BME BSDF data, technical notes

BSDF,BRDF measurements

• Terminology: BRTF (bidirectional reflectance transmission function), BSDF (bidirectional scatter distribution function), BRDF (bidirectional reflectance distribution function) and various other four-letter-B-acronyms of the same family (BRDF, BRTD, etc) label the same physical quantity. And there's no conceptual difference between the transmission and reflective side. Please be referenced to ASTM E2387-05 Standard Practice for Goniometric Optical Scatter Measurements.
• Data is presented for a finite set of incident angles (around 10 to 14) and a fairly large number of outgoing angles (around 80000 per incident angle). Data is out-of-plane BRDF data measured relative to the integral value of the scanned unscattered beam (incident power). Thus, numerical integration of the data results in the direct-hemispherical reflection (ρ), which is given in the data tables.
  Datasets and visualisations on these pages show measured data divided by cos(θ_out), so they show the BRDF. The unit for angles is degrees, coordinate system is standard spherical.
• With the automatic sample changer, which was used for this series, plus the Halogen light-source used, outgoing angles are valid (not shaded by sample mount) up to θ_out = 85 degrees. To avoid unnecessary errors and skew, the fitting algorithm uses unobstructed data only: Data values beyond 85 degrees and values that are shaded by the supporting sample column are automatically discarded before fitting (as shown in the visualisation screen-dumps).
  For reference, the download-able dataset includes the full data, with θ_out up to 90 degrees.
• Incident angles are typically stepped from θ_in=10 degrees to 70 degrees, and φ_in=0 and 90 degrees. With these materials, the two φ_in angles are mainly to crosscheck for asymmetric scattering, most materials are not specifically selected for asymmetric scattering.
• Since these examples focus on the spatial shape of the BRDF, all data is treated as spectrally grey, integrated from 300nm to 700nm.

technical notes on Radiance version

• Source code of normal.c from Radiance CVS, which codes the Ward BRDF for plastic. Fitting is based on version 2.60 .

technical notes on visualisation

• Visualisation and processing is handled with mountain, a proprietary program by pab Ltd.
• 2D cuts are plotted using the very useful gnuplot, many thanks to them.
• Fitting is done by Levenberg-Marquardt method, as explained in the informative Numerical Recipes book. Adaptation to the pgII database, model handling and web-page generation was written at pab Ltd.
  More information on data fitting is given on the model page.

data formats on these web-pages

• Formats are JPEG and PNG for images and SVG for vector plots. It has been brought to our attention that some older versions of Microsoft Internet Explorer on PCs don't render SVG graphics, apparently for ''political'' reasons. We wish to apologise for inconveniences this may cause and recommend to use other browsers (Firefox, Opera, etc) or another operating systems (Apple, Linux, or other UNIX flavours).
  To support users without SVG support, all 2D plots have been generated as PDF too. A file with all plots combined is available from the sample page.

links

• BSDF survey, Szymon Rusinkiewicz, Cornell, CS348c, Winter 1997
• Notes on the Ward BRDF, Bruce Walter, Technical Report PCG-05-06, Cornell Program of Computer Graphics, April 29, 2005
• A Comparison of Four BRDF Models, Stephen H. Westin, Hongsong Li, and Kenneth E. Torrance, Cornell University Program of Computer Graphics, 2004
• MIT Csail
• Radiance materials description
• other sites on Radiance materials: http://archbps1.campus.tue.nl/bpswiki/index.php/Radiance, http://www.bozzograo.net/radiancewiki/doku.php?id=material_collections

Further details on models and fitting is found on the models page. Back to BME start page, back to sample gallery