Effect of change in intensity of infrared LED on a photoplethysmogram

Peripheral pulse captured with the help of optical sensor placed on an appendage, is known as photoplethysmography (PPG). PPG can be recorded by using reflectance or transmittance type of probe. Transmittance type of probe is used only at the sites were tissue denseness is lower. While the reflectance type of probe is independent of tissue denseness, if it is operated at a higher intensity. The aim of the project is to develop a reflectance type photoplethysmographic sensor to study the intensity dependent penetration of IR light into the human tissue. The designed PPG sensor consists of IR LED of 860 nm wavelength and 5 mm diameter with OPT 101 as a detector. The system was tested for the real signals. The photoplethysmographic signals were obtained from 6 female subjects of different age group. It was seen that the penetration of IR light into the human tissue is more at higher intensity than the penetration IR light at lower intensity. Higher the intensity, higher is the peak to peak voltage of PPG waveform. As the intensity decreases, the peak-peak voltage of PPG waveform also decrease, making it noisier. The recorded decrease in the PPG output from highest to lowest intensity was 35% - 40%for all the subjects.