# Dual-Slope Diffuse Reflectance Instrument for Calibration-Free Broadband Spectroscopy
This method is also effective at suppressing motion artifacts [24] and allows calibration-free saturation measurements with Continuous-Wave (CW) methods after assuming the wavelength dependence of scattering [25]. The original purpose of the SC method was to achieve measurements of absolute optical properties with FD-NIRS. However, recently the optode geometry inspired by the SC method has been extended to the use of only the amplitude or phase of the complex reflectance in FD-NIRS [26; 27]. This extension was named Dual-Slope (DS) since it relies on the average of two slopes measured with this special optode configuration. One of the limitations of time-resolved NIRS methods, such as FD-NIRS, is the added instrumental complexity compared to CW. There has been work in broadband time-resolved spectroscopy [28]; however, such instruments require complex instruments (super-continuum lasers, single photon counting, and instrument response calibration). Because of this, the norm in time-resolved NIRS methods is the use of two or a few wavelengths as opposed to a continuous broadband spectrum [1]. In contrast, CW methods may use White-Light or broadband light sources, such as halogen lamps, and spectrometer detectors, since there is no need for measurements of fast temporal characteristics. These spectroscopic methods in CW are so-called Diffuse Reflectance Spectroscopy (DRS) due to their collection geometry and use of a spectrometer as a detector (instead of avalanche photodiodes or photomultiplier tubes typical in time-resolved methods). Therefore, CW broadband DRS (CW-bDRS) has the advantage of collecting data over many wavelengths but the disadvantage that absorption and scattering are coupled and inseparable due to the use of CW illumination. A solution that has become more and more common is to combine a time-resolved NIRS instrument at few wavelengths with a CW-bDRS system [29; 30; 31; 32; 33; 34]. Such a technique allows extrapolation of scattering from few to many wavelengths, by assuming a power law decay of the reduced scattering coefficient with wavelength [2], as well as decoupling of absorption from scattering in the CW-bDRS data. At this point, a small note on nomenclature is valuable. The acronym MD FD-NIRS refers to a frequency-domain method capable of measuring absolute optical properties (absorption and scattering) at discrete wavelengths from measurements at multiple distances between source and detector. The acronym CW-bDRS refers to a continuous-wave technique capable of measuring wavelength-resolved data (which depends on absorption and scattering) over a range of many wavelengths. Further, to achieve a calibration free full spectral technique, DS CW-bDRS is introduced. It is noted that the distinction between SC (self-calibrating) and DS (dual-slope) is the use of complex reflectance in SC FD-NIRS versus continuous-wave reflectance in DS CW-bDRS, for example. The main focus of this work will be to develop the DS CW-bDRS technique in combination with MD FD-NIRS to decouple absorption and scattering contributions to the absorbance spectra. In this paper, a new DS CW-bDRS/MD FD-NIRS instrument is described and validated by measuring absolute absorption spectra of highly scattering media, or phantoms. The novelty of this work is the design of a CW instrument for broadband spectroscopy that implements DS methods for robust and calibration free measurements of absorbance spectra. The addition of MD FD-NIRS measurements at two wavelengths allows for the translation of absorbance spectra into quantitative absorption spectra. The organization of the paper is as follows. The methods, in Section 2, are split into techniques, in Section 2.1, where FD-NIRS and CW-bDRS are described, and is followed by experiments, Section 2.2, which lays out a phantom experiment, and then by an analysis, Section 2.3, which explains methods to calculate optical properties. Next, the results, Section 3, report measured phantom absorption spectra.