An LCTF-Based Multispectral Imaging System for Estimation of Apple Fruit Firmness: Part I. Acquisition and Characterization of Scattering Images

Fruit firmness is still routinely measured with the Magness-Taylor penetration method in the fruit industry. Thisresearch developed a multispectral imaging system with a low-cost CCD (charge-coupled device) camera and a liquid crystaltunable filter (LCTF) to capture spectral scattering images from apple fruit in 10 nm increments from 650 to 1000 nm. RedDelicious and Golden Delicious apples were tested in this research. A modified Lorentzian distribution (MLD) function withfour independent parameters was proposed to characterize spectral scattering images. The MLD function describedscattering profiles accurately for wavelengths between 650 and 1000 nm with an average correlation coefficient (r) of 0.999.The four MLD parameters for individual wavelengths were used to predict fruit firmness. The correlation between MLDparameters and fruit firmness changed with wavelength; the best firmness estimates for a single wavelength were obtainedwith r = 0.58 and SEE = 8.81 N for Red Delicious apples, and r = 0.50 and SEE = 8.64 N for Golden Delicious apples. Themultispectral imaging system, along with the MLD scattering profile analysis method, provides a means for quantifyingspectral scattering characteristics in apples and finding optimal wavelengths for better prediction of fruit firmness.