This paper presents an opto-physiological model (OPM) to quantify the absorbance of multi-layered tissue in imaging photoplethysmography (IPPG). The approach used to generate such a model is to revise the path length of the Beer Lambert law through the Monte Carlo (MC) simulation of multi-layered tissue. The OPM can mathematically quantify the effect of optical properties on the absorbance of multilayered tissue. Subsequently, the absorbance measured from homogeneous, multi-layered tissue phantoms compares with model predictions. To this end, the model is validated to predict the widest range of experimental outcomes while maintaining the highest possible level of accuracy. This study brings a new approach to understand the principle of IPPG.