone mapping operators has also been given. However, it has been found that these techniques lack in providing quality of service visualization of high dynamic range images. This paper has tried to highlight the drawbacks in the existing traditional methods so that the tone-mapped techniques can be enhanced