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

Abstract

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.

Structured Imagery Processing Strategies for Spectroscopic Measurement in Dense Solutions

“…The objectives of this work have been achieved by the use of chlorophyll and coffee solutions at different concentration levels, and they lead satisfactory results. Note that our proposed data processing algorithms, which is based on PCA and Mean Value, made it possible to calculate extinction coefficients e µ approximately equal to those obtained with the standard method [26]. Owing to the fact that they have a good computation time and are simpler concept and implementation than the standard technique, they also lead to a minimal risk of error.…”

Optical Methods Based on Ultraviolet, Visible, and Near-Infrared Spectra to Estimate Fat and Protein in Raw Milk: A Review

“…Then, the light–milk interaction will be governed by the relationship between the size of particles suspended and the wavelength of incident light [ 3 , 6 , 7 , 8 ]. Considering that the lipids have a diameter between 0.1 to 10 µm [ 7 , 9 ], and a casein protein has a diameter ranging from 50 to 680 nm [ 9 ], the milk–light interaction will be described by the Lorenz–Mie theory [ 10 ]. This considers the molecules involved as a spherical particle and the wavelength of incident light as smaller than the size of the molecules; this relationship is described by , where is the particle size, and is the wavelength of the incident light [ 11 , 12 ].…”

“…It is important to mention that most of the scattered light goes in a forward direction ( Figure 1 ). Besides, the milk usually has a temperature around 37 °C when it leaves the udder; then, the particles are slowly suspended and the Brownian motion is avoided [ 10 ].…”

“…This technique explores the light-milk interaction when the light crosses the components inside the raw milk; however, multiple scattering effects are presented in turbid liquids like milk [ 10 ].…”

A New Method for Total Fat Detection in Raw Milk Based on Dual Low-Coherence Interferometer

“…This technique analyzes the transmission spectra of the light that crosses the liquid sample. By using the Beer–Lambert–Bouguer law, it is possible to obtain the absorbance and correlate compounds from the sample [9]. Despite this method being widely used, the technique has undesired scattering effects produced by optically dense liquids like milk [9].…”