Measuring > 8A Samples

 

Measuring > 8A using the PerkinElmer Lambda 1050 UV/Vis/NIR Spectrometer

Introduction 

The PerkinElmer Lambda 1050 is a state of the art research UV/Vis/NIR spectrometer utilizing 3D (three detector) technology. Because the Lambda 1050 incorporates holographic gratings in a true Littrow double monochromator design, the stray radiation level is specified at < 0.00007 %T in the UV/Vis range. Because the amount of stray radiation limits the dynamic range of a spectrometer, the ultra-low stray light level of the Lambda 1050 allows the instrument dynamic range to be specified to 8A. High absorbance measurements to 8A require reference beam attenuation, a process where neutral density screens are inserted in the reference beam path to help balance the sample and reference beam energies when high absorbance samples are being measured. Reference beam attenuation will improve the signal-to-noise levels when high absorbance samples are measured. The capability to measure to 8A is incorporated in the Lambda 1050 by the inclusion of automated sample/reference beam attenuator screens. The neutral density screens included allow attenuation levels of 1%T (2A) and 0.1 %T (3A) to be programmed in the method. When required, the screens will be automatically inserted in the sample or reference beam paths, or both. The 1%T attenuator works well with sample up to 5-6A, and the 0.1%T attenuator works well when sample absorbance reaches 6-8A range.  Though the Lambda 1050 is specified to 8A, valid data can be acquired in the 8-10A range as long as certain procedures are followed. The instrument’s parameters need to be optimized for wide bandpasses, slow scan speeds (high integration times), enable dark current (0%T) correction, and the use of supplemental reference beam attenuation. Discussed in this application note are the proper procedures for acquiring absorbance data >8A. A bandpass filter with out of band blocking specified at grater that 8A will be used as an example.  A PerkinElmer Lambda 1050 (S/N 1050L1109146) was used for measurements.

Experimental
MicroCoatings™ 337 nm bandpass filter was used an as example. This filter is pictured in Figure 1.
This filter is specified with an out of band blocking greater than 8A. The filter is a sandwich type, with one side mirrored, the other side a glass surface. For measurements, the glass side surface was inserted to face the beam. A standard solid sample holder (PE # B0080822) was used to hold the filter properly in the beam.

Figure 1. MicroCoatings 337 nm Bandpass Filter

A reference beam attenuator kit (PE Part Number L1160560), was used to provide supplemental reference beam attenuation (Figure 2). This kit consists of 5 circular magnetic ringed screens having different attenuation levels (32, 14, 6, 4, and 1%T) which can be adhered to the reference beam magnetic window. The attenuators in this kit provide a uniformly flat response as a function of wavelength. If needed, multiple reference beam attenuators can be stacked to produce a specific level of attenuation in the reference beam.

   Figure 2. Supplemental Reference Attenuation Screen Kit (PE # L1160560)

To construct a method to properly scan above 8A, the use of wide slits (bandpasses), slow scan speeds-high integration times, enabling dark current correction (0%T correction), and use of reference beam attenuation are all required. Because the Lambda 1050 incorporates motorized screen attenuators in sample and reference beams, a unique software feature can be used where the attenuation level is selected on the reference side, and the sample side attenuator set to “Automatic”. When the method is executed, this enables a three step measurement of the internal attenuators 1) Sample side, 2) Reference side, and 3) Attenuators in both sample and reference. This type of correction solves one of the problems of only using reference beam attenuation alone. By just adding screens to the reference beam path, the signal-to-noise at lower absorbance levels will be significantly degraded to obtain the benefit of better S/N at higher absorbance. By correcting with attenuators in both sample and reference, and then correcting for the contributions of the attenuators, the signal-to-noise will be enhanced not only at high absorbance but at low absorbance as well.

Figure 3. Scan parameter selections influencing high absorbance scanning include data interval (1), scan speed (2), response time (3), slit (4), and attenuator settings (5),

Under the Corrections menu in UVWinlab V6, is a check box that allows the 0%T dark current correction to be done. Correction at 0%T allows better accuracy when the sample absorbance exceeds 8A. In this example the MicroCoatings filter was scanned from 750 to 350 nm, using a 5 nm data interval, a 5 second response time, a 5 nm slit, and a reference attenuation setting of 0.1% and the front attenuation setting of Automatic. With the goal of verifying out of band blocking greater that 8A, it is shown in Figure 4 that that was accomplished without the use of supplemental reference beam attenuation. Because no part of the scan in the 670 to 400 nm range dipped below 8A, the conclusion that this filter meets is stated requirements can be proven.

