Applied Optics, vol. 48, N. 19, 3643-3653, July 2009
Photometric-stereo techniques are based on the fact that image intensity depends
upon the orientation of
the surface with regard to the source of the illumination and its spectral
reflectance. They are of special
interest when dealing with rough surfaces because they usually present shadowed
regions where sudden
illumination changes might be found. In the present work we introduce an extension
of the four-source
photometric-stereo algorithm to color images that is able to recover the surface
spectral reflectance of
objects captured with a red–green–blue (RGB) camera. This method
allows image rendering, even for
rough-textured surfaces, under different directions of the impinging illumination.
In addition, the introduction
of spectral recovery techniques applied to the albedo and spectral reflectance
from rough surfaces
offers the possibility of image rendering for scenes captured under sources
of illumination differing in
spectral distribution. Using albedo instead of RGB information helps to avoid
any shadows or highlights
that might falsify results. One of the advantages of this spectral-based photometric-stereo
method is that
it can recover not only the albedo values, but also the spectral reflectance
spectrum of an object’s surface
on a pixel-by-pixel basis, as can be done with more complex hyperspectral imaging
devices involving a
camera coupled to an extensive set of narrowband filters. © 2009 Optical
Society of America
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Am. J. Phys. 76 (12), 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. © 2008
American Association of Physics Teachers.
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Applied Optics, vol. 47, N. 20, 3574-3584, July 2008
A linear pseudo-inverse method for unsupervised illuminant recovery from natural
scenes is presented.
The algorithm, which uses a digital RGB camera, selects the naturally occurring
bright areas (not necessarily
the white ones) in natural images and converts the RGB digital counts directly
into the spectral
power distribution of the illuminants using a learning-based spectral procedure.
Computations show a
good spectral and colorimetric performance when only three sensors (a three-band
RGB camera) are
used. These results go against previous findings concerning the recovery of
spectral reflectances and
radiances, which claimed that the greater the number of sensors, the better
the spectral performance.
Combining the device with the appropriate computations can yield spectral information
about objects
and illuminants simultaneously, avoiding the need for spectroradiometric measurements.
The method
works well and needs neither a white reference located in the natural scene
nor direct measurements of
the spectral power distribution of the light. © 2008 Optical Society of
America
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Applied Optics, vol. 46, N. 19, 4144-4154, July 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 how accurately
the spectra of artificial fluorescent light sources can be recovered with a
digital CCD camera. The redgreen-
blue (RGB) sensor outputs are modified by the use of successive cutoff color
filters. Four algorithms
for simplifying the spectra datasets are used: nonnegative matrix factorization
(NMF), independent component
analysis (ICA), a direct pseudoinverse 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; the PCA results are also
given for the sake of
comparison. The results show that all the algorithms surpass the PCA when a
reduced number of sensors
is used. The experimental results suggest a significant loss of quality when
more than one color filter is used,
which agrees with the previous results for reflectances. Nevertheless, an RGB
digital camera with or
without a prefilter is found to provide good spectral and colorimetric recovery
of indoor fluorescent lighting
and can be used for color correction without the need of a telespectroradiometer.
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Color Research and Application, Vol. 32, N. 5, 352-360, August 2007.
Many spectral-recovery methods
using RGB digital cameras assume the underlying smoothness of illuminant
and reflectance spectra, and apply low-dimensional linear models. The aim of the present work was to test
whether a direct-mapping method could be used instead of a linear-models approach
to recover spectral radiances and reflectances from natural scenes with an
RGB digital camera and colored filters. In computer simulations, a conventional RGB
digital camera with up to three colored filters was used to image scenes drawn
from a hyperspectral image database. Three measures were used to evaluate
recovery with the direct-mapping method: goodness-of-fit, root-mean-square
error, and a color-difference metric. It was found that with two and three
filters both spectral radiances and reflectances could be recovered sufficiently accurately
for many practical applications. With little increase in computational complexity,
an RGB camera and a few colored filters can provide significantly better recoveryof natural scenes than an RGB camera alone.
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Color Research and Application, Vol. 32, N. 5, pp. 284-292, August 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.
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Color Research and Application, Vol. 31, N. 5, 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.
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Applied Optics, vol. 44, N. 27, 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.
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Applied Optics, vol. 43, N. 9, 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.
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Color Research and Application, vol. 29, N. 3, 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.
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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.
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Journal of the Optical Society of America A, vol. 20, N. 9, 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.
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Optical Review, vol. 10, N. 4, 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.
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Color Research and Application, Vol. 28, N. 1, 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.
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