where d t c {\displaystyle \mathrm {d} t_{c}} and d t d {\displaystyle \mathrm {d} t_{d}} are respectively the d t {\displaystyle \mathrm {d} t} for constriction and dilation measured in milliseconds, T c {\displaystyle T_{c}} and T p {\displaystyle T_{p}} are respectively the current and previous simulation times (times since the simulation started) measured in milliseconds, S {\displaystyle S} is a constant that affects the constriction/dilation velocity and varies among individuals. The higher the S {\displaystyle S} value, the smaller the time step used in the simulation and, consequently, the smaller the pupil constriction/dilation velocity.