Transmittance Measurement Presented by Dr Richard Young VP

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

Outline of Presentation Ø Types of transmittance: § Regular § Diffuse Ø Factors affecting measurements: § Regular Transmittance • Beam geometry • Reflections § Diffuse Transmittance • Beam geometry • Reflections Optronic Laboratories, Inc.

Types of Transmittance This is called regular transmittance. And It hits thisthe changes detector, the producing amount Consider ofsignal light a parallel hitting s. Now the we put detector. beam a sample of If light. the innew the detector beam. signal is s’: Detector Optronic Laboratories, Inc.

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

Types of Transmittance or t c e Det tor Detector So if we want to measure diffuse transmittance, we need to measure at all angles. . . Optronic Laboratories, Inc.

Types of Transmittance Detector . . . or have a detector that collects all the light regardless of angle. Optronic Laboratories, Inc.

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

Types of Transmittance Ø However, there are limitations to both types of accessories caused by the interaction of the sample with the accessory. Ø 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. Optronic Laboratories, Inc.

Regular Transmittance Beam Geometry Parallel is such an exact Remember term. Whatwe if itsaid wasa parallel converging beam ofor light. diverging? Detector Optronic Laboratories, Inc.

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

Regular Transmittance Beam Geometry Ø So if the beam over-fills the detector, a diverging beam can lead to too high transmittance values. Ø By the same reasoning, converging beams can give too low values. Ø 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. Ø The under-filled detector must also respond uniformly. Optronic Laboratories, Inc.

Regular Transmittance Beam Geometry Ø 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. Optronic Laboratories, Inc.

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

Regular Transmittance Beam Geometry Half-Angle Ø 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. Radians, For f >> r Optronic Laboratories, Inc.

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

Regular Transmittance Reflections from the detector Let us go can back hit the to the sample againparallel and bebeam. 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%. Optronic Laboratories, Inc.

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

Regular Transmittance 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. Optronic Laboratories, Inc.

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

Regular Transmittance Reflections 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. Optronic Laboratories, Inc.

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

Diffuse Transmission 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 Beam Geometry Optronic Laboratories, Inc.

Diffuse Transmission The rest of the light is omitted. Detector Beam Geometry Only this part of the light is measured. Optronic Laboratories, Inc.

Diffuse Transmission 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. Detector Beam Geometry Optronic Laboratories, Inc.

Diffuse Transmission Beam Geometry Ø 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. Ø Placing the sample close to the sphere entrance port gives another problem: § Reflections. Optronic Laboratories, Inc.

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

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

Diffuse Transmission Beam Geometry Baffles Because constrain light isthe view ofreversible, the detector, so measurements it “sees” the same can area also be with made andreversing without the beamsample. and detector Detector Baffles Optronic Laboratories, Inc.

Diffuse Transmission ØOne problem with using integrating spheres is Measuring their low efficiency. diffuse § A typical sphere lose 99. 9% or more of the in the IR can light transmission entering. be VERY difficult, inrequiring ØMeasuring transmission the IR can be difficult because: long procedures and § Detectors are less attention to sensitive. detail to achieve § Detectors are more noisy. accuracies of several § There are many strong atmospheric percent. absorption bands, particularly water and CO 2. § Atmospheric absorption can change rapidly. Optronic Laboratories, Inc.

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