Transmittance Measurement Presented by Dr. Richard Young VP of Marketing & Science Optronic Laboratories, Inc

Optronic Laboratories, Inc.Optronic Laboratories, Inc.

Transmittance Transmittance MeasurementMeasurementTransmittance Transmittance MeasurementMeasurement

Presented byPresented by

Dr. Richard YoungDr. Richard YoungVP of Marketing & ScienceVP of Marketing & Science


Presented byPresented by

Dr. Richard YoungDr. Richard YoungVP of Marketing & ScienceVP of Marketing & Science



Outline of PresentationOutline of PresentationOutline of PresentationOutline of Presentation

Types of transmittance:Types of transmittance: RegularRegular DiffuseDiffuse

Factors affecting measurements:Factors affecting measurements: Regular TransmittanceRegular Transmittance

• Beam geometry• Reflections

Diffuse TransmittanceDiffuse Transmittance• Beam geometry• Reflections

Types of transmittance:Types of transmittance: RegularRegular DiffuseDiffuse

Factors affecting measurements:Factors affecting measurements: Regular TransmittanceRegular Transmittance

• Beam geometry• Reflections

Diffuse TransmittanceDiffuse Transmittance• Beam geometry• Reflections


Types of TransmittanceTypes of TransmittanceTypes of TransmittanceTypes of Transmittance

DetectorDetector

Consider a parallel Consider a parallel beam of light.beam of light.

Consider a parallel Consider a parallel beam of light.beam of light.

It hits the detector, It hits the detector, producing signal producing signal ss. Now . Now we put a sample in the we put a sample in the

beam.beam.

It hits the detector, It hits the detector, producing signal producing signal ss. Now . Now we put a sample in the we put a sample in the

beam.beam.

And this changes the And this changes the amount of light hitting amount of light hitting the detector. If the new the detector. If the new

detector signal is detector signal is s’s’::

And this changes the And this changes the amount of light hitting amount of light hitting the detector. If the new the detector. If the new

detector signal is detector signal is s’s’::

s

s'nceTransmitta

s

s'nceTransmitta

This is called This is called

regular regular transmittance.transmittance.


regular regular transmittance.transmittance.


Now if we take the same Now if we take the same parallel beam of light...parallel beam of light...

Now if we take the same Now if we take the same parallel beam of light...parallel beam of light...

But when we put a But when we put a sample in the beam, it sample in the beam, it scatters the light in all scatters the light in all

directions.directions.

But when we put a But when we put a sample in the beam, it sample in the beam, it scatters the light in all scatters the light in all

directions.directions.


diffuse diffuse transmittance.transmittance.


diffuse diffuse transmittance.transmittance.


DetectorDetector

s

s'nceTransmitta

s

s'nceTransmitta

Now the detector signal, Now the detector signal, s’s’, does not represent , does not represent

all the transmitted light.all the transmitted light.

Now the detector signal, Now the detector signal, s’s’, does not represent , does not represent

all the transmitted light.all the transmitted light.



So if we want to So if we want to measure diffuse measure diffuse

transmittance, we need transmittance, we need to measure at all to measure at all

angles...angles...

So if we want to So if we want to measure diffuse measure diffuse

transmittance, we need transmittance, we need to measure at all to measure at all

angles...angles...

DetectorDetectorDetectorDetectorDetectorDetectorDetectorDetector

s

dds

2

0

2

0

..,'

nceTransmitta

s

dds

2

0

2

0

..,'

nceTransmitta



...or have a detector that ...or have a detector that collects all the light collects all the light regardless of angle.regardless of angle.

...or have a detector that ...or have a detector that collects all the light collects all the light regardless of angle.regardless of angle.

DetectorDetector


Types of TransmittanceTypes of TransmittanceTypes of TransmittanceTypes of Transmittance Regular and diffuse transmittance represent Regular and diffuse transmittance represent

the extremes of material properties.the extremes of material properties. Real samples show some elements of both Real samples show some elements of both

behaviours.behaviours. Generally, Generally,

accessories for regular transmittance can accessories for regular transmittance can ONLY measure regular transmittance. ONLY measure regular transmittance.

accessories for diffuse transmittance can accessories for diffuse transmittance can measure regular transmittance as well.measure regular transmittance as well.

Regular and diffuse transmittance represent Regular and diffuse transmittance represent the extremes of material properties.the extremes of material properties.

Real samples show some elements of both Real samples show some elements of both behaviours.behaviours.

