| Zugriffsnummer | 37017 |
| Dokumenttyp | Konferenzartikel |
| Sprache | Englisch |
| Titel | Polarization effects in diffuse reflectance measurements - comparison of white standards and special-effect pigment samples |
| Autor(in); Institution |
Quast, Tatjana; 4.2, Bild- und Wellenoptik, PTB-Braunschweig
Hauer, Kai-Olaf; 4.2, Bild- und Wellenoptik, PTB-Braunschweig
Schirmacher, Alfred; 4.2, Bild- und Wellenoptik, PTB-Braunschweig
|
| Quelle/Jahr | Proceedings of the 4th CIE Expert Symposium on Colour and Visual Appearance:(2016), 30 - 39 |
| Schriftenreihe | CIE X: 043 |
| ISBN | 978-3-902842-59-6 |
| Verlag | Vienna: CIE |
| Konferenzangaben | 4th CIE Expert Symposium on Colour and Visual Appearance, Prague, 06-07, September, 2016, Czech Republic |
| Zusammenfassung | To precisely characterize the bidirectional reflectance distribution function (BRDF) of any sample under test, the polarisation of the incident and reflected light has to be taken into account. While the influence of polarization is expected to be small in standard geometries in the case of white standards, the polarisation dependence is especially interesting for special-effect pigments. Such pigments are widely used in a number of different branches of industry (e.g. automobile, printing, cosmetics) due to their different visual effects like sparkle or goniochromatism. However, these effects in turn make it challenging to characterize the pigments because the BRDF depends strongly on the geometry investigated so that a large number of geometries deviating from standard cases have to be sampled. For the measurement of BRDF of radiance factor PTB uses a robot-based gonioreflectometer in which the angles of illumination and detection can chosen. As also the orientation of the sample can be controlled many different in-plane and out-of-plane geometries can be realized. By using a broadband light source and different detectors, the wavelength range of 250 nm to 1750 nm is available for measurements. With this experimental setup it is possible to determine the spectral radiance factor as an absolute quantity. The gonioreflectometer has been equipped with a polarization-analyzer unit. It consists of a quarter-wave plate whose fast axis can be rotated perpendicular to the direction of detection by computer control, and a linear polarizer behind with fixed transmission axis. By measuring the spectral radiance factor at different angles of the wave plate, it is possible to determine the Stokes parameters I, M, C and S of the reflected beam. This set of parameters fully describes the polarization state of the reflected beam of light in terms of linear (parameter M for s/p and C for +/-45 degrees orientation of the fast axis, respectively) and circular polarization (parameter S) and intensity I. In this way one can gain a quick overview of the polarization dependence of the spectral radiance factor. However, determining the Stokes parameters for each geometry would increase the measurement time by a factor of eight, since an eight-step technique is used in our experiments. In order to measure the spectral radiance factor efficiently one has to descriminate between geometries which exhibit a strong polarization dependence and those where polarization only plays a minor role. In the former case it is advisable to determine the Stokes parameters whereas in the latter case the polarization dependence can be omitted. Also in most cases it is expected that circular polarization is close to zero in the reflection process and therefore its determination could be avoided. In this study, the polarization properties of the gonioreflectometer setup itself and of a variety of samples have been examined. It could be shown that the incident light can be considered unpolarized as is expected from an isotropically emitting integrating sphere source. Thus, any detected polarization dependence is caused by the sample under test. Different kinds of samples have been studied, among them special-effect pigments which exhibit a strong goniochromatism, interference pigments, monochrome samples with glossy surface and white standards with matte surface. The geometries range from "typical" measurement geometries (e.g. illumination under 45°, detection under 0 ° relative to the surface normal of the sample) to more unusual ones. In this way it is possible to evaluate the influence of the polarization on the spectral radiance factor for a variety of experimental situations. The results of this study are supposed to give an overview to which extent a polarization dependence exists, which parameters have the most pronounced effect and to supply information on parameter dependence, e.g. with respect to sample type, surface finish, measurement geometry and wavelength. It will give information on such cases where it is most pronounced and therefore should be taken into account and in which cases it may be neglected. |
Zitierung
Quast, T., Hauer, K.-O., & Schirmacher, A. (2016). Polarization effects in diffuse reflectance measurements - comparison of white standards and special-effect pigment samples. 4th CIE Expert Symposium on Colour and Visual Appearance, Prague, 06-07, September, 2016, Czech Republic.