Xiaping, Fu, et al. “Quantitative and Qualitative Measurement of Pear Firmness Based on Near Infrared Spectroscopy and Chemometrics”. International Journal of Agricultural and Biological Engineering, vol. 1, no. 1, Sept. 2008, pp. 69-74, https://www.ijabe.migration.pkpps06.publicknowledgeproject.org/index.php/ijabe/article/view/18.