Generally, Generally, accessories for regular transmittance can accessories for regular transmittance can

ONLY measure regular transmittance. ONLY measure regular transmittance. accessories for diffuse transmittance can accessories for diffuse transmittance can

measure regular transmittance as well.measure regular transmittance as well.



However, there are limitations to both However, there are limitations to both types of accessories caused by the types of accessories caused by the interaction of the sample with the interaction of the sample with the accessory.accessory.

Most often, no sample is present during Most often, no sample is present during calibration.calibration. But air and samples have very different But air and samples have very different

properties.properties.• Reflection and refraction by the sample,

which are not there during calibration, can introduce errors in measurement.

However, there are limitations to both However, there are limitations to both types of accessories caused by the types of accessories caused by the interaction of the sample with the interaction of the sample with the accessory.accessory.

Most often, no sample is present during Most often, no sample is present during calibration.calibration. But air and samples have very different But air and samples have very different

properties.properties.• Reflection and refraction by the sample,

which are not there during calibration, can introduce errors in measurement.


Remember we said a Remember we said a parallelparallel beam of light. beam of light.Remember we said a Remember we said a parallelparallel beam of light. beam of light.

Parallel is such an exact Parallel is such an exact term. What if it was term. What if it was

converging or converging or diverging?diverging?

Parallel is such an exact Parallel is such an exact term. What if it was term. What if it was

converging or converging or diverging?diverging?

Regular TransmittanceRegular TransmittanceBeam GeometryBeam Geometry


DetectorDetector


Here the beam over-fills Here the beam over-fills the detector. When we the detector. When we put the sample in the put the sample in the

beam…beam…

Here the beam over-fills Here the beam over-fills the detector. When we the detector. When we put the sample in the put the sample in the

beam…beam…



...the beam is refracted ...the beam is refracted inward, causing a slight inward, causing a slight focussing effect on the focussing effect on the

detector.detector.

...the beam is refracted ...the beam is refracted inward, causing a slight inward, causing a slight focussing effect on the focussing effect on the

detector.detector.

The increase in detector The increase in detector signal gives a higher signal gives a higher transmittance value transmittance value

than it should.than it should.

The increase in detector The increase in detector signal gives a higher signal gives a higher transmittance value transmittance value

than it should.than it should.

In extreme cases, this In extreme cases, this can lead to values of can lead to values of

more than 100%, which more than 100%, which is impossible.is impossible.

In extreme cases, this In extreme cases, this can lead to values of can lead to values of

more than 100%, which more than 100%, which is impossible.is impossible.

DetectorDetector




So if the beam over-fills the detector, a So if the beam over-fills the detector, a diverging beam can lead to too high diverging beam can lead to too high transmittance values.transmittance values.

By the same reasoning, converging beams By the same reasoning, converging beams can give too low values.can give too low values.

This means if non-parallel beams are This means if non-parallel beams are used, it is important that BOTH the used, it is important that BOTH the calibration and sample measurements are calibration and sample measurements are done with the detector under-filled.done with the detector under-filled.

The under-filled detector must also The under-filled detector must also respond uniformly.respond uniformly.

So if the beam over-fills the detector, a So if the beam over-fills the detector, a diverging beam can lead to too high diverging beam can lead to too high transmittance values.transmittance values.

By the same reasoning, converging beams By the same reasoning, converging beams can give too low values.can give too low values.

This means if non-parallel beams are This means if non-parallel beams are used, it is important that BOTH the used, it is important that BOTH the calibration and sample measurements are calibration and sample measurements are done with the detector under-filled.done with the detector under-filled.

The under-filled detector must also The under-filled detector must also respond uniformly.respond uniformly.




In addition to the sample altering the beam In addition to the sample altering the beam geometry, a converging or diverging beam geometry, a converging or diverging beam will include light that enters the sample at will include light that enters the sample at an angle.an angle. This means different parts of the beam This means different parts of the beam

travel different distances through the travel different distances through the sample.sample.

So the transmittance is an average of So the transmittance is an average of several path lengths.several path lengths.

Different beam geometries can Different beam geometries can therefore give different transmittance therefore give different transmittance values.values.

In addition to the sample altering the beam In addition to the sample altering the beam geometry, a converging or diverging beam geometry, a converging or diverging beam will include light that enters the sample at will include light that enters the sample at an angle.an angle. This means different parts of the beam This means different parts of the beam

travel different distances through the travel different distances through the sample.sample.

So the transmittance is an average of So the transmittance is an average of several path lengths.several path lengths.

Different beam geometries can Different beam geometries can therefore give different transmittance therefore give different transmittance values.values.


