“…As this miniature sensor has small dimensions of 2.7 x 3.2 x 1.4 mm (W x L x H), 40 units are densely arranged at sampling points on the face capture mask, as shown in Figure 5. Following the standard practice of face recognition [24,25,33], we picked eyebrows, cheeks and around eyes and mouth as the sampling points of photo reflective sensors. We ensure that the sensors do not come into contact with the eyes, nose and mouth of the mask wearer.…”