Exploring the influence of concentration and optical path on nonlinearity in VIS&NIR dynamic spectrum

Abstract

Studying the effects of nonlinearity caused by scattering in complex solution composition analysis, avoiding and even utilizing the nonlinearity are of great significance for improving the accuracy of non-invasive blood component analysis based on dynamic spectrum (DS). The variable optical path spectral correlation coefficient (VOPSR) is proposed in this paper. The degree of nonlinearity in the dynamic spectrum is characterized by calculating the correlation coefficient of the spectrums at two different optical paths of the same sample. By comparing the average variable optical path spectral correlation coefficient (M-VOPSR) of samples under different concentration and optical path groupings, the effects of hemoglobin concentration and optical path differences on nonlinearity were studied. The results show that the nonlinearity is more serious in the 954.1–103.8 nm band. Under different concentration groups (low to high concentration), the VOPSR of different groups in the 954.1–103.8 nm band are 0.5514, 0.5436 and 0.480, respectively. That is, the greater the hemoglobin concentration, the more severe the nonlinearity caused by scattering. However, under different optical path (PPG signal amplitude) groups (short to long optical path), the VOPSR of different groups in the 954.1–1043.8 nm band is 0.0974, 0.6047 and 0.8733, respectively. The influence of the optical path on the nonlinearity is much weaker than the influence on the signal-to-noise ratio (SNR), that is, the influence of optical path on nonlinearity is annihilated in the influence of SNR and is difficult to be directly utilized. This also explains from another angle that in the dynamic spectrum non-invasive blood component analysis, it is a more effective measure to adopt a more efficient method to increase the amplitude of the PPG signal to improve the SNR.