Measurement of optical properties of onion skin and flesh at 633 nm

Evaluating onion quality using optical techniques is challenging because of the presence of dry outer skin and the layered structure of onion fleshy tissues. To better understand the light propagation in onions, the optical properties of dry skin and fleshy tissues from two cultivars were measured at 632.8 nm by using a single integrating sphere-based system. Onion tissues were cut into 30 mm square pieces and sandwiched by Borofloat glass slides. The total diffuse reflectance, the total transmittance, and the collimated transmittance of the onion samples were measured by an integrating sphere system with a VIS-NIR spectrometer. The absorption coefficient (µa), the reduced scattering coefficient (µs’), and the anisotropy coefficient (g) of onion tissue samples were estimated using the inverse adding-doubling method based on the measured spectra. The light propagation in onion tissues were modeled based on the calculated optical parameters using Monte Carlo simulations. The results indicated that onion tissues are high albedo biological media. Dry onion skins have much higher absorption and reduced scattering coefficients than onion fleshy tissues. Comparisons between the two onion cultivars showed that the optical properties of onions could vary with cultivars. The results of this study can be used to develop appropriate optical approaches for onion quality inspection.


Figure 1. The schematic of the system configurations for measuring the total reflectance (A), the total transmittance (B), and the collimated transmittance (C)


Figure 2. (II) demonstration of the onion tissue samples: the outermost dry skin (A), the outer skin (B), the first layer of fleshy scale (C), and the second layer of fleshy scale (D).


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Figure 3. The measurement of optical properties of onion samples at 632.8 nm: (a) anisotropy, (b) absorption coefficients, (c) scattering coefficients, and (d) relative transmittance

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