A device including: a processor being configured to: instruct an image sensor to capture first image data of a patch applied to a user's skin unprotected against an ultraviolent spectrum of radiation, the patch having a reflection coefficient in the ultraviolent spectrum; measure a first reflectance from the patch based on the reflection coefficient; determine a second reflectance from the unprotected skin adjacent to the patch based on the measured first reflectance; instruct the image sensor to capture second image data of the user's skin after application of a sunscreen; determine a third reflectance from the skin adjacent to the applied patch for the skin having the applied sunscreen based on the measured first reflectance; and determine a time extending factor for the applied sunscreen based on the second and third reflectance.