This may not be directly on point to your example of blinding cavalry but a particularly famous--if legendary--example of sunlight as a weapon comes from the Siege of Syracuse by the Romans (214-212 BCE) during the Second Punic War. According to legend, Archimedes developed many ingenious war machines to keep the Roman fleet at bay, including a heat ray created by focusing sunlight. From Zonaras and Tzetzes (or fragments thereof):

At last in an incredible manner he burned up the whole Roman fleet. For by tilting a kind of mirror toward the sun he concentrated the sun's beam upon it; and owing to the thickness and smoothness of the mirror he ignited the air from this beam and kindled a great flame, the whole of which he directed upon the ships that lay at anchor in the path of the fire, until he consumed them all.

And Lucian makes passing mention of it:

Such an engineer we are told, was Archimedes, and also Sostratus of Cnidus. The latter took Memphis for Ptolemy without a siege by turning the river aside and dividing it ; the former burned the ships of the enemy by means of his science.

I should point out that its debatable whether actually happened, or even if it's physically possible, with everyone from Descartes to Mythbusters weighing in. The consensus seems to be that it never happened (or that if it did it would have been ineffective or done for the purpose of blinding and confusing the ships' crews) but the stories at least indicate that writers in the ancient world were aware of the possibility of using sunlight in warfare.