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.
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.
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.
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.
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.
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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