Regular Transmittance Regular Transmittance Beam GeometryBeam Geometry


A lens focused on an A lens focused on an aperture is a common aperture is a common

way of producing a way of producing a collimated (parallel) collimated (parallel)

beam.beam.

A lens focused on an A lens focused on an aperture is a common aperture is a common

way of producing a way of producing a collimated (parallel) collimated (parallel)

beam.beam.

But light from But light from different parts of the different parts of the

aperture are not aperture are not parallel to each other.parallel to each other.

But light from But light from different parts of the different parts of the

aperture are not aperture are not parallel to each other.parallel to each other.

ApertureApertureApertureApertureLensLensLensLens

Half-AngleHalf-AngleHalf-AngleHalf-Angle




The simplest formula for calculating the The simplest formula for calculating the half-angle half-angle (degree of collimation) is (degree of collimation) is where where rr is the radius of the aperture is the radius of the aperture

and and ff is the lens focal length. is the lens focal length.

The simplest formula for calculating the The simplest formula for calculating the half-angle half-angle (degree of collimation) is (degree of collimation) is where where rr is the radius of the aperture is the radius of the aperture

and and ff is the lens focal length. is the lens focal length.

Half-AngleHalf-AngleHalf-AngleHalf-Angle

f

r1tan

f

r Radians,

For f >> r

Radians,

For f >> r


Unfortunately, Unfortunately, collimation is collimation is

achieved at only one achieved at only one wavelength.wavelength.

Unfortunately, Unfortunately, collimation is collimation is

achieved at only one achieved at only one wavelength.wavelength.

Since the refractive Since the refractive index changes with index changes with

wavelength, so does the wavelength, so does the focal length. At long focal length. At long

wavelengths the beam wavelengths the beam diverges and at short diverges and at short

wavelengths it wavelengths it converges.converges.

Since the refractive Since the refractive index changes with index changes with

wavelength, so does the wavelength, so does the focal length. At long focal length. At long

wavelengths the beam wavelengths the beam diverges and at short diverges and at short

wavelengths it wavelengths it converges.converges.



Note: This does not happen in Note: This does not happen in all-mirror collimators. Lens all-mirror collimators. Lens

collimators need to be optimized collimators need to be optimized for the wavelengths of interest.for the wavelengths of interest.

Note: This does not happen in Note: This does not happen in all-mirror collimators. Lens all-mirror collimators. Lens

collimators need to be optimized collimators need to be optimized for the wavelengths of interest.for the wavelengths of interest.


Let us go back to the Let us go back to the parallel beam.parallel beam.

Let us go back to the Let us go back to the parallel beam.parallel beam.

Reflections from the Reflections from the detector can hit the sample detector can hit the sample again and be reflected back again and be reflected back

to the detector.to the detector.

Reflections from the Reflections from the detector can hit the sample detector can hit the sample again and be reflected back again and be reflected back

to the detector.to the detector.

Regular TransmittanceRegular TransmittanceReflectionsReflections


DetectorDetector

The detector may reflect The detector may reflect 50% or more. The sample, 50% or more. The sample, perhaps 8%. This gives an perhaps 8%. This gives an error in transmittance of error in transmittance of

4%.4%.

The detector may reflect The detector may reflect 50% or more. The sample, 50% or more. The sample, perhaps 8%. This gives an perhaps 8%. This gives an error in transmittance of error in transmittance of

4%.4%.


By setting the detector By setting the detector at an angle, this at an angle, this

unwanted reflection can unwanted reflection can be eliminated.be eliminated.

By setting the detector By setting the detector at an angle, this at an angle, this

unwanted reflection can unwanted reflection can be eliminated.be eliminated.

Regular TransmittanceRegular Transmittance ReflectionsReflections

Regular TransmittanceRegular Transmittance ReflectionsReflections

DetectorDetector


Reflections also apply Reflections also apply when the parallel beam when the parallel beam

is focussed at the is focussed at the detector.detector.

Reflections also apply Reflections also apply when the parallel beam when the parallel beam

is focussed at the is focussed at the detector.detector.



DetectorDetector

There are two ways to There are two ways to reduce reflection reduce reflection

effects, and both may effects, and both may be applied together.be applied together.

There are two ways to There are two ways to reduce reflection reduce reflection

effects, and both may effects, and both may be applied together.be applied together.


One way is to rotate the One way is to rotate the detector, as with the detector, as with the

parallel beam.parallel beam.

