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

3. Outline of Presentation  Types of transmittance:  Regular  Diffuse  Factors affecting measurements:  Regular Transmittance Beam geometry Reflections  Diffuse Transmittance Beam geometry Reflections

4. Types of Transmittance Detector 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’: This is called regular transmittance.

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

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

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

8. 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.

9. 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.

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

11. Here the beam over-fills the detector. When we put the sample in the beam… Regular Transmittance Beam Geometry...the beam is refracted inward, causing a slight focussing effect on the detector. 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

12. Regular Transmittance Beam 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.

13. Regular Transmittance Beam 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.

14. Regular Transmittance Beam Geometry 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. But light from different parts of the aperture are not parallel to each other. Aperture Lens Half-Angle

15. 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 Radians, For f >> r

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

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

18. By setting the detector at an angle, this unwanted reflection can be eliminated. Regular Transmittance Reflections Detector

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

20. One way is to rotate the detector, as with the parallel beam. Regular Transmittance Reflections The reflected beam then misses the detector. Detector

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

22. 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. Only half the sphere is shown so you can see what happens inside. Detector

23. Diffuse Transmission Beam Geometry Detector 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.

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

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

26. 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.

27. Diffuse Transmission Beam Geometry Detector 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.

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

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

30. 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 CO 2.  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.

31. Conclusions  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 Regular Transmittance Diffuse Transmittance In summary…