Do I need sun protection factor when exposed to light therapy?

No, because sunscreens with a protection factor block the harmful UV rays which are not present in light therapy. This means that they do not affect how much red and near-infrared light from the red light treatment reaches your tissues, if it is not comprehensive sun protection, for example zinc paste, which can block light of all wavelengths. However, it is on such a small body surface that it does not matter in practice.