A contrast enhancing agent can be applied to the skin to enhance or enable the response of a sensor element in a dermatological handpiece for measurement of one or more positional parameters of the handpiece. The positional parameter(s) can be used as part of a feedback system that controls the treatment dosage. A bleaching agent is applied to the skin surface to reduce the visibility of the topically applied contrast enhancing agent as part of a treatment with a dermatological handpiece wherein at least one positional parameter of the handpiece is measured and used to control the treatment dosage.