Development of a Multispectral Imaging Prototype for Real-Time Detection of Apple Fruit Firmness

Authors: Lu, Renfu; PENG, YANKUN - MICHIGAN ST UNIVERSITY

Submitted to: Optical Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/13/2007
Publication Date: 12/1/2007
Citation: Lu, R., Peng, Y. 2007. Development of a Multispectral Imaging Prototype for Real-Time Detection of Apple Fruit Firmness. Optical Engineering. 46(12):123201.

Interpretive Summary: Firmness is a key parameter in assessing the overall quality of apples. Sorting and grading apples for firmness has long been an important issue for the fruit industry. Several mechanical methods (including impact and sonic) have been developed for nondestructive measurement of fruit firmness. However, these methods do not correlate well or consistently with the standard destructive firmness measurement. Recently, we developed a new optical sensing technique for measuring fruit firmness, which is based on measuring light scattering in the fruit and correlates fruit spectral scattering characteristics to the standard firmness testing method. This paper reports on the development of a laboratory prototype for real-time detection of apple fruit firmness. The prototype, which mainly consisted of a multispectral imaging unit coupled with four specially designed lasers, was able to acquire four spectral scattering images simultaneously at a speed up to two fruit per second. Experiments were performed for real-time measurement of spectral scattering characteristics of ‘Golden Delicious’ and ‘Red Delicious’ apples. The prototype, incorporated with the newly developed mathematical models, achieved good predictions of fruit firmness with the correlation coefficient of 0.86 for both apple varieties. The technique is fast and relatively easy to implement. With further improvements in speed and performance, this new technology will be useful for online sorting and grading of fruit for firmness with an additional capability of measuring fruit sugar content. The technology will provide the fruit packers an effective means for delivering premium quality fruit to the marketplace and better meet the consumer and/or market needs.

Technical Abstract: Multispectral scattering is a promising nondestructive technique for assessing the firmness of fruit. This paper reports on the development of a laser-based multispectral imaging prototype for real-time detection of apple fruit firmness. The prototype consisted of a common aperture multispectral imaging unit, a specially designed multi-laser unit and a fruit handling unit, which captured and processed four spectral scattering images simultaneously. The multispectral imaging system was tested for detecting the firmness of ‘Golden Delicious’ and ‘Red Delicious’ apples at a rate of one fruit for every two seconds. The original two-dimensional scattering images were corrected by removing noise pixels and incorporating fruit size into the calculations of the scattering distance and intensity. The one-dimensional scattering profiles were fitted with a four-parameter Lorentzian distribution function. Multi-linear regression models were developed using the four Lorentzian parameters for the four wavelengths to predict the firmness of apples. The multispectral imaging system achieved good firmness predictions with the correlation coefficient of 0.86 for both ‘Golden Delicious’ and ‘Red Delicious.’ The technique is fast and relatively easy to implement, and it has the potential to meet the requirement for online sorting and grading of apple fruit.