The erythema protection efficacy of sunscreens is characterized by the sun protection factor (SPF), which is related to the UVB part of the solar spectrum. UVA protection, which is an important factor in immune suppression, is not taken into account by the SPF. The combination of the suction blister technique and UV spectroscopy allows the determination of the energy and intensity distribution of photons entering into the living epidermis. In contrast to the SPF measurements, the determination of the energy and intensity distribution of UV photons below the epidermis allows the direct determination of UVB and UVA photon transition into the living skin.