Author: Michaela Kastanek, ASD Chemometric Specialist
In the world of NIR spectroscopy the words “calibration”, “white reference”, and “baseline” are often used interchangeably. Essentially these are all describing the same thing – the use of a reference material that has essentially 100% reflectance to calibrate the instrument to create a reflectance spectrum. But that raises a few questions. What is this white reference? Why is it needed? And is it truly 100% reflective?
What is it?
ASD field and lab based spectrometers all use a sintered polytetrafluoroethylene material better known by its commercial name, Spectralon, as a white reference. Produced by LabSphere and sold with all ASD instruments, Spectralon has a few unique qualities that make it an ideal material for optimizing ASD spectrometers. It comes in large and small round disks and square flat panels. It is rugged enough for field work and relatively easy to clean, and it is thermally and chemically stable.
Why is it needed?
Spectralon is uniquely suited as a white reference material because of its property of Lambertian reflectance. It reflects light at all angles equally creating nearly perfect diffuse reflectance. No matter the angle of the fiber optic input, the instrument will receive the same amount of light. This property is useful when optimizing the instrument – no one portion of the spectrum is weighted more than another.
Is it truly 100% reflective?
The short answer is ‘No’. Spectralon is nearly 100% reflective. However, as white, featureless materials go Spectralon is probably one of the whitest and most featureless you will ever come across. The NIR spectrum of Spectralon, shown below, displays 97% or higher reflectance for the regions 350-2000 nm. Where the reflectance level does fall off in the higher wavelengths, the values are still 93% or better. As far as the spectrometer is concerned, the value is 100% - it is the whitest, most reflective thing the instrument will see, especially compared to a sample. When calculating relative reflectance, the digital number of the sample is ratioed against the digital number of the Spectralon. Because the instrument assumes 100% for the Spectralon reflectance, this measurement is termed “relative reflectance”.
If the ASD system is to be used with satellite or overflight sensor data, then the Spectralon panel must be calibrated as shown below so that the spectrum produced by the ASD system is “absolute reflectance”. The exact same Spectralon material is used for both relative and absolute reflectance and the difference between these is that one assumes 100% reflectance and the other uses a calibration file to account for wavelengths where the spectralon is not at 100%.
Although Spectralon can be cleaned using wet/dry sandpaper, we do not recommend this for calibrated panels; the cleaning procedure may alter the Lambertian properties of the calibrated panel, thus requiring it to be recalibrated.
ASD leverages the unique properties of Spectralon as a white reference in our instruments and accessories. It is found in specialized caps for the Contact Probe, multipurpose round disks and as part of the automatic white reference mechanism found in TerraSpec® Halo.
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