One way is to rotate the One way is to rotate the detector, as with the detector, as with the

parallel beam.parallel beam.



The reflected beam The reflected beam then misses the then misses the

detector.detector.

The reflected beam The reflected beam then misses the then misses the

detector.detector.

DetectorDetector


The other way is to move The other way is to move the detector back from the the detector back from the focus, and use an aperture focus, and use an aperture

to mask reflections.to mask reflections.

The other way is to move The other way is to move the detector back from the the detector back from the focus, and use an aperture focus, and use an aperture

to mask reflections.to mask reflections.



Most of the reflections Most of the reflections from the detector are from the detector are

blocked and do not get blocked and do not get back to the sample.back to the sample.

Most of the reflections Most of the reflections from the detector are from the detector are

blocked and do not get blocked and do not get back to the sample.back to the sample.

DetectorDetector


Diffuse Transmission Diffuse Transmission Beam GeometryBeam Geometry


For diffuse transmittance, For diffuse transmittance, light is scattered at all light is scattered at all

angles. We need a angles. We need a collection optic that collection optic that

responds equally responds equally regardless of angle – an regardless of angle – an

integrating sphereintegrating sphere

For diffuse transmittance, For diffuse transmittance, light is scattered at all light is scattered at all

angles. We need a angles. We need a collection optic that collection optic that

responds equally responds equally regardless of angle – an regardless of angle – an

integrating sphereintegrating sphere

When the beam of light When the beam of light (without the sample) hits (without the sample) hits the sphere it is scattered the sphere it is scattered

many times within the many times within the sphere. It emerges from sphere. It emerges from

the exit port and is picked the exit port and is picked up by the detector.up by the detector.

When the beam of light When the beam of light (without the sample) hits (without the sample) hits the sphere it is scattered the sphere it is scattered

many times within the many times within the sphere. It emerges from sphere. It emerges from

the exit port and is picked the exit port and is picked up by the detector.up by the detector.

Only half the sphere Only half the sphere is shown so you can is shown so you can

see what happens see what happens inside.inside.

Only half the sphere Only half the sphere is shown so you can is shown so you can

see what happens see what happens inside.inside.

De

tect

orD

ete

ctor




De

tect

orD

ete

ctorWhen we insert the When we insert the

sample to be measured, it sample to be measured, it has to be close to the has to be close to the

sphere entrance to avoid sphere entrance to avoid omitting some of the omitting some of the

transmitted light.transmitted light.

When we insert the When we insert the sample to be measured, it sample to be measured, it

has to be close to the has to be close to the sphere entrance to avoid sphere entrance to avoid

omitting some of the omitting some of the transmitted light.transmitted light.




De

tect

orD

ete

ctor

Only this part Only this part of the light is of the light is

measured.measured.

Only this part Only this part of the light is of the light is

measured.measured.

The rest of the The rest of the light is light is

omitted.omitted.

The rest of the The rest of the light is light is

omitted.omitted.




De

tect

orD

ete

ctor

Putting the sample close Putting the sample close to the sphere entrance to the sphere entrance port means all the light port means all the light

is collected.is collected.

Putting the sample close Putting the sample close to the sphere entrance to the sphere entrance port means all the light port means all the light

is collected.is collected.

However, some light can However, some light can still be lost as the beam still be lost as the beam

spreads within the sample spreads within the sample if the beam is too big.if the beam is too big.

However, some light can However, some light can still be lost as the beam still be lost as the beam

spreads within the sample spreads within the sample if the beam is too big.if the beam is too big.




Diffuse transmittance is fairly Diffuse transmittance is fairly insensitive to beam geometry, but…insensitive to beam geometry, but… The beam must be much smaller The beam must be much smaller

than the sphere entrance port.than the sphere entrance port. The sample must be close to the The sample must be close to the

sphere entrance port.sphere entrance port.Placing the sample close to the Placing the sample close to the

sphere entrance port gives another sphere entrance port gives another problem:problem: Reflections.Reflections.

Diffuse transmittance is fairly Diffuse transmittance is fairly insensitive to beam geometry, but…insensitive to beam geometry, but… The beam must be much smaller The beam must be much smaller

than the sphere entrance port.than the sphere entrance port. The sample must be close to the The sample must be close to the

sphere entrance port.sphere entrance port.Placing the sample close to the Placing the sample close to the

sphere entrance port gives another sphere entrance port gives another problem:problem: Reflections.Reflections.




De

tect

orD

ete

ctor

Without the sample, some Without the sample, some of the light in the sphere is of the light in the sphere is lost through the entrance lost through the entrance

port.port.

