The output of the optical sensor was affected by the change
in the ambient light, as shown in Fig. 4(a), even though we
used an active light source. We can completely eliminate the
effect of ambient light using a different output of the optical
sensor, as shown in Fig. 4(b). The different output was
generated using the difference between the output of the sensor
when the active light was turned on and that when the active
light was turned off. In Fig. 4, ① and ② represent conditions
in which a desk lamp is on and off, respectively. The output of
the sensor was low when the user’s eyes were open and high
when the user’s eyes were closed by the eyelids.