Alejandro Ferrero

• PhD
• Spanish National Research Council

Using a modeled look-up chart developed in this work, we show that accurate photometric measurements of characteristics like illuminance or luminous intensity of test LED sources can be measured in one step. This is a simple broadband measurement that may substitute for complicated and costly spectral measurements currently in place. To develop the...

We built an improved 3D rendering framework to accurately visualize the complete appearance of effect coatings, including metallic effects, sparkle and iridescence. Spectral reflectance measurements and sparkle indexes from a commercially available multi-angle spectrophotometer (BYKmac-i) were used together with physics-based approaches, such as fl...

The bidirectional scattering-surface reflectance distribution function (BSSRDF) is the function that describes the variation of the radiance of the elementary areas of a surface with respect to the directionally incident radiant flux on that surface. Measurements of the BSSRDF are important for characterizing the translucency of objects and for obt...

Large effect pigments, widely used in various fields of industrial applications, produce characteristic visual textures known as sparkle and graininess, which need to be quantified by objective or subjective methods. The development of preliminary measurement scales for sparkle and graininess, whose recommendation is now under discussion in the Int...

We validate a physically based and spectral rendering framework with improved color reproduction. With a recently developed model, we take into account both the colorimetric specifications of the rendering display as well as the spectral and angular characteristics of lighting and also the spectral reflectance of the objects. Therefore, it should p...

The bidirectional reflectance distribution function (BRDF) and the bidirectional scattering - surface reflectance distribution function (BSSRDF), which relate radiance at the surface to irradiance and radiant flux, respectively, are regarded as the most fundamental scattering quantities used to determine the reflectance of objects. However, for mat...

The Bidirectional Reflectance Distribution Function (BRDF) of iridescent (or goniochromatic) surfaces may vary notably with both spectral and angular variables, and, therefore, finite spectral bandwidth and collection solid angles inherent to any measuring instrument introduce a deviation from the correct value. Experimental data of highly goniochr...

Large-effect pigments, due to their strongly specular reflectance, produce a special visual texture known as sparkle. The use of these pigments in many industries (automotive, cosmetic, paper, architecture...) makes the control of this visual texture necessary. Sparkle measurands have been defined in this article, so that traceability of sparkle me...

Computer Aided Design (CAD) is increasingly used as a tool in industries varying from automotive to interior design. Digital visualization allows users to design their working and living spaces, and to select materials and colors for future products. The rendering software that is currently available often suggests photorealistic quality. However,...

The dependence of color differences on the illumination and viewing directions for two widely used gray scales for color change (SDCE and AATCC) was evaluated through measuring the spectral bidirectional reflectance distribution function (BRDF) by a gonio-spectrophotometer of metrological quality. Large incidence and viewing angles must be speciall...

When measuring the Bidirectional Reflectance Distribution Function (BRDF) to characterize typical materials, the effect of the polarization of the light source or the dependence of the detector on the polarization is not usually considered, even though many studies have proven their impact in the uncertainty of the BRDF of most materials, including...

Digital visualization of gonio-chromatic colors nowadays is a very hot topic since these new color effects are widely used in many industries and especially in automotive coatings. This popularity is due to the special effects they produce by changing its color with the observation and illumination geometries. This effect is produced by the presenc...

The assumption that the reflectance of white diffuse reflectance standards is identical to that of the perfect reflecting diffuser (PRD) allows these standards to be used to characterize reflectance or radiance factors of any surface at any irradiation/collection geometry simply by comparison. However, this assumption is only true within certain li...

The bidirectional reflectance distribution function (BRDF) is a well-defined quantity that allows the bidirectional reflectance of surfaces to be described. However, the light propagation in a specific direction cannot be perfectly realized in practice, because the physical apertures are not infinitesimal but finite. Consequently, a BRDF measuremen...

In this paper, the fisheye camera method is validated for spatial non-uniformity corrections in luminous flux measurements with integrating spheres. The method was tested in eight integrating spheres with six LED lamps, and the determined angular intensity distributions and spatial non-uniformity correction factors were compared with the results of...

