Short-wavelength infrared reflectance spectroscopy of minerals by supercontinuum illumination and speckle reduction

The advent of broadband supercontinuum (SC) light sources offers an opportunity to improve techniques such as reflectance spectroscopy. In this study, a SC laser, providing a broad spectral bandwidth and higher-intensity light than traditional halogen lamps, is used to illuminate mineral samples in the short-wavelength infrared (SWIR; 1.10– 2.35 μm) spectrum. To overcome speckle interference originating from the intrinsic coherence of the SC laser illumination, a rotation-based technique is applied to achieve high signal-to-noise interference-free reflectance spectra of the three minerals studied (muscovite, kaolinite and calcite) and these spectra are compared to reference spectra obtained from the United States Geological Survey spectral library. We report a speckle magnitude, σSpk < 2×10−3 (35 times better than without rotation) resulting in a measurement time < 1.6 s with a repeatability of > 99%. The benefits and limitations of using SC laser illumination over halogen lamp illumination are discussed with regards to the measurement time, speckle, laser operation safety and cost. The technique described could be used for time-efficient (< 1.6 s) in-lab mineral assay of minerals having distinct SWIR absorption features or developed for real-time (within few milliseconds) on-conveyor mineral assay of crushed ores.