Parabolic Concentration of Diffusely Transmitted near Infrared Radiation in an Acousto-Optic Tunable Filter Spectrometer

The research model near infrared (NIR) spectrometer described in this paper, worthy of quantitative use and suited for industrial applications, is the result of stepwise refinement of the acousto-optic tunable filter (AOTF)-based monochromator and other optical components. A sensitive and rapid response extended-range indium gallium arsenide (InGaAs) detector (covering the 1100–2500 nm range) was selected to match the high speed of acousto-optic TFS electronic wavelength switching. Excellent quantitative performance has been demonstrated in comparison with commercial grating monochromator and Fourier transform (FT)-NIR instruments for multi-component liquids, films, pumped fluids and even supercritical fluids. Software-controlled wavelength switching, data acquisition and programmed or interactive time averaging at each wavelength demonstrated analytically-enhanced performance. Throughput is not limited by slits, and exposure of the sample to white light, as with a diode array or an FT instrument, is avoided. Although focusing optics between sample and detector were found to work well for concentrating radiation on a small surface area InGaAs detector, detector size limited performance in the case of optically difficult samples because of the low solid angle of collection. Use of an experimental non-focusing multistage parabolic concentrator immediately after the sample vessel and before the 1 mm2 detector was found to offer an advantage for highly scattering specimens. Although the reflection optics of the locally fabricated concentrator directed a lower intensity of light transmitted by clear specimens than from the standard lens arrangement, successful analysis was extended to include highly scattering samples. The percentage of incident radiation attenuated via scatter was reduced, and for scatter beyond a point, the absolute intensity of radiation falling on the detector was greater, even though the absolute maximum of non-scattered intensity was less, thus establishing proof of principle. We anticipate that further improvement could be attained for highly scattering samples with a commercial optics manufacturer's version of the homemade concentrator.