Knowledge of transmittance and reflectance, defined as the fraction of transmitted and reflected light to any direction, is not enough to describe these scatter phenomena. The optical properties leading to the observations given above can only be explained by the directional dependency of reflection and transmission. This dependency applies to the direction from the transmitting or reflecting object towards both the light source and the observer.
[Figure: Light scatter on a structured glass pane]
The twofold directionally dependent characteristics of transmission and reflection are described by the Bidirectional Scattering Distribution Function (BSDF). This function relates the intensity of light that is transmitted and reflected towards any direction to the intensity and direction of the light incident on that object. In practice, both the incident source direction and the receiver direction, towards which the light is scattered, are each written as pairs of elevation and azimuth angles. This leads to the basic formulation of light scattering as a function of four dimensions BSDF (θs,φs,θi,φi ). If different wavelength ranges are to be considered separately, the wavelength is added as the fifth dimension of the function. Typical applications for such wave-length-resolved BSDF are in appearance models as a field of computer graphics, where scattering is treated differently for each color channel, and in building science, where the visible light range needs to be handled separately from the invisible near-infrared wavelength range.
[Figure: Directions to light source (θs,φs) and receiver (θi,φi ), sampling aperture A on a reflective surface]
The BSDF as a measureable quantity describing light scattering is established in applications in
- optics, e.g. for the characterization of surfaces and the development of models for optical simulation,
- computer graphics for the realistic modeling of appearance of materials and surface finishes,
- surface engineering to characterize surface qualities and for comparisons in quality control and the selection of products,
- increasingly in building science and engineering to describe and model advanced facades in building simulation,
- the computation of derived quantities such as surface roughness or direct-hemispherical transmission and reflection.
[Figure: Scattering of light from a laser, incident from top left, on three aluminum sheets with different structured surfaces]
Measurement and data processing
The raw measurement data is recorded in a documented format and can be further processed for analysis, comparison with other measurements or visualization purposes. For applications in simulation, data-driven BSDF models can be compiled in various formats. Such models can be imported, e.g. into optical modeling or building simulation software.
A brief overview on the possible measurement configurations and typical output formats of the processed measurement data is given in the section "Technical Specifications" below. To achieve an optimum adaption of the measurement configuration to specific requirements, consultation with the experts at CC EASE is encouraged.
[Abbildung: Goniophotometer am CC EASE]
Die Rohdaten des Instruments liegen in einem dokumentieren Format vor und können zur weiteren Analyse, zum Vergleich mit anderen Messungen oder zur Visualisierung aufbereitet werden. Für Anwendungen im Bereich der Simulation besteht die Möglichkeit, datenbasierte BSDF-Modelle in verschiedenen Formaten zu erzeugen, die z.B. in optischen Simulationen oder im Bereich der Gebäudesimulation weiterverwendet werden können.
Eine kurze Übersicht über die Möglichkeiten der Messkonfigurationen und zu typischen Ausgabeformaten der aufbereiteten Messdaten findet sich im Abschnitt «Technische Daten». Um eine optimale Anpassung an die spezifischen Anforderungen sicherzustellen, empfiehlt sich in jedem Fall eine Beratung mit den Experten am CC EASE.
[Figures: Photograph of light from a laser scattered on a rough aluminum surface, projection and profile of the measured BSDF x cos(θs), variable-resolution BSDF model as displayed by the LBL BSDF-viewer and applied in a simulation with Radiance]
Services and opportunities for collaboration
Instrumentation and competences at CC EASE are made available both as services and within Research and Developlement (R&D) collaborations with partners from industry or other research institutes. Before the commissioning of measurement services, the experts at CC EASE will prepare a detailed quotation according to your requirements after an initial consultation. This ensures that the highly flexible instrumentation is configured according to your requirements and that the results can be prepared for a seamless transfer into your application. R&D collaborations are possible within short-term collaborations similar to services, but generating scientific results that shall be published. More extensive R&D collaborations, potentially supported by public funding agencies, allow the application and further development of technology and know-how both at the school and in the partner company over a project duration of normally 2-3 years. The experts at CC EASE can inform about possible funding schemes and the possibilities to initiate such collaboration within a direct consultation.
[Figure: Workshop on measurement and application of BSDF at CC EASE with partners (2014)]
Instrument class: Goniophotometer
Measurement method: Far-field measurements, sequential scan by continuous movement of the detector head orbiting the sample
Manufacturer, model: PAB advanced optics, pg-II
Absolute measurement independent of reflection or transmission standard
Measurement of Bidirectional Reflection Distribution Function (BRDF) and Bidirectional Transmission Distribution Function (BTDF) merged to Bidirectional Scattering Distribution Function (BSDF)
Typical sample dimensions >= 100mm x 50mm
Maximum sample dimensions 1000mm x 900mm, 25kg
Diameter of the measured area on the sample (sampling aperture) under orthogonal light incidence 7mm - 65mm
Distance of the detector to the center of the sampling aperture 1000mm
Configurable directional resolution of the measurement up to < 1 degree, accuracy < 1mrad
Optionally locally adaptive resolution according to the measurement including irregular distributions
High Dynamic Range measurement by one out of three detectors:
- Silicon detector, response approx. 350nm - 1050nm
- Silicon detector with v(λ )-filter for photometric measurements, response approx. 380nm - 780nm (according to human eye response)
- Indium-Gallium-Arsenide-(InGaAs)-detector for measurements in the near infrared, response approx. 900nm - 2500nm
Configurable illumination system:
- Halogen or high pressure arc-lamp
- Application of filters to limit the measurement to specific wavelength ranges
- Configurable beam diameter and focus
- Output of measurement data following ASTM E2387 recommendations
- Compilation of measurement data into data-driven models of fixed or variable resolution, according to Berkeley XML format for application, e.g. in LBL Window or Radiance
- Compilation of data-driven BSDF-models in tabular formats of fixed resolution, e.g. for applications in LightTools
- Visualization of measurement data, e.g. as profile plot in the scatter plane, surface plots of the BSDF for given source directions