Imaging Sphere Measurement of

Luminous Intensity, View Angle, and

Scatter Distribution Functions

Hubert Kostal, Vice President of Sales and Marketing

Radiant Imaging, Inc.

22908 NE Alder Crest Drive, Suite 100

Redmond, WA 98053

kostal@radiantimaging.com

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Overview

• Imaging Sphere Theory – Brief Description

• Luminous Intensity Distribution Measurement

• View Angle Performance Measurement

• BSDF (Scatter) Measurement

• System Function and Assessment vs. Traditional Measurement Methods

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IMAGING SPHERE THEORY

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Imaging Sphere Design and Theory (1)

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Imaging sphere is a hemisphere.

For luminous intensity distribution and view angle performance measurement, the

DUT (light source or display) is placed a geometric center of sphere

Convex mirror placed to one side of DUT acts as a fisheye lens

The imaging photometer/radiometer/colorimeter captures an image of the entire

inner surface of the sphere (2π steradians) in a single measurement

MEASUREMENT

METHOD

Imaging Sphere Design and Theory (2a)

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For BRDF measurement, a probe beam illuminates the material sample placed at the

Imaging Sphere aperture

The angle of incidence of the probe beam can be adjusted from 0 to 180 degrees

The probe beam can be a broadband light source (e.g., metal halide or tungsten

halogen) or a narrowband source (e.g., laser or a narrowband selected by a

monochromator)

ILLUMINATION

METHOD

Imaging Sphere Design and Theory (2b)

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The light scatter off of the material illuminates the interior of the Imaging Sphere dome

The imaging colorimeter captures the light distribution

Using a 1024x1024 pixel resolution, full-frame, imaging colorimeter results in seamless

web of over 750,000 simultaneous luminance and color measurements

Typical measurement time is a few seconds for photopic and about 10 seconds for color

measurement; times will vary based on the material

SCATTER

MEASUREMENT

Anatomy of the Imaging Sphere

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Imaging sphere

500mm diameter dome

with approximately 20%

diffuse reflectance interior

coating

Probe beam

For illumination; can be a wideband or narrowband source.

Imaging Colorimeter:Radiometric response (350-

1100nm)

CIE photopic response V()CIE tristimulus response (XYZ)

Material to be tested is placed at the aperture for BRDF and BTDF measurement

Convex mirror

is imaged by the

colorimeter

Goniometric platformCan be set to any illumination angle 0 to 80

(and 110 to 180 degrees with Transmission

Arm attachment) for BRDF (and BTDF)

measurement

LUMINOUS INTENSITY

DISTRIBUTION MEASUREMENT

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Imaging Sphere Luminous Intensity Measurement

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Measurement example

Green LED

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Measurement example

Defective Blue LED

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IS-LI blue LED scan

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Comparison: SIG and IS-LI Measurements

Source Imaging Goniometer

[near-field] measurement

Imaging sphere [far-field]

measurement

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Additional comparisons show IS accuracy

LXHL-LW5C IS vs SIG Vertical Cross Section

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0 20 40 60 80 100

Inclination (degrees)

No

rmalized

In

ten

sit

y

IS

SIG

LXHL-LW5C IS vs SIG Horizontal Cross Section

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0 20 40 60 80 100

Inclination (degrees)

No

rmalized

In

ten

sit

y

IS

SIG

Luxeon model LXHL-LW5C LED was scanned in a Radiant Imaging SIG300 for reference

purposes. Subsequently, it was measured with the IS-LI. Intensity data was exported from

IS-LI for this measurement for each 1 degree step in inclination and azimuth with a 2 degree

virtual detector. These data were compared with the same information from the SIG 300.

