This chapter explores key functional metrics that extend beyond standard visual acuity, focusing on how light and contrast affect visual experience. These include disability glare (light scatter reducing contrast and visibility), discomfort glare (subjective irritation from bright light), visual range (distance an object can be identified against its background), photo-stress recovery time (duration to visual recovery after bright light exposure), and chromatic contrast (ability to distinguish between colors). Each measure is described along with its clinical or environmental significance and methods of assessment, such as veiling luminance, Unified Glare Rating (UGR), Daylight Glare Probability (DGP), Michelson contrast, ΔE values, and recovery time metrics. These indices offer important insight into real-world visual function, particularly in glare-prone, high-contrast, or color-critical environments like driving, aviation, and outdoor activities.