Fig. 1.

Illustration of the optical model for motion artifact generation. The light illuminated by the source optode is either partly reflected at hairs to get short-circuited, or travels through the tissues. Both are received by detector optode. $I_0$, illumination intensity; $I_1(t)$, intensity of short-circuited light; $r_s(t)$, transmittance at source–scalp gap; $R(t)$, transmittance at head tissues; $r_d(t)$, transmittance at optode–scalp gap; $I_2(t)$, intensity of tissue-traveling light; $I(t)$, total detection intensity.