The Imaging Sphere™

The Imaging Sphere™
The Imaging Sphere™
The First Appearance Meter?
Robert Yeo
Pro-Lite Technology LLP
www.pro-lite.uk.com
[email protected]
+44 (0) 1234 436110
on behalf of
Douglas Kreysar
Radiant Imaging, Inc.
www.radiantimaging.com
[email protected]
+1 425 844-0152
Introduction
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Can an instrument truly measure appearance or is this just a
tabloid headline used simply to grab your attention?
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This paper will review why we need to perform angular light
and colour output measurements and how this impacts on the
measurement of the optical properties of materials, on light
sources and on displays
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Traditional measurement devices will be compared with the
newly developed Imaging Sphere™ from Radiant Imaging
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Results from initial applications studies conducted using the
Imaging Sphere™ will be presented
2
Presentation Outline
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Why do we need angular light intensity & colour
measurements?
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Traditional instrumentation solutions
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New technology - the Imaging Sphere™
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Applications Survey
3
Presentation Outline
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Why do we need angular light intensity & colour
measurements?
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Traditional instrumentation solutions
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New technology - the Imaging Sphere™
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Applications Survey
4
Luminous Intensity
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Luminous intensity is the far-field photometrically-scaled
radiation emitted by a source in a specific direction
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LI measurements quantitatively characterise light source
output
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LI data is useful for optical design purposes
5
View Angle
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View angle measurements describe display appearance
as a function of angle (luminance, colour and contrast
ratio as a function of angle)
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View angle measurements may also include scattered
light as a function of angle
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View angle data enables prediction of display appearance
at various view angles
6
Scatter Analysis
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BRDF (bidirectional reflectance distribution function)
describes light scatter by a surface under specific
illumination conditions
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BTDF (bidirectional transmittance distribution function)
describes light scattered as light passes through a
material
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Enables us to determine how surface characteristics will
affect overall surface appearance under various
illumination conditions
7
What is Appearance?
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The appearance of a material is not just a function of its
colour; as important is the variation of reflectance with
angle
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Knowing both sets of data allows us to quantify the
appearance of an object for different illumination and
viewing conditions
Specular Reflectance
Combination
Diffuse Reflectance
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Scatterometry
z
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The intensity and
angular distribution
of the light
scattered from a
surface tells us
about its surface
profile
Scatter
parameters are
TIS & BRDF
ϕout
ϕ, θin
θout
9
BRDF
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BRDF is the ratio of directional reflected radiance to the
incident irradiance. Units are inverse steradians (sr-1)
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BRDF of a perfect Lambertian surface is 1/π at all points
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Cosine corrected BRDF (CCBRDF) is BRDF x cos(θ)
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Presentation Outline
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Why do we need angular light intensity & colour
measurements?
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Traditional instrumentation solutions
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New technology - the Imaging Sphere™
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Applications Survey
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Traditional Solutions
Luminous Intensity
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Goniophotometer/gonio-spectroradiometer
View Angle
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Goniophotometer/gonio-spectroradiometer
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Conoscope
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Imaging goniometer (e.g. Radiant Imaging flat panel
measurement system)
Scatter
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Gloss meter
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Gonioreflectometer (scatterometer)
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Luminous Intensity - Goniophotometer
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Dual-axis goniometer used to rotate source relative to a
fixed detector
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Uses a spot photometer for luminance or intensity
measurements or a spot colorimeter or
spectroradiometer for luminance and colour
measurements
13
Luminous Intensity - Goniophotometer
z Used for LED measurements
z Advantages
z Colour accuracy (if spectroradiometer)
z Disadvantages
z Speed
z Cost
14
View Angle - Goniophotometer
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Dual-axis goniometer used to rotate FPD relative to the
detector
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Uses a spot photometer for luminance or intensity
measurements or a spot colorimeter or
spectroradiometer for luminance and colour
measurements
15
View Angle – Gonio-spectroradiometer
z Used for FPD measurements
z Advantages
z Colour accuracy (if spectroradiometer)
z Less sensitive to stray light
z Disadvantages
z Cost
z Inconsistent measurement
spot size
z Speed
z Moving parts
16
View Angle - Conoscope
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Utilises exotic (Fourier) optics that map an emitting spot
to a CCD so that each pixel corresponds to a different
emission angle
17
View Angle - Conoscope
z Measures FPD luminance and colour vs. angle
z Advantages
z Speed
z Consistent measurement spot size across all view
angles
z Good angular resolution
z Disadvantages
z Expensive
z Inaccuracy due to stray light
z Limited measurement spot size (~2 mm ) or larger
spot (~ 7mm) with reduced field of view
18
View Angle - Imaging Goniophotometer
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Dual-axis goniometer used to rotate the source relative
to an imaging photometer or colorimeter – all positions
for one angle at a time
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Imaging capabilities provide precise spatial
measurements of luminance and colour
19
View Angle - Imaging Goniophotometer
z Measures FPDs and FPD components
z Advantages
z Measures entire display surface, not just a few spots
z Provides uniformity as a function of view angle
z Provides view angle performance for any number of
separate locations
z Selectable spot size
z Spot size constant with measurement angle
z Gives “infinity” type view perspective
z Disadvantages
z Speed (all locations but only one angle at a time)
20
Scatter - Gloss Meter/Multi-Angle Colorimeter
z Simple gloss meter
z Measures specular reflectance for a single
illumination and collection geometry (e.g. 0/45°)
z Multi-angle Colorimeter
z Measures reflected colour in several directions (e.g.
