Experiment on Laser Speckle Imaging of Apples Using A CMOS Camera


Laser Speckle Imaging recently has become a promising tool to assess fruit quality and can be applied in fruit sorting. It is a non-destructive, optical method which uses light scattered of fruit surface after illuminated by laser light. Laser speckle imaging methods use He-Ne laser or diode laser as the light source yet both lasers have differences in performance and price. Diode lasers are more preferable due to their efficiency, low cost, small and compact, and varied wavelength.  In this research, an optical system which consisted of a laser source and a CMOS camera was used for Laser Speckle Imaging.  A 632,8 nm He-Ne laser and a 650 nm diode laser were used, their performances were compared. The samples were two kinds of apples, Red Delicious and Fuji, five samples for each cultivar. The laser light was expanded using a beam expander hence illuminated on the apple surface at 30o angle.  A monochrome Thorlabs CMOS camera with camera lens was used to record the speckle patterns of the apple surface. Both lasers were kept at the same laser power by Neutral Density Filters. ImageJ software was used to calculate the gray value of the speckle pattern for each sample, the speckle gray values were compared for different laser light sources and apple types. The results showed that there is a significant difference in gray value level between both apples. Higher maximum gray values were found on the Fuji apples compared to the Red Delicious apples which were about 21.7 % when using He-Ne laser and 18.3 % when using diode laser.  Higher maximum gray value for Fuji apples could be due to their rounder shape, firmer skin and flesh, they scatter more light.  The curvature and the firmness of the fruits affected the gray value level. For each apple type, there was a slight difference in maximum gray values for both laser sources. Higher gray values were obtained when using diode laser compared to the He-Ne laser, 22.0 % difference for Fuji apples and 25.2 % difference for Red Delicious apples. These could be because of the less coherence, wider bandwidth, and irregular beam shape of the diode laser that it scatters more light and suppresses speckles.

[1] S. Sankaran, A. Mishraa, R. Ehsania, and C. Davis, A Review of Advanced Techniques for Detecting Plant Diseases, Comput Electron Agric, 72, 1–13, (2010).

[2] J. A. Abbott, Quality measurement of fruits and vegetables, Postharvest Biol Technol, 15, 207–225, (1999).

[3] H. Sun, in A Practical Guide to Handling Laser Diode Beams, Chapter 2, Springer, 2015.

[4] J. A. Kodagali and S. Balaji, Computer Vision and Image Analysis based Techniques for Automatic Characterization of Fruits, a Review International Journal of Computer Applications, 50, 0975–8887, (2012).

[5] R. Mahendran, G. C. Jayashree, and K. Alagusundaram, Application of Computer Vision Technique on Sorting and Grading of Fruits and Vegetables, J Food Process Technol, no. S5, (2011).

[6] G. F. Rabelo, A. Roberto, B. Junior, and I. M. D. Fabbro, Laser Speckle Techniques In Quality Evaluation Of Orange Fruits, Rev Bras Eng Agric Ambient, 9, no. 4, 570–575,(2005).

[7] A. Zdunek and J. Cybulska, Relation of biospeckle activity with quality attributes of apples, Sensors (Basel), 11, 6317–6327, (2011).

[8] G. G. Romero, C. C. Martinez, E. E. Alanis, G. A. Salazar, V. G. Broglia, and V. GL. Alvarez, Bio-speckle activity applied to the assessment of tomato fruit ripening, Biosyst Eng, 103, 116–119, (2009).

[9] D. Briers, D. D. Duncan, E. Hirst, S. J. Kirkpatrick, M. Larsson, W. Steenbergen, T. Stromberg, and O. B. Thompson, Laser speckle contrast imaging: theoretical and practical limitations, J Biomed Opt, 18, p. 066018, (2013).

[10] J. W. Goodman, Some fundamental properties of speckle, J Opt Soc Am, 66, 1145– 1150, (1976).

[11] B. Dobrzañski, J. Rabcewicz, and R. Rybczyñski, Handling of Apple: transport techniques and efficiency vibration, damage and bruising texture, firmness and quality, B. Dobrzañski Institute of Agrophysics Polish Academy of Sciences, LUBLIN, ISBN 83-89969-55-6