WHITE PAPER. Application of Imaging Sphere for BSDF Measurements of Arbitrary Materials

Transcription

1 Application of Imaging Sphere for BSDF Measurements of Arbitrary Materials

2 Application of Imaging Sphere for BSDF Measurements of Arbitrary Materials Abstract BSDF measurements are broadly applicable to material characterization, quality assessment, and computer modeling. The Imaging Sphere is optical measurement technology that allows BSDFs to be obtained quickly and accurately for wide variety of materials Optical Society of America OCIS codes: ( ) Scattering; ( ) BSDF, BRDF, and BTDF; ( ) Instrumentation, measurement, and metrology; ( ) Surface measurements, roughness; ( ) Scattering measurements. Introduction Accurate modeling of surface appearance for arbitrary materials requires either complex simulations to account for subtle surface variations, or the use of actual measured BSDF (bi-directional scatter distribution function) data. Traditionally, BSDF measurements are performed using goniometric systems, but these instruments are relatively slow, potentially requiring hours to measure a sample under a full range of illumination and observation angles. The Imaging Sphere has been successfully applied to obtain BSDF data for materials as diverse as metals, plastics, paper, wood, painted surfaces, polished surfaces, transparent and translucent films, holographic films, grating structures and human skin and hair. By taking advantage of a novel optical configuration and the ability of an imaging system to capture hundreds of thousands to millions of measurements simultaneously, Imaging Sphere technology allows capturing 2π steradians of scattered light in a single measurement, dramatically reducing the time required to obtain a BSDF measurement. These measurements can be readily used for material characterization, quality assessment, and for building libraries of BSDF measurements for computer modeling. This paper describes the measurement principals used in the Imaging Sphere, provides illustrative measurement results for a variety of materials, and briefly the breadth of applications for this data. The Imaging Sphere The imaging sphere was developed specifically to address the need for rapid, accurate and complete characterization of BSDF. The two main optical components of the imaging sphere system are an imaging colorimeter and a hemispherical measurement chamber arranged so that the imaging colorimeter sees the full inner surface of the measurement chamber with the aid of a convex mirror (Figure 1). 2 I Radiant Vision Systems, LLC

3 Figure 1 - Main optical elements of an Imaging Sphere The imaging colorimeter is a CCD camera utilizing calibrated color filters to obtain accurate color images by combining 3 sequential exposures. With proper design and calibration, accurate luminance and color data is obtained for all CCD pixels. Conceptually, this is like a matrix of spot detectors all measuring simultaneously. The Imaging Sphere measurement chamber is a hemisphere with a diffuse, low reflectance coating on its inner surface, a curved secondary mirror placed somewhat off center on its base plate, and a light source that can be moved to various illumination angles. The light source is usually a known white light source with stable illumination properties, but other light sources, including laser sources, have been used as well. The surface of the material to be measured is positioned outside the hemisphere at an aperture at the center of the baseplate and then illuminated. When illuminated from the inside of the hemisphere, both specular and diffuse reflections from the test surface strike the inner surface of the coated hemisphere, which functions as a curved screen capturing essentially all of the returned light. The BSDF measurements taken with the Imaging Sphere can be plotted to show scatter, color coordinates or tristimulus values as function of scatter angle; they can be summarized as 2D and 3D isoplots, histograms and on CIE color charts. The convex mirror acts as a fish-eye lens enabling the camera to image the entire inner surface of the hemisphere at once. Thus, the imaging sphere captures the entire 2π steradians of scattered and reflected light from the surface at this one illumination angle in a single exposure taking just seconds. Multiple image captures through color filters allow color information to be gathered. The angular resolution of the system is determined by the number of pixels on the camera s image sensor, and is typically <0.5 for current systems. The Imaging Sphere measurement described yields BRDF for the sample. By adjusting the illumination so that it is through the material into the measurement chamber, BTDF can also be measured (Figure 2). For a given sample, full BSDF characterization will consist of BRDF and BTDF measurements at different illumination angles, potentially with the material being rotated relative to the illumination plane. All measurement setup, data capture, and data presentation is done via the control software. A typical color BRDF measurement sequence measuring scatter at 5 illumination angles, say 0º, 15 º, 30 º, 45 º, and 60 º, at a specific alignment of the sample to the illumination plane requires approximately 1 minute for a highly reflective material. Somewhat longer exposure times may be required for more optically diffuse or absorptive materials to capture enough light at the CCD sensor. 3 I Radiant Vision Systems, LLC

