Transmittance Measurement

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

2. Outline of Presentation • Types of transmittance: • Regular • Diffuse • Factors affecting measurements: • Regular Transmittance • Beam geometry • Reflections • Diffuse Transmittance • Beam geometry • Reflections

3. Types of Transmittance This is called regular transmittance. Consider a parallel beam of light. It hits the detector, producing signal s. Now we put a sample in the beam. And this changes the amount of light hitting the detector. If the new detector signal is s’: Detector

4. Types of Transmittance This is called diffuse transmittance. Detector Now if we take the same parallel beam of light... But when we put a sample in the beam, it scatters the light in all directions. Now the detector signal, s’, does not represent all the transmitted light.

5. Types of Transmittance Detector Detector Detector Detector So if we want to measure diffuse transmittance, we need to measure at all angles...

6. Types of Transmittance Detector ...or have a detector that collects all the light regardless of angle.

7. Types of Transmittance • Regular and diffuse transmittance represent the extremes of material properties. • Real samples show some elements of both behaviours. • Generally, • accessories for regular transmittance can ONLY measure regular transmittance. • accessories for diffuse transmittance can measure regular transmittance as well.

8. Types of Transmittance • However, there are limitations to both types of accessories caused by the interaction of the sample with the accessory. • Most often, no sample is present during calibration. • But air and samples have very different properties. • Reflection and refraction by the sample, which are not there during calibration, can introduce errors in measurement.

9. Regular TransmittanceBeam Geometry Remember we said a parallel beam of light. Parallel is such an exact term. What if it was converging or diverging? Detector

10. Regular TransmittanceBeam Geometry Here the beam over-fills the detector. When we put the sample in the beam… The increase in detector signal gives a higher transmittance value than it should. In extreme cases, this can lead to values of more than 100%, which is impossible. Detector ...the beam is refracted inward, causing a slight focussing effect on the detector.

11. Regular TransmittanceBeam Geometry • So if the beam over-fills the detector, a diverging beam can lead to too high transmittance values. • By the same reasoning, converging beams can give too low values. • This means if non-parallel beams are used, it is important that BOTH the calibration and sample measurements are done with the detector under-filled. • The under-filled detector must also respond uniformly.

12. Regular TransmittanceBeam Geometry • In addition to the sample altering the beam geometry, a converging or diverging beam will include light that enters the sample at an angle. • This means different parts of the beam travel different distances through the sample. • So the transmittance is an average of several path lengths. • Different beam geometries can therefore give different transmittance values.

13. Regular Transmittance Beam Geometry Half-Angle A lens focused on an aperture is a common way of producing a collimated (parallel) beam. But light from different parts of the aperture are not parallel to each other. Aperture Lens

14. Radians, For f>> r Regular Transmittance Beam Geometry • The simplest formula for calculating the half-angle  (degree of collimation) is • where r is the radius of the aperture and f is the lens focal length. Half-Angle

15. Regular Transmittance Beam Geometry Since the refractive index changes with wavelength, so does the focal length. At long wavelengths the beam diverges and at short wavelengths it converges. Unfortunately, collimation is achieved at only one wavelength. Note: This does not happen in all-mirror collimators. Lens collimators need to be optimized for the wavelengths of interest.

16. Regular TransmittanceReflections Let us go back to the parallel beam. Reflections from the detector can hit the sample again and be reflected back to the detector. Detector The detector may reflect 50% or more. The sample, perhaps 8%. This gives an error in transmittance of 4%.

17. Regular TransmittanceReflections Detector By setting the detector at an angle, this unwanted reflection can be eliminated.

18. Regular TransmittanceReflections Reflections also apply when the parallel beam is focussed at the detector. Detector There are two ways to reduce reflection effects, and both may be applied together.

19. Regular TransmittanceReflections One way is to rotate the detector, as with the parallel beam. Detector The reflected beam then misses the detector.

20. Regular TransmittanceReflections The other way is to move the detector back from the focus, and use an aperture to mask reflections. Detector Most of the reflections from the detector are blocked and do not get back to the sample.

21. Diffuse Transmission Beam Geometry For diffuse transmittance, light is scattered at all angles. We need a collection optic that responds equally regardless of angle – an integrating sphere When the beam of light (without the sample) hits the sphere it is scattered many times within the sphere. It emerges from the exit port and is picked up by the detector. Detector Only half the sphere is shown so you can see what happens inside.

22. Diffuse Transmission Beam Geometry When we insert the sample to be measured, it has to be close to the sphere entrance to avoid omitting some of the transmitted light. Detector

23. Diffuse Transmission Beam Geometry The rest of the light is omitted. Detector Only this part of the light is measured.

24. Diffuse Transmission Beam Geometry Putting the sample close to the sphere entrance port means all the light is collected. Detector However, some light can still be lost as the beam spreads within the sample if the beam is too big.

25. Diffuse Transmission Beam Geometry • Diffuse transmittance is fairly insensitive to beam geometry, but… • The beam must be much smaller than the sphere entrance port. • The sample must be close to the sphere entrance port. • Placing the sample close to the sphere entrance port gives another problem: • Reflections.

26. Diffuse Transmission Beam Geometry Without the sample, some of the light in the sphere is lost through the entrance port. When the sample is in place, some of that light is reflected back into the sphere, making the transmittance appear higher than it really is. This effect of reflection cannot be eliminated but are reduced significantly with higher sphere-to-port diameter ratios. Detector

27. Diffuse Transmission Beam Geometry Because light is reversible, measurements can also be made reversing the beam and detector. Detector

28. Diffuse Transmission Beam Geometry Because light is reversible, measurements can also be made reversing the beam and detector Baffles constrain the view of the detector, so it “sees” the same area with and without the sample. Detector Baffles

29. Diffuse Transmission • One problem with using integrating spheres is their low efficiency. • A typical sphere lose 99.9% or more of the light entering. • Measuring transmission in the IR can be difficult because: • Detectors are less sensitive. • Detectors are more noisy. • There are many strong atmospheric absorption bands, particularly water and CO2. • Atmospheric absorption can change rapidly. Measuring diffuse transmission in the IR can be VERY difficult, requiring long procedures and attention to detail to achieve accuracies of several percent.

30. Depends on beam geometry Reflections from the sample gives an error Errors can be eliminated by good design Depends on beam size Reflections from the sample gives an error Errors can only be minimized by good design Conclusions In summary… Regular Transmittance Diffuse Transmittance