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Application of Imaging Sphere for BSDF Measurements of Arbitrary Materials

2 I Radiant Vision Systems, LLC

AbstractBSDF 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. ©2008 Optical Society of America OCIS codes: (290.0290) Scattering; (290.1483)

BSDF, BRDF, and BTDF; (120.0120) Instrumentation, measurement, and metrology;

(120.6660) Surface measurements, roughness; (120.5820) Scattering measurements.

IntroductionAccurate 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.

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 SphereThe 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).

Application of Imaging Sphere for BSDF Measurements of Arbitrary Materials

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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.

3 I Radiant Vision Systems, LLC

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 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 set-

up, 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.

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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.

4 I Radiant Vision Systems, LLC

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 ExamplesThe 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.

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.

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Radiant Vision Systems IS-SA (Imaging

Sphere for Scatter Appearance

Measurement) is a high-speed, highly-

flexible 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.

5 I Radiant Vision Systems, LLC

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 MeasurementsThese 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. 101-104.

2. R. Rykowski, K. Chittim & S. Wadman, “Imaging Sphere,” Photonics Spectra,

September 2005, pp. 64-68.

3. 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).

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© 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.770-4009-02 1/15

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.

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