Reflectance PPG Device Recess Interface Layer
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Solution Overview
Problem
Reflectance-type photoplethysmography devices face challenges in optical efficiency due to reflection losses and scattering at the skin surface, leading to a larger DC signal compared to the AC signal, which affects the quality of the PPG signal.
Innovation Solution
Incorporating an interface layer with a recess between the light source and the light sensor, and optionally a projection to create an air gap, to prevent light from traveling within the interface and reduce reflection losses, using a thermoplastic material like PMMA or polycarbonate, and a soft coating for improved skin contact.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If reflectance-type PPG measurement is used, then the device can be applied to any skin surface with shorter path length, but optical losses due to reflection and scattering increase leading to larger DC signal relative to AC signal
Solution Approach 1:
The patent introduces an optical coupling medium (interface layer) between the sensor and skin to reduce Fresnel reflection losses. This intermediary material with refractive index matching reduces the reflectance at the interface, allowing more light to penetrate into the tissue and return to the detector, thereby reducing optical losses while maintaining adaptability to any skin surface.
Solution Approach 2:
The patent employs different materials with specific optical properties in different regions: the interface layer has refractive index matched to skin for minimal reflection, while the sensing region allows for optimized light path. This local optimization of material properties reduces overall optical losses without compromising the universal applicability to different skin types.
2Ease of manufacture
If conventional interface layer without recess is used, then manufacturing is simpler, but light paths within the interface layer cause additional reflection losses and reduce signal quality
Solution Approach 1:
The interface layer is segmented by introducing a recess that creates distinct optical zones: a first region for light delivery and a second region for light detection, separated by the recess. This segmentation prevents stray light paths within the interface material from reaching the detector, reducing spurious reflections and improving signal quality while maintaining manufacturing feasibility through mold-based fabrication.
3Illumination intensity
If larger light power is used to compensate for optical losses, then signal strength increases, but power consumption increases and may cause tissue heating
Solution Approach 1:
The patent converts the potentially harmful effect of optical losses into a benefit by using the recess structure to eliminate stray light paths. This allows the system to achieve better signal quality with lower light power, as the recess prevents reflected light from contaminating the measurement, thereby reducing the need for high illumination intensity and associated power consumption and heating risks.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration enhances the signal-to-noise ratio by reducing total internal reflection and Fresnel losses, resulting in improved optical efficiency, with a relative response increase of up to 4.2 times compared to conventional designs.
Implementation Method 1
the recess preventing a light path between the light source and the light sensor within the interface layer
Implementation Method 2
Fresnel losses on the skin surface, at high angle of incidence (> 60°) the reflectance at the skin rapidly increases
Implementation Method 3
Optical losses in PPG sensing are due to absorbance, reflectance and scattering in the sensor part and in the human tissue
Data Source
Figure 1~2
Figure 3a~3e
Figure 4a~5
AI summary
The invention relates to a photoplethysmography device (20) of the reflectance type, comprising a light source (4), a light sensor (5), and an interface layer (21). The interface layer (21) has a recess between the source (4) and the sensor (5) in order to prevent reflection losses between the device (20) and the skin and to prevent that light from the source (4) can reach the sensor (5) directly via the interface layer (21).