Biostrap utilizes a clinical-quality PPG sensor with RED and InfraRed LEDs, whereas FitBit (and most of other wearables) use GREEN LEDS.
Red & IR are capable of penetrating much deeper into tissue which helps us get very high definition pulse waveform. Another advantage of infrared light over green is that it is less sensitive to skin tone variations and perfusion level due to its better tissue penetration characteristics. In fact most of the medical grade devices use the similar technology.
We use the high definition pulse to derive various vital biometric markers using complex patented algorithms. Currently we read Heart Rate, Heart Rate Variability, Respiration and Oxygen Saturation, and more to come in near future.
Green LEDs on other hand, are more resilient to motion, therefore can read active-HR. But can not penetrate deeper, and therefore can’t gather any detailed biometric data.
So it is a trade-off between getting accurate biometric reading versus active-HR.