Optical properties of healthy and rotten onion flesh from 700 to 1000 nm

To facilitate the design of an optical system for detecting internal rot in onions, the light propagation properties in healthy and rotten onions were studied in the 700 to 1000 nm range. The absorption and reduced scattering coefficients of internal onion tissues were measured using a combination of measurements on excised onion slices, using the Inverse-Adding Doubling method (IAD), and light transmittance measurements on extracted onion juice. Standard NIRS transmittance measurements were made on intact onions, revealing a transmission window in the range of 670 to 950 nm. From 700 to 750 nm, more severely rotten onions had lower transmittance, whereas from 850 to 900 nm onions with larger rotten tissue had higher transmittance. A multi-laser light transmittance system was implemented to make accurate measurements for different source-detector positions around intact onions, enabling calculations of attenuation coefficients at 636, 700, 728 and 804 nm. The results showed the onion juice measurements yielded much more accurate absorption coefficients than the IAD method on the onion slices. The attenuation coefficients calculated using accurate optical properties were generally matched with the light transmission characteristics, and were in good agreement (6% on average) with the measured attenuation coefficients for both healthy and rotten onions at 700, 728 and 804 nm.