OPTICAL PROPERTIES OF HEALTHY AND DISEASED ONION TISSUES IN THE VISIBLE AND NEAR-INFRARED SPECTRAL REGION

Optical techniques such as spectroscopy and hyperspectral imaging are promising tools for the nondestructive inspection of onions. To apply optical techniques on onions appropriately, it is important to understand the fundamental optical properties of onion tissues. In this study, the light absorption coefficient (mu(a)), reduced scattering coefficient (mu(s)'), and scattering anisotropy (g) of onion tissues were estimated in the wavelength ranges of 550-880 nm and 950-1650 nm. Dry skin and flesh samples of healthy onions, Burkholderia cepacia-infected (causing sour skin) onions, and Botrytis aclada-infected (causing neck rot) onions were tested. The total diffuse reflectance, total transmittance, and collimated transmittance spectra of the samples were collected using spectroscopic systems that consisted of an integrating sphere, fiber optic guide, and spectrometer. Based on the collected spectra, the mu(a), mu(s)', and g values of the onion tissues were calculated using the inverse adding-doubling method. The results indicated that onion dry skin and flesh were scattering-dominated biological tissues at wavelengths below 1300 nm. The mu(a) values of the dry skin of onions with sour skin were significantly greater than those of healthy onions in the near-infrared region. Dry skin samples from onions with neck rot were statistically different from those of healthy onions in terms of mu(a) in 550-750 nm and mu(s)' in 550-1650 nm. The flesh of onions with sour skin or neck rot was significantly different from that of healthy onions in terms of mu(a) in 550-1100 nm and mu(s)' in 550-1650 nm. This study demonstrates the feasibility of detecting diseased onions by investigating their optical characteristics. The measured optical properties of healthy and diseased onion tissues can be used in theoretical modeling and simulations of light-onion interactions for developing onion quality inspection systems.