Designing good sampling optics: Estimation of optical scattering and absorption properties of real process samples

A present study addresses some cornerstones in designing high performance optical instruments. Very often the practical work starts via feasibility studies on whether something can be measured or not. Either lab or process instruments are used to find response in a given spectral range. Other specifications address signal-to-noise ratio, sensitivity, selectivity, application-related linearity as well as instrument related photometric linearity, accuracy, and repeatability. Much of the work focuses on establishing a correlation between primary lab method and secondary spectral method. Less attention is often paid to scattering and absorption properties of samples. At some later point in application development, all method developers pay attention to these issues at latest when calibrating spectral instruments with chemometric tools, either knowingly or unknowingly. In many industries, both near and mid infrared spectroscopy rely on sample chemistry and specific absorptions while down-weighting the physical interferences caused by packing density variations, particle size distribution differences, surface topography variations, and/or other optical path length variations. Chemometric tools such as standard normal variate (SNV), spectral interference subtraction (SIS), inverted signal correction (ISC), multiplicative signal correction (MSC), and their extensions (EISC and EMSC) correct for these physical interferences and greatly simplify calibrations. While these methods solve the problem for calibration, there are areas where this is not enough, namely, the design processes of optical instruments. This work illustrates some tools developed and used routinely in VTT Optical Instrumentation Center prior to designing new optical instruments. Particularly, these tools address the estimation of optical properties of sample materials representative to real processes. This work illustrates how experimental data can be collected using a specially designed hardware accessory and high performance double-beam lab UV-vis-NIR spectrometer and how tailored software tools can be used to study these data and compute optical properties of sample materials. Though these optical properties may not be exact estimates, they are very important and accurate enough for the design of tailored optical instruments for performance-critical customer applications. The study shown here presents results from near infrared (NIR) measurement made on industrial applications. The results and methods shown here are directly applicable both for studying sample properties and for designing appropriate sampling optics for existing commercial or tailor-made optical instruments whether the working range is in ultraviolet (UV), visible, fluorescence, Raman, NIR, or mid infrared (MIR). This study shows how the above methods can bring along significant benefits in the design phase of optical instruments. To conclude this presentation, we present also general guidelines how to design a high performance optical instrument