Joaquin Campos

• Doctor in Physics
• Managing Director at Spanish National Research Council

Camouflage is often considered a daytime phenomenon based on light and shade. Nocturnal camouflage can also occur, but its mechanistic basis remains unclear. Here, we analyze the conditions for background matching (BM) of avian predators against the night sky. Such concealment is achieved when the contrast between the predator and the sky is smalle...

Nonlinearity characterization of detectors is necessary for performing measurements at different radiant flux levels, as it is the case in bidirectional reflectance distribution function (BRDF) measurements. The differential spectral responsivity (DSR) method is one of the typical procedures for this characterization. In this work, we use attenuati...

Addressing the effect of partially correlated components of spectral irradiance data on spectral integrals poses a substantial challenge. This study employs Monte Carlo methods to conduct an uncertainty analysis of spectral integrals, with a focus on the uncertainty of the spectral mismatch correction factor. The investigation encompasses the prima...

In recent years, a growing demand for the capability of performing accurate measurements of the bidirectional transmittance distribution function (BTDF) has been observed in industry, research and development, and aerospace applications. However, there exists no calibration and measurement capabilities-entry for BTDF in the database of the Bureau I...

The bidirectional scattering-surface reflectance distribution function (BSSRDF) describes the radiance originated by the volume scattering and the surface reflectance of any position on the surface of a sample, when it is directionally irradiated at any other position. The present state of the technology for appearance rendering requires a more det...

An erratum is presented to correct the measurement equation used for the calculation of the BSSRDF in our manuscript [Opt. Express 29 (21), 34175 (2021)10.1364/OE.439108]. This correction affects slightly the results showed in Figs. 7 and 8 of the primary manuscript, so they are replaced by Fig. S1 and Fig. S2, respectively, of the current erratum....

Main text This report describes the Key Comparison of the Consultative Committee for Photometry and Radiometry CCPR-K1.a.2017 for Spectral Irradiance in the wavelength range of 250 nm to 2500 nm. Tungsten quartz halogen lamps of the FEL type (1000 W) were used as artefacts. Each participant used its own set of lamps. The star scheme of measurements...

In recent years, there has been a growing interest in the measurements of the bidirectional reflectance distribution function (BRDF) in industry and research and development. However, there is currently no dedicated key comparison to demonstrate the scale conformity. To date, scale conformity has been proved only for classical in-plane geometries,...

Main text The metrological equivalence of national measurement standards in the field of photometry and radiometry is determined by a set of key comparisons chosen and organised by the Consultative Committee of Photometry and Radiometry (CCPR) of the Comité international des poids et mesures (CIPM), working closely with the Regional Metrology Organ...

Rendering of effect coatings is a very active hot topic since this type of coatings changes considerably its visual attributes such as color and texture with the illumination/viewing geometry. The digital reproduction of such pigments demands an extra effort due to the physical complexity of their surfaces. Special BRDFs (bidirectional reflectance...

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...

In this work, we evaluate the capability of a physically based rendering framework to reproduce the color flop phenomena of effect coatings through psychophysical tests. For this task, a digitally simulated lighting environment (the Byko-spectra effect light booth) is built to set up the visual tests, in which physical objects inside the physical l...

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...

We report new developments in instrumentation and techniques for both acoustic (speed of sound) and radiometric primary thermometry methods. These include both new cylindrical resonators for extending acoustic gas thermometry to higher temperatures and absolute radiation thermometers incorporating InGaAs detectors to extend primary radiometry to lo...

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...

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"....

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...

After measuring thermodynamic temperatures in the range from 900 °C to 2500 °C, The Centro Español de Metrologia (CEM) in collaboration with the Instituto de Óptica of the Consejo Superior de Investigaciones Científicas (IO-CSIC) are currently involved in the measurement of thermodynamic temperatures down to 400 °C using radiometers based on InGaAs...

After measuring thermodynamic temperatures in the range from 900 °C to 2500 °C, The Centro Español de Metrologia (CEM) in collaboration with the Instituto de Óptica of the Consejo Superior de Investigaciones Científicas (IO-CSIC) are currently involved in the measurement of thermodynamic temperatures down to 400 °C using radiometers based on InGaAs...

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...

Predictable quantum efficient detectors (PQEDs), based on photoelectric effect in silicon semiconductor, have proved their potential as optical radiant power primary standard. At present, simulations for semiconductor devices are the common method to predict the response of a PQED. As an alternative, this work investigates the feasibility of an exp...

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...

The evolving mise en pratique for the definition of the kelvin (MeP-K) [1, 2] will, in its forthcoming edition, encourage the realization and dissemination of the thermodynamic temperature either directly (primary thermometry) or indirectly (relative primary thermometry) via fixed points with assigned reference thermodynamic temperatures. In the la...

