Using Calibrated Specular Reflectance Standards for Absolute and Relative Reflectance Measurements
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1 Using Calibrated Specular Reflectance Standards for Absolute and Relative Reflectance Measurements Applications Overview here are two fundamental techniques for measuring specular reflectance with a UV/VIS/NIR spectrometer. In relative reflectance a baseline (autozero) is performed with a standard of known reflectance in the measurement position, and the standard is replaced with the unknown sample for testing. Absolute reflectance measurements require the use of a device that is capable of collecting a baseline without the direct use of a standard. his note reviews the use of calibrated standards in both of these techniques and outlines the calculations necessary to obtain true reflectance values. opics are covered very briefly, and experts may disagree on some of the opinions expressed here. or more extensive discussions please contact the Optical Reference Laboratory. Relative Reflectance R elative specular reflectance is typically measured using some variation of a spectrometer accessory depicted schematically in igure 1, which shows the baseline configuration with the standard in place (A), replaced by the sample in the measurement configuration (B). he angle of the incident beam on the sample is determined by the angles and positions of the mirrors relative to the sampling platform. he simplest of these accessories are set at fixed angles, but designs for variable angles are also available. he greatest advantage of this technique is that the same beam configuration is used for baseline collection and for sample measurement. Having no moving parts provides the potential to be more accurate than absolute reflectance; however, because a standard is used to set the baseline, the result of the measurement can be no more accurate than the standard itself. urther, since the performance of the standard can degrade with age and frequent use, it must be verified or replaced often. 1 igure 1. Schematic diagram of a relative reflectance accessory.
2 he measurement process begins with an instrument baseline, performed in this first example using an aluminum standard mirror with known reflectance values. In essence, this sets the standard mirror at 100% R for the purposes of the measurement. Shown in igure 2, a laser line mirror, highly reflective around the target wavelength of 532 nm, is placed in the sampling position and scanned. At its peak wavelength range, this sample is more reflective than aluminum, so its raw (or relative) spectrum exceeds 100%. In the final step, the true reflectance of the sample is calculated by multiplying the raw spectrum point by point times the known standard mirror values. o keep the units correct for the calculation, the values of the standard are expressed as a decimal fraction rather than percent. igure 2. Measuring a high reflector relative to an aluminum mirror. Angle of incidence is 8 degrees. he measurement of an uncoated glass sample relative to an aluminum standard mirror is shown in igure 3. ollowing the procedure in the previous example, it can be seen that the reflectance of the glass sample (raw scan) is considerably lower than aluminum throughout the scan. ypical of these measurements, the signature spectral minimum of aluminum at around 820 nm appears as a peak in the raw spectrum of the glass. Upon correction (multiplying the raw spectrum times the aluminum standard values) this peak disappears, and the relatively flat spectral profile of the glass is obtained. igure 3. Measuring a glass sample relative to an aluminum mirror. Angle of incidence is 8 degrees. he relative reflectance technique is used again in the third example for measurement of an antireflective sample, shown in igure 4. he uncoated glass sample which was measured against the aluminum mirror in the previous example is used as the standard. his relatively low reflector is preferred by many users over aluminum as a reference material when measuring antireflective samples, which can exhibit values of less than 0.05% R. he reason for this preference is that most spectrometers give better results when the energy in the sample beam during baseline correction more closely matches the energy during sample measurement. 2 igure 4. Measuring an antireflective sample relative to an uncoated glass standard. Angle of incidence is 8 degrees.
3 W hen using double beam instruments, there has been a long tradition of using two relative reflectance accessories, one in the sample beam and one in the reference. A replica of the standard is left on the sampling platform in the reference beam during baseline collection and sample measurement. he object of this technique is to attenuate the reference beam energy to achieve better balance with the sample beam. However, most modern instruments exhibit excellent performance with respect to dynamic range and detector electronics, and as a result the reference beam can generally be left open. Only when measuring very low reflectors such as antireflective coatings would reference beam attenuation need to be considered, and other materials such as perforated metal screens and neutral density filters are available to serve this purpose equally well. he use of relative reflectance methods requires frequent handling of the standard, exposing it to potential harm. Especially when using vacuum deposited first surface metallic mirrors, it is good practice to create replicas of the original standard for routine use, and discard them when they become damaged. hese working standards can be similar to the original, or they can be made from more durable materials such as polished metals, electroplated metals and second surface mirrors. Absolute Reflectance S everal types of accessories have been designed to measure absolute specular reflectance. Common to all of them is the requirement that the baseline be established using only the optical elements within the accessory itself. Insertion of the sample into the optical path adds one or two reflections to those of the accessory, allowing the sample reflectance to be measured without the direct use of a standard. he beam geometry must be preserved; that is, the number of bounces of the beam on the accessory mirrors, the angles of those bounces, and the total path length of the beam must be kept the same for baseline as well as sample configurations. P erhaps the earliest absolute reflectance devices employed the VW design, shown in igure 5. In the baseline configuration the radiation beam reflects from Mirror M1. When the sample is in place, two reflections from the sample and one from Mirror M2 are seen. his configuration requires that M1 and M2 be identical or that M1 moves reproducibly to the M2 position for measurement. he two sample reflections result in the raw measurement of the square of the reflectance; therefore the square root of the igure 5. Schematic diagram of the absolute reflectance VW design. raw spectrum must be calculated in order to obtain the true reflectance values for the sample. While this design is quite useful for many types of samples, it is of limited use for antireflective coatings, because two reflections from a very low reflector often result in too little energy reaching the detector. 3