Figure 4. Scan of MicroCoatings 337 nm bandpass filter for verification of out of band blocking performance. The filter does exceed 8A in the range of 670 to 400 nm.

To help reduce the noise above 8A, additional reference beam attenuation was applied by adding the 1%T screen from the attenuator screen kit, affixing this to the reference beam magnetic window. Additional reference attenuation often will reduce the noise seen above 8A. The spectrum shown in Figure 5 is the same MicroCoatings filter scan with an additional 1%T screen added to the reference beam. Note that the spectra of the screens from the attenuator kit were scanned earlier to allow the absorbance from the selected screen to be easily added to the sample spectrum (reference attenuation will reduce the absorbance by the amount of the screen – to derive the actual absorbance the screen absorbance is added to the sample absorbance spectrum).

Figure 5. The same filter shown in Figure 4 is rescanned with an additional 1.8A (1%T) attenuator screen inserted in the reference beam. The additional reference attenuation has the benefit of improving the signal to noise at high absorbance. In this example, not only can it be shown that the filter exceeds the stated performance of >8A, but it actually exceeds 9A!

 

Figure 6. The mean absorbance of this filter calculated from 660 to 400 nm, was determined to be 9.4613 A.

 

Conclusion

The Lambda 1050 is a state-of-the-art double monochromator research UV/Vis/NIR spectrometer with ultra-low stray radiation levels In this system the use of holographic gratings used in a double monochromator Littrow design allows the dynamic range to be specified to 8A. In instances where the absorbance levels exceed 8A, valid data can still be obtained when the instrument parameters are properly configured.  In the examples presented here, verification of > 8A out of band blocking can be achieved using the standard automated attenuators. To achieve better signal-to-noise when the sample absorbance start to approach 10A, additional attenuators can be used in the reference beam path, allowing sample absorbance measurements to exceed 9A
.

Calculating % Transmission from Energy Spectra

 

Calculating Transmission Spectra from the E1 and E2 Energy Spectra

Using a PerkinElmer Lambda spectrometer, it is possible to calculate the transmission spectra (uncorrected) from the E1 (sample) and E2 (reference) energy spectra. The procedure to do this is described herein. A PerkinElmer Lambda 1050 running UVWinlab V6 is used for this procedure example.

The first step in the procedure requires the user to determine the correct PMT energy setting required to operate properly in the E1 and E2 modes.
After power up, and loading of UVWinlab, from UVWinlab Explorer, click on the Instrument menu, and then click on the Lambda instrument icon and then click on the Manual Control icon.
Using the Manual Control instrument settings enter a wavelength of 520 nm, a 2 nm slit, and then set the ordinate mode to E1 – then click on the Apply button. The maximum energy in the UV/Vis range is typically around 520 nm. The wavelength of maximum UV energy is typically around 250 nm, but this will be less than the signal at 520 nm.
Observe the E1 signal level on the live display. If the energy is between 95 and 100, the detector will be saturated. The energy needs to be dropped. The energy level is controlled by the PMT Gain setting. The default is 30. Enter a value of 15 to start and then click on Apply. The value should drop somewhere in the 10 to 30 range. Note that the bandpass selected will have an effect on the displayed E1 signal level, as well as the type of detector module being used. A PMT gain of 15 can be set for the standard detector module, and a slit of 2 nm.
After the PMT gain is determined, close the Manual Control mode, and click on a Scan method. Set an ordinate mode to E1, and the PMT Gain determined to be correct (i.e., 15). In the example shown here, a scan range from 700 to 400 nm was set, E1, and a PMT Gain of 15.



Click on the Sample Info menu in the method, and enter three sample names, called… E1_Raw E2_Raw E1_Sample
Click on Start to collect E1_Raw. When prompted for the next sample, click on Cancel, return to the Data Collection screen, and enter E2 for the ordinate mode. Click on Start to collect E2_Raw. An example of the overlaid E1 and E2 spectra is shown below. It is important not to change any other settings between the E1 and E2 scans.

The ratio of (E1/E2)*100 is the uncorrected transmission baseline. This spectrum can be calculated in UVWinlab V6 by clicking on the Processing section, and then adding an equation. For Equation 1, click to set the equation (E1_Raw/E2_Raw)*100.

