Submitted to: Transactions of the ASABE
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/16/2005
Publication Date: 2/22/2006
Citation: Peng, Y., Lu, R. 2006. A LCTF based multispectral imaging system for estimation of apple fruit firmness: Part I: acquisition and characterization of scattering images. Transactions of the ASABE. 49(1):259-267. Interpretive Summary: Firmness is an important parameter in grading apple fruit, and it directly influences consumer acceptance. Currently, the Magness-Taylor (MT) firmness tester, the standard destructive technique, is routinely used in the fruit industry to measure fruit firmness. A nondestructive firmness sensing technique would be valuable for determining optimal harvest time and implementing appropriate postharvest handling and marketing strategies. A multispectral imaging system with a low cost digital camera was developed for measuring spectral scattering from 'Red Delicious' and 'Golden Delicious' apples over the visible and near-infrared (longer than the visible) region to predict fruit firmness. The system can be electronically tuned for rapid selection of any narrow wavebands between 650 nm and 1000 nm. A proposed mathematical model accurately described light scattering for different wavelengths; its parameters were useful for predicting fruit firmness. Correlation between fruit firmness and scattering parameters for single wavelengths was generally low (less than 0.58), suggesting the need of using multiple wavelengths for prediction of fruit firmness. The multispectral imaging system provides an effective means for determining optimal wavelengths for firmness measurement. The research demonstrated that a cost effective multispectral imaging system can be developed for measuring and monitoring fruit firmness nondestructively, thus assuring the delivery of superior, consistent fresh products to the marketplace.
Technical Abstract: Fruit firmness is important for consumers, and it is routinely measured by the Magness-Taylor penetration method in the fruit industry. This research developed a compact multispectral imaging system with a low cost CCD (charge-coupled device) camera and a liquid crystal tunable filter (LCTF) to capture spectral scattering images from Red Delicious and Golden Delicious apples in a wavelength increment of 10 nm between 650 nm and 1000 nm. A modified Lorentzian distribution (MLD) function was proposed to characterize spectral scattering images and its parameters were then used to predict fruit firmness. The MLD function with four independent parameters described scattering profiles accurately, with average r = 0.999 and the standard error of estimate (SEE) of 2.1 x 10**-4. The spectral scattering broadened as fruit firmness decreased and its profile varied with wavelength. The correlation between MLD parameters and fruit firmness changed with wavelength; best firmness estimates for one single wavelength were obtained with r = 0.58 and SEE = 8.81 N for Red Delicious, and r = 0.50 and SEE = 8.64 N for Golden Delicious. The multispectral imaging system, along with the MLD scattering profile analysis method, provides us with a means of finding optimal wavelengths for better prediction of fruit firmness.