Optronic Laboratories, Optronic Laboratories, Inc. Inc. Transmittance Transmittance Measurement Measurement Presented by Presented by Dr. Richard Young Dr. Richard Young VP of Marketing & Science VP of Marketing & Science Optronic Laboratories, Optronic Laboratories, Inc. Inc.
30
Embed
Transmittance Measurement Presented by Dr. Richard Young VP of Marketing & Science Optronic Laboratories, Inc.
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Optronic Laboratories, Inc.Optronic Laboratories, Inc.
Optronic Laboratories, Inc.Optronic Laboratories, Inc.
Types of TransmittanceTypes of TransmittanceTypes of TransmittanceTypes of Transmittance
...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
Optronic Laboratories, Inc.Optronic Laboratories, Inc.
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.
Optronic Laboratories, Inc.Optronic Laboratories, Inc.
Types of TransmittanceTypes of TransmittanceTypes of TransmittanceTypes of Transmittance
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.
Optronic Laboratories, Inc.Optronic Laboratories, Inc.
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
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.
Optronic Laboratories, Inc.Optronic Laboratories, Inc.
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.
Optronic Laboratories, Inc.Optronic Laboratories, Inc.
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
Optronic Laboratories, Inc.Optronic Laboratories, Inc.
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.
Optronic Laboratories, Inc.Optronic Laboratories, Inc.
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
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%.
Optronic Laboratories, Inc.Optronic Laboratories, Inc.
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.
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
Optronic Laboratories, Inc.Optronic Laboratories, Inc.
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
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
Optronic Laboratories, Inc.Optronic Laboratories, Inc.
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.
Optronic Laboratories, Inc.Optronic Laboratories, Inc.
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.
Optronic Laboratories, Inc.Optronic Laboratories, Inc.
Because light is Because light is reversible, reversible,
measurements can also measurements can also be made reversing the be made reversing the
beam and detectorbeam 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 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.
Optronic Laboratories, Inc.Optronic Laboratories, Inc.
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.
Optronic Laboratories, Inc.Optronic Laboratories, Inc.
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
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 sizesize
Reflections from Reflections from the sample gives the sample gives an erroran error
Errors can only be Errors can only be minimized by good minimized by good designdesign
Depends on beam Depends on beam sizesize
Reflections from Reflections from the sample gives the sample gives an erroran error
Errors can only be Errors can only be minimized by good minimized by good designdesign