4 A diagram of the VN design for absolute reflectance is shown in igure 6. Mirror M1 moves and M2 rotates when changing from the baseline or V configuration to the measurement or N setting. he requirements to maintain constant angles of reflection and total beam path length are preserved. he added single bounce of the beam on the sample results in the direct measurement of its true reflectance, with no calculations required. As such, materials of all types, including antireflective coatings, are easily measured. B y altering the positions of the various mirrors, the angle of incidence on the sample can be changed. Reproducible mirror settings and proper alignment of the optical path are critical to good performance, and many times an integrating sphere is used to collect the beam in order to make the alignment process more forgiving. Variations on this single bounce design have been developed to include continuously variable angles and motorized software control. igure 7. Schematic diagram of an absolute reflectance design using a movable detector. igure 6. Schematic diagram of the absolute reflectance VN design. igure 7 shows a simple absolute reflectance design using a rotating sample stage and a detector element that moves in an arc around the stage. he baseline is performed with the detector positioned at 180 degrees from the incoming beam. he sample is mounted at the desired angle, and the detector swings to twice that angle so the specular reflectance can be captured. he detector is usually mounted in a small collection sphere to facilitate alignment. Also, because the position of the detector changes during this process, the reference beam must either follow this movement or utilize a second detector. A bsolute reflectance techniques have proven to be very convenient and popular with users. However, there is a common misconception that standards are not necessary when using absolute reflectance. While it is true that the baseline is not defined directly by the use of a standard, the following statement should be emphasized: he term absolute reflectance refers to the design of the measurement device, as discussed above. Use of this technique does not guarantee that the correct reflectance values will be obtained. his is highly dependent on the proper alignment and function of the measurement system, and the accuracy of results should be periodically evaluated through the use of a calibrated standard. 4
5 igure 8 shows a spectrum of an aluminum mirror taken in absolute reflectance, along with the NIS traceable values for the same mirror. In the VIS and NIR range, the scan is about 0.3 to 0.4% higher than the calibration data. he most obvious way to improve the accuracy of the scan is to adjust the alignment of the measurement system so that the values match the calibration data more closely. Sometimes this approach is not entirely successful, and greater accuracy can be achieved by employing a correction strategy. irst, a ratio file is obtained by dividing the NIS calibration data by the mirror scan data. his ratio file can be used to correct scans of subsequent samples. igure 9. Correction of an absolute reflectance spectrum of a silver mirror using a ratio file, 8 degrees incidence. igure 8. Absolute reflectance spectrum of an aluminum mirror and its corresponding calibration file, 8 degrees incidence. he continuation of the correction process is illustrated in igure 9, using the example of a silver mirror measured in the NIR. he ratio file from the previous step is multiplied times the raw absolute reflectance spectrum of the silver to obtain the corrected spectrum. his spectrum should more closely represent the true reflectance of the silver mirror. Care must be taken to scan the unknown sample and the standard mirror under the same instrument parameters such as incident angle, spectral bandpass, beam spot dimensions and polarization conditions. Creation of a new ratio file with each sampling session is recommended, to ensure compatibility with current samples. General Considerations I f any standards or samples are transparent, it is important to prevent back surface reflections from reaching the detector optics, thereby contributing to the measurements. Especially when measuring antireflective coatings, even a tiny amount of back surface reflection can have a large impact on the results. igure 10 shows several ways to eliminate back surface effects. or example, neutral density absorbers can be incorporated into standard materials (i.e. black glass), or their back surfaces can be ground and blackened. Refractive index matching oils and igure 10. Elimination of back surface reflections in transparent samples. 5 wedges are available for use with transparent samples that can not be altered.
6 I t is best to mount standards on the measurement device by their edges or rims, to avoid touching the optical surface, especially when items are being held with spring tension. If this is not possible, a thin spacer can be used to support the standard by its edges and elevate the center of the optical surface away from the measurement platform. A business card with an appropriate hole, just smaller than the standard, can be used as a spacer. A very thin O-ring or gasket can also serve this purpose. In many spectrometer systems this small displacement will not affect the accuracy of the results, but it should be carefully studied. It may be necessary to mount the standards and samples on the same type of spacer in order to preserve beam geometry and measurement accuracy. G ood standards can be made from many different materials. As seen in the earlier example with the antireflective coating (igure 4), they need not possess the highest possible reflectivity. It is usually more important that their spectral profiles are relatively flat across the wavelength range of interest and that their reflectance values are accurately known. igure 11 shows typical spectra of several materials that could be used as standards, all of which can be more durable than vacuum deposited metallic coatings. igure 12. Spectra of an aluminum mirror, taken at 45 degrees incidence, S and P polarization. igure 11. ypical spectra of possible materials for use as standards, 8 degrees incidence. or angles of incidence greater than about 15 degrees, polarization effects must be considered. Calibration values provided with standards should include polarization data which are appropriate for the angles at which they are being used. igure 12 shows example spectra of an aluminum mirror at 45 degrees incidence, S and P polarization orientations. he green line represents random polarization, calculated by averaging the S and P curves. or further information contact: Optical Reference Laboratory LLC info@opticalreferencelab.com Applications Overview 1203 B 2012, Optical Reference Laboratory 6
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