An Experimental Comparison of Simple NIR Spectrometers for Fruit Grading Applications by C. V. Greensill, D. S. Newman

Abstract

Three simple near infrared (NIR) spectrometers, with differing dispersion elements (single equilateral prism, two equilateral prisms in series, and a ruled diffraction grating) were constructed to compare their relative performance for suitability for in-line fruit sorting. Performance in terms of resolution, spectral range, efficiency, and linearity across the spectral range was characterized using a HgAr discharge source. Further, a practical application was assessed by comparison of spectra of transmitted radiation from whole immature and mature limes (Citrus aurantifola) over the wavelength range 650-1050 nm. Light from a 100 W quartz-halogen light source was focused into samples of whole fruit and the transmitted radiation collected, spectrally dispersed, and focused onto a 2048-pixel linear charge-coupled device. The dual-prism configuration demonstrated a relatively uniform wavelength-independent optical transmission efficiency and dispersion. In comparison, the diffraction grating efficiency was a non-linear function of wavelength, blaze angle, incident angle, and coating material. Hence, although all designs performed adequately, the dual-prism instrument demonstrated a higher potential for more reliable, rapid sorting than those based on either the diffraction grating or the single prism.