Quantitative measurements of turbid liquids via structured laser illumination planar imaging where absorption spectrophotometry fails.

A comparison between the commonly used absorption spectrophotometry and a more recent approach known as structured laser illumination planar imaging (SLIPI) is presented for the characterization of scattering and absorbing liquids. Water solutions of milk and coffee are, respectively, investigated for 10 different levels of turbidity. For the milk solutions, scattering is the dominant process, while the coffee solutions have a high level of absorption. Measurements of the extinction coefficient are performed at both λ=450  nm and λ=638  nm and the ratio of their values has been extracted. We show that the turbidity limit of valid transmission measurements is reached at an optical depth of OD∼2.4, corresponding here to an extinction coefficient of μ_e=0.60  mm^-1 when using a modern absorption spectrometer having a spatial Fourier filter prior to detection. Above this value, errors are induced due to the contribution of scattered and multiply scattered photons reaching the detector. On the contrary, the SLIPI measurements were found to be very reliable, even for an extinction coefficient three times as high, where μ_e=1.80  mm^-1. This improvement is due to the capability of the technique in efficiently suppressing the contribution from multiple light scattering.