Mental workload monitoring is becoming increasingly important in demanding and safety-critical settings, where excessive cognitive load may negatively affect attention, decision-making, and performance. In this context, physiological monitoring offers a valuable solution by providing continuous and objective information on the operator’s state. Among the available techniques, contact photoplethysmography (PPG) is especially attractive because of its low power requirements, simple integration, and ability to derive both cardiac and respiratory information from the same optical measurement. When positioned on the face or head, PPG sensors can be embedded into wearable equipment already used by the operator. With this background, this thesis investigates whether a single multi-wavelength PPG sensor placed on the face can provide acceptable estimates of both heart rate (HR) and respiratory rate (RR), while also identifying the facial site and optical channel configuration that best support this dual task.