Design of the Optical System for a grating front-dispersive near-infrared spectrometer with a large-spot

Classical grating front-dispersive near-infrared (NIR) spectrometers, constrained by their small illumination spot size (typically around 1 cm × 2 cm), often require destructive sample pretreatment and repeated measurements at multiple positions to ensure data representativeness when analyzing nonuniform samples. These limitations significantly increase the overall measurement time. To address this issue, we designed a grating front-dispersive large-spot NIR spectrometer optical system, which enables rapid and non-destructive detection of nonuniform samples by increasing the illumination spot size. The proposed system integrated a tungsten halogen light source and employed a cross-asymmetric Czerny-Turner optical configuration. A concave mirror was used to expand the beam to form a 4 cm × 2 cm illumination spot, while an integrating sphere and an InGaAs detector were used to uniformly collect and detect the diffuse-reflection signals. Key parameters of the integrating sphere, such as height and port size, were optimized through optical simulations using the LightTools platform and further validated through experimental testing to determine the optimal sizes. The results demonstrated that the system maintained acceptable optical throughput while providing good spectral repeatability, with absorbance noise as low as 0.0006 Abs. The system was successfully applied to the non-destructive discrimination of tobacco leaves from the upper, middle, and lower stalk positions. A partial least squares discriminant analysis (PLS-DA) model exhibited outstanding classification performance, achieving high classification correct rate and rational recognition rate. This design provides a reliable technical solution for the non-destructive detection and real-time monitoring of nonuniform samples in agriculture.