[Show abstract][Hide abstract] ABSTRACT: 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 source, a six-axis robot-arm to rotate the sample and a spectroradiometer that may revolve around the sample to be able to vary the source-to-detector angular separation. This gonio-spectrophotometer makes it possible to perform spectral measurements in the visible range, both inside and outside the incidence plane, as well as measurements in retroreflection conditions. This fully automated system is able to measure autonomously a sample's complete spectral BRDF (comprising around 1000 different angular configurations) in less than 4 h.
[Show abstract][Hide abstract] ABSTRACT: 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 the multivariable analysis technique of Principal Components Analysis (PCA) applied to the experimental BRDF data of a ceramic colour standard. It will be shown that the result may be linked to the various reflection processes occurring on the surface, assuming that the incoming spectral distribution is affected by each one of these processes in a specific manner. Moreover, this procedure facilitates the task of interpolating a series of BRDF measurements obtained for a particular sample.
[Show abstract][Hide abstract] ABSTRACT: 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 matrix-based formalism to allow this effect to be compensated from the measured BSDF and from the previous knowledge of the solid angles involved in the measurements. This deconvolution is important to obtain a better insight of the surface gloss, since it provides a more accurate absolute peak measurement and a better angular resolution in the directional scattering determination.
No preview · Article · May 2011 · Proceedings of SPIE - The International Society for Optical Engineering
[Show abstract][Hide abstract] ABSTRACT: 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.
No preview · Article · Jan 2011 · Optica Pura y Aplicada
[Show abstract][Hide abstract] ABSTRACT: [ES] En este trabajo se analiza el cambio del color con el ángulo de observación de muestras de cartón-yeso iluminadas en la dirección normal a superficie de la muestra. Para ello se ha medido la distribución espectral de la radiancia emitida mediante un espectro-gonio-fotómetro construido y diseñado en el Instituto de Física Aplicada (CSIC). Se han estudiado muestras de cartón-yeso estándar y pintadas, encontrándose diferente comportamiento entre ellas, si bien todas divergen del difusor perfecto. Las diferencias de color encontradas son todas perceptibles, proviniendo de un cambio de claridad para los ángulos de observación más próximos al de incidencia y de un cambio de claridad y croma para los ángulos más alejados. [EN] Colour coordinates change of plaster card-board versus observation angle under normal illumination is analysed in this job. Spectral radiance distribution has been measured at different observation angles by using a spectro-gonio-photometer designed and built at the Instituto de Fisica Aplicada (CSIC). Standard plaster card-board samples as well as painted ones have been studied. Their angular distribution is different but they differ from the perfect diffuser in all cases. Colour differences found are perceptible, coming from a brightness change at the smaller angles and from a brightness and chroma change at the largest angles apart from the incidence. Peer reviewed
[Show abstract][Hide abstract] ABSTRACT: 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 other existing models, although this experimental fact was already known. The well-known phenomenon of supraresponsivity is also explained with this model. Finally, we show the dependence of the internal quantum efficiency on the variables involved in the model.
[Show abstract][Hide abstract] ABSTRACT: The absolute spectral responsivity at some infrared wavelengths of two germanium photodiodes has been determined by two different methods: comparison with an absolute radiometer (ECPR) and calibration with respect to a black-body simulator. The same geometry and spectral width have been used in both methods. The results, uncertainty sources and their evaluation, and the agreement between the methods, are discussed in this paper.
[Show abstract][Hide abstract] ABSTRACT: A bilateral comparison of illuminance responsivity scales between the UME and the IFA-CSIC was carried out, where the IFA-CSIC acted as the pilot and link to the key comparison CCPR-K3.b. The ratio of the measured illuminance responsivities (UME/IFA) was 1.0003 with expanded uncertainty of 0.0084 (k = 2) including the uncertainty of the comparison and the uncertainties of the realization of the scales.
To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.
The final report has been peer-reviewed and approved for publication by the CCPR, according to the provisions of the Mutual Recognition Arrangement (MRA).
[Show abstract][Hide abstract] ABSTRACT: Reference-frequency generation for optical fiber instrumentation is now restricted to the frequency bands in which the reference materials have well-resolved absorption lines. We study analytically and experimentally the possibility of generating reference wavelengths by use of Raman-enhanced four-photon mixing in an optical fiber. We show that it is possible to generate efficiently frequencies that are 10-40 nm away from the absorption bands of the usual reference materials: acetylene (12C2H2), hydrogen cyanide (HCN), and similar or derived species. As a demonstration we use an acetylene cell to generate reference frequencies that cover the whole C (1530-1565 nm) and some part of the L (1565-1625 nm) transmission bands of the optical fiber.
[Show abstract][Hide abstract] ABSTRACT: This paper describes the approach used to realize the candela from illuminance measurements made with a large-area, partial-filtering photometer. The methods followed to determine the absolute responsivity at 555 nm, traceable to an absolute cryogenic radiometer, and to measure the relative spectral responsivity are discussed, including the uncertainty values of the measured quantities. Finally, preliminary results obtained on comparing the photometer measurements with the previous luminous intensity scale are presented.