Without the sample, some Without the sample, some of the light in the sphere is of the light in the sphere is lost through the entrance lost through the entrance

port.port.

When the sample is in When the sample is in place, some of that light is place, some of that light is

reflected back into the reflected back into the sphere, making the sphere, making the

transmittance appear transmittance appear higher than it really is.higher than it really is.

When the sample is in When the sample is in place, some of that light is place, some of that light is

reflected back into the reflected back into the sphere, making the sphere, making the

transmittance appear transmittance appear higher than it really is.higher than it really is.

This effect of reflection This effect of reflection cannot be eliminated but cannot be eliminated but are reduced significantly are reduced significantly

with higher sphere-to-port with higher sphere-to-port diameter ratios.diameter ratios.

This effect of reflection This effect of reflection cannot be eliminated but cannot be eliminated but are reduced significantly are reduced significantly

with higher sphere-to-port with higher sphere-to-port diameter ratios.diameter ratios.




De

tect

orD

ete

ctorBecause light is Because light is

reversible, reversible, measurements can also measurements can also be made reversing the be made reversing the

beam and detector.beam and detector.

Because light is Because light is reversible, reversible,

measurements can also measurements can also be made reversing the be made reversing the

beam and detector.beam and detector.




DetectorDetector



beam and detectorbeam and detector



beam and detectorbeam and detector

BafflesBafflesBafflesBaffles

Baffles constrain the Baffles constrain the view of the detector, so view of the detector, so it “sees” the same area it “sees” the same area

with and without the with and without the sample.sample.

Baffles constrain the Baffles constrain the view of the detector, so view of the detector, so it “sees” the same area it “sees” the same area

with and without the with and without the sample.sample.


Diffuse TransmissionDiffuse TransmissionDiffuse TransmissionDiffuse TransmissionOne problem with using integrating spheres is One problem with using integrating spheres is

their low efficiency.their low efficiency.A typical sphere lose 99.9% or more of the A typical sphere lose 99.9% or more of the light entering.light entering.

Measuring transmission in the IR can be Measuring transmission in the IR can be difficult because:difficult because:Detectors are less sensitive.Detectors are less sensitive.Detectors are more noisy.Detectors are more noisy.There are many strong atmospheric There are many strong atmospheric absorption bands, particularly water and absorption bands, particularly water and COCO22..

Atmospheric absorption can change rapidly.Atmospheric absorption can change rapidly.

One problem with using integrating spheres is One problem with using integrating spheres is their low efficiency.their low efficiency.A typical sphere lose 99.9% or more of the A typical sphere lose 99.9% or more of the light entering.light entering.

Measuring transmission in the IR can be Measuring transmission in the IR can be difficult because:difficult because:Detectors are less sensitive.Detectors are less sensitive.Detectors are more noisy.Detectors are more noisy.There are many strong atmospheric There are many strong atmospheric absorption bands, particularly water and absorption bands, particularly water and COCO22..

Atmospheric absorption can change rapidly.Atmospheric absorption can change rapidly.

Measuring diffuse Measuring diffuse transmission in the IR can transmission in the IR can be VERY difficult, requiring be VERY difficult, requiring

long procedures and long procedures and attention to detail to achieve attention to detail to achieve

accuracies of several accuracies of several percent.percent.

Measuring diffuse Measuring diffuse transmission in the IR can transmission in the IR can be VERY difficult, requiring be VERY difficult, requiring

long procedures and long procedures and attention to detail to achieve attention to detail to achieve

accuracies of several accuracies of several percent.percent.


ConclusionsConclusionsConclusionsConclusions

Depends on beam Depends on beam geometrygeometry

Reflections from Reflections from the sample gives the sample gives an erroran error

Errors can be Errors can be eliminated by good eliminated by good designdesign

Depends on beam Depends on beam geometrygeometry


Errors can be Errors can be eliminated by good eliminated by good designdesign

Depends on beam Depends on beam sizesize


Errors can only be Errors can only be minimized by good minimized by good designdesign

Depends on beam Depends on beam sizesize


Errors can only be Errors can only be minimized by good minimized by good designdesign

Regular TransmittanceRegular TransmittanceRegular TransmittanceRegular Transmittance Diffuse TransmittanceDiffuse TransmittanceDiffuse TransmittanceDiffuse Transmittance

In summary…In summary…In summary…In summary…

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Transmittance Measurement Presented by Dr. Richard Young VP of Marketing & Science Optronic Laboratories, Inc