15°, 25°, 45°, 75°, 110°) for a single illumination
direction (e.g. 45°)
21
Scatter - Gloss Meter/Multi-Angle Colorimeter
z Measures paints, plastics, metals, skin (etc)
z Advantages
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Inexpensive
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Simple
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Fast
z Disadvantages
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Only one illumination direction
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Only measures reflectance in a single direction or limited
number of directions, and only in the same plane as the
illumination
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Cannot provide meaningful characterisation of appearance
for all illumination or viewing conditions
22
Scatter - Gonioreflectometer
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Variously known as a scatterometer or a
gonioreflectometer
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Measures the in-plane BRDF (BTDF) of a sample
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Photodetector normally rotates about the sample
ϕin, ϕout
ϕin, θin
23
Scatter - Gonioreflectometer
z Measures paints, plastics, metals, skin (etc)
z Advantages
z Accuracy
z Resolution
z Disadvantages
z Speed – very, very slow; must measure reflected
radiance sequentially at one angle of elevation and at
one azimuth angle for each angle of illumination
z Cost
z Complexity
24
Instrumentation Summary
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There has not been an instrument for either luminous
intensity, colour, view angle or BRDF measurements that
offers speed, angular resolution and low cost – until now
25
Presentation Outline
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Why do we need angular light intensity & colour
measurements?
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Traditional instrumentation solutions
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New technology - the Imaging Sphere™
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Applications Survey
26
In the Beginning
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1996: technology developed and
patented by Philips Applied
Technologies (Eindhoven, The
Netherlands)
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Philips called this the
Parousiameter (after the Greek
word for “appearance”)
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Originally developed to address the
mismatch of appearance of the
cabinet, front piece, stand and VCR
recorder of Philips “Matchline” TV
sets
Image © Philips
27
Genesis of the Parousiameter
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These used plastic parts with metallic finishes
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Made at different factories worldwide
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Parts met the specified colour point and gloss level but
didn’t match
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Parousiameter showed that this was due to the
directionality of the anisotropy of the reflections when
viewing in the direction of the short axis on one part and
the long axis on the adjacent part
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This caused a brightness difference in one direction of
view, that could flip over in another viewing direction
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Imaging Sphere™ Latest Developments
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Philips sought a partner to commercialise its invention
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Radiant Imaging was chosen for its expertise and
leadership in CCD-based light and colour instrumentation
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Radiant Imaging and Philips sign a cross-licence
agreement in February 2005
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April 2005: Radiant receives its first order for an
Imaging Sphere™
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October 2005: First commercial Imaging Sphere™
delivered
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June 2006: series production begins
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July 2006: first deliveries of production-specification
units begins
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An Imaging Sphere™ Comprises
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A 508mm (20”) diameter dome with an approx. 20%
diffuse reflectance interior coating
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Radiant Imaging ProMetric™ CCD camera with either:
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Radiometric response (350-1100nm)
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CIE photopic response V(λ)
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CIE tristimulus response (XYZ)
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Imaging Sphere™ software (& PC)
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Probe beam(s) - for passive devices
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What Does an Imaging Sphere™ Look Like?
