Paper Abstracts
Javier Hernández-Andrés
Departamento de Óptica
Facultad de Ciencias
Universidad de Granada
Granada - 18071
SPAIN



Linear bases for representation of natural and artificial illuminants

Javier Romero, Antonio García-Beltrán, and Javier Hernández-Andrés
Department of Optics, Faculty of Sciences, University of Granada, 18071 Granada, Spain

Journal of the Optical Society of America A, Vol. 14, N. 5, p. 1007-1014, May 1997.
By the principal-value decomposition process, we have obtained two linear bases for representing the spectral power distributions of illuminants, applicable for algorithms of color synthesis and analysis in artificial vision: one from experimental measurements of daylight and another combining both natural and artificial illuminants. The first basis adequately represents daylight with dimension 3, in accordance with the previous results of Judd et al. [J. Opt. Soc. Am. 54, 1031 (1964)]; however, it does not adequately represent artificial illuminants, even with a higher dimension. In the case of the second basis, many good results are obtained in the reconstruction of the spectral power distribution both of daylight and of artificial illuminants, including some fluorescent lights, with dimension 7 or even less. In consequence, we show the possibility of obtaining linear bases of a low dimension, even when the set of illuminants that we try to represent presents a certain variability in shape.

Testing linear models on spectral daylight measurements

Javier Hernández-Andrés, Javier Romero, Antonio García-Beltrán, and Juan L. Nieves
Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.

Applied Optics, Vol. 37, N. 6, p. 971-977, 20 February 1998.
We have analyzed the results of the reconstruction quality of 252 daylight spectral curves measured at Granada, Spain, using four bases obtained from measurements in different areas of the world. For these reconstructions we used two different methods (orthogonality of characteristic vectors and chromaticity coordinates) to study the influence of the wavelength range and spectral resolution. The reconstruction method from chromaticity coordinates presents difficulties for the spectral recovery of daylight spectral power distributions regardless of the basis used. The orthogonality method makes clear that the best bases were those proposed by the CIE, but more than two characteristic CIE vectors were needed for good reconstruction.

Linear bases for spectral reflectance functions of acrylic paints

Antonio García-Beltrán, Juan L. Nieves, Javier Hernández-Andrés and Javier Romero
Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.

Color Research and Application, Vol. 23, N. 1, p. 39-45, February 1998.
A linear model for representing reflectances has been developed from a group of 5574 samples of acrylic paint on paper. Using acrilyc paints makes easy the generation of a large variety of samples by mixing, due to the high miscibility among these kinds of pigments: this point was the key to achieve a great spatial homogeneity in our samples. Besides, these kinds of paints keep their chromatic properties stable over time. The first 7 vectors of the so-called overall linear basis were sufficient for a more than adequate mathematical representation of the spectral-reflectance curves. A study by hue groups of the mathematical properties of these curves indicates that the use a hue basis of representation implies, on the average, a reduction in 1 or 2 of the number of vectors needed in order to achieve results analogous to those of the over-all basis

Calculating correlated color temperatures across the entire gamut of daylight and skylight chromaticities

Javier Hernández-Andrés1, Raymond L. Lee, Jr2, and Javier Romero1
1Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.
2Oceanography Department, United States Naval Academy, Annapolis, Maryland 21402, EE.UU.

Applied Optics, Vol. 38, N. 27, p. 5703-5709, 20 September 1999.
Natural outdoor illumination daily undergoes large changes in its correlated color temperature (CCT), yet existing equations for calculating CCT from chromaticity coordinates span only part of this range. To improve both the gamut and accuracy of these CCT calculations, we use chromaticities calculated from our measurements of nearly 7000 daylight and skylight spectra to test an equation that accurately maps CIE 1931 chromaticities x and y into CCT. We extend the work of McCamy [Color Res. Appl. 12, 285-287 (1992)] by using a chromaticity epicenter for CCT and the inverse slope of the line that connects it to x and y. With two epicenters for different CCT ranges, our simple equation is accurate acros wide chromaticity and CCT ranges (3000 K-106 K) spanned by daylight and skylight.
Tesis Doctoral (Ph.D. Dissertation, in spanish)

Características espectrales y colorimétricas de la luz-día y luz-cielo en Granada