The Centro Español de Metrologia (CEM) in collaboration with the Instituto de Óptica of the Consejo Superior de Investigaciones Científicas (IO-CSIC) are able to calibrate filter radiometers/radiation thermometers with direct traceability to the radiant watt at 650 nm wavelength. This allows CEM to measure thermodynamic temperature in the range fro...

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 eutectic alloys rhenium-carbon, platinum-carbon and cobalt-carbon have been proposed as reference standards for thermometry, with temperature and uncertainty values specified within the mise en pratique of the definition of the kelvin. These alloys have been investigated in a collaboration of eleven national measurement institutes and laborator...

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...

CIELAB color coordinates are characteristics of visible object's spectra widely used in color science, and a reliable evaluation of the uncertainty associated with measured color coordinates is desirable. The calculation of CIELAB color coordinates requires the application of a nonlinear transformation. We consider two ways to calculate the uncerta...

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 evolving mise en pratique for the definition of the kelvin (MeP-K) [1,2] will, in its forthcoming edition, encourage the realisation and dissemination of the thermodynamic temperature either directly (primary thermometry), or indirectly (relative primary thermometry) via fixed points with assigned reference thermodynamic temperatures. In the la...

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...

The purpose of this mise en pratique, prepared by the Consultative Committee for Photometry and Radiometry (CCPR) of the International Committee for Weights and Measures (CIPM) and formally adopted by the CIPM, is to provide guidance on how the candela and related units used in photometry and radiometry can be realized in practice. The scope of the...

The thermodynamic temperature of the point of inflection of the melting transition of Re-C, Pt-C and Co-C eutectics has been determined to be 2747.84 ± 0.35 K, 2011.43 ± 0.18 K and 1597.39 ± 0.13 K, respectively, and the thermodynamic temperature of the freezing transition of Cu has been determined to be 1357.80 ± 0.08 K, where the ± symbol represe...

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...

Currently, the dissemination of the kelvin at high temperatures, according to the International Temperature Scale (ITS-90), is realised at Centro Español de Metrología (CEM) by using the fixed points of Ag and Cu and a standard radiation thermometer. Recently, absolute radiometry has been proposed by the CCT Working Group 5 [1] to be included in fu...

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...

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...

Ancient Greek and Roman sources report that the statue of Zeus in Olympia had a head, and in particular eyes, similar to the description of Zeus by Homer, so we think that the statue was visible to the human eye. Since the temple was 12 m high, and had a small door and no windows, the illumination of the statue by conventional media is questionable...

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...

Passive performance of buildings is nowadays one of the key points, not only for reducing energy consumption of buildings, but also for decreasing “fuel poverty”. Among the constructive systems in buildings, façades are the ones having higher influence on thermal performance in urban spaces. Lime renders are specialized systems which can improve no...

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...

Centro Español de Metrología (CEM) is disseminating the International Temperature Scale (ITS-90), at high temperatures, by using the fixed points of Ag and Cu and a standard radiation thermometer. However, the future mise-en-pratique for the definition of the kelvin (MeP-K) will include the dissemination of the kelvin by primary methods and by indi...

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...

The European Metrology Research Program (EMRP) is a metrology-focused program of coordinated Research and Development (RD) funded by the European Commission and participating countries within the European Association of National Metrology Institutes (EURAMET). It supports and ensures research collaboration between them by launching and managing dif...

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...

This report contains the results of the regional comparison EUROMET PR-K2.b (registered in the KCDB under the identifier EURAMET.PR-K2.b). Ten laboratories took part in it, including the pilot. In general the results are consistent, with a few exceptions as explained in the report. The comparison gives international linkage in spectral responsivity...

This report contains the results of the regional comparison EUROMET PR-K2.b. Ten laboratories took part in it, including the pilot. In general the results are consistent with a few exceptions as explained in the report. The comparison gives international linkage in spectral responsivity from 300 nm to 1 000 nm to seven European laboratories: Bundes...

This chapter shows a review about the most important characteristics of photodiodes: speed of response, responsivity, sensitivity, response linearity and the basic radiometrical behavior of any detector.

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 processor is oriented to studies in the extra-galactic astronomy as well as to searching the extra-solar planets, so that algorithm of the space-and-time integrating is desirable for a wideband spectrum analysis with an improved resolution. It includes 1D-acousto-optic cells as the input devices for a 2D-optical data processing. The importance...

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...

Both a high level of developing the spatially spot-like and one-dimensional input devices and the flexibility of a design inherent in two-dimensional optical systems with similar modulating components make it possible to realize various high-bit-rate opto-electronic processors. This is why a one-dimension acousto-optic technique has been involved i...

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...

InP-photodiodes from different manufacturers have got rather low noise level, a good response uniformity over the sensitive surface and a wide dynamic range. Therefore they are good devices to built radiometers in the NIR spectral region. As in any photodiode, the spectral shortcircuit responsivity is determined by the wavelength and the photodiode...