João M. M. Linhares

Effect of sport-tinted contact lenses for contrast enhancement on retinal straylight measurements

Purpose:  To investigate the effect of two tinted contact lenses (CL) designed for outdoor sports activity on the psychometric determination of retinal straylight using the compensation comparison method.

Visual sensitivity to color errors in images of natural scenes

Simple color-difference formulae and pictorial images have traditionally been used to estimate the visual impact of color errors introduced by image-reproduction processes. But the limited gamut of RGB cameras constrains such analyses, particularly of natural scenes. The purpose of this work was to estimate visual sensitivity to color errors introduced deliberately into pictures synthesized from hyperspectral images of natural scenes without gamut constraints and to compare discrimination thresholds expressed in CIELAB and S-CIELAB color spaces. From each original image, a set of approximate images with variable color errors were generated and displayed on a calibrated RGB color monitor. The threshold for perceptibility of the errors was determined in a paired-comparison experiment. In agreement with previous studies, it was found that discrimination between original and approximate images needed on average a CIELAB color difference DeltaEab* of about 2.2. Although a large variation of performance across the nine images tested was found when errors were expressed in CIELAB units, little variation was obtained when they were expressed in S-CIELAB units.

Chromatic effects of metamers of D65 on art paintings

The visual impression of an artistic painting is influenced both by the colour and by the specific spectral structure of the rendering light source. The relationship between illuminant spectral structure and visual appearance assumes particular relevance with the advent of light sources with almost arbitrary spectral distribution, like modern LED based lighting. The aim of this work was to study, computationally, chromatic effects on paintings of illuminants with the same colour as D65 but different spectral profile. Hyperspectral data from twenty oil paintings were used in the analysis. A large collection of metamers of D65 was generated and the radiance reflected from each pixel of the paintings was estimated for each of the metamers. The number of discernible colours produced for each painting and illuminant was computed, and correlated with the spectral structure of the metamers. It was found that the number of colours generated varied considerably across the collection of metamers and that the metamers producing more colours were spectrally more structured, that is, less uniform. This result suggests that it may be beneficial to explore appropriate spectral tuning in practical illumination.

Colour rendering of indoor lighting with CIE illuminants and white LEDs for normal and colour deficient observers

The goal of this work was to evaluate the colour rendering of indoor lighting with CIE illuminants and white LEDs by estimating the chromatic diversity produced for normal and colour deficient observers. Reflectance spectra of a collection of scenes made of objects typically found indoors were obtained with hyperspectral imaging. Chromatic diversity was computed for 55 CIE illuminants and five LED light sources by estimating the number of different colours perceived in the scenes analysed. A considerable variation in chromatic diversity was found across illuminants, with the best producing about 50% more colours than the worst. For normal observers, the best illuminant was CIE FL3.8 which produced about 8% more colours than CIE illuminant A and D65; for colour deficient observers, the best illuminants varied with the type of deficiency. When the number of colours produced with a specific illuminant was compared against its colour rendering index (CRI) and gamut area index (GAI), weak correlations were obtained. Together, these results suggest that normal and colour deficient observers may benefit from a careful choice of the illuminant, and this choice may not necessarily be based only on the CRI or GAI.

A chromatic diversity index based on complex scenes

We propose a chromatic diversity index based on the Munsell set capable of predicting illuminant induced changes in chromatic diversity of complex scenes. The color differences between complex scenes derived from hyperspectral data under a test and under a reference CIE D65 illuminant were computed and compared with the corresponding differences for the Munsell set. It was found that the average color difference between the complex scenes correlates well with the color differences of the Munsell samples with an average correlation of about 0.94, a result indicating that the Munsell set can be used to predict chromatic changes in complex scenes.