Beam Pattern Width

IS SIG300

FWHM

(degrees) 124 124.5

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VIEW ANGLE PERFORMANCE

MEASUREMENT

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Imaging Sphere View Angle Performance Measurement

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Black State of LDC TV

17” TN Monitor horizontal luminance cross section

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White Luminance (Horiz)

0

50

100

150

200

250

300

-80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80

Goniometer

Conoscope

Imagine Sphere

Goniometer

Conoscope

Imaging Sphere

32 inch SPVA TV horizontal luminance cross-section

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White Luminance (Horiz)

0.0

50.0

100.0

150.0

200.0

250.0

-80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80

Goniomet er

Conoscope

Imagine Sphere

Goniometer

Conoscope

Imaging Sphere

Measurement of 42” 3D display

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SCATTER AND APPEARANCE

MEASUREMENT

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IS-SA: Scatter and Appearance Measurement

Quantitative measurement and characterization of surface appearance, roughness, color, translucence, total reflection and transmission, and bi-directional scatter distribution function (BRDF, BTDF)

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Imaging Sphere Scatter Measurement

A full hemisphere of luminance and color BSDF data is captured in a single measurement

The illuminating angle for BRDF and BTDF measurement can be automatically adjusted (and scanned)

Material sample can be positioned relative to plane of illumination (for measurement of anisotropic materials)

Allows selection of illuminating light source and wavelength

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Total reflectance - the total integrated scatter (TIS)

Scatter as a function of angle - bi-directional scatter distribution

function (BSDF)

BSDF = BRDF + BTDF

(R = Reflectance; T = Transmission)

Imaging Sphere Measurement Example

BRDF of a metal surface as a function of angle

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True Color Image False Color Image

Imaging Sphere Measurement Example

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BRDF of a grating (Compact Disk) as a function of angle

Imaging Sphere Measurement Example

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BRDF of color changing paint

True Color ViewFalse Color View (BRDF)

Imaging Sphere Measurements

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The Imaging Sphere has been successfully used for BSDF measurement of:

Films (BEF, anti-reflection, etc.)

Holographic films

Metals

Plastics

Textiles

Glass

Papers

Painted surfaces

Polished surfaces

Human skin and hair

And more

Example: Rendering Uniform + Environmental Lighting

CONCLUSIONS

Radiant Imaging - Proprietary Information

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Imaging Spheres: Variations on a Theme

Portable Imaging Sphere

Imaging Sphere for

Luminous Intensity Distribution

and View Angle Performance

Measurement

Imaging Sphere for

Scatter and

Appearance

Measurement

… With Transmission

Arm Attachment

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Summary

• The Imaging Sphere is a rapid, efficient way to measure:– Luminous intensity distributions for small light sources

– Local view angle performance for displays

– BRDF, BTDF and TIS for arbitrary materials

• Testing to date has shown excellent accuracy when compared to traditional measurement technologies

• Careful calibrations provide very good absolute accuracy in measurements

• Repeatability allows ready determination of fine scale differences between DUTs / samples

• New spectral measurement capabilities are currently being added to the Imaging Sphere design

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References

• R. Rykowski, D. Kreysar, and S. 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.

• R. Rykowski, K. Chittim & S. Wadman, “Imaging Sphere”, Photonics Spectra, September 2005,pp. 64-68.

• S. Wadman & S. Baumer, “Characterisation of Appearance by a Parousiameter”, Annual Proceedings SPIE, Vol. 48, August 2003.

• R. Rykowski and H. Kostal, “Novel Approach for LED Luminous Intensity Measurement,” SPIE Photonics West, January 2008.

• R. Rykowski and J. Lee, “ Novel Technology for View Angle Performance Measurement,” IMID/IDMC/Asia Display 2008 Digest, October 2008.

• H. Kostal, D. Kreysar, and R. Rykowski, “Application of Imaging Sphere for BSDF Measurements of Arbitrary Materials,” OSA Frontiers in Optics Conference, October 2008.

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Hubert Kostal,

Vice President of Sales and Marketing

Radiant Imaging, Inc.

22908 NE Alder Crest Drive, Suite 100

Redmond, WA 98053

kostal@radiantimaging.com