Spectra directly calculated with equations will be placed into the Results section, under Custom. For each equation the spectra will be sequentially named Equation1, Equation2, etc. The calculated uncorrected transmission spectrum can be view there. An example is below.

To scan a sample in the energy mode, insert the sample into the sample side cell holder. Set the ordinate mode to E1. Leave all other Data Collection settings intact. Click on Start. The example below is a holmium oxide filter scanned in the E1 mode overlaid with the E1_Raw curve.

To calculate the E1 sample curve, in Processing, and another equation (Equation 2) to subtract E1_Sample from E1_Raw. See example. Again, the calculated spectrum will be placed in to Custom tab under Results, called Equation2.Sample. The curve calculated Equation 2 is shown below.

To calculate the transmission spectrum, the spectrum from Equation 2 needs to be ratioed to E2_Raw. In processing, add another equation (Equation 3) and set the final calculation as… [1-[Equation2/E2_Raw]]*100

This will calculate an uncorrected transmission spectrum and place the result into the Custom tab. Shown below is the E1 and E2 calculated transmission of holmium oxide, overlaid with the corrected spectra of holmium oxide scanned in the normal %T mode.

URA Flexibility: PMT Window Coating

 

Just Another Big, Bulky Sample

Sample – The coated window material of an intact photo-multiplier tube, viewed end-on
There are many “real world” coatings that do not reside on nice flat one inch sized substrates. Many modern quality control specular reflectance samples are the finished manufactured part.
The picture at left shows such a sample.
The placement of this type of large sample on the Universal Reflectance Accessory is shown in the bottom left picture. Since the sampled area is flat and covers the entire beam entry aperture, there is no need for additional light cover. There spectra of the window coating are shown at bottom right. The PMT contents behind the window are far enough removed from the window (1 cm) so as not to interfere.

Sample Preparation
1) Sample is placed on the URA stage with the coated side contacting the opening in the sample stage.
2) The internal Common Beam Depolarizer was used to prevent polarization artifacts at incidence angles higher than 20 degrees.
Placement of the sample on the URA accessory is seen below.

Sample on URA

The spectral results of  an angular study of the reflectance of the PMT widow coating is seen below.

URA Specular Reflectance Results

Conclusion: The design of the URA , with it’s gravity held sample placement and lack of either width or height obstructions, makes it the reflectance accessory of choice for large, bulky, irregular shaped samples. For more info go to http://www.perkinelmer.com

Creating %RC Correction Files in UVWinlab

Introduction

Reflectance measurements can either be relative or absolute. Relative simply means that the data collected from the spectrometer is relative to the standard that was used to background the accessory. Absolute reflectance means that the acquired data is independent of the standard or optical components being used to background the accessory to set a 100% R baseline. Absolute reflectance can be obtained using the proper absolute reflectance accessory (i.e., such as the Universal Reflectance Accessory) or by mathematically correcting the relative data to absolute using a known, calibrated standard.

An important consideration is that relative data typically cannot be compared across spectrometers because different standards are used in different laboratories. Even the “same” type of standard (i.e., a front surface aluminum mirror) can vary significantly from manufacturer, and typically will change response with age and use. Relative spectral data collected using a mirror reference will be different than spectral data acquired using a BK-7 reference, or a Spectralon white plate reference Absolute reflectance data by comparison is comparable across spectrometers and laboratories.

This technical note will describe how to create reference correction files that can be used in UVWinlab V6. This will allow relative reflectance data to be converted to absolute reflectance data. Note that this procedure requires a calibrated standard of some sort, i.e., a NIST traceable mirror or a Labsphere calibrated Spectralon standard are common. The more accurate the standard is specified the more accurate the conversion to absolute reflectance data. This procedure is valid for integrating sphere accessories, and relative specular reflectance accessories.

 

Creating %RC Correction Files

A calibrated standard is required. Typically, this will be a NIST traceable mirror, or a calibrated Spectralon white plate from Labsphere. The %RC option in UVWinlab V6 is available under the Corrections menu, when the ordinate mode of the method is set to %R.

When %RC is selected as the Correction Type, options will be shown for Light Spectral Reference, and Dark Spectral Reference. The Light Spectral Reference is the calibration data file that was supplied with the standard. This is a required entry. The Dark Spectral Reference is optional, and usually applies to integrating sphere measurements. An entry here depends if the measurements are being performed near 0%R, such as with AR coatings. It is recommended to use a Dark Spectral Reference for measurements routinely below 1% R. A standard is not required for the Dark Spectral Reference.

