Validated multi-wavelength simulations of light transport in healthy onion

Multi-wavelength light transport simulations of individual brown onions have been validated in the wavelength range 710-850 nm by comparing the simulations with experimental results. The simulations used the Finite Element Method (FEM) implemented in the software code NIRFast in Matlab. NIRFast required tissue geometry, optical scattering and absorption properties as inputs. The scattering values were obtained from Inverse Adding Doubling (IAD) measurements of onion slices. The absorption properties of several components were investigated. They included water, chlorophyll and onion absorption values measured with the IAD method or using transmission measurements on onion juice. These inputs to NIRFast enabled simulations of the transmission through model onions with different light attenuation characteristics. Error between simulation and measurement was minimized using absorption values from onion juice. An imposed realistic water concentration constraint (∼88%) removed cross talk between scattering and absorption. This resulted in a set of optical properties, based on a small set of component absorbers and Mie scattering theory, which accurately modelled light transport in each onion. The input absorption values were then checked for completeness with inverse simulations at each wavelength independently. Results indicated no major component was missing from the absorption model. The resulting optical property estimates were used to predict a range of transmission measurements from a high speed conveyor system.