Development of a multispectral imaging system for apple firmness prediction

Multispectral imaging has been studied in recent years as a means of assessing fruit firmness. Here we report on the development of a static multispectral imaging system (MSI) that was used to validate the potential of the technique for high-speed commercial grading. The system consists of a high-performance CMOS camera, four lasers, electronically controlled shutters and a location control system. It captures four spectral scattering images on the fruit. In this study 100 `Royal Gala' apples have been measured using the static MSI. The MSI measurements were applied on the intact apple and also on a flat surface of internal flesh exposed by removing a thick skin slice. The scattering profiles were fitted with a standard light diffusion model and a heuristic modified Lorentzian model. The results showed that the correlations between penetrometer firmness and model parameters were poor, with the correlation coefficient, R, ranging from 0.4 to 0.8 in the best circumstance of the flat surface measurements. Although multivariate models can improve the correlations, this work suggests that laser scattering information on its own is not sufficient to predict the firmness.