The original Philips
“Parousiameter” with its
inventor, Sipke Wadman
The Production
Imaging Sphere™
Image © Philips
31
Imaging Sphere Operation
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Hemisphere coated with a diffuse, ~ 20% reflectance
coating
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Imaging photometer/radiometer/colorimeter with optics
that enable entire inner surface of hemisphere to be
viewed
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Imaging Sphere™ Operation
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Device Under Test (DUT) is placed at geometric center of
sphere – can be emissive or passive
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Light emitted, reflected from or transmitted through the
DUT strikes the inner surface of the sphere
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Sphere coating produces diffuse reflection with minimal
multiple reflections
33
Imaging Sphere™ Operation
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The imaging photometer/radiometer/colorimeter
captures an image of the entire inner surface of the
sphere (2π steradians) in an instant
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Convex mirror placed to one side of DUT acts as a
fisheye lens to enable this
34
The Three Configurations
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Far-field (luminous intensity) for emissive devices (e.g.
LEDs)
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Near-field (luminance) for emissive devices (e.g. LCDs)
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Far-field intensity for passive devices
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All three versions can be equipped for radiometric,
photometric or colorimetric measurements (or any
combination thereof)
35
IS-LI (Luminous Intensity)
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LED luminous intensity distribution
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Small light source luminous intensity
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Production-line testing of LEDs
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IS-LI (Luminous Intensity)
Benefits
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Speed
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Angular resolution
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Cost
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Reliability (no moving parts!)
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IS-VA (View Angle)
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LCD, PDP, OLED, Backlights, etc.
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Brightness vs. Angle
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Colour vs. Angle
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Contrast Ratio vs. Angle
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IS-VA (View Angle)
Benefits
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Rapid
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Economical
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Angular resolution
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Large spot size possible (up to 40mm)
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Not sensitive to stray light
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Consistent spot size over all angles
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IS-SA (Scatter Analysis)
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Uses a probe beam to illuminate the material at various
incident angles
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Analyse scattered light from the surface
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Can be used in both transmission and reflection
40
IS-SA Alternative Illumination Schemes
Single Lamp, Variable Angle*
Multiple Lamps, Fixed Angles
* Single lamp, variable angle scheme has been
chosen as standard configuration for production sphere
41
IS-SA (Scatter Analysis)
Benefits
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Speed
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Cost
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Can be used in ambient light conditions
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Characterisation of reflectance for all viewing and
illumination directions
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Modularity
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All three IS embodiments use the same basic hardware
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This enables a single instrument, along with various
hardware and software modules, to be used for multiple
measurement types
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This offers the ultimate in cost effectiveness
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Performance Parameters
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Angular range:
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Azimuth: 0 -360°
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Inclination: 0 - 85°
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IS-LI (luminous intensity): 0.1 – 107 cd
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IS-VA (luminance): 1.0 – 4.0 x 107 cdm-2
z
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Aperture (max. DUT) size: 40mm (on 508 mm dome)
IS-SA (min. reflectance): 1.0%
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Measurement spot size: 2-10 mm
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Dynamic range: 10-14 bits (1,024 – 16,384 grey scale levels)
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Min. measurement time for 512 x 512 pixel ProMetric CCD:
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1 sec (photopic)
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5 sec (colour)
All performance specifications are subject to change without notice
44
Angular Resolution
z The two limiting factors are:
z CCD pixel resolution – with 512 x
512 pixels (on a full frame CCD),
field of view of each pixel is 0.35°
z Size of DUT:
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With a 4mm aperture, the angular
uncertainty θ is calculated per
arc tan (4/508) = ± 0.45°
z
10mm aperture: ± 1.13°
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40mm aperture: ± 4.50°
θ
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Validation Data
Colour (Cx)
0.4
0.35
0.3
Luminance
0.25
Series1
0.2
Series2
1.8
0.15
1.6
0.1
1.4
0.05
1.2
0
-100
-80
-60
-40
-20
0
1
20
40
60
80
Series1
100
Series2
0.8
White LED, Imaging
Sphere™ versus
goniometer
0.6
0.4
0.2
0
0
20
40
60
80
100
120
140
160
180
200
46
Software
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Imaging Sphere™ software enables instrument
calibration, data acquisition, and a diverse range of data
analysis capabilities
True Color
Luminous Intensity
CIE Coordinates
47
Software
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Imaging Sphere™ software enables instrument
calibration, data acquisition, and a diverse range of data
analysis capabilities
Cross Sections
3D Plots
48
Software
z
Imaging Sphere™ software enables instrument
calibration, data acquisition, and a diverse range of data
analysis capabilities
z True Colour
z Luminous Intensity, Luminance
z CIE Coordinates (x, y; u’, v’; L*a*b*)
z Tristimulus values (XYZ)
z Correlated Colour Temperature (CCT)
z View angle
z BRDF and CCBRDF
49
Presentation Outline
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Why do we need angular light intensity & colour
measurements?
