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Quantitative photoplethysmography: Lambert-Beer law or inverse function incorporating light scatter
M Cejnar1, H Kobler, S N Hunyor
1Department of Cardiology, Royal North Shore Hospital, St Leonard's (Sydney), Australia.
Journal of Biomedical Engineering
|March 1, 1993
Summary
Accurate finger blood volume measurement requires accounting for light scatter. An inverse function, not the traditional Lambert-Beer law, better models infra-red light transmittance for improved arterial compliance assessments.
Area of Science:
- Biomedical Engineering
- Optical Physics
- Physiology
Background:
- Infra-red (IR) light transmittance is used to measure finger blood volume.
- The Lambert-Beer law, designed for non-scattering media, is typically applied.
- Light scatter around the phalangeal bone can affect accuracy.
Purpose of the Study:
- To compare the accuracy of the Lambert-Beer exponential function versus an inverse function for modeling IR transmittance and finger blood volume.
- To determine the optimal model for measuring arterial compliance.
Main Methods:
- Simultaneous measurement of IR transmittance and finger volume in seven subjects.
- Fitting data using both the Lambert-Beer exponential function and an inverse function for scattering media.
- Comparing goodness of fit using standard errors of estimate (SEE).
Main Results:
- The inverse function provided a significantly better data fit in six out of seven subjects.
- Mean SEE for the inverse function was 1.9 (SD 0.7), compared to 4.6 (2.2) for the exponential function (p < 0.02).
- This indicates improved accuracy when accounting for light scattering.
Conclusions:
- An inverse function, derived from a light attenuation model for scattering materials, offers more accurate finger blood volume estimation than the traditional Lambert-Beer law.
- This improved accuracy is crucial for reliable arterial compliance measurements.
- The findings suggest a revision of standard methods for optical-based physiological monitoring.