Abstract
Spatial frequency domain imaging (SFDI) is a non-contact wide-field optical imaging technique for optical property detection. This study aimed to establish an SFDI system and investigate the effects of system calibration, error analysis and correction on the measurement of optical properties. Optical parameter characteristic measurements of normal pears with three different damage types were performed using the calibrated system. The obtained absorption coefficient μa and the reduced scattering coefficient μ’s were used for discriminating pears with different surface damage using a linear discriminant analysis model. The results showed that at 527 nm and 675 nm, the pears’ quadruple classification (normal, bruised, scratched and abraded) accuracy using the SFDI technique was 92.5% and 83.8%, respectively, which has an advantage compared with the conventional planar light classification results of 82.5% and 77.5%. The three-way classification (normal, minor damage and serious damage) SFDI technique was as high as 100% and 98.8% at 527 nm and 675 nm, respectively, while the classification accuracy of conventional planar light was 93.8% and 93.8%, respectively. The results of this study indicated that SFDI has the potential to detect different damage types in fruit and that the SFDI technique has a promising future in agricultural product quality inspection in further research.
Highlights
During the collection and transportation of fruit, collisions are likely to occur, causing surface defects such as bruises, scratches and abrasions, while damaged fruits are prone to decay and affect other normal fruits
The specific objectives of this study were (1) to develop a Spatial frequency domain imaging (SFDI) system and its control software that can be used for optical property measurements; (2) to perform projector–camera calibration of the SFDI system, and perform keystone correction and frequency calibrations based on the calibration results; (3) to use liquid phantoms to calibrate the measurement errors of the system and calculate the μa and μ’s of ‘crown’ pears; (4) to discriminate the normal pears and pears with different damage types based on the obtained μa and μ’s
An optical calibration and correction method was proposed for the SFDI system
Summary
During the collection and transportation of fruit, collisions are likely to occur, causing surface defects such as bruises, scratches and abrasions, while damaged fruits are prone to decay and affect other normal fruits. The commonly used rapid and non-destructive detection methods are mainly visible light imaging and hyperspectral imaging techniques based on reflectance measurements [1,2], and Raman techniques based on inelastic scattering [3]. These techniques find it difficult to achieve the detection of fine defects and initial damage in fruits [4,5]. SR technique can only perform point measurements, the equipment is cheaper and can be non-destructive These methods were described in detail in a recent review by Lu et al and can be consulted by those interested [10]
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