Development of a Handheld Spectrometer Based on a Linear Variable Filter and a Complementary Metal-Oxide-Semiconductor Detector for Measuring the Internal Quality of Fruit

Abstract

Visible and near infrared spectroscopy has long been used to predict fruit internal quality, with portable instrumentation advantageous for in-field use. We developed a handheld spectrometer using a linear variable filter (LVF) and a complementary metal-oxide-semiconductor (CMOS) linear detector array. The LVF is a bandpass filter with a centre wavelength changing linearly in one direction and can replace a grating as the light-dispersion component. An LVF was designed and fabricated specifically to work in the 620–1080 nm region and for the analysis of fruit. The optical design used an improved collimator and an LVF to yield a compact, stable and low-cost optical engine. By using a CMOS detector and other suitable electronics, the spectrometer achieved a low power consumption. The spectrometer can analyse spectral data using an onboard prediction model and can be operated from a remote smartphone, tablet or laptop computer. This paper details the design of the spectrometer and the results of its resolution and stability tests. The spectrometer operated with a resolution of less than 1.5% centre wavelength and a signal-to-noise ratio of up to 5000. The spectrometer was then used to predict the sugar content in pears. The optimised model provided an R2c value of 0.96, standard error of calibration value of 0.29 °Bx and standard error of prediction value of 0.46 °Bx. The results indicated that this LVF-based spectrometer is promising for measuring the internal quality of fruit.