Abstract: The present invention provides a personal, portable and self-powered ultraviolet light intensity (UVI) monitor and a method for determining the duration of safe exposure to the sun or, alternatively, the value of the sun protection factor (SPF) of a sun protection product required for a given duration of exposure to the sun. The monitor is a substrate imprinted with indicia and non-uniformly overlaid with photosensitive material. Depending on the intensity of UV light to which the monitor is exposed, the photosensitive material changes physical characteristics and exposes some and conceals other indicia to indicate a duration of safe exposure to the sun for given conditions or the SPF value of the sun protection product used to be used, or both.