Abstract
The multi-wavelength photoplethysmography sensors were introduced to measure depth-dependent blood volume based on that concept that the longer the light wavelength, the deeper the penetration depth near visible spectrum band. In this study, we propose an omnidirectional optical sensor module that can measure photoplethysmogram while using multiple wavelengths, and describe implementation detail. The developed sensor is manufactured by making a hole in a metal plate and mounting an LED therein, and it has four wavelength LEDs of blue (460 nm), green (530 nm), red (660 nm), and IR (940 nm), being arranged concentrically around a photodetector. Irradiation light intensity was measured by photoluminescent test, and photoplethymogram was measured with each wavelength simultaneously at a periphery of the human body such as fingertip, earlobe, toe, forehead, and wrist, in order to evaluate the developed sensor. As a result, the developed sensor module showed a linear increase of irradiating light intensity according to the number of LEDs increases, and pulsatile waveforms were observed at all four wavelengths in all measuring sites.
Highlights
Photoplethysmography (PPG) is a non-invasive technique for optically measuring the blood volume changes in the vascular bed under the skin [1]
As the blood volume of the microvascular bed increases due to cardiac contraction, the amount of light absorbed increases as the number of hemoglobin increases, and the amount of light that passes through the measurement site, such as the finger, decreases
The fabricated multiwavelength optical sensor was designed to change the number of LEDs and fabricated multiwavelength optical sensorwas was designed to change the number of LEDs inputThe current by each wavelength
Summary
Photoplethysmography (PPG) is a non-invasive technique for optically measuring the blood volume changes in the vascular bed under the skin [1]. Multi-wavelength PPG has been proposed for analyzing pulse waves in blood vessels of different depths using light sources of various wavelengths [5,6,7,8,9,10]. It is known that wavelengths in the range of 390−600 nm penetrate to the superficial tissue, and longer wavelengths in the range of 600−1100 nm, which penetrate further (Figure 1) [11] Based on these characteristics, attempts have been made to obtain PPG measurements from various blood vessel depths while using different wavelength light sources. Previous studies have shown that placing multiple LEDs around a photodetector can improve the quality of the PPG [13]; the use of multiple light sources is common in wrist watch type pulse monitors that are already commercially available. The new sensor has the potential to provide a solution to the problems of directionality between the light source and the photodetector, the non-uniform distance between the light source and the photodetector, and the light amount control function
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