A.T. Robertson Commentary John 9:5

"When I am in the world, I am the light of the world." — John 9:5 (ASV)

When I am in the world (οταν εν τω κοσμω ω). Indefinite relative clause with οταν and present active subjunctive ω, "whenever I am in the world." The Latin Vulgate renders here οταν by quamdiu so long as or while as if it were εως. But clearly Jesus here refers to the historic Incarnation (17:11) and to any previous visitations in the time of the patriarchs, prophets, etc. Jesus as God's Son is always the Light of the World (1:4,10; 8:12), but here the reference is limited to his manifestation "in the world."

I am the light of the world (φως ειμ του κοσμου). The absence of the definite article (το φως in 8:12) is to be noted (Westcott). Literally, "I am light to the world, whenever I am in the world." "The display of the character varies with the occasion" (Westcott).