There is a scene a little more than half-way through the movie where Frost & Blade are talking in a park during the day. It is explained that Frost is able to be out in daylight because he's wearing sunscreen (a dubious theory at best, but beside the point). However, there is no way he could be wearing sunscreen on his eyes. So, when he takes his sunglasses of (as he does in the scene) his eyeballs would have been burned from the sockets.