4 Figure 2 - Imaging Sphere with track for illumining light source to support BRDF and BTDF version. The material sample being measured is placed at the aperture on the face of the Imaging Sphere. Measurement Examples The Imaging Sphere can be readily applied to measure BRDF and BTDF for many different materials (Figures 3, 4). Generally the sample should be placed flat at the aperture, and, if necessary, any surface structure (e.g., grating or grain) on the material should be aligned as desired relative to the illumination plane. A measurement sequence can then consist of a single measurement at any selected angle or a series of measurements at various angles and sample orientations. The BSDF measurements taken with the Imaging Sphere can be plotted to show scatter, color coordinates, or tristimulus values as function of scatter angle; they can be summarized as 2D and 3D iso-plots, histograms, and on CIE color charts. In addition BSDF measurement files from the Imaging Sphere can also be exported for use in analysis or optical design software packages. The Imaging Sphere has been successfully applied to obtain BSDF data for materials as diverse as metals, plastics, paper, wood, painted surfaces, polished surfaces, transparent and translucent films, holographic films, grating structures, and human skin and hair. Radiant Vision Systems IS-SA (Imaging Sphere for Scatter Appearance Measurement) is a high-speed, highlyflexible system for comprehensive BRDF, BTDF and TIS measurement. It is designed for use in both R&D and production quality control applications for material characterization and quality assessment. Figure 3 - BRDF measurement for a grating (the surface of a compact disk) at 45º illumination; true color image and 3D plot of BRDF showing multiple side lob structure of scatter distribution. 4 I Radiant Vision Systems, LLC

5 Figure 4 - BRDF measurement for a paint sample at 45º illumination; the paint appears either purple or green depending on the scatter angle. Shown are a false color plot of BRDF distribution, true color image, and color chart for a cross section along the illumination plane. Applications of Imaging Sphere BSDF Measurements These measurements can be readily used for material characterization, quality assessment, and for building libraries of BSDF measurements for computer modeling. Absolute BSDF measurement values can be used to characterize different materials. Comparative measurements of the BSDF at various illumination angles for two materials or surface treatments are particularly informative in describing the differences in their perceived appearance. Comparison of the BSDF against a standard BSDF or an objective allows the use of this data for quality assurance or production quality control. An unrelated, but intriguing, application is the use of measured (rather than theoretical) BSDF data for optical system modeling and computer rendering. For such applications, the Imaging Sphere can be used to build a BSDF library for modeling. Because the Imaging Sphere allows rapid and accurate measurement of BSDF data, this method allows investigation of scattering phenomena, comparison of materials, and optical modeling of systems much more readily than traditional measurement methods. The breadth of applications is still being discovered. References 1. Rykowski, Kreysar, and Wadman, The Use of an Imaging Sphere for High-Throughput Measurements of Display Performance Technical Challenges and Mathematical Solutions, SID Symposium Digest of Technical Papers, June 2006, pp R. Rykowski, K. Chittim & S. Wadman, Imaging Sphere, Photonics Spectra, September 2005, pp S. Wadman & S. Baumer, Characterisation Prepublication Copy Paper Accepted for Illumination Modeling Workshop at the 2008 Frontiers in Optics Laser Science XXIV, Conference (October 19-23, 2008, Rochester, NY). 5 I Radiant Vision Systems, LLC

6 Radiant Vision Systems Imaging Spheres are based on a novel optical configuration to allow simultaneous imaging of a target device or material from all directions on a hemisphere at once, resulting in faster, more complete and often more accurate measurement than can be achieved with traditional goniometric or conoscopic solutions. Contact us to find out more. Contact Us Today Worldwide Radiant Vision Systems, LLC NE Alder Crest Drive, Suite 100 Redmond, WA USA T F info@radiantvs.com RadiantVisionSystems.com RadiantVisionSystems.com 2015 Radiant Vision Systems, LLC. Radiant Vision Systems, ProMetric and TrueTest are trademarks of Radiant Vision Systems, LLC. All other marks are the property of their respective owners /15