 

Creating the Light Spectral Reference File

1. Examine the calibration sheet supplied with the standard. If not electronic, you will need to get this data into Excel. Examine the data interval - if the data interval is not uniform, you will need to interpolate this data in Excel to a uniform data interval, typically every 5 nm or 10 nm.

2. The calibration data needs to be descending from high to low wavelength. If not, reverse the order of the data in Excel. The data can entered in Excel as R or %R.

3. The next step is to scan a “dummy” file in %R on the Lambda XXX from the starting wavelength of the standard (commonly 2500 nm) to the ending wavelength (commonly 250 nm), at the data interval for the calibration data (typically 5 or 10 nm). Background in %R using the 100% and 0% baseline corrections (under the Corrections menu, make sure the 0%T option is checked).

Note - The quality of the data is not important, as it will be overwritten.

If using a sphere accessory, acquire scan with the standard white Spectralon plates in place on the sample and reference reflectance ports. If this is a relative specular reflectance accessory, acquire the scan with the supplied first surface mirror.

4. When completed, right-click on the filename and select “Save as ASC”, and save to a location on your PC.

5. Next browse to this file and open this ASCII spectral file in Excel, by right-clicking on the file and selecting “Open with…” and select Excel.

6. You will see about 80 lines of header info, and beneath that, starting with #DATA line the two column data, as wavelength and ordinate. Important entries to note are 1. Ordinate mode 2. Data interval and 3. Number of data points in the scan… The number of points must match between the calibration data and the dummy file. This is calculated by… ((starting wavelength – ending wavelength/data interval)+1).

7. Open the calibration data file in a separate copy of Excel, and copy the two columns calibration data (starting at the first wavelength) by selecting and then Copy.

8. Paste this data into the “Dummy” file Excel spreadsheet over the top of the prior data. Note if R data is being pasted, change the line 77 label to R from %R.

9. Save the modified Excel spreadsheet as text (click on the file type selector - important – don’t save an XLS).

10. Using Windows Explorer, browse to the saved file and make sure the extension is .ASC. If the extension is .txt, highlight and change to .ASC.

11. Copy this file to C:\Program Files\PerkinElmer\UVWinLab\V6.0\Data\Corrections Data.

12. The file is now ready to be used in the Corrections menu for Light Spectral Reference. You will need to browse to your file. Important – when browsing to your created reference file, you should see the spectrum graph be displayed for this file. If it does, it is a valid correction file. If an error is given, it is not a valid correction file, and likely a mistake was made in its creation. In this case, re-do the procedure to regenerate the file.

 

Creating the Dark Spectral Reference File

If using relative specular reflectance accessories a Dark Spectral Reference file will not be needed, as the specular accessory without a mirror in place will read 0%R.The Dark Reference File is typically only recommended with integrating spheres, and especially if the measurements are in the low %R range. For integrating spheres, the Dark Spectral Reference option corrects for the small offset from true 0% R due to the light traveling the length of the sphere (the detectors will see a small amount of air scattered light). To generate a Dark Spectral Reference file for the 150 mm integrating sphere accessory, perform the following steps…

1. Using the same range and data interval of the Light Spectral Reference “dummy” scan, background in %R using the 100% and 0% baseline corrections using the standard white plates.

2. When the background is completed, remove the Spectralon white plate from the sample reflectance port and replace the cover. The cover will act as a light trap.

3. Collect a scan. The observed ordinate readings will typically be low, somewhere in the 0.1 to 0.5% range. High ordinate readings (i.e., 5%) may indicate that the light beam is not freely passing through the sphere, and may be clipping on the edge of the port. Re-check beam alignment using the Align button on the top of the toolbar. If necessary adjust the sphere optics top center the sample beam on the %T port and the %R port so not clipping can be observed.

4. When completed, right-click on the filename and select “Save as ASC”, and save to C:\Program Files\PerkinElmer\UVWinLab\V6.0\Data\Corrections Data. The file is now ready to be used in the Corrections menu for Dark Spectral Reference.

 

Using %RC

After the reference files have been created, set the ordinate mode to %R in the method, and in the Corrections menu, select a Correction type of %RC. Assign you newly created Light Spectral Reference file, and optionally the Dark reference file.

