Circular Polarization Filtering in Imaging Photoplethysmography to Reduce the Influence of Specular Reflections

Abstract

Accurate measurement of blood flow changes in living tissues is an important and challenging task, especially in surgery. One promising method for solving this problem is imaging photoplethysmography (iPPG). Polarization filtering is an important component of the iPPG optical system and is usually implemented using two crossed linear polarizers, that reduces the influence of specular reflections from the skin and motion artifacts on the recorded signal, thereby increasing the signal-to-noise ratio. However, such scheme requires careful adjustment, which is not always feasible in surgery. We propose to implement polarization filtering using only one optical element, a circular polarizer (CPL-filter). This is a combination of linear and circular polarization filters. In our iPPG module, the CPL-filter covers both the camera lens and the LEDs in such a way that the light emitted by the LEDs first hits the linear polarizer. In this way, the subject's skin is illuminated with circularly polarized light, the polarization of which remains circular after mirroring, thereby preventing the reflected beam from passing through the CPL-filter. On the contrary, after interacting with red blood cells, the light becomes elliptically polarized and passes through the CPL-filter. Thus, the CPL-filter provides filtering of specular reflections in the same way as a pair of crossed linear polarizers, but its use has several advantages: the position of the filter during experiments does not require adjustment; easy integration into optical systems with small apertures, such as endoscopic and laparoscopic. The work is supported by the Russian Science Foundation (grant No. 25-15-00400).

Speaker

Mariia Asalkhanova
Institute of Automation and Control Processes FEB RAS
Russia

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