Media Characterization under Scattering Conditions by Nanophotonics Iterative Multiplane Spectroscopy Measurements

Abstract

Characterizing materials is preferably done by multiple wavelengths. In opaque materials, the scattering poses a challenge due to the additional complexity to the spectroscopic measurements. We have previously demonstrated an iterative multiplane method for characterizing materials using the reflection from turbid media. Initial studies were performed in the red wavelength regime (632.8 nm) which is optimal for biomedical applications. However, in order to differentiate between materials, it is better to use multiple wavelengths, as spectroscopy may detect the material fingerprint. In this paper, our iterative multiplane optical property extraction (IMOPE) technique is presented in the blue regime (473 nm). Agar-based solid phantom measurements were conducted and compared to our theoretical model. Compatibility between experiments in the red and blue wavelengths shows the robustness of our technique.

Figure 1

Reflection-based IMOPE. (a) Schematic description of the IMOPE algorithm for extracting μ_s′. After running the multiplane GS algorithm as described elsewhere, the estimated phase φ̂_j is retrieved. The RMS is computed and compared to the theory to produce the estimated μ_s′. (b) Experimental setup in the blue regime for recording light intensity images. The camera records images at multiple planes with equal intervals between them (dz). The light source is a DPSS laser with a wavelength of λ = 473 nm, and the focal length of the lens is 75 mm; polarizers were added for optical clearing purposes. The solid phantoms were set on a three-axis micrometer plates and can be adjusted in the x–y–z directions.

Figure 2

Calculated reduced scattering coefficient for different IL concentrations in the blue regime. For liquid phantoms (black line), μ_s′ was obtained by an extrapolation from Assadi et al. to the range of the IL concentrations we used. According to Cubeddu et al., there is a factor (smaller than 1) between liquid and solid phantoms. Here, a factor of 0.55 was used to receive the relation between μ_s′ and IL concentrations for solid phantoms (red line). The equations for both liquid and solid phantoms are located on the graph with proximity to their respective curves.

Figure 3

Phase RMS obtained theoretically (black) and experimentally (blue) for different μ_s′ values. The phase RMS was computed from two regimes: single-scattering regime (black dashed line and blue stars correspond to theory and experiment, respectively) and multiple-scattering regime (black straight line and blue circles correspond to theory and experiment, respectively). The theoretical results were received for the theoretical adjustments of λ = 473 nm, μ_a = 2 × 10^–5 mm^–1, and g = 0.85.⁻ For both regimes, the RMS obtained from experiments produced a standard deviation <0.014.

Figure 4

Reconstructed phase images obtained following the multiple-measurement GS algorithm for solid phantoms in the blue wavelength with (a) μ_s′ = 1.21 mm^–1 and (b) μ_s′ = 1.44 mm^–1. The border between the single- and multiple-scattering regimes is marked with the white circles. The phase was reconstructed using a distance between images of dz = 0.635 mm, a total propagation distance of 3.81 mm, and an image size of 3.35 mm × 3.35 mm.

Figure 5

Spectral comparison of the phase RMS for different μ_s′. The black line represents the theoretical phase RMS received with the respective parameters of the blue regime. Red and blue circles correspond to the experimental phase RMS obtained by using red and blue wavelengths for the multiple-scattering regimes. The standard deviation for both wavelengths was less than 0.017