Measuring scattering samples’ absolute optical properties is valuable in bio-medicine, agriculture, material characterization, and beyond. These measurements may be achieved by analyzing the sample’s frequency-domain diffuse reflectance or transmittance. However, successfully achieving these absolute measurements is complicated by the need for calibration. We present a calibration-free method to conduct these measurements. This method, dubbed dual-ratio, creates a measured data type that cancels most coupling and calibration factors that confound traditional reflectance or transmittance measurements, specifically multiplicative factors associated with source emission, detector efficiency, or optical coupling with the sample. Furthermore, we have applied the dual-ratio method to measure absolute optical properties of a small-volume sample (i.e., the size of a standard cuvette). Applications include tissue hemodynamics and oxygenation assessment (brain, muscle, etc.), water turbidity and chemical analysis, food quality determination, and more. This work builds on our previous work developing the dual-ratio method for a cuvette-sized sample volume. We also expand on other previous work, combining frequency-domain and continuous-wave measurements to achieve absolute broadband absorption spectra. Optical properties recovered by small-volume dual-ratio agree well with semi-infinite medium multi-distance scanning, which we consider the gold standard. Such calibration-free methods may make sample quantitative analysis more accessible and allow for easy quantitative measurements outside the traditional laboratory setting.
In this work, a miniaturized heterodyning FD-NIRS instrument was presented. The device uses a dual slope probe, which removes the need for pre-calibration. The low-footprint system consists of only the circuit board, the probe and a Raspberry-Pi. Four lasers (685 nm and 830 nm) and two avalanche photodiodes are used, where the lasers are modulated with 80 MHz and the APD signals are amplified and downconverted by the analog front end of the instrument: a custom designed fully differential ASIC in 130 nm CMOS technology. Solid phantom measurements revealed <9% error and significant stability for long-term measurements.
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