Bidirectional Geometry Research Articles

Development of primary reference facilities and measurement comparison of standard artifacts for the bidirectional transmittance distribution function

To determine the bidirectional transmittance distribution function (BTDF) of diffusely transmitting materials, two new primary facilities have been developed at the Physikalisch-Technische Bundesanstalt (PTB) and Aalto University (Aalto). A detailed description of both facilities and the different approaches used are presented in this paper. The performance of both facilities is compared by determining the BTDF of two different diffuser types in both in-plane and out-of-plane bidirectional geometries at four different wavelengths in the visible spectral range. Due to delayed completion of PTB’s primary facility, the measured BTDF values are compared between Aalto’s primary facility and another PTB setup, whose measurement scales are successfully transferred to PTB’s primary facility by an internal comparison. A thorough analysis of the measurement uncertainty is presented, leading to a combined k = 1 standard uncertainty of 0.8%–1.2% for PTB’s primary facility and 1.3%–1.7% for Aalto’s primary facility. The BTDF results obtained agree well within their expanded k = 2 uncertainty. This indirect bilateral comparison shows that Aalto’s and PTB’s new facilities are suited as primary reference setups for the determination of the BTDF. These studies also reveal action points to improved measurement capabilities and for a reduction of the measurement uncertainty, depending on the type of diffuser under test.

Monitoring sub-surface chemical reactions in heterogeneous materials using wavefront-shaping-assisted bidirectional focusing.

We have developed a bidirectional focusing microscope that utilizes feedback-assisted wavefront shaping to focus light inside a heterogenous material in order to monitor sub-surface chemical reactions. The bidirectional geometry is found to provide superior intensity enhancement relative to single-sided focusing, owing to increased mode control and long-range mesoscopic correlations. Also, we demonstrate the microscope's capability to measure sub-surface chemical reactions by optically monitoring the photodegradation of a Eu-doped organic molecular crystal embedded in a heterogeneous material using both fluorescence and Raman spectroscopy as probe techniques.

Deviation of white diffuse reflectance standards from perfect reflecting diffuser at visible and near-infrared spectral ranges

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 limits, and, for some applications, requirements may be out of those limits. PTB and IO-CSIC have studied the variation of the reflectance with respect to the bidirectional geometry for the four most typical white diffuse materials (barium sulfate, opal glass, ceramic and Spectralon), at in- and out-of plane geometries and at spectral range from 380 nm to 1700 nm. We have defined descriptors in order to more clearly quantify the spectral reflectance variation with the bidirectional geometries. The values obtained for these descriptors have been separately presented for the visible and near-infrared spectral ranges. In both spectral ranges, deviations of white diffuse reflectance standards with respect to the PRD were found, regarding both Lambertian behaviour and spectral constancy. The observed deviation from the BRDF is in general very large for high incidence and collection angles (reaching in many cases 20%). Therefore, it is not possible to assume Lambertianity in standards at those geometries when calibrating measuring systems.

Polarization properties and microfacet-based modelling of white, grey and coloured matte diffuse reflection standards

To elucidate the influence of polarization in diffuse reflectometry, we performed a series of measurements in several bidirectional geometries and determined the Stokes parameters of the diffusely reflected radiation. Different types of matte reflection standards were used, including several common white standards and ceramic colour standards. The dependence of the polarization on the sample type, wavelength and geometry have been studied systematically, the main influence factors have been identified: The effect is largest at large angles of incidence or detection and at wavelengths where the magnitude of the reflectance is small. The results for the colour standards have been modelled using a microfacet-based reflection theory which is derived from the well-known model of Torrance and Sparrow. Although the theory is very simple and only has three free parameters, the agreement with the measured data is very good, all essential features of the data can be reproduced by the model.

Characterization of a versatile reference instrument for traceable fluorescence measurements using different illumination and viewing geometries specified in practical colorimetry—part 1: bidirectional geometry (45:0)

