The conventional view was that cones are responsible for pupil constriction in photopic lighting conditions. With the discovery of intrinsically photosensitive retinal ganglion cells (ipRGC), it was found that signals from ipRGCs along with cones mediated the pupil light reflex in photopic lighting conditions. Although both signals contributed, it was unclear how these signals were summed. In the work reported here, steady-state pupil size was measured with an infrared camera under LED lighting conditions with different color temperatures and luminance. A formula was then derived for pupil size according to the linear summation of cirtopic and photopic luminance. This formula allowed direct calculations to predict pupil size well when LED photopic luminance ranged from about 50 cd/m2 to 300 cd/m2, which is the general luminance level range for computer and smartphone screens.