where formula_2 is the pupil diameter measured in millimeters and formula_3 is the luminous intensity reaching the retina in a time formula_4, which can be described as formula_5: luminance reaching the eye in lumens/mm2 times the pupil area in mm2. formula_6 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. formula_7, formula_8 and formula_9 are the derivatives for the formula_10 function, pupil diameter formula_2 and time formula_4.