Diversidade cromática de cenários complexos digitalizados com imagiografia hiperespectral

Tese de doutoramento em Ciências (área do conhecimento em Física)Os métodos utilizados presentemente na avaliação da qualidade de fontes luminosas apresentam grandes limitações, em particular, com o aparecimento de fontes luminosas baseadas em LEDs. Alguns dos métodos até agora apresentados (“Colour Rendering Index” - CRI, “Gamut Area Index” - GAI ou o “Colour Quality Scale” - CQS) incluem parâmetros que vão para além da capacidade de reprodução das cores, como, por exemplo, a diversidade cromática que são capazes de gerar. Apesar da diversidade de índices, estes não estão orientadas para a estimativa da diversidade cromática em cenários complexos. Além disso, estão vocacionados para observadores normais, não contemplando observadores com deficiências da visão das cores. O objectivo deste trabalho é estimar a diversidade cromática em cenas naturais complexas a partir de dados obtidos por imagiografia hiperespectral e utilizar imagens hiperespectrais de cenários naturais, pinturas artísticas e cenários de interior para estimar a influência da distribuição espectral de diferentes iluminantes na diversidade cromática de cenários complexos, para observadores normais e deficientes da visão das cores. Pretende-se assim propor um índice que permita auxiliar na escolha de iluminantes com aplicação específica. Verificou-se que a diversidade cromática de cenas naturais é substancialmente inferior à prevista pelo volume do sólido de cor-objecto e que o número de cores discerníveis é um bom indicador desta diversidade, permitindo avaliar a influência de lentes coloridas ou de iluminantes com diferentes distribuições espectrais, em cenários de grande complexidade, para observadores normais ou deficientes da visão das cores. Com base na estimativa da variação da diversidade cromática em cenários complexos, é possível apresentar um índice – o CDI (“Chromatic Diversity Index”) que avalia as alterações cromáticas em tais cenários complexos provocadas por diferentes iluminantes, utilizando um reduzido número de amostras coloridas (1269 amostras de Munsell), para observadores normais e deficientes da visão das cores. Estes resultados permitem sugerir a utilização de determinados iluminantes para a observação de pinturas artísticas e cenários de interior com o objectivo de aumentar a diversidade cromática, para observadores normais e deficientes da visão das cores, resultados que poderão ser do interesse de museus, galerias de arte e utilizadores de ambientes específicos onde a discriminação cromática seja importante.The methods presently available to assess the quality of light sources have several limitations, in particular with the introduction of light sources based on LEDs. Some of the methods available (the Colour Rendering Index - CRI, the Gamut Area Index – GAI or the Colour Quality Scale - CQS) include in its computations parameters that extend the scope of the index beyond the colour reproduction capabilities as, for example, the colour gamut generated by the test illuminant. Although there are several indices they do not provide the assessment of the chromatic diversity on complex scenarios. Apart from this limitation they are only used with normal observers leaving aside the colour deficient observers. The purpose of this work was to estimate the chromatic diversity of complex scenes using data acquired using hyperspectral imaging and to use hyperspectral images of natural scenes, artistic paintings and indoor scenarios to estimate the influence on the chromatic diversity in complex scenes perceived by normal and colour deficient observers of illuminants with different spectral profiles. The aim is to present an index that will assist in the selection of illuminants for a particular task. It was found that the chromatic diversity of natural scenes is considerably inferior when compared to the expected result from the theoretical object colour volume and that the number of discernible colours is a good predictor of this diversity, enabling the estimate of the influence of coloured lens or illuminants with different spectral profiles on complex scenarios for normal and colour deficient observers. The Chromatic Diversity Index – CDI developed in the course of this work uses the computation of the chromatic diversity in complex scenarios to assess the chromatic changes induced by different illuminants by using a small set of coloured samples (the 1269 samples from the Munsell Colour Book), for normal and colour deficient observers. These results assist in the selection of an illuminant to view artistic paintings and indoor scenarios by proposing specific illuminants that will enhance the chromatic diversity of such scenes, not only for normal observers but also for colour deficient observers. The findings of this work could be of the interest of museums, art galleries or users of specific environments where chromatic discrimination is important.Fundação para a Ciência e a Tecnologia (FCT) - Bolsa de Doutoramento SFRH BD 35874 2007

Addendum: Behavior of a bipartite system in a cavity

This note is an Addendum to our previous article [Phys. Rev. A \textbf{81}, 053820 (2010)]. We show that under the assumption of a Bose-Einstein distribution for the thermal reservoir, zero-temperature properties of the entangled states considered there are not changed by heating, for temperatures up to the order of room temperatures. In this case, the system is dissipative in free space and presents stability for a small cavity, both for T=0 and for finite temperature.Comment: Revtex, 04 pages no figures, Version as accepted for publication in Phys. Rev.

Magnetic effects on spontaneous symmetry breaking/restoration in a toroidal topology

We study temperature and finite-size effects on the spontaneous symmetry breaking/restoration for a scalar field model under the influence of an external magnetic field, at finite chemical potential. We use the 2PI formalism and consider the large-$N$ limit. We find that there is a minimal size of the system to sustain the broken phase, which diminishes as the applied field increases but is independent of the chemical potential. We analyze the critical curves and show that the magnetic field enhances the broken-phase regions, while increasing the chemical potential leads to a diminishement of the critical temperature.Comment: Five pages, five figures, version as accepted for publication in Phys. Rev.

A note on the infrared behavior of the compactified Ginzburg--Landau model in a magnetic field

We consider the Euclidean large-$N$ Ginzburg--Landau model in $D$ dimensions, $d$ ($d\leq D$) of them being compactified. For D=3, the system can be supposed to describe, in the cases of d=1, d=2, and d=3, respectively, a superconducting material in the form of a film, of an infinitely long wire having a rectangular cross-section and of a brick-shaped grain. We investigate the fixed-point structure of the model, in the presence of an external magnetic field. An infrared-stable fixed points is found, which is independent of the number of compactified dimensions. This generalizes previous work for type-II superconducting filmsComment: LATEX, 6 pages no figures. arXiv admin note: 80% of text overlaps with arXiv:1102.139

Color diversity index : the effect of chromatic adaptation

Common descriptors of light quality fail to predict the chromatic diversity produced by the same illuminant in different contexts. The aim of this paper was to study the influence of the chromatic adaptation in the context of the development of the color diversity index, a new index capable of predicting illuminant-induced variations in several types of images. The spectral reflectance obtained from hyperspectral images of natural, indoor and artistic paintings, and the spectral reflectance of 1264 Munsell surfaces were converted into the CIELAB color space for each of the 55 CIE illuminants and 5 light sources tested. The influence of the CAT02 chromatic adaptation was estimated for each illuminant and for each scene. The CIELAB volume was estimated by the convex hull method and the number of discernible colors was estimated by segmenting the CIELAB color volume into unitary cubes and by counting the number of non-empty cubes. High correlation was found between the CIELAB volume occupied by the Munsell surfaces and the number of discernible colors and the CILEAB color volume of the colors in all images analyzed. The effects of the chromatic adaptation were marginal and did not change the overall result. These results indicate that the efficiency of the new illuminant chromatic diversity index is not influenced by chromatic adaptation