Miniature near-infrared spectrometer using linear variable filter and metal capillary cavity with lens for detecting water in ethanol

A compact and highly sensitive near-infrared spectrometer based on linear variable filter (LVF) and metal capillary cavity (MCC) with lens was proposed, which can simultaneously enhance both received light intensity and spectral resolution, because the light output from MCC can be easily collimated into a small parallel beam and passes vertically through the LVF. The spectrometer was applied to detect water in ethanol (WIE). The transmission spectrum of WIE in MCC was measured in the wavelength range of 930-1670 nm, and the weakest and strongest response to water content was found at wavelength 1.07 μm and 1.3 μm, which were chosen as the calibration wavelength ( λ c ) and measurement wavelength ( λ m ), respectively. Through spectrum calibration at λ c , the relative standard deviation (RSD) of replicated detection was reduced from 0.2% to 0.07%, and the water content in WIE can be directly discriminated from the transmission spectrum at λ m with a limit of detection (LOD) of 200 ppm. The LOD is 50-fold lower than that of commercial spectrometer (1 cm cuvette) and lower than that (400~1000 ppm) of previously reported spectrometers with complicated chemometric methods. The spectrometer features miniaturization, high sensitivity, fast detection, and low cost. • A spectrometer based on metal capillary cavity and linear variable filter. • The received light intensity and spectra resolution can be simultaneously enhanced. • The spectrometer was applied to detect water content in ethanol with LOD of 200 ppm. • Spectrum calibration reduced the relative standard deviation from 0.2% to 0.07%. • The spectrometer features miniaturization, high sensitivity and low cost.