Autor: Javier Hernández-Andrés
Director de Tesis: Dr. Javier Romero Mora

Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.
12 Noviembre 1999, Nota: Sobresaliente Cum Laude
Es esencial poseer, por sus diversas y extensas aplicaciones, un extenso y detallado conocimiento de las distribuciones espectrales (y sus características colorimétricas) de la luz natural recibida sobre la superficie terrestre. La evidencia constatada de la falta de campañas de medidas espectrorradiométricas sistemáticas, en un largo periodo de tiempo, de la luz natural, tanto en España como en el resto del mundo, ha provocado que hayamos llevado a cabo tres campañas de medidas (luz-día, luz-cielo y luz-día durante crepúsculos) utilizando un espectrorradiómetro portátil Li-1800 de LI-COR, constituyendo éstas una utilísima base de datos con aplicaciones en distintas áreas de la Ciencia, Industria y Técnica.
En la primera campaña de medidas, desarrollada durante dos años completos, se midieron irradiancias espectrales de luz-día recibida sobre una superficie horizontal y procedente de toda la bóveda celeste, en el rango espectral comprendido entre 300 y 1100 nm. Esta campaña fue diseñada como respuesta a las deficiencias presentadas por otras campañas publicadas en la literatura y llevadas a cabo principalmente en los años 60. Con un total de 2600 medidas esta campaña se ha convertido en la más extensa temporalmente y en la que posee un mayor número de curvas espectrales, de todas las existentes en la literatura.
El principal propósito de la segunda campaña, llevada a cabo durante siete meses, fue medir distribuciones espectrales de radiancia (con campos de visión de 3 grados), procedente de un número extenso y bien repartido de puntos sobre la bóveda celeste, en días con cielos despejados para distintas elevaciones solares y analizar la dependencia de las características colorimétricas de la luz-cielo con ciertas variables (elevación solar, posición sobre la bóveda y ángulo de esparcimiento).
En la tercera y última campaña hemos medido irradiancias espectrales sobre una superficie horizontal en intervalos de 30 segundos durante crepúsculos con cielos despejados y cielos cubiertos, con la intención de actualizar y mejorar el conocimiento científico sobre la evolución temporal tanto de la cromaticidad como la iluminancia durante los crepúsculos, dado que los últimos datos existentes en la literatura fueron publicados hace más de cuarenta años.


Colorimetric and spectroradiometric characteristics of narrow-field-of-view clear skylight in Granada, Spain

Javier Hernández-Andrés1, Javier Romero1 and Raymond L. Lee, Jr.2
1Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.
2Mathematics and Science Division, United States Naval Academy, Annapolis, Maryland 21402, EE.UU.

Journal of the Optical Society of America A, Vol. 18, N. 2, p. 412-420, Feb. 2001.
As part of our ongoing research into the clear daytime sky's visible structure, we analyze over 1500 skylight spectra measured during a seven-month period in Granada, Spain. We use spectral radiances measured within 3° fields of view (FOV's) to define colorimetric characteristics along four sky meridians: the solar meridian and three meridians at azimuths of 45°, 90°, and 315° relative to it. The resulting clear-sky chromaticities in 44 different view directions (1) are close to but do not coincide with the CIE daylight locus, (2) form V-shaped meridional chromaticity curves along it (as expected from theory), and (3) have correlated color temperatures (CCT's) ranging from 3800 K to infinity K. We also routinely observe that sky color and luminance are asymmetric about the solar meridian, usually perceptibly so. A principal-components analysis shows that three vectors are required for accurate clear-sky colorimetry, whereas six are needed for spectral analyses.

Color and spectral analysis of daylight in southern Europe

Javier Hernández-Andrés1, Javier Romero1, Juan L. Nieves1 and Raymond L. Lee, Jr.2
1Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.
2Mathematics and Science Division, United States Naval Academy, Annapolis, Maryland 21402, EE.UU.

Journal of the Optical Society of America A, Vol. 18, N. 6, pp. 1325-1335, June 2001.
We have analyzed the colorimetric and spectral characteristics of 2600 daylight spectra (global spectral irradiances on a horizontal surface) measured for all sky states during a two year period at Granada, Spain. We describe in detail the chromaticity coordinates, correlated color temperatures (CCT), luminous efficacies, and relative UV and IR contents of Granada daylight. The chromaticity coordinates of Granada daylight lie far above the CIE locus at high CCTs (> 9000 K), and a CCT of 5700 K best typifies this daylight. Our principal-component analysis shows that Granada daylight spectra can be adequately represented by using six-dimensional linear models in the visible, whereas seven-dimensional models are required if we include the UV or near-IR. Yet on average only three-dimensional models are needed to reconstruct spectra that are colorimetrically indistinguishable from the original spectra.

Fizeau fringes at home

Javier Hernández-Andrés, Eva Valero, Juan L. Nieves and Javier Romero
Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.

