The pupil light reflex (PLR) is a key indicator of visual and neurological function, governed by a complex pathway involving retinal ganglion cells (RGCs) and intrinsically photosensitive RGCs (ipRGCs). While PLR dynamics are well-documented in clinical disorders (e.g., Alzheimer’s, glaucoma), recent studies reveal its utility for exploring cognitive processes like attention and decision-making. However, high-cost pupillometry systems (e.g., video pupillometers: $5000–$20,000; research-grade eye trackers: > $100,000) limit accessibility. Here, we present two infrared video-based methods to quantify relative pupil changes in response to photopic light stimuli of varying energy. We recorded pupillary responses from three participants under nine illumination conditions, analyzing normalized area and diameter metrics. Results demonstrated illumination-dependent PLR magnitudes, with contractions ranging from <10% to >50%. Area-based measurements captured larger contractions than diameter, though both metrics showed consistent trends. Model fits revealed a three-phase latency reduction: An abrupt decline at low energy, a plateau, and a further decrease at high energy. Maximum contractions (MC) scaled logarithmically with light energy, with transformed diameter and area curves converging closely (despite higher variability in diameter-derived data). Our findings highlight the feasibility of low-cost infrared video for precise PLR assessment, offering a scalable alternative to commercial systems. The illumination-energy relationship underscores the PLR’s utility in probing both physiological and cognitive states.