That second stanza really hammers home Sassoon's point: for soldier's blinded by war, the loss of vision matters very deeply. But there's more than one way to be blind. And by the end of the poem, it's clear that these folks surrounding the soldier are blind in a much more damaging way.
Line 6: The speaker asks if losing one's sight matters, to which we say, absolutely. Line 7: In what is perhaps history's most pathetic attempt to comfort someone ever, the speaker assures the soldier that "there's such splendid work for the blind." Work is a loaded word here. It seems to mean both employment, but also things to do, as in "there's so much a blind guy can do." Sarcastic much, dear speaker?
Line 10: The soldier sits on a terrace with his face to the light. Of course he's talking about the sun here, but it could be a metaphor for something like truth or understanding.
Lines 13-14: Okay, so these lines don't have any light imagery. But we think they're the perfect example of what Sassoon saw as the blindness of the folks on the home front, whom the veterans came home to—the folks who assume that the vets are doing fine because they don't show that they mind having lost so much (see line 3).