American Journal of Physics, Vol. 70, N. 7, pp. 684-688, July 2002

We propose an easy and inexpensive laboratory experiment to obtain equal-thickness optical interference using a flatbed scanner and a personal computer to produce Fizeau fringes. Valuable discussions about multiple-beam interference, Fresnel equations, the theory of partial coherence, and colorimetry arise naturally from the interference-fringe results.
Libro (Book, in Spanish)

Introducción a la Teoría Difraccional de la Formación de Imágenes

Juan Luis Nieves, José Ramón Jiménez y Javier Hernández-Andrés
Universidad de Granada, España, Junio 2002
ISBN: 84-607-4970-3
Dep. Legal: J-280-2002
ÍNDICE
1.- Óptica de Fourier
2.- Teoría escalar de la difracción
3.- Difracción de Fresnel y de Fraunhoffer
4.- Coherencia parcial de la luz
5.- Teoría difraccional de la formación de imágenes (I)
6.- Teoría difraccional de la formación de imágenes (II)
7.- Holografía
Apéndice 1: Introducción a la teoría vectorial de la difracción
Apéndice 2: Problemas propuestos
Bibliografía



Spectral-reflectance function recovery for improved colour-constancy experiments

Juan L. Nieves, F. Pérez-Ocón, Javier Hernández-Andrés and Javier Romero
Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.

Displays, Vol. 23, N. 5, pp. 213-222, November 2002
A set of symmetric memory-matching data is presented to analyse some implications of long-term memory factors within classical colour-constancy paradigms and separation algorithms. Using simulated Mondrian-type colour surrounds on a CRT monitor, subjects make a series of colour matches between a test and a matching surface; the surfaces are rendered under the same standard illuminant (equal-energy illuminant). The 16 test surfaces used were categorised into four apparent-hue collections. The analysis of the colour differences show that subjects maintained good mental representations of the surfaces, although a shift in luminance was found. With these results, we investigated how errors in remembering surface colours might be translated into errors in reconstructing surface reflectances. Thus, a description of the remembered surfaces is provided, and the spectral differences are analysed via a goodness-of-fit coefficient (GFC). As it is derived from colour-differential thresholds and GFC values, the analysis of the recalled spectral-reflectance functions shows little loss of information in the observer's task, despite imperfect mathematical recovery of the surfaces. The similarities between test and matching surfaces suggest that colour-constancy algorithms could benefit of memory matches when an illuminant change takes place, and use spectral-tolerance bands defined over the surfaces comprising a scene to improve their implementation.

Measuring and modeling twilight's purple light

Raymond L. Lee, Jr.1 and Javier Hernández-Andrés2
1Mathematics and Science Division, United States Naval Academy, Annapolis, Maryland 21402, EE.UU.
2Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.


Applied Optics, Vol. 42, N. 3, pp. 445-457, January 2003
During many clear twilights, much of the solar sky is dominated by pastel purples. This purple light’s red component has long been ascribed to transmission through and scattering by stratospheric dust and other aerosols. Clearly the vivid purples of post-volcanic twilights are related to increased stratospheric aerosol loading. Yet our time-series measurements of purple-light spectra, combined with radiative transfer modeling and satellite soundings, indicate that background stratospheric aerosols by themselves do not redden sunlight enough to cause the purple light’s reds. Furthermore, scattering and extinction in both the troposphere and stratosphere are needed to explain most purple lights.

Color and luminance asymmetries in the clear sky

Javier Hernández-Andrés1, Raymond L. Lee, Jr.2 and Javier Romero1
1Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.
2Mathematics and Science Division, United States Naval Academy, Annapolis, Maryland 21402, EE.UU.

Applied Optics, Vol. 42, N. 3, pp. 458-464, January 2003
A long-standing assumption about the clear sky is that its colors and luminances are distributed symmetrically about the principal plane. As useful as this approximation is, our digital image analyses show that clear-sky color and luminance routinely depart perceptibly from exact symmetry. These analyses reconfirm our earlier measurements with narrow field-of-view spectroradiometers [Hernández-Andrés et al., J. Opt. Soc. Am. A 18, 2001], and they do so with much higher temporal and angular resolution across the entire sky dome.

Color coordinates and color appearance of objects with daylight changes

Javier Romero, Javier Hernández-Andrés, Juan L. Nieves and José A. García
Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.

Color Research and Application, Vol. 28, N. 1, pp. 25-35, February 2003.
Colorimetric changes were analyzed for a broad set of natural and artificial objects that were illuminated by daylight measured at different solar elevations on separate days, under diverse meteorologic conditions. The changes in L*-, a*-, and b*-color coordinates of the objects, when illuminated with daylight at the maximum solar elevation and at twilight, normally exceeded 3 CIELAB units. However, color differences were not significant when evaluated during the middle hours of the day. Nor were significant differences found in the color of an object on different days, when evaluated during the middle hours. Color appearance attributes of the objects at intervals during the day were also calculated based on the CIECAM97s color appearance model, showing the trends with daylight changes.

