Abstract
Ambient light (AML) limits the usage of absorption spectroscopy to strictly-controlled environments due to its impact on the quality of the signals. In medical applications for Optical instruments that require high reliability and validity results, it is indispensable to control AML impact. In this work, we present a multi-wavelength optical acquisition method, here called SHADE. The technique increases the quality of the optical signals, dynamically assesses AML, and enables mitigating the AML influence on the data. Our proposed technique involves four primary functions: (1) Multiplexing/demultiplexing of target wavelengths using a frequency-division method; (2) Robust signal recovery using inverse notch filters; (3) Concurrent AML intensity estimation; (4) A simple post-processing (offline) suppression of AML interference. We introduce the mathematical framework of SHADE to demonstrate the theoretical scope and limitations. SHADE was also experimentally tested by using a digital signal processing board under different conditions. The results confirmed the performance of the AML reconstruction and the potential of such a method for further improvements of the signal quality.
Highlights
The interaction between light and a specific sample can determine its chemical species due to optical phenomena such as absorption or reflection
The low-level hardware gathers the data from an analog-to-digital converter (ADC), and the personal computers (PC) is entitled to store and plot the data
We investigated the conformity of measured ambient light (AML) with the control setup and its estimation by SHADE implemented in MCU
Summary
The interaction between light and a specific sample can determine its chemical species due to optical phenomena such as absorption or reflection. Passive methods to counteract the AML issues can fall under some headings including covering by shield-cloth [9,10], limiting the experiment in a dark environment [11], attaching of a reference detector [12], insertion of optical filters inside the system [13], and using polarizers [14,9,10] These passive techniques will complicate the measuring schemes and do not alleviate the aforementioned non-stationary characteristics of AML, such as AML leakage. The lock-in-amplification seems to be the most practiced technique in continuous-wave optical tomography devices, in brain-computer interfaces (BCI) based on functional near-infrared spectroscopy (fNIRS) [19,20,21,22,23] This narrow-band detection technique is usually implemented with the trade-off of losing information relating to the measurement of AML [19,20,21,22,23].
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