Integrating Spheres in Molecular...

© 2012 Perkin Elmer

Integrating Spheres in Molecular Spectrophotometry

Theory and Practice

© 2012 Perkin Elmer

General Sphere Theory

Integrating Spheres

3

Types of Sphere Measurements

Total Reflectance (Specular + Diffuse)

Diffuse Only Reflectance

Scatter Transmission

Center Mount Absorbance

4

Scatter Transmission Configuration

5

The Center Mount

6

Diffuse (Lambersion) Reflectance Sphere Theory

Radiant Flux From Sphere Wall After N Reflections

One Wall Reflection N Wall Reflections

Specular vs. Diffuse Background Correction Problem

The “Specular” Component “Hot Spot” Problem

Diffuse

Sample

10% Specular

Sample

90% Specular

Sample

The Effect of Sample Behavior on Detector Output

Typical Specular Samples With Spectralon Background Correction

NIST Mirror

Polished Aluminum

Silica Plate

60 mm Sphere

Silica Plate Sample Using Different Materials in Background Correction

Green – Spectralon Background Correction

Blue – NiIST Mirror Background Correction

Red – NIST Mirror %RC Mode

60 mm Sphere

Low Glass Reflectance: Different Materials in Background Correction

60 mm Sphere

Green – Spectralon Background Correction

Blue – NIST Mirror Background Correction

Red – NIST Mirror Background Correction, %RC Mode

Sphere Port Fraction

The port fraction is defined as the ratio of the total port area relative to the total internal surface area of the sphere.

A low port fraction ensures good integration of the sample signal before it reaches the sphere’s detector.

The port fraction of a 150 mm sphere is 2.5 %

A 60 mm sphere has a port fraction of 11.3%.

CIE color recommends lower than 10%

ASTM D1003-95 (haze) lower than 4%

60 mm Sphere Pros & Cons

PROS Sphere Efficiency – Smaller spheres are more efficient collectors

Noise Level – Higher throughput systems, therefore, signal-to-noise is usually better

Sample Beam Size – Smaller sample beam spot size better matches small test samples

Cost – Less expensive

CONS Port Fraction – High port fraction: typically above 10%.

Measurement Accuracy – Sphere errors or hot spots may occur in small spheres: errors may not be completely corrected by a sphere’s baffles due to space constraints

Substitution Errors

Sample Beam Size – Small sample beam size means multiple locations must be measured on inhomogeneous samples.

150 mm Sphere Pros & Cons

PROS Port Fraction – Low port fraction: typically 2-4%. Meets CIE color measurement specifications.

Measurement Accuracy – Highest measurement accuracy is achieved with large integrating spheres since sphere errors can be minimized, resulting in very homogeneous light flux and minimal hot spots in sphere.

Sample Beam Size – Large sample beam size: good coverage of inhomogeneous samples.

CONS Sphere efficiency – Not as efficient as smaller spheres: large sphere diameter attenuates the sample beam energy more than a small sphere of similar design.

Noise Level – Signal-to-noise may be lower for highly absorbing samples (may have to perform scans at larger slit widths, slower scan speeds, or with reference beam attenuation to compensate).

Sample Beam Size – Large sample beam spot size overfills small test samples, requiring masking or small spot kits which lead to additional energy loss.

Cost – More expensive

© 2012 Perkin Elmer

Considerations When Using Spheres

150 mm Sphere: How Low is Zero

20

Red - Black Spectralon

Black – Open Port Green – Light Trap

Light trap is lowest, but not zero due to air scatter inside sphere

60 mm Sphere: How Low is Zero

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Red - Black Spectralon

Black – Open Port Green – Light Trap

Open port is lowest, and almost zero

Lowest Reflectance Sample for Each Sphere Size in %R

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Red – Light Trap, 150 mm Sphere Size

Green – Open Port, 60 mm Sphere Size

60 mm sphere size lower due to shorter diameter of sphere

Low Reflectance Sphere Comparison with Absorbance Analog Scale

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Red – Light Trap, 150 mm Sphere Size

Green – Open Port, 60 mm Sphere Size

Maximum Absorbance Values: 150 mm Sphere vs. Standard Detector

24

Problems Due to Thick Non-Chamfored Sphere Ports

Leads to a lower %R artifact

Problems Due to Recessed Sample Position

Leads to a lower %R artifact

Problems Due to Lateral Diffusion by Translucent Sample

Leads to a lower %R artifact

© 2012 Perkin Elmer

Non-Homogeneous Sample Texture

Asymmetric Sample: Different Positions of a Woven Fabric

Woven Fabric Spectral Sample Set 1: Full Sphere Wavelength Range

Note Wavelength Dependent Variations at Longer Wavelengths

Woven Fabric Spectral Set 1: Detector and Grating Change

Woven Fabric Spectral Sample Set 2: Full Sphere Wavelength Range

Note Fabric Sample Difference From Set 1

© 2012 Perkin Elmer

The UL 270 Large Integrating Sphere

Problems With Sphere Scatter Transmission Measurements of Non–Lambertian Samples

Transmittance Measurement of “Pure” Specular Sample

Transmittance Measurement of a Lambertian Diffuse Sample

Transmittance Measurement of Any Sample

A Non-Lambertian Sample: Pyramid Glass for Solar Cells

150 mm Integrating Sphere: Excellent for Diffuse and Total Reflection

Limitations When Measuring Scatter Transmission of Some Non-Lambertian

Scatter Transmission Of A Non-Lambertian Diffuse Sample

transmitance at 550 nm

0.24

0.25

0.26

0.27

0.28

0.29

0.30

0.31

20 40 60 80 100 120 140 160 180 200

sphere port diameter in mm

As a Function of Sphere Scatter Transmission Port Diameter

Spectrophotometer Beam Through Pyramid Glass

screen

A Non-Lambertian Sample

Problems in Measuring Pattern Glass Samples

• Sphere ports are to small to capture all transmitted or reflected radiation

• Sphere wall uniformity is compromised by ports with different target materials

• Screening is insufficient for diffuse transmission

• The spectrophotometer beam is small compared to surface structures

• The beam size in the NIR is wavelength dependent

• Maximum sample size is too small for tempered glass

An Experiment to Simulate Different Port Diameters

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Simulating a Larger Port Size

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Port Experiment Step 1

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Port Experiment Step 2

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Port Experiment Step 3

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Port Experiment Step 4: Add Spectral Measurements

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The UL 270 Integrating Sphere

Note the large port area

The UL 270 Sphere Design

The UL 270 Can Measure Both Transmission and Reflectance

The UL 270 Diffuse Transmission Mode

The UL 270 Diffuse Reflection Mode

Sphere Energy Comparison

Energy Transmission

Sample date 270 mm Sphere,

sample as is 150 mm Sphere, sample

polished

05/12/2008 91.18 91.08

07/05/2008 91.26 91.03

10/05/2008 91.26 90.77

12/05/2008 90.99 90.89

© 2012 Perkin Elmer

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