Colour appearance of surfaces as affected by different time-varying colour-adaptation sequences.

Juan L. Nieves, Eva M. Valero, Javier Hernández-Andrés, and Javier Romero
Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.

Optical Review, vol. 10, N. 4, pp. 221-230, July 2003
We report on experiments in which observers judged colour appearance within the context of time-varying colour adaptation. We used pairs of contextual images consisting of a rapid succession of colour surfaces reproduced under different illuminants to analyse the effect of temporal colour adaptation rather than a spatial context on asymmetric matching and also to judge its influence upon the cone excitation components. We used adaptation colour distributions along the red–green and yellow–blue axes (selective conditions) and random colour distributions (non-selective condition). The results of observers' matches for both conditions showed approximate colour-constant appearance. Although light adaptation did not fully compensate the colour changes, we obtained average colour-constancy index values of 0.6. The results for the two opponent conditions showed similar contextual effects. No significant differences between each condition were found for the L- and S-cone mechanisms and the three test illuminants. On the contrary, some degree of interaction between the comparison-field cone excitations and the colour axis can be seen when the colour mechanisms are analysed separately. This seems to be more pronounced for the S-cone mechanism and suggests that the selective condition of the adaptation sequence may well affect the observer's chromatic matching response.

Color-signal filtering in the Fourier-frequency domain

Javier Romero, Eva Valero, Javier Hernández-Andrés, and Juan L. Nieves
Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.

Journal of the Optical Society of America A, vol. 20, N. 9, pp. 1714-1724, September 2003
We have analyzed the Fourier-frequency content of spectral power distributions deriving from three types of illuminants (daylight, incandescent, and fluorescent) and the color signals from both biochrome and nonbiochrome surfaces lit by these illuminants. As far as daylight and the incandescent illuminant are concerned, after filtering the signals through parabolic (low-pass) filters in the Fourier-frequency domain and then reconstructing them, we found that most of the spectral information was contained below 0.016 c/nm. When fluorescent illuminants were involved, we were unable to recover either the original illuminants or color signals to any satisfactory degree. We also used the spectral modulation sensitivity function, which is related to the human visual system’s color discrimination thresholds, as a Fourier-frequency filter and obtained consistently less reliable results than with low-pass filtering. We provide comparative results for daylight signals recovered by three different methods. We found reconstructions based on linear models to be the most effective.

Virtual tunnels and green glass: The colors of common mirrors

Raymond L. Lee, Jr.1 and Javier Hernández-Andrés2
1Mathematics and Science Division, United States Naval Academy, Annapolis, Maryland 21402, EE.UU.
2Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.


American Journal of Physics, Vol. 72, N.1, pp. 53-59, January 2004

When a pair of common second-surface plane mirrors face each other, repeated mirror-to-mirror reflections form a virtual optical "tunnel" with some unusual properties. One property readily analyzed in a student laboratory experiment is that objects’ colors steadily grow darker and greener the deeper we look into the mirror tunnel. This simple observation is both visually compelling and physically instructive: measuring and modeling a tunnel’s colors requires students to blend colorimetry and spectrophotometry with a knowledge of how complex refractive indices and the Fresnel equations predict reflectance spectra of composite materials.


Spectral daylight recovery using only a few sensors

Javier Hernández-Andrés, Juan L. Nieves, Eva M. Valero, and J. Romero
Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.

Journal of the Optical Society of America A, vol. 21, N. 1, pp. 13-23, January 2004.
Linear models have already been proved accurate enough to recover spectral functions. We have resorted to such linear models to recover spectral daylight via the response of no more than a few real sensors. We performed an exhaustive search to obtain the best set of Gaussian sensors with a combination of optimum spectral position and bandwidth. We have also examined to what extent the accuracy of daylight estimation depends upon the number of sensors and their spectral properties. A set of 2,600 daylight spectra [Hernández-Andrés et al., J. Opt. Soc. Am. A 18, 2001] were used to determine the basis functions in the linear model and also to evaluate the accuracy of the search. The estimated spectra are compared with original ones for different spectral daylight and skylight sets of data within the visible spectrum. Spectral similarity, colorimetric differences and integrated spectral irradiance errors are all taken into account. We compare our best results with those obtained using a commercial CCD, revealing its potential as a daylight-estimation device.

Spectral-reflectance linear models for color pattern recognition

Juan L. Nieves, Javier Hernández-Andrés, Eva M. Valero and Javier Romero
Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.

