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As one of the most important ingredients of our diet, onion is the second largest fresh vegetable in the U.S. To meet the increasing quality demand of consumers, modern optical techniques like hyperspectral imaging have been investigated to evaluate onion quality nondestructively. To better apply these techniques, the optical properties of the dry skin and the flesh of healthy and sour skin infected onions were measured in the wavelength range of 450-1000 nm. The total diffuse reflectance, total transmittance, and collimated transmittance spectra of onion tissues were collected by using an integrating sphere system with a VIS-NIR spectrometer. The absorption coefficient (a), reduced scattering coefficient (s), and anisotropy (g) of the onion tissues were calculated using the inverse adding-doubling method based on the measured spectra. The results showed that both onion dry skin and flesh were scattering dominated biological tissues. The estimated a and s values of onion dry skins and flesh were comparable to the optical properties of other fruits and vegetables reported in the literature. In the Vis-NIR range, the a and s of the flesh of sour skin-infected onions were significantly different from those of the healthy onions, which proved the possibility to inspect quality of onion flesh by optical techniques. The results also suggested that onion dry skins should be handled properly in optical measurements since they can significantly affect the light propagation. The results of this study can be used to develop appropriate optical techniques to improve quality control of onions.