The standard for the Light Spectral Reference needs to be used for the background correction (autozero). If using a relative specular reflectance accessory, position the certified mirror on the accessory to obtain a baseline. If using the 150 mm integrating sphere, place the NIST traceable mirror or calibrated Spectralon plate on the sample reflectance port and acquire the background.

To measure a sample, replace the standard with your sample. Data that is collected from the spectrometer is corrected point-by-point in real time. Note that it is normal not to observe 100% R readings after a baseline has been acquired, because the correction data is being applied to the live display for the standard. Important - If the ordinate data is 100 times higher than it should be, then the line 77 label (R or %R) needs to be edited to match the data format in the Light Spectral Reference file. Note that some earlier versions of UVWinlab stored an “nm” as the ordinate label when files were saved as ASC. If an “nm” label is observed change to %R. This has been corrected in V6.

 

Summary

With properly prepared %RC Reference files, and the appropriate standards, absolute reflectance data can be acquired from the PerkinElmer UV/Vis and UV/Vis/NIR spectrometers.

Specular Reflectance Measurements with Spheres

 

Introduction

One of the most common measurements made by the solar energy industry today is quantification of a material’s surface reflectance. These materials are as diverse as metal coatings, semiconductor coatings, anti-reflective coatings on window material, as well as the window material itself. These measurements are most commonly made between 300 nm and 1500 nm. This is where the solar cell is responsive to energy from the sun. Reflection comes in two varieties, specular and diffuse.
Specular reflection (part A in Figure 1) is generated by a smooth surface. The light ray’s angle of incidence is equal to the angle of reflection; therefore, specular materials frequently produce images on their surface (mirror). Specular reflectance is measured by a number of different types of accessories (VW, VN, IV, and Universal Reflectance Accessory).


Diffuse reflection (part B in Figure 1) is generated by a rough surface. Here the light ray’s incidence angle gives rise to a multiplicity of reflection angles; therefore, images are not produced. Diffuse reflectance is how people see the world. This is because the vast majority of objects in the world are diffuse reflectors. Diffuse reflection is measured by an integrating sphere, which comes in two sizes, small (60 mm diameter) and large (150 mm diameter).
One of the unique requirements of the solar industry is for specular sample to be measured on an integrating sphere. Why? Because the solar industry needs to measure total reflection (specular + diffuse) even if the sample is predominately a specular reflector. Integrating spheres are the only devices which excel at total reflection measurements.

The 60 mm Integrating Sphere
The 60 mm integrating sphere is a low cost reflectance accessory that measures both “diffuse only” and total reflectance. It consists of a hollow 60 mm sphere of highly reflective Spectralon polymer with two holes for the sample and reference beam to enter and two ports for placement of sample and reference material. Background collection (autozero) is performed by placing two Spectralon plates at the sample and reference ports. Since Spectralon has a diffuse reflectance of 99.0+ %R, the reflectance of spheres can be assumed to be close to absolute %R. Because of the size of the sphere and the lack of “baffling” of the detectors in the sphere from first bounce sample reflectance; this sphere type is subject to several types of spectral artifacts such as incorrect %R values and steps at instrumental filter and detector change points.
In the solar industry however, total reflection measurements must be obtained for highly specular samples. Under these circumstances the 60 mm integrating sphere can provide acceptable spectra for diffuse, specular,  and combination diffuse/specular samples. We will show here that although the 60 mm sphere can be prone to artifacts, with the use of the proper background correction techniques, excellent and accurate specular reflectance data can be obtained on this type of sphere.

Figure 2

What happens when you measure a totally specular sample with little or no diffuse component in a sphere? In order to answer this question we need to understand what happens to the light inside the sphere. Figure 2 depicts how both diffuse and specular light reflect off the sample and into the sphere. The diffusely reflected light will evenly illuminate the entire area inside the sphere through 380 degrees (Figure 2, left). The specular reflected light, however, will strike only an area along the midline of the sphere in the vicinity of the transmittance port (Figure 2, right).

Figure 3A

Photographs of this area, taken via a webcam inserted inside the sphere, are shown in Figures 3 A, B, and C. The photograph in Figure 3A shows the even illumination of the sphere from a 100% diffuse reflectance sample; however, if a specular sample is placed at the sample port a “hot spot” is formed on the wall of the sphere.