Applied Optics, vol. 43, N. 9, pp. 1880-1891, March 2004
We suggest a new method for color pattern recognition using a linear description of spectral reflectance functions and the spectral power distribution of illuminants containing very few parameters. We report on methods in which these spectral functions are derived from linear models based on principal component analysis (PCA). The correlation is made in the subspace spanned by the coefficients that describe each reflectance according to a suitable basis. The method is first illustrated in a control experiment where the scenes are captured under known illuminant conditions. The discrimination capacity of the algorithm improves upon conventional RGB multi-channel decomposition when scenes are captured under different illuminant conditions, and is comparable to color recognition based on CIELab system. Then we tested the coefficient method in situations where the target is captured under a reference illuminant and the scene containing the target under an unknown spectrally different one. We show that the method avoid false alarms under changes in the illuminant. The correlation results are satisfactory even for the reduced dimensional basis used here to represent the surface reflectance function of the image pixels.


Changes in contrast thresholds with mean luminance for chromatic and luminance gratings: transition from the DeVries-Rose to Weber regions revisited

Eva M. Valero, Juan L. Nieves, Javier Hernández-Andrés, and José A. García
Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.

Color Research and Application, vol. 29, N. 3, pp. 177-182, June 2004.
We examine the influence of the mean luminance level on the detection thresholds for luminance and red-green chromatic gratings of three different spatial frequencies. The changes in detection thresholds with mean luminance level reflect the two different regions found in previous studies: the DeVries-Rose and Weber ranges. We determine the transtion luminance between both regions by a new procedure. The results for luminance gratings suggest that this luminance is proportional to the spatial frequency of the grating. Predictions based on the constant-flux hypothesis indicate that the transition luminance is proportional to the square of the spatial frequency of the grating, and thus they are not adequate to describe the distributions of luminance constrast thresholds. The results suggest that the transition luminance is related to the peak spatial frequency and to the bandwidths of visual mechanisms that respond to chromatic and luminance gratings.

Group theoretical structure of color spaces

Reiner Lenz1, Thanh Hai Bui1 and Javier Hernández-Andrés2
1Media Group, Campus Norrkoping, Linkoping University, Bredgatan, SE-60174 Norrkoping, Sweden
2Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.


J. Math. Imaging and Vision, Vol. 23, N. 3, pp. 297-313, November 2005
It is known that for every selection of illumination spectra there is a coordinate system such that all coordinate vectors of these illumination spectra are located in a cone. A natural set of transformations of this cone are the Lorentz transformations. In this paper we investigate if sequences of illumination spectra can be described by one-parameter subgroups of Lorentz-transformations. We present two methods to estimate the parameters of such a curve from a set of coordinate points. We also use an optimization technique to approximate a given set of points by a one-parameter curve with a minimum approximation error. In the experimental part of the paper we investigate series of blackbody radiators and sequences of measured daylight spectra and show that one-parameter curves provide good approximations for large sequences of illumination spectra.


Short-term variability of overcast brightness

Raymond L. Lee, Jr.1 and Javier Hernández-Andrés2
1Mathematics and Science Division, United States Naval Academy, Annapolis, Maryland 21402, EE.UU.
2Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.

Applied Optics, vol. 44, N. 27, pp. 5704-5711, September 2005

Overcasts seen from below seldom are uniform, unchanging cloud shields, yet little is known about their short-term photometric variability (periods <=2 h). Visible-wavelength spectra of daytime and twilight overcast skies measured at 30-s intervals reveal unexpected temporal variability in horizontal illuminance Ev and zenith luminance Lv. Fourier analysis of these time series shows peak fluctuations at periods of 2-40 min. Factors such as cloud type and optical depth, presence of fog or snow, and instrument field of view can affect overcast brightness variability. Surprisingly, under some circumstances overcast twilight Ev exceeds clear-sky Ev at the same sun elevation.



Designing a practical system for spectral imaging of skylight

Miguel A. López-Álvarez1, Javier Hernández-Andrés1, J. Romero1, Raymond L. Lee, Jr.2,
1Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.
2Mathematics and Science Division, United States Naval Academy, Annapolis, Maryland 21402, EE.UU.

Applied Optics, vol. 44, N. 27, pp. 5688-5695, September 2005

In earlier work [Hernández-Andrés et al., J. Opt. Soc. Am. A 21, 13-23 (2004)], we showed that a combination of linear models and optimum Gaussian sensors obtained by an exhaustive search can recover daylight spectra reliably from broadband sensor data. Thus our algorithm and sensors could be used to design an accurate, relatively inexpensive system for spectral imaging of daylight. Here we improve our simulation of the multispectral system by (1) considering the different kinds of noise inherent in electronic devices such as CCDs or CMOSs and (2) extending our research to a different kind of natural illumination, skylight. Because exhaustive searches are expensive computationally, here we switch to a simulated annealing algorithm to define the optimum sensors for recovering skylight spectra. The annealing algorithm requires us to minimize a single cost function, and so we develop one that calculates both the spectral and colorimetric similarity of any pair of skylight spectra. We show that the simulated annealing algorithm yields results similar to the exhaustive search but with much less computational effort. Our technique lets us study the properties of optimal sensors in the presence of noise, one side effect of which is that adding more sensors may not improve the spectral recovery.