Sparkle, as defined by ASTM E284 -17 Standard Terminology of Appearance is "the aspect of the appearance of a material that seems to emit or reveal tiny bright points of light that are strikingly brighter than their immediate surround and are made more apparent when a minimum of one of the contributors (observer, specimen, light source) is moved"....

This study examined the design and daylight performance of a new louvre screen for office buildings. The screen was evaluated for three different material finishes: specular aluminium as a traditional material commonly used in louvres, and two types of ceramic finishes, with the intention to reduce the systems’ environmental impact. The new system...

The digital representation of three dimensional objects with different materials has become common not only in the games and movie industry, but also in designer software, e-commerce and other applications. Although the rendered images often seem to be realistic, a closer look reveals that their color accuracy is often insufficient for critical app...

Effect pigments in coatings produce eye-catching colour and texture effects and are widely used in automotive, cosmetics, coatings, inks, flooring, textile or decoration. One of these texture effects is graininess, which is the perceived texture exhibited when the effect coating is observed under diffuse illumination. To date there is not a standar...

The General V(λ) Mismatch Index, f 1 ′ , was defined for a general description of the photometric performance of photometers. This index is widely-used in photometry, and it is very relevant for selecting photometers for low-uncertainty photometric measurements. It quantifies the spectral mismatch between the relative spectral responsivity of a pho...

General lighting is undergoing a revolutionary change towards LED-based technologies. To provide firm scientific basis for the related colorimetric and photometric measurements, this paper presents the development of new white-LED-based illuminants for colorimetry, and their evaluation to recommend a new reference spectrum for calibration of photom...

Ultra-weak photon emission (UPE) is the spontaneous emission from living systems mainly attributed to oxidation reactions, in which reactive oxygen species (ROS) may play a major role. Given the capability of the next-generation electron-multiplying CCD (EMCCD) sensors and the easy use of liquid crystal tunable filters (LCTF), the aim of this work...

Determining reflectance factor and its variability across reference sites for Earth observation satellites is a problem involving large amounts of data and measurement time. Principal component analysis (PCA) may be used to simplify this problem by reducing the size of the data and by highlighting spectral features that could be related to physical...

At present, numerical simulations are the common method to predict the response of quantum efficient detectors based on induced-junction diodes. As alternative, this work proposes an analytical model based on photocurrent analysis where surface and bulk losses are modelled considering design and operation photodiode parameters and the characteristi...

Following the strategy in a previous work, we have evaluated the potential Blue-Light Hazard (BLH) of high colour rendering index LED sources, on people whose crystalline lenses have been removed through a surgical procedure to implant intraocular lenses. Starting from the spectral transmittance of various models of commercial intraocular lenses (I...

The potential of predictable quantum efficient detectors (PQEDs) as optical radiant power primary standard, based on photoelectric effect in silicon semiconductor, has been proved. Until now, the internal quantum efficiency (IQE) of a PQED is only predicted, from the design and setup parameters of the two photodiodes of this radiometer, by means of...

Digital simulation by rendering gonio-apparent coatings and thereby visualizing the variation of visual attributes such as color and texture with geometry is requested for different industries. Recently, quantum dots (QD) displays appeared in the market with attractive characteristics such as a wide color gamut. Therefore, a proper color characteri...

This work reports a versatile and efficient production of periodic structures of alloy nanoparticles (NPs) with customized diffraction patterns by using the technique of phase mask laser interference. This technique uses interfering single nanosecond laser pulses to induce the periodic dewetting of bilayer (Ag/Au) films on glass produced by pulse l...

The field of spectral radiance factor (SRF) measurements has seen growing interest in recent years. Scale conformity has so far only been established between the national metrology institutes (NMIs) of Germany and the USA. This study aims at a bigger, multilateral scale comparison. For this purpose, a total of six NMIs participated in a scale compa...