Number of discernible colors for color-deficient observers estimated from the MacAdam limits

We estimated the number of colors perceived by color normal and color-deficient observers when looking at the theoretic limits of object-color stimuli. These limits, the optimal color stimuli, were computed for a color normal observer and CIE standard illuminant D65, and the resultant colors were expressed in the CIELAB and DIN99d color spaces. The corresponding color volumes for abnormal color vision were computed using models simulating for normal trichromatic observers the appearance for dichromats and anomalous trichomats. The number of colors perceived in each case was then computed from the color volumes enclosed by the optimal colors also known as MacAdam limits. It was estimated that dichromats perceive less than 1% of the colors perceived by normal trichromats and that anomalous trichromats perceive 50%-60% for anomalies in the medium-wavelength-sensitive and 60%-70% for anomalies in the long-wavelength-sensitive cones. Complementary estimates obtained similarly for the spectral locus of monochromatic stimuli suggest less impairment for color-deficient observers, a fact that is explained by the two-dimensional nature of the locus.

Statistics of colors in paintings and natural scenes

Painters reproduce some spatial statistical regularities of natural scenes. To what extent they replicate their color statistics is an open question. We investigated this question by analyzing the colors of 50 natural scenes of rural and urban environments and 44 paintings with abstract and figurative compositions. The analysis was carried out using hyperspectral imaging data from both sets and focused on the gamut and distribution of colors in the CIELAB space. The results showed that paintings, like natural scenes, have gamuts with elongated shapes in the yellow-blue direction but more tilted to the red direction. It was also found that the fraction of discernible colors, expressed as a function of the number of occurrences in the scene or painting, is well described by power laws. These have similar distribution of slopes in a log-log scale for paintings and natural scenes. These features are observed in both abstract and figurative compositions. These results suggest that the underlying chromatic structure of artistic compositions generally follows the main statistical features of the natural environment.

Radiometric characterization of a novel LED array system for visual assessment

Light that enters the eye can be distorted due to several factors leading to a poor visual performance. The purpose of this paper is to describe and characterize the light-emitting diode (LED) display system to be used in a new device to assess visual quality under high glare conditions. The device has a central white LED and surrounding white LEDs distributed in a radial manner. Each LED is controlled independently using special designed software. The spectral power distribution and color of the LEDs were assessed at different voltage intensities to test the response in terms of output luminance and spectral distribution. It was found that the typical maximum luminance was about 2800 cd/m2 and 6 cd/m2 for the central and surrounding LEDs, respectively. Their color was found to be within the ΔE∗ab range of 2.6 and 0.23, respectively, if the minimum and maximum intensities are considered. The characterization of this device was proved successfully, which might indicate its usefulness in future visual assessments.

Assessing the effects of dynamic luminance contrast noise masking on a color discrimination task

The aim of this work was to assess the influence of dynamic luminance contrast noise masking (LCNM) on color discrimination for color normal and anomalous trichromats. The stimulus was a colored target on a background presented on a calibrated CRT display. In the static LCNM condition, the background and target consisted of packed circles with variable size and static random luminance. In the dynamic LCNM condition, a 10 Hz square luminance signal was added to each circle. The phase of this signal was randomized across circles. Discrimination thresholds were estimated along 20 hue directions concurrent at the color of the background. Six observers with normal color vision, six deuteranomalous observers, and three protanomalous observers performed the test in both conditions. With dynamic LCNM, thresholds were significantly lower for anomalous observers but not for normal observers, suggesting a facilitation effect of the masking for anomalous trichromats.

Estimating the Colors of Paintings

Observers can adjust the spectrum of illumination on paintings for optimal viewing experience. But can they adjust the colors of paintings for the best visual impression? In an experiment carried out on a calibrated color monitor images of four abstract paintings obtained from hyperspectral data were shown to observers that were unfamiliar with the paintings. The color volume of the images could be manipulated by rotating the volume around the axis through the average (a*, b*) point for each painting in CIELAB color space. The task of the observers was to adjust the angle of rotation to produce the best subjective impression from the paintings. It was found that the distribution of angles selected for data pooled across paintings and observers could be described by a Gaussian function centered at 10o, i.e. very close to the original colors of the paintings. This result suggest that painters are able to predict well what compositions of colors observers prefer.