Figure 3B

 

Figure 3C

This “hot spot” is photographed in Figures 3B and 3C for a 10% R and a 90% R specular sample respectively. Note that there is little illumination of the sphere interior, only the concentrated “hot spot” shows up. The brightness of the “hot spot” is directly related to the reflectivity of the specular sample. Errors result from the fact that the background correction performed with the diffuse Spectralon plate illuminates the sphere differently (see Figure 3A) than the specular sample “hot spot” illumination (see Figures 3B and 3C). The resulting different sphere interior images that the detector measures between background and sample can cause errors and artifacts.

The Test Samples
Four sample types that span a wide range of specular reflectance intensities were investigated. A piece of polished aluminum represents high reflectance (around 90%R), a wafer of silica has medium reflectance (around 50%R), and a piece of dark glass as well as a clear glass slab with back side protection represent low reflectors (around 4%R). The samples were measured in the total reflectance mode on a 60 mm PMT/InGaAs integrating sphere. A fixed 2 nm slit was used in the UV/Vis range and the servo slit mode at medium gain was used for the NIR region. Data was collected every 1 nm with a data point collection time of 16 milliseconds.

The Problem
Figures 4A and 4B graphically display the problems associated with measuring specular samples on a small integrating sphere. The data are obtained with a autozero correction using the typical Spectralon plate.

Figure 4A

The first problem is that the two highest %R samples, the NIST mirror (green) and the polished aluminum (red), have values over 100% R. This is an obvious physical impossibility and is due to the intense “hot spot” in a small, non-baffled sphere. The second problem is the step at the UV/Vis-NIR detector change for all samples. Note that if we calculated the size of the step as a percentage of %R, the step is the same size (between 5%R to 6%R) for all samples regardless of their %R intensity. Parameter juggling (UV/Vis silt vs. NIR gain) will not decrease or eliminate these steps. They are a physical reality of the specular “hot spot” inside a small sphere.

Figure 4B

When we measure diffuse samples under the same instrument conditions, there are no problems and the data are excellent. As seen in Figure 5, the spectra of the white, gray, and black Spectralon plates lack the artifacts observed with specular samples.

Figure 5

 

 

The Possible Solution
Since we know that the artifacts due to specular samples are generated by the difference in internal sphere illumination between a diffuse autozero target (white Spectralon plate) and the “hot spot” illumination of a specular sample, would it be possible to improve the situation by using a specular autozero target. To investigate this we used a NIST front surface aluminum mirror for the autozero target and then measured the test samples under the same instrument conditions as the data above. The results are displayed in Figure 6. The artifacts that appeared when a white Spectralon plate was used for the autozero target appear to be gone when we use the specular mirror in its place; however, on closer inspection we see that we may have traded on set of artifacts for another. The %R values are all below 100 and the steps at the detector change are gone, but there appears to be a “bump” in the spectra at about 818 nm and are the %R values correct?

Figure 6

How could using a NIST mirror as an autozero target cause artifacts? At this point a little information on how a UV/Vis/NIR instrument autozeros is appropriate. When a UV/Vis/NIR instrument is turned on it is literally “as dumb as a stump”. A procedure must be performed to calibrate 100 % (or 0 absorbance) on the instrument. The autozero (background correction) sets the 100 % level for the instrument and is usually performed with a “blank” or sample that is 100% R. A white Spectralon plate fits this criteria well since Spectralon is over 99% R for its usable ranger of 250 nm to 2500 nm. So when Spectralon is used as an autozero target, 100% R is properly calibrated.
But what happens when an aluminum mirror is used for an autozero target? Figure 7 shows the spectrum for a front surfaced NIST aluminum mirror. As we can see the reflectivity is well below 100% R over the entire spectral range. This means that when this mirror is employed for the autozero procedure, a %R value of less that 100% R be set in the instrument’s calibration file to that value. This means that an error in photometric accuracy (%R) is commensurate with a mirror autozero correction. In addition, the spectral features of the mirror in the background correction will be introduce into spectra measured with that correction. As a result the sharp downward peak at 818 nm (or any other additional spectral features) appear as an artifact in all spectra measured with a mirror correction.
Fortunately there is a easy solution to both of these problems.

Figure 7

 

 

The Final Solution (%RC)
 The solution is a simple mathematical calculation that uses the known values for the autozero target mirror to eliminate both the photometric and wavelength dependent artifacts (see article “Procedure for Creating %RC Correction Files in UVWinlab V6”). Equation 1 below displays the math used for this correction. The R100 spectrum corresponds the the autozero target mirror values, while the R0 spectrum corresponds to the spectrum obtained with a light trap at the sample port or open sample port. The R0 spectrum corrects for the small amount of atmospheric scattered light in the sphere.

