A mobile device, movable across a surface by a user, that provides ultraviolet light for a process of sanitizing or sterilizing the surface that comprisesan ultraviolet light source for illuminating the surface with the ultraviolet light to thereby sanitize or sterilize the surface,a movement sensor operatively coupled to a microprocessor that receives movement data from the movement sensor, andan indicator that is operatively coupled to the microprocessor,with the microprocessor comprising programmingto perform calculations comprising the movement data and an intensity of the light at the surface to calculate a calculated dosage of the light at the surface andto perform a comparison of the calculated dosage to a predetermined dosage,wherein the microprocessor, based on the comparison, controls the indicator to provide instructions to the user.