Multispectral synthesis of daylight using a commercial digital CCD camera

Juan L. Nieves1, Eva M. Valero1, Sergio M.C. Nascimento2, Javier Hernández-Andrés1, Javier Romero1
1Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.
2Departamento de Física, Campus de Gualtar, Universidade do Minho, 4710-057 Braga

Applied Optics, vol. 44, N. 27, pp. 5696-5703, September 2005

Performance of multispectral devices in recovering spectral data has been intensively investigated in same areas as in spectral characterization of art paintings, but received little attention in the context of spectral characterization of natural illumination. This study investigated the quality of the spectral estimation of daylight-type illuminants using a commercial digital CCD camera coupled or not with a set of broadband colored filters. Several recovery algorithms that did not need information about spectral sensitivities of the camera sensors nor eigenvectors to describe the spectra were tested. Tests were carried out both with virtual data, using simulated digital counts, and real data obtained from real measurements. It is found that it is possible to recover daylight spectra with high spectral and colorimetric accuracy with a reduced number of 3 to 9 spectral bands.



Colors of the daytime overcast sky

Raymond L. Lee, Jr.1 and Javier Hernández-Andrés2
1Mathematics and Science Division, United States Naval Academy, Annapolis, Maryland 21402, EE.UU.
2Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.

Applied Optics, vol. 44, N. 27, pp. 5712-5722, September 2005

Our time-series measurements of daylight spectra beneath unexpectedly wide gamut of pastel colors. Analyses of these wavelengths, overcasts are far from spectrally neutral transmitters tops. Colorimetric analyses of overcasts indicate that they make amount of bluing increases with cloud optical depth. Radiative observed bluing: multiple scattering within optically thick clouds selective absorption by water droplets. However, identifying measured overcast colors is often problematic.



Crepuscular and nocturnal illumination and its effects on color perception by the nocturnal hawkmoth Deilephila elpenor

Sönke Johnsen1, Almut Kelber2, Eric Warrant2, Alison M. Sweeney1,
Edith A. Widder3,
Raymond L. Lee, Jr.4 and Javier Hernández-Andrés5
1Biology Department, Duke University, Durham, NC, USA
2Department of Cell and Organism Biology, Lund University, Sweden
3Marine Science Division, Harbor Branch Oceanographic Institution, Ft. Pierce, FL, USA
4Mathematics and Science Division, United States Naval Academy, Annapolis, Maryland 21402, EE.UU.
5Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.

The Journal of Experimental Biology, vol. 209, N. 5, pp. 789-800, February 2006

Recent studies have shown that certain nocturnal insect and vertebrate species have true color vision under nocturnal illumination. Thus, their vision is potentially affected by changes in the spectral quality of twilight and nocturnal illumination, due to the presence or absence of the moon, artificial light pollution and other factors. We investigated this in the following manner. First we measured the spectral irradiance (from 300 to 700 nm) during the day, sunset, twilight, full moon, new moon, and in the presence of high levels of light pollution. The spectra were then converted to both human-based chromaticities and to relative quantum catches for the nocturnal hawkmoth Deilephila elpenor, which has color vision. The reflectance spectra of various flowers and leaves and the red hindwings of D. elpenor were also converted to chromaticities and relative quantum catches. Finally, the achromatic and chromatic contrasts (with and without von Kries color constancy) of the flowers and hindwings against a leaf background were determined under the various lighting environments. The twilight and nocturnal illuminants were substantially different from each other, resulting in significantly different contrasts. The addition of von Kries color constancy significantly reduced the effect of changing illuminants on chromatic contrast, suggesting that, even in this light-limited environment, the ability of color vision to provide reliable signals under changing illuminants may offset the concurrent three-fold decrease in sensitivity and spatial resolution. Given this, color vision may be more common in crepuscular and nocturnal species than previously considered.



Spectral sensitivity of sensors for a color-image descriptor invariant to changes in daylight conditions


Javier Romero, Javier Hernández-Andrés, Juan L. Nieves, and Eva M. Valero

Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.


Color Research and Application, Vol. 31, N. 5, pp. 391-398, October 2006.
Previous authors (J.A. Marchant and C.M. Onyango, J Opt Soc Am A 17, 1952 (2000) and G.D. Finlayson and S.D. Hordley, J Opt Soc Am A 18, 253 (2001)) have proposed a pixel-by-pixel image descriptor that is invariant to certain changes in illumination. We have studied the possibility of applying such an invariant descriptor to scenery illuminated by natural light by choosing sensors that allow the invariant to behave satisfactorily under daylight. We obtained different triads of monochromatic sensors by using an exhaustive-search method and compared the results with those obtained with other triads proposed by different authors. We extended our study to Gaussian sensors centered in the wavelengths considered for the monochromatic sensors and to commercial CCD camera sensors. Satisfactory results are achieved for Gaussian sensors with maximum sensitivities at different locations and when CCD camera sensors are sharpened.