Coatings with diffraction pigments present high iridescence, which needs to be characterized in order to describe their appearance. The spectral bidirectional reflectance distribution functions (BRDFs) of six coatings with SpectraFlair diffraction pigments were measured using the robot-arm-based goniospectrophotometer GEFE, designed and developed a...

The spectral bidirectional reflectance distribution function (BRDF) is the key quantity to specify the spectral reflectance of materials for any condition of irradiation and detection, and its characterization is quite important for surfaces with a high dependence on these conditions, such as iridescent coatings. In order to evaluate the calibratio...

We propose a method based on Zernike polynomials to characterize photometric quantities and descriptors of light emitting diodes (LEDs) from measurements of the angular distribution of the luminous intensity, such as total luminous flux, BA, inhomogeneity, anisotropy, direction of the optical axis and Lambertianity of the source. The performance of...

New methods to characterize coatings with effect pigments are necessary to quantify their complex appearance. A description of the challenges faced by their characterization is given in this contribution.

The color of automotive coatings varies with illumination and detection angles. For 3D rendering of these coatings currently two categories of methods exist, aiming at either color accuracy or at computational speed. Current methods that aim for color accuracy are based on measurement data from BRDF instruments. Since many hundreds of measurement g...

In an effort to create more dynamic looking automobiles, there is an ever increasing trend among automobile manufacturers towards the use of gonio-apparent coatings in car bodies. These coatings consist of transparent pigments mixed with metallic or interference flakes. The flakes in the coating cause a change in color and brightness of the finish...

For suitable illumination and observation conditions, sparkles may be observed in metallic coatings. The visibility of these sparkles depends critically on their intensity, and on the paint medium surrounding the metallic flakes. Based on previous perception studies from other disciplines, we derive equations for the threshold for sparkles to be vi...

Sparkle is a non-uniform texture effect consisting of very bright spots distributed on a dark surround. To date the only commercially-available instrument able to quantify sparkle is BYK-mac. This instrument allows sparkle parameters to be obtained, which should correlate with the visual experience. However, the measurands corresponding to these pa...

The goniospectrophotometer GEFE, designed and developed at IO-CSIC (Instituto de Optica, Agencia Estatal Consejo Superior de Investigaciones Cientí ficas), was conceived to measure the spectral Bidirectional Reflectance Distribution Function (BRDF) at any pair of irradiation and detection directions. Although the potential of this instrument has la...

Colors of special-effect coatings have strong dependence on illumination/viewing geometry and an appealing appearance. An open question is to ask about the minimum number of measurement geometries required to completely characterize their observed color shift. A recently published principal components analysis (PCA)-based procedure to estimate the...

We report a nonlinearity effect related to the integration time in a double-beam spectroradiometer equipped with two negative-module metal–oxide semiconductor (NMOS) sensors. This effect can be explained by the addition of photoelectrons produced by the radiant flux on the sensors during the readout phase to the photoelectrons produced during the m...

Flake-based parameters were recently introduced as a physical concept to predict a series of measurement geometries producing similar reflection data for effect paints. We derive expressions to calculate these so-called isochromatic lines, connecting the two Helmholtz-reciprocal in-plane geometries with a series of out-of-plane geometries. Thus iso...

Radiometrically characterizing a camera means relating the response of every pixel of the camera to the radiance emitted by a light source. In order to accomplish this task, not only the responsivities of the pixels have to be characterized, but, in addition, their non-ideal response (dark response or non-linearity) or the radiant flux impinging on...

The increase in the bidirectional reflectance distribution function around the retro-reflection direction was characterized for four common diffuse standard reflectance materials: Spectralon, matte white Russian opal glass, matte white ceramic standard and pressed barium sulphate (BaSO4) powder. The characterized increase or surge is high enough to...

A representation of the color gamut of special effect coatings is proposed and shown for six different samples, whose colors were calculated from spectral bidirectional reflectance distribution function (BRDF) measurements at different geometries. The most important characteristic of the proposed representation is that it allows a straightforward u...

