Alicia Pons

Spectral and geometrical variation of the bidirectional reflectance distribution function of diffuse reflectance standards

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 variation respect to a reference geometry was assessed from the experimental data. Several descriptors were defined in order to compare the spectral BRDF variation of the four materials.

Improvements for determining the modulation transfer function of charge-coupled devices by the speckle method

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 power-spectral-density values that arise from the field and CCD response non-uniformities. In the high-spatial-frequency region the results show that the distance between the CCD and the aperture is critical and significantly influences the MTF; a variation of 1 mm in the distance can cause a root-mean-square error in the MTF higher than 10%. We propose a simple correction that minimizes the experimental error committed in positioning the CCD and that diminishes the error to 0.43%.

Spectral BRDF-based determination of proper measurement geometries to characterize color shift of special effect coatings

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 (BRDF). The analysis of experimental spectral BRDF was carried out by means of principal component analysis. From this analysis, a set of nine measurement geometries was proposed to characterize special effect coatings. It was shown that, for two different special effect coatings, these geometries provide a good prediction of their complete color shift.

Variables separation of the spectral BRDF for better understanding color variation in special effect pigment coatings

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 decorrelated spectral components, and the weight that they have at every geometrical configuration of irradiation/viewing is established. This method was applied to the spectral BRDF measurement of a special effect pigment sample, and four principal components with relevant variance were identified. These four components are enough to reproduce the great diversity of spectral reflectances observed at different geometrical configurations. Since this representation is able to separate spectral and geometrical variables, it facilitates the interpretation of the color variation of special effect pigments coatings versus the geometrical configuration of irradiation/viewing.

Colorimetric and spectral evaluation of the optical anisotropy of metallic and pearlescent samples

The visual and optical anisotropies of metallic and pearlescent samples have been evaluated using a multi-gonio-spectrophotometer and by adapting a gonio-template over the instrument aperture for measuring the relative spectral reflectance factor when the sample is rotated around an axis normal to its surface. The results show that spectral data greatly vary as the azimuth angle changes, especially in pearlescent samples. Spectral and colorimetric data of the same sample for the interference and aspecular lines seen in complementary angle pairs have been compared, resulting in hue angle being kept constant, but lightness L* and chroma variations are high. In particular, the colour differences ΔEab calculated for pearlescent and metallic samples show that these visual differences can be clearly appreciable, even in metallic samples, but especially in pearlescent ones. Therefore, these types of optical materials do not verify the Helmholtz reciprocity, which is usually applied for digital modelling of material appearance.

Low-uncertainty absolute radiometric calibration of a CCD

This work presents an experimental method for the low-uncertainty calibration of the spectral radiant exposure responsivity of a CCD detector. It comprises a description of the experimental set-up as well as an analysis of the various sources of uncertainty. The overall result shows that the calibration procedure-related uncertainty is only 0.18% as long as the temperature is kept constant and the field of view is kept lower than 13°.

Calibration of near-infrared transfer standards at optical-fibre communication wavelengths by direct comparison with a cryogenic radiometer

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.

New model for the internal quantum efficiency of photodiodes based on photocurrent analysis

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.

Color representation and interpretation of special effect coatings

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 understanding of the color shift to be done both in terms of conventional irradiation and viewing angles and in terms of flake-based parameters. A different line was proposed to assess the color shift of special effect coatings on a*,b*-diagrams: the absorption line. Similar to interference and aspecular lines (constant aspecular and irradiation angles, respectively), an absorption line is the locus of calculated color coordinates from measurement geometries with a fixed bistatic angle. The advantages of using the absorption lines to characterize the contributions to the spectral BRDF of the scattering at the absorption pigments and the reflection at interference pigments for different geometries are shown.

Principal components analysis on the spectral bidirectional reflectance distribution function of ceramic colour standards

The Bidirectional Reflectance Distribution Function (BRDF) is essential to characterize an objects 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.