Analysis of Spectrophotometer

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• Analysis of Spectrophotometer
• Specular Performance Using
• Goniophotometric Information
• David R. Wyble
• Munsell Color Science Laboratory
• Rochester Institute of Technology
• wyble_at_cis.rit.edu

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Introduction

• All integrating sphere spectrophotometers are not
  created equal
• Standards allow a wide range of conforming
  devices
• Sample gloss and specular port configuration can
  significantly affect measurements

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A Pathological Case
Reflectance factor
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A more expected example
Reflectance factor
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Purpose

• Determine a relationship between the size of the
  specular port and the effective performance of
  the spectrophotometer in SPEX mode
• Create a method to correct measurements to allow
  comparison of results from instruments of
  differing configurations

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Outline

• CIE 15.2 Colorimetry
• Theory on Effective Specular Port Size
• Samples and Goniometric Measurements
• Results Conclusions

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CIE 15.2 Colorimetry
1.4 Illuminating and viewing conditions for
reflecting specimens c) Diffuse/normal (symbol
d/0) The specimen is illuminated diffusely by an
integrating sphere. The angle between the normal
to the specimen and the axis of the viewing beam
should not exceed 10. The integrating sphere may
be of any diameter provided the total area of the
ports does not exceed 10 of the internal
reflecting sphere area. The angle between the
axis and any ray of the viewing beam should not
exceed 5. c) Diffuse/normal (symbol
d/0) (Similar angular specifications)
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Reflectance d/8 SPEX
Specular cap black cap excludes
Detector
Incident light
Baffle
Sample
8 viewing, diffuse illumination, SPIN
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Reflectance 8/d SPEX
Incident light
Detector
Baffle
Sample
Diffuse viewing, 8 illumination, SPEX
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CIE 15.2 Colorimetry
1.4 Illuminating and viewing conditions for
reflecting specimens c) Diffuse/normal (symbol
d/0) The specimen is illuminated diffusely by an
integrating sphere. The angle between the normal
to the specimen and the axis of the viewing beam
should not exceed 10. The integrating sphere may
be of any diameter provided the total area of the
ports does not exceed 10 of the internal
reflecting sphere area. The angle between the
axis and any ray of the viewing beam should not
exceed 5.
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CIE 15.2 Colorimetry
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CIE 15.2 Colorimetry
1.4 Illuminating and viewing conditions for
reflecting specimens c) Diffuse/normal (symbol
d/0) The specimen is illuminated diffusely by an
integrating sphere. The angle between the normal
to the specimen and the axis of the viewing beam
should not exceed 10. The integrating sphere may
be of any diameter provided the total area of the
ports does not exceed 10 of the internal
reflecting sphere area. The angle between the
axis and any ray of the viewing beam should not
exceed 5.
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CIE 15.2 Colorimetry
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CIE 15.2 Colorimetry
1.4 Illuminating and viewing conditions for
reflecting specimens Note 1 For the conditions
diffuse/normal and normal diffuse the
regularly reflected component of specimens with
mixed reflection may be excluded with a gloss
trap. If a gloss trap is used, details of its
size, shape, and position should be given,
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CIE 15.2 Colorimetry
By inference, the angle from the normal to the
gloss trap will not exceed 10.
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CIE 15.2 Colorimetry
No guidance on angular size of gloss trap.
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CIE 15.2 Colorimetry

• All we are told about the specular port is to
  report the configuration used
• Still a range of configurations that meet the
  specification

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Specular Port Size
As port size grows
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Specular Port Size
more of the specular information
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Specular Port Size
is lost
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Specular Port Size
but how much?
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Where do we go now?
We need to know the details of how our
instruments handle this component of the
reflected light.
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Where do we go now?
To do this, we first need to accurately
characterize a set of samples, by measuring their
reflectance characteristics as a function of
angle.
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MCSL Goniometer
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MCSL Goniometer Technical description

• Light source
• 100 cm sphere
• 2 interior 19.6V GE bulbs
• Lamp current monitored and manually maintained at
  6.00A
• IR filter
• Collimation lens
• Detector
• Photoresearch PR704 spectroradiometer
• Aperture 3
• Measurement units are integrated radiance

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MCSL Goniometer Technical description

• Sample and detector angles independently
  adjustable within physical constraints
• Vernier scales allow repeatable angle settings
  to 0.5

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MCSL Goniometer
q
Detector
0
Sample
-q
Incident light
Physical constraints limit measurement angles to
-8 to 75, always referenced to the specular
angle.
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Theory

• Measure spectral reflectance using SPIN and SPEX
  modes
• Calculate the average difference between SPIN and
  SPEX data, in percent reflectance
• Determine the effective size of the specular port
  that would account for the above difference

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SPIN and SPEX
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Theory