Equation 1

When this methodology is employed and the calibrated mirror is measured for the autozero and then run as a sample, you obtain the spectrum seen in Figure 7. Here we do not see the familiar flat 100 %R spectrum obtained before, but rather the actual values for the calibrated (NIST) mirror. With the combination of a mirror used for autozero and the %RC correction math, a spectrum of a specular sample can be obtained that is both photometrically accurate and free of any wavelength dependent artifacts.
In Figures 8, 9, and 10 we see the spectra for the polished aluminum, silica wafer, and back side protected clear glass respectively. The spectra represented in each graph are:
Green Spectrum = white Spectralon plate target for autozero
Blue Spectrum = mirror target for autozero
Red Spectrum = calibrated mirror target and %RC correction used

Figure 8

 

Figure 9

 

Figure 10
 

Polarization in Specular Reflectance Measurements

Radiation emitted by a spectrophotometer’s light source is by nature unpolarized; however, this light will inevitably become partially polarized by a spectrometer’s optical components. The primary causes of this polarization are due to:
(1) The reflection type diffraction grating used in the instrument’s monochromator. This is because of the very narrow ruling distance of the lines that make up the grating, radiation with an electric vector parallel to the ruling is reflected preferentially over the perpendicular electric component.
(2) Each of the spectrometer's mirrors has the potential to contribute additional polarization. The degree to which each mirror polarizes light is a function of the angle of the incident light. For aluminum mirrors at near normal angles (less than 10 degrees from the perpendicular to the mirror surface) the polarization contribution is zero; however, as the angle of incidence increases so does the polarization. Polarization is total at incident angles close to 56 degrees from normal (Brewster’s angle).
(3) Narrow monochromator slits introduce additional polarization. To achieve the minimum spectral bandwidth of 0.05 nm on the Lambda 900 spectrometer, a geometrical bandwidth of 18 micrometers is needed.
(4) The photomultiplier tube detector for the UV-VIS range exhibits slightly different sensitivity for different directions of polarization.
As seen in Figure 1 below, the measure of polarization is generally non-zero for any given spectrometer. Most spectrophotometers have their radiation partly polarized parallel to the slit. This effect is especially strong at the wavelengths for Wood's anomalies, which reside at around 500 nm and 2000 nm for the Lambda 900. This figure clearly shows the characteristic polarization profile for each of the respective gratings used in the UV-Visible and NIR spectral ranges. The instrumental polarization increases as a function of increasing wavelength in a similar fashion for both grating regions; thereby, reinforcing the fact that the diffraction grating is by far the most dominant element contributing to the inherent polarization of the spectrophotometer’s light.

Figure 1

One possible way to control the effects of polarization would be to take into account the degree of native polarization for a given instrument; however, this can never be applied in practice because of the number of dependencies involved. Another possibility would be to use deliberately polarized radiation; however, the most practical procedure is the removal of existing instrumental polarization, i.e., to depolarize the instrument beam. The best choice for this is an optical device, called a depolarizer, which is mounted in the instrument’s light beam and generates pseudo-depolarized radiation by scrambling; however, absolute depolarization is not easy to perform across the entire wavelength range of the instrument.
The depolarizer used in the Lambda 950/1050 is of the Hanle type, which consists of two wedges of differing optical material fastened together. The first wedge is made of double refracting natural quartz material, while the second wedge, manufactured from silica, is used to correct the direction of the beam. In order to achieve maximal depolarization the angle of rotation of the depolarizer must be individually adjusted for the spectrometer in which it is installed. This is accomplished by rotating the depolarizer around the instrument’s beam axis in a trial-and-error fashion until maximal depolarization is obtained.
The spectra in Figure 2 is from a NIST mirror measured at 30 degrees on a URA. The red spectrum was measured with a common beam depolarizer in place. The black spectrum was measured with the depolarizer removed. In the previous figure we saw the dramatic difference in the polarization state of the Lambda 950/1050 at the detector/grating change. This is a result of the fact that the change occurs at the low wavelength end of the NIR grating where the polarization is highly negative and the high wavelength end of the UV-Vis grating where the polarization is highly positive. These opposite polarization states of the two gratings are incorporated as artifacts into the spectrum of a polarizing sample when there is no depolarization of the sample beam.