To completely characterize the color of effect coatings, the spectral BRDF should be measured for a large number of illumination/detection geometries. This is possible with the GEFE, the gonio-spectrophotometer developed at Instituto de Óptica in CSIC (IO-CSIC). Twenty-four effect coating samples were prepared by AkzoNobel, containing metallic and/...

A goniophotometer to characterize the angular distribution of the averaged luminous intensity of LEDs has been developed at Optics Institute of CSIC. Distributions of for four low-power LEDs transfer standards (red, blue, green and white) for luminous flux and luminous intensity are shown. A proper characterization of the angular luminous intensity...

Flakes in effect coatings, which are responsible for its color shift, are not perfectly horizontally oriented, producing a non uniform texture when observed from a distance of around one meter or less. If the coating is illuminated by a diffuse source, a granular appearance is observed, called graininess. But when the coating is illuminated by unid...

A reduced set of measurement geometries allows the spectral reflectance of special effect coatings to be predicted for any other geometry. A physical model based on flake-related parameters has been used to determine nonredundant measurement geometries for the complete description of the spectral bidirectional reflectance distribution function (BRD...

A study on the variation of the spectral bidirectional reflectance distribution function (BRDF) of four diffuse reflectance standards (matte ceramic, BaSO 4 , Spectralon, and white Russian opal glass) is accomplished through this work. Spectral BRDF measurements were carried out and, using principal components analysis, its spectral and geometrical...

This paper presents the description and the characterization of the gonio-spectrophotometer GEFE (the acronym for ‘Gonio-EspectroFotómetro Español’). This device has been designed and built for the low-uncertainty absolute measurement of the bidirectional reflectance distribution function (BRDF). It comprises a fixed, collimated and uniform light s...

A type of representation of the spectral bidirectional reflectance distribution function (BRDF) is proposed that distinctly separates the spectral variable (wavelength) from the geometrical variables (spherical coordinates of the irradiation and viewing directions). Principal components analysis (PCA) is used in order to decompose the spectral BRDF...

In this paper we study the angular and spectral distribution of the emission of conventional white LEDs using the low uncertainty Spanish Gonio‐spectro‐photometer (GEFE). With this instrument we obtain the emission profiles for different observation planes, so that you can get the information needed to characterize the emissions from any source in...

The Bidirectional Reflectance Distribution Function (BRDF) is essential to characterize an object’s reflectance properties. This function depends both on the various illumination-observation geometries as well as on the wavelength. As a result, the comprehensive interpretation of the data becomes rather complex. In this work we assess the use of th...

The Bidirectional Scattering Distribution Function (BSDF) of a surface depicts how it scatters the optical radiation, being therefore fundamental in order to explain the visual appearance of the objects. This work assesses the effect that the use of non-zero solid angles of illumination and detection has in the BSDF measurement, and it proposes a m...

A gonio-spectrophotometer developed and built at CSIC for bidirectional scattering distribution function (BSDF) low-uncertainty measurements is described. The measurement equation and the uncertainty analysis are explained, and first results on the BSDF of ceramic colour standards are shown.

The Watcher robotic telescope was developed primarily to perform rapid optical follow-up observations of Gamma-Ray Bursts (GRBs). Secondary scientific goals include blazar monitoring and variable star studies. An automated photometry pipeline to rapidly analyse data from Watcher has been implemented. Details of the procedures to get image zero-poin...

Electron Multiplying CCD is a CCD technology sensor reduces read-out noise to less than one electron. We study the way the usage of this technology affects to the astronomical photometry and to the improvement of the temporal resolution of the measurements. We show the effect of this technology on individual celestial sources and in the limiting ma...

This work provides a thorough insight into the performance of a new approach to the measurement of astronomical magnitudes. This new photometric procedure only considers as part of the source those pixels with a value higher than the sky background value within a specified confidence interval. The resultant procedure is robust, simple and fast, and...

Watcher is a 40 cm Classical Cassegrain robotic telescope located at Boyden Observatory, South Africa. In the period of operation from May 2006 to May 2008, 224 GRBs were detected by different gamma-ray satellites. Watcher responded to 141 of these; 106 within 1 day of the GRB, 27 within 1 hour and 6 within 1 minute. A summary of results to date ar...