z
Traditional instrumentation solutions
z
New technology - the Imaging Sphere™
z
Applications survey
50
LEDs
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True color
view shows
little difference
between units
51
LEDs
z
A 3D plot of
luminous
intensity
displays the
subtle
differences
52
LEDs
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The Imaging Sphere™ can be used to make quantitative
LED measurements in production
53
LEDs
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This is the same LED rendered in 3D
54
LEDs
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This is an LED rendered in 3D and as an isometric plot –
it is clearly defective
55
Brightness Enhancing Films
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Performance of FPD
brightness enhancing
films can be tested
using the Imaging
Sphere™
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Measurements are
performed with a
diffuse backlight
illuminator
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Detailed output
structure can be
visualised
True Colour
56
Brightness Enhancing Films
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This can be
enhanced in a
false colour plot
of the luminous
intensity
Luminous Intensity
57
Brightness Enhancing Films
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Quantitative performance measurements can be
obtained
True Colour
58
Brightness Enhancing Films
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Colour/luminance
difference can be
plotted
Colour/Luminance Difference
59
BEF – Quantifying the Difference
Backlight – 1 BEF Layer
No BEF
Backlight – 2 BEF Layers
2 BEF Layers
60
Surface Analysis
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The ability to measure BRDF enables quantitative
analysis of surface appearance under a variety of
illumination conditions.
z
This can be utilised with plastics, metals, glass, painted
surfaces and even human skin (etc)
61
Surface Analysis
Decorative Foil
Hologram on €10 Bank Note
62
Special Effect Finishes
Images ©
www.mg-rover.org
63
Helicone Paint 0° Illumination
Cross Sectional Plot at 45° Azimuth Angle (u’,v’)
64
Helicone Paint 15° Illumination
Cross Sectional Plot at 45° Azimuth Angle (u’,v’)
65
Helicone Paint 30° Illumination
Cross Sectional Plot at 45° Azimuth Angle (u’,v’)
66
Helicone Paint 45° Illumination
Cross Sectional Plot at 45° Azimuth Angle (u’,v’)
67
Paper
Plain Paper
Glossy Paper
True Colour
CCBRDF
68
Metallic Surfaces
Un-brushed Aluminum
Parallel Orientation
CCBRDF
Un-brushed Aluminum
Perpendicular Orientation
CCBRDF
69
Metallic Surfaces
Brushed Aluminum
Parallel Orientation
False Colour
Brushed Aluminum
Offset Orientation
False Colour
70
Metallic Surfaces
Un-brushed Aluminum
Perpendicular Orientation
CCBRDF
Brushed Aluminum
Perpendicular Orientation
CCBRDF
71
Metallic Surfaces
Un-brushed Aluminum
CCBRDF
Brushed Aluminum
CCBRDF
72
Skin
Caucasian Male Cheek
73
Skin
Caucasian Male Cheek
74
Skin
Asian Male Cheek
75
Skin
Asian Male Cheek
76
Skin
True Colour - Forehead
CCBRDF - Forehead
77
Summary
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The Imaging Sphere™ enables fast measurement of
luminous intensity, view angle performance and scatter
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It can be used to quantify appearance
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The same basic set of hardware can perform all the
measurements providing maximum flexibility
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The Imaging Sphere™ is a cost effective tool for both
R&D, process development and statistical process control
78
References
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R. Rykowski, D. Kreysar & S. Wadman, “The Use of an
Imaging Sphere for High-Throughput Measurements of
Display Performance – Technical Challenges and
Mathematical Solutions”, SID 06 Digest, pp. 101-104
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R. Rykowski, K. Chittim & S. Wadman, “Imaging Sphere”,
Photonics Spectra, September 2005,
pp. 64-68
z
S. Wadman & S. Baumer, “Characterisation of Appearance
by a Parousiameter”, Annual Proceedings SPIE, Vol. 48,
August 2003
79
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