Selecting algorithms, sensors and linear bases for optimum spectral recovery of skylight


Miguel A. López-Álvarez, Javier Hernández-Andrés, Eva M. Valero, and J. Romero

Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.


Journal of the Optical Society of America A, Vol. 24, N. 4, pp. 942-956, Abril 2007
In a previous work [Appl. Opt. 44, 5688 (2005)] we found the optimum sensors for a planned multispectral system for measuring skylight in the presence of noise by adapting a linear spectral recovery algorithm proposed by Maloney and Wandell [J. Opt. Soc. Am. A 3, 29 (1986)]. Here we continue along these lines by simulating the responses of three to five Gaussian sensors and recovering spectral information from noise-affected sensor data by trying out four different estimation algorithms, three different sizes for the training set of spectra, and various linear bases. We attempt to find the optimum combination of sensors, recovery method, linear basis, and matrix size to recover the best skylight spectral power distributions from colorimetric and spectral (in the visible range) points of view. We show how all these parameters play an important role in the practical design of a real multispectral system and how to obtain several relevant conclusions


Sensor-response-ratio constancy under changes in natural and artificial illuminants


Javier Romero, Daniel Partal, Juan L. Nieves and
Javier Hernández-Andrés
Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.


Color Research and Application, vol. 32, N. 4, 284-292 (2007)

We have analyzed the constancy of the response ratio for cones, second-stage mechanisms, and CCD sensors when daylight or an artificial illuminant (A, F2, F7, and F11) is changed to an equal-energy illuminant (E) in scenes containing natural and artificial objects. The response ratios were always found to be roughly constant for all the sensors. For daylight, we have deduced mathematical expressions, which relate the values of these ratios with the correlated color temperature (CCT) and applied these expressions to the synthesis of images of a scene viewed under different daylights corresponding to a variety of CCTs. The results were highly satisfactory in a rural scene for any CCT. In the scene with artificial objects, the results were also good for nonextreme CCTs. We also included in our study artificial illuminants, with which we achieved very good image syntheses for illuminants A and F7.



Recovering fluorescent spectra with an RGB digital camera and color filters using different matrix factorizations

Juan L. Nieves, Eva M. Valero, Javier Hernández-Andrés and Javier Romero
Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.

Applied Optics, vol. 46, 4144-4154 (2007)

The aim of a multispectral system is to recover a spectral function at each image pixel, but when a scene is digitally imaged under a light of unknown spectral power distribution (SPD), the image pixels give incomplete information about the spectral reflectances of objects in the scene. We have analyzed here how accurately the spectra of artificial fluorescent light sources can be recovered with a digital CCD camera. The RGB sensor outputs are modified by the use of successive cut-off color filters. Three algorithms for simplifying the spectra datasets are used: non-negative matrix factorization (NMF), independent component analysis (ICA), a direct pseudo-inverse method, and principal component analysis (PCA). The algorithms are tested using both simulated data and data from a real RGB digital camera. The methods are compared in terms of the minimum rank of factorization and the number of sensors required to derive acceptable spectral and colorimetric SPD estimations; PCA results are also given for the sake of comparison. The results show that all the algorithms surpass PCA when a reduced number of sensors is used. Experimental results suggest a significant loss of quality when more than one color filter is used, which agrees with previous results for reflectances. Nevertheless, an RGB digital camera with or without prefilter is found to provide good spectral and colorimetric recovery of indoor fluorescent lighting and can be used for color correction without need of a telespectroradiometer.



A simple experiment to distinguish between replicated and duplicated compact discs using Fraunhofer diffraction


J. Fernández-Dorado, J. Hernández-Andrés, E.M. Valero, J. L. Nieves and J. Romero
Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.

American Journal of Physics, Vol. 76, N.12, pp. 1137-1140, December 2008


Compact discs are a useful tool for studying interference and diffraction. We propose an easy and inexpensive experiment to distinguish between replicated and duplicated compact discs based on Fraunhofer diffraction. The nonvisible differences of the surface of compact discs depend on the way that they have been manufactured and can be seen by using a laser beam in a simple diffraction experiment. The method has been tested on many different brands of CDs and is reliable.



Developing an optimum computer-designed multispectral system comprising a monochrome CCD camera and a liquid-crystal tunable filter

Miguel A. López-Álvarez, Javier Hernández-Andrés and Javier Romero
Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.

