Method for Measuring Absolute Optical Properties of Turbid Samples in a Standard Cuvette

TL;DR

This paper presents a diffusion theory-based frequency-domain method to accurately measure the absolute optical properties of highly scattering turbid samples in a standard cuvette, overcoming limitations of commercial spectrometers.

Contribution

The method introduces a self-calibrating approach using diffuse transmittance measurements at two source-detector distances, specifically addressing scattering confounds in turbid samples.

Findings

Works best for highly scattering samples with reduced scattering coefficient above 1 per mm

Achieves 4% error in absorption coefficient and 1% in scattering coefficient under simulated noise

Accuracy depends on the sample's index of refraction knowledge

Abstract

Many applications seek to measure a sample's absorption coefficient spectrum to retrieve the chemical makeup. Many real world samples are optically turbid, causing scattering confounds which many commercial spectrometers cannot address. Using diffusion theory and considering absorption and reduced scattering coefficients on the order of 0.01 per mm and 1 per mm, respectively, we develop a method which utilizes frequency-domain to measure absolute optical properties of turbid samples in a standard cuvette (45 mm by 10 mm by 10 mm). Inspired by the self calibrating method, which removes instrumental confounds, the method uses measurements of the diffuse complex transmittance at two sets of two different source-detector distances. We find: this works best for highly scattering samples (reduced scattering coefficient above 1 per mm); higher relative error in the absorption coefficient compared to the reduced scattering coefficient; accuracy is tied to knowledge of the samples index of refraction. Noise simulations with 0.1 percent amplitude and 0.1 deg (1.7 mrad) phase uncertainty find errors in absorption and reduced scattering coefficients of 4 percent and 1 percent, respectively. We expect that higher error in the absorption coefficient can be alleviated with highly scattering samples and that boundary condition confounds may be suppressed by designing a cuvette with high index of refraction. Further work will investigate implementation and reproducibility.