For highest accuracy fluorescence colorimetry, standardizing organizations recommend the use of a two-monochromator method with a bidirectional illumination and viewing geometry (45:0 or 0:45). For this reason, reference fluorescence instruments developed by National Measurement Institutes (NMIs) have largely conformed to this bidirectional geometry. However, for many practical applications in colorimetry where the samples exhibit texture, surface roughness or other spatial non-uniformities, the relevant standard test methods specify a sphere geometry with diffuse illumination or viewing (e.g. d:8 or 8:d) which gives improved measurement precision. This difference in the measurement geometry between the primary instrument used to realize the fluorescence scale and the secondary testing instruments used for practical measurements, compromises the traceability of these fluorescence calibrations. To address this metrology issue, a two-monochromator goniospectrofluorimeter instrument has been developed at the National Research Council of Canada (NRC). This instrument can be configured for different illumination and viewing geometries to conform with international standards for different colorimetric applications. To improve the traceability chain for measurements using different geometries, the instrument has been thoroughly characterized and validated by means of comparison measurements with NRC’s other spectrophotometric and fluorescence reference instruments. This uncertainty analysis has been carried out in a step-wise manner; first, for a bidirectional geometry (45:0) and then for a sphere geometry (8:d) to provide an uninterrupted traceability to primary radiometric scales. The first paper in this two paper series reviews the background to this work and provides details of the basic design of the new instrument and its characterization for measurements using a bidirectional geometry (45:0), including a representative uncertainty budget. In part 2, the major sources of sphere error are described and minimized in a modified sphere design. The instrument characterization and validation are then extended to a sphere geometry (8:d) to provide direct traceability for practical fluorescence colorimetry.

Software for the Absolute Correction Color Values and Calibration of Spectrometers, An Instrument Correction Program

We developed a software program that corrects known, systematic spectrophotometric errors seen in commercial spectrophotometers today. This enables users of spectrophotometers to correlate color values from around the world so that they will be nearly identical. This includes instruments manufactured by different manufacturers, and instruments manufactured by the same manufacturer. The program works on six different modalities; including bi-directional geometries; that is, 45°/0°; hemispherical, either d/0° or d/8° geometries; or multi-angle geometries. Multiangle geometries are used to characterize and assess gonioapparent inks and coatings. It is not necessary that both instruments be of the same geometry.We summarize the performance of the software in the following statement: For modern instruments that are close in performance; that is less than 1 DE*ab one can expect that the errors will be reduced to an average of less than 0.10 DE*ab unit across 14 BCRA tiles. This agreement or correlation between two instruments can be relative; that is one instrument to another, or absolute; that is, correlating one or more instruments to a reference international standardization laboratory.ISO and CGATS.5 recommend methods to improve interinstrument agreement. The software uses traceable artifact standards to do the training. Implementing this program allows conformance to in-house certification programs, ISO requirements, and CGATS. This is of particular importance to those who utilize ICC profiling.The program is designed to be readily incorporated into your existing software. There are multiple implementation methodologies available. For instance; it can be incorporated into your existing software through a DLL, Dynamic Linked Library. It can be used externally in post processing modes with an MS Excel spreadsheet, or it is available in a full-featured, customizable, quality control package. This program operates transparently to the user. Included with the software are all the support programs and documentation required to utilize this application.

Polarization- and interval-independent wavelength conversion at 2.5 Gb/s by means of bidirectional four-wave mixing in semiconductor optical amplifiers

We report interval-polarization-independent wavelength conversion at 2.5 Gb/s using four-wave mixing in a single semiconductor optical amplifier. These results have been obtained with a dual-pump configuration in a bidirectional geometry. Bit-error-rate (BER) curves are reported for a conversion shifts of -1 and +1 THz. The signal polarization was automatically scrambled during the BER measurement. The penalty with respect to the back-to-back line is 2.4 and 1.6 dB, respectively.

The effect of measuring geometry on computer color matching

AbstractThe debate over the best optical geometry for color measurement dates back to the development of the first recording reflectance spectrophotometer. It is hypothesized that a bidirectional geometry conforms best to the procedure used to inspect colored surface samples visually. This hypothesis was tested using two spectrophotometers of nearly identical design, one utilizing d/8 geometry and the other utilizing 0/45 geometry. The results indicate that bidirectional geometry may be superior to integrating‐sphere geometry when matching standards with varying gloss. The bidirectional measurements are quite sensitive to the size of the source and the detector‐aperture half angle.

When are spectral reflectance curves comparable?

Spectral reflectance curves of flat laboratory samples of the carbonaceous chondrite Allende, a basalt, and the ordinary chondrite Bruderheim measured in a bidirectional geometry are shown to differ from those measured using an integrating sphere. In general, reflectance curves obtained by the bidirectional method are redder than those obtained with an integrating sphere. The degree of difference increases with increasing absolute reflectance. When spectral reflectance curves obtained by the two methods are compared to the reflectance curves expected for spherical and aspherical planets covered with the same materials, it is found that in general the integrating sphere measurements provide a better match to a planet at small phase angles. As the phase angle increases, bidirectional reflectance curves provide a closer match.