Imaging Sphere Measurement of Luminous Intensity, View Angle, and Scatter Distribution Functions

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1 Imaging Sphere Measurement of Luminous Intensity, View Angle, and Scatter Distribution Functions Hubert Kostal, Vice President of Sales and Marketing Radiant Imaging, Inc NE Alder Crest Drive, Suite 100 Redmond, WA

2 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 2

3 IMAGING SPHERE THEORY 3

4 Imaging Sphere Design and Theory (1) MEASUREMENT METHOD 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 4

5 Imaging Sphere Design and Theory (2a) ILLUMINATION METHOD 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) 5

6 Imaging Sphere Design and Theory (2b) SCATTER MEASUREMENT 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 6

7 Anatomy of the Imaging Sphere Material to be tested is placed at the aperture for BRDF and BTDF measurement Goniometric platform Can be set to any illumination angle 0 to 80 (and 110 to 180 degrees with Transmission Arm attachment) for BRDF (and BTDF) measurement Probe beam For illumination; can be a wideband or narrowband source. Convex mirror is imaged by the colorimeter Imaging sphere 500mm diameter dome with approximately 20% diffuse reflectance interior coating Imaging Colorimeter: Radiometric response ( nm) CIE photopic response V( ) CIE tristimulus response (XYZ) 7

8 LUMINOUS INTENSITY DISTRIBUTION MEASUREMENT 8

9 Imaging Sphere Luminous Intensity Measurement 9

10 Measurement example Green LED 10

11 Measurement example Defective Blue LED 11

12 IS-LI blue LED scan 12

13 Comparison: SIG and IS-LI Measurements Source Imaging Goniometer [near-field] measurement Imaging sphere [far-field] measurement 13

14 Normalized Intensity Normalized Intensity Additional comparisons show IS accuracy 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. LXHL-LW5C IS vs SIG Vertical Cross Section LXHL-LW5C IS vs SIG Horizontal Cross Section IS SIG IS SIG Inclination (degrees) Inclination (degrees) Beam Pattern Width IS SIG300 FWHM (degrees)

15 VIEW ANGLE PERFORMANCE MEASUREMENT 15

16 Imaging Sphere View Angle Performance Measurement Black State of LDC TV 16

17 17 TN Monitor horizontal luminance cross section White Luminance (Horiz) Goniometer Conoscope Imaging Imagine Sphere

18 32 inch SPVA TV horizontal luminance cross-section White Luminance (Horiz) er Goniometer Conoscope Imaging Imagine Sphere Sphere

19 Measurement of 42 3D display 19

20 SCATTER AND APPEARANCE MEASUREMENT 20

21 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) 21 21

22 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 Total reflectance - the total integrated scatter (TIS) Scatter as a function of angle - bidirectional scatter distribution function (BSDF) BSDF = BRDF + BTDF (R = Reflectance; T = Transmission) 22

23 Imaging Sphere Measurement Example BRDF of a metal surface as a function of angle True Color Image False Color Image 23

24 Imaging Sphere Measurement Example BRDF of a grating (Compact Disk) as a function of angle 24

25 Imaging Sphere Measurement Example BRDF of color changing paint False Color View (BRDF) True Color View 25

26 Imaging Sphere Measurements 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 26

27 CONCLUSIONS Radiant Imaging - Proprietary Information 27

28 Imaging Spheres: Variations on a Theme Imaging Sphere for Scatter and Appearance Measurement Portable Imaging Sphere Imaging Sphere for Luminous Intensity Distribution and View Angle Performance Measurement With Transmission Arm Attachment 28 28

29 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 29

30 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 R. Rykowski, K. Chittim & S. Wadman, Imaging Sphere, Photonics Spectra, September 2005,pp S. Wadman & S. Baumer, Characterisation of Appearance by a Parousiameter, Annual Proceedings SPIE, Vol. 48, August R. Rykowski and H. Kostal, Novel Approach for LED Luminous Intensity Measurement, SPIE Photonics West, January R. Rykowski and J. Lee, Novel Technology for View Angle Performance Measurement, IMID/IDMC/Asia Display 2008 Digest, October H. Kostal, D. Kreysar, and R. Rykowski, Application of Imaging Sphere for BSDF Measurements of Arbitrary Materials, OSA Frontiers in Optics Conference, October

31 Hubert Kostal, Vice President of Sales and Marketing Radiant Imaging, Inc NE Alder Crest Drive, Suite 100 Redmond, WA