A new spectral data processing scheme based on the standard deviation of collected spectra is compared with the traditional ensemble-averaging of laser-induced breakdown spectroscopy (LIBS)-based spectral data for homogenous (i.e., pure gas phase) systems and with a LIBS-based traditional conditional spectral analysis scheme for non-homogenous (e.g...

Aims: With this paper we want to investigate the highly variable afterglow light curve and environment of gamma-ray burst (GRB) 060526 at $z=3.221$. Methods: We present one of the largest photometric datasets ever obtained for a GRB afterglow, consisting of multi-color photometric data from the ultraviolet to the near infrared. The data set contain...

Laser induced breakdown spectroscopy (LIBS) was demonstrated in stand-off mode as a detection technique for polydisperse liquid aerosols. The goal of this work was to assess the possibility of producing optical breakdown in an aerosol at 10 meters of distance, and to detect the analytical signal resulting from the optical discharge. The doublet cha...

In this work it is proposed that an easy solution based directly on linear algebra in order to obtain the relation between a spectrum and a spectrum base. This solution is based on the algebraic determination of an unknown spectrum coordinates with respect to a spectral library base. The identification capacity comparison between this algebraic met...

A theoretical study of atmospheric extinction mechanisms of optical radiation (molecular/aerosol scattering and absorption) has been carried out in order to assess their influences on stand-off laser-induced breakdown spectroscopy (LIBS) measurements. The atmospheric extinction of laser radiation at wavelengths commonly used in laser-induced breakd...

INTEGRAL detected GRB 080414 during an open time observation of the Galactic disk. The long (duration ∼10 s) GRB occurred at an off-axis angle of 11.5 degrees with a peak 20-200 keV flux of 1 ph cm-2 s -1 as reported by [1]. The Watcher robotic telescope started observations of GRB 080414 33 sec post GCN trigger. Initial analysis of the first image...

The principal component analysis is used to identify and quantify spatial distributions of relative photoresponse as a function of the exposure time for a visible CCD array. The analysis shows a simple way to define an invariant photoresponse nonuniformity and compare it with the definition of this invariant pattern as the one obtained for long exp...

We present and evaluate two corrections applicable in determining the modulation transfer function (MTF) of a charge-coupled device (CCD) by the speckle method that minimize its uncertainty: one for the low frequency region and another for the high frequency region. The correction at the low-spatial-frequency region enables attenuation of the high...

We analyze in detail the apparent violation of the reciprocity law that occurs for an interline CCD array owing to its responsivity that varies with exposure time, and we investigate other parameters having an influence on this variation, such as the numerical aperture of the system or the illumination wavelength. The variation can be explained fro...

What we believe to be a novel procedure to correct the nonuniformity that is inherent in all matrix detectors has been developed and experimentally validated. This correction method, unlike other nonuniformity-correction algorithms, consists of two steps that separate two of the usual problems that affect characterization of matrix detectors, i.e.,...

The use of CCD detectors for radiometric measurements has proven to be very interesting for a variety of fields. This work presents an experimental method for the low-uncertainty calibration of the spectral radiant exposure responsivity of a CCD detector. It contains the description of the experimental setup as well as the analysis of the various s...

A simple and straightforward method to assess the relative temporal fluctuation of the individual pixels in a CCD array with respect to the fluctuation of the spatial average is proposed. This procedure is based on the comparison of the low-frequency temporal noise of the individual pixels and the value of this magnitude averaged over the entire CC...

A new expression for the internal quantum efficiency of a photodiode is presented. It is obtained from the analysis of the photocurrent generated within the diode, considering the power and the cross-sectional diameter of the incident beam. The model explains variations of the internal quantum efficiency with irradiance that are not explained by ot...

An interpolation procedure for CCD responsivity that takes into account physical considerations about the pixel is presented. The procedure has been applied to the responsivity values of a CCD and the results are shown.