[Show abstract][Hide abstract] ABSTRACT: This work analyses the reflectance of silicon photodiodes as a function of incident beam polarization, angles of incidence and acceptance, and thickness of the passivating layer. The contributions of these effects to the polarization sensitivity of real trap devices are considered. An analytical model is used to calculate changes in responsivity due to polarization in traps with angular misalignments and traps constructed from different types of silicon photodiode. The results show that sensitivity to polarization should be considered when high-accuracy measurements are required.
[Show abstract][Hide abstract] ABSTRACT: This paper describes an experimental setup, based on an optical-fibre system, to calibrate infrared photodiodes at 1300 nm and 1550 nm by comparison with a cryogenic radiometer. An integrating-sphere radiometer used as a transfer standard for optical-fibre power measurements has been characterized and calibrated, obtaining an uncertainty of less than 0.1% at these wavelengths.
[Show abstract][Hide abstract] ABSTRACT: An experimental set-up designed for the calibration of near-infrared detectors against a cryogenic radiometer, using a tuneable laser diode as source, is described. A complete characterization of the cryogenic radiometer has been performed at the Instituto de Fisica Aplicada (IFA, Madrid), at wavelengths of 1300 nm and 1550 nm. Two different secondary standards, an electrically calibrated pyroelectric radiometer and an InGaAs-based integrating-sphere radiometer, have been calibrated with relative uncertainties of less than 10−3 at these wavelengths.
[Show abstract][Hide abstract] ABSTRACT: In 1991, the last international comparison of spectral responsivity with germanium detectors showed relative differences between national laboratories of up to 10−2 at wavelengths of 1300 nm and 1550 nm. The 1997 EUROMET international comparison of optical-fibre power meters showed relative differences between European laboratories of 10−2. The coincidence of the results from the two comparisons suggests that the radiometric standards used by these laboratories in the calibration of their power meters are the main cause of such differences, rather than the optical-fibre or transfer devices used in the comparisons. To test this assumption, three different electrical-substitution radiometers (ESRs), working at room temperature and used at the same wavelengths as radiometric standards in optical-fibre laboratories, were compared. The radiometers were an optical calorimeter, a pyroelectric radiometer and a thermopile. The relative differences obtained were within 5 × 10−3.
[Show abstract][Hide abstract] ABSTRACT: A facility has been designed to perform a variety of detector characterizations in the air-ultraviolet in the spectral range 230 nm to 400 nm. The facility allows the measurement of spatial uniformity, linearity and the spectral responsivity of different detectors, irradiated by a He-Cd laser or by a monochromator and Xe discharge lamp. With this technique, a set of Si photodiodes, Si detectors in trap configuration, and ultraviolet (UV) radiometers built with SiC detectors in an integrating sphere, have been calibrated and their absolute responsivity at the He-Cd laser UV line has been determined with respect to an absolute cryogenic radiometer.
[Show abstract][Hide abstract] ABSTRACT: Correlated colour temperature (CCT) is an important parameter of light sources. It is estimated from the position of the chromatic coordinates (u, v) in the Commission Internationale de l'Éclairage (CIE) 1960 colour space. The International Organization for Standardization (ISO) Guide is difficult to use when trying to obtain CCT uncertainty from chromaticity coordinates and their uncertainties because there is no clear analytical expression relating them. In this paper, an analytical method of obtaining the CCT uncertainty from the coordinates (u, v) and their uncertainties, without approximations, is proposed. The validity of the method is proven by comparing results with examples in the literature and with those obtained using a geometrical projection method.
[Show abstract][Hide abstract] ABSTRACT: A systematic study on the uncertainty in spectral responsivity of silicon photodiodes due to the calibration spectral bandwidth is presented. Two of the most common types of silicon photodiodes in the field of optical power measurements have been used in this work: one with thickness of about 100 nm of SiO2 and the other with thickness about 27 nm of SiO2. It is shown that the spectral responsivity error will be negligible (≤5×10-4) using a spectral bandwidth up to 20 nm in the calibration if the effective wavelength of the spectral transfer function, including the distribution of the radiation source, is calculated and the measured responsivity is assigned to that effective wavelength. In other cases, the error depends not only on the extent of the interval isolated by the spectral analysis system but also on the shape of the spectral isolation function.
Full-text · Article · Oct 2001 · Measurement Science and Technology
[Show abstract][Hide abstract] ABSTRACT: An optical method to measure time response in scanning spectrophotometers is described. The method is wavelength independent and can be used to check both the raise time and the fall time. The method was applied to a scanning spectrophotometer and the results were compared to those obtained for the same instrument using a kinetic method. The validity of the new method was demonstrated by the fact that the agreement between the results obtained using the two methods was complete.
[Show abstract][Hide abstract] ABSTRACT: Describes the use of a fully automatic, computer-controlled absolute spectroradiometer as a precision colorimeter. The chromaticity coordinates of several types of light sources have been obtained with this measurement system.
Full-text · Article · Nov 2000 · Journal of Physics E Scientific Instruments
[Show abstract][Hide abstract] ABSTRACT: The dependence of the reflectance of standard aluminium mirrors on the polarization status of the incident light has been calculated and measured for an angle of incidence close to normal , so that the systematic uncertainty introduced by not controlling the light polarization status in spectrophotometric measurements of specular reflectance can be known and then eliminated or minimized.
No preview · Article · Dec 1998 · Measurement Science and Technology