• Measure spectral reflectance using SPIN and SPEX
  modes
• Calculate the average difference between SPIN and
  SPEX data, in percent reflectance
• Determine the effective size of the specular port
  that would account for the above difference

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Average Spectral Difference
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Theory

• Measure spectral reflectance using SPIN and SPEX
  modes
• Calculate the average difference between SPIN and
  SPEX data, in percent reflectance
• Determine the effective size of the specular port
  that would account for the above difference

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Effective Specular Port Calculation

• Measure radiance vs angle on goniometer
• Calculate cumulative volume as a function of
  radius
• Determine the radius that results in the percent
  cumulative volume that matches the SPIN-SPEX
  difference

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Radiance
Radiance

• Detection angle

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Radiance vs 2D Detection Angle
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Effective Specular Port Diameter

• Measure radiance vs angle on goniometer
• Calculate cumulative volume as a function of
  radius. Normalize this volume to the average
  reflectance data for the sample.
• Determine the radius that results in the percent
  cumulative volume that matches the SPIN-SPEX
  difference

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Cumulative Volume vs Radius
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Effective Specular Port Calculation

• Measure radiance vs angle on goniometer
• Calculate cumulative volume as a function of
  radius
• Determine the radius that results in the percent
  cumulative volume that matches the SPIN-SPEX
  difference

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Effective Specular Port Calculation
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Effective Specular Port Calculation
reff
volume inside that radius
current radius
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Effective Specular Port Calculation

• effective radius equation

Search through radii until we match the average
SPIN-SPEX spectral difference
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Searching
radiance
Detection angle
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Searching
radiance
Detection angle
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Searching
radiance
Detection angle
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Searching
radiance
Detection angle
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Searching
radiance
Detection angle
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Found
SPIN-SPEX
SPIN total
radiance
Detection angle
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ExperimentalSpectrophotometers

• Four commercial-grade benchtop devices
• Datacolor Spectraflash 600
• Macbeth Coloreye 7000
• BYK-Gardner The Color Sphere
• Minolta 3600-d
• All are d/0 devices
• All have reasonable calibration status

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ExperimentalSpectrophotometers

• Specifically chosen for their range of specular
  port configurations, from lt1 to 2
• One (Minolta) has a detector in place of the
  specular port. SPIN and SPEX are calculated using
  the signal gathered by that detector

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ExperimentalSamples

• Two different automotive plastic samples (gray,
  tan)
• Three levels of gloss
• Total of five samples, two highly glossy, and
  three at two levels of matte surface
• Underlying color identical, since various gloss
  levels are stamped in the same piece of plastic

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ExperimentalSamples
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ExperimentalSample Set
Color Description
gray Glossy Smooth Matte Rough Matte
tan Glossy Smooth Matte
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ExperimentalSamples
Smooth matte
Rough matte
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For ReferenceMeasured Gloss
Color Level 20 60 85
gray glossy smooth matte rough matte 49.2 0.7 0.5 65.6 4.7 3.5 94.3 22.3 6.5
tan glossy smooth matte 56.2 0.6 69.7 4.5 94.1 20.1
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Goniometric Resultsmeasured radiance
Note alternate ordinate axis for glossy samples.
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Experimental ResultsCumulative volume
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Experimental ResultsCumulative volume
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Experimental ResultsEffective Specular Port
Size ()
Datacolor SF600 Macbeth CE7000 BYK-Gardner TCS Minolta 3600-d
Gray Glossy 1.9 1.9 2.0 2.0
Gray Smooth Matte 7.6 4.2 3.2 5.1
Gray Rough Matte 8.3 5.0 3.9 4.7
Tan Glossy 3.7 3.2 3.3 3.3
Tan Smooth Matte 6.2 4.8 5.8 7.5
Actual 4.7 3.0 3.6
Minolta 3600-d has specular sensor
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Experimental ResultsEffective Specular Port
Size ()
TCS
CE7000
SF6000
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Experimental ResultsEffective Specular Port
Size ()
Minolta data at arbitrary port width
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Assumptions and Limitationsor Opportunities

• Specular port vs sphere wall
• Specular port uniformity
• More comprehensive sample set
• Viewing/illumination beam
• Relationship to Gonio collimation

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Conclusions

• Overall trend among spectrophotometers agrees
  with the physical measure
• Ability to compare traditional designs to those
  with electronic port detection
• May aid in device selection to best accommodate
  the application
• Goal of inter-instrument correction not realized

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Acknowledgements

• This work was supported by the
• Munsell Color Science Laboratory
• Special thanks are due to
• Danny Rich
• Mark Fairchild
• Roy Berns
• Mitch Rosen
• who all helped tremendously with many fruitful
  discussions and emails.

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• Thats all.Thanks for listening!