What does "light spent" mean?

"On His Blindness" is a sonnet written by Milton as a lament of sorts for his lost vision. Milton is contemplating the fact that he has been stricken with blindness, as the title implies, in a way that makes his "one Talent" very difficult to use. Effectively, Milton is saying that because he is now blind, it is more difficult for him to write in praise of God; but he comes to the conclusion that God does not require man to continually serve him through deeds for the entirety of their lives—on the contrary, "they also serve who only stand and waite."
The first line of the poem, then, reflects Milton's concern about the fact that his "light is spent"—his vision has been lost—"e'ere half my days" (before I have lived half of my allotted lifetime). His "light" here generally seems to be a metaphor for his vision: he is saying that his capacity to see the world has been exhausted too soon. "Light" could also have other connotations: Milton connects his vision to his talent, so there is an implication that, in losing his vision, he also loses his enlightenment, his sense of understanding, and his fresh outlook on the world. Without his "light," he is concerned that he can no longer contribute or make use of his "talent." The word "spent," also, gives the suggestion that there was only ever a finite amount of "light" to be used up, and the poet is concerned that he has exhausted all of it in an unbalanced way—as if his "light" should really have lasted him for the whole of his life, rather than only half of it.