Visual functions refers to the multitude of functions comprising of acuity of vision, colour vision, acuity of alignment, the perception of movement, and perception of changing luminous intensity or differentiating luminous intensity. ICD-10 Version: 2015 classify visual impairment into no or mild, moderate, severe, and blindness. The visual field in the centre is light adaptive and have a high acuity of vision with good colour sensitivity and works efficiently with high illumination (photopic system). The visual field in the periphery is dark adapted and have reduced acuity and poor colour sensitivity and works effectively with minimum illumination (scotopic system). Thus, the central field is more important for essential activities, and peripheral field is necessary for activities in the dark and mobility. A defect in the central and inferior field is more disabling than that of the superior field and hence derives more weight for a defect in the central and inferior field. Impairment of temporal field obtains higher impairment than nasal field because of the larger field area. Low/poor contrast sensitivity derives an impairment of 5%–10% because of the risk of accidents during driving in the night. If night blindness is irreversible even after maximum medical management, it obtains an impairment of 25%. If photophobia persists even after maximum medical management, it imparts an impairment of 10%. “Integrated Evaluation of Disability” assigns maximum impairment of 25% for loss of colour perception. Diplopia derives a maximum impairment of 25% if the diplopia score is between 16 and 25. The formula A + [B(100 – A)/100] combines impairment of acuity of vision, the field of vision, double-vision, colour-vision, night-blindness, and photophobia.