Applied Optics, vol. 47, N.24, 4381-4390 (2008)

In a previous work [J. Opt. Soc. Am. A 24, 942 (2007)] we made a complete theoretical and computational study of the influence of several parameters on the behavior of a planned multispectral system for imaging skylight, including the number of sensors and the spectral estimation algorithm. Here we follow up this study by using all the information obtained in the computational simulations to implement a real multispectral imaging system based on a monochrome CCD camera and a liquid-crystal tunable filter (LCTF). We were able to construct the optimum Gaussian sensors found in the simulations by adjusting
the exposure times of some of the transmittance modes of the LCTF, hence obtaining really accurate spectral estimations of skylight with only a few optimum sensors.



Using a trichromatic CCD camera for spectral skylight estimation

Miguel A. López-Álvarez, Javier Hernández-Andrés, Javier Romero, Francisco J. Olmo, Alberto Cazorla and Lucas Alados-Arboledas
Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.

Applied Optics, vol. 47, N.34, H31-H38 (2008)

In a previous work [J. Opt. Soc. Am. A 24, 942–956 (2007)] we showed how to design an optimum multispectral system aimed at spectral recovery of skylight. Since high-resolution multispectral images of skylight could be interesting for many scientific disciplines, here we also propose a nonoptimum but
much cheaper and faster approach to achieve this goal by using a trichromatic RGB charge-coupled device (CCD) digital camera. The camera is attached to a fish-eye lens, hence permitting us to obtain a spectrum of every point of the skydome corresponding to each pixel of the image. In this work we show
how to apply multispectral techniques to the sensors’ responses of a common trichromatic camera in order to obtain skylight spectra from them. This spectral information is accurate enough to estimate experimental values of some climate parameters or to be used in algorithms for automatic cloud detection, among many other possible scientific applications.



Retrieval of the optical depth by an all-sky CCD camera

Francisco J. Olmo, Alberto Cazorla and Lucas Alados-Arboledas, Miguel A. López-Álvarez, Javier Hernández-Andrés and Javier Romero
Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain

Applied Optics, vol. 47, N.34, H182-H189 (2008)

A new method is presented for retrieval of the aerosol and cloud optical depth using a CCD camera equipped with a fish-eye lens (all-sky imager system). In a first step, the proposed method retrieves the spectral radiance from sky images acquired by the all-sky imager system using a linear pseudoinverse
algorithm. Then, the aerosol or cloud optical depth at 500nm is obtained as that which minimizes the residuals between the zenith spectral radiance retrieved from the sky images and that estimated by the radiative transfer code. The method is tested under extreme situations including the presence of
nonspherical aerosol particles. The comparison of optical depths derived from the all-sky imager with those retrieved with a sunphotometer operated side by side shows differences similar to the nominal error claimed in the aerosol optical depth retrievals from sunphotometer networks



Quantifying the "milky sky" experiment

Stanley D. Gedzelman, Miguel A. López-Álvarez, Javier Hernández-Andrés and Robert Greenler
Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.

Applied Optics, vol. 47, N.34, H128-H132 (2008)

Spectra of direct and scattered light that passed through a tank of water mixed with up to 25 ml of homogenized skim milk were measured with a spectroradiometer in a classic experiment used to illustrate why the sky is blue and why the Sun turns red near the horizon. The direct light penetrating the tank was reddened by preferential scattering of short waves by the milk particles (protein casein micelles and fat globules). Scattered light was blue near the light source when the optical thickness was small and red far from the source when the optical thickness was large. The measured radiance spectra and Mie theory were used to estimate that the optically effective mean diameters of protein casein micelles and fat globules were 170 and 610 nm.



Fountain rainbows

Stanley D. Gedzelman and Javier Hernández-Andrés
Departamento de Óptica, Facultad de Ciencias, Universidad de Granada, Granada 18071, Spain.

Applied Optics, vol. 47, N.34, H220-H224 (2008)

We present the first measurements of radiance spectra of rainbows. The bows on two sunny days (3 and 6 June 2008) were produced by the fountain in the Parque de las Ciencias, Granada, Spain, that consists of a rectangular perimeter of 40 spray nozzles. Optical thickness of the spray from each nozzle was approximately 0.5. Spectral purity of the primary bow was highest for orange and blue, reaching values of 23% and 7%, respectively, while skylight 90° from the Sun had a color purity of 34% (on 6 June). The secondary bow had much lower color purity with red absent because the regions around the bows and in Alexander’s dark band were pale blue. The narrow sickle shape of the chromaticity curves for the primary bows and the absence of supernumerary bows indicated that the drop radius was between 0.2 and 0:4 mm


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