A Comparison of optical measurements in turbid media

Accuracy and precision are typically among the most important factors in the design of an optical measurement system for quantitative assay of natural products. Often the variation expected in the constituent and absorber of interest is small - perhaps less than 20% of the mean as for soluble-solid concentration in Kiwifruit, for example. Most natural products in their raw form are not pure absorbers: they also scatter light. This complicates the measurement problem as scatter can introduce variation far greater than the absorption we are trying to measure. We describe an experimental system, revolving around the measurement of temperature in the liquid phantom Intralipid. Many absorption bands, including those in water, respond to temperature as subtly as they do to concentration in turbid media, yet temperature can be easily controlled and modulated. We have found this useful for quantifying the precision and accuracy of optical measurements in scattering samples. Two methods, intensity-modulated optical spectroscopy and continuous wave coupled with diffusion theory, have been applied to estimate temperature from the absorption coefficient of Intralipid. The precision of these measurements is compared to more conventional near-infrared multi-spectral and partial least-squares calibrations.