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.