Phototesting is used to assess individual sensitivity to ultraviolet (UV) radiation in order to determine adequate UV dosage for phototherapy1 . In the standard procedure, small skin areas are exposed to increasing doses of UV radiation. The lowest UV dose that induces a delineated erythema at 24±2 h after UV exposure defines the minimal erythema dose (MED)2 . Visual assessment is the gold standard for MED determination; however, it is prone to observer variability3 . Optical methods have been considered to quantify the magnitude of erythema response. However, they are limited by light scattering therefore high-resolution is restricted to depths of <200 μm resulting in unreliable measurements4,5 .