Dual-Slope Diffuse Reflectance Instrument for Calibration-Free Broadband Spectroscopy

Abstract

This work presents the design and validation of an instrument for dual-slope broadband diffuse reflectance spectroscopy. This instrument affords calibration-free, continuous-wave measurements of broadband absorbance of optically diffusive media, which may be translated into absolute absorption spectra by adding frequency-domain measurements of scattering at two wavelengths. An experiment on a strongly scattering liquid phantom (milk, water, dyes) confirms the instrument's ability to correctly identify spectral features and measure absolute absorption. This is done by sequentially adding three dyes, each featuring a distinct spectral absorption, to the milk/water phantom. After each dye addition, the absorption spectrum is measured, and it is found to reproduce the spectral features of the added dye. Additionally, the absorption spectrum is compared to the absorption values measured with a commercial frequency-domain instrument at two wavelengths. The measured absorption of the milk/water phantom quantitatively agrees with the known water absorption spectrum (R ² = 0.98), and the measured absorption of the milk/water/dyes phantom quantitatively agrees with the absorption measured with the frequency-domain instrument in six of eight cases. Additionally, the measured absorption spectrum correctly recovers the concentration of one dye, black India ink, for which we could accurately determine the extinction spectrum (i.e., the specific absorption per unit concentration). The instrumental methods presented in this work can find applications in quantitative spectroscopy of optically diffusive media, and particularly in near-infrared spectroscopy of biological tissue.

Keywords: absorption spectra; broadband diffuse reflectance spectroscopy; dual-slope; frequency-domain near-infrared spectroscopy.

Figure 1.

Frequency-domain near-infrared spectroscopy methods to achieve measurements of absolute optical properties. (a) Render of multi-distance scan done on diffuse optical phantoms. (b) Schematic of eleven different source positions realized during the multi-distance scan.

Figure 2.

Broadband diffuse reflectance spectroscopy methods to achieve measurements of absolute absorption spectra. (a) Render of diffuse reflectance spectroscopy probe on a diffuse optical phantom. (b) Schematic of dual-slope diffuse reflectance spectroscopy device. Acronyms: Universal Serial Bus (USB) and Transistor-Transistor Logic (TTL). (c) Schematic of the source (1 and 2) and detector (A and B) positions on the dual-slope diffuse reflectance spectroscopy probe.

Figure 3.

Expected spectral features of near-infrared dyes in water, provided by the manufacturer (QCR Solutions, Palm City, FL, USA). [37] (a) Normalized Absorbance (A) versus wavelength (λ) of NIR746A dye (N7). (b) Normalized A of NIR869A dye (N8).

Figure 4.

Results from phantom experiment. (a) Absolute absorption (μ_a) spectra as a function of wavelength (λ). Showing results from Dual-Slope Continuous-Wave broadband Diffuse Reflectance Spectroscopy (DS CW-bDRS) and Multi-Distance Frequency-Domain Near-Infrared Spectroscopy (MD FD-NIRS) measurements. Spectra shown for the following phantoms: Milk and Water (MW), MW plus India Ink (II), MW plus II plus NIR746A (N7), and, finally, MW plus II plus N7 plus NIR869A (N8). DS CW-bDRS points show individual wavelength measurements and lines show smoothed (moving average) spectra for visualization. Dashed line shows the expected spectrum for MW modeled as water and lipid. (b) Change in absorption (Δμ_a) from adding N8 (i.e., ${\mu }_a^{MW+II+N7+N8}-{\mu }_a^{MW+II+N7}$), (c) Δμ_a from adding N7 (i.e., ${\mu }_a^{MW+N7}-{\mu }_a^{MW+II}$), and (d) Δμ_a from adding II (i.e., ${\mu }_a^{MW+II}-{\mu }_a^{MW}$).

Figure 5.

Comparison of wavelength (λ) normalized Absorbance (A) peak locations for two dyes. Four types of spectra shown: Diffuse Reflectance (Diff Refl, i.e., in milk), provided by the Manufacturer (Manu; (QCR Solutions, Palm City FL, USA) [37], i.e., in water), Diffuse Transmission (Diff Tran, i.e., in milk), and Transmission (Tran, i.e., in water). (a) NIR746A (N7) dye. (b) NIR869A (N8) dye.