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In attempting to simulate the reflection and transport processes of light
propagation, it is first necessary to measure and compare the material light
reflection models (BRDF's) as well as measure the goniometric diagrams and
spectral distribution of the illumination sources. For this reason, we have
established a sophisticated light measurement laboratory with NSF and industry
support.
Equipment in the Light Measurement Laboratory (circa 1995)
This laboratory includes:
- A Photometrics PXL1300 CCD camera as a 2D imaging spectroradiometer
suitable for producing quantitative spectral images of real world scenes
- An Optronics 125mm 77400 spectroradiometer/reflectometer
- A custom-built gonioreflectometer
for measuring bidirectional reflectance
distribution functions (BRDF's) of physical samples
- An Oriel Multispec/Instaspec II spectroradiometer
- A Labsphere US-120-SF uniform light source integrating sphere
- A surface profilometer
The CCD camera has been calibrated as a spectroradiometer to provide fast
spectral images of an illuminated environment, using broadband filters and
computer-based spectral inversion. The equipment has been used to measure
hemispherical and bidirectional reflectivities over a wide wavelength range
and to assess new wave-optics based bidirectional reflectance models. Through
this equipment we have acquired quantitative images of real-world scenes for
verification of local light reflectance models and global illumination
algorithms.
Difficulties for the measurement experiments are numerous and basically fall
into three categories:
- Information on the calibration and precision of measurement equipment is
scarce.
- Physical constraints sometimes make the measurement a complex task,
particularly for systems involving dense data sets.
- Certain phenomena play a large role in reflection models, and yet are
extremely difficult to measure
We have calibrated our high-resolution liquid-cooled CCD camera to determine
the behavor and response of the entire camera system over its full dynamic
range before using the measurements. We have also modified our
gonioreflectometer equipment several times to obtain almost all of the
hemispherical reflection values, especially from the grazing region where some
of the most interesting characteristics occur, such as the off-specular peak.
Measurements and Comparisons
We use the calibrated CCD camera to capture a twelve-bit image of a physical
scene to then directly compare with a
simulated rendering of the same scene, using identical geometry, material
properties, light source chacteristics, and camera location. The Cornell box data used for these experiments are now
available for researchers to use in generating reference images of the
identical scene for comparison as a test of their own rendering algorithms.
Furthermore, we present various measurements of emission spectra,
transmission spectra, response curves, and full hemispherical and spectral
BRDF data.
Publications
- Sing-Choong Foo and Kenneth Torrance, Equipment Acquisition for the Light
Measurement Laboratory of the Cornell Program of Computer Graphics, PCG-95-5,
Program of Computer Graphics Technical Report, Cornell University, September,
1995.
- J. Wei-Chieh-Li, Ken Torrance, S. Pattanaik, S. Chen. Preliminary
Calibration of the Photometrics PXL1300L CCD Camera. PCG-96-1, Program of
Computer Graphics Technical Report, Cornell University, March, 1996.
- Sing-Choong Foo. A Gonioreflectometer for Measuring the Bidirectional
Reflectance of Material for Use in Illumination Computation. M.S. Thesis,
Cornell University, August, 1997.
- Donald P. Greenberg, Kenneth Torrance, Peter
Shirley, James Arvo, James Ferwerda, Sumanta Pattanaik, Eric Lafortune, Bruce
Walter, Sing-Choong Foo, and Ben Trumbore.
A framework for realistic image synthesis.
In Turner Whitted, editor, SIGGRAPH 97 Conference Proceedings,
Annual Conference Series, pages 477--494. ACM SIGGRAPH, Addison Wesley,
August 1997.
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Eric P. F. Lafortune, Sing-Choong Foo, Kenneth E. Torrance, and Donald P.
Greenberg.
Non-linear approximation of reflectance functions.
In Turner Whitted, editor, SIGGRAPH 97 Conference Proceedings,
Annual Conference Series, pages 117--126. ACM SIGGRAPH, Addison Wesley,
August 1997.
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Sumanta N. Pattanaik, James A. Ferwerda, Kenneth E. Torrance, and Donald P.
Greenberg.
Validation of global illumination solutions through CCD camera
measurements.
In Proceedings of the Fifth Color Imaging Conference, Society
for Imaging Science and Technology, pages 250--253, November 1997.
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Stephen R. Marschner, Stephen H. Westin, Eric P. F. Lafortune, Kenneth E.
Torrance, and Donald P. Greenberg.
Image-based brdf measurement including human skin.
In Eurographics Workshop on Rendering, 1999.
Support
The original development of the laboratory was funded in part by NSF
Engineering Research Equipment Grant number CTS 9213183 from 1992-1995, under
the direction of Professor Kenneth E. Torrance.
The gonioreflectometer was donated by the Eastman Kodak Company.
Research support has been provided by the NSF Science and Technology Center
for Computer Graphics and Scientific Visualizaton (STC) and by the Hewlett-Packard
Corporation.
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