Figure 2

The next two figures display the results of the effects from lack of proper depolarization as a function of reflection angle on URA measurements.
As can be seen in Figure 3, there are no apparent polarization effects at angles below 15 degrees. The first effects are noted in the spectrum at 20 degrees as a minimal step at the detector/grating change point. The step continues to increase in size with increasing angle at 30 degrees.

Figure 3

As can be seen in Figure 4, the step continues to increase in size as the angle approaches Brewster's Angle (56 degrees) for aluminum where the step would be maximal.

Figure 4

Visible-NIR Detector/Grating Change Steps

Steps at the detector change point in a UV/Vis/NIR instrument equipped with a specular reflectance accessory are due to four primary causes:
    (1) Lack of total depolarization of the native light beam in the instrument.
    (2) Change of either the size or shape of the beam image on the detector from the background correction to the sample.
    (3) Difference in the noise profile between the UV/Vis and NIR regions.
    (4) Lack of proper optical alignment in the beam path of the instrument.

Only two of the items above (1 and 4) are relevant to a serviceman performing an instillation or alignment of a reflectance accessory.
Item 2 is usually caused by the sample and the step can be more or less severe depending on the individual sample's optical characteristics.
Item 3 is controlled by the energy related parameters set in the method.
Items 2 and 3 are very important because they can be the source of detector steps in instruments where the reflectance accessory and instrument are properly aligned.

Steps at the detector/grating change due to noise are fairly easy to identify. These types of steps may be problematic for the customer but they do not indicate an optical alignment problem that needs to be fixed. This type of step can be minimized by adjusting the energy related parameters of the method (such as slit size, common beam mask size, and integration time. A noise step can be easily identified by performing multiple scans on the same sample under identical instrument parameters. A noise step will not be reproducible. It's direction and magnitude will be random from measurement to measurement. Here are some examples.
Figure 1 below shows three separate measurement runs on a NIST mirror at 8 degrees with a Universal Reflectance Accessory (URA). The appearance of a step at the detector/grating change point is due to the elevated noise of the NIR region in relation to the less noisy UV/Vis region. Note the random nature of both the size and direction of the step due to the random nature of the noise. The high %R of the mirror keeps the step to a minimal level.

Figure 1

Figure 2 shows two separate measurement runs of the blackened quartz window from a microcell. It was measured on a URA at 8 degrees. The %R of this type of sample is an order of magnitude less then the NIST mirror and is an excellent example of a low reflectance sample. Because of the lowered reflectance of this sample, the noise in the NIR region is larger and as a result the step appears more pronounced. But there is no doubt that this step is due to noise and not alignment.

Figure 2

Now let's take a look at another type of sample. Figure 3 is a spectrum of a low reflectance optical coating measured at 8 degrees on a URA. On this scale everything looks good, but what happens when we expand the scale?

Figure 3

This is the same low reflectance coating as above, but with the scale increased. Now it becomes apparent that we have a step at the detector/grating change that is not from noise. The spectrum in Figure 4 has a clear off-set for the %R value between the NIR and UV/Visible. To try and judge the size of the step by an empirical visual means will not be satisfactory. Visual inspection is too dependent on arbitrary scaling to be useful. Let's try another way...

Figure 4

In Figure 5 we have displayed the spectrum full-scale. There is no question at this scale that we have a step that is greater then the noise envelope on either side of the step.
To size the step we obtain the ordinate value on the high wavelength side of the step and the corresponding ordinate value for the low wavelength side of the step. Subtraction of the lower value from the higher yields the delta value of the step. This delta value is meaningless in itself and must be compared to the ordinate value (%R) to be meaningful. If we divide the delta value by the ordinate value at the step wavelength and multiply by 100, we obtain a relative percentage of the step to the ordinate signal.
For the spectrum below the step is about 1.6% of the ordinate value.

Figure 5

In Figure 6 is shown a spectrum of a NIST mirror measured on a URA. The URA is out of alignment and as a result has a step at the detector/grating change. The same procedure was used to obtain the relative step size. The value calculated for the step was about 0.2%R. Note here how looks can be deceiving. Although the relative step magnitudes are about the same for the spectrum below and the one above, the step in the spectrum here appears smaller than the step from the spectrum above. This is due to scaling differences. Note also that the relative here is 0.2% which is much lower than the 1.6% from the spectrum above; thus, the step below is small when compared to the level of reflectance signal.

Figure 6

Expressing the step as a percentage of the total reflectance signal is an excellent way to compare the relative magnitude of the detector change step anomalies.