where D {\displaystyle D} is the pupil diameter measured in millimeters and Φ ( t − τ ) {\displaystyle \Phi (t-\tau )} is the luminous intensity reaching the retina in a time t {\displaystyle t} , which can be described as Φ = I A {\displaystyle \Phi =IA} : luminance reaching the eye in lumens/mm2 times the pupil area in mm2. τ {\displaystyle \tau } is the pupillary latency, a time delay between the instant in which the light pulse reaches the retina and the beginning of iridal reaction due nerve transmission, neuro-muscular excitation and activation delays. d M {\displaystyle \mathrm {d} M} , d D {\displaystyle \mathrm {d} D} and d t {\displaystyle \mathrm {d} t} are the derivatives for the M {\displaystyle M} function, pupil diameter D {\displaystyle D} and time t {\displaystyle t} .