The present invention is a system and methods of establishing a Melanocyte Protection Factor (MPF), which indicates the level of protection against DNA damage to a target cell, such as the level of protection a particular sunscreen offers against UVA rays when compared to the unprotected case, i.e., no sunscreen. The present invention determines and records levels of stable melanin radicals (SMR) in a target cell. Light is applied to the target cell forming light-induced melanin radicals (LIR). The levels of SMR and intensity of LIR are measured to determine the amount of incident light reaching the target cell. Since LIR is proportional to the square root of light intensity reaching the target cell, the ratio of light reaching the target cell is defined as the MPF: MPF = UV ⁢ ⁢ reaching ⁢ ⁢ melanocyte without ⁢ ⁢ sunscreen UV ⁢ ⁢ reaching ⁢ ⁢ melanocyte with ⁢ ⁢ sunscreen = [ SMR + screen LIR + UV + screen ] 2 [ SMR control LIR ⁢ + UV ⁢ ⁢ control ] 2 .